comment and reply on weathering before the advent of land plants evidence from detrital k feldspars in cambrian ordovician arenites

middle to late jurassic tectonic evolution of the klamath mountains california oregon the geochronology stratigraphy and spatial relationships of middle and late jurassic terranes of the klamath mountains strongly suggest that they were formed in a single west facing magmatic arc built upon older accreted terranes a middle jurassic arc complex is represented by the volcanic rocks of the western hayfork terrane and consanguineous dioritic to peridotitic plutons new u pb zircon dates indicate that the middle jurassic plutonic belt was active from 159 to 174 ma and is much more extensive than previously thought this plutonic belt became inactive just as the 157 ma josephine ophiolite which lies west and structurally below the middle jurassic arc was generated late jurassic volcanic and plutonic arc rocks rogue formation and chetco intrusive complex lie outboard and structurally beneath the josephine ophiolite u pb and k ar age data indicate that this arc complex is coeval with the josephine ophiolite both the late jurassic arc complex and the josephine ophiolite are overlain by the galice formation a late jurassic flysch sequence and are intruded by 150 ma dikes and sills the following tectonic model is presented that accounts for the age and distribution of these terranes a middle jurassic arc built on older accreted terranes undergoes rifting at 160 ma resulting in formation of a remnant arc back arc basin island arc triad this system collapsed during the late jurassic nevadan orogeny 150 ma and was strongly deformed and stacked into a series of east dipping thrust sheets arc magmatism was active both before and after the nevadan orogeny but virtually ceased at 140 ma

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

early jurassic deformation in the pine forest range northwest nevada and implications for cordilleran tectonics triassic and jurassic rocks in the black rock desert of northwest nevada comprise part of the mesozoic volcanic arc successions found throughout the westernmost u s cordillera detailed structural studies in the pine forest range located in the western black rock desert provide new insight into the jurassic structural history of this part of the mesozoic arc system regional jurassic deformation and metamorphism in the pine forest range affected a thick 10 5 km section of lower paleozoic through triassic strata that were tilted after deformation and are now exposed in approximately cross sectional view at high stratigraphic levels deformation produced a slaty cleavage and relatively open folds both of which are concentrated in rheologically incompetent rock types and was accompanied by metamorphism at low grade at deeper stratigraphic levels the regional foliation intensifies and becomes more pervasive folds tighten and metamorphic grade increases through greenschist into amphibolite grade at the deepest stratigraphic levels the rocks are amphibolite grade l s tectonites with a pronounced mylonitic fabric the 2 km thick zone of mylonites is interpreted as a ductile shear zone that formed when the stratigraphic section was still upright whereas the less deformed younger rocks are interpreted to occupy the upper plate of the shear zone and to record a strain gradient away from the shear zone a variety of structural and regional relations indicate that the shear zone is a ductile thrust fault along which younger rocks moved to the northwest in present day coordinates with respect to older rocks the timing of regional jurassic deformation and metamorphism is constrained by relations in and around two plutons dated by the u pb zircon method the 201 1 ma big creek pluton was intruded as a sill like body along the upper levels of the shear zone and is associated with a variety of features indicating syntectonic intrusion including internal textural features in plutonic rocks and metamorphic changes in wall rocks with proximity to the pluton the 185 1 ma theodore pluton which intrudes across part of the shear zone has a thermal aureole which statically overprints the shear zone fabric deformation was thus ongoing at 201 ma and over by 185 ma the early jurassic age of regional shortening deformation in the pine forest range contrasts with the middle to late jurassic 170 150 ma age of regional shortening documented in many other parts of the mesozoic arc system including the klamath mountains sierra nevada and mojave desert regions of california it also contrasts with the record of early jurassic extensional tectonism documented in the sierra nevada mojave desert and western arizona segments of the mesozoic arc system these conflicting relations indicate a complex tectonic evolution for the jurassic u s cordillera plate margin and lend support to the idea that disparate parts of this plate margin were juxtaposed along younger strike slip faults

breakup of young oceanic lithosphere in the upper part of a subduction zone implications for the emplacement of ophiolites it is investigated by means of numerical modelling whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate first a thermal model of the upper part of a subduction zone the region between the trench and the volcanic arc is presented temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases culminating in the arrival of a spreading ridge at the trench temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology next it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere recently created at a spreading center in the vicinity of a subduction zone can lead to its breakup our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench if breakup occurs some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate the detached sheet of thin oceanic lithosphere will not be subducted to mantle depths instead subduction of the trailing plate at the opposite side of the ridge can lead to the incorporation of this sheet into the forearc region our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc trench region and eventually upon closure of the ocean basin into an orogenic belt it is shown that a large subset of ophiolites those with a harzburgite dominated mantle section has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase

inferred high primary volatile contents in lavas erupted in an ancient back arc basin california several recent studies have demonstrated that high volatile contents are characteristic of many lavas erupted in modern back arc basins the josephine ophiolite is believed to have formed in a back arc basin in the late jurassic and high primary volatile contents are inferred for pillow lavas of the ophiolite based on the presence of pillows with very vesicular margins although the vesicular pillows could be the result of eruption in shallow water geologic evidence suggests eruption in very deep water this evidence includes the following 1 a near absence of vesicles in some pillow lavas 2 depths of modern back arc basins are generally greater than 2200 m 3 siliceous pelagic rocks overlie the ophiolite suggesting deposition below the ccd and 4 trace fossils in the flysch sequence overlying the pelagic rocks suggest deposition in abyssal or perhaps lower bathyal depths the occurrence of isolated and broken pillow breccias in the ophiolite suggests that they may form in deep water and the

tectonically driven organic fluid migration in the dabashan foreland belt evidenced by geochemistry and geothermometry of vein filling fibrous calcite with organic inclusions abstract fibrous calcite veins with organic inclusions have been widely considered as indicators of oil and gas generation and migration under overpressure abundant fibrous calcite veins containing organic bearing inclusions occur in faulted lower paleozoic through triassic hydrocarbon source rocks in the dabashan foreland belt dbf 13cpdb and 18opdb values of the fibrous calcite range from 4 8 to 1 9 and 12 8 to 8 4 respectively which is lighter than that of associated carbonate host rocks ranging from 1 7 to 3 1 and 8 7 to 4 5 a linear relationship between 13cpdb and 18opdb indicates that the calcite veins were precipitated from a mixture of basinal and surface fluids the fibrous calcite contains a variety of inclusions such as solid bitumen methane bearing all liquid inclusions and vapor liquid aqueous inclusions homogenization temperatures of aqueous inclusions range from 140 to 196 c with an average of 179 c salinities of aqueous inclusions average 9 7 wt nacl independent temperatures from bitumen reflectance and inclusion phase relationships of aqueous and methane bearing inclusions were used to determine fluid pressures results indicate high pressures elevated above typical lithostatic confining pressure from 150 to 200 mpa the elevated salinity and high temperature and pressure conditions of the fibrous calcite veins argue against an origin solely from burial overpressure resulting from clay transformation and dehydration reactions instead fluid inclusion p t data and geochemistry results and regional geology indicate abnormally high pressures during fluid migration these findings indicate that tectonic stress generated fracture and fault fluid pathways and caused migration of organic bearing fluids from the dbf during the yanshan orogeny

a field chemical and stable isotope study of subseafloor metamorphism of the josephine ophiolite california oregon the josephine ophiolite is a large complete ophiolite generated in a late jurassic back arc basin along western north america the ophiolite underwent subseafloor metamorphism under a steep thermal gradient as indicated by downward changes in mineralogy and 18o values characteristic mineral zonation includes mica orthoclase and hematite in the lower extrusives greenschist facies assemblages in the upper sheeted dikes and amphibolite facies assemblages in the lower sheeted dikes and high level gabbro cumulate gabbros are only incipiently altered but are depleted in 18o indicating high temperature alteration with fluids that evolved at low water w rock r ratios a second off axis circulation system is recorded by metalliferous sediments that occur 8 21 m above the ophiolite alteration in the upper sheeted dike complex and especially the extrusive sequence is heterogeneous at outcrop scale as indicated by large variations in mineralogy chemistry and 18o values within individual pillows and dikes the heterogenity is due to 1 variations in w r ratio and starting material during alteration by downwelling seawater e g pillow cores versus glassy rims and 2 localized discharge mineralization mostly epidosites superimposed on the background recharge alteration epidosites occur largely as dike parallel stringers irregular replacement of pillow and massive lava and replacement of interpillow and pillow breccia matrices the chemistry of most extrusives and sheeted dikes is characterized by loss of ca and gain in na and mg consistent with alteration by downwelling fluids in addition the sheeted dikes are depleted in k2o and the lower sheeted dikes and high level gabbros are depleted in zn and cu the 18o and d values for most samples from the extrusive sequence indicate alteration by seawater at 50 rotation of the entire crustal sequence at the spreading axis suggests that the ophiolite formed at a slow spreading center where magma chambers were episodic

paleomagnetic data from the redding section of the eastern klamath belt northern california a paleomagnetic analysis of units of devonian to triassic age within the redding section of the eastern klamath mountains indicates clockwise rotation of 83 116 with respect to stable north america in the kinematic model presented here shear zones within the franciscan complex with separation antithetic to the san andreas are used to effect this rotation

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

mineralization alteration and hydrothermal metamorphism of the ophiolite hosted turner albright sulfide deposit southwestern oregon the turner albright sulfide deposit part of the josephine ophiolite formed on and below the seafloor during late jurassic volcanism at a back arc spreading center ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition the uppermost massive sulfide formed on the seafloor at hydrothermal vents similar to sulfide depositing hot springs on modern spreading centers the bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization infiltration of hydrothermal fluid into the hyalo clastite altered the rock to quartz fe chlorite and quartz sericite fe chlorite intense alteration resulted in replacement of the protolith by quartz sulfide the fluid responsible for the hydrothermal alteration was evolved seawater with low ph and mg and high fe sulfide 534s values average 4 7 and indicate contribution of sulfur from both basalt and seawater sulfate the average value of sulfide 34s and the difference between sulfide and contemporaneous seawater sulfate 34s values are similar to ophiolite hosted sulfide deposits in cyprus chlorite rich hydrothermally altered basalt has 18o values of 3 0 6 8 due to high temperature 250 350 c reaction with a seawater derived ore fluid quartz in silicified basalt has higher 18o values of 12 3 15 5 probably reflecting lower temperature recrystallization of amorphous silica formed during initial alteration of basalt mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore transporting hydrothermal fluid but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust this subseafloor metamorphism produced a mineral assemblage typical of prehnite pumpellyite facies metamorphism exchange with altered seawater increased the whole rock 18o of the basalts to values of 9 4 11 2

significance of mid mesozoic peridotitic to dioritic intrusive complexes klamath mountains western sierra nevada california a mid mesozoic plutonic suite ranging from ultramafic gabbroic rocks to dioritic rocks and commonly intruded by younger granitoids is widespread in the klamath mountains western sierra nevada california the ultramafic gabbroic rocks are clinopyroxene rich and commonly vary from wehrlite to olivine hornblende clinopyroxenite and melagabbro associated dioritic rocks include biotite two pyroxene diorite monzodiorite and the granitic rocks typically range from tonalite to granodiorite these plutonic complexes are coextensive and broadly coeval with a suite of weakly metamorphosed volcanic rocks ranging in composition from basalt to basaltic andesite to andesite the volcanic rocks are chiefly volcaniclastic and are characterized by several distinct phenocryst assemblages clinopyroxene plagioclase olivine or hornblende the plutonic complexes and the associated volcanic rocks are restricted to terranes that lack continental sialic crust and that at least locally contain juvenile ophiolitic crust generated adjacent to the locus of magmatism thus their petrogenesis may be related to recurrent fracturing and magmatism within a rifted ensimatic arc

variations in the degree of crustal extension during formation of a back arc basin abstract ophiolite complexes in southern chile represent the remnants of the mafic portion of the floor of a cretaceous back arc basin which widened markedly from north to south over a length of 600 km detailed field and geochemical studies of ophiolites in the northern sarmiento complex and southern tortuga complex extremities of the originally wedge shaped back arc basin floor indicate significant north south differences in the mode of emplacement of basaltic magmas into the pre existing continental crust during the formation of the basin in the northern narrow extremity of the original basin mafic melts intruded into the continental crust over a diffuse zone causing extensive remobilization and reconstitution of the sialic continental crust in the southern wider part of the original basin mafic magmas appear to have been emplaced at a localized oceanic type spreading centre the observed north south variations resulted in formation of back arc floor with crustal characteristics ranging from intermediate between continental and oceanic to typically oceanic these variations are interpreted as representing different stages of evolution of a back arc basin which formed due to a subtle interplay between subduction induced back arc mantle convection and the release of stress across the convergent plate boundary possibly related to ridge subduction prior to the release of stress heat transferred from mantle diapirs to the base of crust caused widespread silicic volcanism in south america with the release of stress mantle derived melts erupted to the surface along structural pathways resulting in extensive basaltic volcanism in a linear belt behind the island arc and the cessation of silicic volcanism initially basaltic magmas intruded the continental crust over a diffuse region causing reconstitution of sialic crustal rocks progressive localization of the zone of intrusion of mafic magmas from the mantle eventually resulted in the development of an oceanic type spreading centre observations in southern chile and elsewhere suggest that variability in horizontal stress across a convergent plate boundary may be the overriding factor in determining the regional response of continental crust to subduction induced back arc convection and hence the mechanism of emplacement into the crust of mafic mantle melts the various lithologies observed in southern chile could also be expected to form during the opening phase of major ocean basins and to currently underlie atlantic type continental margins

time relations and structural stratigraphic patterns in ophiolite accretion west central klamath mountains california new geochronological data and published structural and stratigraphic data show that two distinctly different ophiolitic assemblages formed in general proximity to one another at nearly the same time and were subsequently imbricated along a regional thrust zone the josephine ophiolite constitutes a complete oceanic crust and upper mantle sequence which lies within the western jurassic belt of the klamath province within the study area the josephine ophiolite was formed by seafloor spreading at about 157 m y before present it was immediately covered by a thin pelagic and hemipelagic sequence which grades into a thick flysch sequence both of which comprise the galice formation the galice flysch was derived from volcanic arc and uplifted continental margin orogenic assemblages a major nonvolcanic source for the galice flysch appears to have been the western paleozoic and triassic belt of the klamath province exposed to the east proximal volcanic arc activity migrated to the site of the josephine galice section by 151 m y and is represented by numerous dikes and sills which intrude the ophiolite and galice formation the preston peak ophiolite is a polygenetic assemblage consisting of 1 a pre mid jurassic tectonitic peridotite amphibolite substrate which represents disrupted and unroofed basement of the western paleozoic and triassic belt and 2 an upper mafic complex which was intruded through and constructed above the tectonite substrate at about 160 m y the mafic complex consists primarily of diabase hypabyssal rocks that are overlain by diabase clast breccia and hemipelagic deposits a major arc plutonic complex was emplaced into the preston peak ophiolite in at least two pulses at 153 and 149 m y major phases of this complex consist of wehrlite gabbro pyroxene diorite and hornblende diorite the josephine ophiolite is interpreted as the remnants of interarc basin crust the preston peak ophiolite is interpreted as either a primitive remnant arc complex or a rift edge facies for the josephine interarc basin the galice formation represents a submarine fan complex that was built on juvenile crust of the josephine basin floor during the time interval of 153 to 149 m y the locus or arc magmatism migrated to an area which included the interarc basin floor and the remnant arc or basin edge the basin shortly thereafter closed by convergent tectonics during the nevadan orogeny resulting in the imbrication of the josephine and preston peak ophiolites and their superimposed arc assemblages the transition from seafloor spreading generation of josephine ophiolite to its tectonic accretion by convergence and basin closure occurred within 5 to 10 m y the process of rifting and ophiolite formation in series with convergence and ophiolite accretion is considered an important mechanism for generating and displacing allocthonous terranes in the klamath mountains sierra nevada region and perhaps throughout the western cordillera

tharsis tholus an unusual martian volcano tharsis tholus is unusual martian shield volcano in that the edifice is cut by a series of large normal faults that appear to penetrate the entire volcano northeast trending narrow graben also cut the flank the large normal faults may be caused by loading of a ductile subsurface layer allowing failure of the edifice the narrow graben are typical tensional faults the flank is heavily mantled by aeolian material despite the bulbous appearance the overall morphology of tharsis tholus suggests it is a basaltic shield crater counts indicate an age of early hesperian placing tharsis tholus in the middle of the period of activity that built the other small tharsis volcanoes

late proterozoic evolution of the northern part of the hamisana zone northeast sudan constraints on pan african accretionary tectonics new structural data from the red sea hills northeast sudan add constraints to accretionary tectonic processes that acted in the arabian nubian shield during the late proterozoic geometric constraints provided by field data are used together with landsat thematic mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages in contrast to interpretations reported elsewhere these studies indicate that the hamisana zone a prominent structural feature in the northern red sea hills post dated terrane accretion it is characterized by east west crustal shortening and by steep folds and thrust faults calc alkaline magmatism was contemporaneous with and outlasted main phase deformation thus deformation occurred in an intra arc setting thrust faults were reactivated after east west contraction compromising their usefulness in delineating 9sutures9 or as piercing points across older faults pan african accretionary processes may have been analogous to phanerozoic ophiolite and island arc accretion in the western north american cordillera where penetrative deformation occurred in response to periodic intra plate shortening events rather than an ultimate collision of unrelated crustal fragments such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long lived convergent margin setting consistent with the long standing observation that pan african magmatism became more evolved with time this view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening in contrast to models where crustal shortening results directly from collision of a volcanic arc microcontinent or other buoyant features

fe ti basalts and propagating rift tectonics in the josephine ophiolite the jurassic josephine ophiolite of northwest california and southwest oregon is one of the largest and most complete ophiolites in north america and clearly formed in a suprasubduction zone setting the geochemistry of lavas and dikes is highly variable because of a range in magma types and different degrees of fractionation basalts rich in fe and ti occur in the uppermost part of the extrusive sequence and as late dikes cutting oceanic serpentinite they have morb mid oceanic ridge basalt affinities and appear to be unrelated to the rest of the ophiolite that has affinities dominantly transitional between iat and morb iat island arc tholeiite this late fe ti morb suite was erupted while the ophiolite was undergoing the last half of 50 of tilting of the entire crustal sequence and after widespread serpentinization of ultramafic cumulates and uppermantle peridotite related to freezing of the axial magma chamber fe ti basalts are characteristic of propagating spreading centers on mid ocean ridges and in at least one backarc basin geochemical structural stratigraphic and regional geologic constraints suggest formation of the josephine ophiolite by propagation of a spreading center into rifted island arc lithosphere that is preserved along the margin of the ophiolite another possible indication of propagating rift tectonics is the presence of off axis metalliferous sedimentary rocks that may have been deposited as hydrothermal plume fall out from a second propagating spreading center the lau basin provides modern analogues including spreading centers propagating into rifted arc lithosphere into older backarc ocean crust and across the arc into the forearc

pre middle jurassic accretionary metamorphism in the southern klamath mountains of northern california usa high p t metamorphic parageneses are preserved within two late palaeozoic to early mesozoic assemblages of the southern klamath mountains that show contrasting structural styles and mineral parageneses reflecting formation in different parts of a subduction zone regime blueschist facies tectonites of the stuart fork terrane represent a coherent subduction complex formed at relatively deep crustal levels whereas the chaotic metasedimentary melange of the eastern hayfork terrane contains a diverse range of metamorphic parageneses reflecting complex structural mixing of metamorphic components at shallower levels the convergent margin type accretionary metamorphism evident in both terranes pre dates middle jurassic low p t metamorphism resulting from regional tectonic contraction and magmatism the epidote to lawsonite zone stuart fork blueschists and eclogites locally formed at pressures of about 6 11 kbar and temperatures of 250 400 c deformed matrix material of the eastern hayfork melange formed at similar temperatures but lower pressures on the order of 3 6 kbar the melange contains a diverse assemblage of tectonic blocks that formed under a range of p t conditions including those of the blueschist pumpellyite actinolite greenschist and upper greenschist to amphibolite facies the succession of mineral parageneses and inferred p t conditions of the eastern hayfork blocks reflect those of igneous protolith formation structural mixing subduction zone metamorphism olistolith transport and tectonic and erosional denudation although temporal relations are not well constrained the evolution of these terranes is consistent with formation within a single convergent margin system

magma emplacement during arc perpendicular shortening an example from the cascades crystalline core washington it is increasingly inferred by others that active arcs at all crustal levels are undergoing arc perpendicular extension and that magma utilizes this extension or extension along arc parallel strike slip faults during emplacement however a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc perpendicular shortening without the aid of strike slip faulting this conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens evaluation of host rock material transfer processes that operated during emplacement of the 93 ma 10 to 14 km deep mount stuart batholith washington provides one example and includes the following 1 ductile downward flow in a narrow 0 2 body radii structural aureole 2 stoping and of lesser importance 3 regional folding thrusting and subhorizontal arc parallel extension we see no evidence that arc perpendicular extension or local extension along strike slip faults made space for this pluton nor for most other plutons in contractional arcs instead we argue that both localized and rapid around plutons and regional and slower downward flow during growth of crustal roots is a common process in such arcs

tectonic significance of post accretion rifting of a mesozoic island arc terrane in the northern sierra nevada california this paper presents a new interpretation for the tectonic evolution of the smartville complex that forms the western belt of the foothills metamorphic belt in the northern sierra nevada california new geological petrological and geochronological information suggest that the smartville complex may form part of an early jurassic island arc terrane that was probably built on a pre existing oceanic basement inferred oceanic basement is represented by multiply deformed and metamorphosed ophiolitic rocks that crop out in the central belt north and east of the smartville complex that are intruded by numerous lower jurassic dikes of an incipient island arc character lower jurassic volcanic and plutonic rocks of the slate creek and possibly the lake combie complexes in the central belt are interpreted as fragments of the early jurassic arc terrane accretion of the ensimatic arc terrane into the continental margin of north america may have occurred along east vergent thrusting around middle jurassic time and

speculative reconstruction of the mojave snow lake fault implications for paleozoic and mesozoic orogenesis in the western united states available evidence suggests that the mojave snow lake fault a major early cretaceous dextral fault with up to 400 km displacement formerly existed in what is now the axial part of the sierra nevada batholith regional relations suggest that the fault formerly continued in western nevada the mojave desert and in rocks now exposed in the salinian block restoration of 400 km of early cretaceous dextral displacement on the mojave snow lake fault leads to speculative but testable reconstructions of mesozoic and paleozoic tectonic features of california and nevada insights derived from these reconstructions include the following 1 two regionally extensive jurassic thrust belts of opposed vergence appear to have intersected in the northern sierra nevada the eastern east vergent thrust belt which includes the fencemaker and liming thrusts in western nevada and several east vergent thrusts in the northern sierra may be a contractional belt along which an early jurassic interarc or marginal basin was closed in middle to late jurassic time the western west vergent thrust belt is part of the classic nevadan orogen of the western klamath mountains and the western sierra metamorphic belt in which collisions and or thrusting of various island arc terranes occurred during middle and late jurassic time 2 the early mesozoic continental arc included paired belts of triassic jurassic blueschist and melange and igneous rocks including late triassic and younger plutons and volcanic rocks this conclusion is not new but the reconstruction provides a slightly different geometry than previously thought 3 sonomia the late paleozoic island arc terrane of the western u s may have formed a relatively narrow belt extending from northwestern nevada to the latitude of the western mojave desert the geologic development of the paleozoic basement complex of sonomia was very different from that of the cordilleran miogeocline in death valley and the mojave desert requiring that a continuation of the sonoma suture belt or golconda thrust must have extended southward through the sierra nevada batholith to the mojave region 4 a heterogeneous belt of permian to triassic or triassic deformation in the mojave and eastern sierra nevada together with the golconda allochthon in nevada may represent the protracted response to the collision of sonomia with the western margin of north america the contractional deformation continued during the evolution of the early mesozoic continental arc 5 several predictions can be made about the nature of concealed basement within the sierra nevada batholith and within western nevada the reconstruction suggests that elements of the antler and sonoma orogenic belts should underlie areas of the batholith and western nevada as a result of northward displacement along the mojave snow lake fault from the western mojave and southern sierra nevada

north american apparent polar wander plate motion and left oblique convergence late jurassic early cretaceous orogenic consequences the north american apparent polar wander apw path indicates an episode of unusually rapid absolute northward motion of western north america between 150 and 135 ma during this time the northward component of absolute motion of points along the washington oregon california coast was in excess of 150 km m y and perhaps as high as 230 km m y we believe that such high absolute northward velocity for north america probably ensures that relative motions of oceanic plates and terranes influenced by them were to the south at this time the inception of rapid northward motion and left oblique convergence was abrupt and should be recorded in the geology of the western cordillera it is tempting to correlate this period of unusual pacific basin north american interaction with the nevadan orogeny in the klamath mountains as well as with left lateral strike slip structures such as the pine nut fault and bear mountains fault zone significant differences exist between north american plate motion recorded by the late jurassic cretaceous apw path and that predicted by a fixed hotspot model we believe that this discrepancy reflects uncertainty associated with pre late cretaceous hotspot tracks and poorly constrained relative plate motions during the cretaceous normal polarity superchron

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

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading