remagnetization of the coast range ophiolite and lower part of the great valley sequence in northern california and southwest oregon overprinted magnetizations have been found at four localities in the middle jurassic coast range ophiolite and the overlying upper jurassic and lower cretaceous great valley sequence in northern california and at one locality in the partially correlative lower cretaceous days creek formation in southwest oregon at del puerto canyon on the east side of the diablo range a pilot study of the upper jurassic lotta creek formation gives in situ paleomagnetic directions grouped around the present geomagnetic field suggesting magnetic overprinting long after deposition and folding we suggest that the loss of original magnetization could be the result of long burial at about 7 km depth followed by late tertiary uplift the possibility of chemical remagnetization however can not be excluded paleomagnetic data from sandstones from the lower cretaceous great valley sequence in the wilbur springs area suggest cenozoic remagnetization that could be related to pliocene and pleistocene volcanic and hydrothermal activity in the area the results from calcareous concretions in the wilbur springs area from 21 sites from basalts and overlying sedimentary rocks of the great valley sequence at stonyford and from several sites in various rock types in the coast range ophiolite and great valley sequence near paskenta suggest unblocking of magnetization during the long period of burial at depth of 7 km or more and remagnetization during tectonic uplift in the late cretaceous or tertiary chemical remagnetization however can not be excluded the paleomagnetic data from six sites in lower cretaceous sedimentary rocks of the days creek formation in southwest oregon which overlies a dismembered ophiolite fail the fold test in situ paleomagnetic directions group near the expected tertiary field directions these sedimentary rocks may have been remagnetized during a major eocene tectonic event known to have occurred in this region the paleomagnetic results confirm that the coast range ophiolite and the seemingly little deformed great valley sequence in northern california as well as correlative rocks in southwest oregon have had a complex tectonic history once the nature and timing of these events are better understood the timing of remagnetization may be further constrained which in turn could give further insight into the nature of the tectonic events

mesozoic transpression transtension subduction and metallogenesis in northern and central california middle paleozoic to middle jurassic terrane assemblies in the klamaths and sierran foothills consist of mafic ultramafic complexes fine grained terrigenous strata derived from previously accreted continental margin belts sutured oceanic terranes reflect c 230 myr of margin parallel slip involving chiefly transtension and transpression quartzofeldspathic clastic rocks and blueschists eclogites are very rare little devolatilization occurred at magmagenic depths hence coeval hydrothermal ore deposits and granitoids are uncommon in contrast nearly head on cretaceous subduction of the farallon plate generated the massive klamath sierra nevada volcanic plutonic arc reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone immature great valley forearc and franciscan trench deposits shed from the arc record c 70 myr of rapid crustal growth au bearing solutions rising from magmagenic depths exsolved from plutons and expelled from heated wall rocks were mobilized attending arc construction precipitation of gold bearing quartz veins occurred where h2o co2 bearing fluids encountered major geochemical discontinuities in the wall rocks

ophiolitic thrust nappes in western nevada implications for the cordilleran orogen new field and geophysical evidence from jurassic mafic and ultramafic rocks in western nevada indicate the presence of two major ophiolite complexes tectonically overlying the rocks of the north american continental margin coupled with the existence of jurassic and older island arc marginal basin complexes and overthrust relations in the adjacent sierran klamath region these ophiolitic thrust nappes in nevada support collision models for the middle jurassic deformation the geometry of the observed structures and their relative age relationships suggest that this deformation might have resulted from the collision of the north american continent with a west dipping subduction zone which resulted in ophiolite emplacement and continental underthrusting continued westward motion of north america beneath the allochthonous oceanic terranes during the late mesozoic developed extensive thrust and possible strike slip faulting in the western us cordillera and caused migration of deformation towards the east this eastward propagation of deformation is interpreted to have resulted in the development of the sevier foreland thrust belt of latest jurassic to latest cretaceous age these collisional processes and the geometry of the proposed collision orogen are reminiscent of the alpine type collision belts

petrofacies and provenance of late mesozoic forearc basin northern and central california data from the great valley group sequence represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc this information is useful in documenting tectonic and magmatic events within the arc and related terranes and forms the basis for the establishment of petrostratigraphic units for mapping and correlation sandstone and conglomerate compositions are controlled by changes in provenance many of which were basinwide and synchronous clay mineral composition is controlled primarily by burial metamorphism careful attention to sample collection sample preparation and petrographic techniques is essential for uniform results seven petrographic parameters p f lv l m qp q q f and l listed in decreasing importance to petrofacies discrimination define eight petrofacies stony creek platina lodoga grabast boxer cortina los gatos and rumsey listed in approximate order of decreasing age the upper jurassic lower cretaceous petrofacies stony creek platina and lodoga contain higher lithic contents supracrustal sources whereas the upper cretaceous petrofacies especially the rumsey contain higher proportions of plutoniclastic components quartz feldspar and micas the proportion of potassium feldspar increases from near zero in the upper jurassic to nearly 50 of all feldspars in the uppermost cretaceous the lower part of the great valley group upper jurassic and lower cretaceous contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes tectonic highlands formed by the arc arc collision nevadan orogeny that occurred prior to initiation of the franciscan great valley sierra nevada arc trench system the klamath mountains area provided a major proportion of this detritus ophiolite and serpentinite detritus was deposited locally near the base of the great valley group as a result of deformation along the east side of the growing franciscan subduction complex volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the sierra nevada area during the cretaceous as the volcanic cover was stripped off plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin crustal components were more continental in the southern sierra nevada and more oceanic in the northern sierra nevada as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south by the middle of the late cretaceous extensive batholithic terranes provided potassium feldspar rich arkosic detritus to the entire forearc basin by the paleogene arc magmatism had migrated eastward sufficiently that deeper levels of the california part of the arc were exposed by erosion tectonic activity decreased in the forearc basin and the basin was filled to sea leve in most parts

tectonic controls on mudrock geochemisry mesozoic rocks of eastern oregon and western idaho u s a implications for cordilleran tectonics abstract tectonic models for the mesozoic evolution of accreted island arcs and subduction complexes in the blue mountain province bmp of eastern oregon and western idaho vary widely the bmp is situated between coeval accreted terranes of southern british columbia and the western u s and is critical for interpreting mesozoic paleogeography of the u s cordillera in this study we interpret the triassic jurassic evolution of the bmp using geochemistry as a tool to evaluate the provenance of mudrocks in fine grained turbidites temporal and spatial variations in mudrock geochemistry indicate that during late triassic to early jurassic time the wallowa terrane was an intra oceanic island arc and the olds ferry terrane was a pericratonic continental fringing island arc subsequently a regionally extensive middle to late jurassic marine basin received input from continental sources and sediment may also have been derived from coeval terranes to the south comparison of provenance data from the bmp with similar data from accreted terranes to the north and south suggests that triassic jurassic basins of the western u s share a long term provenance history within a complex system of continent fringing and intra oceanic island arcs in contrast it appears that the quesnel terrane of southern british columbia had a distinct provenance with a more direct link to laurentia during triassic time early to middle jurassic sedimentary rocks from southern british columbia and the western u s show a high degree of geochemical provenance similarity this may indicate reorganization of formerly discrete tectonic regions into a nascent integrated western laurentian margin prior to complete development of a cretaceous andean type margin the results of this study demonstrate the utility of mudrock geochemistry in tectonic analyses and show that major and trace element geochemistry of mudrocks is useful for developing a comprehensive understanding of regional tectonic evolution

genesis and evolution of a permian jurassic magmatic arc accretionary wedge and reevaluation of terranes in the central klamath mountains investigation of key areas of the central klamath mountains demonstrates that genetic relationships connect several regionally significant lithologic units that were previously considered to be separate tectonostratigraphic terranes the rocks embody an intraoceanic tholeiitic to alkaline magmatic arc and contiguous accretionary wedge formed in permian to middle jurassic time the eastern hayfork and st claire creek units comprise permian jurassic oceanic sedimentary rocks deposited nearby and coeval with eruption of the north fork salmon river volcanoplutonic arc the more inboard st claire creek unit consists of stratigraphically coherent chert and minor argillite probably deposited in a back arc setting whereas the more outboard eastern hayfork sedimentary rocks were disrupted during formation of an accretionary wedge the volcanoplutonic north fork salmon river arc extends for 200 km in the central klamath mountains and was active episodically or continuously from permian through middle jurassic time alkalic members of the mafic igneous suite may have erupted following subduction of a spreading center a varied heating history has produced an amalgam of submarine and regional low pressure moderate temperature metamorphism overprinted locally by contact metamorphism geochronology of weakly metamorphosed mafic dikes sills 10 new 40ar 39ar analyses suggests that significant heat was retained regionally into early cretaceous time

an expanded view of jurassic orogenesis in the western united states cordillera middle jurassic pre nevadan regional metamorphism and thrust faulting within an active arc environment klamath mountains california basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic ultramafic intrusive complexes delineate a middle jurassic arc terrane within the klamath mountain province of northern california exposures of the supracrustal volcaniclastic rocks are restricted to a single fault bounded terrane but the deeper level intrusive complexes were emplaced into most if not all the pre late jurassic terranes of the klamath mountain region the pre late jurassic terranes thus constitute the basement of the middle jurassic arc u pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus k ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of 177 159 ma the active arc and its basement were imbricated by a compressive deformational event the signature of which included thrust faulting isoclinal folding and regional metamorphism several diverse lines of evidence including k ar dating of metamorphic rocks crosscutting relations of dated intrusive complexes to thrust faults and u pb dating of synmetamorphic intrusive complexes establish a distinctly pre nevadan middle jurassic age ongoing at 169 ma and over by at least 161 ma for this compressive deformational episode outboard and structurally beneath the middle jurassic arc and its basement are several terranes that collectively comprise the western jurassic belt these terranes were deformed and regionally metamorphosed during the late jurassic nevadan orogeny which occurred within the time interval of 157 150 ma as upper jurassic plutons with 150 to 142 m y old zircon ages have contact aureoles that overprint the nevadan fabric and the 157 m y old rogue formation was deformed in the nevadan event the middle and late jurassic compressive deformational events were thus distinct and separated by as much as 15 20 m y the relation between middle and late jurassic magmatism and deformation suggests that the klamath mountain province records the evolution of a considerably long lived arc system that evolved above an east dipping subduction zone in addition we suggest that this are system may represent an oceanic continuation of the long recognized early mesozoic arc terrane of the western u s cordillera

diverse sources for igneous blocks in franciscan melanges california coast ranges igneous blocks in franciscan melanges are of three chemical petrologic types 1 tholeiitic basalts of both arc and spreading center origin with depletions in light relative to heavy rare earth elements 3 tio 2 1 high y zr and y ti ratios and relict augites that generally have low al and ti and well defined iron enrichment trends 2 basalts of probable seamount origin with marked enrichments in light relative to heavy rare earth elements 5 tio 2 1 lower y zr and y ti than 1 and ti al rich augites showing little if any iron enrichment trends and 3 hypabyssal intrusives having sio 2 52 tio 2 1 flat or only slightly fractionated rare earth abundance patterns and diopsidic augites that are very low in ti and al and show no iron enrichment trends all of the blocks are metamorphosed most are undeformed pumpellyite bearing greenstones and a few contain sodic amphibole lawsonite sodic pyroxene the melanges are probably olistostromal in origin deriving the

tectonic significance of low temperature blueschist blocks in the franciscan melange at san simeon california abstract the blueschist bearing franciscan melange exposed near san simeon is very similar to the melange terrane forming the central belt north of san francisco epidote bearing low t blueschist blocks at san simeon n 3 are the same age as high t garnet bearing blocks in the northern and diablo range melange terranes 155 150 ma this age approximates the time of initiation of franciscan subduction during the initial stages of franciscan subduction part of the top volcanic sequence from the subducting oceanic plate and small volumes of overlying sediments were imbricated at the bottom of the overriding plate and metamorphosed under high p conditions this underplated material was variably retrograded as subduction refrigerated the forearc blocks actinolitic rinds formed at the contacts between blueschists and intercalated mantle under relatively static conditions as the base of the overriding plate became serpentinized a model is proposed in which exhumation was facilitated by gravity driven seaward oriented extensional thinning of the forearc prism caused by the decrease in subducting plate dip starting around 80 ma that caused the laramide orogeny as this occurred blueschist and graphite schist blocks were plucked from the bottom of the hanging wall incorporated into the shale and water rich shear zone at the plate interface subduction channel shear zone and exhumed during the upward flow of melange driven by the movement of the downgoing plate while subduction was still active

regional detrital zircon provenance of exotic metasandstone blocks eastern hayfork terrane western paleozoic and triassic belt klamath mountains california abstract the addition of oceanic material through accretion underplating was an important process in construction of the convergent transpressive cordilleran margin of north america according to radiolarian ages the late triassic or younger eastern hayfork terrane represents trench deposits that accreted outboard of the coeval north fork immature island arc it consists chiefly of chert and argillite but contains tectonic and olistostromal blocks of preexisting rock units derived from exotic sources that include inboard klamath terranes fine grained terrigenous debris was shed from these eastern terranes but the eastern hayfork melange also contains blocks of quartzofeldspathic metasandstone of uncertain provenance new detrital zircon u pb ages from five blocks of immature variably metamorphosed arkosic to subarkosic sandstone range from late proterozoic to late archean with modes of 1804 1843 1970 2300 and 2550 2585 ma the adjacent inboard north fork terrane contains zircons with phanero

permo triassic accretionary subduction complex southwestern klamath mountains northern california the western paleozoic and triassic subprovince of the southwestern klamath mountains has previously been subdivided into three relatively steeply dipping fault bounded litho tectonic terranes in descending structural order from east to west these terranes are 1 the north fork terrane 2 the hayfork terrane and 3 the rattlesnake creek terrane in a general fashion each of these terranes exhibits its own particular set of lithologic structural and temporal relationships that sets it apart from adjacent terranes this threefold subdivision of the southwestern klamath mountains needs modification only as far as the hayfork terrane is concerned the hayfork terrane is actually a composite terrane consisting of a structurally lower assemblage of predominantly basaltic metavolcaniclastic rocks and a structurally higher assemblage of chaotically disrupted metasedimentary rocks juxtaposed along a previously unrecognized major regional thrust these two lithologically and structurally distinct assemblages have been elevated to the status of a terrane in this report and are herein referred to as the western hayfork terrane structurally lower metavolcaniclastic assemblage and the eastern hayfork terrane structurally overlying metasedimentary assemblage and the fault which juxtaposes these two terranes is herein referred to as the wilson point thrust this paper will focus on the eastern hayfork and north fork terranes and their relationship to other roughly coeval tectonic elements of the klamath mountain province the eastern hayfork terrane is best described as a tectono stratigraphic unit consisting of a complex chert argillite and quartzose sandstone melange and broken formation that contains radiolarian cherts as young as late triassic in age scattered throughout this terrane are various types of blocks some of which are clearly exotic metachert is by far the most common type of metamorphic block and these blocks appear to have been derived via sedimentary processes from the paleozoic central metamorphic belt and or the mesozoic stuart fork formation shallow water permian limestone blocks are also plentiful and are interpreted as reefal accumulations which flanked seamounts that became incorporated into the melange and broken formation as proto pacific ocean floor was subducted beneath the western margin of north america during at least the permo triassic to early jurassic the north fork terrane has at its base basaltic rocks tentatively interpreted as an oceanic crustal remnant generated within an open ocean ridge setting that was removed from any appreciable source of continental or volcanic arc derived detritus overlying the basaltic rocks is a sequence of pelagic hemipelagic terrigenous and tuffaceous metasedimentary rocks that have yielded a late triassic and early or middle jurassic radiolarian fauna the metasedimentary rocks of the north fork terrane are lithologically very similar to those of the eastern hayfork terrane both terranes consist predominantly of chert and argillite contain blocks of metamorphic rock and permian limestone and are at least in part of overlapping age however the metasedimentary rocks of the north fork terrane are structurally coherent unlike the chaotic melange and broken formation of the eastern hayfork terrane taken together the north fork and eastern hayfork terranes bear remarkable similarities to both modern and ancient rock assemblages that occupy sites within zones of lithospheric plate convergence and are interpreted as tectonic elements of an underthrust plate margin developed oceanward of a volcanic arc represented by the permo triassic to middle jurassic volcanic strata of the eastern klamath belt

extensional fault uplift of regional franciscan blueschists die to subduction shallowing during the laramide orogeny regional structural and stratigraphic relations in the northern california coast ranges require an episode of extensional tectonic unroofing of coherent blueschist terranes of the eastern belt of the franciscan complex along a crustal scale low angle normal fault system here named the cost range detachment this tectonic unroofing occurred during the latest cretaceous and early tertiary contemporaneous with the compressional laramide orogeny of the north american interior dynamic models of accretionary wedges predict that shallowing of subduction as has been inferred during the laramide orogeny will impart extensional stresses in the wedge above the subducting plate the authors therefore suggest a single casual event gradual shallowing of subduction in the latest cretaceous and early tertiary to link the extensional fault uplift of regional franciscan blueschists to laramide compression in the north american interior

polygenetic ophiolite belt of the california sierra nevada geochronological and tectonostratigraphic development the assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology however this assumption has only been verified in a few locations by an integrated geochronological and structural stratigraphic approach many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind such reconstructions are prone to error without adequate age control on each of the reconstructed fragments this is a significant problem in structurally complex regions where more than one generation of ophiolite may be present in this paper new pb u zircon ages are presented for key locations along a 375 km segment of the western sierra nevada ophiolite belt these age data are combined with structural stratigraphic observations and published ages and significant tectonic implications for the ophiolite belt emerge three different ophiolitic assemblages are recognized with igneous ages of about 300 200 and 160 m y b p rocks of the 300 m y assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite matrix melange fragments of upper paleozoic seamounts occur in association with the ophiolitic melange and together these assemblages constitute the basement framework for the western sierra pb u and k ar isotopic systematics are complex within this framework and indicate a polymetamorphic history systematics in the 200 and 160 m y assemblages are less complex and give tighter igneous age constraints rocks of the 200 m y assemblage are in a semi intact state with only local tectonite and melange zones rocks of the 160 m y assemblage are intact but nevertheless deformed both the 200 and 160 m y assemblages have equivalent age basinal volcanic sedimentary sequences that lie unconformably above the ophiolitic melange basement in each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age equivalent ophiolite sections these relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement the sierran ophiolite belt is considered to have formed by a multistage process initiated by the early mesozoic tectonic accretion of upper paleozoic sea floor in general proximity to the ancient continental margin regional metamorphism and ophiolitic melange resulted this accretionary nucleus became the basement of jurassic age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m y ophiolites the rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic sedimentary sequences non volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes contact zones between pre existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles protoclastic deformation zones and cross cutting dikes such edge zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges both the 200 and 160 m y basins were destroyed by compressional orogenic episodes shortly after their formational episodes destruction of young ophiolite floored basins may be a common course of events when small oceanic type plates are generated along continental margin environments such tectonic settings are ideal for the emplacement of young ophiolite sheets

hydro and morpho dynamic modeling of breaking solitary waves over a fine sand beach part ii numerical simulation abstract a comprehensive numerical model is developed to predict the transient wave propagation sediment transport morphological change and the elastodynamic responses of seabed due to breaking solitary waves runup and drawdown over a sloping beach the individual components of the numerical model are first validated against previous analytical numerical and experimental results the validated numerical model is then used to simulate breaking solitary wave runup and drawdown over a fine sand beach where the experimental results are presented in young et al 2010b hydro and morpho dynamic modeling of breaking solitary waves over a fine sand beach part i experimental study the strengths and weaknesses of the model are assessed through comparisons with the experimental data based on the results sediment transport mechanisms and wave seabed interactions in the nearshore region are discussed

tectonic implications of diverse igneous blocks in franciscan melange northern california and southwestern oregon more than 50 of the igneous blocks from melanges of the franciscan subduction complex in northernmost california and from the equivalent dothan formation of southwestern oregon are more silica rich than basalt and some contain supra subduction zone geochemical signatures such as elevated th ta ratios in contrast blocks from more southerly parts of the franciscan melange belt are more commonly mid ocean ridge and ocean island seamount basalts the data indicate a north south variation in the block population due to differences in source terrane or in the processes that provided the blocks and transported them into the melange or both we suggest that in the northern franciscan a relatively greater fraction of igneous melange blocks was emplaced by block slumping from the forearc crust into the trench than was the case to the south

franciscan subduction off to a slow start evidence from high precision lu hf garnet ages on high grade blocks lu hf analyses of garnet from metabasic amphibolite glaucophane schist and eclogite facies blocks from the franciscan complex give highly precise ages that allow us to place new constraints on the early thermal history of the franciscan subduction zone garnets yield 176 lu 177 hf ratios ranging from 1 5 to 28 with the highest ratios from garnets with high spessartine pyrope ratio sulphuric acid leaching sal of garnets revealed the presence of inclusions with significantly higher lu hf ratios than those of garnet itself most likely apatite their removal by sal brings the 176 lu 177 hf ratios in garnets down by as much as 40 this suggests that 176 lu 177 hf ratios of apparently pure garnets can be greatly overestimated due to the presence of such inclusions sm nd garnet analyses were dominated by inclusions mainly sphene and failed to provide precise and accurate age information the oldest lu hf ages are 168 7f0 8 and 162 5f0 5 ma on plagioclase bearing garnet amphibolite from panoche pass and the berkeley hills respectively which suggests initiation of the subduction zone at about 169 ma coeval with the formation of the tectonically overlying coast range ophiolite relatively high temperature conditions persisted for about 14 ma as indicated by 153 4f0 8 ma garnet growth recorded in epidote amphibolite and 157 9f0 7 in eclogite from ring mountain and jenner respectively a 146 7f0 7 ma age was obtained from garnet glacuophane schist metamorphosed at around 400 jc the sequence of ages from central and northern california shows a younging trend with decreasing metamorphic grade which supports previous suggestions that the high grade metamorphic blocks and slices resulted from progressive underthrusting and underplating in a cooling subduction system combining geothermometry with geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist facies conditions as ca 15 jc ma the thermal history requires high initial geothermal gradients within both the footwall and the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km ma during the initial stages of franciscan subduction such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre the data also suggest slow exhumation rates and significant residence time at depth of the earliest franciscan rocks

isotopic systematics of pb u zircon and 40ar 39ar biotite hornblende from rocks of the central foothills terrane sierra nevada california pb u zircon and 40 ar 39 ar biotite horn blende studies in conjunction with detailed structural and textural studies clarify the age and structural relations of a unique cluster of plutons in the central sierra nevada western metamorphic belt the plutons range from gabbro to tonalite trondhjemite and include the guadalupe igneous complex magmatic emplacement ages for the plutons are for the most part between 150 and 145 m y as recorded in both the zircon and hornblende isotopic systems additional ages of 138 and 123 m y were determined on small plutons having key structural settings in general the pb u and 40 ar 39 ar isotopic systems are well behaved second order complications are locally present in the pb u systems owing to the entrainment of older zircon from wall rocks and in 40 ar 39 ar systems owing to minor compositional differences in mica and amphibole phases the age data and structural relations of the plutons in conjunction with 40 ar 39 ar metamorphic wall rock biotite and hornblende age data constrain regional ductile deformation in the study area to have occurred between 150 and 125 ma such deformation was accompanied by upper greenschist to lower amphibolite facies metamorphism and is at least in part closely related to pluton emplacement such deformation and metamorphism has in the past been attributed to the late jurassic nevadan orogeny this study shows clearly however that ductile deformation and metamorphism in the study area are post nevadan if modern concepts of the nevadan orogeny are strictly adopted

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

accretionary tectonics of the north american cordillera continental geology stands on the threshold of a change that is likely to be as fundamental as plate tectonic theory was for marine geology ongoing seismic reflection investigations into the deep crustal structure of north america are verifying that orogenic zones are underlain by low angle faults of regional extent brown et al 1981 the growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture bounded fragments particularly within the north american cordillera such interdisciplinary studies are leading to a consensus that the cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in mesozoic and early cenozoic time such crustal growth is referred to as accretionary tectonics in this paper we review some of the important concepts in accretionary tectonics discuss the nature of the materials accreted between central alaska and southern california in jurassic and cretaceous time and consider the general relations between cordilleran accretion and the movement of lithospheric plates the concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest with the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen along with mechanisms for crustal attrition accretion mechanisms include the growth of imbricated sedimentary prisms along inner trench walls slicing off of submarine topographic irregularities within subducting plates and collision of continents and volcanic arcs by ocean basin closure tectonic attrition mechanisms include rifting transform faulting and strike slip or underthrust removal of inner trench wall materials coincident with or in place of accretionary prism growth growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments an important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins since attrition by nature leaves little material evidence of having operated one of the major problems confronting cordilleran geologists lies in the recognition of such attrition within the ancient record particularly when interspersed with accretionary events the spectrum of accretion and attrition mechanisms viewed at cm yr 1 plate transport rates over time scales of 100 m y leads one to suspect a highly mobile history for continental margin orogens the serial arrangement of subducting transform and rifting links along the modern cordillera plate juncture system and both serial and parallel arrangements in the western pacific systems show the complex interplay of such mechanisms through space similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the cordillera which presently resemble a collage of crustal fragments davis et al 1978 recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and earth history the first problem to be considered is the recognition of native north american crust from exotic fragments that have been accreted to its edge

large pressure gap between the coastal and central franciscan belts northern and central california abstract the nature of the metamorphism of the central belt of the franciscan complex has long been debated did the whole central belt or its melange matrix undergo high p t metamorphism to solve this problem we have investigated the regional distribution of aragonite in these rocks in northern and central california we have discovered more than 300 new localities of aragonite using the staining method by friedman 1959 aragonite occurs as network veins or pods in both metagraywacke and matrix forming metashale and is invariably partially replaced by calcite on its rim thus we conclude that aragonite is stable in all parts of the franciscan central belt from the lower boundary between the coastal and central belts to the topmost part of the central belt and that both the matrix forming melange shale and exotic blocks of the central belt suffered high p t metamorphism we traced the boundary between the central and coastal belts in northern california using the staining method along the gently eastward dipping thrust plane between the underlying coastal and overlying central belts more than 2 5 kbar pressure break is obtained through the lawsonite laumontite aragonite quartz relations such an observed large pressure gap was formed during the exhumation process of the central belt of the franciscan complex

field investigation of tsunami impact on coral reefs and coastal sandy slopes abstract the objective of this study was to investigate the effects of tsunamis on coral reefs and sediment beds in the near shore region a remotely operated vehicle rov was used to survey the coral reef and collect sediment samples from the coastal regions surrounding american samoa five weeks after the tsunami generated by a magnitude 8 3 earthquake on september 29th 2009 the surveys recorded extensive damage to the coral reefs as evident by the broken corals of almost every type in the region and by debris from onshore along the coastal regions of poloa 14 19 11 s 170 50 11 w and leone 14 20 30 s 170 47 1 w the field observations suggest that sand was carried from the beach out to sea by the tsunami drawdown causing net beach erosion which is consistent with findings from model scale studies presented in young et al 2010 the field observations also suggest that the high sheer stresses created by the rapidly retreating seaward flow and the sudden release of the sediments and debris entrained in the water during the drawdown process are responsible for majority of the damage to the coral reef

evolution of intra oceanic arc trench systems abstract intra oceanic island arc trench systems represent a locus of crustal thickening through the addition of mantle derived magmas which form the volcanoes and subjacent stratiform plutons the crust of immature island arcs and adjacent back arc basins is oceanic in chemistry in that it is dominated by mafic e g tholeiitic magmas a magma sequence is recognized in many arc systems which progress from boninite to arc tholeiite to calc alkaline and in some arcs to shoshonites the adjacent back arc basins are formed of morb like tholeiite arc systems may be dominated by extension rather than compression if the subducted and overriding plates are decoupled extension of the arc system may be associated with seaward retreat or roll back of the trench axis alternate cycles of extension and compression of the arc system can lead to both tectonic erosion of the fore arc and accretion of fragments from the incoming sea floor the izu bonin mariana tonga yap and palau trench arc systems are examples of different stages in a dynamic process which starts when subduction begins along old fracture zone traces crustal extension causes both fore arc subsidence and opening of back arc basins island arc volcanic chains and back arc basins are new crust formed largely of fractional melts of the mantle the evolution of an arc system may stop when blocking of the subduction zone or changes in relative plate motion causes overthrusting of back arc lithosphere on to former volcanic arcs many ophiolites preserve remnants of arc system crust and upper mantle

post middle miocene accretion of franciscan rocks northwestern california deformed sedimentary rocks assigned to the franciscan assemblage in the king range south of cape mendocino northern california are dominantly deep water argillite and sandstone occurring as thick to thin bedded locally channelized marine turbidites of arkosic to andesitic volcani clastic composition these rocks structurally overlie late cretaceous coniacian to campanian basaltic pillow flows flow breccias diabase sills and arkosic sandstone turbidites that also are assignable to the franciscan assemblage and are well exposed along the coast at point delgada radiolarians and planktic and benthic foraminifers indicate that the oldest rocks in the king range may be early tertiary and that the youngest rocks are middle miocene or younger the youngest rocks yet reported from the franciscan assemblage partly coeval miocene relizian and luisian to quaternary rocks representing shallower sedimentary facies of the inner shelf to outer continental slope occupy the eel river basin and isolated fault bounded slivers adjacent to the king range on the north and east wide zones of penetrative shearing that may delineate a middle miocene or younger suture separate the deep water franciscan rocks of the king range from less severely deformed neogene rocks of the eel river basin the king range appears to be a displaced terrane of oceanic basement overlain by paleogene and neogene sedimentary and igneous rocks of continental and oceanic derivation this terrane may have been displaced north or eastward and obductively accreted to california 15 m y ago before or perhaps during northward passage of the pacific farallon north american plate junction

mid paleozoic olistoliths in eastern hayfork terrane melange klamath mountains implications for late paleozoic early mesozoic cordilleran forearc development the western paleozoic and triassic belt in the southern klamath mountains contains several convergent margin tectonic assemblages that show great variation in lithology and structural patterns many preserve evidence of intermediate to high pressure facies series metamorphism consistent with a subduction and or accretionary origin prior to regional mid jurassic magmatism and shortening retention of a pre middle jurassic record is revealed by new 40ar 39ar data from the eastern hayfork terrane chert argillite melange which document unexpectedly old cooling ages of 346 360 ma for micas and amphiboles in a variety of blocks within melange matrix these early mississippian cooling ages and petrologic criteria indicate that the blocks probably originated as olistoliths derived from an exhumed central metamorphic belt in the late paleozoic minor low temperature discordance of the block ages suggests low grade reheating between about 240 and 280 ma coincident with an age on eastern hayfork matrix of 263 ma that reflects early permian or younger melange formation the mineral spectra indicate regional temperatures have not significantly exceeded 300 c since the mississippian this early permian accretionary history may only be preserved in the southern klamath mountains where mid jurassic granitoids are sparse we suggest that a late paleozoic eastern hayfork accretionary complex was constructed proximally to denuded eastern klamath forearc basement in an environment analogous to the modern sumatran convergent margin

a plate tectonic model for late jurassic ophiolite genesis nevadan orogeny and forearc initiation northern california recently published age and structural data allow the reconciliation of previously conflicting models for late jurassic genesis of the josephine smartville and coast range ophiolites and the nevadan orogeny in the klamath mountains and sierra nevada the resulting model is consistent with the mode of initiation location and geometry of the great valley forearc basin and with the lack of a significant forearc basin west of the klamath mountains the coast range ophiolite formed by backarc spreading west of an east facing intraoceanic arc soon thereafter a remnant arc was calved off the west side of this arc and the smartville ophiolite formed by backarc interarc spreading during this time the sierran phase of the nevadan orogeny began as the intraoceanic arc encountered the west facing continental margin arc of north america an east west trending calcalkaline dike swarm in the sierra nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision simultaneously a new subduction zone was initiated west of the collision suture zone and this new trench propagated southward thus trapping the coast range ophiolite in the new forearc area south of the klamath area intense deformation in the sierran region resulted from this collision and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted continued westward relative movement of the north american arc was permitted north of the sierra nevada owing to the lack of a colliding intraoceanic arc the result was the westward rifting of the continental margin arc by intraarc spreading which formed the josephine ophiolite in the klamath area the klamath phase of the nevadan orogeny resulted from contraction of the west facing intraoceanic arc and josephine backarc basin beneath the continental margin basal sediments of the great valley forearc basin were derived primarily from the sutured arc ophiolite terranes and were deposited on top of the coast range ophiolite the southern edge of the klamaths and the western side of the sierra nevada a new late mesozoic magmatic arc was superposed across the previously accreted terranes and formed the primary sediment source for the cretaceous forearc basin

u pb sm nd and rb sr geochronological and isotopic study of northern sierra nevada ophiolitic assemblages california distinct ophiolitic assemblages occur as oceanic basement within three of the four regional tectonic belts of the northern sierra nevada new u pb zircon sm nd and rb sr data are presented for each assemblage providing critical geochronological and isotopic constraints on the petrogenesis and tectonic evolution of the ophiolitic and associated ensimatic assemblages ophiolitic assemblages include from west to east the smartville complex central belt and feather river belt the smartville complex represents an island arc volcanic plutonic sequence with a major late stage sheeted dike swarm the sm nd systems from a wide compositional spectrum of rocks record a 178 21 ma petrogenetic age and an nd t 9 2 0 6 zircon u pb systems on an uppermost dacite yield a 164 2 ma age and on a number of plagiogranite screens and dikes from the sheeted complex 162 1 ma ages the central and feather river belts are structurally complex polygenetic assemblages the u pb zircon and sm nd systems record major 205 ma and 315 ma petrogenetic events respectively both involving depleted mantle derived magmas such magmatism probably occurred in marginal basin transform systems developed within an older oceanic depleted mantle basement regime both sm nd and u pb zircon systems show local components of proterozoic sialic material the sialic contaminants were probably introduced into the system as craton derived detritus it is doubtful that any of the ophiolitic assemblages studied represent genetically related crust upper mantle sequences generated during the development of new oceanic lithosphere integration of the geochronological data with geological relations reveals a pattern of petrogenesis and tectonics whereby progressively younger ensimatic terranes were added to the continental margin through time by plate convergence and were ultimately welded into north american sial by a crosscutting batholithic belt this accretionary pattern is reflected in both the protolith ages and deformation metamorphic ages of each of the regional belts which progressively young westward crustal components of the accreted ensimatic terranes grew by mainly basaltic igneous activity within island arc marginal basin and leaky transform systems adjacent to the continent edge prior to final tectonic accretion such complexities are suggested to be typical of cordilleran type ophiolites and representative of the circum pacific erogenic style

geology and petrology of quaternary volcanic rocks garibaldi lake area southwestern british columbia summary recent geochemical studies of convergent margin volcanoes have been strongly influenced by the concept that calc alkaline andesites are primary magmas formed by partial melting of wet mantle peridotite above subducted oceanic crust fountain 1979 rogers and ragland 1980 suzuki and shiraki 1980 partial melting of the peridotite is thought to occur through access of volatiles released by dehydration of the descending oceanic crust mcbirney 1969 best 1975 or of volatile rich silicate melts generated by partial fusion of the crustal portion of the subducted plate ringwood 1975 such an interpretation is critically dependent upon the assumption that the composition of melts erupted at the surface has been little modified by crystal fractionation this paper examines the petrogenesis of pleistocene holocene andesites from the garibaldi lake area of southwestern british columbia on the light of observed mineralogical and chemical variations these andesites provide evidence that crystal fractionation processes have significantly modified magma compositions evolution of british columbia9s continental margin during the past 10 m y has been dominated by a complex pattern of plate interactions riddihough 1977 keen and hyndman 1979 the present plate tectonic setting of southwestern british columbia is depicted in figure 1 the queen charlotte fault system which parallels the edge of the continental shelf from southeastern alaska to a poorly defined ridge trench fault triple junction just north of vancouver island marks the locus of right lateral transcurrent motion between the pacific and north american plates en echelon spreading axes juan de fuca ridge explorer ridge dellwood and j tuzo wilson knolls offset by short transform segments form the juan de fuca pacific and explorer pacific plate boundaries the explorer plate detached from the juan de fuca plate as the spreading ridge system rotated to become more nearly orthogonal to the queen charlotte fault has been shown to be moving independently during the past 3 m y riddihough 1977 left lateral motion between the juan de fuca and explorer plates occurs across the nootka tranform fault system which extends north easterly from the northern end of the juan de fuca ridge to nootka island off the west coast of vancouver island keen and hyndman 1979 south of the nootaka fault the boundary between the juan de fuca and north american plates is a zone of convergence seismic data show that the oceanic lithosphere dips at 5 toward the margin and reaches depths of 4 to 5 km before disappearing under vancouver island riddihough 1979 the absence of a clearly defined eastward dipping seismic zone and the lack of a major bathymetric trench at the foot of the continental slope have contributed to uncertainty as to whether subduction has taken place recently riddihough and hyndman 1979 ando and balazs 1979 and riddihough 1979 however have reviewed relevant geological geodetic and geophysical data and concluded that the juan de fuca plate is now continuously underthrusting western north america the plate motion is largely aseismic but a few benioff type events at about 50 to 60 km beneath southern georgia strait keen and hyndman 1979 provide some weak constraints on the present position of the subducted plate whether the explorer plate is now being subducted beneath the continent is less certain riddihough 1977 suggested the possibility that the oceanic lithosphere east of dellwood knolls has become locked to the north american plate

varying mantle sources of supra subduction zone ophiolites ree evidence from the zambales ophiolite complex luzon philippines the zambales ophiolite complex a supra subduction zone ophiolite is characterized by three volcanic hypabyssal rock units the coto block volcanic hypabyssal rocks the coto dikes intruded into the coto residual peridotites and the acoje block volcanic hypabyssal rocks the first two groups exhibit transitional mid ocean ridge island arc characteristics while the latter reveals island arc affinity furthermore these three volcanic rock suites are characterized by differing bulk ree major element trace element and mineral chemistries the coto block volcanic hypabyssal rocks ceyb n 0 4 1 0 tio2 0 50 1 50 wt zr 31 76 ppm y 13 31 ppm have higher ree ti zr and y than the coto dikes ceyb n 0 3 0 8 tio2 0 52 0 94 wt zr 15 55 ppm y 10 27 ppm and the acoje block volcanic hypabyssal rocks ceyb n 0 2 0 3 tio2 0 26 0 86 wt zr 11 45 ppm y 10 23 ppm arguing for an increasing degree of melting of the sources from the former to the latter these data suggest the involvement of several mantle sources which have undergone different degrees of partial melting and lree addition as evidenced by the presence of lree enriched basalts ceyb n 3 5 among the coto block volcanic hypabyssal rocks these results further illustrate the complexities involved in the generation and evolution of supra subduction zone ophiolites

the rattlesnake creek terrane klamath mountains california an early mesozoic volcanic arc and its basement of tectonically disrupted oceanic crust the rattlesnake creek terrane in the south western klamath mountains consists of a serpentinite matrix melange basement unconformably overlain by a coherent cover sequence of upper triassic and lower jurassic volcanic hemipelagic and clastic sedimentary rocks both the meelange and cover sequence are intruded by a suite of ca 193 207 ma u pb zircon gabbroic to quartz dioritic plutons these rocks collectively record a complex multistage evolution through several distinct tectonic environments blocks in the meelange basement include peridotite greenstone amphibolite pillow basalt various generally mafic plutonic rocks metachert and limestone all suspended in a matrix of sheared serpentinite trace element chemistry indicates that basalt amphibolite and greenstone blocks were derived from normal to enriched mid oceanic ridge basalt n morb to e morb and within plate basalt wpb protoliths there is no geochemical evidence that any blocks were derived from a magmatic arc nor do any of the sedimentary rocks contained within the meelange reflect deposition near a terrigenous sediment source collectively these relations indicate that the basement meelange was derived by disruption of oceanic crust and upper mantle in a setting far removed from either an arc or a terrigenous landmass and are most consistent with an interpretation that meelange formation oc curred in an oceanic fracture zone the overlying cover sequence is divisible into a stratigraphically lower salt creek assemblage and a stratigraphically higher dubakella mountain assemblage both deposited in a marine setting volcanic rocks in the lower assemblage are basaltic pillow lava and pillow breccia with the trace element characteristics of a primitive island arc tholeiite suite whereas those in the upper assemblage are predominantly chnopyroxene phyric basalt to basaltic andesite and compositionally similar volcaniclastic rocks with the geochemical characteristics of a more evolved cale alkaline to shoshonitic volcanic arc suite the geochemical character of these volcanic rocks is indistinguishable from that of the coeval early mesozoic plutons leading to the conclusion that these plutons represent the intrusive roots of the volcanic succession collectively these rocks reflect construction of a late triassic to early jurassic island arc and indicate that subduction had initiated within or near the under lying fracture zone assemblage by at least the late triassic the latest stages of crustal disruption in the basement thus probably occurred in a subduction zone environment both cover sequence units also contain inter layered chert and argillite as well as coarse epiclastic rocks with an abundance of quartzose sedimentary and metasedimentary grains these elastic rocks provide the first evidence for terrigenous input into the rattle snake creek terrane and indicate that the early mesozoic arc developed in proximity to a terrigenous sediment source probably paleozoic rocks of the western north american cordillera several lines of evidence suggest that early mesozoic arc magmatism was accompanied by an episode of extensional tectonism these include structural evidence for an early mesozoic phase of high angle faulting that appears to have localized cover sequence deposition in fault bounded grabens sedimentologic evidence indicating that both the basement and terrigenous sediment sources experienced episodic pulses of uplift and erosion during cover sequence deposition and widespread intrusion of arc plutons into the upper crustal levels of the are carapace both the basement complex and the early mesozoic arc assemblage of the rattlesnake creek terrane were then deformed by thrust faulting folding and penetrative foliation development during over thrusting from the east by the western hayfork terrane in the middle jurassic regional de formation was in response to northeast south west directed shortening and was accompanied by metamorphism to subgreenschist grade insofar as the western hayfork terrane represents part of a middle jurassic arc whose plutonic roots intrude most klamath mountain terranes east of and including the rattle snake creek terrane it is clear that the rattlesnake creek terrane formed part of the klamath mountain province by middle jurassic time the paleogeographic and tectonic relation of the rattlesnake creek terrane to the rest of the klamath mountains prior to the middle jurassic however is unclear

geochemical evidence for a subducted infant arc in franciscan high grade metamorphic tectonic blocks some of the evidence of tectonic and chemical processes that occur in subduction zones is preserved in blueschists and eclogites as validated by the geochemical data presented from high grade metamorphic tectonic blocks from the franciscan complex in central california combining major elements compatible and incompatible trace element concentrations and ratios and sr nd pb isotopic data we conclude that the protoliths of these high grade franciscan blocks were arc lavas with no continental crust derived components although the protoliths underwent high grade metamorphism at the inception of subduction the tectonic affi nity of the protoliths can be distinguished by our geochemical study our geochemical data are signifi cant in that 1 the protoliths of these high grade blocks have a different mode of origin than the protoliths of the lower grade franciscan basalts that were incorporated later in the subduction complex as the latter generally are considered to be oceanic island basalts oib and derived from mid oceanicridge basalt morb and 2 the protoliths of the high grade blocks were formed over a pre franciscan subduction zone based on the geochemical evidence for an arc origin and the available geochronological data we suggest that protoliths of the highgrade tectonic blocks of the franciscan complex and the coast range ophiolite formed in the same infant arc setting initiation of franciscan subduction in this arc crust led to the formation of the high grade blocks beneath the young coast range ophiolite continued subduction resulted in subduction of older oceanic crust of morb and oib origin some of which was scraped off to form lower grade metabasites within the franciscan complex

facies history of the upper jurassic lower cretaceous great valley sequence response to structural development of an outer arc basin abstract the facies history of the great valley sequence is closely linked to structural events in an arc trench system upper jurassic lower cretaceous marine rocks as much as 8 500 m thick along the western margin of the sacramento valley northern california were deposited on slopes and submarine fans that extended over an unstable basement floored by ophiolitic rocks paleocurrent and paleoslope analyses indicate that eastward subduction initiated as early as kimmeridgian created an outer arc ridge which bounded the west side of the basin debris was shed dominantly from the north and east but significant contributions were ridge derived sediment fabrics structures and bedding styles indicate that earliest deposition on basin plains and on eastward facing slopes was followed at le st until the valanginian by a stable sedimentological framework of prograding and avulsing submarine fans characterized by broad shallow midfan channels depositional lobes and abundant thick deposits of interchannel mud the great valley rocks are transected by northwest trending faults that produced strike separation of the east dipping strata smooth facies transitions but with abrupt thickness variations across faults suggest that faulting with some normal component of motion was contemporaneous with deposition during latest jurassic and earliest cretaceous later movement either strike slip or reverse brought coarse inner fan facies that were deposited on klamath basement into fault contact with basin plain facies that were deposited on ophiolitic basement

tectono stratigraphy and evolution of the mesozoic pindos ophiolite and related units northwestern greece the northwest pindos mountains expose mesozoic and tertiary thrust sheets which include the jurassic pindos ophiolite composed of ultramafic and mafic oceanic crustal rocks the tectono stratigraphy of these units is i mainly ultramafic ophiolitic thrust sheet and basal metamorphic sole dramala complex and loumnitsa unit ii late cretaceous platformal limestones orliakas group iii dismembered intrusive and extrusive ophiolitic rocks aspropotamos complex iv tectonic melange and olistostromes dominated by triassic jurassic volcanic and sedimtntary rocks avdella melange v coherent thrust sheets of late jurassic late cretaceous deep water sediments dio dendra group vi early tertiary flysch pindos flysch immobile trace element studies indicate that triassic and jurassic extrusives of the volcanic sedimentary melange formed mainly at within plate and or mid ocean ridge settings by contrast structurally overlying ophiolitic extrusives include boninitic volcanics and island arc tholeiites of supra subduction zone origin initial ophiolite displacement c 165 ma is recorded in metamorphic sole formation including basal peridotite mylonite amphibolites and greenschists the sole rocks have morb and wpb chemical affinities in our favoured model the pindos ophiolite formed above a mid jurassic westerly dipping intra oceanic subduction zone accompanied by accretion to form melange followed by collision and emplacement with the pelagonian margin final suturing of the pindos ocean however did not take place until early tertiary times

the volcanic and intrusive rocks of the quebec appalachian ophiolites canada and their island arc setting abstract the early to middle ordovician quebec appalachian ophiolites contain three geochemically distinct groups of extrusive and intrusive rocks the predominant group consists of ni and cr rich olivine clinopyroxene phyric lavas and dikes grading in composition from basalt and andesite to rhyodacite these rocks are geochemically similar to arc related boinites they are closely associated at the base of the extrusive sequence with aphyric lavas and dikes which have geochemical affinities with volcanic arc basalts the third group is restricted to a single locality mount orford at the southwestern end of the ophiolite belt it consists of alkaline diabases occurring in interdigitation with arc basaltic diabases to form a 1000 m thick sheeted sill complex the thetford mines ophiolite is interpreted to be typical of ocean crust created by fore arc rifting in a subduction zone environment the mount orford ophiolite with its alkaline rocks is considered to be remnant of the back arc section of the same island arc system

garibaldi group volcanic rocks of the salal creek area southwestern british columbia alkaline lavas on the fringe of the predominantly calc alkaline garibaldi cascade volcanic arc abstract the salal creek area at the north end of the main group of vents for the quaternary garibaldi cascade volcanic belt southwestern british columbia was the site of several small eruptions of mafic lava during the past 1 ma in contrast to the calc alkaline character of all other parts of the garibaldi belt and the geographically nearly coincident miocene and older pemberton volcanic belt the salal creek area quaternary lavas are predominantly alkaline basalt and hawaiite with typical alkaline volcanic petrography chemistry and fractionation trends trace elements ti zr y show within plate character for the suite as for other garibaldi belt volcanic rocks rb is low rb sr very low and 87 sr 86 sr ratio is low averaging 0 7032 the oxygen isotopic composition average 18 o 5 9 is normal for mantle derived volcanic rocks this distinct change in magma type at the end of a volcanic are may be the consequence of a smaller degree of melting melting at a slightly greater depth than calc alkaline magma production or a descending plate edge effect ponded flows and pillow palagonite accumulations indicate that several salal creek area eruptions occurred in proximity to ice which filled major valleys during pre wisconsin glacial periods

timing of arc construction and metamorphism in the slate creek complex northern sierra nevada california late paleozoic to early mesozoic rocks in the sierra nevada and klamath mountains western united states preserve a record of lateral growth of continental crust by incorporation of ophiolites volcanic arcs and associated sedimentary rocks deciphering the timing of arc construction and metamorphism is critical for elucidating tectonic and thermal evolution during this type of crustal growth but is difficult in complex accretionary terranes in this study we address the timing of arc construction and regional metamorphism in the slate creek complex a volcano plutonic terrane in the central part of the northern sierra nevada new 40 ar 39 ar ages from relict volcanic hornblende demonstrate that the youngest volcanic unit in the slate creek complex is ca 170 ma at least 30 m y younger than previous estimates of ca 200 ma for metaplutonic rocks in the lower part of the complex consequently the slate creek complex is polygenetic with two distinct episodes of arc magmatism the distribution of mineral assemblages and textures indicates that greenschist and epidote amphibolite facies metamorphism is younger than 170 ma and is associated with crosscutting plutons that yield cooling ages of 150 ma and younger amphiboles from foliated rocks in the slate creek complex yield plateau ages of 156 1 0 6 and 152 0 0 7 ma that date cooling subsequent to or dynamic recrystallization during the metamorphic event age and lithologic similarities suggest that the lower parts of the slate creek complex correlate with plutons ca 200 ma and the volcanic cover sequence of the rattlesnake creek terrane in the klamath mountains the uppermost volcanic unit correlates with the ca 170 ma western hayfork terrane that structurally overlies the rattlesnake creek terrane metamorphism in the slate creek complex correlates broadly with a similar late metamorphic event in the klamath mountains

implications of estimated magmatic additions and recycling losses at the subduction zones of accretionary non collisional and collisional suturing orogens

paleo glacier reconstruction in southwestern british columbia canada a glaciovolcanic model abstract volcanic rocks erupted within glacial environments offer powerful paleoenvironmental insights these glaciovolcanoes and their deposits serve as proxies that inform on the paleo presence distribution and thickness of past glaciers we investigate the paleoenvironmental implications of three mid to late pleistocene volcanic deposits in the garibaldi volcanic belt of southwestern british columbia canada we use these data to inform a simple geometric model that constrains paleo glacier distributions in the southern canadian cordillera the three volcanoes are used to recover i a coalesced mountain ice sheet in late marine isotope stage mis 4 and possibly into mis 3 up to 49 1 5 5 ka ii major and rapid degradation of glaciers during the late mis 6 to 5e transition by 141 12 9 ka and iii a coalesced mountain ice sheet or continental ice sheet that existed during mis 15 598 7 5 ka

detrital zircon evidence for progressive underthrusting in franciscan metagraywackes west central california we present new u pb ages for detrital zircons separated from six quartzose metagraywackes collected from different franciscan complex imbricate nappes around san francisco bay all six rocks contain a broad spread of late jurassic cretaceous grains originating from the klamath sierra nevada volcanic plutonic arc units young structurally downward consistent with models of progressive underplating and offscraping within a subduction complex the youngest specimen is from the structurally lowest san bruno mountain sheet at 52 ma it evidently was deposited during the eocene none of the other metagraywackes yielded zircon ages younger than 83 ma zircons from both el cerrito units are dominated by ca 100 160 ma grains the upper el cerrito also contains several grains in the 1200 1800 ma interval these samples are nearly identical to 97 ma metasedimentary rock from the hunters point shear zone zircon ages from this melange block exhibit a broad distribution ranging from 97 to 200 ma with only a single pre mesozoic age the albany hill specimen has a distribution of pre mesozoic grains from 1300 to 1800 ma generally similar to that of the upper el cerrito sheet however it contains zircons as young as 83 ma suggesting that it is significantly younger than the upper el cerrito unit the skaggs spring schist is the oldest studied unit its youngest analyzed grains were ca 144 ma and it is the only investigated specimen to display a significant paleozoic detrital component sedimentation and subduction accretion of this tract of the trench complex took place along the continental margin during early to early late cretaceous time and perhaps into eocene time franciscan and great valley deposition attests to erosion of an andean arc that was active over the entire span from ca 145 to 80 ma with an associated accretionary prism built by progressive underthrusting we use these new data to demonstrate that the eastern franciscan complex in the northern and central coast ranges is a classic accretionary prism where younger structurally lower allochthons are exposed on the west and older structurally higher allochthons occur to the east in the heavily studied san francisco bay area

the colima volcanic complex mexico part ii late quaternary cinder cones since the late pleistocene eleven cinder and lava cones have erupted on the floor of the southern colima graben ne and nw of the large active andesitic volcano colima scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites 9 leucite basanites 3 and minettes 15 the commonest variety of mica lamprophyre these samples represent primitive high temperature magmas with 47 6 50 3 sio2 7 4 15 3 mgo 2 5 4 4 k2o and 2 2 9 9 normative nepheline all members of this basic alkalic suite contain mg olivine fo75 94 chromite augite and late plagioclase and titanomagnetite the petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile bearing phases phlogopite apatite and analcime during late stages of crystallization as these phases increase in abundance presumably reflecting a rise in magmatic volatile content there are corresponding increases in the whole rock concentrations of 16 incompatible elements although these incompatible elements are relatively abundant even in the basanites many are highly concentrated in the minettes ba 4 200 ppm sr 3 100 ppm zr 550 ppm ce 190 ppm hf 18 ppm among the incompatible elements the degrees of enrichment in the minettes relative to the basanites decrease in the order h th ce la nd zr hf u ba sm eu pb p nb sr ti these enrichments may reflect the increasing importance of minor incompatible element rich mantle phases during partial melting the concentrations of alkali metals k na rb and cs do not correlate with these other elemental enrichments the leucite basanties have similar incompatible element contents to many minettes differing from them only in the presence of leucite rather than analcime and ti f rich groundmass phlogopite rather than hydrous phlogopite phenocrysts thus the leucite basanites represent relatively dry equivalents of minettes two of the eleven cinder cones are calc alkaline in nature and do not belong to the basanite minette group the easternmost cone is constructed of high al basalt and the northernmost of basaltic andesite the high al basalt 49 5 sio2 9 3 mgo 221 ppm ni closely approximates a parental magma to the post caldera andesitic suite of v colima 56 5 61 6 sio2 the basaltic andesite is relatively enriched in incompatible elements compared to the high al basalt v colima trend the ne normative basanite minette samples are highly enriched in incompatible elements while the contemporaneous hy qz normative calc alkaline suite encompassing the high al basalt and v colima s andesites is characterized by relatively low abundances of these elements no likely mineral assemblage can relate the alkaline and calc alkaline suites through crystal fractionation they probably represent fundamentally different melting events during the quaternary the main focus of andesitic volcanism in the southern colima graben has migrated southward with time volcan colima marks its present position 5 km south of the pleistocene volcano nevado de colima and another 15 km from the still older volcan cantaro the eruptions of basic alkalic magma probably occurred during the late stages of nevado s life and through the life of v colima they generally migrated from west to east with time towards v cantaro the most recent cone v apaxtepec is the only one east of the andesitic colima cantaro axis the oldest and the two youngest cones produced basanites while minettes dominated at cones of intermediate ages the cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of v colima the southern end of the colima graben can be viewed then as the volcanic analog of many classical post plutonic hypabyssal lamprophyre localities

age of the barremian aptian boundary and onset of the cretaceous normal superchron abstract the age assigned to the boundary of the barremian and aptian stages remains one of the most poorly constrained post pangean stratigraphic boundaries the lack of a global boundary stratotype section and point gssp for the stage boundary has hampered efforts to calibrate the absolute age of the cretaceous period in the geological time scale the current definition of the barremian aptian boundary also approximates the onset of magnetic polarity chron m0r the end of this chron denotes the start of the cretaceous normal polarity superchron that is of fundamental importance for plate reconstructions currently there is up to 5 discrepancy in the age estimates of the barremian aptian boundary ca 126 121 ma and the start of the cretaceous normal superchron here we review available geochronological information from the late barremian and early aptian stages collected from the pacific ocean china california the ontong java nui large igneous province and the high arctic large igneous province by utilizing only robust geochronological data including u pb and recalibrated 40ar 39ar ages from sites with magnetic polarity information and or paleontological constraints we calculate a best estimate of between 123 8 and 121 8 ma for the barremian aptian boundary and the onset of chron m0r at 2 confidence using estimates of the duration of chron m0r 0 49 0 10 myr 2 we conservatively compute the start of the cretaceous normal superchron to between 123 4 and 121 2 ma 2 using an age of 83 07 0 15 ma 2 for the end of the cretaceous normal superchron the duration of the superchron is also constrained to between 38 0 and 40 5 myr 2 these age ranges for the barremian aptian boundary the onset of the cretaceous normal superchron and the duration of the superchron currently provide the best estimates until a gssp is formally ratified

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

tectonic setting of the jurassic smartville and slate creek complexes northern sierra nevada california jurassic ophiolitic complexes in california and oregon are widely distributed outboard of a jurassic and older continental margin whether these outboard complexes were exotic and collided with the jurassic continental margin or whether they were simply part of a wide forearc to a jurassic continental magmatic arc has long been at issue we have studied two such complexes and crosscutting intrusions in the fault bounded central and western belts of the northern sierra nevada we report new and revised u pb zircon dates for the early jurassic and younger slate creek complex ca 203 171 ma in the central belt the late jurassic smartville complex 159 3 ma in the western belt and crosscutting intrusions ranging from middle jurassic to early cretaceous age the new ages imply that the central and western belts have been part of the same terrane since middle jurassic time and that the fault separating them has accommodated only minor displacements since ca 160 ma isotopic compositions of common pb indicate derivation from mantle sources but evidence for inheritance of pre cambrian zircon is common in plutons within the early and late jurassic complexes and in middle jurassic late jurassic and early cretaceous crosscutting intrusions although there are several potential sources for such contamination the ages of the inherited zircon are consistent with a north american origin this contamination would have been accomplished most plausibly near the north american margin

magmatic origin of low t mafic blueschist and greenstone blocks from the franciscan melange san simeon california abstract the franciscan melange exposure near san simeon contains abundant greenstone and minor blueschist blocks that were tectonically boudinaged while encased in the shale matrix tectonic deformation of the blueschists is evident from variable amounts of cataclastic flow along their margins and in pinched tails major trace and rare earth elemental analyses indicate that blueschist and greenstone blocks in this area of the melange were derived from sources with morb like composition along with some having trace element and ree patterns similar to oib compositions most blocks are low lree basalts that probably formed in an open ocean ridge setting but some blocks have high lree contents similar to off axis seamounts linear trends of incompatible elements for both blueschist and greenstone blocks indicate that both lithologies were probably derived from a similar variably fractionated tholeiitic magma blueschist blocks with sodic amphibole lawsonite epidote were pervasively recrystallized at 300 350 c and foliated during ductile deformation that included folding their protolith can only be identified as mafic a few blocks contain very small amounts of metasedimentary materials indicating some were probably seafloor basalts but some may have been diabase or even gabbro where interlayered sediment was present the mafic protolith was enriched in k rb and na greenstones on the other hand contain abundant pseudomorphic evidence of magmatic textures alteration to albite chlorite and pumpellyite at temperatures of 100 200 c is intense especially in cataclastic margins and pinched tails some of the basaltic greenstones have attached radiolarian chert and a few have relict diabasic textures the mafic blueschists and greenstones in the melange near san simeon are probably fragments from the uppermost part of the farallon plate the blueschists may be mostly mafic slabs uprooted from the subducting crust underplated to the base of the north american plate and later detached and carried back towards the surface in upwelling melange the greenstones are most likely fragments detached from seamounts subducted during times of melange upwelling

kinematic evidence for extensional unroofing of the franciscan complex along the coast range fault northern diablo range california franciscan metagraywacke immediately below the del puerto ophiolite an outlier of the coast range ophiolite in the northern diablo range was sheared during top to the east displacement on the coast range fault this represents normal faulting and extensional offset it was accompanied by attenuation of the coast range ophiolite and great valley sequence in the hanging wall along layer parallel normal faults that sole into the coast range fault extension occurred as the franciscan complex moved relatively west out from under north american lithosphere and across the subducting ocean plate below this effected a lengthening and thinning in the wedge of material above the down going plate presumably in response to instability brought about by subduction shallowing krueger and jones 1989 and accretion of the franciscan central belt in the latest cretaceous to early paleocene as a result blueschist facies terranes of the uppermost part of the franciscan complex are now juxtaposed directly against hanging wall units that bear only low grade metamorphism

very low temperature metamorphism of shales from the diablo range franciscan complex california new constraints on the exhumation path we have determined illite crystallinity coal rank percent 2m 1 white k mica polytype and phase assemblages for shales and shale matrix melange from various areas in the diablo range additional information concerning metamorphic conditions and fluid composition was derived from stable isotope work and fluid inclusion measurements this study presents new constraints on the pressure temperature evolution of the franciscan units from the diablo range several shales indicate a lower pressure possibly higher temperature overprint on the peak pressure assemblages peak pressure assemblages include jadeitic pyroxene sodic amphibole lawsonite aragonite and phengite reactions which indicate a lower pressure possibly higher temperature overprint are the transition from aragonite to calcite the formation of albite from jadeitic pyroxene quartz the compositional gradient from phengite to muscovite in white k mica and the transition from sodic to calcic amphibole the assemblage albite calcic amphibole lawsonite and chlorite found in a single shale from pacheco pass indicates the transition from blueschist to greenschist facies conditions we suggest that this transition occurred at a pressure and temperature of 5 8 kilobars and 300 350 c fluid inclusion data from post kinematic quartz veins from the mount hamilton area support the idea of a lower pressure possibly higher temperature metamorphic event pressure temperature conditions of trapping early h 2 o and ch 4 rich fluids were 4 5 kilobars and 230 c geothermal gradients for the transition from blueschist to greenschist facies and the formation of early fluid inclusions are estimated to be 14 c km illite crystallinity for all areas indicates diagenetic to high grade anchizonal conditions coal ranks are between 1 6 and 2 5 mean ro these estimates are not consistent with metamorphic conditions deduced for two different shale assemblages from pacheco pass suggesting that illite crystallinity and coal rank are retarded poor illite crystallinity is thought to result from small coherent scattering domain sizes along c of white k mica crystals rich in a phengite component dalla torre et al 1994a in press the exact cause of the retardation of coal rank is not clear the high pressure in the diablo range may have affected the maturation of organic matter our results imply that coal rank data from high pressure terranes must be interpreted with caution if used as an absolute paleothermometer

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

accretion of the franciscan complex attending jurassic cretaceous geotectonic development of northern and central california scattered igneous activity marks the triassic margin of california but an andean volcanic plutonic arc began to form in the sierra nevada white inyo range and klamath mountains at ca 170 ma during underflow of the oceanic lithosphere this calc alkaline belt supplied volcanogenic erosional debris to the mariposa galice proximal overlap sequence by ca 165 160 ma transpression also generated high pressure low temperature basaltic eclogites garnet blueschists and amphibolites along the middle and late jurassic convergent plate junction at ca 170 155 ma most of these returned surfaceward as tectonic blocks only during the mid cretaceous the high grade metamafic metamorphosed mafic blocks that formed during construction of the emergent arc and derived mariposa galice strata predate the onset of franciscan trench deposition near the end of jurassic time the klamath salient evidently migrated 200 km westward an earliest cretaceous westward stepout of the convergent junction apparently formed directly offshore from the klamath imbricate orogen but to the south preexisting oceanic crust capped lithosphere was trapped landward as the coast range ophiolite andean arc detritus began to accumulate on this ophiolitic basement within the great valley forearc and outboard franciscan oceanic trench at ca 145 140 ma because earliest cretaceous relatively continuous great valley strata were deposited on the stable north american plate protected from both surface and subcrustal erosion forearc terrigenous sedimentation also signaled coeval deposition in the outboard trench 10 25 m y after middle and late jurassic initiation of the continental margin calc alkaline arc voluminous sedimentation and the accretion of franciscan and san joaquin sections of great valley rocks took place during mid and late cretaceous time after a flare up in igneous arc activity at 125 120 ma the youngest sierran granites have ca 85 ma ages reflecting extinction of the magmagenic zone beneath northern and central california and presumably reflecting subhorizontal oceanic plate underflow attending the laramide orogeny feebly zeolitized upper cretaceous and lower tertiary franciscan coastal belt rocks derived from the now inactive arc underwent only low pressure burial metamorphism reflecting shallow offloading and or large scale transpression they were not appreciably subducted although coastal belt units are members of the franciscan lithotectonic assemblage their origin contrasts with the rest of the subduction complex

igneous petrogenesis of magnesian metavolcanic rocks from the central klamath mountains northern california mafic meta igneous supracrustal rocks in the east central part of the western triassic and paleozoic belt are interlayered with and appear to predominantly overlie fine grained clastic and cherty metasedimentary strata this complex constitutes a lithostratigraphic terrane exposed in the vicinity of sawyers bar california basaltic flows dikes and sills are informally referred to as the yellow dog greenstones dark colored amygdaloidal flow breccias rich in titanium iron phosphorus and light rare earth elements lree9s constitute the lowest mildly alkalic members of the mafic flow series intimately interlayered with fine grained terrigenous detritus element proportions suggest that the dark ti fe p rich basaltic lavas may have been extruded in an oceanic intraplate setting interstratification with distal turbiditic strata indicates concurrent submarine volcanism and deep sea sedimentary deposition the main mass of the stratigraphically higher extrusive sequence is a light colored massive tholeiite series poorer in ti fe p lree9s hypabyssal rocks belong exclusively to this second igneous suite compositions of the pale colored overlying basaltic diabasic mass of the yellow dog greenstone section are consistent with eruption in an immature magmatic arc dikes presumably comagmatic with terminal stage yellow dog volcanism cut the overlying more eastern stuart fork terrane the yellow dog greenstones contain relict magmatic clinopyroxene and or pargasitic hornblende both melt series include mg rich units ave 17 mgo 14 4 1 9 wt cr 650 200 ppm ni 270 100 ppm sea floor alteration apparently affected these mafic rocks in the marine environment following eruption as reflected by 18 o mgo and cao al 2 o 3 values and by minor alkali and silica metasomatism yellow dog igneous activity was extinguished during mid jurassic suturing of the oceanward sawyers bar terrane against and beneath the pre existing landward stuart fork blueschist complex of late triassic metamorphic age subsequently the terrane amalgam was invaded by middle late jurassic granitoids minor chemical alteration including 18 o enrichment also probably accompanied later regional and especially contact metamorphism nevertheless many major minor and rare earth element variations in the yellow dog greenstones are compatible with igneous fractional crystallization trends but exclude the assimilation of significant amounts of sialic crust

hydrothermal alteration of volcanic rocks at the mclaughlin gold deposit northern california the plio pleistocene mclaughlin mine is a large hot spring gold silver mercury deposit located in the northern coast ranges of california two distinctly different suites of volcanic rocks host mineralization an upper jurassic basalt part of the coast range ophiolite and an upper cenozoic basaltic andesite part of the clear lake volcanic field the two suites of volcanic rocks range from least altered rocks showing primary igneous mineralogy to intensely altered rocks composed of an assemblage of quartz montmorillonite sericite and adularia mass balance calculations indicate almost total loss of na ca mg and sr and large gains in and ba as a result of the hydrothermal alteration the mass flux corresponds to the replacement of plagioclase feldspar with sericite and adularia and mafic minerals with montmorillonite trace elements show no evidence for mobility rocks adjacent to the hydrothermal system have the greatest 18o values and the highest calculated mass flux suggesting the greates

calaveras reversed westward younging is indicated samples of limestone collected from strata in the southern part of the western metamorphic belt of the sierra nevada which traditionally have been assigned to the calaveras formation have yielded early triassic conodonts and samples of metavolcanic rock indicate an early jurassic rb sr age these ages together with previously published fossil and isotopic data are consistent with general westward younging of units rather than with eastward younging as has been assumed until recently by many workers chiefly on the basis of sparse observations of bedding facings the rocks are strongly deformed and the possibility that tectonism rather than stratigraphic succession accounts for the age pattern cannot be dismissed

hydro and morpho dynamic modeling of breaking solitary waves over a fine sand beach part i experimental study abstract the objectives of this work are 1 to examine the mechanisms that influence tsunami erosion and deposition mechanisms in the littoral zone via physical simulations of breaking solitary waves over a fine sand beach and 2 to provide experimental data for validation of numerical models to predict tsunami erosion and deposition processes the experiments were carried out in a 48 8 m 2 16 m 2 1 m wave flume the flume was instrumented to observe free surface elevations cross shore velocities suspended sediment concentrations vertical and cross shore pore pressure gradients near the shoreline and morphological changes in addition wave sediment interactions were observed via underwater video cameras the results are systematically analyzed to investigate the roles of wave breaking bore runup wave drawdown and wave induced pore pressure variations on tsunami erosion and deposition processes the studies showed that the wave plunging on a thin layer of water prior to reaching the shoreline did not cause much sediment suspension while the waterjet impinging directly on the beach entrained substantial amounts of sand the suspended sediments were subsequently pushed up the slope by fluid momentum as the broken wave transformed to a turbulent bore a small net deposition region was observed near the maximum runup point where both the flow velocity and the water depth were near zero a significant amount of the sediment transport occurred during the wave drawdown in the form of thick sheet flow which resulted in net erosion of the shore face and the beach a hydraulic jump formed near the wave breaking region toward the end of the drawdown which caused most of the suspended sand to deposit in the wave breaking region consequently breaking solitary waves over a sloping fine sand beach led to net erosion of the shore face and the beach net deposition in a small region immediately seaward of the max excursion point and net deposition in the wave breaking zone

deep crustal fracture zones control fluid escape and the seismic cycle in the cascadia subduction zone abstract seismic activity and non volcanic tremors are often associated with fluid circulation resulting from the dehydration of subducting plates tremors in the overriding continental crust of several subduction zones suggest fluid circulation at shallower depths but potential fluid pathways are still poorly documented using receiver function analysis in the cascadia subduction zone we provide evidence for a seismic discontinuity near 15 km depth in the crust of the overriding north american plate this interface is segmented and its interruptions are spatially correlated with conductive regions of the forearc and shallow swarms of seismicity and non volcanic tremors these observations suggest that fluid circulation in the overriding plate is controlled by fault zones separating blocks of accreted terranes these zones constitute fluid escape routes that may influence the seismic cycle by releasing fluid pressure from the megathrust

palaeoceanography and tectonic setting of the jurassic coast range ophiolite central california evidence from the extrusive rocks and the volcaniclastic sediment cover abstract the primary thesis of this paper is that the character of sedimentary rocks within or directly on top of an ophiolite complex can be used to constrain tectonic models of oceanic crustal genesis late jurassic c 169 161 ma coast range ophiolite remnants are exposed e and w of the san andreas fault in central california ophiolite volcanic rocks are mainly sub alkaline tholeiitic basalts but include subordinate intermediate and silicic flows and breccias available immobile major and trace element data indicate that the coast range ophiolite formed above a subduction zone possibly in a fore arc setting as suggested by the presence of high magnesian andesites and boninites the ophiolite extrusives mainly comprise pillow lava and lava breccia formed on a rugged fault controlled ocean floor interlava pelagic sediment is restricted to very small amounts of interstitial red chert and nannoplankton limestone widespread occurrence of vesicular lavas probably reflects high primary volatile contents rather than eruption in shallow water local interbeds of arc derived silicic tephra in the uppermost extrusives suggest that the ophiolite and cover sediments formed together in a single tectonic setting depositionally overlying deep water successions up to 600 m thick comprise radiolarian rich tuffaceous and volcaniclastic sediments of callovian mid tithonian age 165 148 ma depositional processes included airfall turbidity currents mass flow and current reworking small si fe mn lenses along the lava sediment contact formed by hydrothermal replacement of basal siliceous tuffaceous sediments or by direct chemical precipitation at one locality llanada the ophiolite is overlain by up to 1 8 km of younger tithonian basic intermed and acid lava flows submarine lahars and mainly silicic pyroclastic sediments of inferred volcanic arc origin the volcaniclastic cover passes conformably upwards into late tithonian and younger terrigenous deep water clastic sediments of the great valley group deposited in a fore arc setting the coast range ophiolite and its cover are interpreted as an incipient intraoceanic volcanic arc possibly developed above an eastward continent ward dipping subduction zone within the equatorial pacific ocean in this scenario short lived 5 8 ma supra subduction zone spreading to form the ophiolite was followed by callovian tithonian arc volcanism shedding tuffaceous debris onto the adjacent fore arc subduction was active separately along the western american continental margin with the result that the coast range arc ophiolite complex eventually collided with north america triggering the late jurassic nevadan orogeny and initiating franciscan subduction accretion the coast range ophiolite remnants at least those west of the san andreas fault were apparently translated northwards to their present positions after late cretaceous

the franciscan complex california usa the model case for return flow in a subduction channel put to the test the model of return flow in a subduction channel rfsc is put to the test by opposing key assumptions of the model to an integrated review of field data petrology geochronology and structures of high p melanges of the franciscan subduction complex fsc to which rfsc has been first applied key assumptions are i the subduction channel sc is a low viscosity zone of finite width restrained by the rigid subducted and overriding plates ii forced return flow in the sc is associated with formation and km scale displacements of small tectonic blocks 1 km in a highly deformable matrix iii this results in apparently random distributions and close associations of blocks exhumed from different depths hence the block in matrix structures and exotic blocks in high p melanges would reflect the structural state at depth of the sc on the basis of the predictions ensuing from the key assumptions of the rfsc model on the rock record it is detailed why the actual record of the high p melanges of the fsc does not comply with this perception the record rather reflects recurring subduction underplating associated decollement erosion and mingling of high p rocks by sedimentary processes in the trench all occurring in an 80 m y lasting period distributed deformation of the melange matrix is unrelated to formation of apparently chaotic block in matrix structures since i creation subduction ascent and cooling of exotic blocks on counterclockwise p t paths predates deposition of the bulk of the metasediments constituting the matrix by 20 40 m y ii deformation of the matrix occurs during a second loop of high p metamorphism postdating creation and ascent of the exotic blocks by at least 20 40 m y iii the magnitudes of distributed deformation of the matrix are too low to cause juxtaposition of blocks different in p t paths against each other from km scale vertical distances instead narrow few tens of metres wide fault shear and decollement zones formed at locally high pore fluid pressures accumulate the major tectonic transport during high p metamorphism they bound several 100s of metres wide tabular tectonic bodies that are coherent sedimentary processes at the active continental margin formed the bulk of the melanges tectonically formed block in matrix structures within the shear and decollement zones are unrelated to juxtaposition of exotic blocks universal diagnostic criteria are developed to identify return rfsc as a mechanism that potentially amalgamates heterogeneous metamorphic terrains by using field data petrology and geochronology

geochemical evidence for the tectonic setting of the coast range ophiolite a composite island arc oceanic crust terrane in western california the middle to late jurassic age coast range ophiolite cro of california contains two geochemically distinct volcanic rock associations that formed in different tectonic settings volcanic rocks from the southern cro point sal cuesta ridge stanley mountain llanada quinto creek and del puerto and parts of the northern cro healdsburg elder creek are similar to low k tholeiites and calc alkaline rocks of the island arc suite the thin volcanic sections of these ophiolite remnants suggest formation by intra arc rifting in contrast volcanic rocks from stonyford seamount and paskenta in the northern cro are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid ocean ridge basalts or ocean island tholeiites these rocks are associated with tithonian radiolarian cherts and may be part of the franciscan complex alternatively they may represent a change in tectonic setting within the cro during the late jurassic regardless the cro as currently conceived cannot be considered a single terrane with one mode of origin

terranes early faults and pre late jurassic amalgamation of the western sierra nevada metamorphic belt california the western sierra nevada metamorphic belt is subdivided into six tectonic terranes which are juxtaposed along mostly cryptic pre late jurassic faults the northern sierra terrane contains a basement of imbricated and locally chaotic lower paleozoic quartz sandstone mafic rocks chert argillite and other rocks overlain unconformably by upper devonian middle jurassic volcanic and siliceous strata the feather river terrane is a paleozoic possibly polygenetic ophiolite the red ant terrane is a small terrane with an early mesozoic blueschist facies overprint the calaveras terrane is a chaotic assemblage of upper paleozoic triassic argillite chert volcanic rock and limestone contained in two regionally mapped melange units the tuolumne river terrane is a stratigraphic assemblage of disrupted paleozoic ophiolite overlain by middle triassic lower jurassic siliceous and volcanic strata both intruded by 200 ma lower jurassic plutons the slate creek terrane is a 200 ma pseudostratigraphic sequence of cumulate ultramafic plutonic and volcanic rocks the northern sierra and calaveras terranes contain permian mccloud fossils whereas the tuolumne river terrane contains permian mccloud and tethyan fossils in olistoliths within lower mesozoic deposits these terranes are bounded mostly by steeply dipping faults of the late jurassic foothills fault system but locally are juxtaposed along earlier highly deformed low angle faults that are remnants of major terrane bounding faults a system of early mostly cryptic east dipping faults juxtapose in descending structural order the northern sierra feather river red ant calaveras and tuolumne river terranes each of the four boundaries that separate these five terranes have three or more independent pre late jurassic upper age brackets including crosscutting plutons sedimentary provenance and shared structural fabrics the slate creek terrane tectonically overlies the tuolumne river calaveras and red ant terranes along an isoclinally folded west dipping fault that is cut by a 165 ma pluton callovian kimmeridgian strata and coeval intrusive rocks including the 155 165 ma volcanic rocks sheeted dikes and zoned mafic plutons of the smartville complex are deposited on and intruded into the older previously juxtaposed terranes and constitute an overlap sequence the well known faults of the foothills fault system including the melones fault cut kimmeridgian strata of the overlap sequence and thus cannot include ancient sutures these faults reactivated many of the earlier terrane bounding faults pre late jurassic amalgamation of the sierra nevada metamorphic belt rules out previous models of late jurassic arc continent collision earlier collisions and or noncollisional tectonic processes must be invoked to explain strong contractional deformation in the western sierra nevada metamorphic belt

petrotectonic evolution and melt modeling of the penon blanco arc central sierra nevada foothills california the penon blanco arc of the jurassic triassic arc belt in central california is composed of the jasper point formation penon blanco formation and coeval don pedro intrusive suite all exposed in the core of the cotton creek anticline the jasper point formation consists of 900 m of massive to pillowed lavas and up to 50 m of depositionally overlying chert and transitional basalts it passes upward into the penon blanco formation which is made up of 700 m of crystal lithic basaltic tuff 1 3 5 km of augite rich volcaniclastic rocks and up to 3 5 km of massive to brecciated flows of augite phyric basalt the penon blanco formation is paraconformably overlain by the oxfordian kimmeridgian mariposa formation which provides a minimum age of juxtaposition for the penon blanco arc against the inboard calaveras complex new geochemical data from the penon blanco arc show that the two volcanic suites are geochemically distinct jasper point basalts are tholeiitic and are characterized by high large ion lithophile element lile abundances moderate high field strength element hfse and heavy rare earth element hree abundances and low ti v ratios penon blanco basalts are calc alkaline have higher lile abundances lower hfse and hree abundances and lower ti v ratios geochemical modeling of melt sources indicates that both units formed by melting of a depleted spinel bearing mantle source at 30 60 km depth by low to moderate amounts of partial melting 3 5 for jasper point and 5 7 5 for penon blanco the geochemical modeling and field data suggest that the jasper point basalts are similar to normal mid ocean ridge basalt n morb and were associated with forearc rifting while the penon blanco basalts represent the transition to arc volcanism and head on subduction this model is consistent with interpretations for a single ensimatic arc on the jurassic margin of north america

implications of new jurassic stratigraphic geochronometric and paleolatitudinal data from the western klamath terrane smith river and rogue valley subterranes combined biostratigraphic chronostratigraphic and geochronometric studies of the rogue and galice formations rogue valley subterrane southwestern oregon indicate that the oxfordian kimmeridgian boundary should be placed at 154 1 5 ma rather than at 156 6 ma as advocated in the decade of north american geology 1983 geologic time scale in the smith river subterrane northwestern california southwestern oregon well preserved radiolaria were recovered not only from strata overlying the josephine ophiolite but also from within the volcanic member of the ophiolite u pb geochronometric data constrain the age of the mirifusus first occurrence event to after 162 1 ma and to being 157 1 5 ma or slightly older radiolarian faunal data indicate that the josephine ophiolite originated at central tethyan paleolatitudes during the latest callovian 162 ma and was carried northward to northern tethyan and southern boreal paleolatitudes during the oxfordian

sandstone matrix olistostrome deposited on intra subduction complex serpentinite franciscan complex western marin county california abstract detailed field mapping in the franciscan complex in western marin county california offers insight into the origins and development of melanges as well as accretionary complex architecture from structurally high to low the study area units comprise a structurally high sandstone matrix olistostrome serpentinite a structurally intermediate sandstone matrix olistostrome early cretaceous nicasio reservoir terrane greenstone and coherent cretaceous greywacke all exhibiting prehnite pumpellyite burial metamorphism the structurally high sandstone matrix olistostrome includes volcanic blocks up to 700 m in diameter and chert blocks up to 100 m in diameter whereas the structurally intermediate olistostrome unit comprises a lithic rich sandstone matrix enclosing volcanic blocks 1 20 m in diameter the youngest detrital zircons in the high olistostrome and the coherent greywacke have u pb ages of 20 m y older than the inferred depositional ages of the units themselves suggesting that the bulk of the detritus comprising these units was derived by exposure and sedimentary recycling of significantly older previously accreted sediments the matrix of both olistostrome units exhibits minimal strain with weak local development of pressure solution cleavage these findings demonstrate introduction of franciscan exotic blocks and development of franciscan block in matrix fabrics through sedimentary sliding and support sedimentary interpretations for more highly strained shale and serpentinite matrix melanges additionally these data suggest that sandstone matrix olistostromes melange may constitute a larger part of the franciscan and other accretionary complexes than previously believed the structurally high olistostrome preserves a depositional contact on serpentinite verifying an intra subduction complex origin for the serpentinite the serpentinite lacks block in matrix structure and likely represents a tectonically emplaced unit derived from the subducting plate as a coherent nappe rather than a sedimentary serpentinite deposit multi kilometer scale west vergent overturned folds with subhorizontal fold axes deform the nappes of the field area and a later generation of open northeast trending folds overprints the earlier folds

provenance of franciscan graywackes in coastal california a systematic comparison of available detrital modes for graywacke sandstones of the franciscan subduction complex and for coeval sandstones of the great valley sequence in the california coast ranges indicates that both were apparently derived from the same general sources the inferred provenance terrane was the ancestral sierran klamath magmatic arc from which mixed volcanic and plutonic detritus readily entered the adjacent great valley forearc basin at intervals along the trend of the arc trench system arc derived detritus also bypassed the forearc region through submarine canyons that fed the franciscan trench longitudinal flow along both the great valley trough and the franciscan trench achieved wide dispersal of the turbidite sediment suites of both franciscan and great valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic mean framework modes of 17 franciscan suites comprising 203 individual samples and of 23 great valley suites comprising 410 individual samples range from 14 to 44 quartz grains 15 to 54 feldspar grains and 7 to 71 total lithic fragments the ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes the compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz and feldspar derived principally from erosion of underlying plutons despite major overlap in the compositions of the two sets of samples some franciscan sandstones are somewhat more feldspathic and less lithic than any known great valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical great valley sources higher proportions of non volcanic to volcanic lithic fragments in some franciscan sandstones probably reflect complex recycling processes on the trench slope diagenetic effects in many franciscan suites include apparent wholesale replacement of k feldspar by albite present age control is inadequate to test fully for time dependent trends in the compositions of franciscan sandstones analogous to the known stratigraphic variations in the composition of great valley sandstones this question ought to be investigated in future studies

stratigraphic and sedimentologic evidence of a submarine island arc volcano in the lower mesozoic penon blanco and jasper point formations mariposa county california detailed mapping in the foothills of the central sierra nevada has revealed that the lower jurassic penon blanco formation and the newly named jurassic triassic jasper point formation occupy the core of a previously unrecognized anticline of late jurassic age that plunges gently southeastward exposing the upper jurassic mariposa formation on its limbs the jasper point formation consists of 900 m of massive pillowed and brecciated basalt overlain by as much as 100 m of varicolored radiolarian chert these units resemble layers 2 and 1 of the oceanic crust respectively the penon blanco formation is composed of as much as 5 km of coarse augite and plagioclase bearing volcaniclastic rocks and lesser flows that appear to have originated at a common vent fine tuff at the base of the formation is interbedded with and overlies the jasper point chert gradationally a restored section of the penon blanco formation suggests that the volcanic rocks define a single composite volcano that probably originated as part of an island arc voluminous volcanidastic detritus that composes most of the volcano may have been produced at great depth possibly by hyaloclasis autoclasis or mass wasting much of the detritus was deposited as turbidites of tuff and tuff breccia that define an extensive volcanic apron that thickens and coarsens upward over hundreds of metres suggesting progradation submarine clastic processes thus dominated the construction of the volcano fragmentation of most of the erupted material probably occurred at or near the vent and subsequent distribution of the detritus around the vent by epiclastic processes formed a distinctive stratigraphic sequence these hypotheses may be tested by dredging young deeply submerged island arc volcanoes

lillooet glacier basalts southwestern british columbia canada products of quaternary glaciovolcanism the retreat of lillooet glacier lg has exposed a succession of pillow basalt and subordinate amounts of breccia and hyaloclastite the lithofacies and physiographic setting suggest that the deposits have a glaciovolcanic origin and represent a partially dissected basaltic pillow dominated tindar chemically the lg volcanic rocks are basalt to basaltic andesite and as a group they represent the highest silica quaternary mafic products in the garibaldi volcanic belt gvb like other northern gvb alkaline basalts they lack the nb ta depletion signature typically associated with subduction related products geochemical and petrologic analysis indicates that the lg basalts are comagmatic and that chemical variations within the suite are consistent with sorting of the observed phenocryst assemblage olivine plagioclase thermodynamic modeling establishes shallow crustal pre eruptive storage conditions at 2 kbar 1 kbar 100 mpa or 7 5 km and an h2o content of 0 5 1 wt we estimate that the

tectonics of formation translation and dispersal of the coast range ophiolite of california data from the coast range ophiolite and its tectonic outliers in the northern california coast ranges suggest that the lower part of the ophiolite formed 169 to 163 ma in a forearc or back arc setting at equatorial latitudes beginning about 156 ma and continuing until 145 ma arc magmatism was superimposed on the ophiolite and concurrently a transform developed along the arc axis or in the back arc area rapid northward translation of this rifted active magmatic arc to middle latitudes culminated in its accretion to the california margin of north america at about 145 ma this late jurassic episode of translation arc magmatism and accretion coincided with the nevadan orogeny and a proposed major plate reorganization in the eastern pacific basin the high rate of poleward motion necessary to translate the coast range ophiolite to middle latitudes during this time implies that the ophiolite traveled north on a fast moving plate of the eastern pacific basin here termed plate x plate x probably was driven by a cryptic ridge east northeast of the pacific farallon izanagi ridge triple junction structural relations indicate that following late jurassic time parts of the coast range ophiolite were displaced from the west side of the great valley province and incorporated into the central belt of the franciscan complex along steep dipping to low angle reverse faults having dominant components of dextral shear a northwest trending eastern zone of these right laterally displaced outliers shows strong affinities to the main coast range ophiolite of the northwestern sacramento valley the elder creek terrane in that the outliers include ophiolitic breccias of oxfordian to kimmeridgian age a southwestern zone of outliers lacks ophiolitic breccia and instead includes latest oxfordian or kimmeridgian to tithonian arc derived volcanic rocks like those found in the del puerto and stanley mountain terranes of the main ophiolite whereas outliers of the northeastern outlier zone are right laterally displaced no more than 260 km from the western side of the sacramento valley outliers of the southwestern zone are displaced a minimum of 169 to 249 km this displacement occurred between about 60 and 52 ma ophiolitic rocks in the decatur terrane of western washington that have recently been correlated with the coast range ophiolite and the great valley sequence of california were apparently displaced at least 950 to 1200 km from the west side of the great valley between early tertiary and early cretaceous time derived rates of northward translation for the ophiolite outliers in california are in the range of 1 to 4 cm yr rates for the decatur terrane are in the range of 2 5 to 4 cm yr if translation was initiated 90 ma but as much as 11 9 to 15 cm yr if it was not initiated until 60 ma the lower rates for the decatur terrane are consistent with the rates derived for the california outliers and with the northward component of relative motion between the farallon and north american plates from 90 to 50 ma the higher rates require northward transport on the kula plate or on a fast moving microplate the higher translation rates derived for the decatur terrane are also consistent with paleomagnetically determined rates for some limestones of the central belt this may indicate that outliers of the coast range ophiolite dispersed in the central belt of the franciscan complex record only part of the total displacement which occurred along the late cretaceous to early tertiary western margin of north america

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

recognition of ocean plate stratigraphy in accretionary orogens through earth history a record of 3 8billion years of sea floor spreading subduction and accretion abstract ocean plate stratigraphy ops is a term used to describe the sequence of sedimentary and volcanic rocks deposited on oceanic crust substratum from the time it forms at a spreading center to the time it is incorporated into an accretionary prism at a convergent margin in this study we review the major geological characteristics of relict cenozoic to early archean oceanic crust and ops persevered in alaska japan california franciscan complex central asia british isles canada slave province australia pilbara craton and greenland isua and ivisaartoq belts an assessment of ops in accretionary orogens spanning the duration of earth s rock record shows remarkable similarities between ops of all ages in terms of structural style major rock components sequence of accretion and trace element geochemical signatures volcanic rocks preserved in the orogenic belts are characterized predominantly by oceanic island arc basalts island arc picrites mid ocean ridge basalts back arc basalts oceanic plateau basalts ocean island basalts and boninites with extremely rare komatiites this demonstrates that sea floor spreading lateral movement of oceanic plates with accompanying sedimentation over the oceanic substratum and accretion at convergent margins have been major earth processes since at least 3 8 ga ago there have been some secular changes in the rock types in ops such as changes in carbonates and radiolarian cherts whose sources were in the biota in existence in phanerozoic times but absent in the precambrian but overall there have been few changes in the style of ops accretion with time komatiites and banded iron formations occur predominantly in archean orogenic belts reflecting higher mantle temperatures and less oxic seawater composition respectively before 2 5 ga this is clear documentation that plate tectonics including the lateral movement of oceanic lithosphere has been a major heat loss mechanism on earth since the early precambrian

the stonyford volcanic complex a forearc seamount in the northern california coast ranges jurassic age volcanic rocks of the stonyford volcanic complex sfvc comprise three distinct petrological groups based on their whole rock geochemistry 1 oceanic tholeiites 2 transitional alkali basalts and glasses 3 high al low ti tholeiites major and trace element and sr nd pb isotopic data indicate that the oceanic tholeiites formed as low degree partial melts of normal mid ocean ridge basalt n morb source asthenosphere similar in isotope composition to the east pacific rise today the alkalic lavas were derived from an enriched source similar to that of e morb the high al low ti lavas resemble second stage melts of a depleted morb source asthenosphere that formed by melting spinel lherzolite at low pressures trace element systematics of the high al low ti basalts show the influence of an enriched component which overprints generally depleted trace element characteristics tectonic discrimination diagrams show that the oceanic tholeiite and alkali suites are similar to present day basalts generated at mid oceanic ridges the high al low ti suite resembles primitive arc basalts with an enriched alkali basalt like overprint isotopic data show the influence of recycled components in all three suites the sfvc was constructed on a substrate of normal coast range ophiolite in an extensional forearc setting the close juxtaposition of the morb like olivine tholeiites with alkali and high al low ti basalts suggests derivation from a hybrid mantle source region that included morb source asthenosphere enriched oceanic asthenosphere and the depleted supra subduction zone mantle wedge we propose that the sfvc formed in response to collision of a mid ocean ridge spreading center with the coast range ophiolite subduction zone formation of a slab window beneath the forearc during collision allowed the influx of ridge derived magmas or the mantle source of these magmas continued melting of the previously depleted mantle wedge above the now defunct subduction zone produced strongly depleted high al low ti basalts that were partially fertilized with enriched alkali basalt type melts and slab derived fluids

the ophiolitic north fork terrane in the salmon river region central klamath mountains california the north fork terrane is an assemblage of ophiolitic and other oceanic volcanic and sedimentary rocks that has been internally imbricated and folded the ophiolitic rocks form a north trending belt through the central part of the region and consist of a disrupted sequence of homogeneous gabbro diabase massive to pillowed basalt and interleaved tectonitic harzburgite u pb zircon age data on a plagiogranite pod from the gabbroic unit indicate that at least this part of the igneous sequence is late paleozoic in age the ophiolitic belt is flanked on either side by mafic volcanic and volcaniclastic rocks limestone bedded chert and argillite most of the chert is triassic including much of late triassic age but chert with uncertain stratigraphic relations at one locality is permian the strata flanking the east side of the ophiolitic belt face eastward and depositional contacts between units are for the most part preserved the strata on the west side of the ophiolitic belt are more highly disrupted than those on the east side contain chert argillite melange and have unproven stratigraphic relation to either the ophiolitic rocks or the eastern strata rocks of the north fork terrane do not show widespread evidence of penetrative deformation at elevated temperatures except an early tectonitic fabric in the harzburgite slip fiber foliation in serpentinite phacoidal foliation in chert and mafic rocks scaly foliation in argillite and mesoscopic folds in bedded chert are consistent with an interpretation of large scale anti formal folding of the terrane about a north south hinge found along the ophiolitic belt but other structural interpretations are tenable the age of folding of north fork rocks is constrained by the involvement of triassic and younger cherts and crosscutting late jurassic plutons deformation in the north fork terrane must have spanned a short period of time because the terrane is bounded structurally above and below by middle or late jurassic thrust faults the north fork terrane appears to contain no arc volcanic rocks or arc derived detritus suggesting that it neither constituted the base for an arc nor was in a basinal setting adjacent to an arc sediment source details of the progressive accretion and evolutionary relationship of the north fork to other terranes of the klamath mountains are not yet clear

tectonic evolution of the jurassic cretaceous great valley forearc california implications for the franciscan thrust wedge hypothesis interpretation of seismic reflection data and restoration of depositional geometries of cretaceous forearc basin strata in the northwest great valley of california provide important controls on structural reconstructions of the western margin of the sacramento valley and northern coast ranges monoclinal eastward dips of great valley group strata and fault systems striking northwest southeast which are features proposed as evidence for a west dipping blind great valley franciscan sole thrust and related backthrusts instead are expressions of bedding geometry that resulted from folding of the paskenta and related synsedimentary normal faults depositional onlap and a major structural stratigraphic discontinuity the discontinuity separates east dipping aptian and younger great valley group strata from beds of lower great valley group and coast range ophiolite that were deformed and erosionally or structurally truncated by mid cretaceous time dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient great valley forearc but is also observed in modern forearc basins advocates of the franciscan thrust wedge model have also proposed that west dipping shingled patterns of seismic events imaged beneath the sacramento valley are imbricate thrust slices of the great valley group this hypothesis however is incompatible with borehole potential field and seismic refraction data that characterize the sacramento valley basement as ophiolitic seaward dipping reflections in the ophiolitic basement of the sacramento valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges thus late stage tectonic mechanisms are not required to interpret a forearc that owes much of its present day bedding architecture to processes coeval with deposition thickening of the great valley group stratigraphic section valanginian turonian in the hanging walls of the paskenta elder creek and cold fork fault zones combined with attenuation or complete omission of preextensional units including the coast range ophiolite and geometric evidence based on seismic reconstructions suggest that these faults are jurassic cretaceous normal faults that developed in a submarine setting down structure views of the great valley outcrop belt simplify otherwise complex map relations and portray the paskenta and related faults as half graben bounding faults that accommodated significant northwestward tectonic transport of hanging wall rocks it is significant that these faults sole into the coast range fault an enigmatic forearc structure that juxtaposes rocks of the franciscan complex blueschists with rocks of the coast range ophiolite and great valley group that have sustained only zeolite grade metamorphism discovery of jurassic cretaceous crustal scale extension in the great valley forearc suggests that a significant part of coast range fault related attenuation developed early in the history of the subduction complex

cross shore sediment transport under breaking solitary waves 1 laboratory experiments were performed to examine the cross shore sediment transport processes under breaking solitary waves on a fine sand beach the initial beach slope of 1 12 was exposed to a positive solitary wave eight times the beach was rebuilt and exposed to a negative solitary wave eight times the wave motion and sediment transport were not affected much by the beach profile change from the initial profile the positive solitary wave plunged violently near the shoreline and suspended a considerable volume of sand the plunging wave with no seaward flow impeding its run up caused large run up on the foreshore the strong downrush following the large run up resulted in erosion on the foreshore and deposition seaward of wave run down on the other hand the negative solitary wave collapsed against the seaward flow induced by the free surface slope of the negative wave and caused less sediment suspension the wave run up against the seaward flow was much smaller the weak downrush following the small run up resulted in deposition on the foreshore and erosion near the wave collapsing point these limited laboratory experiments indicate the importance of the initial wave profile for swash sediment dynamics and the capacity of a single wave in causing noticeable beach profile changes

extensional faulting in southern klamath mountains california large northeast striking normal faults in the southern klamath mountains may indicate that substantial crustal extension occurred during tertiary time some of these faults form grabens in the jurassic and older bedrock of the province the grabens contain continental oligocene or miocene deposits weaverville formation and in two of them the oligocene or miocene is underlain by lower cretaceous marine formations great valley sequence at the la grange gold placer mine the oligocene or miocene strata dip northwest into the gently southeast dipping mylonitic footwall surface of the la grange fault the large normal displacement required by the relations at the la grange mine is also suggested by omission of several kilometers of structural thickness of bedrock units across the northeast continuation of the la grange fault as well as by significant changes in bedrock across some northeast striking faults elsewhere in the central metamorphic and eastern klamath belts the trinity ultramafic sheet crops out in the eastern klamath terrane as part of a broad northeast trending arch that may be structurally analogous to the domed lower plate of metamorphic core complexes found in eastern parts of the cordillera the northeast continuation of the la grange fault bounds the southeastern side of the trinity arch in the eastern klamath terrane and locally cuts out substantial lower parts of adjacent paleozoic strata of the redding section faults bounding the northwestern side of the trinity arch generally trend northeast and juxtapose stacked thrust sheets of lower paleozoic strata of the yreka terrane against the trinity ultramafic sheet geometric relations suggest that the tertiary extension of the southern klamath mountains was in nw se directions and that the redding section and the southern part of the central metamorphic terrane may be a large tertiary allochthon detached from the trinity ultramafic sheet paleomagnetic data indicate a lack of rotation about a vertical axis during the extension we propose that the trinity ultramafic sheet is structurally analogous to a metamorphic core complex if so it is the first core complex to be described that involves ultramafic rocks we infer that mesozoic terrane accretion produced a large gravitational instability in the crust that spread laterally during tertiary extension

development of concepts concerning the genesis and emplacement of tethyan ophiolites in the eastern mediterranean and oman regions this article uses evidence mainly from the eastern mediterranean and oman to help understand the origin and emplacement of ophiolites field studies in these regions emphasize the diversity of ophiolite related sedimentary and tectonic processes topics to be discussed here include the sedimentary covers of ophiolites their metamorphic soles related melanges and emplaced continental margin slope rise units modes of ophiolite emplacement will also be considered and tectonic reconstructions that have a direct bearing on the origin and emplacement of ophiolites the writer argues that there is compelling evidence that many ophiolites formed by spreading above intraoceanic subduction zones evidence from their intact sedimentary covers however indicates that such subduction was only incipient in most cases as directly associated volcanic arcs or back arc basins are rare metamorphic soles record fragments of oceanic lithosphere derived from settings ranging from open ocean to continental slope rise settings underlying melanges record the subduction of most of the ocean lithosphere formed at spreading ridges continental margin units beneath ophiolites demonstrate that these ophiolites were formed in ocean basins that were mainly initiated by continental rifting in the eastern mediterranean and oman regions ophiolite emplacement was commonly achieved by trench margin collision although strike slip transpression and large scale accretion along subducting margins played a role in some cases taking account of the mesozoic palaeotectonic setting of the eastern mediterranean region it is widely believed that ophiolites of two main ages mid jurassic and late cretaceous were formed in several small oceanic basins separated by rifted microcontinents the history of tethys in pre mesozoic time remains more open to question lastly giant subduction related ophiolites as in oman cyprus or the balkans may not be forming anywhere in the world ocean today but may instead relate to rare events in earth history when regional to global plate boundaries were drastically reorganised

contrasting early and late mesozoic petrotectonic evolution of northern california devonian middle jurassic terrane assemblies in the klamath mountains and sierra nevada foothills consist chiefly of ophiolite chert argillite sequences mafic ultramafic complexes are oceanic whereas associated fine grained deep water terrigenous sediments were derived mainly from adjacent previously docked klamath sierran terranes coeval calc alkaline arc rocks are volumetrically rare geologic and petrochemical relations suggest a rifted arc origin for klamath mafic metavolcanic units inter layered with distal turbidites in the 170 200 ma north fork terrane detrital zircon u pb ages indicate that the clastic debris had a regional eastern klamath source the eastern hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby eastern klamath antelope mountain quartzite the seaward 200 ma rattlesnake creek terrane is an ophiolitic melange with north fork petrotectonic affinities the north fork eastern hayfork rattlesnake creek amalgam correlates with the calaveras complex and the outboard jura triassic arc belt in the sierran foothills geochemical bulk rock and zircon u pb age data support interpretation of the 200 ma jura triassic arc as an adjacent offshore mafic belt overlying a 300 ma ophiolitic basement these oceanic complexes were sutured against the central metamorphic belt eastern klamath feather river northern sierra terrane backstop before deposition and deformation of the outboard upper jurassic galice and mariposa formations klamath sierran terrane assemblies reflect 230 m y of transpression transtension involving only minor episodes of subduction producing ubiquitous ophiolite chert argillite lithologies and rare felsic arc rocks in contrast the late jurassic to largely cretaceous klamath sierra nevada quartzo feldspathic volcanic plutonic arc attests to massive calc alkaline magmatism attending a strong eastward component of underflow by the farallon plate the coeval galice mariposa formations followed by the cretaceous great valley forearc and franciscan trench deposits are first cycle felsic debris shed mainly from the klamath sierran arc these units record 70 m y of rapid sialic crustal growth attending major periods of approximately margin normal convergence this profound transition in northern california included devonian middle jurassic rifting drifting and stranding of ophiolite chert argillite terranes along an adjacent curvilinear continental margin then nearly head on cretaceous subduction that resulted in massive calc alkaline igneous activity the erosion of which generated the felsic great valley group forearc basin and franciscan complex trench clastic sedimentary units

radioisotopic and biostratigraphic age relations in the coast range ophiolite northern california implications for the tectonic evolution of the western cordillera the coast range ophiolite cro in northern california includes two distinct remnants the elder creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites layered gabbro wehrlite pyroxenite quartz diorite a mafi c to felsic dike complex and mafi c felsic volcanic rocks the entire suite is cut by late mid oceanic ridge basalt morb dikes and overlain by ophiolitic breccia the stonyford volcanic complex sfvc comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite structurally below this seamount are melange blocks of cro similar to elder creek u pb zircon ages from plagiogranite and quartz diorites at elder creek range in age from 165 ma to 172 ma u pb zircon ages obtained from cro melange blocks below the sfvc are similar 166 172 ma 40 ar 39 ar ages of alkali basalt glass in the upper sfvc are all younger at 164 ma radiolarians extracted from chert lenses intercalated with basalt in the sfvc indicate that the sedimentary strata range in age from bathonian unitary association zone 6 6 of baumgartner et al 1995a near the base of the complex to late callovian to early kimmeridgian unitary association zones 8 10 in the upper part the sfvc sedimentary record preserves evidence of a major faunal change wherein relatively small sized polytaxic radiolarian faunas were replaced by very robust oligotaxic nassellarian dominated faunas that included praeparvicingula spp we suggest that cro formation began after the early middle jurassic 172 180 ma collision of an exotic or fringing arc with north america and initiation of a new or reconfi gured east dipping subduction zone the data show that the cro formed prior to the late jurassic nevadan orogeny probably by rapid forearc extension above a nascent subduction zone we infer that cro spreading ended with the collision of an oceanic spreading center ca 164 ma coincident with the oldest high grade blocks in the structurally underlying franciscan assemblage we further suggest that the classic nevadan orogeny represents a response to spreading center collision with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in north american plate motion to rapid northward drift j2 cusp causing sinistral transpression and transtension in the sierra foothills these data are not consistent with models for late jurassic arc collision in the sierra foothills or a back arc origin for the cro

platinum group element resources in podiform chromitites from california and oregon rock samples from about 280 podiform chromite deposits in california and oregon were analyzed for pt pd rh and ir by fire assay atomic absorption and spectrographic techniques to estimate by product platinum group element potential of chromite mining these deposits include ophiolites from the sierra nevada klamath mountains coast ranges and the canyon mountain complex the deposits vary from paleozoic to mesozoic age and vary in size shape texture and degree of metamorphism the maximum platinum group element contents in ppm measured for chromitites are pt 2 53 pd 0 2 rh 0 14 ru 4 93 and ir 2 93 correlations between pt and rh 0 60 rh and ir 0 64 rh and ru 0 71 and ir and ru 0 85 are significant at the 1 percent level relatively small but significant differences in platinum group element content exist between geologic terranes chromite production from california and oregon amounts to about 692 000 metric tons 1914 1985 the mean contained platinum group element amounts in troy ounces are pt 850 pd 100 rh 260 ru 4200 and ir 1 700 using estimated grades previous investigators have suggested that relatively few samples can represent the platinum group element content of a podiform chromite deposit therefore these analyses are assumed to approximate grades total world resources of platinum group elements in podiform chromite deposits are estimated in troy ounces to be pt 210 000 pd 25 000 rh 64 000 ru 1 000 000 and ir 420 000

significance of klamath rocks between the franciscan complex and coast range ophiolite northern california small fault bounded slabs of low grade prehnite pumpellyite bearing slate metagraywacke and greenstone occur between the coast range ophiolite and south fork mountain schist for at least 60 km south of the klamath mountains northern california the metagraywacke slabs differ from typical franciscan complex metagraywacke to the west by the absence of blueschist facies minerals and the abundance of quartz and chert clasts and they differ from sandstone and mudstone of the great valley sequence to the east by the presence of a penetrative cleavage and low grade metamorphism the metasedimentary rock in the slabs is petrographically identical to the jurassic galice formation which occurs along the western and southern margins of the klamath mountains upward drag along a west dipping reverse fault best explains the present structural position of the slabs and is consistent with available geological and geophysical data these data imply that the coast range ophiolite structurally overlies sierran klamath basement at least locally

constraints on early franciscan subduction rates from 2 d thermal modeling abstract mesozoic subduction along the western margin of north america in california jumped west in mid jurassic time to form the franciscan accretionary wedge outboard of the coast range ophiolite and the great valley forearc basin based on mesozoic plate motion analysis for the pacific region overall rates of subduction are likely to have been on the order of 50 km m y new and previously published lu hf garnet growth ages on high grade blocks in the franciscan however suggest that early franciscan metamorphism took place in a slowly cooling environment characteristic of slow subduction rates in order to assess rates of early franciscan subduction 2 d thermal modeling was carried out based on the following assumptions 1 subduction was initiated at 170 ma based on the age of the oldest tectonic block and on the age of the coast range ophiolite believed to have formed in a forearc setting behind the newly initiated subduction zone 2 at the start of subduction the hanging wall mantle wedge was at asthenospheric temperatures consistent with initiation of subduction at the ophiolite spreading center 3 the earliest subducted lithosphere was 10 m y old at the start of subduction 4 tectonic blocks were metamorphosed at an average pressure of 1 4 gpa corresponding to a depth of 43 km beneath mantle rocks assuming 3 3 g cm 3 subduction rates were investigated between 1 and 300 km m y and the best fit between the ages and estimated metamorphic temperatures of the blocks was found to be 10 5 km m y this is five times slower than the 50 km m y determined from plate motion analysis for that time and suggests that some reappraisal of the configuration and velocities of the oceanic plates may be needed

subducted sedimentary serpentinite melanges record of multiple burial exhumation cycles and subduction erosion abstract serpentinite matrix melanges give insight into large scale convergent plate margin processes particularly because of the derivation of the serpentinite from oceanic mantle similar to shale matrix melanges a field geologist may easily recognize the sedimentary origins of little deformed serpentinite matrix melanges but melanges within accretionary prisms have undergone significant deformation and recrystallization of matrix serpentinite melanges of the franciscan subduction complex of california have a seemingly intact and foliated matrix such exposures contrast sharply with the granular undeformed sedimentary serpentinite melanges of the coeval great valley group gvg forearc basin deposits that depositionally overlie coast range ophiolite that structurally overlies the franciscan nonetheless franciscan serpentinite melanges display evidence of sedimentary origins including sedimentary breccia composed of exotic block material tolay ridge sedimentary serpentinite breccia panoche pass road basal serpentinite conglomerate with exotic clasts sunol regional wilderness and serpentinite sandstones and conglomerates including a basal conglomerate overlying coherent metagraywacke tiburon peninsula these examples record two burial exhumation cycles to blueschist facies depths in addition a melange breccia in the panoche pass area may have components that record three burial exhumation cycles to blueschist or greater depth exhumation rates for various cycles ranged from about 1 2 to 10 mm year the tiburon peninsula serpentinite melange occupies the structurally highest horizon in the franciscan of the san francisco bay area and regional field relationships indicate deposition at ca 100 ma apparently about 65 ma of subduction erosion non accretion followed initiation of franciscan subduction in this region the oldest franciscan serpentinite melanges are at least 35 ma younger than sedimentary serpentinites of the gvg subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench evidence of multiple burial exhumation cycles indicates reworking of subduction complex material consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages