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

magnetic fingerprint of the late holocene inception of the rio de la plata plume onto the southeast brazilian shelf abstract measurement of changes in the assembly of magnetic particles in sediment records is useful both for paleoclimatic and paleoceanographic reconstructions and for assessing provenance and pathways of sediment transport in this study we provide detailed rock magnetic analysis along with scanning electron microscopy sem ti ca ratio and particle size distribution of the decarbonated fraction of sediments to trace terrigenous source changes during the last 6 cal kyr bp in core 7610 collected on the se brazilian continental shelf magnetic mineralogy data indicate a gradual shift between two distinct sources of sediments 1 dust from southern south america patagonia central western argentina and puna altiplano plateau and 2 sediments eroded from the weathered volcanics of parana basin these materials reached the continental margin off argentina and uruguay by different pathways and were transported northward by the brazilian coastal current bcc the chronology of events related to the environmental record of core 7610 indicates that between 6 and 4 7 cal kyr bp dry conditions over continental south america resulted in enhanced eastward eolian transport of the argentinean loess and a limited discharge of the rdlp river load into the atlantic ocean at this time the sedimentary magnetic fraction that reached the se brazilian shelf reflect predominantly loessic sources transported from the pampean terraces to the argentinean and uruguay shelves and then distributed northward by the shelf current system from 4 7 to 2 cal kyr bp the climate became gradually wetter over the continent and the contribution of oxidized phases from the weathered parana basin mainly from the basaltic and acidic volcanics started to become important this change in climate overlaps in time a with the late holocene sea level fall recorded along the se south american coast later from 2 up to 0 9 cal kyr bp even wetter conditions were prevalent in s and se south america and the rdlp sedimentary load became the main source of sediments transported to the se brazilian shelf by the bcc the magnetic record of core 7610 reported here together with other paleoenvironmental tracers palynology geochemistry and stable isotopes thus indicates that the inception of the rdlp plume onto the se brazilian shelf started as early as 4 7 cal kyr bp and has been strengthened since then mostly due to changes in precipitation over south america and to a smaller extent by sea level changes

coastal evolution changes in sea level and molluscan fauna in northeastern argentina during the late quaternary abstract the northern argentine shoreline underwent a remarkable progradation during the last 6890 years equivalent to about 4000 km 2 the shoreline moved approximately 30 km seaward on the average two types of accretionary shorelines were formed high energy environments produced barrier islands and barrier spits which began to form approximately synchronously low energy environments located between those of high energy consisted of tidal flats and salt marshes four groups of molluscan species represent the high energy palaeoenvironments of the barrier islands and barrier spits the other fossils represent a low energy environment of tidal flats the formation of the barrier islands began 6000 years b p it ended 3500 years ago due to a slight fall in relative sea level of 1 5 m above m s l and subsequent formation of barrier spits seaward as a consequence a very rapid faunal change occurred in less than 5000 6000 years the dominant high energy fossil fauna was replaced by a modern low energy fauna over most of the area palaeoenvironmental changes were probably greater and more rapid during the high energy episode when the fossils were more varied than those living now in the same area at samborombon bay whilst along the oceanic coastal area the molluscan fauna increased in diversity to the more varied extant one

multi proxy evidence of late quaternary environmental changes in the coastal area of puerto lobos northern patagonia argentina abstract a multidisciplinary approach involving geomorphology taxa composition of mollusks and taphonomy performed on bivalve shells linked to mineralogical and microstructural analyses provide evidence of late quaternary environmental changes in the coastal area of puerto lobos in northern patagonia digital elevation models dems showed a beach ridge system developed parallel to the coast indicating sea level variations during mis1 mis5e and earlier in the pleistocene these ridges can be correlated with other marine ridges along the patagonian coast showing a similar geomorphological pattern and similar temporal evolution in relation to faunal composition when comparing the pleistocene with the holocene the most relevant differences are the presence of tegula atra and mactra patagonica in the pleistocene sediments and a diversification of taxa in the holocene during the late holocene the coastal area of puerto lobos also recorded a faunal shift in which species belonging to the magellan province displaced the fauna of the argentinean province to the north probably in coincidence with the little ice age lia taphonomic analysis on glycymeris longior and venus antiqua shells two common taxa in these beach ridges indicate greater energy in the depositional environment correlated with the youngest holocene beach ridge these shells exhibited taphonomic differences between them in relation to fracturing explained on the basis of higher elasticity in the g longior shell than in v antiqua shell because of its crossed lamellar microstructure

a critical review of mid to late holocene sea level fluctuations on the eastern brazilian coastline abstract the eastern coast of brazil is 7000 km long and has been the subject of systematic paleo sea level investigations for more than 35 years more than 1000 samples have been radiocarbon dated and paleo sea level trends have been determined for 14 coastal sectors these trends have in common a mid holocene sea level maximum pmt above present sea level and a subsequent fall to the present time the time and elevation of the pmt the time when relative sea level rose above present mean sea level and the nature of late holocene sea level fall can however differ significantly discrepancies are observed not only between neighboring coastal sectors but also between different studies in a same coastal sector this paper discusses all the key radiocarbon dated samples used to establish regional trends in relative sea level on the eastern coast of brazil it is concluded that although many of the key sea level indicators are imprecise in time and space there is widespread evidence 30 of the data set for a progressive decline possibly with uneven rates of relative sea level since the end of the pmt

the post last glacial maximum transgression in the de la plata river and adjacent inner continental shelf argentina abstract deposits resulting from the last glacial maximum transgression have been studied in the lowermost regions of the del plata fluvial basin in eastern argentina which include the de la plata river and adjacent inner shelf and coastal plains after applying the concepts of sequence stratigraphy those deposits were considered as a unit and defined as a depositional sequence composed of two systems tracts respectively representing the transgressive event and the highstand of sea level the sequence was deposited in different but genetically linked depositional environments such as high and low energy coasts with coastal barriers beach ridges coastal plains estuaries and deltas as well as inner shelf the boundary between the sequence and the underlying pre transgressive units is represented by the transgressive surface or unconformity modeled by the ravinement process which extends from the outer edge of the shelf to the coast this surface is time transgressive and can be recognized by distinct physical characteristics above and below it depending on the successive sedimentary environments developed in deep shallow or coastal waters as sea level fluctuated

stateline fault system a new component of the miocene quaternary eastern california shear zone the eastern california shear zone is an active north northwest trending zone of intraplate right lateral shear that absorbs 25 of pacific north america relative plate motion the stateline fault system sfs which includes several previously recognized discontinuously exposed quaternary structures along the california nevada border is in this paper defined as a continuous 200 km long zone of active dextral shear that includes from south to north the mesquite pahrump and amargosa valley segments recognition of this system expands the known extent of the eastern california shear zone 50 km to the east northeast from its traditionally recognized boundary along the death valley fault system proximal volcanic and rock avalanche deposits offset across the mesquite segment of the sfs indicate 30 4 km of slip on this structure since 13 1 0 2 ma this offset is an order of magnitude larger than previous estimates across this section of the sfs but it is consistent with larger offsets previously proposed for the central and northern sections the total offset and averaged slip rate since mid miocene time 2 3 0 35 mm yr are similar to those of other major faults across this portion of the basin and range which from east to west include the death valley panamint valley hunter mountain and owens valley fault systems however in contrast to these faults the average post mid miocene slip rate on the sfs is approximately twice that estimated from present day geodetic observations and an order of magnitude greater than estimates of average post mid pleistocene slip rates this discrepancy between long term short term and geodetically derived slip rates differs from other geologic geodetic slip rate discrepancies in the eastern california shear zone where geodetic slip rates are significantly faster than both long term and short term geologic slip rates this suggests that either the slip rate on the sfs has diminished over time such that the system is an abandoned strand of the relatively young eastern california shear zone or that the present day slip rate represents a transient period of slow slip such that strands of the shear zone must accommodate a complex spatial and temporal distribution of slip

sea level fluctuations during the last 8600 years in the de la plata river argentina abstract the de la plata river considered from the geomorphologic point of view can be defined as a unit composed of a set of features genetically linked together which were formed as a result of the post last glacial maximum transgression regional stratigraphical morphological and topographical studies carried out in the de la plata river and surrounding areas of eastern argentina determined that the holocene evolution of the area was the result of processes resulting from relative sea level fluctuations in order to date the different stages of evolution a curve of sea level fluctuations was constructed after a selective choosing of samples with reliable ages supported by field geological evidence the curve shows a rapid initial transgressive event and the maximum highstand of mean sea level at 6000 yr bp at a relative elevation of 6 5 m above present the following regressive event was characterized by a discontinuous fall in sea level geomorphological evidence suggests that sea level fall could have experienced minor changes in rate as well as moments of stability that are not shown by the curve short interruptions in the geological record can be explained either by the same minor changes in sea level or by a change in wave regime correlation of transgressive regressive events with paleoclimatic and biogeographic information proposed that holocene sea level fluctuations in the de la plata river were mainly controlled by climatic changes

warmer or cooler late holocene marine palaeoenvironments interpreting southeast australian and brazilian sea level changes using fixed biological indicators and their δ18o composition abstract relic inter tidal assemblages of sessile organisms which are referred to as fixed biological indicators fbis are shown to be useful proxies for late holocene climatic and environmental changes as well as direct indicators of short term fluctuations in sea level a detailed comparison of change in the inter tidal assemblages of two southern hemisphere sites at comparable latitudes port hacking new south wales south east australia and the laguna imbituba region of southern brazil indicates periods of marked similarities in the timing and nature of changes at 5200 3800 cal years bp and differences after 2400 cal years bp there is sufficient concurrence of cooling phases and falling sea levels to strongly suggest that in some cases global rather than regional forcing factors are at work the value of inter tidal organisms as indicators of changed environments is developed by identifying the past and present distributions of the common marker species tubeworms barnacles and mussels the 18 o content of these extant specimens at port hacking confirmed changes in ocean water temperature and associated sea level behaviour corresponding to invasion or extinction of temperature sensitive species a similar pattern is repeated although in a less marked fashion in the laguna imbituba region of southern brazil

holocene benthic foraminifera from bahia blanca estuary a review and update of systematic and palaeoenvironmental aspects assemblages of benthic foraminifera in a holocene core from the outer area of bahia blanca estuary in argentina were studied the systematic of the group was updated and provided 28 genera distributed among 59 species six species with nomenclature aperta the main species recorded were buccella peruviana f campsi boltovskoy ammonia beccarii linne elphidium gunteri cole elphidium galvestonense kornfeld elphidium articulatum d orbigny and elphidium discoidale d orbigny these species allowed an estuarine environment to be determined qualitative and quantitative studies conducted on the faunistic content in the core allowed identification of three subenvironments closely linked to sea level fluctuations during the mid holocene a lower zone 6350 yr bp corresponding to a high intertidal environment an intermediate zone 2460 yr bp representing a low intertidal environment related to a system of channels and variations in environmental energy and an upper zone characterized by the developmen

variation of coastal dynamics during the last 7000 years recorded in beach ridge plains associated with river mouths example from the central brazilian coast abstract the reconstruction of relative sea level changes during the last 7000 years has shown that the central brazilian coast was been subjected to submergence until approximately 5100 yr b p however the emergence after 5100 yr b p was interrupted by two important fluctuations with an amplitude of approximately 2 3 m and a duration of 200 300 yr this evolutionary history which is quite different from that of several other regions in the world played an essential role in the development of the central brazilian coastal plains whether or not they are associated with important river mouths the periods of submergence characterized by erosional phases introduced noticeable changes in the geometry of the coastal deposits the periods of emergence gave rise to sandy terraces covered by beach ridges whose orientation is determined by the longshore drift elsewhere on a low sandy coast the direction of longshore sand transport depends on the orientation of the swell in a coast subjected to various swell patterns the efficient swells are defined as those which determine the resulting longshore transport direction such swells are not necessarily the most prevalent for instance along the central brazilian coast the southern sector waves in spite of their infrequent occurrences are much more powerful than the northern sector waves and therefore generate a longshore sand transport from south to north when fossil beach ridges are present as is the case on the central brazilian coast their geometry reflects the past directions of longshore sand transport this makes it possible to determine the provenance of past efficient swells and to establish the past wind patterns a detailed study of the beach ridge geometry of some sections of the central brazilian coast showed a sequence of reversals of the longshore drift during the last 5100 yr with a duration ranging from 10 to 100 yr these reversals represent changes in the direction of effective waves which determine the longshore transportation of sediments and consequently indicate changes in the wind pattern

new age controls on initiation and timing of foreland belt thrusting in the clark mountains southern california new geochronologic data from the clark mountain area eastern california better define the regional development of the cordilleran magmatic arc and the timing of hinterland and foreland deformation at this latitude the new ages show that sevier belt thrusting started in the clark mountain thrust complex in late jurassic time this is consistent with other studies that suggest a similar age of initiation of the sevier belt in areas to the north the clark mountain thrust complex is located at the intersection of the mesozoic magmatic arc and the east sierran and sevier thrust belts new u pb zircon ages for granitic rocks establish the timing of deformation within the thrust complex the pachalka thrust structurally the highest fault in the complex carries plutonic rocks eastward above cambrian rocks hanging wall and footwall rocks are mylonitic near the thrust hanging wall plutonic rocks previously considered precambrian are dated at 146 62 ma and thus are late jurassic folds kinematically related to thrusting in footwall metasedimentary rocks are overturned eastward and cut by the pachalka pluton dated at 142 7 ma this brackets thrusting and related deformation in structurally high levels of the complex to be latest jurassic the ivanpah and striped mountain plutons in the ivan pah mountains have been dated at 147 7 ma and 141 11 ma respectively these plutons are cut by the keaney mollusk mine and morning star mine thrusts which are in turn cut by the middle to late cretaceous teutonia batholith these data indicate that these thrusts are early to middle cretaceous in age

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

case of the phantom foredeep early cretaceous in west central utah a reconstruction of the structure in west central utah shows that a major foredeep with sedimentary deposits possibly as thick as 6 km now eroded probably existed at the present site of the sevier desert during early cretaceous time and conceivably during late jurassic time concurrent emplacement and erosion of the canyon range thrust sheet provided the load and sediment much like the willard paris thrust sheet did for the better documented early cretaceous foredeep in northern utah and eastern idaho the onset of major thrusting in western utah may have been as early as callovian or as late as albian a 50 m y time span

basement involved thrust faulting in a thin skinned fold and thrust belt death valley california usa although dominated by a strong late tertiary extensional overprint the turtlebacks of the black mountains of death valley display evidence for earlier possibly laramide age basement involved thrust faulting the thrust faults consist of now highly deformed contacts in which 200 m thick bodies of basement gneiss structurally overlie upper proterozoic metasedimentary rock in addition the faults contain locally preserved east directed mylonitic fabrics in their footwalls that are demonstrably early tertiary or older the association of these fabrics with laramide age pegmatite suggests that they may have formed concurrently with the intrusions these basement thrust faults are significant because they formed in a region long considered to have exclusively thin skinned deformation their presence also places new constraints on this part of the fold and thrust belt s preextension geometry by limiting the amount of late tertiary extension specifically these faults disallow the correlation of thin skinned structures that currently exist to the east and west of the black mountains

phyllosilicate and hydrated silica detections in the knobby terrains of acidalia planitia northern plains mars here we report detections of fe mg phyllosilicates and hydrated silica in discrete stratigraphic units within the knobby terrains of acidalia planitia made using data acquired by compact reconnaissance imaging spectrometer for mars fe mg phyllosilicates are detected in knobs that were eroded during southward retreat of the dichotomy boundary a second later unit now eroded to steep sided platforms embaying the knobs contains hydrated silica which may have formed via localized vapor weathering thin film leaching or transient water that resulted in surface alteration these are then overlain by smooth plains with small cones hypothesized to be mud volcanoes which previous studies have shown to have no hydrated minerals in spite of acidalia s location within the putative northern ocean collectively the data record a history of aqueous processes much like that in the southern highlands with progressively less intensive aqueous chemical alteration from the noachian to amazonian

comment on the butte valley and layton well thrusts of eastern california distribution and regional significance by chester t wrucke calvin h stevens and joseph l wooden

40ar 39ar analyses of artificially mixed micas and the treatment of complex age spectra from samples with multiple mica populations abstract artificial mixtures of two age populations of muscovite and of biotite were prepared at relative weight percents of 3 1 1 1 and 1 3 and analyzed by the 40ar 39ar vacuum furnace step heating method the original micas are late jurassic and late cretaceous samples that yield flat age spectra with plateau ages defined by 97 100 of the gas release and no evidence for excess argon the age spectra from mica mixtures yield patterns that systematically decrease with decreasing jurassic mica content and increasing cretaceous component however the mixed muscovite spectra are relatively flat consistent with the two original samples showing similar degassing patterns during vacuum furnace heating in contrast the mixed biotite spectra are highly discordant with an overall decrease in age during the step heat analysis this is consistent with the degassing patterns of the original mica samples which indicate the jurassic biotite high fe mg outgases at lower temperatures and the cretaceous biotite low fe mg remains retentive until the higher temperature steps the individual degassing patterns and compositions show consistencies with known compositional controls on argon diffusivity indicating that micas may maintain argon retention characteristics rooted in crystal chemistry during vacuum furnace heating all but one of the mixed samples failed to yield ages reflecting the age of the original micas these results imply that complex age spectra obtained for multiply deformed rocks with multiple mica populations may define geologically meaningless ages as a result of simultaneous degassing i e mixing of sample reservoirs during heating comparison of laboratory degassing rates and crystal chemistry indicates compositional controls on argon retention are likely preserved during vacuum furnace step heating and micas may degas retain argon in a predictable manner these conclusions support assertions that recovery of fossil age gradients in slowly cooled samples may be possible using furnace step heating

mid holocene evolution and paleoenvironments of the shoreface offshore transition north eastern argentina new evidence based on benthic microfauna abstract a sedimentary record spanning 5792 5511 cal yr bp and 3188 2854 cal yr bp was recovered at 36 45 43 s 56 37 13 w south west south atlantic the sedimentological features and micropaleontological benthic foraminifera and ostracoda content were analyzed in order to reconstruct paleoenvironmental conditions considerable environmental fluctuations are indicated by all these proxies five different stages were distinguished stage 1 ca 5800 5000 cal yr bp consists of muddy sand with abundant microfossils in this interval species typical for inner marine shelf environments maintained a high abundance stage 2 consists of plastic light greenish grey clays barren of microfossils and probably represents fluvial input from the de la plata river to the shelf contemporaneous of a lowering of sea level stage 3 is composed of brownish yellow sandy silts and represents increasing marine conditions in the area as reflected by higher faunal diversity and typical foraminifera of inner shelf environments stage 4 is made of homogeneous mud barren of microfossil which represents a new pulse of fluvial input to the shelf in consequence of a new fall in sea level the final part of the core stage 5 is a coarsening upward sequence grading from greeny brown clayey sandy silts to coarse shelly sands and represents the modern sedimentation in the area this interpretation strengthens the stepped model of late holocene sea level fall between 5511 5792 cal yr bp and 2854 3188 cal yr bp in buenos aires coast and agrees with the relative sea level history previously proposed by some authors from western south atlantic coasts

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

biogeomorphology of a mojave desert landscape configurations and feedbacks of abiotic and biotic land surfaces during landform evolution abstract terrestrial ecosystems can be more holistically understood by investigating the morphology of landscape mosaics the assemblage of their ecological communities and the linkages and feedbacks between the mosaics and communities the overarching objectives of this study were to 1 study the abiotic and biotic configurations of landform units as mosaics within a mojave desert chronosequence and 2 elucidate their potential feedbacks interactions and dynamics during landform evolution seven landform units distributed over three geomorphic ages were identified including young bars and swales intermediate aged flattened bars flattened swales and bioturbation units and old desert pavements and shrub zones these landform units were characterized according to abiotic and biotic land surface properties landform units were statistically distinct and predictable based on a specific suite of abiotic and biotic properties vascular plant functional group and biological soil crust community diversity varied with geomorphology with greatest diversity associated with bars and shrub zones and lowest diversity associated with desert pavements biological soil crust communities were controlled by geomorphic age surface rock size and protruding rocks with young bar units having the highest abundance and diversity perennial forbs were observed in old shrub zones with small rocks and few protruding rocks a high clast density and a finer sized clast distribution were found particularly in desert pavements and flattened swales and generally inhibited biological soil crust and plant cover evolutionary trajectories for landforms of a lower piedmont landscape can be dominated by either abiotic and biotic landform processes these two trajectories are distinctly different and are associated with their own unique linkages feedbacks and dynamics of abiotic and biotic land surface properties producing a highly diverse desert landscape

late quaternary productivity changes in the equatorial indian ocean odp hole 716a abstract the equatorial indian ocean is swept by the indian ocean equatorial westerlies iew which intensify in monsoon transitions during april may and october november driving eastward equatorial current eec to the upper ocean the eec brings significant changes in the surface productivity in the equatorial region influencing the life style of the surface and deep sea fauna this study is aimed at understanding 300 kyr record 450 150 kyrs ago of productivity changes linked to variations in iew eec in the equatorial indian ocean using high resolution microfaunal record from integrated ocean drilling program iodp hole 716a maldives ridge in total 451 core samples of 10 cm 3 volume from the 300 kyr long sequence ranging 450 150 kyrs ago were analyzed at every 1 cm interval to generate census data of the benthic foraminiferal fauna factor and cluster analyses using percentages of 53 highest ranked benthic foraminiferal species were run that enabled us to identify five biofacies indicating the varied nature of deep sea environments during the late quaternary the results show a major shift roughly 300 kyr ago across marine isotope stages 9 and 8 mis 9 8 recorded in this study as mid brunhes transition at hole 716a the present study also combines published planktic foraminiferal fauna and pteropod records from hole 716a to understand if the surface to shallow and intermediate water paleoenvironments were coupled with deep water changes during the late quaternary in the maldivian archipelago biofacies robulus nicobarensis trifarina reussi rn tr uvigerina porrecta reussella simplex upo rs and cymbaloporetta squammosa bolivinita sp cs bsp dominate before the mid brunhes transition and document high organic flux with relatively low oxygen paleoenvironment similar to the planktic foraminifer globigerina bulloides population while benthic foraminiferal biofacies hoeglundina elegans miliolinella subrotunda he ms and uvigerina peregrina quinqueloculina seminulum upe qs record high seasonality in food supply with well oxygenated deep water following the mid brunhes climatic transition 300 kyr ago at hole 716a these changes are also recorded in the population of planktic foraminiferal species globigerinoides ruber and neogloboquadrina dutertrei as well as pteropods organic carbon flux linked to changes in the iew eec strength appears to be the most plausible factor controlling faunal assemblages and hence paleoproductivity dynamics in the maldivian region our results suggest that the strength of the iew is inversely related with the indian ocean dipole iod the iod strengthened across mis 9 8 during which time the iew weakened leading to major changes at hole 716a

the pinto shear zone a laramide synconvergent extensional shear zone in the mojave desert region of the southwestern united states the pinto shear zone is one of several late cretaceous shear zones within the eastern fringe of the mesozoic magmatic arc of the southwest cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening we demonstrate an extensional origin for the shear zone by describing the shear zone geometry and kinematics hanging wall deformation style progressive changes in deformation temperature and differences in hanging wall and footwall thermal histories deformation is constrained between w74 and 68 ma by 40 ar 39 ar thermochronology of the exhumed footwall including multi diffusion domain modeling of k feldspar we discount the interpretations applied in other areas of the mojave desert region that widespread late cretaceous cooling results from refrigeration due to subduction of a shallowly dipping laramide slab or to erosional denudation and suggest alternatively that post intrusion cooling and exhumation by extensional structures are recorded widespread crustal melting and magmatism followed by extension and cooling in the late cretaceous are most consistent with production of a low viscosity lower crust during anatexis and or delamination of mantle lithosphere at the onset of laramide shallow subduction q 2005 published by elsevier ltd

late mid holocene sea level oscillation a possible cause abstract sea level oscillated between 5500 and 3500 years ago at murrells inlet south carolina chezzetcook and baie verte nova scotia and montmagny quebec the oscillation is well constrained by foraminiferal marsh zonations in three locations and by diatoms in the fourth one the implications are 1 there was a eustatic sea level oscillation of about 2 10 m in the late mid holocene on the southeast coast of north america south carolina to quebec that is not predicted by present geophysical models of relative sea level change 2 this oscillation coincides with oceanographic cooling on the east coast of canada that we associate with melting ice and 3 this sea level oscillation climatic event coincides exactly with the end of pyramid building in egypt which is suggested to have resulted from a climate change i e drought cooling this sea level climatic change is a prime example of feedback where climatic warming in the mid holocene promoted ice melt in the arctic which subsequently caused climatic cooling by opening up arctic channels releasing cold water into the inner labrador current that continued to intensify until 4000 years ago this sea level event may also be the best way of measuring when the final ice melted since most estimates of the ages of the last melting are based on end moraine dates in the arctic which may not coincide with when the last ice actually melted out since there is no way of dating the final ice positions

isotopic complexities and the age of the delfonte volcanic rocks eastern mescal range southeastern california stratigraphic and tectonic implications combined u pb zircon rb sr 40 ar 39 ar laser fusion and conventional k ar geochronology establish a late early cretaceous age for the delfonte volcanic rocks u pb zircon analyses define a lower intercept age of 100 5 2 ma that is interpreted as the crystallization age of the delfonte sequence argon studies document both xenocrystic contamination and postemplacement ar loss rb sr results from mafic lavas at the base of the sequence demonstrate compositionally correlated variations in initial 87 sr 86 sr ratios sr i from 0 706 for basalts to 0 716 for andesitic compositions this covariation indicates substantial mixing of subcontinental lithosphere with proterozoic upper crust correlations between rb sr and sr i may result not only in pseudoisochrons approaching the age of the crustal component but also in reasonable but incorrect apparent ages approaching the true age ages obtained in this study require that at least some of the thrust faulting in the mescal range clark mountain portion of the foreland fold and thrust belt occurred later than ca 100 ma and was broadly contemporaneous with emplacement of the keystone thrust plate in the spring mountains to the northeast comparison of the age and rb sr systematics of ash flow tuff boulders in the synorogenic lavinia wash sequence near goodsprings nevada with those of the delfonte volcanic rocks supports a delfonte source for the boulders the 99 ma age of the lavinia wash sequence is nearly identical to the delfonte age requiring rapid erosion transport and deposition following delfonte volcanism

modelling holocene relative sea level observations from the caribbean and south america abstract holocene data from the caribbean and the atlantic coast of south america have been critically assessed and a subset of the best quality data are presented these data cover a large north south geographic extent and display a distinct spatial variation we make the first comparisons of this data set to predictions based on a realistic model of glaciation induced sea level change with the main aims of understanding the cause of the observed spatial trend and estimating a eustatic signal for the holocene the spatial variation is dominated by the influence of the ice and ocean mass redistribution on sea level change with the ice induced effect dominating the observed north south trend a best fitting model is applied to estimate a holocene eustatic signal from the observations we find that the model corrected data are consistent with a relatively rapid rise of 7 8 mm yr in the early holocene with a marked reduction in this rate around 7 cal kyr bp from this time until present the model corrected data suggest that the volume of mass transfer between ice sheets and oceans was no more than 1 m eustatic sea level equivalent

holocene climate variability although the dramatic climate disruptions of the last glacial period have received considerable attention relatively little has been directed toward climate variability in the holocene 11 500 cal yr b p to the present examination of similar to50 globally distributed paleoclimate records reveals as many as six periods of significant rapid climate change during the time periods 9000 8000 6000 5000 4200 3800 3500 2500 1200 1000 and 600 150 cal yr b p most of the climate change events in these globally distributed records are characterized by polar cooling tropical aridity and major atmospheric circulation changes although in the most recent interval 600 150 cal yr b p polar cooling was accompanied by increased moisture in some parts of the tropics several intervals coincide with major disruptions of civilization illustrating the human significance of holocene climate variability c 2004 university of washington all rights reserved

an impact origin for hydrated silicates on mars a synthesis 1 recent mars orbiting spectrometers continue to detect surface materials containing hydrated silicates particularly clays and amorphous phases e g silica glasses concentrated within the heavily cratered noachian highlands crust this paper provides a review summary and synthesis of observations from terrestrial impact structures with current martian data it is suggested that numerous and frequent impacts into the volatile rich silicate crust of mars through direct and indirect impact generated mechanisms represent a plausible hypothesis that can explain the widespread distribution of hydrated silicates in the surface and subsurface of the heavily cratered noachian highlands crust largely independent of climate in addition to impact generated hydrothermal activity devitrification autometamorphism and the voluminous production of impact damaged materials that are susceptible to alteration must be considered when taken together a drastically different early climate on mars in which water is stable at the surface for extended periods of time cannot be ruled out however it is noted here that these additional impact mechanisms can operate and thereby extend the range of possible alteration settings to include climate conditions that may have been predominately colder and drier such a climate would not be dissimilar to the conditions of today with the important exceptions of a higher geothermal gradient and punctuated thermal disturbance to the cryosphere and hydrosphere from igneous activity and an exponentially higher impact flux

time of initial thrusting in the sevier orogenic belt idaho wyoming and utah reexamination of the distribution of fossils found in the earliest preserved synorogenic conglomerates within the sevier thrust belt suggests that initial thrust movement may be no older than aptian age this interpretation is corroborated by subsidence analyses of sedimentary sequences lying within and east of the idaho wyoming and utah thrust belts a major episode of middle cretaceous aptian cenomanian subsidence is interpreted as recording the initiation of thrust loading deformation in the adjacent sevier orogenic belt an earlier subsidence event took place during middle jurassic time more than 30 m y prior to the cretaceous event and may be the result of tectonic events to the west that are unrelated to thrust deformation in the idaho wyoming and utah thrust belts we find no verifiable evidence to support previous interpretations that sevier belt deformation and uplift began in late jurassic time

age of jurassic volcanism and tectonism southern owens valley region east central california remnants of the eastern fringe of the volcanic and volcanogenic sedimentary cover of the mesozoic sierra nevada batholith are exposed in the southern inyo mountains and adjacent alabama hills of east central california six new u pb dates on volcanic units and crosscutting intrusions reveal that the upper parts of both the inyo mountains and alabama hills sections accumulated during middle and late jurassic time during this same interval both sections were steeply tilted and locally folded during one or more episodes of contractile deformation occurring in the east sierran thrust belt differences between the largely undated lower parts of the inyo mountains and alabama hills sections suggest that they were once located farther apart then later brought into proximity by thrust faulting or strike slip faulting the inyo mountains and alabama hills sections are similar to partly coeval strata in the white mountains in that both contain abundant sedimentary strata that were in part deposited in or near terrain of moderate topographic relief together these areas seem to compose a distinctive arc marginal depositional province different than that represented by partly coeval strata preserved in pendants to the west

hydrous minerals on mars as seen by the crism and omega imaging spectrometers updated global view 1 the surface of mars has preserved the record of early environments in which its basaltic crust was altered by liquid water these aqueous environments have survived in the form of hydrological morphologies and alteration minerals including clays and hydrated salts because these minerals probe on earth aqueous environments compatible with biotic activity understanding their formation processes on mars is of great exobiological relevance and also offers insight into earth s now erased ancient water environments using remote sensing we conducted a large scale investigation of the distribution composition age and geomorphic settings of hydrous minerals on mars providing a sharpened global view of the early aqueous environments and their evolution with time aqueous alteration seems to have produced clays on a planetary scale but these are found to be restricted to the oldest observable terrains on mars 4 gyr however very diverse aqueous environments have also been found which suggest widespread complex aqueous settings from the surface to kilometric depths and spanning over 1 gyr by building a robust statistical sample of detections the global trends inferred here attempt to provide a broad view of our current understanding of hydrous minerals on mars and provide context for more localized in depth analyses collectively these trends suggest that at least transient conditions have existed on mars which may have been favorable for pre biotic to biotic activity

timing of exhumation wheeler pass thrust sheet southern nevada and california late jurassic to middle cretaceous evolution of the southern sevier fold and thrust belt zircon u th he zhe thermochronologic data for 46 samples from the wheeler pass thrust sheet which carries thick passive margin strata and is discontinuously exposed across the southern part of the sevier fold and thrust belt record late jurassic cooling related to exhumation during early thrust slip and local middle cretaceous cooling related to footwall basement imbrication within the frontal part of the wheeler pass sheet exposed in the northwest spring mountains zhe ages decrease from ca 160 to 140 ma over a paleodepth interval of 6 to 9 km in devonian to lower cambrian strata interpreted to record exhumation and enhanced cooling above a frontal ramp followed by slow cooling a similar pattern of zhe ages occurs over a more limited paleodepth range exposed within the resting spring range to the south in the nopah range zhe ages of ca 140 ma in lower cambrian strata record early cooling followed by slow cooling until ca 100 ma zhe ages in underlying paleoproterozoic basement rocks decrease from ca 100 to 85 ma over a paleodepth interval of 9 to 12 km interpreted to record uplift exhumation and cooling during footwall basement imbrication as slip was transferred eastward onto the keystone thrust system late jurassic slip on the wheeler pass thrust overlapped with deformation along the nw trending east sierran thrust system early hinterland crustal thickening and growth of the sierra magmatic arc and may have been correlative with early deformation in the central nevada thrust belt slip on the wheeler pass thrust preceded major slip on other thrust sheets that carried thick passive margin strata in more northern parts of the sevier belt probably reflecting the influence of different initial widths and tapers of the passive margin sedimentary wedge

physical and chemical response of zircons to deformation an investigation of u pb isotopic systematics in zircons from mylonitized henderson gneiss sinha and glover 1978 revealed that selected zircon fractions from the mylonite zone suffered total loss of radiogenic pb at 460 m y to further investigate the relationship between pb loss u gain and grain size reduction associated with increasing strain in the shear zone we have characterized the chemistry and morphology of zircons in the mylonitic rocks using both electron microprobe analysis and scanning electron microscopy

relative sea level changes in the last 5500 years in southern brazil laguna imbituba region santa catarina state based on vermetid ages abstract twenty six new radiocarbon dates from vermetid shells collected in the southernmost sector of the brazilian rocky coast presented dates ranging from 5410 80 to 190 65 years b p with associated paleosea levels varying from 2 10 m to 0 20 m above present sea level the overall suggested trend of the relative sea level rsl declining until at least 190 years b p is somewhat contradictory to a proposed rsl rise in the last 1000 years in southern brazil the data also seem to undermine a more widely accepted rsl trend that suggests that at least two negative rsl oscillations occurred between 4100 and 3800 years b p and between 3000 and 2700 years b p the maximum elevation of the rsl in the holocene in southern brazil was possibly lower than that observed in most of the brazilian eastern coast discrepancies between ancient sea levels of similar ages are attributed to coincidental methodological problems to imprecisions in determining past relative sea levels and to possible changes in the geomorphology and wave climate close to shore during the last 5000 years a general trend of increasing 18 o with a reduction in age in the studied samples may suggest a gradual reduction of water temperature in the region during the same period

high resolution spatial rupture pattern of a multiphase flower structure rex hills nevada new insights on scarp evolution in complex topography based on 3 d laser scanning fault scarps represent the most obvious expression of tectonic activity at the earth9s surface studies on scarp morphology place constraints on fault kinematics and scarp degradation processes and were often based on geomorphic dating techniques fault scarps exposed in areas of simple topography facilitate data acquisition and interpretation whereas little work had been done where fault scarps are superimposed on complex dissected topography fault scarps developed in complex topography are commonly observed along flower structures and at tips of strike slip faults such structures are important elements for evaluating the evolution and linking of strike slip fault systems and appear to be scale independent from several meters to hundreds of kilometers we examined the detailed meter to hundred meter scale structure and surface expression of a flank of one fault scarp bounded pressure ridge rex hills flower structure by combining field mapping with high resolution digital elevation model dem analysis based on terrestrial laser scanning we generated a detailed dem and extracted high resolution topographic cross sections which enabled us to identify fault scarps and to determine their relative ages and geometry our study site is located on the transpressional left bend between the pahrump and amargosa segments of the dextral stateline fault system the topography is characterized by alternating valleys and ridges each 100 m long relief of 4 m we observed the following the southern rex hills slope exhibits three fault scarps related to three reverse fault branches the basal scarp scarp 1 is most continuous and exhibits five segments the upper two scarps scarps 2 and 3 are less continuous furthermore fault scarps exposed on ridge crests are more numerous up to four to five scarps and smaller 5 m high valleys often exhibit single large 10 m high smoothed scarps to easily detect differences between the scarps we evaluated the height and slope angle of the scarps using topographic cross sections our analysis indicates that scarp shape is influenced by fault dip lithology and degradation processes resulting in large scatter and broad overlap in scarp height slope angle space the analysis further indicates that scarp degradation is stronger in the valleys and that the preservation potential of small individual fault scarps is therefore greater on the ridge crests we compared our fault scarp data with published calibrated data yielding an age of 2 ka for the rex hills scarps consistent with an earlier finding this suggests that the scarp shape mainly reflects progressive degradation since the most recent surface rupture our approach of analyzing high resolution topographic data of closely spaced fault scarps is promising especially when combined with subsurface data as well as geochronological and paleoseismic data and it provides a basic scheme for analyzing scarp populations in a complex topographic region despite the absence of subsurface data our approach allowed the study of complex high resolution fault scarp morphologies across a flower structure for the first time

the brazilian sea level curves a critical review with emphasis on the curves from the paranaguaand cananeia regions abstract relative sea level curves for the mid to late holocene have been proposed for eight different sectors along the brazilian coast this paper aims to review the data used to construct the curves proposed for two sectors the paranaguaand cananeia regions in southeast brazil more specifically we analyze the palaeo sea level indicators vermetid tubes shells wood fragments and shell midden deposits related to 1 the sea level maximum of the mid holocene and 2 the two secondary oscillations that would have occurred between 4100 3800 years b p and 3000 2700 years b p the sedimentary deposits of the coastal plain as well as vermetid datings suggest a mid holocene sea level maximum between 3 5 and 4 m the great majority of the indicators used to infer the secondary oscillations in previous studies were derived from shell middens besides being unreliable palaeo sea level indicators these data also showed inconsistencies that undermine the existence of such oscillations the plot of all the data derived from vermetid samples found along the brazilian coast also indicate a rather smoother sea level fall in the last 5100 years

timing and magnitude of holocene sea level changes along the middle and south patagonian atlantic coast derived from beach ridge systems littoral terraces and valley mouth terraces abstract geomorphologic and chronostratigraphic investigations in various coastal localities along the middle and south patagonian atlantic coast from bahia vera 44 s in the north to san julian 49 s in the south confirm a rich sequence of holocene beach ridge systems and littoral and valley mouth terraces beach ridges are present up to 10 m above current highest tide level htw and date to the early mid holocene transgression maximum beach ridge deposits represent high energy wave dominated coastal environments whereas valley mouth terraces and littoral terraces elevations of 5 5 6 5 m above htw developed in wave protected coastal environments the surface elevation of these littoral forms suggest that the middle and south patagonian atlantic coast is likely undergoing a slow glacio isostatic uplift on the order of 0 3 0 4 mm a since the mid holocene the early holocene sea level rise reached the modern coastline by 8100 14 c years ago c 8600 cal bp at the latest sea level at that time was most likely near its present position the holocene transgression maximum lasted from 6900 to at least 6200 14 c bp c 7400 to 6600 cal bp raising relative sea level to about 2 m to a maximum of 3 m above the present level after which sea level declined to the present level two significant sea level falls of at most 1 m occurred a between 6200 and 6000 14 c bp c 6600 to 6400 cal bp and b between 2600 and 2400 14 c bp c 2300 to 2050 cal bp we assume that both significant sea level falls were predominantly driven by eustatic changes of ocean volume whereas both thermo steric changes and changes of tides may also have contributed to these relatively strong sea level drops gravitational changes driven by greenland ice growth sensu mitrovica et al 2001 may also have amplified any eustatic portion of sea level fall the trend of a general sea level decline since the mid holocene transgression maximum seems to be predominantly glacio isostatically driven