geology of the victorville region california late precambrian and paleozoic strata of the cordilleran continental margin miogeocline form a westward thickening wedge of sedimentary rocks in the southern great basin region stewart 1970 facies boundaries and isopachs of these rocks can be traced across this region into the eastern mojave desert southeastern california where these older trends are crosscut by the younger mesozoic age magmatic arc terrane fig 1 miogeoclinal rocks involved in the cordilleran fold and thrust belt in the eastern mojave desert become more highly deformed and metamorphosed southwestward as the magnatic arc is approached the geologic complexity of the central and western mojave desert however has posed a major stumbling block in attempts to reconstruct the paleogeography and tectonic history of the southern part of the cordilleran orogenic belt in the central and western mojave desert bed rock exposures consist chiefly to tertiary volcanic rocks and mesozoic granitic rocks because of the lack of exposure of older rocks in this terrrane the southwestward continuation of miogeoclinal facies and isopach trends and the timing and nature of structural events in this region are very poorly known in comparison to what is known about the eastern mojave desert region

a petrologic comparison of triassic plutonism in the san gabriel and mule mountains southern california triassic magmatism in the southwest u s cordillera forms a semicontinuous magmatic arc extending from northwestern nevada to southeastern california quartz monzodioritic and quartz monzonitic rocks and associated diorites and granites are widespread in southeastern california and we suggest that these rocks represent exposure of a structurally deeper part of the triassic arc where it was emplaced into comparatively thick proterozoic crust elemental and isotopic data suggest that triassic quartz monzodiorites and quartz monzonites in the mule and san gabriel mountains were derived from a relatively undepleted nonradiogenic mafic lithospheric source with virtually no upper crustal interaction very limited data for associated triassic diorites indicate a wide range in composition and a surprisingly radiogenic isotopic signature younger triassic granites record a strong geochemical signature of interaction with continental crust including inherited zircon and high initial sr ratios but comparatively less radiogenic pb isotopic compositions the major and trace element geochemistry of late triassic plutonic rocks in southeastern california is similar in many respects to alkalic components of the triassic arc in the mojave desert however contemporaneous rocks farther north have a calc alkalic signature perhaps reflecting the variation in age and composition of lithosphere across which the triassic arc was constructed

structural aspects of suspect terranes and accretionary tectonics in western north america abstract the regional structural relationships between the cordilleran suspect terranes and cratonic north america and within the suspect terranes themselves are reviewed for eastern alaska western canada and united states and northern mexico a distinctive characteristic of the relationships between the suspect terranes and the north american craton is that although much modified by post accretionary strike slip faulting thrust faulting and extensional detachment faults the generally more distal and or oceanic terranes are frequently found as rootless nappes or thin thrust sheets sitting upon more inboard terranes or upon the craton itself the amount of tectonic transport implied is large strike slip faulting has been a very significant aspect of the structural evolution of the north american cordillera with total distributed displacements of over 2000 km documented on geologic relationships alone and much more implied by paleomagnetic evidence at least since later mesozoic time to the present movements were such that the terranes moved northward post accretionary intraplate telescoping along thrust faults transpressional strike slip faulting and delamination at mid to upper crustal levels were important processes in cordilleran tectonic evolution

the laurel convict fault eastern sierra nevada california a permo triassic left lateral fault not a cretaceous intrabatholithic break the laurel convict fault is a prominent northwest striking highangle fault exposed in paleozoic rocks of the mount morrison pendant in the eastern sierra nevada of california the fault zone averages 25 50 m in width and consists of narrow anastomosing domains of ductile deformation separating lenses of paleozoic metasedimentary rock the fault which cuts structures in middle permian metasedimentary rocks is intruded by a relatively undeformed quartz porphyry felsite dike that has yielded a u pb zircon upper intercept age of 225 16 ma displacement on the laurel convict fault is therefore constrained to the interval between middle permian and late triassic displacement criteria including curvature and offset of bedding in the fault zone rare mesoscopic ductile shear indicators and restoration of offset lower paleozoic stratigraphy and structure across splay faults all indicate a moderate amount of apparent leftlateral strike slip displacement our data do not support previous interpretations of the laurel convict fault as a thrust faulted terrane boundary or a major right lateral shear zone the laurel convict fault is not therefore an exposed segment of intrabatholithic break 3 ibb3 proposed cretaceous right lateral displacement of the sr i 0 706 isopleth on ibb3 must be accommodated either east of the mount morrison pendant in the owens valley or west of the pendant in the eastern sierra nevada

the early permian thysanophyllum coral belt another clue to permian plate tectonic reconstructions early permian species of the related massive coral genera thysanophyllum stylastraea and protowentzelella are restricted geographically to a narrow belt stretching discontinuously from the southern ural mountains to texas by way of vestspitsbergen canadian arctic islands and the western cordillera british columbia and the basin and range of idaho utah nevada arizona and california in addition limited data suggest that this belt may have extended as far south as peru this coral belt developed at low and intermediate latitudes during early permian time along the northwestern and western margins of the pangaean supercontinent land a very wide paleopacific ocean and temperature barriers prevented these tropical to subtropical shallow water corals from invading similar warm water environments in the tethys ocean on the opposite coast of pangaea where faunas including different types of massive corals developed today the thysanophyllum association is represented in several geosynclinal belts which in the past have been difficult to relate to one another this coral belt therefore may constitute an important key for understanding the relationships between several of these great belts of sediment accumulation and in locating the position of permian continental margins and interpreting some subsequent plate margin tectonic events

displacements on late cenozoic strike slip faults of the central mojave desert california field studies demonstrate that displacements on northwest striking wrench faults of the central mojave desert are too small to support hypotheses suggesting large interior translations and associated rotation of the province during late cenozoic time the margin of an early miocene structural belt provides the marker with which to establish lateral displacement on individual faults displacement values for faults are as follows lenwood fault 1 5 3 km camp rock fault 1 6 4 0 km calico fault 8 2 km and rodman pisgah faults 6 4 14 4 km cumulative displacement on all the major northwest striking faults of the mojave desert is about 26 7 38 4 km most if not all regionally distributed right shear presumably related to pacific north american plate interaction developed in the central mojave desert after 20 m y b p right shear was preceded by significant amounts of kinematically unrelated northeast southwest crustal extension

evolution of the ordovician through middle pennsylvanian carbonate shelf in east central california carbonate shelf rocks marked by various sedimentary features suggesting intertidal to supratidal deposition or bearing rich benthic faunas and their corresponding slope and basin rock sequences containing planktic fossils and sediment gravity flow deposits have been identified in the inyo mountains argus range region of east central california for all of the paleozoic from ordovician through middle pennsylvanian four major depositional sequences each beginning with a marine transgression and all but the third ending with a progradation were developed representing 1 early ordovician into early silurian time 2 early silurian into earliest middle devonian time 3 middle to late devonian time and 4 early to middle late mississippian time a period of basinal infilling in the late mississippian was followed by a fifth major marine transgression in the early pennsylvanian the carbonate shelf itself evolved from a homoclinal ramp in the ordovician to an accretionary rimmed margin by the early devonian this margin was drowned probably in the middle devonian by the early mississippian a new homoclinal ramp had been developed before the end of the mississippian this ramp became distally steepened and then was transformed into an accretionary rimmed margin the early to middle pennsylvanian marine transgression gave rise to still another homoclinal carbonate ramp subsidence of 2 300 m occurred between early ordovician and middle pennsylvanian time in the area immediately east of the inyo mountains it increased westward to probably 3 500 m at the edge of the owens valley as the continental margin subsided and bent downward 1 between early ordovician and middle late mississippian time as a result of subsidence and relatively low rates of sedimentation the water depth in the western part of the study area increased from close to sea level at the beginning of the ordovician to as much as 3 000 m in the late mississippian because of higher rates of sediment accumulation and lower rates of subsidence in the eastern part of the region shallow water environments were maintained throughout most of this time interval even so the shelf slope break retreated at least 45 km southeastward from middle ordovician to middle late mississippian time and at least 35 km farther southeastward by middle pennsylvanian time

continuation of antler and sonoma orogenic belts to the eastern sierra nevada california and late triassic thrusting in a compressional arc strong stratigraphic and structural similarities exist between metamorphic rocks of the saddlebag lake pendant eastern sierra nevada california and rocks of the antler and sonoma orogenic belts in west central nevada these similarities indicate that the roberts mountains and golconda allochthons can be traced to the eastern edge of the sierra nevada batholith in california newly recognized thrust faults that imbricate the antler and sonoma rocks and younger metamorphic rocks are of late triassic age and are cut by a 219 ma pluton these thrusts apparently were active during a major episode of late triassic arc volcanism and they constrain the age of the golconda thrust as pre latest triassic the continuation of the antler and sonoma orogenic belts to the saddlebag lake pendant means that large strike slip displacements cannot have occurred on northwest trending faults along the eastern edge of the sierra nevada however a large structural break may terminate the antler and sonoma belts at or near the western edge of the saddlebag lake pendant

structure and regional significance of the late permian sierra nevada death valley thrust system east central california abstract an imbricate system of north trending east directed thrust faults of late early permian to middle early triassic most likely late permian age forms a belt in east central california extending from the mount morrison roof pendant in the eastern sierra nevada to death valley six major thrust faults typically with a spacing of 15 20 km original dips probably of 25 35 and stratigraphic throws of 2 5 km compose this structural belt which we call the sierra nevada death valley thrust system these thrusts presumably merge into a decollement at depth perhaps at the contact with crystalline basement the position of which is unknown we interpret the deformation that produced these thrusts to have been related to the initiation of convergent plate motion along a southeast trending continental margin segment probably formed by pennsylvanian transform truncation this deformation apparently represents a period of tectonic transition to full scale convergence and arc magmatism along the continental margin beginning in the late triassic in central california

upper ordovician planktic and benthic graptolites and a possible hydroid from the tafilalt biota southeastern morocco

correlation of metamorphosed paleozoic strata of the southeastern mojave desert region california and arizona isolated outcrops of deformed regionally metamorphosed paleozoic strata are scattered within the southeastern mojave desert region of california and western arizona these strata unconformably overlie a basement of proterozoic crystalline rocks and are overlain in turn by metamorphosed mesozoic sedimentary rocks the strata can be correlated lithostratigraphically with the classic cratonal paleozoic section of the western grand canyon arizona and with nonmetamorphosed paleozoic sections transitional between cratonal and miogeoclinal in the ship marble and providence mountains california the strata evidently were once continuous with paleozoic epicontinental strata exposed throughout the southern great basin and colorado plateau outcrops of paleozoic strata and of the underlying proterozoic basement in the southeastern mojave desert region define a terrane that has been disrupted by mesozoic thrust faults and by tertiary detachment faults but that nevertheless retain a gross paleogeographic coherence this coherent terrane extends at least as far west and southwest as the big maria palen and calumet mountains and possibly beyond to include paleozoic exposures in the san bernardino mountains and near victorville poorly understood tectonic boundaries separate the area of paleogeographic coherence from known or suspected allochthonous terranes in the western mojave desert and the eastern transverse ranges

metamorphic evolution of exhumed middle to lower crustal rocks in the mojave extensional belt southern california usa the waterman metamorphic complex of the central mojave desert was exposed as a consequence of early miocene detachment dominated extension however it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment s tectonic thickening by overthrusting and two periods of extension the metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble calc silicate quartzite mafic granulite pyribolite amphibolite migmatite and biotite schist the latter group of rocks was affected by an initial series of high grade metamorphic events m1 and m2 and a localized lower grade overprint m3 the initial metamorphism m1 can be separated into two stages along its high grade p t path m1a a granulite facies metamorphism at 800 850 c and 7 5 9 kbar and mlb an upper amphibolite facies overprint at 750 800 c and 10 12 kbar m1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of h2o and is associated with intense ductile deformation d1 and minor local partial melting m1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and h2o activity and is accompanied by little or no deformation m2 developed at lower pressures and temperatures 650 750 c 4 5 5 5 kbar and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins effects of m3 are confined to the mitchel detachment zone an anastomosing early miocene detachment fault and are characterized by local ductile brittle deformation d2 of the pre existing high grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions 300 350 c 3 5 kbar the initial overprint m1a represents metamorphism devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and possibly magmatic thickening the increasing pressure transition of m1a to m1b reflects a period of continued compressional tectonism thrusting and influx of h2o in part locally related to crystallization of partial melts the near isothermal decompression between m1b and m2 probably represents a pre 112 ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust following the initial extensional event the metamorphic complex remained at depths of 10 17 km for at least 90 ma until it was uplifted following miocene extension m3 develops locally in response to this second extensional period resulting from the early miocene detachment faulting

the end ordovician mass extinction a single pulse event abstract the end ordovician mass extinction eome is widely interpreted as consisting of two pulses associated with the onset and demise of the gondwana glaciation respectively with the second pulse eradicating the distinctive glacially related hirnantian benthic biota hbb a global review of occurrence data of latest ordovician benthic marine organisms reveals that virtually all warm water benthic assemblages previously assigned to the hbb comprise two distinct and clearly postglacial faunas both younger middle and late hirnantian respectively than the cool water hirnantia fauna latest katian to early hirnantian the newly recognised three transitional benthic faunas i e tbfs 1 3 can be closely tied to graptolite conodont and chitinozoan biozonations the hirnantian isotope carbon excursion hice and the glaciation thereby providing an integrated much higher resolution timescale for understanding the tempo and nature of the eome at this finer resolution we postulate a more profound impact of the first pulse of the eome than hitherto envisaged as evidenced by opportunistic expansion of the hirnantia fauna globally and the complete absence of metazoan reefs in its immediate aftermath we also argue based on high quality data from well documented benthic groups in south china i e brachiopods tabulate and rugose corals trilobites and sponges that the magnitude of the second pulse of the eome caused by the deglaciation has been overestimated because the two postglacial faunas i e tbfs 2 3 were part of a subsequent recovery phase of marine ecosystems rather than contributing to biodiversity decline thus it is more plausible to reinterpret the eome as a single pulse rapid event that was followed by a prolonged initial recovery intermittently impeded by climatic shocks through the hirnantian prior to the onset of a progressive reestablishment of marine ecosystems during the early silurian rhuddanian and aeronian associated with an overall amelioration of climatic conditions

interpretation of the last chance thrust death valley region california as an early permian decollement in a previously undeformed shale basin abstract the last chance thrust discontinuously exposed over an area of at least 2500 km2 near the south end of the cordilleran foreland thrust belt in the death valley region of east central california is controversial because of its poorly constrained age and its uncertain original geometry and extent we interpret this thrust to be early permian in age to extend throughout a sedimentary basin in which deep water mississippian shale overlain by pennsylvanian and earliest permian limestone turbidites accumulated to represent about 30 km of eastward displacement and to be related to convergence on a northeast trending segment of the early permian continental margin last chance deformation occurred between the times of the antler and sonoma orogenies of late devonian early mississippian and late permian ages respectively and followed early to middle pennsylvanian truncation of the continental margin by transform faulting in the western part of the mississippian shale basin in east central california the originally recognized exposures of the last chance thrust show neoproterozoic and early paleozoic strata above lower plate mississippian shale farther east faults subparallel to bedding above below and within the mississippian shale are interpreted to mark the thrust zone and to represent a regional decollement at the eastern margin of the basin upper plate thrust slices of deep water late paleozoic strata are interpreted to have piled up against the margin of the mississippian carbonate shelf to form a large antiformal stack above the lee flat thrust which we regard as the easternmost exposure of the last chance thrust thrust loading depressed the western part of the shelf creating a new sedimentary basin in which about 3 5 km of younger early permian deep water strata were deposited against the antiformal stack later probably in the late permian other thrusts including the inyo crest thrust which was subsequently overlapped by early to middle triassic marine strata cut across the last chance thrust we interpret the last chance thrust as similar in many ways to appalachian type decollements in which the zone of thrusting is localized along a shale interval the last chance thrust however has been dismembered during later geologic events so that its original geometry has been obscured our model may have unrecognized analogs in other structurally complex shale basins in which the initial deformation was along a major shale unit

a speculative reconstruction of the middle paleozoic continental margin of southwestern north america geologic relations and data from strontium and oxygen isotopic studies are used together to speculate on the paleogeographic positions of various crustal blocks or terranes in southwestern north america with respect to the middle paleozoic continental margin the southwestern margin of the middle paleozoic continent based on these data is quite irregular in its present configuration but most of the irregularities can be accounted for by strike slip displacement of continental fragments during several post middle paleozoic tectonic events we conclude that large scale late paleozoic sinistral displacements and mesozoic and cenozoic dextral displacements have been the major events affecting the configuration of this margin our preferred model includes the following elements 1 500 km of left lateral displacement of a large part of the margin during a late paleozoic truncation 2 slivering of the margin by right lateral faulting during the mesozoic amounting to about 275 km of total displacement and 3 further deformation of the margin during the cenozoic primarily by right lateral faulting oroclinal bending and crustal extension

permian and triassic rocks of the mojave desert and their implications for timing and mechanisms of continental truncation upper paleozoic and lower mesozoic rocks in the mojave desert and adjacent areas record the change from a passive continental margin to a subduction margin in the southwestern united states this change was apparently accompanied by truncation and reorientation of the continental margin in the mojave desert and western basin and range province from late proterozoic to mississippian time the western margin of north america had a northeast southwest trend through nevada and eastern california truncation of the margin began in pennsylvanian time when a northwest trending left lateral strike slip fault zone formed across the antler orogenic belt the cordilleran miogeocline and the more cratonal character mojave desert rocks of the antler belt were apparently displaced southward from east central california to their present position in the northwestern mojave desert sedimentary rocks deposited during this interval record an active continental borderland characterized by alternately subsided and uplifted marine basins this transcurrent faulting continued to late permian time magmatic activity and deformation started in the mojave desert in late permian time and affected both miogeoclinal cratonal and displaced antler rocks magmatic activity ceased and marine sedimentation resumed in early triassic time lower triassic rocks constitute an overlap sequence on both deformed and undeformed miogeoclinal cratonal rocks and possibly on displaced antler rocks facies trends in these rocks are northwest southeast indicating the new margin trend truncation and reorientation of the margin were completed therefore by early triassic time or possibly as early as late permian time upper permian magmatism and deformation in the mojave desert are older than and distinct from the early triassic sonoma orogeny of nevada lower triassic rocks in the mojave desert are post tectonic with respect to deformation in that region whereas correlative lower triassic rocks in nevada are pretectonic to syntectonic with respect to the sonoma orogeny this difference reflects the variation in deformation and tectonic setting along a newly developed cordilleran margin the southern part of the cordillera was apparently a subduction margin as early as late permian or middle triassic time while the central nevada segment was the site of collision tectonics

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

correlation of permian and triassic deformations in the western great basin and eastern sierra nevada evidence from the northern inyo mountains near tinemaha reservoir east central california geologic relationships exposed near tinemaha reservoir southeast of big pine east central california provide key chronological and structural constraints for linking permian and triassic deformational events recognized in the white and inyo mountains in the western great basin with those in the eastern sierra nevada particularly the mount morrison and saddlebag lake pendants permian to earliest triassic deformation in the tinemaha reservoir area produced a large north to northwest trending east vergent originally recumbent syncline mule spring syncline cut by an overriding thrust strange hill thrust we correlate this deformation with a middle permian to earliest triassic contractional deformation recognized in the southern inyo mountains and with a major episode of folding and thrust faulting in the eastern sierra nevada after a period of tectonic quiescence and marine sedimentation in the early triassic rocks in the tinemaha reservoir area were refolded twice producing distinctive sets of steeply plunging folds similar structures in the mount morrison pendant that formed prior to intrusion of a 225 16 ma dike along the laurel convict fault are correlated with those in the tinemaha reservoir area the timing of these folding events may be similar to that of displacement on the golconda and or lundy canyon thrusts in the saddlebag lake pendant close structural and stratigraphic ties suggest that rocks in the northeastern part of the mount morrison pendant and tinemaha reservoir area now separated by 65 km of dextral displacement along the cryptic tinemaha fault originally lay adjacent to one another this offset postdates triassic folding and is inferred to predate emplacement of the latest triassic wheeler crest granodiorite which crops out across the projected fault trace

post paleozoic alluvial gravel transport as evidence of continental tilting in the u s cordillera the western united states contains three thin but remarkably widespread alluvial conglomeratic units that record episodes of large scale tilting across the u s cordilleran orogen in post paleozoic time these units are 1 the shinarump conglomerate of late triassic age exposed in northern arizona and adjacent parts of utah nevada and new mexico 2 lower cretaceous gravel deposits that overlie the morrison formation throughout the rocky mountain region and 3 the gravel rich parts of the miocene pliocene age ogallala group in western nebraska and adjacent southeastern wyoming paleoslopes of the rivers depositing these units were in the range of 10 24 to 10 23 based on paleohydraulic calculations however depositional thickness trends of these units are not sufficient to have generated such steep paleoslopes thus long wavelength tilting of the earth s surface must have occurred for these gravels to be transported although these units were deposited adjacent to large tectonic features including an evolving and migrating continental arc and for the ogallala group the northward propagating rio grande rift the tilting occurred over wavelengths too broad to be directly generated by these features these widespread gravel units attest to the interplay between the creation of subduction related isostatic and dynamic topography and continental sedimentation hence paleotopography as determined from calculated transport gradients of sedimentary deposits provides a

the rb sr isochron age of the kennack gneiss and its bearing on the age of the lizard complex cornwall an acid vein from a borehole in the kennack gneiss has given a whole rock rb sr isochron age of 369 12 ma this is interpreted as a metamorphic age recording the time of emplacement of the lizard complex this emplacement in late devonian times was part of an early phase of the hercynian orogeny which also affected the gramscatho 9series9 of s cornwall