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

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

structure and evolution of the east sierran thrust system east central california 1 a belt of arc parallel northeast vergent contractional deformation the east sierran thrust system ests crops out for 150 km along the east side of the sierran continental margin arc the ests is nowhere wider than 20 km and it accommodated an estimated minimum of 9 3 km of horizontal shortening remarkably it experienced repeated episodes of broadly coaxial and coaxial planar contractional deformation beginning prior to 188 ma and continuing past 140 ma we postulate that the ests resulted primarily from episodic underthrusting of the back arc lithosphere beneath the east edge of the sierran arc facilitated by a buttressing effect of the arc as a result of this process rocks along the east flank of the batholith including the ests were episodically shortened against the arc buttress the ests experienced significant deformation during the nevadan orogeny indicating that contractional to transpressive deformation affiliated with this event affected the eastern wall rocks of the arc as well as its western wall rocks index terms 8102 tectonophysics continental contractional orogenic belts 9609 information related to geologic time mesozoic 8015 structural geology local crustal structure 9350 information related to geographic region north america keywords eastern california mesozoic structure cordilleran thrust system citation dunne g c and j d walker 2004 structure and evolution of the east sierran thrust system east central california tectonics 23 tc4012 doi 10 1029 2002tc001478

large magnitude miocene extension in the central mojave desert implications for paleozoic to tertiary paleogeography and tectonics this is the published version copyright 1990 american geophysical union all rights reserved

structural geology and 40ar 39ar geochronology of the goldstone lane mountain area mojave desert california metasedimentary rocks near goldstone california consist in ascending structural order of 1 a calc silicate hornfels unit 2 a quartzite schist and radiolarian chert unit and 3 a calcareous and pelitic sequence of rocks that contains minor basaltic or andesitic composition dikes sills and flows unit 1 is of unknown but probably pre mesozoic age unit 2 is lower paleozoic in part ordovician and is correlative to lower paleozoic eugeoclinal rocks in the el paso mountains north of the garlock fault unit 3 is probably correlative to upper paleozoic permian basinal sequences in the el paso mountains these rocks were strongly deformed during a progressive deformational event that caused isoclinal to tight folding about northwest southeast trending fold axes and northeast dipping axial planes tectonic transport during this deformation was in a southwesterly direction similarity of structural trends to those in the el paso mountains suggests that the deformation in these two areas is the same age and predates intrusion of permo triassic plutons in the el paso mountains regional relationships suggest that deep water metasedimentary rocks in the northwestern mojave desert and in the el paso mountains are allochthonous with respect to miogeoclinal and cratonal paleozoic rocks elsewhere in the mojave the emplacement of the allochthonous rocks postdates deposition of permian strata and predates permo triassic to earliest triassic deformation and plutonism this deformation involved both eugeoclinal and miogeoclinal sequences and took place within an andean arc setting the subsequent mesozoic history of these two parts of the mojave desert has been the same 40 ar 39 ar data on hornblende biotite and muscovite in the goldstone and lane mountain quadrangles indicate that a plutonic complex consisting of gabbro quartz diorite and tonalite was intruded during the late jurassic probably slightly earlier than 148 m y ago voluminous granitic rocks were intruded during the late cretaceous about 85 90 m y ago these younger plutonic rocks caused variable resetting of all k ar systems in older rocks cooling of late cretaceous plutons to about 280 40 c occurred by about 78 4 m y regional uplift of these rocks to a crustal level where no further diffusive loss of argon took place occurred at about 45 m y ago

paleogeographic and tectonic implications of the geology of the tiefort mountains northern mojave desert california geologic mapping petrologic and structural analyses and u pb dating in the tiefort mountains northern mojave desert california provide data on paleozoic paleogeography and mesozoic intra arc tectonics metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late precambrian cordilleran passive margin facies recognition of precambrian augen gneiss and metasedimentary rocks with craton margin protoliths suggests that the craton miogeocline boundary is in or near the tiefort mountains the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area the metasedimentary units are intruded by middle jurassic granite and diorite orthogneisses dated by u pb as ca 164 and ca 160 ma deformed rocks record amphibolite facies metamorphism and mylonitization during middle to late jurassic southeast vergent ductile shear pervasively annealed microstructures indicate that recrystallization outlasted deformation at south tiefort mountain the mylonitic fabric is deformed by northwest trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 14 ma at tiefort mountain synkinematic biotite granite gneiss is dated as ca 105 ma and mylonitic fabrics are folded about north trending axes the presence of the cretaceous orthogneiss suggests that two deformational events jurassic and cretaceous in age may have affected part of the study area dikes dated as ca 82 ma that crosscut all fabrics are interpreted to be part of a regional intrusive event the timing and vergence of shear zones at south tiefort mountain are similar to contractional shear zones in adjacent areas forming a belt of contractional deformation from the central mojave desert to the east sierran thrust system this deformation may be related to a period of late middle to late jurassic sinistral oblique subduction

structure and kinematics along a segment of the mojave sonora megashear a strike slip fault that truncates the jurassic continental magmatic arc of southwestern north america 1 similarities between crystalline crust and overlying pre jurassic cover in northwestern sonora mexico and in the inyo mountains death valley region california are attributed to left lateral displacement along the mojave sonora megashear a late jurassic transform fault related to the opening of the gulf of mexico three exposures of mylonitic rocks extending 25 km along the postulated fault occur within sierra de los tanques sonora mexico the exposures 13 km southwest of sonoita reveal a tripartite assemblage of 1 jurassic volcanic and volcaniclastic strata to the northeast 2 an axial zone of mylonitic rocks and 3 masses of granitic rocks and compositionally banded quartzo feldspathic gneiss of proterozoic and late triassic age to the southwest mylonitic foliation in rocks within the axial zone strikes northwesterly and lineations plunge shallowly microstructues record left lateral shear shallow thrusts folded mylonite and steeply dipping mylonitic foliation with downdip mineral lineation record transpression triassic and precambrian crystalline rocks are emplaced above shallow thrusts onto ductilly deformed jurassic rocks northeast of the principal fault in the central and northwestern exposures pervasively foliated and lineated rocks are cut by weakly foliated cretaceous granite that also may contain inclusions of mylonite the northwesterly strike of steep foliation left lateral kinematics and relationships with cretaceous intrusive rocks preclude correlation with episodes of cretaceous contraction and tertiary extension that are recognized in the southwestern united states the fault is interpreted to have formed in the late jurassic and records a change in tectonic setting from arc magmatism to rifting and basin development

characteristics of basement rocks exposed near the cajon pass scientific drill hole basement rocks exposed near the drill site to the northeast of the san andreas fault consist of mesozoic granitoid intrusions and amphibolite facies migmatitic gneiss a possible tongue or lobe shaped pluton of granodiorite is exposed on cleghorn ridge squaw peak and whale mountain adjacent gneiss contains marble calcsilicate rocks quartzite and aluminous schist of sedimentary origin metagabbro and dioritic gneiss of possible igneous origin are also present peak metamorphic temperatures during emplacement of granodiorite are estimated to have been between 600 to 750 c at a pressure between 3 6 kb

large magnitude permian shortening and continental margin tectonics in the southern cordillera the last chance allochthon with displacement estimated to be 55 to 105 km and lateral extent possibly exceeding 300 km was inferred to be middle triassic and thus kinematically distinct from a belt of permian triassic deformation in the western mojave desert correlations of structures forming the death valley thrust belt which includes the last chance allochthon several subjacent structures and their inferred lateral equivalents suggest that prior to tertiary extension individual structures were laterally continuous for distances exceeding 150 km along strike from the nevada test site area southwest to the darwin plateau their inferred mesozoic age and the presence of an early triassic overlap sequence on the darwin plateau precluded projection of these structures farther south in spite of long recognized strain compatibility problems associated with large displacement on thrusts that apparently terminated there without suitable accommodation structures such as tear faults or lateral ramps these problems may be resolved if the last chance allochthon is permian consistent with structural correlations new isotopic data from post tectonic stocks and stratigraphic evidence a large permian turbidite basin near the darwin plateau can be interpreted as a marine foredeep resulting from emplacement of the 5 km thick allochthon rather than a local trough caused by transcurrent faulting during inferred permian triassic continental truncation i propose that anomalous eugeoclinal rocks in the western mojave desert have been emplaced against cratonic strata there by the last chance allochthon which foreshortened an originally narrow continental margin

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

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

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

north south contraction of the mojave block and strike slip tectonics in southern california the mojave block of southern california has undergone significant late cenozoic north south contraction this previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate tectonic estimates of pacific north american plate motion and for part of the big bend in the san andreas fault in the eastern mojave block contraction is superimposed on early miocene crustal extension in the western mojave block contractional folds and reverse faults have been mistaken for extensional structures the three dimensional complexity of the contractional structures may mean that rigid block tectonic models of the region based primarily on paleomagnetic data are unreliable

cretaceous dikes within the jurassic independence dike swarm in eastern california field relationships and new u pb geochronology reveal that a significant number of dikes associated with the independence dike swarm both within the type locality and elsewhere are of cretaceous age a new u pb zircon age for a diorite dike in the type locality of the independence dike swarm in california confirms that some mafic dikes in the swarm are coeval with previously dated late jurassic felsic dikes cretaceous dikes in the eastern sierra nevada batholith are located near and may be genetically related to coeval mafic and felsic plutons however the presence of cretaceous dikes throughout the swarm demonstrates that this is not a local phenomenon and caution therefore should be exercised when using independence like dikes as temporal markers we have not identified any consistent petrographic or chemical criteria that distinguish jurassic independence dikes from cretaceous dikes however jurassic dikes in the sierra nevada commonly contain a sinistral shear fabric that is absent in both dated cretaceous dikes and undated dikes that intrude dated cretaceous plutons despite intruding at least 60 m y apart the orientations and petrology of the jurassic and cretaceous dikes are remarkably similar because dikes were intruded over a time period characterized by large changes in the orientation of plate convergence west of the sierran arc it is unlikely that the uniform structural orientation of jurassic and cretaceous dikes resulted from regional stresses imposed by the subducting plate instead the orientation of jurassic and cretaceous dikes and of many sierran cretaceous plutons may have been controlled by preexisting faults or by the orientation of the continental margin

cenozoic evolution of the mojave block of southern california the recorded cenozoic history of the mojave desert region of southern california began in the latest oligocene when intense volcanism and tectonism interrupted a long early tertiary silence volcanism commenced across the region in an east west band ca 24 22 ma northwest of barstow volcanism was accompanied by intense crustal extension and development of a metamorphic core complex outside of this relatively restricted area extension was minor or absent after extension ceased ca 18 ma volcanism shifted to small volume eruptions of basalt post extensional deformation has largely been by strike slip faulting along northwest striking dextral faults and west striking sinistral faults and total dextral slip across the mojave desert region since the early miocene is 45 60 km strike slip deformation has been overprinted locally by intense north south contraction that is the dominant style of deformation in the western mojave block paleomagnetic data indicate that parts of the mojave block were rotated clockwise although the magnitude and timing of this rotation are poorly determined the best evidence for large 45 rotation comes from the area east of barstow where large clockwise declination anomalies and mesozoic and cenozoic dikes with anomalous strikes may reflect early miocene clockwise deflection along the mojave river fault volcanism and tectonism in the mojave block resulted from interactions among the north american pacific and various oceanic plates patterns of volcanism and tectonism do not correlate with growth of slab windows beneath the continent but do correlate with the position of the subducted mendocino fracture zone plate circuit reconstructions suggest that the driving force for extension was divergence between the pacific and north american plates along the transform margin that separated the two this hypothesis accounts for the direction magnitude and rate of extension in the mojave block

petrological and geochemical investigations at the cajon pass deep drillhole chemical analyses of cores from the cajon pass deep drillhole confirm vertical lithologic diversity of the basement rocks they are metaluminous and calcalkaline weak major element correlations with wide ranging sio 2 suggest they are not a simple cogenetic suite quartz contents correlate with the thermal gradient profile supporting conduction as the principal thermal transport mechanism abundances of heat producing elements are variable and somewhat lower than average crustal values cross calibration with other measures of th u and k are underway ages of surface granitoids and core 17 are late cretaceous other plutonic rocks appear older but late mesozoic initial lead isotopic signatures of the plutonic rocks change dramatically with depth in the hole

the geochemical evolution of the catalan potash subbasin south pyrenean foreland basin spain over the last decade our studies in ancient evaporitic basins have been based on a detailed study of a single borehole record the detailed findings in medium to large sized evaporitic basins were shadowed with a relevant question can interpretations from a representative evaporitic record in a single borehole be extended to the whole evaporitic basin this paper addresses that question the results obtained are compared with results from another distant point within the basin the general methodology not only proves its reliability in interpreting the evolution of evaporitic basins from a single borehole but reveals its capability to obtain detailed palaeoenvironmental interpretations the chemical evolution of an upper eocene evaporitic sequence from the south pyrenean foreland basin spain has been investigated along the suria 19 borehole record detailed petrographic and mineralogical study x ray microanalysis of frozen primary inclusions trapped in halite cryo sem eds systematic isotopic analysis d 34 s and d 18 o in sulphates and computer based evaporation models have been integrated in a multi proxy methodology this study revealed that a variable amount of ca excess is required throughout different parts of the marine lower halite unit lhu for sylvite instead of k mg sulphates to form this ca excess is in turn different from that required for the western sector of the same evaporitic basin navarrese subbasin quick and variable changes in ca rich brines or equivalent dolomitization required are explained as internal processes within the basin rather than secular variations in seawater chemistry the general hydrological evolution of the catalan subbasin is explained as a restricted subbasin with a first marine stage in which continental input up to 50 of total input had an important control on the geochemistry of the subbasin a second stage was determined during potash precipitation in which the subbasin was cut from any seawater input to end up in its last stage as a purely continental evaporitic basin coupling evaporation models and analytical results we have obtained the proportions of recycling and their sources estimated to change from a 100 total mass of sulphate eocene source to 20 eocene and 80 triassic keuper towards the latest stage of potash precipitation the results obtained have been compared with results from the navarrese subbasin allowing an integrated interpretation of the hydrological evolution of the whole upper eocene south

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

late cretaceous ductile deformation metamorphism and plutonism in the piute mountains eastern mojave desert late cretaceous deformation in the piute mountains involved ductile thrusting on a network of anastomosing northeast and southwest directed shear zones the shear zones often separate proterozoic lithologies with strong competency contrasts suggesting that proterozoic crustal anisotropies were important in controlling the geometry of the shear zone network shear zones now divide the crust into wedge and lozenge shaped blocks the piute mountains can be separated into two kinematic domains each characterized by a dominant sense of shear thrusting in the southwest directed kinematic domain ended at 85 7 ma the age of the late synkinematic east piute pluton thrusting in the northeast directed kinematic domain could have lasted until 74 3 ma the age of the lazy daisy pluton postthrusting northwest southeast shortening produced a second generation of upnght open folds with a northeast striking subvertical crenulation cleavage f2 strain increases toward the south and is concentrated around the margins of the lazy daisy pluton this deformation was synchronous with pluton emplacement late cretaceous peak metamorphism outlasted all ductile deformation peak metamorphic grade increases from upper greenschist facies in the north to upper amphibolite facies as the lazy daisy pluton is approached temperature and pressure of approximately 450 c and 2 5 4 0 kbar were achieved during ductile thrusting temperatures of 500 540 c existed at the onset of upright open folding and peak temperatures as high as 620 c near the lazy daisy pluton occurred after f2 upright folding all penetrative deformation ended before 72 71 ma by which time the area had cooled to below 300 c foster et al 1989

progressive deformation and superposed fabrics related to cretaceous crustal underthrusting in western arizona u s a abstract in the maria fold and thrust belt a newly recognized e trending cretaceous orogenic belt in the southwestern united states ductile thrusts large folds and superposed cleavages record discordant emplacement of crystalline thrust sheets across previously tilted sections of crust style of deformation and direction of thrusting are in sharp contrast to those of the foreland fold thrust belt in adjacent segments of the cordillera the net effect of polyphase deformation in the maria belt was underthrusting of paleozoic and mesozoic metasedimentary rocks under the proterozoic crystalline basement of north america the structure of the maria belt is illustrated by the granite wash mountains in west central arizona where at least four non coaxial deformation events d 1 d 4 occurred during the cretaceous sse facing d 1 folds are associated with s directed thrusts and a low grade slaty cleavage d 1 structures are truncated by the gently dipping hercules thrust zone d 2 a regional sw vergent shear zone that placed proterozoic and jurassic crystalline rocks over upturned paleozoic and mesozoic supracrustal rocks exposures across the footwall margin of the hercules thrust zone show the progressive development of folds cleavage and metamorphism related to thrusting d 3 and d 4 structures include open folds and spaced cleavages that refold or transect d 1 and d 2 folds the d 2 hercules thrust zone and a d 3 shear zone are discordantly crosscut by late cretaceous plutons

path and amount of dextral fault slip in the eastern california shear zone across the central mojave desert new total fault slip estimates for the central mojave desert portion of the eastern california shear zone support through going dextral shear of 18 km that is transferred northward to the garlock fault the eastern california shear zone accommodates 25 of the plate boundary shear with the transversely oriented garlock fault forming its northern boundary with the walker lane belt total fault slip estimates for the major faults in the central mojave desert were determined using detailed geologic mapping supplemented with aeromagnetic data offset data for the blackwater fault show a consistent 1 8 0 1 km of dextral slip along 55 km of strike length and they indicate that slip initiated at or after 3 8 ma the faults adjacent to the blackwater fault have dextral slip of 4 8 0 3 km harper lake fault 2 9 0 5 km mount general fault 1 0 0 7 km lockhart fault and 3 2 0 3 and 0 53 0 05 km paradise fault these slip data augment published data for the central mojave desert and allow evaluation of the strain path in the central mojave desert showing consistent along strike through going dextral slip contractional and extensional step overs in the central mojave desert accommodate a change from focused slip in the southern central mojave desert to a wider and more dispersed fault system northward with a shift in the locus of dextral shear westward we interpret these changes in the fault system to be a response to the garlock fault which inhibits through going faulting but allows through going dextral shear via clockwise deflection of its trace

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

speculative tectonic model for the early mesozoic arc of the southwest cordilleran united states the early mesozoic continental arc of arizona california and western nevada is here interpreted to have occupied a continuous graben depression more than 1000 km long similar to the modern extensional are of central america high calculated rates of subsidence the presence of syndepositional normal faults and fault talus breccias the abundance of calderas and high level intrusions and interstratification of cale alkalic and alkalic volcanic rocks provide support for this model the arc graben depression acted as a long lived more than 40 m y trap for craton derived quartz sands which were funneled along the length and across the width of the depression

timing and kinematics of deformation in the cronese hills california and implications for mesozoic structure of the southwestern cordillera a southeast vergent ductile shear zone is exposed in the cronese hills in the central mojave desert this shear zone formed under greenschist facies conditions and placed mylonitic metaplutonic and metavolcanic rocks upon folded triassic metasedimentary rocks u pb zircon ages of prekinematic and postkinematic plutons constrain the age of deformation to between 169 and 154 ma late middle or early late jurassic the authors believe that the cronese shear zone forms part of the southern continuation of the east sierran thrust system mesozoic thrust faults may have controlled preservation of jurassic arc related rocks and may be responsible for structural stacking of paleozoic eugeoclinal and miogeoclinal strata in the central mojave desert

high temperature anomalies oceanward of subducting slabs at the 410 km discontinuity abstract our p wave whole mantle tomography revealed a low velocity region oceanward of the northern honshu slab of the pacific plate at depths around the 410 km seismic discontinuity resolution tests and scrutiny of the traveltime residuals for the ray paths passing through the low velocity region indicate that this anomaly is a resolvable feature and not an artifact due to the strong slab anomalies the existence of the slow anomalies is also supported by the analysis of the p wave records from the j array a large aperture seismic array in japan for a bonin earthquake the p arrivals to northern honshu at epicentral distances of 13 20 are strongly triplicated because of the 410 km discontinuity the later arrivals along the retrograde branch where ray paths pass through the low velocity region are anomalously slow comparison of the observed and synthetic waveforms indicates not only slow anomalies but also depression of the 410 km discontinuity this depression represents the direct evidence for the low velocity zone of primarily thermal origin an excess temperature of 200 k and the associated fractional melt of less than 1 can explain both the results of the tomographic and waveform analyses