composition of modern sand and cretaceous sandstone derived from the sierra nevada california usa with implications for cenozoic and mesozoic uplift and dissection abstract the california sierra nevada represents the roots of a primarily cretaceous magmatic arc that is presently being dissected extensively in the south and to a lesser degree in the north where it is covered by cenozoic volcanic rocks the cenozoic magmatic arc built on top of the cretaceous arc has been dissected concurrently with northward migration of the mendocino triple junction south of which the magmatic arc is inactive a north to south transect from the modern cascade arc along the sierra nevada therefore acts as a proxy for evolution from an active undissected magmatic arc to a transitional arc to a dissected arc to intensely uplifted basement discriminant analysis of cascade and sierra nevada modern alluvial sand defines four compositional groups and clearly distinguishes volcaniclastic undissected arc and plutoniclastic uplifted basement end members two intermediate compositional groups are primarily volcaniplutoniclastic and metamorphiplutoniclastic the former group represents dickinson s 1985 transitional arc the latter represents dissected arc sand composition in the southern sierra nevada reflects significant uplift resulting from a combination of latest cretaceous paleogene uplift and late cenozoic basin and range extension footwall uplift of the western rift shoulder possibly enhanced by mantle lithosphere delamination high concentrations of quartz and feldspar and near absence of aphanitic lithic fragments are typical of the southern sierra nevada and are characteristic of dickinson s basement uplift cretaceous sandstone petrofacies of the great valley group gvg of western california document dissection of the cretaceous magmatic arc as well as erosion of residual orogenic highlands analogous to modern taiwan formed during latest jurassic arc continent collision nevadan orogeny when cretaceous petrofacies data are entered into the modern sand discriminant analysis as unknowns they are classified into the modern cascade sierra nevada groups and a group from taiwan the results confirm that the lowest gvg petrofacies represent erosion of the nevadan suture belt and the undissected post nevadan arc increasing dissection of the arc occurred during the cretaceous but most samples represent transitional arc indicating that even by the latest cretaceous laramide cessation of arc activity in california dissection of the magmatic arc was less than what we see today in the central and southern sierra nevada no cretaceous samples have compositions equivalent to the basement uplift of the southern sierra nevada the application of actualistic sand stone petrofacies in this study provides important constraints on models for the cretaceous to modern history of the sierra nevada as well as implications for arc evolution in general

stable isotope paleoaltimetry and the evolution of landscapes and life abstract reconstructing topography of our planet not only advances our knowledge of the geodynamic processes that shape the earth s surface equally important it adds a key element towards understanding long term continental moisture transport atmospheric circulation and the distribution of biomes and biodiversity stable isotope paleoaltimetry exploits systematic decreases in the oxygen 18o or hydrogen d isotopic composition of precipitation along a mountain front when the interaction of topography and advected moist air masses induces orographic precipitation these changes in 18o or d can be recovered from the geologic record and recent geochemical and modeling advances allow a broad range of proxy materials to be evaluated over the last 10 yr stable isotope paleoaltimetry has witnessed rapidly expanding research activities and has produced a broad array of fascinating tectonic and geomorphologic studies many of which have concentrated on determining the elevation history of continental plateau regions these single site studies have greatly expanded what used to be very sparse global paleoaltimetric data the challenge now lies in disentangling the surface uplift component from the impact of climate change on 18o and d in precipitation the robustness of stable isotope paleoaltimetry can be enhanced when high elevation 18o or d data are referenced against low elevation sites that track climate modulated sea level 18o or d of precipitation through time approach analysis of central andean paleosols documents that differences in 18o of soil carbonate between the subandean foreland and the bolivian altiplano are small between 11 and 7 ma but rise rapidly to ca 2 9 after 7 ma corroborating the magnitude of late miocene change in 18o on the altiplano future advances in stable isotope paleoaltimetry will greatly benefit from addressing four key challenges 1 identifying topographically induced changes in atmospheric circulation and associated teleconnections in the global climate system that affect 18o or d of precipitation 2 evaluating on a case by case basis if temporal and spatial changes in isotope lapse rates influence interpretations of paleoelevation 3 interfacing with phylogenetic techniques to evaluate competing hypotheses with respect to the timing of surface uplift and the diversification of lineages 4 characterizing feedbacks between changes in surface elevation and atmospheric circulation as these are likely to be equally important to the diversification of lineages than changes in surface elevation alone tackling these challenges will benefit from the accelerating pace of improved data model comparisons and rapidly evolving geochemical techniques for reconstructing precipitation patterns most importantly stable isotope paleoaltimetry has the potential to develop into a truly interdisciplinary field if innovative tectonic paleoclimatic and evolutionary biology phylogenetic approaches are integrated into a common research framework it therefore opens new avenues to study the long term evolution of landscapes and life

secular changes in environmental stresses and eukaryotes during the early triassic to the early middle triassic abstract the early triassic following the end permian mass extinction was an interval of severe low diversity increasing amounts of evidence demonstrate that variable environmental stresses were widespread and intense after the end permian mass extinction here we report biomarkers from lowest triassic to lower middle triassic strata in south china qingyan and chaohu sections including biomarkers for environmental stress 2 methyl hopane index and eukaryotic algae steranes and c21 n alkylbenzene ratio using the 2 methyl hopane index we detected the persistence of environmental stress during most of the early triassic using steranes and the c21 n alkylbenzene ratio we found a gradual increase in the biomass of eukaryotic algae during the early to early middle triassic a decrease in environmental stress in the qingyan section leidapo member during the early middle triassic was synchronous with the explosion of the qingyan biota which is characterized by a high abundance and diversity of invertebrate marine animals because the environmental stresses revealed by the 2 methyl hopane index encompass various factors e g ph and temperature we cannot identify the exact stresses at that time however our results reflect the amelioration of harsh environments for life during the interval of complete biotic recovery

orographic clouds in terrain blocked flows an idealized modeling study abstract idealized numerical simulations of moist strongly stratified flow over topography are used to study the processes that control orographic clouds in terrain blocked flows as a joint function of the nondimensional flow parameter nh u the horizontal topographic aspect ratio and the rossby radius of deformation nh f the simulations show the competition between enhanced upstream condensation in the secondary vertically propagating gravity wave and the reduction of condensation owing to enhanced low level flow deflection as nh u increases above about 1 5 the tendency for flow to be deflected around the barrier reduces cloud formation in the primary gravity wave over the ridge while increasing expands low level clouds over a broader upstream area ice clouds may form aloft in the secondary vertically propagating gravity wave and extend upstream for several hundred kilometers in terrain blocked flows more than half of the condensate mass develops upstream of the barrier in the secondary gravi

pace of landscape evolution in the sierra nevada california revealed by cosmogenic dating of cave sediments we report 26 al 10 be based ages of sierra nevada caves that constrain detailed late pliocene and quaternary river incision his tories for five river canyons rapid incision of 0 2 mm yr from 2 7 to ca 1 5 ma slowed markedly to 0 03 mm yr thereafter likely reflecting the combined effects of a transient erosional re sponse to pliocene rock uplift and periodic mantling of riverbeds with glacially derived sediment in the late quaternary while 400 m of incision has occurred in the past 2 7 m y outpacing interfluve erosion and thereby increasing the local relief canyons as deep as 1 6 km existed prior to that time these new erosion rates strength en the case for tectonically driven late cenozoic uplift

the stratigraphic evolution of the el paso basin southern california implications for the miocene development of the garlock fault and uplift of the sierra nevada the ricardo group is a 1 700 m thick sequence of miocene volcanic rocks and continental sedimentary rocks deposited between 19 and 7 ma in the el paso basin near the junction of the garlock fault and the sierra nevada frontal fault in southern california the combination of stratigraphic and structural data from the ricardo group with chronologic and rotational histories derived from magnetostratigraphic and radiometric studies provides some new constraints for the morphotectonic emergence of the sierra nevada the initiation of strike slip movement on the garlock fault and related east west extension in the southern basin and range region the miocene sequence documents 1 volcanism and north south tension without net extension at 18 15 ma 2 relative uplift about 15 13 5 ma 3 the beginning of sinistral slip on the garlock fault and east west extension in the southwestern basin and range region about 10 9 ma and 4 emergence of the sierra nevada as a source area and topographic high by 8 ma stratigraphic evidence from the ricardo group does not indicate significant miocene dip slip movement along the large scale down to the north fault that has been previously hypothesized to have preceded the present garlock fault in the same location during the early tertiary the evolution of this sequence however can be explained by tectonic models which propose a time space linkage between basin evolution east west extension in southern california and the mendocino triple junction system

sierra nevada basin and range transition near reno nevada two stage development at 12 and 3 ma relative and absolute elevations of the sierra nevada and adjacent basin and range province timing of their differentiation and location amount and timing of strike slip movement between them are controversial the provincial boundary near reno developed in two stages 1 at ca 12 ma the 700 km 2 verdi boca sedimentary basin formed across what was to become the boundary probably as a result of a small magnitude but regional extensional episode that affected much of the western basin and range 2 at 3 ma the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern sierran structural and topographic boundary in this area the boundary is really a transition zone with a western edge along the donner pass california fault zone which is farther west than previously placed both episodes appear to have resulted from east west extension only which suggests that northwest motion of the sierra nevada relative to the basin and range shown by geodetic data began after 3 ma or was taken up farther east

a miocene to pleistocene climate and elevation record of the sierra nevada california orographic precipitation of pacific sourced moisture creates a rain shadow across the central part of the sierra nevada california that contrasts with the southern part of the range where seasonal monsoonal precipitation sourced to the south obscures this rain shadow effect orographic rainout systematically lowers the hydrogen isotope composition of precipitation dppt and therefore dppt reflects a measure of the magnitude of the rain shadow hydrogen isotope compositions of volcanic glass dglass hydrated at the earth s surface provide a unique opportunity to track the elevation and precipitation history of the sierra nevada and adjacent basin and range province analysis of 67 well dated volcanic glass samples from widespread volcanic ash fall deposits located from the pacific coast to the basin and range province demonstrates that between 0 6 and 12 1 ma the hydrogen isotope compositions of meteoric water displayed a large 40 decrease from the windward to the leeward side of the central sierra nevada consistent with the existence of a rain shadow of modern magnitude over that time evidence for a miocene to recent rain shadow of constant magnitude and systematic changes in the longitudinal climate and precipitation patterns strongly suggest that the modern first order topographic elements of the sierra nevada characterized the landscape over at least the last 12 million years

three dimensional effects in high drag state flows over long ridges abstract numerical simulations of nonrotating flow with uniform basic wind and stability past long three dimensional 3d ridges are compared to the corresponding two dimensional 2d limit to reveal the importance of 3d effects for mountain heights smaller than the threshold for breaking waves the low level flow over the interior of the ridge is well described by 2d theory when the horizontal aspect ratio is roughly 10 or greater by contrast in flows with wave breaking significant discrepancies between 2d and 3d results remain apparent even for 12 it is found that the onset of wave breaking and the transition to the high drag state is accompanied in 3d by an abrupt increase in deflection of the low level flow around the ridge the increased flow deflection is produced at least in part by upstream propagating columnar disturbances forced by the transition to the high drag state the deflection of the incident flow reduces the amplitude of the mountain wave aloft relative to 2d and acts as a neg

fluvial landscape response time how plausible is steady state denudation whether or not steady state topography and denudation are probable states depends on the timescale of system response to tectonic and climatic perturba tions relative to the frequency of those perturbations this paper presents analytical derivations of algebraic relations for the response time of detachment limited fluvial bedrock channel systems both to tectonic and climatic perturbations detachment limited fluvial erosion is described by the stream power incision model and the derivations are limited to the applicability of that model all factors likely to influence system response time that are not adequately captured by the stream power incision model will tend to increase the response time the calculations presented thus provide minimum estimates of landscape response time and therefore over predict the prob ability of attaining and sustaining steady state topography and denudation the central range of taiwan is used as a case study to estimate response times in a landscape often argued to be in steady state model parameters are fit to modern stream profiles by assuming that the topography represents a quasi steady state form estimated response times generally range from 0 25 to 2 5 ma depending on the non linearity of the incision rule and the magnitude and type of perturbation thus it may be reasonably argued that steady state topography and denudation are likely to prevail during periods of climatic stability response time is sufficiently short compared with plate tectonic timescales however rapid climatic fluctuation in the quaternary appears to preclude the attainment of steady state conditions in modern orogens motivation

paleotopography in the western u s cordillera issue 2 of volume 312 of the american journal of science brings together several new papers bearing on the history of orogenesis paleotopography and paleoclimate in a region whose geologic record has been more important to the modern development of ideas about these processes than any other in

coupled crust mantle dynamics and intraplate tectonics two dimensional numerical and three dimensional analogue modeling 1 tectonic deformation of some intraplate regions may be caused by the rayleigh taylor rt instability of dense subcrustal lithosphere mantle lithosphere as it descends into the mantle we report on a series of scaled three dimensional 3 d analogue and 2 d numerical experiments of coupled crust mantle dynamics that study such a process the models investigate the effects of two geometries of rt instability linear versus axisymmetric and use different rheological stratifications of the crust the growth of the rt instability is strongly influenced by crustal rheology while rt growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself the surface is experiments having a viscous only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions in experiments with a brittle upper crust the surface deformation is much more subdued however strong mantle flow induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust when the lower crust is relatively strong localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability in all the experiments a portion of the ductile lower crustal material is entrained within the downgoing rt instability deep into the mantle the interplay between dynamic topography and crustal thickening thinning induced by the underlying mantle flow governs various time dependent phases of subsidence and uplift of the model surface the experiments help to account for a number of first order tectonic behaviors of the lithospheres of earth and other terrestrial planets for example intraplate orogens and basins complex localized tectonic structure in intraplate settings and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere rt instabilities

the uplift of the sierra nevada and implications for late cenozoic epeirogeny in the western cordillera tilted cenozoic strata in the eastern great valley california record progressive late cenozoic uplift of the sierra nevada the magnitude of tilting is combined with other geologic data using a technique pioneered by grant and others to infer the rate and timing of post late miocene uplift between the kings river and the feather river angular unconformities of late neogene age have been reported throughout the east central great valley typically stratigraphic units of late oligocene through miocene pliocene age beneath the unconformities dip approximately 1 2 1 6 southwest middle to late pliocene units above the unconformities dip approximately 0 6 0 9 southwest and progressively overlap younger units from east to west these angular unconformities are interpreted to represent the onset of late cenozoic uplift of the sierra nevada due to westward tilting dated volcaniclastic units in the east central sacramento valley bracket the onset of tilting there between 3 4 and 8 4 ma stratigraphic and geologic relations from the stanislaus river area eastern san joa quin valley suggest that the late cenozoic tilting probably began approximately 5 ma previously published tilt data from the san joaquin valley combined with new work in the eastern sacramento valley indicate that the rate of post late miocene tilting has been approximately uniform in the east central great valley early to middle pleistocene units typically dip 0 5 0 7 southwest middle pleistocene units dip 0 2 0 4 middle to late pleistocene units dip 0 1 0 15 late pleistocene units dip 0 05 0 1 if it is assumed that major westward tilting began 5 ma these data indicate an approximately uniform tilting rate of 0 28 per million years these data have important implications for models of the uplift simultaneous inception of tilting in the east central great valley at approximately 5 ma does not support models that link uplift of the sierra nevada to the migration of the mendocino triple junction late cenozoic tilting of the sierra nevada block does not appear to coincide temporally with the onset of major basin and range extension the uplift may be due to thinning of the mantle lithosphere beneath the sierra nevada but the tilt data do not afford a direct test of this hypothesis post late miocene tilting of the sierra nevada is temporally associated with late cenozoic uplift of the northern basin and range uplift of the cascades uplift of the colorado plateau and uplift of the southern rocky mountains supporting the contention that tilting of the sierra nevada may have occurred as part of a cordillera wide uplift event

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

oligocene and miocene paleogeography of central california and displacement along the san andreas fault recently completed sedimentologic and petrologic studies of oligocene and miocene strata in the temblor range san joaquin basin and santa cruz mountains la honda basin permit detailed reconstructions of paleogeography as well as new estimates of displacement along the san andreas fault during the oligocene and miocene the san joaquin and la honda basins were contiguous the southwestern margin of the san joaquin la honda basin was tectonically unstable and paleogeographically complex with several small deep sea fans fed by sediment derived from nearby subaerial uplifts of franciscan and granitic basement one of these deep sea fans represented by the temblor formation of the southern temblor range and the vaqueros sandstone of the central santa cruz mountains apparently has been displaced about 315 to 320 km by post early miocene post zemorrian about 23 ma right lateral slip along the san andreas fault this new estimate of displacement places an additional constraint on interpretations of san andreas slip history but ambiguities remain the time of initiation of san andreas displacement is poorly known and available data do not permit discrimination between a history of relatively continuous slip versus a history of episodic slip

the distribution of radiogenic heat production as a function of depth in the sierra nevada batholith california geochemical analyses and geobarometric determinations have been combined to create a depth vs radiogenic heat production database for the sierra nevada batholith california this database shows that mean heat production values first increase then decrease with increasing depth heat production is 2 aw m 3 within the 3 km thick volcanic pile at the top of the batholith below which it increases to an average value of 3 5 aw m 3 at 5 5 km depth then decreases to 0 5 1 aw m 3 at 15 km depth and remains at these values through the entire crust below 15 km below the crust from depths of 40 125 km the batholith s root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of 0 14 aw m 3 this is higher than the rates from most published xenolith studies but reasonable given the presence of crustal components in the arc root assemblages the pattern of radiogenic heat production interpreted from the depth vs heat production database is not consistent with the downward decreasing exponential distribution predicted from modeling of surface heat flow data the interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the sierra nevada could be generated within the 35 km thick crust this requires a very low heat flux from the mantle which is consistent with a model of cessation of sierran magmatism during laramide flat slab subduction followed by conductive cooling of the upper mantle for 70 m y the heat production variation with depth is principally due to large variations in uranium and thorium concentration potassium is less variable in concentration within the sierran crust and produces relatively little of the heat in high heat production rocks because silica content is relatively constant through the upper 30 km of the sierran batholith while u th and k concentrations are highly variable radiogenic heat production does not vary directly with silica content d 2005 elsevier b v all rights reserved

hydrothermal mixing carbonate dissolution and sulfide precipitation in mississippi valley type deposits a large number of mississippi valley type mvt deposits are located within dissolution zones in carbonate host rocks some genetic models propose the existence of cavities generated by an earlier event such as a shallow karstification that were subsequently filled with hydrothermal minerals alternative models propose carbonate dissolution caused by the simultaneous precipitation of sulfides these models fail to explain either the deep geological setting of the cavities or the observational features which suggest that the dissolution of carbonates and the precipitation of minerals filling the cavities are not strictly coeval we present a genetic model inspired by the textural characteristics of mvt deposits that accounts for both the dissolution of carbonate and precipitation of sulfides and later carbonates in variable volumes the model is based on the mixing of two hydrothermal fluids with a different chemistry depending on the proportion of the end members the mixture dissolves and precipitates carbonates even though the two mixing solutions are both independently saturated in carbonates we perform reactive transport simulations of mixing of a regional groundwater and brine ascending through a fracture both saturated in calcite but with different overall chemistries ca and carbonate concentrations ph etc as a result of the intrinsic effects of chemical mixing a carbonate dissolution zone which is enhanced by acid brines appears above the fracture and another zone of calcite precipitation builds up between the cavity and the surrounding rock sulfide forms near the fracture and occupies a volume smaller than the cavity a decline of the fluid flux in the fracture would cause the precipitation of calcite within the previously formed cavities therefore dissolution of carbonate host rock sulfide precipitation within the forming cavity and later filling by carbonates may be part of the same overall process of mixing of fluids in the carbonate host rock

the cenozoic climatic and topographic evolution of the western north american cordillera herein we present oxygen isotope records from cretaceous to recent terrestrial sediments in the western north american cordillera the purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the cenozoic to do this we constructed 18o maps of surface waters for time intervals that trace the development of topography of western north america these maps are based on 4861 oxygen isotope analyses from both published 4478 and new 383 data we determined the 18o values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors these data suggest that in the late cretaceous to early eocene the sevier hinterland formed a plateau of unknown height around 50 ma a topographic wave developed in british columbia and eastern washington that swept southward reaching northeastern nevada at 40 to 38 ma and southern nevada 23 ma this southward encroachment of an eocene plateau sweep caused reorganization of drainage patterns such that the intraforeland basins of wyoming and utah drainages extended deep within the sevier hinterland as the wave swept southward the landscape within the sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of 4 km and relief of 1 5 km this eocene highland was bordered on the west by a high sierra nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands the spatial and temporal evolution of this highland correlates with the timing of volcanism and extension these observations support tectonic models that call for north to south removal of the farallon slab or piecemeal removal of mantle lithosphere the isotopic data show that prior to growth of this highland the north american monsoon nam penetrated much farther north in the paleocene eocene than today the combined effects of global cooling increasing latitudinal temperature gradients and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the nam front by the oligocene the hydrologic regime that we observe today was in place it has been modified since then as a result of basin and range extension and collapse of the highlands in the mid miocene this collapse allowed the nam to penetrate farther north into the great basin of nevada and utah

testing carbonate associated sulfate cas extraction methods for sulfur isotope stratigraphy a case study of a lower middle ordovician carbonate succession shingle pass nevada usa abstract sulfur isotopes measured from sedimentary rocks are used to reconstruct the global sulfur cycle and chemostratigraphic correlation relative and absolute changes in the sulfur isotopic composition of the ocean 34sseawater are essential for estimating changes in redox conditions in ancient environments particularly for geochemical models that utilize isotope mass balance thus accurate estimates of 34sseawater must be measured using a laboratory protocol optimized for the extraction of primary sulfate preserved as carbonate associated sulfate cas by removing any contaminant secondary sulfate e g from diagenetic pyrite oxidation here we use two cas extraction protocols on lower middle ordovician carbonate rocks to test the degree to which similar absolute values and stratigraphic trends in 34scas are produced by different cas extraction methods one method processes carbonate powders once with a single rinse of a 10 nacl solution to remove sulfate minerals or weakly bonded sulfate ions followed by a rinse in 5 6 bleach to remove organically bound sulfur before dissolution in hydrochloric acid hcl the second method treats carbonate powders with three rinses of nacl without a bleach rinse to more aggressively remove secondary sulfate before dissolution in hcl isotopic results show that samples treated in three nacl rinses produce 34scas values that are on average 7 more positive than samples treated with a single nacl rinse but a similar stratigraphic trend is preserved in samples processed with either method we interpret the difference in 34scas between methods to reflect the incomplete removal of secondary sulfate derived from the oxidation of 32s enriched pyrite when employing only a single nacl rinse surprisingly rocks with low cas concentrations 10 mg kg using both methods show no significant difference in 34scas values as well as no significant difference in pyrite concentrations and pyrite 34s 34spy between methods samples with low cas concentrations are likely altered and secondary sulfate sourced from pyrite oxidation has likely become incorporated into recrystallized carbonate minerals or incorporated into pore filling calcite cement all of which is captured as cas results suggest that both cas extraction protocols can produce broadly similar 34scas trends but a single nacl rinse may not completely remove contaminant sulfate sorbed onto carbonate grains and thus will not yield meaningful data for reconstructing 34sseawater this has clear implications for modeling studies aimed at reconstructing paleo redox conditions using 34s data assumed to record global 34sseawater trends if these 34s data are generated using a non optimized cas protocol

topographic response to mantle lithosphere removal in the southern sierra nevada region california geological studies of mantle xenoliths entrained in late neogene quaternary lavas from the southern sierra nevada region and regional geophysical studies suggest that the highdensity mantle lithosphere that formed beneath the sierra nevada batholith in conjunction with arc magmatism is being convectively removed as a drip structure this structure as imaged seismically is roughly cylindrical in shape with a diameter of 100 km and extends to 225 km depth centered above this structure is a region 120 km in diameter that is undergoing active subsidence relative to adjacent regions such subsidence is seen in the active fluvial alluvial sediment flooding of mountainous topography of the southwestern sierra and in the development of the adjacent tulare lake basin of the san joaquin valley dynamic modeling of such upper mantle drip structures predicts a phase of overlying surface subsidence during the most vigorous phase of drip formation the southern sierra upper mantle drip and the overlying crust appear to be in this phase of their dynamically coupled evolution

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

magnetostratigraphy late devonian iridium anomaly and impact hypotheses paleomagnetism sedimentology and fine scale stratigraphy have been integrated to explain the origin of an iridium anomaly in the late devonian of western australia thermal demagnetization experiments were carried out on 93 specimens of marginal slope limestone form the northern canning basin samples are from a condensed sequence of deep water 100 m frutexites microstromatolites frutexites is a shrublike cyanobacterial organism that probably precipitated hematite or a metastable precursor from sea water when plotted within the microstratigraphic framework for the study area the observed characteristic directions from the sampled interval 14 5 cm thick are in five discrete layer parallel normal and reversed polarity zones the measured northeast southwest declinations and shallow inclinations probably record late devonian magnetostratigraphy on a centimetre scale the frutexites bed studied there occurs close to the frasnian famennian late devonian boundary a time of mass extinction of a wide variety of marine organisms throughout the world anomalously high iridium concentrations observed in the frutexites bed have suggested to some authors that the mass extinction was caused by meteorite impact this study concludes that iridium which is present over the span of five layer parallel magnetic reversals was concentrated over a long period of time by biologic processes thus the canning basin iridium anomalymore may be unrelated to meteorite impact less

middle miocene tectonic uplift of southern san joaquin basin california calculations of the tectonic component of the subsidence history of california s southern san joaquin basin indicate 200 400 m of abrupt tectonic uplift during the relizian about 15 ma this uplift can be explained as an isostatic response to the movement of thin young lithosphere south of the subducted mendocino fracture zone to a position under the basin the calculated uplift cannot be a result of extension and subsidence caused by instability of the mendocino triple junction but earlier early miocene subsidence of the basin may have resulted from triple junction related extension marine strata deposited in the san joaquin basin during the middle miocene are rich sources of petroleum so the inferred uplift and facies shifts may have important implications for ex loration

stable isotopic evidence for a pre middle miocene rain shadow in the western basin and range implications for the paleotopography of the sierra nevada 1 a recently proposed method for approximating the paleorelief of mountain belts makes use of the predictable relationship between the isotopic depletion of precipitation and the net elevation of an orographic barrier over which an air mass rises this rain shadow effect often creates desert regions on the lee side of mountain belts in which precipitation is isotopically light changes in the isotopic composition of precipitation can be estimated from the isotopic composition of authigenic or pedogenic minerals which can then be used to infer both the development of relief during orogenesis as well as the stability of a rain shadow formed by developing mountains the 18o of smectites formed from the weathering of middle miocene to late pliocene volcanic ashes currently exposed in the rain shadow of the modern sierra nevada of california show no indication of large scale late cenozoic surface uplift of the sierra and corresponding regional rain shadow development rather smectite isotope data tentatively suggest that elevations may have decreased over this time by as much as 2000 m toward the southern end of the range and 700 m in regions farther north this suggests that the modern rain shadow cast over the western basin and range has been in existence since pre middle miocene and that the sierra nevada have been a prominent orographic barrier since before this time these interpretations are in accord with several recent studies also suggesting a possible cenozoic elevation loss of an already developed sierra nevada mountain range

time specific aspects of facies state of the art examples and possible causes abstract the term time specific facies tsf was introduced to the scientific community by the late otto h walliser to refer to unique facies typical of particular narrow intervals some of which were related to bioevents in some senses however the concept was recognized much earlier and is even engrained in the very names of some geologic periods the concept of time specific facies is expanded slightly herein to include distinctive or unique regional to global characteristics of the sedimentary record that characterize particular intervals of geologic time the recognition of time specific and widespread processes in the sedimentary record is a critical step in unraveling the interplay between processes of differing scale these range from very short term but widespread facies that overlap with event deposits to general facies types that may persist for intervals up to 10s of millions of years in duration this paper briefly summarizes the history of development of the tsf concept provides examples to illustrate a few of the key aspects of time specific facies and offers a few tentative explanations for this phenomenon among the factors that control tsfs abrupt changes in redox conditions and early diagenetic mineralization sedimentary condensation often associated with abrupt sea level change altered climate and paleoceanography and biotic evolution and extinction seem to be most critical and permit a preliminary genetic classification of tsfs inevitably however many tsfs reflect multiple effects and some remain largely unexplained

paleomagnetic dating of alleghanian orogenesis and mineralisation in the mascot jefferson city zinc district of east tennessee usa abstract the mascot jefferson city m jc mississippi valley type mvt deposits are in the valley and ridge province of the appalachian orogen in east tennessee they have been a major source of zinc for the usa but their age is uncertain and thus their genesis controversial about 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods the sites sample limestones dolostones breccia clasts and sphalerite dolomite mvt mineralisation from mines in the lower ordovician kingsport and mascot formations of the knox group the characteristic remanent magnetisation chrm is carried by magnetite in the limestones by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation mineralized sites have a more intense chrm than non mineralised indicating that the mineralising and magnetisation event are coeval paleomagnetic breccia tests on clasts at the three sites are negative indicating that their chrm is post depositional remagnetisation and a paleomagnetic fold test is negative indicating that the chrm is a remagnetisation and a post dates peak alleghanian deformation the unit mean chrm direction for the a limestones gives a paleopole at 129 e 12 n d p 18 d m 26 n 3 indicating diagenesis formed a secondary chemical remanent magnetisation during the late ordovician early silurian b dolomitic limestones and dolostone host rocks gives a paleopole at 125 3 e 31 9 n d p 5 3 d m 9 4 n 7 recording regional dolomitisation at 334 14 ma 1 and c mvt mineralisation gives a paleopole at 128 7 e 34 0 n d p 2 4 d m 4 4 n 25 showing that it acquired its primary chemical remanence at 316 8 ma 1 the mineralisation is interpreted to have formed from hydrothermal fluid flow either gravity or tectonically driven after peak alleghanian deformation in eastern tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 ma prior to and during peak deformation

atmospheric flow patterns around the southern alps of new zealand and implications for paleoaltimetry the development of relief may generate leeside rain shadows where precipitation 18o values are lower due to rainout on the windward side the magnitude of lowering in paleo 18o sampled from the leeside of a mountain range should at least in principle be related to the elevation of the mountain range in order for leeside proxies to record the highest elevations the majority of air masses need to travel up and over the ridge crest before raining out on the leeside if atmospheric flow patterns around the mountain range are dominated by flow deflection air masses that reach the leeside are less likely to record the highest elevations using the weather research and forecasting model and hybrid single particle lagrangian integrated trajectory hysplit model we demonstrate that modern atmospheric flow patterns in the southern alps of new zealand are not dominated by flow deflection the lack of flow deflection around the southern alps and the relatively low relief throughout the uplift history supports the use of leeside isotope records to constrain the timing of uplift and that uplift likely occurred 5 ma based on leeside isotope records in contrast orogens that are characterized by high elevations and strong flow deflection like the sierra nevada of california may not be good candidates for leeside isotope based paleoaltimetry studies ideal candidates for such studies should be characterized by relatively low elevations low atmospheric flow deflection and relatively low relief throughout the period of uplift

moist dynamics and orographic precipitation in northern and central california during the new year s flood of 1997 abstract the dynamics of moist orographic flows during the january 1997 floods in northern and central california are investigated using numerical simulations computed with the fifth generation pennsylvania state university national center for atmospheric research psu ncar mesoscale model mm5 early in the event 31 december 1996 1 january 1997 the low level winds offshore of california s central coast were blocked by the topography of the santa lucia range and the low level winds in the central valley were blocked by the topography of the central sierra nevada range in contrast moisture laden winds along the northern coast ranges and the northern sierra nevada flowed over topographic barriers as the core of humid air migrated to the south over 24 h the low level barrier jets weakened as the atmospheric stability decreased bringing heavy rainfall to the central and southern sierra nevada at the end of the event the heavy precipitation during this event was largely controlled by the interactio

can stable isotopes ride out the storms the role of convection for water isotopes in models records and paleoaltimetry studies in the central andes abstract globally changes in stable isotope ratios of oxygen and hydrogen o 18 and d in the meteoric water cycle result from distillation and evaporation processes isotope fractionation occurs when air masses rise in elevation cool and reduce their water vapor holding capacity with decreasing temperature as such o 18 and d values from a variety of sedimentary archives are often used to reconstruct changes in continental paleohydrology as well as paleoaltimetry of mountain ranges based on 234 stream water samples we demonstrate that areas experiencing deep convective storms in the eastern south central andes 22 28 s do not show the commonly observed relationship between o 18 and d with elevation these convective storms arise from intermontane basins where diurnal heating forces warm air masses upward resulting in cloudbursts and raindrop evaporation especially at the boundary between the tropical and extra tropical atmospheric circulation regimes where deep convective storms are very common 26 to 32 n and s the impact of such storms may yield non systematic stable isotope elevation relationships as convection dominates over adiabatic lifting of air masses because convective storms can reduce or mask the depletion of heavy isotopes in precipitation as a function of elevation linking modern or past topography to patterns of stable isotope proxy records can be compromised in mountainous regions and atmospheric circulation models attempting to predict stable isotope patterns must have sufficiently high spatial resolution to capture the fractionation dynamics of convective cells

landscape response to tectonic forcing digital elevation model analysis of stream profiles in the mendocino triple junction region northern california the topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock channel incision quantitative field assessments of process based laws are needed to accurately describe landscape uplift and denudation in response to tectonics and climate we evaluate and calibrate the shear stress or similar unit stream power bedrock incision model by studying stream profiles in a tectonically active mountain range previous work on emergent marine terraces in the mendocino triple junction region of northern california provides spatial and temporal control on rock uplift rates digital elevation models and field data are used to quantify differences in landscape morphology associated with along strike northwest to southeast changes in tectonic and climatic conditions analysis of longitudinal profiles supports the hypothesis that the study area channels are in equilibrium with current uplift and climatic conditions consistent with theoretical calculations of system response time based on the shear stress model within uncertainty the profile concavity of the trunk streams is constant throughout the study area 0 43 as predicted by the model channel steepness correlates with uplift rate these data help constrain the two key unknown model parameters the coefficient of erosion k and the exponent associated with channel gradient n this analysis shows that k cannot be treated as a constant throughout the study area despite generally homogeneous substrate properties for a reasonable range of slope exponent values n best fit values of k are positively correlated with uplift rate this correlation has important implications for landscape evolution models and likely reflects dynamic adjustment of k to tectonic changes due to variations in orographic precipitation and perhaps channel width sediment load and frequency of debris flows the apparent variation in k makes a unique value of n impossible to constrain with present data

iridium anomaly in the upper devonian of the canning basin western australia a moderate iridium anomaly about 20 times the local background has been found in upper devonian rocks in the canning basin it occurs at or near the frasnian famennian boundary which is known to be associated with a major massextinction event of global extent the anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions its occurrence suggests a causal link with some form of meteoroid impact moreover carbon isotope data indicate that a large reduction in biomass could have occurred at this level however the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium frutexites iridium platinum iron manganese cobalt arsenic antimony and cerium are preferentially concentrated in filaments of this organism with concentrations ranging from two to five times that of the matrix it is possible that frutexites extracted these elements directly from seawater without the need for their derivation from an extraterrestrial source

evidence of basin and range extensional tectonics in the sierra nevada the durrwood meadows swarm tulare county california 1983 1984 an extensive earthquake swarm occurred at durrwood meadows in the southern sierra nevada of eastern california during late 1983 and 1984 it was located within a 100 km long linear belt of seismicity that cuts through the southern sierra nevada along a north southward strike this seismic belt has been characterized by swarms and was one of the most seismically active features in southern california during 1984 the durrwood meadows swarm itself was characterized by a complex spatial distribution and a simple pattern of focal mechanisms at the beginning of the swarm the earthquakes were located along a northwestward trend later periods of high seismicity were distributed along a northeastward trend forming a y shaped structure and along other northward and northwestward trends in spite of this spatial complexity the focal mechanisms of the 35 m l 3 0 earthquakes within the swarm are all similar to each other with almost pure normal faulting along a north southward strike the strikes of the nodal planes in the focal mechanisms and the spatial distribution of epicenters form an en echelon pattern the consistency of the focal mechanisms with each other and the en echelon pattern of epicenters imply a homogenous stress field and a discontinuous fault structure the 100 km long linear belt of seismicity in an area with no throughgoing fault structure is interpreted as a basin and range normal fault beginning to form within the sierra nevada

relation of mineralization to wall rock alteration and brecciation mascot jefferson city mississippi valley type district tennessee this study was undertaken to assess the relation of mississippi valley type mineralization to wall rock alteration and brecciation in the mascot jefferson city district the largest part of the east tennessee mississippi valley type ore field the main question of interest was whether the mississippi valley type forming brines created or greatly enlarged the breccia system that hosts the ore or whether the breccia system was a preexisting paleoaquifer that simply controlled movement of the mineralizing brines a secondary and closely related question was whether brine wall rock interaction deposited mississippi valley type ore the breccia system that hosts the east tennessee ore field began as karst breccias which formed in the upper part of the late cambrian early ordovician knox group during middle ordovician emergence brecciation which was most common at the paleosurface and in a limestone rich zone about 200 m below the surface took place when limestone solution caused collapse of primary dolostone layers mississippi valley type mineralization consisting of sphalerite and sparry dolomite fills interstices in the breccias that formed in the limestone rich part of the knox group ore is associated with recrystalline dolomite that replaced limestone and there is an inverse correlation between the original limestone and sphalerite abundance suggesting that the ore forming fluids reacted strongly with limestone wall rock possibly dissolving it where alteration was most intense the assessment of a relation between alteration and mississippi valley type mineralization was based on 3 533 surface drill holes covering the 110 km 2 mascot jefferson city district each of which provided stratigraphic data and quantified estimates of mineralization intensity and alteration intensity these data show clearly that as much as 50 percent of the limestone in the mineralized breccia section was lost over enormous areas that extend far beyond significant mineralization the intensity of this effect clearly decreases downdip toward the east away from the probable source of meteoric karst forming waters these relations combined with isotopic analyses and reaction path calculations suggest that breccia formation and limestone dissolution took place during the original karst breccia formation in contrast later mississippi valley type mineralization was associated with replacement of limestone by recrystalline dolomite the main effect of dolomitization on the chemistry of the mississippi valley type brines an increase in their ca mg ratio would not cause sulfide precipitation thus it appears unlikely that mississippi valley type forming brines created much of their ore hosting breccias or that water rock interaction was a major cause of mississippi valley type ore deposition

stable isotope constraints on the elevation history of the sierra nevada mountains california research on the uplift history of the sierra nevada mountain range has yielded seemingly conflicting results some studies argue for substantial uplift within the past 3 5 m y others suggest that high elevations may have existed since the cretaceous the rain shadow across the sierra nevada is associated with a strong isotopic gradient with lower 18o values in precipitation on the leeward side of the range reconstruction of the 18o value of meteoric water as a monitor of paleoelevation has focused mainly on the leeward side of the sierras but interpretation of the results of these studies may be complicated by shifts in global climate and regional moisture sources we address these concerns by analyzing the 18o value of tooth enamel bioapatite from contemporaneous mammalian fossils on either side of the present sierra range by sampling across the range 18o differences induced by a rain shadow can be isolated from other complicating factors our results indicate that the sierra rain shadow has existed since at least 16 ma which is an important constraint on models for the tectonic evolution of the western united states unfortunately temporal resolution for localities is too coarse to differentiate between glacial and interglacial localities during the past 2 m y so we cannot evaluate if there was a latest cenozoic pulse of uplift or elevation loss

late cenozoic surface uplift of the southern sierra nevada california usa a paleoclimate perspective on lee side stable isotope paleoaltimetry late cenozoic surface uplift of the southern sierra nevada sierra california united states is widely debated recent interpretations of lee side isotope records from the southern sierra suggest that the elevation of the southern sierra has been sufficiently high to induce atmospheric flow patterns similar to modern patterns since the mid miocene at ca 12 ma the tendency for flow to deflect around a topographic barrier can be determined by the atmospheric stability barrier height and incoming wind speed we test the sensitivity of flow deflection to elevation to determine what elevation would have been sufficiently high to establish modern flow patterns in the mid miocene using global paleoclimate models and idealized regional weather models we find that the miocene atmosphere was more stable than modern we suggest that in a miocene climate flow patterns similar to modern pat terns could have been achieved for elevations as low as 2 km and that while miocene lee side isotope records from the southern sierra may indicate that the southern sierra has been a longstanding topographic feature they may not have changed significantly in response to proposed late cenozoic surface uplift of the southern sierra

atmospheric flow deflection in the late cenozoic sierra nevada abstract given the intimate links between topography tectonics climate and biodiversity considerable effort has been devoted to developing robust climate and elevation histories of orogens in particular quantitative geochemical reconstructions using stable oxygen and hydrogen isotopes have been applied to many of the world s mountain belts recent advances in atmospheric modeling have suggested that such stable isotope records from leeward sites can be affected by the complicating role that sufficiently elevated topography such as the southern high sierra plays in diverting atmospheric circulation while such terrain blocking effects are a hallmark feature of modern atmospheric circulation in the sierra their evolution remains poorly constrained in order to examine the history of these terrain blocking effects we developed stable isotope records from three late cenozoic sedimentary basins in the eastern sierra and basin and range 1 authigenic clay minerals in the mio pliocene verdi basin vb near present day reno nevada 2 fluvial and lacustrine carbonates from the plio pleistocene coso basin cb in the southern owens valley and 3 miocene to holocene pedogenic fluvial and lacustrine carbonates of fish lake valley flv whereas both the vb and cb are proximal to the sierra crest flv is a distal leeward site east of the white and inyo mountains in the basin and range the cb oxygen isotope record exhibits an increase of 1 2 over the last 6 myr while vb and flv show no significant change these results suggest that terrain blocking around the southern sierra initiated prior to the late cenozoic though it may have been modestly enhanced during the last 6 ma

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

refining paleoaltimetry reconstructions of the sierra nevada california using air parcel trajectories stable isotope paleoaltimetry studies often implicitly assume that atmospheric flow interactions with topography can be simply modeled as a rayleigh distillation process in which air parcels consistently ascend topographic barriers we present a modern a d 1979 2010 air parcel trajectory analysis using the hybrid single particle lagrangian integrated trajectory hysplit model that shows that this fundamental assumption is often violated in the sierra nevada region of the western united states observed trajectory patterns and scaling calculations indicate that windward orographic blocking frequently occurs when trajectories encounter high elevations 2 5 km in the central and southern sierra nevada as a result trajectories reaching the sierran lee commonly travel around rather than over the highest range elevations redirection effects are particularly pronounced at leeward sites distal 150 km to the sierran crest but are also evident in trajectory patterns in the northern sierra nevada this trajectory analysis improves the interpretability of regional meteoric water and proxy isotopic records and has particular relevance to stable isotope based reconstructions of sierran paleoelevations specifically stable isotope methods alone provide only limited insight into the elevation history of the sierra nevada and are likely insufficient to resolve proposed late cenozoic elevation gains on the order of 1 2 km that may have raised the sierra to its modern elevations

tectonic depositional and diagenetic history of monterey formation miocene central san joaquin basin california the miocene monterey formation in the central san joaquin basin of california consists of biogenic siliceous rocks with a significant terrigenous clastic component correlation of electric logs in conjunction with analysis of outcrop sections core samples and paleontologic data suggests that deposition of the monterey was controlled by an interplay of basin geometry san andreas related tectonics fluctuations in sea level and changes in climate the basin is bounded on the east by the sierran magmatic arc complex and on the west by the san andreas fault during the late saucesian through early mohnian 20 10 ma the basin was reshaped by tectonism related to the migrating mendocino triple junction creating a broad gently sloping shelf to the east that cut off the supply of clastics to the central basin with the subsequent evolution of the san andreas fault the salinian block was emplaced along the western basin margin restricting the seaway and permitting low oxygen nutrient rich waters to occupy the basin as a result laminated organic rich fine grained pelagic to hemipelagic sediments of the monterey formation were deposited a warmer climate during saucesian through luisian time with less intensive upwelling le to more calcareous less siliceous sedimentation a cooling trend during late miocene time 10 5 ma increased upwelling activity diatom productivity in the water column and deposition of diatomaceous debris although turbidites accumulated on the basin floor and terrigenous clastics were deposited on the eastern shelf relatively undiluted diatomaceous deposits developed on outer shelves and slopes atop anticlinal banks generated by wrench tectonism and in some areas of the basin floor a mid mohnian eustatic sea level rise caused fine grained deposition to extend onto the eastern shelf slightly later a mid late mohnian period of exceptionally cool temperatures resulted in the most extensive development of upwelling related siliceous sediments throughout the basin with diagenesis these deposits formed the siliceous shales and porcelanites of the monterey formation high sedimentation rates coincident with a eustatic drop in sea level led to clastic progradation in latest miocene time as the basin filled analysis of organic matter from cores of the monterey formation shows that kerogen type reflects overall sedimentation trends samples from northern and eastern areas contain abundant terrestrial organic matter whereas samples from western areas mostly contain marine kerogens source potential is thus strongly influenced by geographic locality as well as lithology maturity of kerogen was assessed using vitrinite reflectance thermal alteration indices and pyrolysis techniques the latter two appear to be the more reliable indicators of organic matter maturity in the monterey formation of the san joaquin valley although much of the monterey formation in the study area may be thermally immature portions are probably generating hydrocarbons today in deep synclinal areas

eocene activity on the western sierra fault system and its role incising kings canyon california combining new and published apatite u th he and apatite 4he 3he data from along the kings river canyon california we rediscover a west down normal fault on the western slope of the southern sierra nevada one of a series of scarps initially described by hake 1928 which we call the western sierra fault system integrating field observations with apatite u th he data we infer a single fault trace 30 km long and constrain the vertical offset across this fault to be roughly a kilometer thermal modeling of apatite 4he 3he data documents a pulse of footwall cooling near the fault and upstream in the footwall at circa 45 40 ma which we infer to be the timing of a kilometer scale incision pulse resulting from the fault activity in the context of published data from the subsurface of the sacramento and san joaquin valleys our data from the western sierra fault system suggests an eocene tectonic regime dominated by low to moderate magnitude extension surface uplift and internal structural deformation of the southern sierra nevada and proximal great valley forearc

iron bacterial and fungal mats bajocian stratotype mid jurassic northern normandy france abstract the oolithe ferrugineuse de bayeux formation is located at the historical bajocian stratotype of sainte honorine des pertes north of bayeux normandy the condensed formation ranges from the base of the humphriesianum zone to the parkinsoni zone and is divided into four beds of decimetric scale three main microfacies are present 1 oncoid rudstones 2 ooid bioclastic packstones and 3 silty burrowed wackestones packstones sedimentation took place in a very quiet environment below the photic zone and below or near the storm wave base the general setting is a distal carbonate ramp its lower part characterized by hemipelagic sedimentation indicated by the presence of planktonic foraminifers the inferred depth is around 100 m free oxygen concentration was low dysaerobic conditions are indicated by a scarcity of benthic macrofauna ferruginous structures are numerous in the first two microfacies and absent in the last hematite staining is not uniform and follows many sedimentary patterns among the more widespread fe structures are perforation infillings with endolithic micro organisms microstromatolites oncoids ooids blisters coatings and hardgrounds these structures can be associated and none are mutually exclusive hematite coated filaments of different sizes and shapes are observed in the micrite matrix the walls of various organisms the calcite crystals associated with the fe cortical laminations the perforations and burrow the hardgrounds and microstromatolites petrographical and sem examinations suggest that the laminated crusts oncoids and hardgrounds are formed by microbial iron mats dominated by filamentous bacteria and fungi seven types of microbes are recognized filaments five morphotypes spheroidal bodies and stalked bodies filamentous microfossils of type 1 to 4 resemble the present day filamentous bacteria beggiatoales and cytophagaceae because of their large diameter and their branching nature filaments of type 5 are possibly filamentous fungi another argument in favor of fungi is the presence of stalked and spheroidal bodies that resemble zoosporangia and oogonia of some oomycota in deep calm and dysaerobic waters many interfaces e g between aerobic and dysaerobic waters are present in the sediments the stability of the soluble reduced state of iron fe 2 is higher at such interfaces and many ferric iron encrusted microbial fossils are observed iron could thus serve as an electron donor for microbial iron oxidation processes other microbial iron deposition pathways are also possible it appears that regardless of geological age paleozoic mesozoic and geographical location the same microbiological mechanisms are probably responsible for the red color in calcareous stratified or unstratified bodies the presence of fossilized iron encrusted bacteria and fungi at interfaces may therefore serve as an indicator of anoxic to dysaerobic conditions in various paleo micro environments

response of meteoric δ18o to surface uplift implications for cenozoic andean plateau growth abstract the timing and magnitude of surface uplift provide important constraints on geodynamic models of orogen formation oxygen isotope 18 o and mass 47 isotopolog 47 compositions from terrestrial carbonate sediments have been used with modern isotope and temperature lapse rates to infer past surface elevations of the andes however these paleoaltimetry interpretations are contentious because variations in the oxygen isotope composition in meteoric water 18 o p are caused by changes in elevation orographic and regional climate here we use a limited domain isotope tracking general circulation model to simulate changes in 18 o p and isotopic lapse rates in response to andean surface uplift and to re evaluate 18 o and 47 changes in late miocene carbonates previously associated with rapid andean growth results indicate that andean surface uplift leads to changes in low level atmospheric circulation and an increase in precipitation along the eastern andean flank which influences isotopic source and amount effects simulated changes in andean 18 o p are not systematic with an increase in surface elevation but are instead a function of orographic thresholds that abruptly change regional climate a 18 o p decrease of 5 over the central andes and an increase in isotopic lapse rates up to 0 8 km 1 coincide with andean surface uplift from 75 to 100 of modern elevation these changes in the isotopic signature could account for the entire 3 4 18 o depletion in late miocene carbonate nodules and suggest an andean paleoelevation of 3000 m 75 of modern elevations before 10 ma

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

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

using tracer thermochronology to measure modern relief change in the sierra nevada california abstract we examine relief change in two major sierra nevada drainages the kings and the san joaquin the modern distribution of bedrock cooling ages is derived by fitting isochrones to a dense distribution of existing cooling ages this analysis indicates southwest tilting of the range by 3 4 0 8 which is consistent with other geologic evidence for tilting after 5 ma the detrital cooling ages from the kings show an excess of young ages relative to what would be expected from uniform erosion whereas the san joaquin drainage shows excess old ages these results indicate increasing relief in the kings the results in the san joaquin are more difficult to interpret due to the dense network of dams which trap sediment however we analyze the effect of storage behind the dams and suggest steady or declining relief in the san joaquin the residence time of sediment in these drainages indicates that these relief estimates are averaged over the last 100 to 10 ka we suggest that relief change is related to localized surface uplift in the southern sierra nevada including the kings drainage caused by ongoing lower lithosphere removal the san joaquin drainage may be north of the lithospheric anomaly which is consistent with stagnating or decaying relief there

global correlation of middle cretaceous sea level events seven albian sea level curves from localities around the globe have been correlated on the basis of the number and timing of transgressive regressive cycles shown the localities chosen were considered to have been tectonically stable there are 11 time lines placed in the different sections by use of graphic correlation they are based on the position of european ammonite zonal subzonal boundaries projected into sections in north america and the middle east the examined sea level sequences with durations of 0 3 2 7 m y appear to vary regionally and their global correlation cannot be demonstrated with the present stratigraphic resolution frequency trends show that albian sea level was statistically stable from the late early to the early late albian whereas it fluctuated rapidly in earliest and latest albian

cenozoic thermal evolution of the central sierra nevada california from uth he thermochronometry apatite u th he cooling ages are reported for igneous apatite samples from the central sierra nevada and compared to published apatite fission track ages and track length data from the same mineral separates helium ages are youngest at low elevations and increase systematically toward higher elevations ranging from 43 to 84 ma at yosemite valley 32 to 74 ma at kings river canyon and 23 to 75 ma at mt whitney helium ages from high elevation samples are generally concordant with corresponding fission track ages while lower elevation helium ages are substantially younger cooling histories inferred from present laboratory derived fission track annealing and helium diffusion models do not match well suggesting that either helium diffusion rates or fission track annealing rates are miscalibrated at temperatures below about 60 c for geologic exposure periods unlike the fission track results the helium data do not indicate a very low geothermal gradient in the sierra nevada during early to middle tertiary time

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

three dimensional seismic model of the sierra nevada arc california and its implications for crustal and upper mantle composition a three dimensional p wave velocity model of south central california from the coast ranges to the sierra nevada shows that the crust under most of the southern sierra nevada batholith has seismic velocities 5 9 6 3 km s below the continental average the crust is not much thicker on average about 35 km than in the adjacent great valley and basin and range province apart from a small northward thickening crustal root under the western sierra nevada that reaches a depth of 42 km crustal velocities above the continental average are observed beneath much of the great valley due to a high velocity body underlying the sedimentary basin and the foothills metamorphic belt 6 4 7 0 km s upper mantle velocities are generally low 7 8 km s but span a wide range 7 4 8 2 km s we display the velocity model in several cross sections and maps of moho depth and average crustal velocity the measured velocities in the upper and mid crust of the sierra nevada batholith are in good agreement with laboratory measurements on sierra nevada tonalites after corrections for density and temperature peridotite xenoliths from the eastern sierra nevada suggest strong upper mantle anisotropy which could explain some of the velocity heterogeneity in the sierra nevada mantle by the time cretaceous subduction related magmatism ceased the sierra nevada arc must have had a thick mafic lower crust yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout a subcrustal layer with velocities below normal pn velocities 7 6 km s may indicate the presence of lower crustal material in eclogite facies

arrested development erosional equilibrium in the southern sierra nevada california maintained by feedbacks between channel incision and hillslope sediment production tributary creeks of the southern sierra nevada have pronounced knickpoints that separate the landscape into an alternating sequence of gently sloped treads and steeply sloped risers these knickpoints and the surrounding stepped topography suggest that the landscape is still responding to pleistocene changes in base level on main stem rivers we tested this hypothesis using cosmogenic nuclides and uranium isotopes measured in stream sediment from widely distributed locations catchment scale erosion rates from the cosmogenic nuclides suggest that the treads are relict surfaces that have adjusted to a previous base level nevertheless erosion rates of relict interfluves are similar to canyon incision rates implying that relief is unchanging in the lower kings and san joaquin rivers in addition our results suggest that much of the southern sierra nevada is in a state of arrested development the landscape is not fully adjusted to and moreover is not responding to changes in base level lowering in the canyons we propose that this can be explained by a paucity of coarse sediment supply which fails to provide sufficient tools for bedrock channel incision at knickpoints we hypothesize that the lack of coarse sediment in channels is driven by intense weathering of the local granitic bedrock which reduces the size of sediment supplied from hillslopes to the channels our analysis highlights a feedback in which sediment size reduction due to weathering on hillslopes and transport in channels is both a key response to and control of bedrock channel incision and landscape adjustment to base level change

from black and white to colour in the silurian abstract ironstones and iron rich limestones regularly occur as components of time specific intervals of the palaeozoic as well as in younger times brett et al 2012 this issue silurian sediments deposited at high latitudes along the peri gondwana border are characterized by black and white limestone and graptolitic shale sequences those in the carnic alps southern austria additionally contain colourful pink to red limestones and ironstones laminated structures such as the ferruginous coatings around skeletal fragments mostly trilobites and some cephalopods and echinoderms and stromatolitic features along discontinuity surfaces display dark red green white and brownish colours due to the presence of goethite magnetite hematite chamosite calcite and subordinate apatite confocal laser raman microscopy and complementary microscopic analysis of these ferruginous laminated structures document the presence of carbonaceous matter associated with fossilized microbial structures in the form of stromatolites filaments and coccoids suggesting a microbial role in the colouring of the silurian world of the carnic alps iron concentrations up to 30 that of matrix and surrounding non ferruginous rocks suggest blooms of iron microbe activity in response to the time specific occurrence of chemically charged sea water during global biotic events

iron bacterial mediation in phanerozoic red limestones state of the art different pathways can be proposed to explain the origin of the red pigmentation present in many phanerozoic limestones we investigate three possible models 1 telogenetic alteration 2 detrital input and 3 role of iron bacteria the third hypothesis is supported by our data for a number of phanerozoic series by this hypothesis not only can we highlight the importance of the microenvironments at the dysoxic anoxic water sediment interfaces for the bacterial fe oxidation but also can conclude that the dred limestonest have no particular paleogeographic meaning the seven studied red successions are rather poor in iron 350 ppm in the ammonitico rosso without large fluctuations the pigmentation is due to the dispersion of submicrometric hydro oxides now hematite formed by bacterial mediation during early diagenesis in various microaerophilitic environments different ferruginous morphs e g filaments coccoids microstromatolites suggest the presence of iron bacteria the limiting factor is the oxygen content which was low in these very quiet and relatively deep environments the presence of iron bacteria is also confirmed by iron isotope analysis on the ammonitico rosso fe isotopic compositions have been analyzed on residues as well as on leachates of red and grey facies of the same interval d 56 fe values are systematically lower in the red residues varying from 1 19x to 0 34x compared to the grey ones 0 27x to 0 09x this indicates a significant accumulation of lighter isotopes in the red parts suggesting that hematite have been formed via a biological pathway d 2005 elsevier b v all rights reserved

role of extrusion of the rand and sierra de salinas schists in late cretaceous extension and rotation of the southern sierra nevada and vicinity the rand and sierra de salinas schists of southern california were underplated beneath the southern sierra nevada batholith and adjacent mojave salinia region along a shallow segment of the subducting farallon plate in late cretaceous time various mechanisms including return flow isostatically driven uplift upper plate normal faulting erosion or some combination thereof have been proposed for the exhumation of the schist we supplement existing kinematic data with new vorticity and strain analysis to characterize deformation in the rand and sierra de salinas schists these data indicate that the schist was transported to the ssw from deep to shallow crustal levels along a mylonitic contact the rand fault and salinas shear zone with upper plate assemblages crystallographic preferred orientation patterns in deformed quartzites reveal a decreasing simple shear component with increasing structural depth suggesting a pure shear dominated westward flow within the subduction channel and localized simple shear along the upper channel boundary the resulting flow type within the channel is that of general shear extrusion integration of these observations with published geochronologic thermochronometric thermobarometric and paleomagnetic studies reveals a temporal relationship between schist unroofing and upper crustal extension and rotation we present a model whereby trench directed channelized extrusion of the underplated schist triggered gravitational collapse and clockwise rotation of the upper plate

evidence for right lateral transfer faulting accommodating en echelon miocene extension mojave desert california in the southwestern united states two early miocene extensional terranes one along the colorado river trough and the other in the central mojave desert have been inferred to be part of an en echelon rift system linked by a ductile detachment at depth evidence from pre tertiary surface geology in the central and western mojave desert suggests that a zone of east to northeast trending right lateral strike slip faulting may have transferred extension between these two areas these data include the apparent offsets of the late jurassic age independence dike swarm a middle to late jurassic age deformation belt triassic jurassic volcanic rocks lower triassic sedimentary rocks and late paleozoic age stratigraphic and tectonic elements this inferred fault zone probably served as the main shallow crustal kinematic link between extension in the central mojave desert and in the colorado river trough

microbial crust with frutexites and iron staining in chalks albian cenomanian boundary hunstanton uk new petrographic observations and stable isotope data help reinterpret the complex sedimentology of the albian cenomanian boundary exposed in the famous red and white chalks of the cliff section at hunstanton in norfolk uk thin section analysis of a prominent crust at the top of the hunstanton red chalk formation reveals fossil microstructures attributable to the microbe frutexites these red chalk microstructures are less bushy that frutexites sensu stricto and poor preservation in part caused by later diagenetic iron migration means they are identified only tentatively as frutexites stable oxygen isotope values from the crust are similar to those from early diagenetic nodular chalks immediately below the crust and to partially altered chalks elsewhere in norfolk the 18o data are interpreted as albian seafloor depositional values albeit slightly altered by subsequent meteoric diagenesis the microbial affinities of frutexites are not yet proven thus the presence of frutexites alone is not diagnostic of either photic zone or deep water sedimentation however the presence of frutexites suggests that the red colour of the hunstanton red chalk is due at least in part to the mediation of iron fixing microbes in the accumulating chalk sediment at a dysoxic anoxic interface centimetre scale columnar and nodular structures above the frutexites crust that project upwards into the basal paradoxica bed of the overlying ferriby chalk formation are sites of localized syndepositional iron staining these nodules are not stromatolitic or microbial and are not evidence for deposition in shallow water or intertidal settings

geochemical constraints on the origin of enigmatic cemented chalks norfolk uk very hard cemented chalk stacks and crusts found locally in the upper part of the cretaceous chalk of north norfolk uk are related to solution features the solution features mainly pipes and caves formed after deposition of the overlying middle pleistocene wroxham crag probably by routing of sub glacial or glacial melt waters derived from late pleistocene glaciers new geochemical particularly stable isotope data shows that cementation of the chalks although related spatially to the solution features was not caused by glacier derived waters the carbon isotope composition of the chalk cements is typically around 9 5 indicative of biologically active soils moreover the oxygen isotope compositions of the cements around 5 are incompatible with water 18 o values much below 9 to 10 which probably precludes isotopically negative glacier derived water as resulting palaeo temperatures are below zero taken together the isotope data suggest chalk cementation occurred under interglacial conditions similar to the present dissolved calcium carbonate for cementation came from dissolution of reworked chalk in overlying mis 12 glacial tills

late cenozoic uplift of the sierra nevada california a critical analysis of the geomorphic evidence from the earliest days of california geology the ramp like profile of the northern half of the sierra nevada mountains and putative signs of recent incision have been interpreted as evidence that the range was formed by the tilting of a rigid block in the late cenozoic over the years various geomorphic analyses have been used to quantify the magnitude of uplift and to establish its timing such as analyzing the gradients of ancient channels examining the tilt of sedimentary beds and reconstructing the incisional history of rivers most studies that have used these methods have supported substantial 500 m recent uplift of the sierra in contrast investigations based on other sources of paleotopographic information such as isotope records thermochronology and detrital zircon geochronology have found that the sierra have been at high elevations for much of the cenozoic this set of contradictory results motivates a re examination of the geomorphic evidence for late cenozoic uplift a critical assessment of these geomorphic studies based on new topographic analyses and field investigations reveals that their conclusions are not well supported for example several studies based their results on reconstructions of ancient channels that would have flowed up and over bedrock ridges as high as 190 m a physical impossibility other weaknesses include unjustified assumptions regarding the original tilt of fluvial deposits misinterpretations of stratigraphic relationships and inadequate recognition of the effect of lithology on channel profiles the studies supporting recent tilting in the northern sierra nevada are inconclusive and rely on observations not unique to tectonic forcing indeed much of the evidence based on the paleogradients of the tertiary channels is consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies in addition analyses are presented to demonstrate that deep northern sierran canyons thought to have been recently incised were instead cut as early as the eocene oligocene two geomorphic studies from the southern sierra are consistent with late cenozoic tilting and uplift although ongoing tectonic activity may be insignificant finally i present a conceptual model of the evolution of the sierran landscape applicable primarily to the northern half of the range illustrating the development of three different drainage networks since the late jurassic

topographic evolution of the sierra nevada measured directly by inversion of low temperature thermochronology at present there are at least two competing ideas for the topographic evolution of the sierra nevada one idea is that the sierra nevada was formed as a monocline in the cretaceous marking the transition from the great valley forearc basin to the west and a high nevadaplano plateau to the east similar to the west flank of the modern altiplano of the andes both the thermochronologic imprint of local relief and the stable isotopic evidence of a topographic rain shadow support this hypothesis a second idea supported by geomorphic observations suggests that the sierra gained a large fraction of its present elevation as recently as the pliocene this recent surface uplift could have been driven by convective removal of the lower part of the lithosphere the isostatic response to basin and range faulting and or by changes in dynamic topography associated with deep subduction of the farallon plate here we present the first comprehensive analysis of low temperature thermochronology in the sierra nevada which indicates that both ideas are likely correct we show that thermochronology can resolve surface uplift as well as exhumation by measuring long wavelength topographic evolution relative to sea level uplift measurements are possible whenever there is sufficient constraint on vertical rock velocity the tilting of isochrones defined as surfaces of equal cooling age provide this constraint in the sierra nevada three new factors further distinguish our analysis the first is that we allow for the local relief and the long wavelength topography to evolve independently second we use al in hb barometry to constrain the initial depth of emplacement for the sierra nevada plutons third our analysis is tied to a sea level datum by using the paleo bathymetric record of the great valley basin where it transitions to the sierra nevada batholith results indicate that westward tilting of the sierra nevada accounts for 2 km of uplift since 20 ma topographic relief increased by a factor of 2 these findings suggest that the sierra nevada lost elevation through most of the tertiary but regained much of its initial elevation following the onset of surface uplift in the miocene

reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals the paleorelief of mountain belts can be estimated from the 18 o value of authigenic minerals development of relief during mountain building often creates lee side rain shadows in which precipitation is depleted in 18 o and d the magnitude of this rain shadow effect is strongly correlated to relief a compilation of 18 o data from surface waters throughout the globe shows a linear relationship between net elevation change and 18 o r 2 0 79 through the use of this relationship we investigated the timing and magnitude of elevation change in the southern alps of new zealand and the sierra nevada of california the 18 o values of kaolinites from new zealand show an 6 decrease in the early pliocene that corresponds to an 2 km elevation change in the southern alps the 18 o of smectites from the sierra nevada show little change since 16 ma suggesting that these mountains have been a long standing topographic feature

deformation of continental crust along a transform boundary coast mountains british columbia new structural paleomagnetic and apatite u th he results from the continental margin inboard of the queen charlotte fault 54 n delineate patterns of brittle faulting linked to transform development since 50 ma in the core of the orogen 250 km from the transform north striking dip slip brittle faults and vertical axis rotation of large crustal domains occurred after 50 ma and before intrusion of mafic dikes at 20 ma by 20 ma dextral faulting was active in the core of the orogen but extension had migrated toward the transform continuing there until 9 ma local tilting in the core of the orogen is associated with glacially driven post 4 ma exhumation integration with previous results shows that post 50 ma dextral and normal faulting affected a region 250 km inboard of the transform and 300 km along strike initially widespread the zone of active extension narrowed and migrated toward the transform 25 ma after initiation of the transform while dextral faulting continued throughout the region differential amounts of post 50 ma extension created oroclines at the southern and northern boundaries of the deformed region this region approximately corresponds to continental crust that was highly extended just prior to transform initiation variation in neogene crustal tilts weakens interpretations relying on uniform tilting to explain anomalous paleomagnetic inclinations of mid cretaceous plutons similarities to the gulf of california suggest that development of a transform in continental crust is aided by previous crustal extension and that initially widespread extension narrows and moves toward the transform as the margin develops

evidence for post early miocene initiation of movement on the garlock fault from offset of the cudahy camp formation east central california the cudahy camp formation located in the el paso basin east central california consists of a 350 m thick volcanic and sedimentary sequence of early miocene age previous investigators determined from paleocurrent indicators that the source for these rocks was to the south and southeast but failed to suggest a specific location for that source restoration of 64 km of left slip on the garlock fault places cudahy camp rocks north and northwest of the early miocene eagle crags volcanic field the unique geochemical nature of basalts found in both locations coincident ages of volcanic rocks identical clast content of volcanic tuffs and identical stratigraphic sequence indicate that the source for the cudahy camp formation was the eagle crags volcanic field this is the first documented occurrence of miocene rocks offset by the garlock fault because the cudahy camp rocks and their sources appear to be offset by the full displacement on the garlock fault we conclude that movement on the garlock started after earliest middle miocene time further constraining initiation of movement on the garlock fault adds significantly to fully understanding the evolution of the western north american plate boundary in the middle to late cenozoic

radiation damage control on apatite u th he dates from the grand canyon region colorado plateau individual detrital apatite grains from the esplanade coconino and moenkopi formations in the grand canyon region of the colorado plateau yield u th he dates from 104 to 5 ma the range of dates within each unit far exceeds analytical uncertainty but correlates with both he concentration he and effective u concentration eu these dates are all signifi cantly younger than the sandstone units indicating partial to complete he loss following deposition recently published laboratory diffusion data suggest that he retentivity in apatite increases with radiation damage forward models predict that the consequences of this effect will be manifested most clearly as a correlation between u th he dates and the he and eu in suites of apatites that 1 are characterized by a large span of eu and 2 had thermal histories in which suffi cient time elapsed for the apatite he diffusion kinetics to diverge prior to reheating and partial resetting apatites in the sedimentary units investigated fi t these cri teria using geologically reasonable deposition burial and unroofi ng histories simulations that include the effect of radiation damage on apatite he retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations these results suggest that a span of u th he dates positively correlated with eu may provide important information regarding a sample s thermal history

the influence of natural radiation damage on helium diffusion kinetics in apatite abstract stepwise degassing diffusion experiments on 39 different apatite samples using radiogenic 4he and proton induced 3he reveal a range in closure temperature tc from 50 to 115 c for a cooling rate of 10 c myr there is no correlation between helium diffusion and apatite chemistry including f cl ratio but the closure temperature is positively correlated with the radiogenic 4he concentration 4he in each sample we argue that 4he is a proxy for a sample s natural exposure to actinide radioactivity below the closure temperature and that helium diffusion in apatite is impeded by radiation induced damage to the apatite structure the kinetics must therefore be an evolving function of time measured diffusivities thus reflect a snapshot in time and cannot alone be applied to the thermochronometric interpretation of a given sample the effect of radiation damage on helium diffusion appears to far exceed other known controls on helium diffusivity including grain size our diffusion data are well described by a previously proposed quantitative model that consists of two arrhenius relations one for volume diffusion through undamaged mineral structure and one for release of helium from radiation damage traps the unknown parameters in this trapping model were determined from the diffusion experiments and allow us to develop a tentative mathematical function that relates diffusivity to temperature and 4he by inserting this function into a 4he production diffusion model we have explored how these results affect the interpretation of apatite u th he thermochronometry the model predicts that the effective 4he closure temperature of apatite will vary with cooling rate and effective u concentration eu and may differ from the commonly assumed tc of 70 c by up to 15 c the 4he partial retention zone will look similar to previous expectations but its depth will depend on accumulation time and on eu most notably samples subjected to reheating after accumulation of substantial radiation damage will be more retentive than previously expected these predictions are consistent with recent observations of unexpected apatite u th he ages in some settings

carson pass kirkwood paleocanyon system paleogeography of the ancestral cascades arc and implications for landscape evolution of the sierra nevada california tertiary strata of the central sierra nevada are dominated by widespread voluminous volcanic breccias that are largely undivided and undated the origin of which is poorly understood these dominantly andesitic strata are interpreted to be eruptive products of the ancestral cascades arc deposited and preserved within paleocanyons that crossed the present day sierra nevada before basin and range faulting began there these strata are thus important not only for understanding the paleogeography of the ancestral cascades arc but also for reconstructing the evolution of the sierra nevada landscape our regional scale mapping shows that paleocanyon fills of the central sierra nevada are dominated by intrusions and vent proximal facies along the present day sierran crest with more distal facies extending westward down the paleocanyons vent proximal facies consist of lava domes that collapsed to generate block and ash flow tuffs which in turn were remobilized down canyon to produce coarse grained volcanic mudflow and dilute flow fluvial deposits lava flows are rare instead magmas invaded wet volcaniclastic sediment to form in situ peperite piles that were partly remobilized to form debris flows or block and ash flows with peperite domains detailed mapping and dating of previously undifferentiated tertiary strata in the carson pass kirkwood area of the central sierra nevada has allowed us to identify 6 unconformity bounded sequences preserved within a paleocanyon as deep as 650 m cut into mesozoic granitic basement the carson pass kirkwood paleocanyon trends ne sw with a paleo transport direction roughly parallel to the modern mokelumne river drainage toward the sw sequence 1 consists of oligocene silicic ignimbrites sourced from nevada sequences 2 through 6 consist of dominantly andesitic rocks of the miocene ancestral cascades arc with 40 ar 39 ar ages ranging from 14 69 0 06 ma to 6 05 0 12 ma these new dates provide constraints on the ages of unconformities vertical relief on the unconformities within the paleocanyon ranges from 12 to 303 meters with paleoslope gradients range from 3 to 48 no evidence exists for widening or deepening of the paleocanyon into mesozoic basement during tertiary time so we infer that unconformity 1 the paleocanyon floor and walls was inherited from cretaceous time the deepest unconformities within the paleocanyon fill reincised into granitic basement in the early miocene unconformity 2 between 14 and 10 ma unconformity 5 and between 10 and 6 ma unconformity 6 the paleocanyon was also beheaded cut off from sources to the east by ca 10 ma we suggest that the early miocene reincision records tectonism related to the onset of arc magmatism in the sierra nevada the middle miocene reincision may record the onset of basin and range faulting in the central sierra the late miocene reincision may correspond to uplift attendant with the northward sweep of the triple junction through the latitude of the central sierra paleocanyons of the central sierra differ from those of the northern and southern sierra by showing steeper local paleorelief and the bouldery stream deposits attest to higher axial gradients than envisioned for other parts of the range this may indicate that the uplift history of the range is not uniform from segment to segment

an evaluation of the scsn moment tensor solutions robustness of the mw magnitude scale style of faulting and automation of the method we have generated moment tensor solutions and moment magnitudes m w for 1700 earthquakes of local magnitude m l 3 0 that occurred from september 1999 to november 2005 in southern california the method is running as an automated real time component of the southern california seismic network scsn with solutions available within 12 min of event nucleation for local events the method can reliably obtain good quality solutions for m w with m l 3 5 and for the moment tensor for events with m l 4 0 the method uses the 1d time domain inverse code tdmt invc software package dreger 2003 the green s functions have been predetermined for various velocity profiles in southern california which are used in the inversion of observed three component broadband waveforms 10 100 sec using data from at least four stations moment tensor solutions have an assigned quality factor dependent on the number of stations in the inversion and the goodness of fit between synthetic and observed data if a minimum quality factor is attained the m l or m w is 5 0 or greater and if the event is in the southern california reporting regions the m w will be the official scsn cisn magnitude the m w from the high quality solutions determined from our method generally correlate very well with reviewed m l except in regions at the perimeter of the network the m w reported here indicates the scsn m l systematically underestimates the magnitude in the mexicali region of baja california mono lakes area coso region and the brawley seismic zone and overestimates the magnitude in the coastal ranges comparisons of the moment tensors determined using this model are made with harvard centroid moment tensors generated for larger earthquakes in the california region and recent 3d models for events in the los angeles region with excellent correlation most of the earthquakes with good quality solutions exhibit strike slip faulting in particular along the major late quaternary strike slip faults thrust faulting on east west striking planes is observed along the southern edge of the transverse ranges while northwest striking thrust faulting is observed in the coastal ranges normal faulting is most common in baja california and southern sierra nevada including the western basin and range region poor quality solutions with unreliable m w are caused by excessive background noise in the waveforms small events m l 4 0 can be affected by ambient noise or teleseisms but larger events can also have unreliable solutions if they follow a recent large regional event

stable isotope composition of waters in the great basin united states 1 air mass trajectories 1 isentropic trajectories calculated using the noaa climate monitoring and diagnostics laboratory s isentropic transport model were used to determine air parcel origins and the influence of air mass trajectories on the isotopic composition of precipitation events that occurred between october 1991 and september 1993 at cedar city utah and winnemucca nevada examination of trajectories that trace the position of air parcels backward in time for 10 days indicated five distinct regions of water vapor origin 1 gulf of alaska and north pacific 2 central pacific 3 tropical pacific 4 gulf of mexico and 5 continental land mass deuterium d and oxygen 18 18o analyses were made of precipitation representing 99 of all cedar city events similar analyses were made on precipitation representing 66 of the precipitation falling at winnemucca during the same period the average isotopic composition of precipitation derived from each water vapor source was determined more than half of the precipitation that fell at both sites during the study period originated in the tropical pacific and traveled northeast to the great basin only a small proportion traversed the sierra nevada the isotopic composition of precipitation is determined by air mass origin and its track to the collection station mechanism of droplet formation reequilibration within clouds and evaporation during its passage from cloud to ground the rayleigh distillation model can explain the changes in isotopic composition of precipitation as an air mass is cooled pseudo adiabatically during uplift however the complicated processes that take place in the rapidly convecting environment of cumulonimbus and other clouds that are common in the great basin especially in summer require modification of this model because raindrops that form in the lower portion of those clouds undergo isotopic change as they are elevated to upper levels of the clouds from where they eventually drop to the ground

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

modeling the miocene climatic optimum part i land and atmosphere abstractthis study presents results from the community climate system model 3 ccsm3 forced with early to middle miocene 20 14 ma vegetation topography bathymetry and modern co2 a decrease in the meridional temperature gradient of 6 5 c and an increase in global mean temperature of 1 5 c are modeled in comparison with a control simulation forced with modern boundary conditions seasonal poleward displacements of the subtropical jet streams and storm tracks compared to the control simulation are associated with changes in hadley circulation and significant cooling of the polar stratosphere consistent with previously predicted effects of global warming energy budget calculations indicate that reduced albedo and topography were responsible for miocene warmth in the high latitude northern hemisphere while a combination of increased ocean heat transport and reduced albedo was responsible for relative warmth in the high latitude southern hemisphere compared to the present model data analysis suggest

quaternary reactivation of the kern canyon fault system southern sierra nevada california the kern canyon fault the longest fault in the southern sierra nevada is an active structure and has been reactivated at discrete times over the past 100 m y in response to changing lithospheric stresses after initiation as a cretaceous transpressional structure the kern canyon fault transitioned into a dextral strike slip shear zone that remained active as it was exhumed into the brittle regime during regional late cretaceous uplift of the sierra nevada batholith the kern canyon fault was reactivated during miocene regional extension as part of a transfer zone between two differentially extending domains in the southern sierra nevada subsequent normal displacement along the fault began in pliocene time new evidence for fault activity which continued into late quaternary time includes its current geomorphic expression as a series of meters high west side up scarps that crop out discontinuously along the fault s 130 km length relocated focal mechanisms of modern earthquakes confirm ongoing normal faulting and geodetic measure ments suggest that the sierra nevada is uplifting relative to the adjacent valleys this evidence for recent activity overturns a long held view that the kern canyon fault has been in active for more than 3 5 m y its reactivation indicates that deformation repeatedly localized along a pre existing crustal weakness a cretaceous shear zone we propose that a system of inter related normal faults including the kern canyon fault is responding to mantle lithosphere removal beneath the southern sierra nevada region the location of the active kern canyon fault within the sierra nevada great valley microplate indicates that deformation is occurring within the microplate

collapse of southwestern north america and the evolution of early miocene detachment faults metamorphic core complexes the sierra nevada orocline and the san andreas fault system plate reconstructions indicate that early miocene pacific north american relative motion was not parallel to the transform but was instead oblique and transtensional we propose that the western edge of the north american plate east of the transform gravitationally collapsed to the southwest in response to this divergence the northern and eastern edges of the collapsed region underwent extension between 24 and 20 ma whereas extension in southeastern california and southern arizona during this interval was coaxial with the overall s50 60 w collapse direction the extension direction along the northern oundary mojave extensional belt was north south and the result of the combined effects of the southwestward collapse and the independent northwestward translation of the region to the north collapse continued between 20 and 16 ma and extension persisted in southeastern california and southern arizona extension along the northern boundary waned owing to the apparent cessation of motion of the region to the north thus the kinematics of the northern boundary changed to dextral shear and extension along what is here named the trans mojave sierran shear zone this east west zone of mainly right shear oroclinally folded the southern tail of the sierra nevada and the central mojave desert rocks within this 83 110 km wide zone were rotated approx 40 60 clockwise about vertical axes finally as collapse moved the north american plate progressively west across the locus of transform shear its western edge became increasingly subject to the motion of the pacific plate over time the edge of north america was truncated by faults of the san andreas system and transferred to the pacific plate

mantle instability beneath the sierra nevada mountains in california and death valley extension the southern sierra nevada mountain range rapidly uplifted at 3 5 ma simultaneously with a pulse of basaltic volcanism xenoliths recovered from volcanics indicate that the range lost a dense crustal root after the miocene the vertical motions and removal of the root have been linked to a fast seismic velocity anomaly that extends 200 km into the mantle but is offset to the west of the range with visco elasto plastic thermo mechanical numerical models we have tested the influence of crustal strength on the kinematics of removal and on the amount of associated uplift we find that delamination of the dense root is the most likely mechanism for gravitational instability to occur the model satisfies the plio quaternary vertical motions the shift of the mantle anomaly to the west of the range and intense miocene extension to the east based on those results we propose the existence of a dynamic link between the sierra nevada mantle instability and death valley rifting

lateral migration of a foundering high density root insights from numerical modeling applied to the southern sierra nevada abstract the southern sierra nevada is a geodynamically complex region where several models have been proposed to explain the rapid removal of lithospheric mantle occurring sometime between 8 0 and 3 5 ma tomographic studies show the presence of an east dipping slab shaped fast seismic anomaly reaching to about 300 km depth below the western sierras and great valley and receiver function studies indicate a broad region of lithosphere removal this work presents thermo mechanical modeling of asymmetric foundering of a high density batholithic root with lateral intrusion of asthenospheric material the predicted evolution is controlled by a the upwelling of buoyant asthenosphere facilitated by the presence of a weakened lithospheric mantle adjacent to a dense batholitic root b the westward inflow enabled by a low viscosity lower crust and c negative buoyancy of a batholithic dense root the dynamics of these models can be characterized as a slowly migrating lithosphere foundering process driven by the density anomaly of the ultramafic root but controlled by the magnitude of the lower crustal viscosity which determines the rate at which asthenospheric material can flow into the opening lower crustal gap final model predicted upper mantle structure is compatible with existing tomographic images and the observed v shape geometry of the moho below the western margin of the southern sierra nevada model predicted topography is also generally consistent with observations and shows a monotonous uplift of the high region since 7 ma and presently ongoing and an area of maximum subsidence west of the area of the v shaped moho due to the pull exerted by the sinking of the high density root

rain shadow development during the growth of mountain ranges an atmospheric dynamics perspective 1 an idealized atmospheric model is used to explore the links between climate and topography in the development of orographic rain shadows during orogenesis the atmospheric dynamics theory of density stratified fluid flow over topography is used to interpret the results the controlling nondimensional parameter is nh u where n is the brunt vaisala frequency a measure of atmospheric stability h is the terrain relief and u is the initial horizontal wind speed rain shadow development is found to be a nonlinear and nonunique function of both topography and atmospheric state indicating that geological records of orographic aridity cannot be interpreted in terms of relief alone when upstream topography exceeds nh u 1 during surface uplift downstream orographic precipitation vanishes and downstream orographic cloud mass decreases by as much as 90 upstream blocking of air flow can generate a forward projecting rain shadow in which a relatively low ridge nh u 1 may be decoupled from the atmospheric flow by a zone of flow stagnation extending upstream of the high terrain such an effect may occur if the valley separating the two ranges is narrower than the length scale of flow stagnation in the model configuration used here lateral widening of a relatively low nh u 1 range increases downstream cloud mass by up to a factor of 3 these results help to refine interpretations of climate tectonic interactions in shaping the geological record of the sierra nevada and andes

paleo geomorphology of the sierra nevada california from u th he ages in apatite new apatite u th he ages from the central sierra nevada california place limits on the morphology and evolution of longitudinal profiles of major transverse river drainages developed in the late cretaceous helium ages from a new orogen parallel constant elevation sample transect are relatively uniform 60 ma and not correlated with topography unlike those from a similar lower elevation transect approx 15 km to the west we interpret the marked difference in the two orogen parallel profiles to reflect a headward decrease in long wavelength transverse relief at the time of cooling consistent with either a concave up stream gradient typical of those observed near the headwaters of modern sierran trunk streams or a convex gradient like those found along slope breaks of eroding plateau edges from the san joaquin drainage we obtained a new helium age versus elevation profile which has a comparable slope to those previously reported for the yosemite valley kings canyon and mt whitney areas this new profile yields slightly older ages at a given elevation as expected from its position adjacent to the largest major drainage the age versus elevation profiles imply that the mean denudation rate of the region in the cenozoic was about 0 04 to 0 05 mm yr we show that long wavelength 70 km relief inferred from longitudinal helium age variations is a strong function of erosion rate such that even relatively subdued relief is detectable with helium age profiling provided that denudation rates were low using the rate implied by sierran age versus elevation profiles we infer long wavelength relief of 1500 500 m in the late cretaceous by analogy with modern orogenic plateaus this value of long wavelength relief suggests a cretaceous interior cordilleran plateau lay at an elevation of at least 3000 m