zircon hf isotopic constraints on the sources of the indus molasse ladakh himalaya india 1 india asia collision has resulted in the largest orogenic event on earth since at least 500 ma and this has had a profound influence on the drainage patterns of asia on the asian monsoon climate on global oceanography as well as on faunal extinctions despite this importance understanding of the paleogene evolution of this system is limited u pb and lu hf isotopic analyses of detrital zircons from the clastic sedimentary rocks of the indus molasse located within the indus suture of the ladakh himalaya india show that most have mesozoic cenozoic ages and juvenile hf isotopic compositions this suggests that the main source of these sediments is the transhimalayan batholith with minor flux from the lhasa karakoram terrane and indian plate the lower indus molasse chogdo formation is inferred to have been deposited at 60 49 ma whereas the upper choksti formation was deposited later than 45 41 ma and contains a greater contribution from the lhasa terrane the transhimalayan batholith was eroded throughout indus molasse sedimentation but increasing exhumation of lhasa karakoram terrane indicates its progressive uplift after initial collision and requires formation of apaleo indus river shortly after collision

flexural subsidence by 29 ma on the ne edge of tibet from the magnetostratigraphy of linxia basin china abstract this study provides a detailed magnetostratigraphic record of subsidence in the linxia basin documenting a 27 myr long sedimentary record from the northeastern edge of the tibetan plateau deposition in the linxia basin began at 29 ma and continued nearly uninterruptedly until 1 7 ma increasing rates of subsidence between 29 and 6 ma in the linxia basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to late oligocene late miocene time by late miocene early pliocene time a decrease in subsidence rates in the linxia basin associated with thrust faulting and a 10 clockwise rotation in the basin indicates that the deformation front of the tibetan plateau had propagated into the currently deforming region northeast of the plateau

significance of clay mineral assemblages in the antarctic ocean abstract typical examples from different morphological and geological settings in the antarctic ocean are reviewed in order to discuss the value of clay mineral assemblages for reconstructing the glacial history of antarctica the paleoceanographic history of the antarctic ocean and the sedimentary processes at the antarctic continental margin the significance of clay minerals for paleoenvironmental reconstructions strongly varies with both the position of the sites under investigation and the age of the sediments in late mesozoic to paleogene sediments clay mineral assemblages are sensitive tools for reconstructing climatic conditions for example the shift from smectite dominated assemblages to illite and chlorite dominated assemblages in the earliest oligocene clearly documents the transition from chemical weathering conditions under a warm and humid climate to physical weathering under cooler conditions submarine elevations such as maud rise and kerguelen plateau give the best record for direct paleoclimatic and paleoceanographic studies at the proximal sites of the continental slope and shelf as well as in the deep sea the paleoclimatic information normally is masked by a variety of processes resulting in sediment redistribution at those sites in contrast the clay mineral assemblages bear a wealth of information on different sedimentary processes after the establishment of a continental east antarctic ice sheet physical weathering prevailed variations in the clay mineral records predominantly reflect the influence of different sediment sources resulting from different glacial hydrographic or gravitational transport processes because these sedimentation processes are generally linked to climatic variations the clay mineral assemblages in most of the neogene and quaternary sediments provide indirect paleoclimatic information the processes are best documented in the clay mineral composition in those areas where changes in source regions with distinct petrographic differences are expected and where distances from the source region are low

timing and pathways of east antarctic ice sheet retreat abstract a lack of reliable chronology often leaves antarctic ice sheet reconstructions incomplete despite being the most heavily investigated region of the antarctic continental shelf the post last glacial maximum lgm record of marine based ice sheet dynamics in the ross sea has remained largely elusive and at odds with fringing terrestrial records of ice sheet decay issues with radiocarbon dating stem primarily from poorly preserved carbonate contamination by glacially recycled carbon and insufficient consideration for glacial geomorphic context and sedimentary facies we assess nearly 70 newly acquired and more than 200 published radiocarbon ages in the context of a consistent sedimentary facies and geomorphic framework to interpret the timing of the local glacial maximum and subsequent retreat of the east antarctic ice sheet eais in the ross sea and potential causes of retreat compound specific radiocarbon ages help to provide good constraints on open marine onset and we demonstrate that with proper corrections and considerations acid insoluble organic aio radiocarbon ages can be reliable in open marine sediments and carbonate ages can be reliable in grounding zone proximal sediments we find that the ice flowing through pennell and joides troughs remained at maximum extent for approximately 10 000 years before retreating at 15 1 and 13 cal ka bp respectively an ice shelf covered the outer continental shelf until 9 5 cal ka bp after which it collapsed in joides trough creating deep iceberg furrows retreat through pennell trough was more episodic marked by grounding zone wedges that backstep upstream and onto lateral banks creating deglacial emergent ice rises as ice retreated from 9 to 4 cal ka bp there was considerable reorganization of drainage and at least one episode of readvance through southern joides trough that began drawdown of terrestrial ice near ross island at 7 8 cal ka bp the ice shelf appears to have retreated to a near present configuration by 2 cal ka bp however due to inconsistencies in calendar ages grounding line position must be inferred from relative chronology based on glacial landforms which suggests grounded ice retreated nearly to drygalski ice tongue and lingered atop crary bank where it was maintained by ice from david glacier our reconstruction further suggests that the eais in the western ross sea was not a discernible contributor to meltwater pulse 1a but likely contributed to eustatic sea level rise well through the middle perhaps even late holocene

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 1 geophysical results in the 1994 1995 and 1998 austral field seasons geophysical research efforts in the ross sea focused on acquiring a high resolution database designed to permit 1 reconstruction of the maximum extent and configuration of the ice sheet during the last glacial maximum i e oxygen isotope stage 2 2 reconstruction of conditions at the base of the ice sheet and 3 assessment of the relative retreat history of the ice sheet following the last glacial maximum five seismic facies are distinguished on the basis of external geometry relationships among features bounding surface amplitude intensity of internal acoustic signature and geometry of internal reflectors seismic facies 1 is a transparent draping unit correlated to diatomaceous mud interpreted as being deposited under open marine conditions seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations these units are interpreted to be grounding zone proximal deposits overridden by the expanded ice sheet seismic facies 2 may be a deforming till unit seismic facies 4a is characterized by an internally massive to chaotic signature an erosional often flat lower surface and a hummocky upper surface displaying glacial lineations this unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion 4a and with an earlier pre last glacial maximum expansion 4b seismic facies 5 is an acoustically laminated ponded and draping deposit interpreted to contain proglacial and sub ice shelf materials deposited continuously since the last interglacial period the associations of these units provide context for the interpretation of the ice edge maximum position conditions at the base of the ice sheet and the relative retreat history of the region there is compelling evidence for a much expanded ice sheet in the ross sea during the last glacial maximum in the western ross sea the maximum grounding position is marked by an isolated grounding zone wedge and placed in the vicinity of coulman island approximately 150 km from the continental shelf edge in the central ross sea the maximum grounding position is close to the continental shelf break based primarily on the presence of an extensive 60 m thick sheet like deposit with a fluted upper surface streaming ice may have occupied bathymetric lows on the continental shelf as suggested by 1 the configuration of bathymetry 2 the presence of glacially eroded troughs 3 the concentration of sediment interpreted to be deforming till within the middle to outer shelf reaches of the troughs and 4 the fluted nature of the upper surface absolute rates of streaming ice flow relative to inter ice stream areas are not implied the western ross sea continental shelf deposits record the retreat history of ice derived predominantly from the east antarctic ice sheet and glaciers of the transantarctic mountains ice flow on the continental shelf is interpreted to have remained fixed in position restricted by the walls of the troughs on the inner shelf of the western ross sea the ice flowed over and eroded lithified sedimentary strata slower moving ice occupied flat topped banks a single grounding zone wedge occurs on the western ross sea central shelf no substantial deposits are observed on the inner continental shelf this lack of grounding zone features reflects a restricted sediment supply and a relatively steady rapid retreat of the ice sheet during retreat ice flow acted independently in each major trough and on the bank tops ice retreated from victoria land basin drygalski trough before it retreated from joides basin ice remained on the bank tops shedding material into the abandoned troughs grounded ice in the central ross sea was derived predominantly from an expanded west antarctic ice sheet the ice sheet remained grounded on the continental shelf edge after retreat of ice from the western ross sea expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf ice stream drainage shifted laterally as recorded by rounded laterally accreting ridges that separate bathymetric troughs the central ross sea is the repository for larger volumes of sediment derived from ice flow across basins of relatively thick unlithified sedimentary deposits two till sheets mark grounding zone positions in the central ross sea reflecting the higher sediment supply and a stepped deglaciation ice remained grounded on the pennell bank to the west a series of moraines marks the retreat of ice across the bank mega scale glacial lineations and other streamlined subglacial geomorphic features occur across the continental shelf primarily within the troughs the lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet this deforming substrate may have contributed to the onset of deglaciation features associated with meltwater are rare or absent suggesting that basal meltwater played a minor role in retreat of the ice sheet

ross sea paleo ice sheet drainage and deglacial history during and since the lgm onshore and offshore studies show that an expanded grounded ice sheet occupied the ross sea embayment during the last glacial maximum lgm results from studies of till provenance and the orienta

mid late pleistocene glacimarine sedimentary processes of a high latitude deep sea sediment drift antarctic peninsula pacific margin the effects of glaciation on sediment drifts is recognised from marked sedimentary facies variation in deep sea cores taken from the continental rise of the antarctic peninsula pacific margin nineteen sediment cores were visually described logged for magnetic susceptibility and x radiographed about 1000 analyses were performed for grain size clay minerals and biostratigraphy foraminifera nannofossils and diatoms four sediment types associated with distinct sedimentary processes are recognised based on textural compositional analysis 1 hemipelagic mud forms the bulk of the interglacial sediment and accumulated from the pelagic settling of bioclasts and ice rafted wind transported detritus 2 terrigenous mud forms the bulk of the glacial sediment and accumulated from a combination of sedimentary processes including turbidity currents turbid plumes and bottom current reworking of nepheloid layers 3 silty deposits occurring as laminated layers and lenses represent the lateral spillout of low density turbidity currents 4 lastly glacial interglacial gravelly mud layers derive from settling of ice rafted detritus five depositional settings are interpreted within sediment drift 7 each characterised by the dominance interaction of one or several depositional processes the repetitive succession of typical sedimentary facies is inferred to reflect a sequence of four climatic stages glaciation glacial deglaciation and interglacial each one characterised by a distinctive clay mineral assemblage and bioclastic content variations in clay mineral assemblage within interglacial stage 5 core sed 06 suggest minor colder climatic fluctuations possibly correlatable with substages 5a to 5e

linking the modern distribution of biogenic proxies in high arctic greenland shelf sediments to sea ice primary production and arctic atlantic inflow the eastern north coast of greenland is considered to be highly sensitive to the ongoing arctic warming but there is a general lack of data on modern conditions and in particular on the modern distribution of climate and environmental proxies to provide a baseline and context for studies on past variability here we present a detailed investigation of 11 biogenic proxies preserved in surface sediments from the remote high arctic wandel sea shelf the entrance to the independence hagen and danmark fjords the composition of organic matter organic carbon c n ratios 13c 15n biogenic silica and ip25 and microfossil assemblages revealed an overall low primary production dominated by benthic diatoms especially at the shallow sites while the benthic and planktic foraminiferal assemblages underline the intrusion of chilled atlantic waters into the deeper parts of the study area the distribution of organic walled dinoflagellate cysts is controlled by the local bathymetry and sea ice conditions the distribution of the dinoflagellate cyst polarella glacialis matches that of seasonal sea ice and the specific biomarker ip25 highlighting the potential of this species for paleo sea ice studies the information inferred from our multi proxy study has important implications for the interpretation of the biogenic proxy signal preserved in sediments from circum arctic fjords and shelf regions and can serve as a baseline for future studies this is the first study of its kind in this area

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

sedimentological signature of a deformable bed preserved beneath an ice stream in a late pleistocene glacial sequence ross sea antarctica abstract subglacial processes are described using a nested set of samples from and 1b drill core collected from beneath the ross ice shelf as part of the antarctic geologic drilling program andrill an interval from the late pleistocene was chosen for study because it is preserved in the sedimentary record yet the depositional processes are applicable to the widely studied glacial conditions at the last glacial maximum a complete glacial interglacial sequence between two glacial surfaces of erosion gses at 41 9 and 47 7 meters below sea floor was studied by centimeter scale core logging based on x radiographs of the archive core halves in addition to core description bulk samples and 45 mm 65 mm thin sections of diamictites for micromorphology analysis above the lower gse 4 6 m of subglacial till occurs including a sequence of thin muddy conglomerate with diverse pebble lithologies massive clast rich muddy diamictite and stratified diamictite with clast rich and clast free beds the sand size fraction of these deposits is dominated by aggregate grains termed till pellets following terminology used for ross sea deposits we present the first detailed description and two different conceptual models of tillpellet formation beneath an ice stream one is related to mechanical processes and the other is related to thermodynamics till pellets are rounded spherical to prolate in form and are associated with turbate structures and aligned grains in thin sections the core of the pellet is either a lithic grain or stiff till with additional clay plastered on the outside forming rounded grains from originally angular ones galaxy structures skelsepic fabric and pressure shadows around grains are indicative of rotational deformation within the till till pellets form in situ within a highly porous deformable bed under an ice stream either by mechanical shearing or by alternating thermal conditions in the till in the first model a slow glacial advance under dry subglacial conditions fractures the till increasing the porosity as shear induces dilation basal melting and high pore water pressure develop as the ice stream thickens decreasing the intergranular effective stress and inducing rotation of the angular till aggregates an alternative model calls for basal freezing with preferential ice growth in larger pore spaces causing rotation of fine grained till aggregates due to concentration of shear stresses in a thin zone beneath the glacier sole survival of till pellets is enhanced in subglacial diamictite as the thin zone of deformation moves upward toward the base of the ice stream a thin granular bed enriched in till pellets is melted out in the grounding zone and this deposit marks the transition from subglacial till to stratified silty claystone representing sub ice shelf deposits

structural evolution of harmaliyah oil field eastern saudi arabia the history of structural growth of the harmaliyah oil field in eastern saudi arabia was studied by means of a series of paleostructural maps to determine if early entrapment of hydrocarbons played an important role in preserving the high porosity of the kimmeridgian upper jurassic arab d reservoir the arab d has been affected by shallow or preburial diagenesis a common feature in arabian upper jurassic calcarenitic reservoirs closure forming the structural trap in the arab d at harmaliyah oil field developed principally during the late turonian late cretaceous during kimmeridgian late jurassic early turonian late cretaceous time shallow or preburial diagenesis of the calcarenitic arab d reservoir rocks seems to have played a major part in preservation of por sity before the porosity was filled with hydrocarbons

metal enrichment and magnetic properties of core sediments from the eastern yellow sea east asia implications for paleo depositional change during the late pleistocene holocene transition abstract to investigate the paleo depositional environments sediment cores yj1 and yj2 from a tidal flat of the eastern yellow sea were analyzed for the vertical variations of metal concentrations particle size clay mineralogy magnetic properties and total organic carbon content toc fe and mn concentrations determined by both partial and total acid extraction increased at 15 m depth in yj1 and 10 m in yj2 the layer directly below these fe rich brownish horizons showed dramatic decreases in clay mineral content and trace metals zn pb cu cr co ni and as but increases in sand fraction magnetic susceptibility and toc all results were inverse in the upper metal rich layer these dramatic changes in multi parameters are likely due to an environmental change during the late pleistocene holocene transition the rapid environmental change sand rich and oxic clay rich and anoxic resulted in aqueous dissolution of redox sensitive metals e g fe and mn from the lower sandy layer materials the dissolved metals diffused upward and concomitantly were immobilized within the upper clay rich layer the reprecipitated fe or mn hydro oxides then scavenged trace metals by coprecipitation and or adsorption this study suggests that metal distribution patterns can provide new insights into the paleo depositional processes during the pleistocene holocene period

geophysical constraints on a modified crust below the rayn anticlines eastern saudi arabia abstract the gravity anomaly signature for a regional trans rayn geophysical traverse trgt across rayn anticlines ra illustrates alternating bouguer anomaly b a highs and lows highs over the anticline axes and lows in flanking sub basins we demonstrate for the first time that the bipolar gravity signatures actually camouflage a more extensive b a low of amplitude 63 mgal and wavelength 750 km developed between the qatar and amar arches 2d gravity modeling for trgt infers a 3 layer crustal model for ra i a regionally extensive upper crustal layer 10 15 km thick of lighter density by 0 02 g cm3 compared to normal upper continental crust ii a mid crustal 10 km thick transition layer of intermediate density and iii the lower mafic crust of higher than normal density the moho dips westward from 37 3 below ghawar anticline ga to 47 and 47 6 km below the kbs amar arches with a lateral density change in the lower crust following the same direction crustal thinning by as much as 22 occurred below ga the altered crust directly correlating to the basement uplift on its top and mantle uplift at its base an inverse correlation between crustal thicknesses and vp vs is observed for the entire ra displaying highest vp vs 1 93 and poisson s ratio 0 443 for the ga crust

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

surface expressions of the ghawar structure saudi arabia abstract the ghawar anticline is approximately 225 km long and 25 km wide in the subsurface but the surface structural expression is not obvious identifying subtle structural imprints in the young mio pliocene sedimentary cover is therefore of great importance in developing a structural growth model for the ghawar field the ghawar area between the jafurah sand desert to the east and the rubayda dahna sand deserts to the west is characterized by a rougher topography when compared to the surrounding rather smooth flat areas this rough geomorphology of the structure can also be noticed on the satellite images a geomorphologic elevation map of the area and a subsurface structural contour map of the top arab d upper jurassic reservoir reveal very similar geometric shapes calcareous sandstone deposits of the youngest hofuf formation mio pliocene cap the structural highs along the axis of the anticline and spectacular fractures caves mesas and monumental geomorphologic features have developed in the escarpments fractures in the dam formation middle miocene are not as conspicuous as in the hofuf formation but indigenous fractures are clearly recognized in this formation a match between the directions of some topographic lineaments and projected surface traces of subsurface faults from seismic cross sections can be observed however at the field locations faults have yet to be defined on the surface surface indications suggest that the structure has been active until the present day elevations of a bedding plane on the shedgum plateau reveal a 0 25 dip angle which cannot be distinguished on the surface by the human eye the movements that occurred during the 4 million years since the pliocene period reveal an average tilting of 0 06 per one million years

foraminiferal assemblages from ice proximal paleo settings in the whales deep basin eastern ross sea antarctica abstract reconstructions of ross sea paleoenvironments based on foraminifera are hindered by the dearth of actualistic data especially from below the ross ice shelf to fill this gap we used a recently developed well resolved deglaciation record from the whales deep basin of the eastern ross sea to understand the ecological affinities of different foraminiferal assemblages during open water conditions similar to the present two benthic foraminiferal communities are strongly dominated by agglutinated species however in the underlying sediments that represent sub ice shelf and grounding zone proximal settings on the outer continental shelf we identified five different assemblages of calcareous foraminifera by antarctic margin standards these calcareous assemblages are periodically truly abundant along with assemblages dominated by benthic calcareous species that are well known from many antarctic settings i e globocassidulina biora trifarina earlandi and astrononion echolsi two important assemblages are dominated by a heretofore undescribed pustulose morphotype of g biora and poorly known spinose morphotype of t earlandi based on our correlations to the deglacial record these two assemblages live near the grounding line or in environments with especially intense bottom water currents as grounded and floating ice retreated south to the middle continental shelf these later sub ice shelf and grounding zone proximal settings were largely devoid of calcareous foraminifera and instead were inhabited by agglutinated taxa this improved understanding of foraminiferal assemblage distributions from a variety of environments within the eastern ross sea may contribute to better use of foraminiferal data to investigate the evolution of environmental changes in grounding line proximal environments

the late quaternary environmental evolution of marine arctic canada barrow strait to lancaster sound abstract a marine sediment core from the east central canadian arctic archipelago core 86027 154 74 22 01 n 89 51 26 w 329 m water depth studied by a multiproxy approach lithostratigraphy biogeochemistry micropalaeontology dinoflagellate cysts other non pollen palynomorphs benthic and planktonic foraminifera ostracods and encompassing 14 ams 14 c dates provides valuable insights into regional deglacial to holocene palaeoenvironments six palaeoenvironmental zones are recognized based on prominent changes in the litho and biostratigraphy the waterlain diamicton of zone i records immediate deglaciation being derived from lift off and calving of previously grounded glacial ice though deglacial timing is complicated by the sparsity of dating materials and the portlandia effect age depth model extrapolation places deglaciation at 11 54 cal ka bp zone ii 11 5 11 0 cal ka bp represents a distinct progression from initially ice proximal to increasingly ice distal conditions interrupted by an interval of pervasive sea ice 11 4 11 2 cal ka bp noteworthy biological activity commences in zone iii 11 0 9 7 cal ka bp with a prominent signal of planktonic foraminifera neogloboquadrina pachyderma this likely signifies penetration of deeper atlantic derived water through the central canadian arctic archipelago upon deglaciation facilitated by the greater glacioisostatically induced water depths 80 m and implies separation of laurentide and innuitian ice sheets by 11 0 cal ka bp zone iv 9 7 7 2 cal ka bp records ameliorated biologically favourable conditions with reduced seasonal sea ice accompanied by high microfossil species diversity and the presence of subpolar taxa zone v 7 2 6 5 cal ka bp signals the exclusion of atlantic derived water concomitant with increasing sea ice simultaneously representing the termination of the dynamic deglacial to early holocene environments zones i iv conditions similar to modern typified by uniform sediment characteristics present day microfossil assemblage structures and sparse benthic foraminifera were established by 5 6 cal ka bp zone vi

late quaternary foraminiferal assemblages from western ross sea antarctica in relation to the main glacial and marine lithofacies abstract investigations on foraminifers from upper pleistocene holocene sediments were carried out on twelve cores from the western ross sea continental margin drygalski joides north victoria land basins as part of a progetto nazionale di ricerche in antartide p n r a multidisciplinary project data on the foraminiferal frequency species diversity tests abundance and their state of preservation were presented as a synthesis of 404 core samples to establish their relationships with the main glacial and marine lithofacies of this area a total of 126 benthic species pertaining to 73 genera have been identified just few taxa such as cibicides spp globocassidulina spp trifarina angulosa and miliammina spp being the most ubiquitous and in some cases the dominant species of these paleoenvironments two variants of neogloboquadrina pachyderma including thin and thick shelled forms have been recovered we propose to use these results to provide the degree of glacial control during the last glacial maximum and the following holocene retreat of the ice sheets high test fragmentation low diversity and density tests reflect higher glacial influence of the ice sheet in the drygalski basin whereas the decreasing percentage of fragmentation and a relative increase of density and diversity in drygalski joides and north victoria land basins indicate the paleoenvironmental passage from the ice sheet to the ice shelf condition the ice shelf retreat is well evidenced in the joides basin by a succession of levels barren of foraminifers alternating with high density levels rich in t angulosa followed by a total disappearance of the calcareous foraminifers open marine settings indicative of lower glacial influence and increased corrosiveness of the water masses is testified by the miliammina foraminiferal assemblage during the holocene in drygalski and joides basins cores on the contrary rich and abundant benthic and planktonic assemblages characterize the holocene paleoenvironment of the north victoria land area indicating that the water masses were less corrosive with respect to the other areas in addition to the glacial reworking of the tests and the dissolution due to the corrosive water mass conditions the volcaniclastic sediments recovered in the north victoria land basin cores also affected the condition of test preservation in volcaniclastic sediments older than about 20 ka bp the foraminifers concentration tends to zero and when present their tests are highly damaged or completely broken

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

late quaternary glacial marine to marine sedimentation in the pennell trough ross sea antarctica abstract late pleistocene and holocene palaeoenvironmental changes were studied in four gravity cores up to 7 8 m long from the pennell trough a nw se trending basin 160 km long and 60 km wide in the central ross sea antarctica with depths occasionally greater than 600 m differences in environments and depositional processes during the last glacial and postglacial epochs were investigated using x rays and volume magnetic susceptibility vms further analyses included bulk and clay mineral composition micropalaeontological studies both benthic and planktic foraminifera and radiometric dating we compare our sedimentological geochemical organic carbon and nitrogen content and geotechnical shear strength and water content results to those on cores previously taken from the region these analyses suggest that prior to the last glacial maximum lgm a glacial marine diamicton

cenozoic shortening budget for the northeastern edge of the tibetan plateau is lower crustal flow necessary 1 two of the most popular mechanisms for thickening the crust beneath the tibetan plateau are 1 pure shear with faulting and folding in the upper crust and horizontal shortening below and 2 flow of lower or middle crust without significant shortening of the upper crust to help discriminate between the relative contributions of these two mechanisms well constrained estimates of upper crustal shortening are needed here we document the cenozoic shortening budget across the northeastern tibetan plateau margin with several balanced cross sections that exploit thermochronological and magnetostratigraphic constraints these sections indicate 11 2 1 east west shortening since middle miocene time and 9 2 3 nne ssw shortening between middle eocene and middle miocene times with little subsequent shortening of this orientation shortening rates accelerate fivefold after middle miocene time given the present day crustal thickness of 56 4 km in northeastern tibet crustal restorations that remove cenozoic shortening suggest that the northeastern tibetan crust was 45 5 km thick prior to the india asia continental collision this precollision thickness estimate is equivalent to average continental crustal thicknesses both adjacent to the tibetan plateau 44 4 km and globally 41 6 km and suggests that pure shear alone may account for cenozoic crustal thickening in northeastern tibet in contrast to eastern tibet where in the absence of significant shortening structures crustal flow has been invoked to explain the addition of crustal material since middle miocene time our results may obviate lower crustal flow as a necessary crustal thickening agent in northeastern tibet

hydrocarbon habitat in main producing areas saudi arabia current hydrocarbon production in saudi arabia is from reservoirs of cretaceous and jurassic age geochemical studies of the sediments and oils suggest that the hydrocarbons were derived from two separate source rock provinces oil production from the large fields in the southern part of the area is from jurassic carbonate reservoirs most of these oils were derived from thermally mature thinly laminated organic rich carbonate rocks of jurassic age callovian oxfordian these source rocks were deposited in an intrashelf basin which is limited to the southern part of the main producing areas extensive vertical migration of oils originating in these sediments is prevented by superjacent evaporite seals deposited during the late jurassic fields in the northern producing areas appear to have derived their hydrocarbons from a source rock province on the north production from cretaceous clastic and carbonate reservoirs is limited to the northeastern part of the producing areas this distribution may be explained by limitation of thermally mature cretaceous source rocks to the northeastern areas or by the local lack of subjacent evaporite seals to separate these reservoirs from jurassic source rocks thermal maturation studies indicate that the hydrocarbons in mesozoic reservoirs migrated into the present traps during the early tertiary

the korudag anticlinorium in the south thrace basin northwest turkey a super giant petroleum trap complex the sedimentary sequence of the south thrace basin northwest turkey comprises upper cretaceous holocene sediments in this basin the korudag anticlinorium which is the subject of this study is located between the aegean sea in the west and the sea of marmara to the east the anticlinorium which is approximately 300 km 186 mi long and 40 km 25 mi wide was formed by effects of the neotethys subduction accretion complex and istranca massif collision during the late early miocene the korudag anticlinorium was deformed to its present day structure by the oldest splay of the north anatolian fault upper middle miocene and the northern branch of the north anatolian fault naf n not earlier than 200 ka organic geochemical analysis oil and gas to source rock correlation and basin modeling studies suggest that the korudag anticlinorium should be charged by hydrocarbons generated from the karaagac ceylan regionally and mezardere formations gas and oil are being produced from the korudag anticlinorium and its subparallel anticlines in the north marmara degirmenkoy seymen cayirdere karacali yulafli tekirdag sevindik and vakiflar gas plus oil fields mapping done as part of this study indicates that the korudag anticlinorium has not yet been tested and explored comprehensively

chronology of late wisconsin ice retreat from the western ross sea antarctica lithologic data from marine sedimentary cores and accelerator mass spectrometer ams radiocarbon dates indicate that grounded ice did not advance to the western ross sea continental shelf edge during the last glacial maximum lgm a chronology of the timing of ice retreat was provided by 26 ams dates obtained from 12 cores dates ranging from 20 to 29 ka suggest that the outer continental shelf beyond ca lat 74 s was not covered by grounded glacial ice prior to and during the early stages of the lgm 14c dates just above transitions from subglacial diamictons to marine muds indicate that the area around the drygalski ice tongue was deglaciated by at least 11 5 ka the ross ice shelf reached its present day position near ross island by about 7 ka a hiatus of about 15 ka is apparent in radiocarbon dates in cores from the outer continental shelf the hiatus is interpreted to represent an absence of sedimentation caused by the presence of an ice shelf

the cenozoic kinematics of qatar evidence for high angle faulting along the dukhan anticline abstract new geological mapping and observations made across qatar favor a fault controlled model for the dukhan structure in the western part of the peninsula over previous fold related and regional erosion models regional stratigraphic relationships and sedimentology record significant fault localized accommodation development this miocene basin development may reflect far field extension in the foreland of coeval thrusting in oman the subsequent change in regional tectonics to convergence across the zagros front is interpreted to have led to basin inversion and the development of the present day topography and outcrop distribution

stable isotopic constraints on the tectonic topographic and climatic evolution of the northern margin of the tibetan plateau abstract this study presents oxygen and carbon isotopic records from lacustrine paleosol alluvial and fluvial carbonate sampled in thirteen cenozoic sedimentary sections that span the northern margin of the tibetan plateau isotopic analyses of 1475 carbonate samples yield 18 o values that range from 13 1 to 38 9 smow and 13 c values range from 11 5 to 3 7 pdb based on these analyses we see two isotopic trends regionally we interpret a paleogene decrease in average oxygen isotope values to result from an increase in the mean elevation of the study region s drainages as they tapped waters from a growing tibetan plateau to the south of the study area we interpret a neogene increase in both carbon and oxygen isotope values to result from the initiation of local mountain building in the study area and the withdrawal of marine waters which increased basin isolation aridity and temperatures the timing of inferred initiation of uplift in this region is consistent with structural thermochronological and sedimentological evidence for a tectonic transition in the early and middle miocene from accommodation of india eurasia convergence through extrusion along the altyn tagh fault to accommodation of convergence through active shortening and mountain building in this region this study also supports tectonic models for the differential uplift of the tibetan plateau in that mountain building in this study region significantly postdates estimates for the attainment of high elevation from isotopic studies of the central tibetan plateau

early cenozoic faulting of the northern tibetan plateau margin from apatite u th he ages models to explain the distributed nature of continental deformation predict the propagation of strain and high topography away from the plate boundary yet a growing body of evidence in the tibetan orogen suggests that deformation occurred at the far northern extent of the modern plateau early in the orogen s history and thus our current mechanical understanding of orogenic plateau development is incomplete new apatite u th he ages from four elevation transects document periods of rapid exhumation related to erosion pulses in hanging wall rocks of major thrust structures accelerated erosion is used as a proxy of fault timing and is interpreted in a larger context of structural data and sediment accumulation in adjacent foreland basins helium results are synthesized with published geologic thermochronometric and sedimentologic data from which a growing picture of regional compressional deformation in middle to late eocene time in northern tibet emerges we relate the early cenozoic period of deformation to the initiation of collision between india and eurasia despite the fact that the plate boundary was located 3000 km to the south regardless of whether or not high topography was built simultaneously as a result of this deformation early cenozoic strain signifies that the modern limit of the orogen has been relatively stationary since continental collision began and that deformation has not significantly propagated farther away from the plate boundary in time

distinguishing subglacial till and glacial marine diamictons in the western ross sea antarctica implications for a last glacial maximum grounding line analyses of lithology stratigraphy and tephra from marine sediment cores collected from the western ross sea during cruises eltanin 32 and 52 and deep freeze 80 and 87 indicate that subglacial till does not extend to the continental shelf edge subglacial till occurs as the lowest unit in most cores landward south of approximately 74 s while seaward of approximately 74 s the lowest diamicton units are glacial marine diamictons glacial marine diamictons are distinguished from subglacial tills by the presence of higher and more variable total organic carbon content downcore distinct tephra layers stratification higher diatom and foraminifera abundances higher sand content and radiocarbon dates in chronological order downcore sand sized tephra layers from two cores on the outer continental shelf are interpreted as single eruptive events one likely to have been derived from the mount melbourne volcano and the other from the pleiades volcano radiocarbon dates from sediment above and below the tephra layer in one of these cores df87 32 show that deposition indicative of open water conditions occurred between 22 and 26 ka in the western ross sea

high resolution stratigraphic correlation of benthic oxygen isotopic records spanning the last 300 000 years abstract in order to compare the response of different oceanographic regions to global climate change very detailed stratigraphic techniques are required the global signal of ice volume changes recorded in the oxygen isotopic composition of foraminifera can provide the tool for developing the necessary high resolution stratigraphy in order to evaluate the resolution of a stratigraphy based on detailed isotopic records two techniques are used to correlate a set of benthic oxygen isotope records from seven piston cores taken in the north and south atlantic the indian and the equatorial and north pacific oceans the first technique was modified from the graphic correlation procedure of shaw 1964 this procedure requires the identification of isotopic events that are correlated from core to core detailed correlations for intervals of the cores between events are provided by a series of straight line segments connecting all common events the second technique developed by martinson et al 1982 uses inverse procedures to define a continuous non linear mapping function that correlates the isotopic records the mapping function maximizes the correlation coefficient between data sets being compared the techniques are independent in that they rely on different criteria for correlating the data series stratigraphic correlations obtained by these procedures are in excellent agreement the mean difference between the correlations is on the order of the sampling intervals of each core and when corrected for sedimentation rates suggests that benthic isotope records from the suite of seven cores can be correlated to a resolution of 2000 to 4000 yrs

transforming the miocene altyn tagh fault slip into shortening of the north western qilian shan insights from the drainage basin geometry although it has long been proposed that active crustal shortening within the qilian shan accommodates slip along the altyn tagh fault questions regarding the temporal and spatial amount of slip transfer remain debated traditional shortening calculations cannot yet be made accurately because the geology is not known well enough following the view that long linear drainage basins in the north western qilian shan developed in response to internal shortening associated with slip transfer along the altyn tagh fault we targeted this special irregular drainage basin geometry to develop an alternative way to estimate shortening we infer a total of 66 km of shortening in the western qilian shan with a north eastward decrease in the magnitude of shortening this 39 shortening strain is comparable to previous estimates estimates of dates of initiation support the view that significant shortening and topographical growth within the north western qilian shan occurred in post middle miocene time

circular geological structures outcropping in the sedimentary basins of saudi arabia abstract a number of circular geological structures outcrop in the sedimentary basins of saudi arabia several are reviewed here to assess their mode of origin they are unrelated to each other are easily seen on aerial imagery and their origins are assessed here on the basis of new fieldwork and reflection seismic data the structures range in size from hundreds of meters to several kilometers in diameter jabal rayah 28 39 n 37 11 30 e is 5 km in diameter with ring faults and anomalously steep dips in siluro devonian strata reflection seismic data demonstrate that underlying strata are structureless and jabal rayah is interpreted as a probable impact structure ash shutbah 21 37 08 n 45 39 21 e is 2 3 km in diameter and consists of a zone of concentric sub horizontal folds in mesozoic carbonates surrounding a central area of disharmonic steeply plunging folds and a mass of stratigraphically anomalous sandstone there are dissolution structures in the vicinity and although this is a viable explanation the uniquely large and complex nature of ash shutbah suggests an alternative origin by impact reflection seismic data show that underlying strata in the vicinity of ash shutbah are structureless a province of at least 100 circular structures occurs in northeast saudi arabia aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features the kidan crater 22 06 53 n 53 26 38 e is a subcircular feature approximately 100 m in diameter exposed in quaternary sabkha between large sand dunes in the rub al khali field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies 3d reflection seismic data shows a small fault underlying the crater leading to a fluid escape interpretation for the depression

sedimentation on the ross sea continental shelf antarctica abstract glacial marine sedimentation has prevailed throughout most of the early glacial history of the ross sea region however since late miocene to early pliocene time the west antarctic ice sheet has advanced onto the continental shelf perhaps several times glacial troughs of the western ross sea may also have formed at this time these glacial troughs border outlet glaciers of the east antarctic ice sheet a widespread lodgement till facies was deposited during this glacial advance holocene retreat of this expanded ice sheet left no conspicuous meltwater record so decoupling of the ice sheet from the seafloor rather than rapid melting is implied modern glacial sediment is far less prevalent than biogenic sediment and reworked glacial sediment except in coastal areas adjacent to small ice shelves and ice tongues little or no debris is being transported beyond the calving line of the ross ice shelf siliceous mud and ooze occurs in the western ross sea where surface circulation is most vigorous and warm deep waters up well this siliceous sediment along with terrigenous silt and clay blankets the seafloor below approximately 300 m water depth most of the ross sea continental shelf is deeper than this surface sediments on shallow banks and coastal platforms have clearly been reworked by marine currents the most vigorous bottom current activity occurs in those areas where geostrophic currents impinge onto the shelf debris flows and turbidity currents are also important in eroding and redistributing glacial sediments on the shelf terrigenous silt and clay are more abundant in surface sediment than basal till the source of this silt and clay is problematic the most plausible sources being onshore transport of suspended sediment by impinging currents and subglacial meltwater input

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 2 sedimentologic and stratigraphic signature sedimentologic geotechnical geochemical and accelerator mass spectrometer ams radiocarbon data from two marine geologic cruises in the ross sea have allowed us to constrain facies relationships and temporal changes in the west antarctic ice sheet the selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality chirp subbottom reflection system a complex but consistent succession of facies documents a number of environments from subglacial to open marine massive mud rich diamictons have low water contents contain in places a calcareous microfossil assemblage show minimal textural variation and contain low and uniform total organic carbon values this reflects a subglacial setting this unit passes upward into a stratified thin granulated facies consisting of pelletized sandy muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble sized clasts this facies reflects the lift off zone or thin water film between the basal debris and sea floor overlying this unit are silty clays that contain a well sorted very fine grained sand component there are no coarse grains within this unit this facies is in part laminated and reflects deposition beneath an ice shelf near and away from the grounding line zone the ice shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open marine setting a sandy volcaniclastic rich subfacies marks the transition from ice shelf to open marine environments found at the calving line the 86 ams radiocarbon dates on organic matter provide an accurate chronology for 19 cores ice shelf conditions were established in the outer drygalski trough by 11 0 25 ka and perhaps earlier this transition took place in the joides basin by 10 8 ka the calving front of the ross ice shelf passed over the drygalski trough at 74 s by 9 5 0 25 ka the timing of deglaciation in the ross sea calls into question current models for the contribution of antarctic glacial ice to holocene sea level rise and suggests that recession was relatively gradual and more closely aligned with northern hemisphere deglaciation and its associated eustatic pulse

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

wrench tectonics in abu dhabi united arab emirates recent studies of the geodynamics and tectonic history of the arabian plate throughout geologic time have revealed that wrench forces played an important role in the structural generation and deformation of petroleum basins and reservoirs of the united arab emirates the tectonic analysis of abu dhabi revealed that basin facies evolution were controlled by wrench tectonics examples are the pre cambrian salt basin the permo triassic and jurassic basins in addition several sedimentary patterns were strongly influenced by wrench tectonics the lower cretaceous shuaiba platform margin and associated reservoirs is a good example wrench faults difficult to identify by conventional methods were examined from a regional perspective and through careful observation and assessment of many factors subsurface structural mapping and geoseismic cross sections supported by outcrop studies and geomorphological features revealed a network of strike slip faults in abu dhabi structural modelling of these wench forces including the use of strain ellipses was applied both on regional and local scales this effort has helped in reinterpreting some structural settings some oil fields were interpreted as en echelon buckle folds associated with ne sw dextral wrench faults several flower structures were interpreted along nw se sinistral wrench faults which have significant hydrocarbon potential synthetic and antitheticmore strike slip faults and associated fracture systems have played a significant role in field development and reservoir management studies four field examples were discussed less

application of landsat imagery to oil exploration in niobrara formation denver basin wyoming the niobrara formation produces oil from fractures in several places in the denver basin the niobrara is an oil prone mature source rock that entered the oil generating window during the laramide orogeny the laramide orogeny began with maximum compressive stress oriented east northeast during the late cretaceous to paleocene and ended with maximum stress oriented to the northeast in the eocene we believe the eocene phase activated northeast trending extension fractures that may have acted as pathways for migration and loci for storage of oil locally generated in the niobrara theoretically the fracture pressures related to oil generation in the niobrara would preferentially open and fill this northeast trending fracture system using landsat imagery to map fracture in the northern part of the denver basin we have identified areas prospective for niobrara oil production within an exploration fairway that is based on subsurface isopach and resistivity mapping support for this concept is the location of wells reported to produce oil from the niobrara along a zone of northeast trending lineaments

oligocene miocene tectonics and sedimentation along the altyn tagh fault northern tibetan plateau analysis of the xorkol subei and aksay basins abstract tertiary basins adjacent to the northern edge of the tibetan plateau include both giant intracontinental sedimentary basins such as tarim and qaidam and smaller basins directly related to the evolution of the altyn tagh strike slip fault system and nearby fold thrust belts three examples of these smaller basins that formed during early evolution of the altyn tagh fault system are the xorkol aksay and subei basins the xorkol basin was the site of nonmarine siliciclastic sedimentation in alluvial fan playa and fluvial environments from at least the mid oligocene through the miocene sediment was delivered to the basin from source areas uplifted by basin bounding thrust faults and from the opposite side of the altyn tagh fault the age of the aksay basin is more poorly constrained but it also contains oligocene miocene nonmarine clastic strata that were derived from the southern side of the altyn tagh fault and adjacent thrust fault bounded uplifts the subei basin is characterized by s

late quaternary and present day rates of slip along the altyn tagh fault northern margin of the tibetan plateau 1 both global positioning system gps measurements and studies of late quaternary faulting are consistent with a slip rate of 10 mm yr along the central segment of the altyn tagh fault and a systematic decrease in that rate toward the eastern end of the fault dates of terraces above and below laterally offset terrace risers yield bounds on quaternary slip rates that range from those that agree with gps measurements to values as much as three times faster we argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace in some cases valleys upstream of the fault have been incised into bedrock and few if any terrace risers can be seen within the valleys such streams debouch onto alluviated floodplains or fans that become incised presumably during climate changes to create terrace risers the terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault and thus the risers become protected from further incision in such cases dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces such dates yield slip rates of 10 mm yr in the central segment of the fault and decreasing rates eastward although we cannot with certainty rule out the higher slip rates along the altyn tagh fault our analysis does show that viable interpretations consistent with gps measurements are more likely at least along some segments of the fault not only do these rates support the view that the tibetan plateau deforms internally by slip on a distributed network of faults in the shallow brittle crust and hence behaves as a continuum at depth but the gradual decrease toward the east also shows that the altyn tagh fault does not separate two effectively rigid lithospheric plates correspondingly the relatively low slip rate and the eastward decrease in slip rate suggest that the altyn tagh fault does not transfer a significant portion of the convergence between india and asia into northeastward extrusion of the tibetan plateau thus large scale extrusion of crustal material in india s path into eurasia seems to be limited largely to the confines of the tibetan plateau