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

late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

connection between igneous activity and extension in the central mojave metamorphic core complex california the development of metamorphic core complexes and associated low angle detachment faults commonly is intimately associated with synextensional igneous activity in most areas studied to date the relation of magmatism to extension is obscured by imprecise dating and by the overprint of later tectonic events we present data from the early miocene central mojave metamorphic core complex cmmcc which indicate that extension was accompanied by igneous activity as reflected by prekinematic synkinematic and postkinematic plutons and coeval volcanic rocks deposited in the associated extensional basins the principal intrusion is an early miocene granite pluton exposed in outcrops across an area greater than 400 km2 dikes adjacent to the pluton are common in the mitchel range at the buttes and at fremont peak the overall orientation of the pluton and associated dikes is west northwest roughly perpendicular to the extension direction results of u pb analyses on zircon from two pluton and two dike samples yield ages of 20 to 23 ma two other dike samples yield inconclusive results synextensional basins formed by detachment faulting during the core complex development rocks in these basins compose the jackhammer and pickhandle formations and filled an elongate nw trending trough more than 50 km long the 4oar 39ar ages for tuff beds are as old as 23 8 0 3 ma near the base of the lower pickhandle formation and as young as 21 3 0 5 ma in the uppermost lower pickhandle hence volcanism and plutonism are coeval the diversity of intrusive relations relative to the timing and development of the mylonitic fabric in the cmmcc precludes any simple cause and effect relationship between magmatism and extensional deformation rather magmatism and extension may have been localized at a releasing bend in a transfer fault system which links extension in the cmmcc with extension in the colorado river area to the east

volume loss fluid flow and state of strain in extensional mylonites from the central mojave desert california abstract compositional changes in mylonitic rocks from the central mojave metamorphic core complex california indicate that large volume loss 20 70 attended formation of mylonite along the waterman hills detachment fault strong silica depletion and significant mobility of normally immobile elements imply large fluid rock ratios during mylonitization possibly as a result of multiple pass hydrothermal convection connecting near surface and mid crustal alteration regimes dissolution of quartz during mylonitization indicates that the fluid was undersaturated with respect to silica and therefore was probably not magmatic or connate large volume loss in a shear zone should result in apparent flattening strains yet mylonites throughout the complex including the mylonites that record large volume loss exhibit constrictional strains this inconsistency raises questions about the applicability of the shear zone model to cordilleran metamorphic core complexes

tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california several authors recently have proposed the rolling hinge model for the formation of extensional detachment faults in metamorphic core complexes in this model isostatic uplift of the denuded footwall of a major normal fault causes the near surface part of the fault to rotate from a higher to a lower dip for the rolling hinge model to be acceptable the footwalls of detachment faults must have undergone the strain pattern dictated by the rolling hinge in this paper the structural development of the footwall of the central mojave metamorphic core complex is used as a field test tertiary structures in the lower plate of the core complex record ne directed ductile extensional shear followed by cataclastic ne side up shear with a significant component of contraction along the previous extension direction this strain history agrees with the rolling hinge model and particularly with a version of the model in which footwall uplift occurs by a combination of elastic flexure and flexurally driven upper crustal failure similar late stage layer parallel contraction appears to be present in some other tectonically denuded terranes of the basin and range suggesting that the flexural failure rolling hinge model for extensional terranes may be widely applicable

mid tertiary structural evolution of the old woman mountains area implications for crustal extension across southeastern california investigations in the old woman mountains area provide insights into mid tertiary extensional tectonism in southeastern california the old woman mountains area is a moderately extended region that lies between two highly extended terranes the central mojave extensional complex and the colorado river extensional corridor two normal faults and a high angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks these blocks correspond to the four mountain ranges of the area the old woman piute little piute and ship mountains other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys deformation occurred in part before but primarily after deposition of the peach springs tuff an 18 5 ma regional stratigraphic marker transport directions of hanging walls are westward on the west side of the old woman mountains area and east southeastward on the east side we suggest that the old woman mountains area is a less extended portion of a continuous extensional terrane that stretches from the central mojave desert across the colorado river to the transition zone of the colorado plateau lower strain in the old woman mountains area than in most of the colorado river extensional corridor and central mojave extensional complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low angle simple shear zone shifted at about 18 20 ma from a position far to the west to just east of the old woman mountains area

stratigraphy structure and potassic alteration of miocene volcanic rocks in the sleeping beauty area central mojave desert california the sleeping beauty area of the southeastern cady mountains central mojave desert contains a sequence of volcanic rocks 3 km thick which was erupted approximately 20 m y ago plate reconstructions indicate that the change from subduction to transform fault tectonics off the coast of california occurred about 20 m y ago as well and so these rocks contain a record of volcanism extensional faulting and potassic metasomatism during this time of tectonic transition the volcanic sequence comprises basalt to rhyolite flows and tuffs which are overlain by the widespread peach springs tuff and by terrestrial sediments bedding below the peach springs tuff generally dips 10 50 to the southwest and is cut by numerous steep northwest to north trending faults tilting predated deposition of the peach springs tuff bedding fault relationships indicate that deformation was not by tilted book geometry in its simplest form no significant low angle faults are exposed but such faults occur to the west and may underlie the area a major open southeast trending anticline in the area may be a drag fold related to the post late miocene right lateral north trending ludlow fault all units below the peach springs tuff were locally affected by severe potassic metasomatism which raised measured k2o contents to as high as 13 3 wt metasomatized rocks occur in irregular zones that follow northwest trending faults and are best developed around northwest trending breccia zones which have jasper matrices ba and mn prospects are invariably found in metasomatized rocks geologic and geochemical constraints indicate that metsomatism occurred at shallow depth 1 or 2 km and low temperature 150 c metasomatism occurred in at least two distinct pulses and apparently predated deposition of the peach springs tuff the k may have been derived from percolating closed basin brines or through hydrogen metasomatism of rocks deeper in the complex

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

nature of the southwestern boundary of the central mojave tertiary province rodman mountains california tertiary volcanic and sedimentary rocks in the central mojave desert are largely confined to the barstow bristol trough an elongate tectonic depression filled with lower miocene strata the southwestern boundary of the trough is sharply defined and separates thick 2 km tertiary sequences from an area to the southwest that is virtually devoid of tertiary rocks segments of this boundary have been interpreted as a buttress unconformity as a right slip sidewall fault that formed the lateral boundary of an early miocene extensional terrane and as a dip slip fault caused by flexure of the north american plate above the subducted mendocino fracture zone the silver bell fault in the rodman mountains central mojave desert is the best exposed segment of the boundary and relations exposed along it can be used to test these hypotheses the tertiary sequence north of the silver bell fault is at least 2 3 km thick and comprises at least 1 9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed tertiary section tuff layers intercalated in the lower part of the clastic deposits are 23 24 m y old indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central mojave metamorphic core complex structural and stratigraphic data indicate that the silver bell fault is primarily a dip slip fault the tertiary section is folded into a west trending upright to overturned syncline located near the southern edge of exposed tertiary rocks the syncline records late cenozoic north south shortening along the northwest striking calico fault which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip southwestward dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension however bedding dip does not decrease upward in the tertiary section paleocurrent directions trend obliquely updip and northeast dipping normal faults are rare stratal tilting postdated the accumulation of the exposed section which therefore was not deposited during rotational normal faulting the new data contradict interpretation of the silver bell fault as a major right slip fault bounding the central mojave extensional terrane although they permit small magnitude extension across the fault the amount of early miocene crustal extension in this area therefore is apparently small in spite of stratal tilting and the lateral limit of large magnitude crustal extension in the central mojave block is farther northwest the data are consistent with the fault having formed by failure during plate flexure above the subducted mendocino fracture zone

patterns of mineral transformations in clay gouge with examples from low angle normal fault rocks in the western usa abstract neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in the elasto frictional regime but neither the conditions nor the processes which wall rock is transformed into clay minerals are well understood yet understanding of these mineral transformations is required to predict the mechanical and seismogenic behavior of faults we therefore present a systematic study of clay gouge mineralogy from 30 outcrops of 17 low angle normal faults lanf s in the american cordillera to demonstrate the range and type of clay transformations in natural fault gouges the sampled faults juxtapose a wide and representative range of wall rock types including sedimentary metamorphic and igneous rocks under shallow crustal conditions clay mineral transformations were observed in all but one of 28 faults one fault contains only mechanically derived clay rich gouge which formed entirely by cataclasis clay mineral transformations observed in gouges show four general patterns 1 growth of authigenic 1m d illite either by transformation of fragmental 2m 1 illite or muscovite or growth after the dissolution of k feldspar illitization of fragmental illite smectite is observed in lanf gouges but is less common than reported from faults with sedimentary wall rocks 2 retrograde diagenesis of an early mechanically derived chlorite rich gouge to authigenic chlorite smectite and saponite mg rich tri octahedral smectite 3 reaction of mechanically derived chlorite rich gouges with mg rich fluids at low temperatures 50 150 c to produce localized lenses of one of two assemblages sepiolite saponite talc lizardite or palygorskite chlorite quartz and 4 growth of authigenic di octahedral smectite from alteration of acidic volcanic wall rocks these transformation groups are consistent with patterns observed in fault rocks elsewhere the main controls for the type of neoformed clay in gouge appear to be wall rock chemistry and fluid chemistry and temperatures in the range of 60 180 c

geology and thermochronology of tertiary cordilleran style metamorphic core complexes in the saghand region of central iran an 100 km long north south belt of metamorphic core complexes is localized along the boundary between the yazd and tabas tectonic blocks of the central iranian micro continent between the towns of saghand and posht e badam amphibolite facies mylonitic gneisses are structurally overlain by east tilted supracrustal rocks including thick 1 km steeply dipping nonmarine siliciclastic and volcanic strata near the detachment the neybaz chatak fault the gneisses are generally overprinted by chlorite brecciation crosscutting relationships along with u pb zircon and 40 ar 39 ar age data indicate that migmatization mylonitic deformation volcanism and sedimentation all occurred in the middle eocene between ca 49 and 41 ma the westernmost portion of the tabas block immediately east of the complexes is an east tilted crustal section of neoproterozoic cambrian crystalline rocks and metasedi mentary strata 10 km thick the 40 ar 39 ar biotite ages of 150 160 ma from structurally deep parts of the section contrast with ages of 218 295 ma from shallower parts and suggest late jurassic tilting of the crustal section these results define three events 1 a late jurassic period of upper crustal cooling of the western tabas block that corresponds to regional jurassic cretaceous tectonism and erosion recorded by a strong angular unconformity below mid cretaceous strata throughout central iran 2 profound approximately east west middle eocene crustal extension plutonism and volcanism ca 44 40 ma and 3 2 3 km of early miocene ca 20 ma erosional exhumation of both core complex and tabas block assemblages at uppermost crustal levels resulting from significant north south shortening the discovery of these and other complexes within the mid tertiary magmatic arcs of iran demonstrates that cordilleran style core complexes are an important tectonic element in all major segments of the alpine himalayan orogenic system

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

unraveling histories of hydrothermal systems via u pb laser ablation dating of skarn garnet abstract skarns are one of the few direct genetic links between magma and base metals in ore deposits yet temporal relationships between skarns and their assumed causative plutons are rarely confirmed with radiometric formation ages skarns have been notoriously difficult to date directly due to a limited primary mineral assemblage generally garnet and pyroxene discrepancies in timescales of skarn formation versus pluton emplacement emphasize our tenuous understanding of temporal relationships between plutons and hydrothermal systems including the duration of hydrothermal activity and skarn mineral growth rates here we apply a new method for u pb dating grossular andradite grandite garnet via la icp ms and present the first comprehensive study to focus on u pb dates of primary skarn garnet we present u pb age constraints on garnets from multiple skarns within the jurassic and cretaceous arcs of the north american cordillera two case studies darwin and empire mountain skarns illustrate the effectiveness of the u pb grandite method by demonstrating that garnet u pb ages agree within error of the pluton and skarn hosted u pb zircon ages two additional study localities black rock and mojave skarns demonstrate the ability of grandite u pb ages to 1 directly identify a causative igneous event and 2 record timing of major tectonic events specifically extensional to trans tensional shifts in the jurassic mojave of california

sedimentation and extensional basin evolution in a miocene metamorphic core complex setting alvord mountain central mojave desert california usa the lower miocene clews formation at alvord mountain mojave desert california comprises an upward coarsening sequence of synextensional continental deposits of variable thickness this sequence pinches out to the west against the alvord mountain pre tertiary basement complex and thickens eastward to c 300 m over a distance of 7 km two stages of sedimentation are recognized in the formation during the initial stage of sedimentation the depocentre was delineated by a lacustrine system that was bounded to the west by small sheetflood dominated alluvial fans and to the north by a southerly flowing fluvial braidplain lacustrine mudstone siltstone and carbonate suggest an initially closed basin conglomerate to the west represents small sheetflood dominated fans igneous clasts and a south east palaeoflow direction suggest a proximal low relief source in the western alvord mountain area pebbly sandstone of the southward prograding braidplain has a metasedimentary provenance in the paradise range to the north the second stage of deposition was dominated by coarse conglomerate and breccia west to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the cronese hills 8 km to the east these strata reflect rapid westward progradation of sheetflood and debris flow dominated alluvial fans that advanced across the axial braidplain and lacustrine system basin development is interpreted to have been controlled by regional ne directed extension on a detachment fault associated with the central mojave metamorphic core complex 30 km to the west the alvord mountain cronese hills region was initially transported as a single hangingwall block on the e dipping detachment initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall subsequent westward progradation of alluvial fans out of the cronese hills is believed to record the propagation of a nw striking sw dipping normal fault antithetic to the low angle e dipping detachment fault

oxygen and carbon isotopic composition of recent larger and smaller foraminifera from the madang lagoon papua new guinea abstract tests of recent benthic foraminifera from the madang lagoon papua new guinea were analyzed for their oxygen and carbon isotopic composition analyses were performed on scuba collected species from backreef and low oxygenated harbor environments covering a depth range of 0 to 45 m both rotaliid and miliolid symbiont bearing species were found to exhibit increasing 18o with depth while only the rotaliids reveal a comparable trend in their 13c composition the general trend of depletion in the heavier isotopes 13c and 18o appears to be depth and light dependant neither the symbiont type nor the nature of the microhabitat appear to affect the magnitude of 18 o 16 o fractionation in larger foraminifers from this environment the carbon isotope patterns of symbiont bearing species revealed distinct differences between rotaliids and miliolids consistently lower 13c values in rotaliid foraminifers may be related to a higher rate of photosynthetic activity the photosynthetic effect of carbon isotope fractionation may be overprinted by the presence absence of an internal carbon pool and or specific modes of biomineralization which may act as isotopic filters foraminifera collected from low oxygenated organic rich sediments in harbor inlets at madang were found to exhibit particularly low 13c values depletion 13c is suggested to be due to oxidative processes in the surrounding organic rich sediment coupled with preferential incorporation of isotopically lighter 12c

stable isotopes in recent larger foraminifera abstract oxygen and carbon isotope analyses have been made on large benthic foraminifera from bermuda the persian gulf and the philippines the foraminifera belong to the two suborders rotallina and miliolina life spans range between one season to more than two years shells contain information about seasonal temperature ranges and life history stages recorded as fluctuations of oxygen and carbon isotopic values within the shell material almost all specimens studied showed the expected variations in 18o with respect to ambient conditions water isotopic composition and seasonal temperature fluctuations the specimens from bermuda seemed enriched in 18o whereas all specimens from the philippines independent of their taxonomic position seemed to the depleted in 18o with respect to calcita equilibrium specimens from the same species showed about the same level and range of 18o values the miliolid species marginopora vertebralis cyclorbiculina compressa archaias angulatus peneroplis proteus and praesorites orbitolitoides cf monensis commonly show carbon isotope values up to 2 5 lighter than expected equilibrium values independent of sample locality the rotaliid species heterostegina depressa operculina sp and calcarina spengleri show carbon isotope values more than 2 lighter than expected equilibrium values also independent of sample locality all analyzed miliolid species show a tendency with age towards lighter than equilibrium 13c values the analyzed rotaliid species showed the reverse that is shells tend toward increased 13 values with age

tertiary extension in the old woman mountains area california evidence from apatite fission track analysis apatite fission track ages and confined track length distributions from the old woman mountains area southeastern california record evidence for periods of rapid cooling during the late cretaceous following mesozoic orogenesis and during miocene crustal extension apatites from the clipper marble and turtle mountains give fission track ages between 62 and 69 ma as a result of cooling of these upper crustal rocks after regional late cretaceous plutonism and metamorphism in the piute mountains apatite fission track ages decrease from 70 ma with mean track lengths of 13 5 m to 14 m from west to east intermediate aged samples from the piute mountains have mean track lengths between 11 and 13 m due to their extended residence in the apatite annealing zone during the early tertiary this pattern is the result of 20 30 of westward tilting of the piute mountains block as indicated by the dip of tertiary strata the apatite data indicate that tilting occurred after 18 ma and caused rapid cooling of rocks exposed on the eastern side of the mountain range knowing the amount of tilting and the apparent paleotemperatures across the piute mountains allows us to estimate that the synextensional geothermal gradient was 50 20 c km apatites from the northeastern old woman mountains yield fission track ages of 68 ma with mean track lengths of 13 8 m these samples record cooling after batholith emplacement at 73 ma in the northwestern old woman mountains apatite fission track ages decrease to 20 ma and have mean track lengths 15 m indicating very rapid cooling we interpret this age pattern to be the result of minor eastward tilting of the old woman mountains block after 18 19 ma or flexural unroofing of the northwestern old woman mountains due to movement on the ship mountains fault extension in the old woman mountains area occurred between 19 and 16 ma which is coincident with extension in the colorado river extensional corridor and later than major extension in the mojave extensional belt

recent surface temperature trends in the interior of east antarctica from borehole firn temperature measurements and geophysical inverse methods 1 we use measured firn temperatures down to depths of 80 to 90 m at four locations in the interior of dronning maud land east antarctica to derive surface temperature histories spanning the past few decades using two different inverse methods we find that the mean surface temperatures near the ice divide the highest elevation ridge of east antarctic ice sheet have increased approximately 1 to 1 5 k within the past 50 years although the onset and rate of this warming vary by site histories at two locations nus07 5 78 65 s 35 64 e and nus07 7 82 07 s 54 89 e suggest that the majority of this warming took place in the past one or two decades slight cooling to no change was indicated at one location nus08 5 82 63 s 17 87 e off the divide near the recovery lakes region in the most recent decade inversion results indicate both cooler and warmer periods at different sites due to high interannual variability and relatively high resolution of the inverted surface temperature histories the overall results of our analysis fit a pattern of recent climate trends emerging from several sources of the antarctic temperature reconstructions there is a contrast in surface temperature trends possibly related to altitude in this part of east antarctica

oxygen and carbon isotopic variation in co existing larger foraminifera from a reef flat at akajima okinawa japan the larger foraminiferal assemblage on the reef flat at akajima ryukyu island arc is comprised of marginopora kudakajimaensis amphisorus hemprichii sorites orbiculus peneroplis planatus heterostegina depressa neorotalia calcar calcarina gaudichaudii baculogypsina sphaerulata and amphistegina lessonii the oxygen and carbon isotopic variations in several specimens of each species living under similar conditions were examined symbiosis had little to no effect on oxygen isotopic composition of the bulk carbonate of the analysed species the isotopic analysis along the ontogenetic stages indicates near equilibrium precipitation of oxygen isotopes in the early life stages and therefore the initial chambers should be microsampled in isotopic analyses for a better estimate of palaeotemperature the order of depletion of 13c parallels the order of accretion of calcium carbonate by the foraminiferal species a poor correlation between 18o and 13c and a large variation in 18o and 13c with size of the shell are the characteristic features of the symbiont bearing benthic foraminifera inhabiting reef flat environments in a deeper water species oxygen isotopic values vary little with size of the shell

tectonic control on southern sierra nevada topography california in this study we integrate the apatite u th he thermochronometric technique with geomorphic structural and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late cenozoic faulting in the southern sierra nevada the geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low relief landscape little affected by internal deformation to a more complex landscape affected by numerous faults regionally the relict landscape surface is readily resolved by age elevation relationships of apatite he ages coupled to geomorphology these relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting on the basis of 63 new apatite he ages and stratigraphic data from proximal parts of the san joaquin basin we resolve two sets of normal faults oriented approximately n s and approximately nw quaternary west side up normal faulting along the n s breckenridge kern canyon zone has resulted in a southwest step over from the owens valley system in the controlling structure on the regional west tilt of sierran basement this zone has also served as a transfer structure partitioning neogene quaternary extension resulting from normal displacements on the nw fault set this fault system for the most part nucleated along late cretaceous structures with late cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

introduction to special section on the california arizona crustal transect cactis part 3 the cactis california arizona crustal transect interim synthesis workshop in may 1988 brought researchers together in flagstaff arizona to discuss the geologic evolution and crustal structure of the southern cordillera between the san andreas fault in southeastern california and the colorado plateau in arizona sass et al 1988 the first set of papers resulting from the workshop appeared in the journal of geophysical research jgr as the special cactis 1 section in january 1990 howard et al 1990 and a second as cactis 2 in november 1990 haxel et al 1990 twelve papers in this issue form part 3 of the cactis series a final collection of papers will appear in the future as a joint special section with the california consortium for crustal studies calcrust

evolution of the southern san joaquin basin and mid tertiary transitional tectonics central california a cenozoic tectonic and sedimentary history is proposed for the southern san joaquin basin ssjb and tehachapi mountains that evolved adjacent to the plate margin off central california seismic reflection borehole field biostratigraphic and paleomagnetic data are integrated into geologic and fault structure maps cross sections and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence timing and distribution of complex tectonically linked events the largely buried structures and strata in the ssjb preserve an unusually complete record of the mid tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the pliocene significant structural relief existing across both extensional and contractile features is preserved in the subsurface and an active fold thrust belt propagates basinward along the margin of the u shaped tejon embayment the cenozoic evolution of the ssjb reflects the regional deformation of central california as different tectonic events followed each other along the adjacent north american plate margin five oligocene miocene basin phases are identified in the ssjb 1 late oligocene early miocene extensional subsidence with high and low angle normal faulting accompanied by volcanism and deposition of coarse syntectonic conglomerates 2 middle miocene uplift 3 later mid miocene transtensional subsidence to lower bathyal depths 4 alternating subsidence and uplift until the late miocene and 5 flexural subsidence due to pliocene to recent contraction reconstructions of mid tertiary california place the southern san joaquin tehachapi extensional terrane as a paleotectonic block located between the western mojave terrane then to the east and the western california terrane then to the south and west regional extension occurred during a long transition period between convergent and transform boundaries along the north american plate margin significant slip along the san andreas transform postdates this extensional event its origin apparently coincided with a regional middle miocene uplift event indicating that the san andreas fault is younger than previously supposed the tertiary ssjb has subsided due to extension transtension crustal tilting and thrust related loading the maricopa subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the sierra nevada and locally has subsided beyond 12 km in contrast the relatively stable tejon embayment underlain by sierran crystalline crust achieved its maximum subsidence in the miocene since pliocene time the ssjb has filled and continued to deepen after the basin was tectonically shut off from the pacific ocean

quantifying dextral shear on the bristol granite mountains fault zone successful geologic prediction from kinematic compatibility of the eastern california shear zone abstractfor regional kinematic compatibility to be a valid boundary condition for continental tectonic reconstructions there must be tests that validate or invalidate kinematic model predictions in several reconstructions of western north america the displacement history of the mojave block continues to be unresolved the magnitude of displacement along the bristol granite mountains fault zone bgmfz which is the eastern margin of the eastern california shear zone ecsz in the mojave block is a key example of a long standing kinematic prediction that has defied a positive field test until now the ecsz is a network of late neogene and quaternary right lateral strike slip faults that extend from the gulf of california north through the mojave desert linking pacific north america plate motion with basin and range extension this network of faults accounts for 15 of post 16 ma plate transform motion geologic estimates of net dextral offset along the mojave portion of the ecsz 53 6 km are appro

the mojave extensional belt of southern california it is proposed that the regional pattern of early miocene low and high angle normal faulting and dike swarm emplacement in the western and central mojave desert is the result of regional extension developed within a roughly east trending zone named here the mojave extensional belt this zone of extension although now disrupted by late cenozoic right slip faulting can be traced from the intersection of the san andreas and garlock faults to the eastern mojave desert near bristol lake and the granite mountains fault restoration of post oligocene movement along the san andreas fault system places the now buried western part of the mojave extensional belt opposite a similar age extended area of west central california surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the mojave extensional belt was low angle normal sense simple shear detachment faulting strain within the mojave extensional belt is partitioned between four domains edwards waterman daggett and bullion terranes that each consist of one or more half grabens each half graben is composed of tilted normal fault bounded blocks that lie above a rooted low angle brittle ductile normal sense shear zone tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping 80 planar to slightly curviplanar normal faults at least one generation of high angle faults postdates the formation of the detachments differential extension between the domains was accommodated by strike slip faults transfer zones similarly the lack of early miocene extensional elements north of the western segment of the garlock fault suggests that the garlock may have been active in the early miocene and served as the northern transfer boundary of the mojave extensional belt the mojave extensional belt initially began to open about 22 m y ago with the major phase of extension occurring between 22 and 20 ma local high angle normal faults cut older detachments and were active between 20 and 17 ma kinematic indicators suggest that the western part of the mojave extensional belt opened in a ne sw direction whereas the eastern portion extended in an ene wsw direction present day reference frame rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the mojave extensional belt was originally oriented n s and was rotated as much as 50 clockwise between 20 and 18 ma early miocene

transient creep of a composite lower crust 2 a polymineralic basis for rapidly evolving postseismic deformation modes postseismic horizontal strain and displacement following the june 28 1992 landers california earthquake mw 7 3 is broad scale and cannot be explained solely by delayed afterslip located at the rupturing fault trace both the observed strain at pinon flat observatory pfo and observed global positioning system receiver velocities evolve rapidly after the landers big bear earthquake sequence the observed exponential decay of these motions with timescales of 4 34 days may reflect a soft creep rheology in the lower crust and brittle ductile transition zone or even within the seismogenic crust itself here a simple model of a two dimensional screw dislocation in a layered maxwell viscoelastic earth is employed in conjunction with a composite rheology to demonstrate that the short timescale transient response modes 4 34 days are consistent with the behavior of a biviscous lower crust the lowest viscosity of this system is derivable from laboratory experimental data on the long term creep of natural quarztites and the highest viscosity is consistent with isostasy related lower crustal flow in a continental extensional tectonic environment the model predicts significant stress relaxation at the base of the seismogenic crust near the base of the seismogenic zone and about 4 km away from the mainshock the rate of predicted relaxation is of the order of 0 01 mpa d 1 during the first 20 days of postseismic flow oblate spheroidal inclusions at 5 concentration levels that are both aligned and fairly flat in shape and that have a viscosity of 3 4 1015 pa s are consistent with both the amplitude and decay time of horizontal crustal strain observed at pfo after the landers mainshock it is speculated that the structures exposed in cross sections and in seismic reflection profiles of the lower crust that have mylonitic associations are in part the cause of such rapid postseismic evolution in southeastern california unmylonitized quartz rich rock at sufficiently elevated temperatures could also contribute to the rapid decay modes

recovery lakes east antarctica radar assessment of sub glacial water extent 1 a fast flowing tributary of recovery ice stream penetrates more than 500 km into the interior of east antarctica recent satellite based studies found surface features in the onset area of this tributary that indicate a significant subglacial hydraulic system including four large smooth basins the typical surface expression of large subglacial lakes as well as eleven smaller areas over which ice sheet surface elevations rapidly changed by discharge filling of subglacial water here we present the first ice penetrating radar evidence of subglacial conditions in this area we identified a distinct ice water interface only over a limited area within the boundaries of the investigated large smooth basins previously hypothesized to be water filled lakes the radar characteristics in some areas are similar to those of a drained lake indicating that parts of the bed are wet but not a typical lake we also find evidence for discrete water bodies outside of the lake boundaries the lines of evidence indicate that the northern most two recovery lakes have recently drained

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

frictional properties of low angle normal fault gouges and implications for low angle normal fault slip abstract the mechanics of slip on low angle normal faults lanfs remain an enduring problem in structural geology and fault mechanics in most cases new faults should form rather than having slip occur on lanfs assuming values of fault friction consistent with byerlee s law we present results of laboratory measurements on the frictional properties of natural clay rich gouges from low angle normal faults lanf in the american cordillera from the whipple mts detachment the panamint range front detachment and the waterman hills detachment these clay rich gouges are dominated by neoformed clay minerals and are an integral part of fault zones in many lanfs yet their frictional properties under in situ conditions remain relatively unknown we conducted measurements under saturated and controlled pore pressure conditions at effective normal stresses ranging from 20 to 60 mpa corresponding to depths of 0 9 2 9 km on both powdered and intact wafers of fault rock for the whipple mountains detachment friction coefficient varies depending on clast content with values ranging from 0 40 to 0 58 for clast rich material and 0 29 0 30 for clay rich gouge samples from the panamint range front detachment were clay rich and exhibit friction values of 0 28 to 0 38 significantly lower than reported from previous studies on fault gouges tested under room humidity nominally dry conditions including samples from the same exposure samples from the waterman hills detachment are slightly stronger with ranging from 0 38 to 0 43 the neoformed gouge materials from all three localities exhibits velocity strengthening frictional behavior under almost all of the experimental conditions we explored with values of the friction rate parameter a b ranging from 0 001 to 0 025 clast rich samples exhibited frictional healing strength increases with hold time whereas clay rich samples do not our results indicate that where clay rich neoformed gouges are present along lanfs they provide a mechanically viable explanation for slip on faults with dips p f 3 overpressures and or correcting for 5 of footwall tilting furthermore the low rates of frictional strength recovery and velocity strengthening frictional behavior we observe provide an explanation for the lack of observed seismicity on these structures we suggest that lanfs in the upper crust depth

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

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

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

magnesium and strontium compositions of recent symbiont bearing benthic foraminifera abstract minor and trace elements in foraminiferal carbonates are potential paleo proxies of climate nutrient and seawater composition there are very few reports of trace element composition of symbiont bearing larger foraminifera that are known to be important constituents of shallow marine modern and ancient carbonates in this paper we examine the range of variation in mg and sr content of recent species of these foraminifera from a lagoon of lakshadweep atoll indian ocean and akajima islands japan two hyaline species amphistegina lessonii and neorotalia calcar and two porcellaneous species amphisorus hemprichii and marginopora vertebralis were collected live from lakshadweep islands mg ca in these foraminifera is of an order of magnitude higher than the values reported for planktonic and symbiont free benthic foraminifera the sr ca values are however comparable with the reported values in other foraminiferal taxa and they are found to vary within a narrow range electron probe micro analysis of three symbiont bearing benthic species indicates spatial heterogeneity of high orders in mg ca composition in all the species the annual variation in temperature and ph of the lagoon water cannot explain the observed amplitude of the compositional variation the photosynthesis and respiration of the symbionts and host foraminifera are possibly the major cause of compositional heterogeneity in individual tests as has also been recently postulated for symbiont bearing planktonic foraminiferal species it highlights the need to isolate biological factors and necessitates species specific paleotemperature scale in paleoclimatic analysis we also analyzed 18 o 13 c ca mg and sr in carefully dissected chambers of a reef dwelling porcellaneous benthic foraminifer marginopora kudakajimaensis collected live in four seasons a moderate positive correlation is observed between mg ca and temperature however large inter and intra test variation in mg limits the precision of mg ca as palaeotemperature proxy the sr ca of the test calcite is unrelated to temperature of the sea water the 13 c of m kudakajimaensis does not correlate with 18 o mg ca or sr ca

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

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

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

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

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

parametric time series analysis of extreme events in earthquake electrical precursors abstract a statistical method to evaluate extreme events in earthquake precursory signals of electrical nature is proposed the occurrence probability of seismo electrical anomalies is computed by means of a parametric time series approach in order to avoid the influence of external noise a preliminary filtering of the electrical signals is performed in a second step an autoregressive model to describe the residual time series is suitably identified and fitted to the data finally parametric inference of extreme events is carried out on the basis of the selected model the performance of the technique has been evaluated by considering geoelectrical time series recorded by an automatic station located on the southern apennine chain italy this region was interested by the november 1980 irpinia basilicata earthquake and it is one of the most active areas of the mediterranean region finally a possible correlation between electrical anomalies and seismic events occurred in the area is discussed