glaucophane schists and ophiolites of the northern california coast ranges isotopic ages and their tectonic implications the geology of a strip of 15 quadrangles across the franciscan complex in the coast ranges of northern california is the basis for interpretation of the significance of potassium argon and fossil ages of glaucophane schists ophiolitic materials and associated sedimentary rocks whole rock and mineral separates of franciscan rocks were analyzed and 69 potassium argon ages were obtained white micas and actinolites from blueschist blocks in melange give apparent ages clustering about 142 and 153 m y but they probably represent a continuum blue amphibole ages from blocks range from 98 to 151 m y the younger ages are from the eastern portion of the central melange belt published u pb ages from the ophiolite belt give crystallization ages from 153 to 165 m y k ar ages extend from 143 to 166 m y whole rock ages from the south fork mountain schist range from 113 to 158 m y but cluster around 124 m y the association of older k ar ages with more coarsely crystalline schist suggests that metamorphism occurred prior to 124 m y ago fossil evidence indicates that the basal sediments of the great valley sequence were deposited upon the disrupted coast range ophiolite within 5 to 10 m y of the oldest crystallization ages for ophiolite fossils within melange units broken formations and unsubducted trench slope deposits provide evidence for melange development by the early cretaceous with youngest melange units generally lying to the west available chronologic evidence is compatible with a tectonic model involving melange formation in a restricted flow channel beneath the accretionary wedge of the subduction complex upflow of melange returned blocks of coarsely crystalline metamorphic rocks that were accreted during the initial stages of subduction when the hanging wall was still hot the viability of this and other proposed tectonic models can be assessed by continued careful field documentation of the nature of contacts between key lithotectonic units and by detailed application of temperature sensitive geochronologic tools

tectonic wedging blueschist metamorphism and exposure of blueschists are they compatible tectonic wedging is currently deforming rocks along the westernmarginofthecentralvalleyofcalifornia structuralrelations in the strata of the mesozoic great valley group fore arc basin indicatethatearlierphasesofwedgingwerecoevalwithsubduction and accretion of the franciscan complex to the west the presentday geometry of tectonic wedging is incompatible with blueschist metamorphism in the franciscan complex because the detachment for the wedge system would have prevented the transport of accretionary wedge materials to great depth we propose a sequence of tectonic events that reconciles these tectonic processes franciscan subduction and metamorphism began at 165 ma and great valley group deposition began in the tithonian shortening affected great valley group rocks in the hauterivian barremian and may havecoincidedwithagapinblueschistmetamorphisminthefranciscan major uplift of franciscan blueschist relative to the unmetamorphosed great valley group rocks began in the albian cenomanian this uplift was accommodated along an east dipping extensional fault system between the franciscan complex and the structurally overlying coast range ophiolite and great valley group movement on this fault system caused the western side of thefore arcbasintosubside resultinginuninterrupteddeposition ofgreatvalleygroupsedimentsfromthecenomaniantothecampanian maastrichtian the youngest exposed blueschists in the franciscan were metamorphosed at 80 90 ma uplift of blueschist relativetogreatvalleygrouprocksmusthavebeencompleteprior todevelopmentofthepresentstyleofeast vergenttectonicwedging this style of deformation probably began between the maastrichtian and paleocene resulting in exhumation and deformation of bothfranciscanandgreatvalleygrouprocks thedevelopmentof wedging was coincident in part with the shallowing of subduction dip and voluminous frontal accretion of the coastal belt of the franciscan

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

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

cretaceous tectonic evolution of coastal southern california insights from the catalina schist the relatively high t high p t subduction related basement in the southern california continental borderland preserves important information regarding the late mesozoic tectonic evolution of coastal southern california integrated petrologic geochemical and geochronological study of the subduction related catalina schist exposed on santa catalina island documents the formation of lawsonite blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling newly initiated subduction zone petrologic and 40ar 39ar thermochronologic data demonstrate similar perhaps shared retrograde pt histories for all but the lawsonite albite facies rocks under pumpellyite actinolite facies conditions as early as 95 100 ma the 40ar 39ar data together with the abundance of pumpellyite actinolite facies vein assemblages and lack of higher p t epidote or lawsonite blueschist over printing in high grade units are consistent with the rapid emplacement of the catalina schist into the shallow to mid crust by 95 ma by combined underplating and tectonic erosional removal of overlying rocks in the accretionary complex cf platt 1986 the catalina schist and the adjacent peninsular ranges batholith potentially represent paired metamorphic belts formed along southwest north america at 120 to 90 ma combining the evidence for the evolution of the catalina schist with the previously advanced hypotheses regarding tectonic evolution of the batholith suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone at 120 115 ma in which the catalina schist was accreted and metamorphosed

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

nappes tectonics of oblique plate convergence and metamorphic evolution related to 140 million years of continuous subduction franciscan complex california this paper presents a new synthesis of franciscan complex tectonics with the emphasis on the pre san andreas fault history of these rocks field relations suggest that the franciscan is characterized by nappe structures that formed during sequential accretion at the trench the presence of these structures along with other field relations including the lack of evidence for large offset of conglomerate suites indicates that strike slip fault systems of large displacement 500 km did not cut the franciscan complex during subduction regional geology and comparisons to modern arc trench systems suggest that strike slip faulting associated with oblique subduction took place inboard east of the franciscan in the vicinity of the magmatic arc the franciscan varies along strike because individual accreted elements packets of trench sediment seamounts etc did not extend the full length of the trench different depths of underplating distribution of post metamorphic faulting and level of erosion pro

melange diapirs into the cambria slab a franciscan trench slope basin near cambria california near cambria california a franciscan trench slope basin of late cretaceous age is intruded by two diapirs of blueschist block bearing mud matrix melange the rise of the diapirs caused major upturning of the bedded strata within a few hundred meters of the contact and complex interfingering of mud matrix melange with basin sediments near the contacts at one locality a fragment of basin sandstone is observed partially detached from the wall rock the mud matrix melange is highly mobile and the intrusive character during diapirism is clearly analogous to that of magma in igneous intrusions diapirism may be responsible for large scale intercalation of slope and slope basin sediments with subduction generated melange at actively convergent plate margins

counterclockwise p t t paths from amphibolites franciscan complex california relics from the early stages of subduction zone metamorphism overprinting mineral relationships in amphibolites from the franciscan complex california suggest metamorphic evolution from high temperature amphibolite facies to the blueschist facies with increasing p t ratio a counterclockwise p t t path early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are in turn cut or rimmed by sodic amphiboles some amphibolites are overprinted successively by eclogite and blueschist facies assemblages clinopyroxenes are zoned with increasing jadeite component from core to rim multiple generations of phengites are present in the same sample and are more si rich with each successive generation of growth garnets in eclogite overprinted amphibolites are zoned with an initial mgo increase away from the core to a maximum mgo zone followed by mgo decrease toward the rim garnets in other amphibolites show continually increasing mgo from core to rim several samples representative of the variety of franciscan amphibolites are describ

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

the effects of sub blocking temperature metamorphism on the k ar systematics of hornblendes 40ar 39ar dating of polymetamorphic garnet amphibolite from the franciscan complex california the effects of sub blocking temperature metamorphism on the k ar system in hornblende as revealed by 40ar 39ar release spectra have been studied in a polymetamorphic knocker from the franciscan complex fc california a primary amphibolite assemblage of horn blende rutile epidote apatite garnet sphene is variably overprinted by a blueschist facies assemblage of blue amphibole lawsonite chlorite white mica pumpellyite sphene the secondary assemblage formed at a temperature of 370 c below that at which rapid ar diffusion is expected in hornblende hornblendes from three variably altered samples of garnet amphibolite yield total gas ages of 147 to 161 ma but the corresponding plateau ages of 163 0 2 8 160 6 2 2 and 161 8 2 2 ma are identical within error hornblende separates with lower total gas ages come from more highly overprinted rocks have excess k compared to that expected on the basis of electron probe analyses and exhibit anomalously high k ca ratios in the low temperature fractions of their incremental heating spectra the reduced total gas ages result from the presence of thin 2 m sheets of younger white mica in hornblendes from the moderately and highly altered amphibolites the secondary micas are difficult to detect because of their small size and low abundance 2 but because their k content is 50 to 100 times that of the host hornblende they contribute significantly to the k and ar budgets of the sample the mica intergrowths are not removed by normal sample preparation but because the mica inclusions degas at lower temperatures than hornblende during vacuum extraction incremental heating analyses can provide precise cooling ages for the hornblendes as well as useful estimates of the age of the mica inclusions the hornblende separate from the most altered sample contained 20 10 younger blue amphibole replacing hornblende but its plateau age was not significantly affected this is consistent with a replacement process in which k and ar loss from the hornblende are coupled leaving the k ar system undisturbed in relict primary grains the k and ar budgets of the sample are not strongly affected by the blue amphibole because of its very low k content because partial replacement of primary amphibole by high k phyllosilicates occurs in many geological environments effects like those described here could be widespread

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

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

ordovician ocean plate stratigraphy and thrust duplexes of the ballantrae complex sw scotland implications for the pelagic deposition rate and forearc accretion in the closing iapetus ocean abstract the ballantrae complex at bennane lea in sw scotland contains important ocean plate stratigraphy basalt chert mudstone sandstone in an accretionary prism that is associated with a classic ordovician ophiolite we used the ocean plate stratigraphy to sub divide the prism into 11 tectonic units to determine the depositional age of bedded cherts zircons were separated from 9 tuff beds from 6 different units all the tuffs have early to middle ordovician ages even though their present positions are mutually distant these ages are consistent with microfossil records of radiolaria and graptolites the stratigraphic structural relationships demonstrate that the ocean plate stratigraphy has been repeated by bedding parallel thrusts this is typical of a modern accretionary duplex we calculated the sedimentation rate of early to middle ordovician bedded cherts at bennane lea on the basis of u pb zircon ages obtained from several tuff beds the data indicate that the depositional rate 0 6 3 m myr was as slow as that of mesozoic cenozoic equivalents defined by radiolaria the age spectra of detrital zircons from ballantrae sandstones show prominent single peaks at ca 467 and 478 ma and a lack of precambrian zircons integration of our new zircon ages with published isotopic data and palaeo geographic maps indicates that the sandstones were deposited near an intra oceanic arc and far from any continent containing precambrian rocks the pelagic to clastic sediments at bennane lea were deposited in the closing iapetus ocean from ca 477 ma to ca 464 ma when they were accreted with the intra oceanic arc of ballantrae

early mesozoic southward migration of cordilleran transpressional terranes oblique plate convergence can cause forearc arc and back arc regions of active margin orogenic belts to behave as semi independent terranes relative to bordering continental and oceanic lithospheric plates these terranes called here transpressional terranes are bound by the subduction zone on one side by arc parallel strike slip faults in the forearc in the volcanic arc or in the back arc region on the other and are underlain by throughgoing basal decollements these terranes undergo shortening in a direction perpendicular to the subduction zone and migrate as coherent structural bodies parallel to the subduction zone along bounding strike slip faults many terranes of the north american cordillera may have behaved in this manner and perhaps did not travel across the pacific basin but rather were always peripheral to north america we propose that during the triassic to mid cretaceous portions of the cordillera borderland generally were displaced southward with respect to the north american craton due to left oblique convergence since the mid cretaceous they moved northward due to right oblique subduction the proposed displacements are consistent with structural paleomagnetic and paleontologic data and with the relative and absolute motions of north america and the pacific basin plates based on hotspot reference frame models

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

age and tectonic setting of mesozoic metavolcanic and metasedimentary rocks northern white mountains california mesozoic metavolcanic and metasedimentary rocks in the northern white mountains eastern california and western nevada are separated from lower paleozoic and precambrian rocks by jurassic and cretaceous plutons the large stratigraphic hiatus across the plutons is called the barcroft structural break recent mapping and new u pb zircon ages of 154 3 1 ma and 137 1 ma from an ash flow tuff and a hypabyssal intrusion respectively indicate that part of the mesozoic section and the barcroft structural break are younger than the 160 165 ma barcroft granodiorite in contrast to previous interpretations the barcroft granodiorite has been thrust westward over most of the mesozoic section it is everywhere in fault contact with overturned metasedimentary rocks on the west side of the range rocks which were previously thought to be upright and the oldest part of the mesozoic section the mcafee creek granite which has a 100 1 ma u pb zircon age postdates thrusting therefore the barcroft structural break is primarily early cretaceous in age

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

kinematics of franciscan complex exhumation new insights from the geology of mount diablo california three major lithotectonic units of an ancient arc trench system are present in western california in ascending structural order they include the franciscan complex a remnant accretionary prism partly including high pressure blueschist facies metamorphic rocks in a subduction melange the relatively unmetamorphosed coast range ophiolite and the great valley group forearc sediments in the mount diablo study area east of san francisco the subduction complex and ophiolite are juxtaposed across the coast range fault and together they represent a structurally attenuated section through the ancestral forearc crust exposed by uplift and folding of the late cenozoic mount diablo anticline the coast range fault is locally associated with a sheared zone in the lower part of the attenuated ophiolite shear sense indicators in the serpentinite such as s c fabrics rotated clasts and slick enfibers record the deformation kinematics and consistently indicate that the ophiolite moved down in a normal sense relative to the franciscan rocks in the modern reference frame based on these kinematic relations and metamorphic facies contrasts between the franciscan and the coast range ophiolite as much as 6 18 km structural attenuation has occurred along the coast range fault after restoration of vertical axis rotation and late cenozoic fold deformation the coast range fault dips at a low angle toward the northeast and shear sense data restore to top to the northeast motion on the coast range fault implying normal displacement of the ophiolite with respect to the structurally underlying franciscan complex these relations are consistent with the hypothesis that the franciscan rocks were exhumed by crustal attenuation along a low angle fault system these data suggest that the coast range fault dominantly accommodated normal displacement and ductile thinning and that these processes were primarily responsible for the structural exhumation of high pressure assemblages in the franciscan complex relative to the structurally overlying forearc crust

formation of taconic melanges and broken formations in the hamburg klippe central appalachian orogenic belt eastern pennsylvania abstract the hamburg klippe of the central appalachian orogenic belt exposed in eastern pennsylvania displays a complex record of poly phase melange and broken formation development in a convergent margin setting it includes an imbricate stack of tectonic slices which consist of upper cambrian to upper ordovician deep water and continental slope sedimentary rocks emplaced by gravity sliding onto the laurentian passive margin during deposition of the upper ordovician martinsburg formation based on their internal structure and stratigraphy the block matrix ratios and relations and the inferred tectonic settings of origin we have differentiated the following melanges and broken formations whose evolutionary stages coincide with specific deformational phases in a complete orogenic cycle from subduction to collision i sedimentary broken formations without exotic blocks formed by in situ and local down slope remobilization during the early stages of closure of the octoraro sea basin in the early ordovician ii two types of sedimentary melanges with exotic blocks formed at the front of the advancing accretionary wedge during the middle ordovician subduction of the laurentian continental margin beneath a microcontinent magmatic arc tectonic assembly iii layer parallel extensional broken formation and a diapiric melange formed in the outer trench and at the toe of the accretionary wedge respectively during the early late ordovician iv precursory olistostromes formed during the late ordovician collisional episodes as the hamburg klippe was emplaced in the martinsburg formation on the downgoing laurentian continental margin and v contractional deformation related broken formations formed at the base of main thrust faults overprinting the previously formed melanges and broken formations this sequential development of different melange types in the central appalachians was strongly controlled by the degree of consolidation of the layered strata and their rheological differences the structural level of melange formation within the accretionary wedge and the kinematics of deformational processes

intrinsic and scattering attenuation images of usu volcano japan we present intrinsic and scattering q attenuation images for usu volcano japan by analyzing over 1800 vertical seismograms by fitting the observed envelopes to the diffusion model we obtained intrinsic and scattering attenuation values at three different frequency bands using a back projection method and assuming a gaussian type weighting function we obtained the 2d images of intrinsic and scattering attenuation resolution tests confirm the robustness and reliability of the obtained images we found that scattering attenuation is the dominant process of energy loss in the frequency range analyzed which suggests strong spatial heterogeneity the resultant scattering attenuation images show an increase of attenuation toward the southwest from toya caldera which may correspond to deepening of the basement we also identify an area of low intrinsic and scattering attenuation at the summit of usu volcano which could be associated with old magma intrusions our results demonstrate a strong spatial relation between structural heterogeneities and attenuation processes in volcanic areas and confirm the efficiency of the method which can be used together with conventional imaging techniques

earthquake source parameters and stress distribution in the adak island region of the central aleutian islands alaska source parameters have been systematically determined for all earthquakes with mw 5 that occurred between 172 w and 179 w longitude in the adak island region of the central aleutian islands during 1977 march 1987 we relocate the events using a plate velocity model developed for the region and use two alternative methods of depth determination the first method uses arrival times of direct and prominent reflected phases primarily pwp the second method uses broadband p wave displacement seismograms in an inversion for source depth the analysis of these events provides a well constrained data set for the study of stress release along an active subduction zone three earthquakes which occurred seaward of the trench axis are located just below the crust mantle interface and show extension nearly perpendicular to the trench axis seven events occurred in the wadati benioff zone sixty four events are located in the main thrust zone and except for five unusual events are characterized by thrust mechanisms with one nodal plane dipping north at a shallow angle in cross section the thrust zone appears as a thin 10 15 km thickness interplate region which extends from 15 to 50 km depth a 10 15 average discrepancy between observed slip vector azimuths and the predicted relative motion direction between the pacific and north american plates exists for these events the observed slip vectors are oriented more normal to the trench than is predicted by plate motions five earthquakes which occurred as aftershocks to the may 7 1986 earthquake mw 8 0 in the crust of the overriding plate have strike slip mechanisms consistent with right lateral motion on arc parallel fault planes observations of slip vectors along the whole aleutian arc show a similar trend to that observed in the adak island region the largest differences 30 between observed and predicted slip azimuths occur around 175 e we propose a model of plate interaction in which a portion of the along arc motion occurs along a weak strike slip shear zone in the upper plate near the volcanic line the slip azimuths in the main thrust zone fit this model well if the amount of transcurrent slip occurring in the upper plate is 60 of the arc parallel relative plate motion a consequence of the model is along arc extension of the overriding plate between the accretionary wedge and the volcanic line especially in the western part of the aleutian arc calculations based on a tectonic model by geist et al 1988 for the formation of arc summit basins through block rotation and translation suggest that along arc extension has been significant since late miocene or early pliocene

evidence of uppermost proterozoic to lower cambrian miogeoclinal rocks and the mojave snow lake fault snow lake pendant central sierra nevada california this is the published version copyright 2010 american geophysical union all rights reserved

exotic terranes of western california numerous distinct geological terranes compose the north american cordillera1 there may be as many as 50 terranes in california alone2 critical to deciphering the history of cordilleran tectonic assembly is an understanding of the displacement history of individual terranes it is therefore important to know 1 whether a terrane has undergone significant motion with respect to the stable craton that is whether it is allochthonous or exotic 2 if so when relative motion started and stopped 3 from where an individual terrane originated and 4 the nature of interterrane movements we consider here the problem of determining whether the now juxtaposed salinian and stanley mountain terranes of california became amalgamated at or near their present position with respect to cratonic north america or if they collided at a considerable distance from their present positions and were later accreted to north america as a composite package the palaeomagnetic data that we present indicate that the latter was the case

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

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

petrogenetic appraisal of early palaeozoic granitoids of kinnaur district higher himachal himalaya india abstract the early palaeozoic felsic magmatism of kinnaur district is represented by more or less equivalent akpa granitoids 477 29 ma and rakcham granitoids 453 9 ma which form an integral part of the central crystalline zone in the southern portion of the tethys himalayan tectogen both akpa granitoids ag and rakcham granitoids rg intrude the rocks of the vaikrita group substantially low magnetic susceptibility values c 0 016 0 187 10 3 si of these early palaeozoic granitoids suggest their nature similar to as ilmenite series reduced type granitoids emplaced in a syn collisional tectonic setting the modal compositions of ag mainly correspond to two mica muscovite biotite monzogranite belonging to the granitoids generated by crustal fusion based on mineral assemblage ms bt tur ap associated skarn type tungsten mineralization occurrence of metasedimentary enclaves and whole rock geochemistry sio2 70 75 72 95 wt tio2 0 02 0 17 wt k2o na2o 1 05 2 32 molar a cnk 0 93 to 1 33 cipw corundum 0 17 to 3 82 wt av sr 304ppm initial 87sr 86sr 0 7206 0 00235 the ag can be typically characterized as peraluminous s type granite as similarly noted for rg higher rb sr ratio and initial 87sr 86sr 0 737 0 002 of rg compared to that of ag however indicate involvement of relatively more crustal component in generation of rg the ag is enriched in ba and light rare earth elements lree the chondrite normalized ree patterns of ag appear weakly fractionated lan lun 1 06 6 43 and show pronounced negative eu anomaly eun eu 0 03 the ag and rg represent typical s type granite suites that largely evolved due to differential degree of anatexis partial fusion of sedimentary protoliths prior to caledonian but after the pan indian thermal orogenic event

formation of an archean tectonic melange in the schreiber hemlo greenstone belt superior province canada implications for archean subduction accretion process the late archean circa 2750 2670 ma schreiber hemlo greenstone belt superior province canada is composed of tectonically juxtaposed fragments of oceanic plateaus circa 2750 2700 ma oceanic island arcs circa 2720 2695 ma and siliciclastic trench turbidites circa 2705 2697 ma following juxtaposition these lithotectonic assemblages were collectively intruded by synkinematic tonalite trondhjemite granodiorite ttg plutons circa 2720 2690 ma and ultramafic to felsic dikes and sills circa 2690 2680 ma with subduction zone geochemical signatures overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation during d1 circa 2695 2685 ma oceanic plateau basalts and associated komatiites arc derived trench turbidites and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex fragmentation of these sequences resulted in broken formations and a tectonic melange in the schreiber assemblage of the belt d2 circa 2685 2680 ma is consistent with an intra arc right lateral transpressional deformation fragmentation and mixing of d2 synkinematic dikes and sills suggest that melange formation continued during d2 the d1 to d2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating the d2 intra arc strike slip faults may have provided conduits for uprising melts from the descending slab and they may have induced decompressional partial melting in the subarc mantle wedge to yield synkinematic ultramafic to felsic intrusions a similar close relationship between orogen parallel strike slip faulting and magmatism has recently been recognized in several phanerozoic transpressional orogenic belts suggesting that as in phanerozoic counterparts orogen parallel strike slip faulting in the schreiber hemlo greenstone belt played an important role in magma emplacement

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

edge deep seismic reflection transect of the eastern aleutian arc trench layered lower crust reveals underplating and continental growth an edge deep crustal seismic reflection transect of the eastern aleutian arc trench traces oceanic crust and moho more than 200 km beneath the accretionary prism to depths of more than 30 km these horizons project beneath a prominent sequence of layered reflectors that extends from about 9 to 35 km beneath the mesozoic core of the prism earthquake hypocenters imply continuity of the downgoing lithosphere from the base of the layered reflectors to beneath and beyond the active augustine volcano rapid lateral growth of the prism in eocene oligocene time coincided with uplift of the mesozoic core of the prism during lateral growth maintenance of critical taper requires thickening either by internal deformation or underplating because exposed rocks show only modest postemplacement shortening thickening most likely occurred by underplating probably of the layered reflectors the overall geometry of the layered reflectors is reminiscent of nappe structures and their emplacement may represent crustal scale duplexing associated with underplating the edge reflection data and borehole results indicate that the self edge is marked by an active out of sequence thrust that separates the paleogene and neogene prisms this thrust apparently developed in response to the prism s need to maintain critical taper and demonstrates that contrasts inmore lithology can result from mechanisms other than terrane emplacement less

correlation between the geologic record and computed plate motions for central california we have attempted to correlate geologic events in part of central california with computed motions of oceanic plates relative to cratonic north america three composite terranes are considered 1 the sierra nevada 2 the coast ranges northeast of the san andreas fault and 3 the salinian block in the sierra nevada jurassic plutonism ending about 147 m y b p and cretaceous plutonism 120 80 m y b p correlate with deduced farallon north america fa na convergence as does the nevadan orogeny 158 153 m y b p however the gap in plutonism 146 121 m y b p outlasted by far an apparent minimum in convergence 145 135 m y b p magmatism ceased in the sierra and uplift slowed during the laramide orogeny 75 45 m y b p when tectonic activity had moved east to the site of the rockies presumably because of a low angle of subduction but volcanisin resumed in the sierra about 33 m y b p indicating that a steeper angle was reestablished apparently east west compression slackened when the pacific north america pa na transform regime developed basalt was erupted in the southern sierra 12 3 m y b p and during this same time the range started its most rapid rise as a normal fault bounded block the coast ranges north east of the san andreas fault are underlain by the franciscan complex the older parts of the complex were probably assembled in a subduction zone far south of their present location but we believe that the same fa na convergence that was cited for sierran plutonism was responsible despite the disparity in latitude as the farallon plate was probably in contact with most of the western margin of north and south america a perceived strong pulse in convergence 100 85 m y b p evidently produced the coast range thrust which on geologic grounds alone most likely originated between 96 and 88 m y b p some of the northwesterly transport of franciscan rocks relative to north america is ascribed to oblique kula north america ku na convergence 85 43 m y b p during which period the coast range thrust was revived about 60 m y b p the farallon plate again influenced the california margin after the kuia plate had moved northward and fa na convergence caused the accretion of the coastal belt and king range terranes of the neo franciscan the advent of pacific north america pa na interaction was marked by local volcanism which advanced from the southeast toward the northwest in consonance with the passage of the mendocino triple junction dickinson and snyder 1978 in the salinian block plutons 107 82 m y old can be ascribed to the same fa na convergence as the contemporary cretaceous plutons of the sierra nevada although the salinian intrusives originated much farther south paleomagnetic results champion et al 1980 from the salinian related pigeon point formation 75 71 m y in age suggest about 2500 km of poleward essentially coastwise movement this postulated large movement during part or all of the last 75 m y poses the greatest dilemma in our study the kuia is the only known plate whose computed obliquely convergent motion is appropriate for the late cretaceous paleogene part of the migration and even this motion is inadequate for the estimated transport within the most likely time window furthermore the farallon plate seems ideally suited for the laramide orogeny 70 45 m y b p both plates could not have been in contact with the same part of the margin at the same time oblique convergence of the kula plate could possibly have caused large coastwise transport and also the laramide orogeny the coast ranges northeast of the san andreas fault and the salinian block both experienced strike slip faulting and en echelon folding correlative with pa na interaction indicating that they were close together by neogene time faulting and folding have been particularly marked since the inception of spreading in the gulf of california and attachment of westernmost california to the pacific plate about 4 m y b p the rise of individual coastal ranges commencing 3 1 m y b p may possibly have been caused by slight convergence which appears in computed pa na relative motions from 5 m y b p to the present

crystallographic preferred orientation of talc determined by an improved ebsd procedure for sheet silicates implications for anisotropy at the slab mantle interface due to si metasomatism

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

normal faulting at convergent plate boundaries mylonitic extensional fabrics in the franciscan subduction complex in del puerto canyon california revisited 1 using a strain and rotation analysis we tested the hypotheses that top east mylonitic extensional structures in the uppermost franciscan subduction complex in del puerto canyon california accomplished exhumation of the franciscan blueschists we found no evidence of strongly noncoaxial deformation instead our data indicate overall coaxial deformation in the proposed zone of mylonitic extensional deformation there are no extensional strains moderate vertical shortening occurred without horizontal extension and was compensated by modest deformation related volume loss there is also no strain gradient toward and within the proposed shear zone therefore the results of our work indicate that mylonitic extensional structures in del puerto canyon do not exist overall we conclude that there is no currently identifiable structure adjacent to or within the franciscan in california that could have accommodated large scale horizontal extension exhumation of the franciscan blueschists was mainly achieved by erosion of an emergent forearc high our strain results and published data are also inconsistent with the development of a supercritically tapered franciscan wedge that ultimately should have triggered normal faulting strain magnitudes are low despite the relatively long residence time of the rocks in the accretionary wedge from this we infer that strain rates were low and that the wedge material flowed with a relatively high viscosity therefore ductile flow probably was not fast enough to form a supercritically tapered franciscan wedge

four dimensional transform fault processes progressive evolution of step overs and bends many bends or step overs along strike slip faults may evolve by propagation of the strike slip fault on one side of the structure and progressive shut off of the strike slip fault on the other side in such a process new transverse structures form and the bend or step over region migrates with respect to materials that were once affected by it this process is the progressive asymmetric development of a strike slip duplex consequences of this type of step over evolution include 1 the amount of structural relief in the restraining step over or bend region is less than expected 2 pull apart basin deposits are left outside of the active basin and 3 local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes this type of evolution of step overs and bends may be common along the dextral san andreas fault system of california we present evidence at different scales for the evolution of bends and step overs along this fault system examples of pull apart basin deposits related to migrating releasing right bends or step overs are the pliopleistocene merced formation tens of km along strike the pleistocene olema creek formation several km along strike along the san andreas fault in the san francisco bay area and an inverted colluvial graben exposed in a paleoseismic trench across the miller creek fault meters to tens of meters along strike in the eastern san francisco bay area examples of migrating restraining bends or step overs include the transfer of slip from the calaveras to hayward fault and the greenville to the concord fault ten km or more along strike the offshore san gregorio fold and thrust belt 40 km along strike and the progressive transfer of slip from the eastern faults of the san andreas system to the migrating mendocino triple junction over 150 km along strike similar 4d evolution may characterize the evolution of other regions in the world including the dead sea pull apart the gulf of paria pull apart basin of northern venezuela and the hanmer and dagg basins of new zealand d 2004 elsevier b v all rights reserved

post nevadan deformation along the bear mountains fault zone implications for the foothills terrane central sierra nevada california southern parts of the foothills terrane sierra nevada california consist of western and eastern sequences of volcanic rocks and overlying slate gray wacke units separated by a multiply deformed and lithologically chaotic central belt structural and strain studies suggest that the intensity of regional ductile deformation decreased from greater than 50 shortening in the western belt to less than 30 shortening in the eastern belt and that much of this ductile deformation is younger than the timing usually assumed for the nevadan orogeny these structures are in turn deformed by a large ductile shear zone representing the southern continuation of the bear mountains fault zone this shear zone separates the western and central belts has an oblique east over west sense of movement and deforms the western margin of the guadalupe igneous complex these observations suggest that the eastern and western volcanic sequences are pieces of arcs amalgamated along the bear mountains fault zone during and after nevadan deformation

evidence for the collision of the salinian block with the franciscan subduction zone california recent detailed field mapping near pilarcitos lake san francisco peninsula california indicates that a subduction related suture between the salinian block continental fragment and the franciscan subduction complex may be preserved in the area in this region a ne dipping mylonite zone separates salinian rocks and structurally overlying franciscan rocks asymmetric augen pressure shadows and and s surfaces in the mylonite suggest down dip movement and a ne over sw sense of shear indicating the thrusting of franciscan over salinian rocks the mylonite is part of a 1 km wide zone of ductile thrust faulting these structures may have formed as a result of a collision between the salinian block and the franciscan subduction zone franciscan rocks being thrust westward over the salinian block because paleocene rocks are involved in the thrusting and clasts of the mylonitized rocks are present in upper paleocene and eocene in the area the age of thrusting is bracketed at approximately 50 60 ma th

implications of franciscan complex graywacke geochemistry for sediment transport provenance determination burial exposure duration and fluid exchange with cosubducted metabasites 1 interpretation of graywacke provenance has long been used to evaluate the record of tectonic process in orogenic belts our geochemical data from graywackes of the franciscan subduction complex california show that the connection between sedimentary record and geologic processes may be more complex than previously believed trace elements and nd sr pb isotopes of franciscan graywackes indicate two sources types one group lacking negative eu anomaly eu eu 0 9 shows slightly concave up heavy rare earth elements arc like trace element patterns and western pacific island arc like pb isotopes reflecting derivation from older accreted oceanic arc terranes in the sierra nevada klamath mountains the other group displays small negative eu anomalies with trace element patterns resembling post archean australian shale and pb isotopes similar to jurassic cretaceous sierran batholith there is no systematic separation of these two groups by depositional ages thus geochemistry of the graywackes may partly reflect variation in location of sediment delivery systems rather than solely reflecting evolution of the neighboring arc variation of nd sr isotopes with stratigraphic age for the graywackes mimics the trends of the coeval great valley group clastic rocks suggesting that 1 they share the same sediment sources 2 there are no exotic sediment sources that fed the franciscan trench and 3 burial exposure cycles for franciscan clastic rocks were comparatively brief comparison of franciscan graywacke and metabasite geochemistry corroborates earlier conclusions that metabasites had little or no chemical exchange with fluids from cosubducted graywacke detrital zircon age populations major element chemistry and detrital framework modes when compared to our data suggest that the former three parameters underrepresent the mafic component of clastic sediment provenance

intra arc strike slip fault exposed at batholithic levels in the southern sierra nevada california the kern canyon fault is a major north trending fault that is continuous for a distance of 140 km in the southern sierra nevada california previous geologic mapping and geochronological work along the northern third of the fault indicate that dextral offset occurred sometime after 80 ma and before 3 5 ma this offset was interpreted to be the result of cenozoic basin and range extension our new results from the central third of the fault kernville lake isabella region indicate an earlier right lateral movement history contemporaneous with emplacement of the largest plutons in the sierra nevada the older structure is termed the proto kern canyon fault zone the cenozoic fault trace is a narrow zone of brittle deformation whereas the cretaceous fault zone is a broad zone of ductile deformation u pb zircon geochronology on plutonic and metavolcanic rocks involved in the ductile deformation as well as a pluton that postdates ductile deformation demonstrate that the proto kern canyon fault zone was active at 85 ma and may have begun to move as early as 105 ma longitudinal strike slip faults are common in modern magmatic arcs where convergence is oblique the proto kern canyon fault zone may have originated in response to a moderate northward component in subduction of the farallon plate or perhaps a strong northward component for the kula plate

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

nature and timing of deformation in the foothills terrane central sierra nevada california its bearing on orogenesis detailed mapping and structural analysis combined with new age dates using u pb and 40 ar 39 ar techniques have allowed us to constrain the timing of pre ductile and ductile deformation in the foothills terrane of the central sierra nevada by using strain and other data it can be shown that rigid rotation of beds folding faulting predated the onset of ductile deformation and probably occurred between 160 and 151 ma ductile structures consisting of continuous and secondary cleavages and associated folds and lineations started forming ca 151 ma in the bear mountains fault zone and then ca 145 ma began moving away from the fault zone forming diachronously over an 30 m y period the last documented ductile structure formed ca 123 ma although some secondary structures may be younger metamorphism of these rocks is generally upper greenschist facies although higher grade belts one bearing staurolite andalusite and sillimanite are present strain was preferentially partitioned into one of these belts of higher metamorphic grade and sporadically elsewhere the structural history here is much more complex and at least one and locally two complete transpositions of the original cleavage have occurred in these zones of complex deformation it is in most cases possible only to identify a composite foliation consisting of new continuous cleavage and relicts of earlier phases all lying mutually parallel timing constraints indicate that the pre ductile structures may correspond to a very late stage of the nevadan orogeny that is 155 3 ma but the ductile structures postdate that orogeny as defined by as much as about 25 30 m y models which relate the ductile structures in the central foothills terrane to nevadan plate tectonic events are untenable in addition recent work indicates that late cretaceous ductile deformation in the central and southern sierra nevada may be relatively widespread indicating that tectonic models for the sierra nevada need to be reassessed

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

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

new evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the franciscan complex california and oregon abstract the early metamorphic history of high grade exotic blocks in the franciscan complex may be more complicated than previously supposed the different assemblages of high grade glaucophane schist eclogite amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust however new textural and mineralogical data presented here suggest that high grade glaucophane schist and eclogite have replaced an earlier epidote amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende rich blocks at least some of the hornblende rich blocks may therefore be well preserved remnants of the earlier metamorphism comparison of the mineral assemblages and element partitioning in the mixed assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier hornblende forming stage

deformation and hydrofracture in a subduction thrust at seismogenic depths the rodeo cove thrust zone marin headlands california we have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths the rodeo cove thrust zone which outcrops north of the golden gate bridge of san francisco imbricates two basalt chertsandstone sequences belonging to the marin headlands terrane franciscan complex the thrust outcrop is a 200 m thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop dominated by basaltic lithologies where zones of concentration of deformation have been mapped disruption is made by variably dense discrete fault systems synthetic to the main thrust r and p fractures these faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defi ned by chlorite and pumpellyite which also constrain the depth of faulting 8 10 km t 200 250 c within the seismogenic zone the central part of the fault also features the densest system of carbonate fi lled veins veins occur in the broken formation matrix and fragments in both cases parallel to the foliation the veins are either folded truncated or pressure solved along the cleavage cementation and hardening of shear surfaces of the fault core may have caused the distribution as opposed to localization of subsequent slip events the fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fl uids occurring through systems of multiple dilatant fractures grossly parallel to the decollement zone the crosscutting relationships between veining and foliation suggest that fl uid injection is cyclic and consequently that large transient variations in permeability and cohesion may occur the repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress or by difference in tensional strength parallel to and perpendicular to the foliation both of which would require extremely high fl uid pressure we interpret the features of the rodeo cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress fi eld in the interseismic phase and an extensional stress fi eld in the immediately postseismic phase

imbricate low angle faulting in uppermost franciscan rocks south yolla bolly area northern california the south yolla bolly area of northern california contains the intersection of the franciscan complex the klamath mountains and the great valley sequence four distinct lithic units among them the south fork mountain schist as well as several subunits have been discerned in the franciscan rocks of this area these units are composed mostly of coherent or semi coherent layered sedimentary and mafic igneous rocks and their metamorphic equivalents rather than melange and are arranged in largely upright parallel imbricate fault slices four major low angle faults have been clearly recognized paleontologic data indicate that at least one fault places older material over younger lithic units exhibit various degrees of differences in metamorphic mineral assemblages textural grade and amount of internal deformation across the faults with more intensely deformed and metamorphosed material occupying successively higher structural positions these contrasts indicate that some of these faults served as surfaces by which deformed rocks were emplaced differentially upward during and or after metamorphism original bedding and compositional layering in all of the coherent units generally parallel the thrust faults two fault bounded units are each composed of a coherent sequence of basalt at the base overlain by bedded chert and further overlain by mudstone and graywacke or their metamorphic equivalents the faults which underlie these two sequences seem to have originated as decollements localized at or near a sedimentary rock mafic crust interface perhaps as discrete underthrust slices the distinct contrasts in original rock composition among these units plus the aforementioned characteristics seem to indicate that these faults had an early history in the accretionary prism apart from their later use as convenient surfaces for differential upward emplacement of metamorphosed material

archaean crustal growth processes in southern west greenland and the southern superior province geodynamic and magmatic constraints abstract eo to mesoarchaean greenstone belts e g 3800 3700 ma isua c 3075 ma ivisaartoq 3071 ma qussuk occur within orthogneisses of the southern west greenland craton greenstone belts are composed mainly of metavolcanic rocks with minor ultramafic and sedimentary schists compositionally volcanic rocks are dominantly tholeiitic basalts boninites and picrites with minor intermediate to felsic volcanic rocks these greenstone belts appear to have formed in convergent margin geodynamic settings detailed field observations contrasting ages and metamorphic and structural histories suggest that this craton was assembled in several accretionary tectonothermal events involving accretion of arcs back arcs forearcs and continental fragments by horizontal tectonics the superior province of canada was also built by the amalgamation of oceanic and continental fragments ranging in age from 3700 to 2650 ma during five discrete tectonothermal events over 40 ma between 2720 and 2680 ma the neoarchaean 2750 2670 ma wawa greenstone belts are composed of tectonically juxtaposed fragments of oceanic plateaux oceanic island arcs back arcs and siliciclastic trench turbidites following juxtaposition these diverse lithologies were collectively intruded by syn to post kinematic granitoids with subduction zone geochemical signatures oceanic island arc lavas are easily distinguished from oceanic plateau counterparts because they possess positively fractionated rare earth element la sm cn 1 and gd yb cn 1 and high field strength element depleted nb th pm pm c 1000 km scale subduction accretion complex that formed along an intra oceanic convergent plate margin during trenchward migration of the magmatic arc axis several first order geological observations on archaean greenstone belts of sw greenland and the superior province suggest that phanerozoic style plate tectonic models can provide an elegant explanation for their structural lithological metamorphic and geochemical characteristics

subnormal geothermal gradients in the great valley forearc basin california during franciscan subduction a fission track study outboard parts of forearcs commonly have subnormal geothermal gradients because relatively cold lithosphere subducts beneath them the strata of the 10 to 15 km thick late jurassic tertiary great valley forearc basin accumulated in the forearc of the franciscan subduction zone of coastal california in this study paleogeothermal gradients in this basin were reconstructed to constrain the tectonic history of the convergent plate margin backstripping was used to reconstruct the burial histories of outcropping basin strata and apatite fission track analysis was used to determine the maximum burial temperatures experienced by 64 rock samples plots of maximum burial temperatures versus maximum burial depths indicate earliest tertiary time of deepest burial forearc basin geothermal gradients of about 9 c km backstripping shows that strata from the lower parts of two sections were deeply buried 6 5 km for the period 90 65 ma without experiencing high temperatures 105 c indicating that gradients were at least moderately subnormal throughout that period the data permit gradients no greater than about 15 c km at 90 ma decreasing to no greater than about 9 c km at 65 ma lower strata in another section were probably deeply buried without experiencing high temperatures from 130 or 120 ma until the latest cretaceous or early tertiary indicating that subnormal gradients probably existed throughout that period k ar ages of franciscan complex blueschists indicate subnormal gradients in the california forearc in the late jurassic and early cretaceous fission track data from the sierra nevada indicate subnormal gradients in the early and middle cenozoic and modern heat flow patterns indicate subnormal gradients in the late cenozoic together these indicators suggest that thermal gradients in the forearc were continuously subnormal from the initiation of franciscan subduction in the late jurassic until the termination of subduction in the late cenozoic gradients in the range that would permit blueschist facies metamorphism in the franciscan complex were thus long lived the broad range of k ar ages noted for franciscan blueschists need not reflect multiple discrete metamorphic events but are also consistent with a continuous extended period of high p t conditions blueschists need not have been uplifted soon after metamorphism to avoid overprinting by normal p t conditions but rather may have remained at depth for extended periods preliminary modeling indicates that an interruption in subduction of longer than 5 20 m y should cause detectable heating of forearc basin strata while a much shorter interruption should heat accretionary wedge rocks since there is no evidence of such heating it appears that convergence at the franciscan trench was essentially continuous if allochthonous terranes traveled northward parallel to the california margin during the late jurassic to late cenozoic period of franciscan subduction they apparently did so during oblique subduction not during extended interludes of pure transform motion

implications of upper mesozoic conglomerate for suspect terrane in western california and adjacent areas several distinctive kinds of conglomerate occur in upper mesozoic rocks in different regions of western california within each region conglomerate in the accretionary wedge franciscan assemblage is similar to conglomerate exposed nearby in the fore arc basin great valley sequence suggesting that both rock units were derived from the same source regions long distance tectonic transport is not necessary to account for the detrital sedimentary rocks in the franciscan the types of conglomerate in the lower part of the great valley sequence relate to their place of deposition in the fore arc basin in the northern coast ranges chert rich conglomerate derived from the klamath mountains was transported longitudinally southward by a relatively large distribution system correlative conglomerate in the diablo range is rich in volcanic and detrital sedimentary rock clasts derived from the western edge of the sierra nevada and transported westward by relatively small distribution systems conglomerate rich in chert and quartzite in suspect terrane of the golden gate gilroy and nacimiento blocks of central california may have been deposited by a large north flowing system in the southern part of the same basin an exotic origin is not required for the suspect terrane in mid cretaceous time the nacimiento block may have been displaced southeastward along the nacimiento fault and juxtaposed against the salinian block a deep and interior part of the magmatic arc in southern california this displacement may be reflected in the clast composition of mid cretaceous conglomerate of the nacimiento block but stratigraphic documentation is still incomplete younger and approximately equal right slip on the san andreas fault system brought both the nacimiento and salinian blocks northwest to their present position a southward increase in quartzite content in conglomerate of post nevadan age is broadly parallel to a similar increase in pre nevadan conglomerate of adjacent parts of the klamath mountains sierra nevada and peninsular ranges making it unlikely that any of these rocks were tectonically derived from remote regions radiolarians from chert pebbles in franciscan conglomerate are similar in age distribution and faunal composition to those from pebbles in the great valley sequence from pebbles in pre nevadan conglomerate and from bedded chert in presently adjacent source areas

simulation of the metamorphic and deformational history of the metamorphic sole of the oman ophiolite emplacement of ophiolites onto less dense continental crust is an enigma the rates of emplacement the mechanisms of emplacement and the stress and temperature histories of the rocks are all incompletely understood the oman ophiolite is well exposed and has been studied extensively by field geologists an integrated thermal and kinematic model of the temperature stress rock type and displacement fields during the early stages of the emplacement of the oman ophiolite is developed here to explain field relations and provide insight into the development of the metamorphic rocks at the base of the ophiolite the thermal evolution was calculated by a finite difference algorithm for heat conduction considering heats of metamorphic reactions deformational heating and heat advection by rock the stress and displacement fields were calculated by an analytical model using a velocity boundary condition power law constitutive relations and a brittle frictional sliding relationship in simulations involving young hot subducted lithosphere a wider portion of the subduction zone deforms by power law creep and deformation occurs at lower stresses and slower strain rates minimizing the deformational heating compared to colder older subducted lithosphere dehydration occurs at an accelerated rate in younger hotter lithosphere removing heat from the subducted plate at an earlier stage and causing the zone of brittle deformation to propagate downward more rapidly power law creep of subducted basaltic rocks is limited to temperatures greater than 500 c and occurs at differential stresses of the order of 100 mpa the simulations predict that metamorphic field gradients and the spatial distribution of rock types in the metamorphic sole of tethyan ophiolites might be used to infer the emplacement direction of the ophiolite

absolute inclination values from deep sea sediments a reexamination of the cretaceous pacific record new data are used to examine anomalously shallow paleomagnetic inclinations derived from previous analyses of cretaceous dsdp sediments of the central western pacific ocean while viscous remanent magnetizations vrms are shown to produce a similar effect the bias toward shallow values is still present with thoroughly demagnetized data superimposed upon this bias is a trend towards maximum flattening at an expected inclination of approximately 55 degree with the flattening falling to zero at the equator and pole flattening due to sediment compaction or inclination error is proposed to explain these observations

a 2 5 ga fore arc subduction accretion complex in the dengfeng granite greenstone belt southern north china craton abstract the dengfeng granite greenstone belt dggb located in the southern segment of the central orogenic belt cob of the north china craton ncc consists of a volcano sedimentary assemblage intruded by tonalite trondhjemite granodiorite ttg suite diorite granites and late mafic dikes the volcano sedimentary assemblage in the dggb mainly consists of tectonically imbricated basaltic amphibolites meta gabbroic rocks with minor ultramafic rocks and metagreywacke marble and quartzite consistent with characterisitics of typical phanerozoic subduction accretion complexes the basaltic amphibolites yield a metamorphic zircon 207pb 206pb age of 2507 26 ma interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction accretion of the basaltic protolith the basaltic amphibolites are characterized by a tholeiitic affinity and flat lree patterns with minor negative nb and zr anomalies based on mixed morb and arc affinities the basaltic amphibolites in the dggb are interpreted to have formed in a fore arc tectonic setting one late potassic granite dike cutting across the fabrics of the volcano sedimentary assemblage yields an intrusion age of 2492 35 ma constraining the minimum deformation age for tectonic assembly of the package the ttg gneisses and diorites intruded the western margin and the center of the subduction accretion complex respectively one ttg sample and one diorite sample yield igneous zircon 207pb 206pb ages of 2514 26 ma and 2518 36 ma respectively constraining their intrusion ages the ttg gneisses display high ratios of la yb cn and sr y and depletion in hfse with negative nb ta and ti anomalies consistent with those of typical archean ttgs the ttg gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and or accreted arc amphibolites the diorites have high concentrations of mgo 2 89 6 05 wt ni 148 178 ppm and cr 85 7 120 6 ppm and highly fractionated ree patterns and are depleted in hfses with negative nb ta zr hf and ti anomalies we suggest that the high mg diorites in the dggb may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction derived melts and or fluids collectively a neoarchean subduction accretion collision event is therefore proposed to have generated the dggb the volcano sedimentary assemblage in the dggb represents a fore arc subduction accretion complex which is interpreted to be related to the suture zone of ca 2 5 ga arc continent collision between a ttg dominated arc terrane in the cob and the eastern block of the ncc we further propose a long n s striking neoarchean suture zone occurring in the eastern margin of the cob mainly consists of the ca 2 5 ga subduction accretion complex in the dggb to the south the ca 2 5 ga melange belt in the zanhuang complex in the central and the ca 2 5 ga ophiolitic melange belt in the zunhua dongwanzi structural belt to the north which separate an arc terrane in the cob from the eastern block of the ncc

proterozoic and lower cambrian miogeoclinal rocks of snow lake pendant yosemite emigrant wilderness sierra nevada california evidence for major early cretaceous dextral translation detailed stratigraphic and structural studies at snow lake pendant have established new evidence for the presence of uppermost precambrian and lower cambrian miogeoclinal strata within the axial region of the sierra nevada batholith lithologic and stratigraphic data along with trace fossils a distinctive triassic overlap sequence fairview valley formation dikes probably related to the jurassic independence dike swarm and structural history suggest that the older rocks of snow lake pendant correlate with the stirling quartzite wood canyon formation zabriskie quartzite and carrara formation in the western mojave desert and san bernardino mountains this correlation implies approximately 500 km of dextral transport of the rocks of snow lake pendant along the proposed mojave snow lake fault movement on the fault probably took place after 148 ma the age of the independence dike swarm and prior to 110 ma the age of plutons in the central part of the sierra nevada batholith

deformation of the eastern franciscan belt northern california abstract the late jurassic and cretaceous eastern franciscan belt of the northern california coast range consists of two multiply deformed blueschist facies terranes the pickett peak and yolla bolly terranes four deformations have been recognized in the pickett peak terrane and three in the yolla bolly terrane the earliest recognized penetrative fabric d1 occurs only in the pickett peak terrane the later penetrative fabrics d2 and d3 occur in both the yolla bolly and pickett peak terranes d1 and d2 apparently represent fabrics that formed during subduction and accretion of the terranes fabrics from both d1 and d2 are consistent with sw ne movement directions with respect to their present geographic positions d3 postdates blueschist facies metamorphism of the terranes and may be related to emplacement of the terranes to higher structural levels a broad regional warping d4 is evident from the map pattern and folding of large metamorphosed thrust sheets d4 folds may be related to deformation associated with oblique convergence along the continental margin in late cretaceous and or early tertiary time

contrasts in thermal maturity within terranes and across terrane boundaries of the franciscan complex northern california the franciscan subduction complex of northern california includes several tectonostratigraphic units we have used the technique of vitrinite reflectance to document levels of thermal maturity for the central belt and the yager coastal and king range terranes spatial variations in thermal maturity help define the thermal structure of each franciscan terrane and each terrane displays a different geometry of thermal structure these differences are controlled by the style of structural deformation the overprinting effects of deformation following metamorphism and thermal overprinting caused by hydrothermal circulation anomalies along and contrasts across terrane boundaries help establish the relative timing and the structural mechanisms of terrane amalgamation for example localized anomalies are evident along the eel river fault central yager boundary and these increases in thermal maturity may be due to shear heating hydrothermal discharge or conductive heat transfer across the fault cross c

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

sandstone petrofacies of the yager complex and the franciscan coastal belt paleogene of northern california paleogene strata of the franciscan coastal belt and the yager complex northern california provide a vivid illustration of the problems encountered during studies of detrital provenance along accretionary continental margins the northern portion of the yager complex contains lower percentages of total quartz and higher percentages of feldspar than do correlative stratigraphic units to the south mean q f l modes are q 30 f 54 l 16 n yager and q 49 f 31 l 20 s yager a similar spatial trend is evident within the coastal belt where mean q f l modes are q 33 f 47 l 20 n cb and q 48 f 33 l 19 s cb a third coastal belt petrofacies l cb is dominated by volcanic lithic grains interpretations of generic provenance for each petrofacies are straightforward the arkosic sands probably were eroded from deeply dissected granitic plutons and the more quartzose sands are attributed to recycling from both orogenic backarc terranes and an adjacent magmatic arc the influx of volcanic detritus proves that magmatism was active within the drainage basin during paleogene time identification of specific source areas is more problematic however perhaps the north to south changes in detrital mineralogy were caused by influx from an idaho batholith source arkosic versus a sierra nevada source quartzose the depositional paleolatitude of franciscan sands is unknown and in the absence of latitudinal control potential sources as far south as mexico remain plausible regardless of these uncertainties the variations common to both coastal and yager terranes demonstrate that amalgamation must have preceded any translational events such that differential slip has not occurred between the two terranes

on the mechanism of tectonic transport in zones of oblique subduction abstract many cases of oblique subduction are known throughout the world today and many more must have existed in the past in at least one modern case sumatra the over riding plate has responded to oblique subduction by forming a strike slip fault that traverses the magmatic arc and displaces its leading edge in the direction of the transverse component of oblique convergence conditions favoring this sort of tectonic response are shown to be 1 1 a high angle of oblique convergence 2 2 a low angle of subduction and 3 3 relative thermal softening of the magmatic arc this mechanism may have been responsible in part for northward transport of western cordilleran magmatic arc and subduction zone rocks in the late cretaceous and earliest tertiary it also may help to explain evidence of southward transport and left lateral shear in northern mexico and the caribbean region in pre jurassic time