shrimp u pb ages and depth of emplacement of ladakh batholith eastern ladakh india abstract the ladakh batholith represents subduction related andean type magmatism intruding an island arc setting the age and emplacement of the batholith was examined along the kharu chang la section having a width of 25 km the shrimp ii u pb zircon age from a granodiorite near its northern boundary with the shyok suture zone ssz is 60 1 0 9 ma whereas a diorite from its southern boundary close to the indus tsangpo suture zone itsz yields an age of 58 4 1 0 ma al in hornblende barometry indicates the depth of crystallization for the diorite to be 16 1 km and granodiorite to be around 8 1 km the data indicate that diorite and granodiorite were emplaced with a depth difference of about 8 km and are now juxtaposed at the present level of erosion exposed with a 2 km elevation difference this can be attributed to tectonic removal of material along the kharu chang la section the depth of emplacement data also indicates northward tilting of the ladakh batholith after crystallization

growth and deformation of the ladakh batholith northwest himalayas implications for timing of continental collision and origin of calc alkaline batholiths abstract the calc alkaline ladakh batholith nw himalayas was dated to constrain the timing of continental collision and subsequent deformation batholith growth ended when collision disrupted subduction of the tethyan oceanic lithosphere and thus the youngest magmatic pulse indirectly dates the collision both u pb ages on zircons from three samples of the ladakh batholith and k ar from one subvolcanic dike sample were determined magmatic activity near leh the capital of ladakh occurred between 70 and 50 ma with the last major magmatic pulse crystallizing at ca documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand tex

environmental and climatic control on seasonal stable isotope variation of freshwater molluscan bivalves in the turkana basin kenya abstract we present growth incremental stable isotope records 18o and 13c of modern and 2 ma fossil bivalve shells from the semi arid turkana basin n kenya s ethiopia these data suggest that seasonal cyclicity in 18o and 13c recorded by modern and fossil shells is driven by wet dry seasonal changes in host water chemistry forced by monsoonal rainfall over the ethiopian highlands fully lacustrine shells show lower amplitude or even absent seasonal cyclicity in comparison with deltaic or riverine shells because of the buffering effect of the large water volume in the lake setting riverine shells arguably have the clearest relation to seasonally variable climate parameters riverine molluscs thus provide a potentially valuable proxy for varying rainfall 18o values in the turkana basin catchment cross plots of molluscan 18o and 13c data reveal dominant environmental control on molluscan isotope values with remarkably large isotopic differences between lacustrine deltaic and riverine environments we interpret this isotope pattern to directly result from the different mixing proportions of omo river source water with evaporated lake water in these environments the interpretation of fossil molluscan 18o and 13c data in a palaeoclimatological context is not straightforward since the potential influence of temporal changes in lake water temperature surface evaporation or river discharge on the 18o budget of the lake is smaller than the isotopic shifts caused by shifting facies patterns in the sedimentary record even though it is clear from the rich molluscan faunas that the 2 ma palaeo lake lorenyang must have been significantly less alkaline than modern lake turkana and likely provided good drinking water and abundant availability of food for the different species of hominins inhabiting the region at that time stable isotope values of molluscan bivalves are not suitable to record the difference in alkalinity between these two settings

structure of the makran subduction zone from wide angle and reflection seismic data abstract makran is one of the largest accretionary wedges on the globe formed by the convergence between the eurasian and the arabian plates it is characterised by an extremely high sediment input of 7 km and a shallow subduction angle we present seismic velocity models from four wide angle seismic lines that image the wedge sediments and the subducted oceanic crust three strike lines show complex but rather one dimensional structures where the observed seismic phases have been studied with the reflectivity method a 160 km long dip line was surveyed coincident with a mcs line collected by cambridge university in 1986 prestack depth migration of this data with the new wide angle velocities shows improved images compared to earlier results interpretation of the wide angle and the mcs data indicates that a decollement is developed along a bright reflector within the turbiditic sequence more than 3 km of sediment bypasses the first accretionary ridges underthrusting and is transported to greater depth this might help to explain the sparse earthquake activity associated with subduction here in the strike lines low velocity layers occur landward of the deformation front in zones where active thrusting or underplating takes place the oceanic crust is characterised by a strong velocity contrast between layer ii and layer iii

multiple monsoon controlled breakdown of oxygen minimum conditions during the past 30 000 years documented in laminated sediments off pakistan abstract late holocene laminated sediments from a core transect centred in the oxygen minimum zone omz impinging at the continental slope off pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years high sw monsoon controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon toc and redox sensitive elements ni v as well as by a low diversity high abundance benthic foraminiferal buliminacea association and high abundance of the planktonic species globigerina bulloides indicative of upwelling conditions surface water productivity was strongest during sw monsoon maxima stable omz conditions reflected by laminated sediments were found also during warm interstadial events preboreal bolling allerod and dansgaard oeschger events as well as during peak glacial times 17 22 5 ka all ages in calendar years sediment mass accumulation rates were at a maximum during the preboreal and younger dryas periods due to strong riverine input and mobilisation of fine grained sediment coinciding with rapid deglacial sea level rise whereas eolian input generally decreased from glacial to interglacial times in contrast the occurrence of bioturbated intervals from 7 to 10 5 ka early holocene in the younger dryas 11 7 13 ka from 15 to 17 ka heinrich event 1 and from 22 5 to 25 ka heinrich event 2 suggests completely different conditions of oxygen rich bottom waters extremely low mass and organic carbon accumulation rates a high diversity benthic fauna all indicating lowered surface water productivity during these intervals the omz was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods the abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the north atlantic heinrich events and dust peaks in the tibetan loess records the correlation of the monsoon driven omz variability in the arabian sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale

episodic unroofing of the kohistan batholith pakistan implications from k feldspar thermochronology new 40ar 39ar and u pb mineral ages from plutonic rocks help constrain the thermal and tectonic evolution of the kohistan island arc terrane following its collision with india in early tertiary time kohistan has experienced a prolonged tectonomagmatic history extending from the early cretaceous through the tertiary thermal histories derived from multi diffusion domain analyses of k feldspar within the kohistan batholith reveal rapid cooling events 70 140 c m y distributed through space and time the cooling histories show a systematic variation along the length of the batholith suggesting that kohistan experienced differential unroofing an episode of rapid cooling in the middle eocene is recognized in western kohistan whereas rapid cooling occurs substantially later during the middle and late miocene in eastern kohistan rapid cooling in western kohistan might have been caused by postemplacement cooling of hot magma against cold country rocks at relatively shallow crustal levels within the nw region of eastern kohistan rapid cooling at 13 12 ma is recorded in k feldspars 60 km from the contact with the nanga parbat haramosh massif nphm whereas cooling does not occur until 11 ma only 2 5 km from the contact this temporal and spatial variation in cooling histories may record differential unroofing in response to the development and propagation of the nphm structure beneath kohistan the nphm has experienced rapid cooling and unroofing over the last 10 m y and our results are consistent with this mid late miocene event

the pianosa contourite depositional system northern tyrrhenian sea drift morphology and plio quaternary stratigraphic evolution abstract the pianosa contourite depositional system cds is located in the corsica trough northern tyrrhenian sea a confined basin dominated by mass transport and contour currents in the eastern flank and by turbidity currents in the western flank the morphologic and stratigraphic characterisation of the pianosa cds is based on multibeam bathymetry seismic reflection data multi channel high resolution mini gi gun single channel sparker and chirp sediment cores and adcp data the pianosa cds is located at shallow to intermediate water depths 170 to 850 m water depth and is formed under the influence of the levantine intermediate water liw it is 120 km long has a maximum width of 10 km and is composed of different types of muddy sediment drifts plastered drift separated mounded drift sigmoid drift and multicrested drift the reduced tectonic activity in the corsica trough since the early pliocene permits to recover a sedimentary record of the contourite depositional system that is only influenced by climate fluctuations contourites started to develop in the middle late pliocene but their growth was enhanced since the middle pleistocene transition 0 7 0 9 ma although the general circulation of the liw flowing northwards in the corsica trough remained active all along the history of the system contourite drift formation changed controlled by sediment influx and bottom current velocity during periods of sea level fall fast bottom currents often eroded the drift crest in the middle and upper slope at that time the proximity of the coast to the shelf edge favoured the formation of bioclastic sand deposits winnowed by bottom currents higher sediment accumulation of mud in the drifts occurred during periods of fast bottom currents and high sediment availability i e high activity of turbidity currents coincident with periods of sea level low stands condensed sections were formed during sea level high stands when bottom currents were more sluggish and the turbidite system was disconnected resulting in a lower sediment influx

large scale sediment underplating in the makran accretionary prism southwest pakistan field relations in the emergent part of the makran accretionary prism show that a mid miocene to early pliocene slope and shelf sedimentary sequence was deposited directly on abyssal plain turbidites without any detectable stratigraphic or structural discordance sedimentological evidence for rapid shoaling however indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period this can be explained by large scale underthrusting and underplating of sediment a mode of accretion that is also favored by mass balance considerations the visible deformation in the coastal makran occurred when the region was 70 100 km north of the contemporary prism front

lake level changes and hominin occupations in the arid turkana basin during volcanic closure of the omo river outflows to the indian ocean in the east african rift the western margin of lake turkana northern kenya exposes mio plio pleistocene lake sediments with dated volcanic horizons constraining basin dynamics at the astronomical time scale since the late pliocene coastal archaeological sites have formed within the lacustrine dynamics here lake levels are reconstructed from 2 4 to 1 7 ma using sedimentary facies and water depth controlled sediment association the lacustrine stratigraphy is measured with a total station and cyclostratigraphy is derived from tephrochronology the water depths are evaluated from paleochemical properties of lake sediments analyzed by inductively coupled plasma optical emission spectrometry and inductively coupled plasma mass spectrometry our reconstruction highlights that lake turkana rose during 100 ka insolation eccentricity maxima periods in response to higher monsoonal inputs of the omo river however lake turkana also expanded through an insolation minimum at 2 17 1 95 ma this asynchronous lake phase coincides with volcanic closure of the omo river and lake turkana outflow sill to the east and the indian ocean an archaeological hiatus occurs during this endorheic lake phase and alkalinity increases at the beginning of the hiatus the lake rose again during insolation eccentricity maxima at 1 9 1 7 ma and a new outflow sill opened to the west and the nile basin hominin coastal occupations return during this exorheic freshwater lake phase

quantitative provenance analysis of sediments review and outlook provenance analysis of sediments is aimed at reconstructing the parent rock assemblages of sediments and the climaticphysiographic conditions under which sediments formed inferring sediment provenance from the final product a basin fill is anything but straightforward because the detrital spectrum evolves as the sediment is transported along the pathway from source to basin successful provenance analysis requires that the nature and extent of compositional and textural modifications to the detrital spectrum be recognised if not quantified the history of quantification in sediment provenance studies is summarised and illustrated by tracking two fundamental ideas the concept of the sediment petrological province or petrofacies and the relation between sandstone composition and plate tectonic environment progress in sedimentary provenance analysis has been closely linked with advancements in measurement technology a brief survey of modern data acquisition tools illustrates the possibilities and limitations of modern provenance research an operational definition of quantitative provenance analysis qpa is presented in which the central role of mass balance is acknowledged extension of this definition to include quantitative predictions obtained by forward modelling computer simulation of sediment production as well as methodological improvements in data acquisition and processing is needed to cover likely future developments in qpa the contributions to the special issue quantitative provenance analysis of sediments illustrate the intrinsic multidisciplinarity and rapid expansion of qpa d 2004 elsevier b v all rights reserved

tectonics of the southern asian plate margin along the karakoram shear zone constraints from field observations and u pb shrimp ages abstract new geological observations recent published data and u pb shrimp zircon dating from the karakoram mountains along the nubra and shyok rivers reveal that the initial subduction of the tethyan oceanic lithosphere took place 110 ma beneath the paleozoic mesozoic platform of the southern edge of the asian plate this has produced the i type plutons within the karakoram batholith complex well before the juxtaposition of the asian plate along the karakoram shear zone within this shear zone u pb zircon crystallisation ages of 75 ma from mylonitised granitoids and 68 ma from undeformed tirit granodiorite constrain the timing of suturing of the karakoram terrain with the trans himalaya between 75 and 68 ma post shearing leucogranite was episodically generated within frontal migmatised karakoram metamorphic belt and emplaced between 20 and 13 ma within the shear zone presence of a low resistivity zone as a possible indication of mid crustal partial molten crust underneath the higher himalaya ladakh karakoram terrains manifests the impingement of the indian plate along the main himalayan thrust at depth physical continuity of the baltoro granite belt into the karakoram batholith is established as well as the continuity of the shyok suture as the shiquanhe suture zone in western tibet through the chushul dungti sector the karakoram shear zone therefore displays a complex geological history of movements since 75 ma and plays a very significant role in the overall india asia convergence rather than merely being a strike slip fault for eastward extrusion of a segment of asia in tibet

internal deformation and compaction of the makran accretionary wedge the makran accretionary wedge is one of the largest on earth a 7 km thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets we present the results of prestack depth migration and focusing error analysis migration velocity analysis performed on a profile across the makran wedge the depth section shows the deformation style of the accreted sediments and the migration velocities allow us to estimate porosity variations in the sediments the thrust sheets show evidence of fault propagation folding with a long wavelength of deformation 12 km and secondary thrusting in the kink bands of the folds such that the central part of each thrust sheet is elevated to form an additional ridge this deformation style and the 15 steep surface slope of the first ridge suggest a high degree of consolidation porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure was estimated for 5 locations along the profile rather than being undercompacted and overpressured as in most accretionary wedges the sedimentary input is normally compacted exponential porosity decay throughout almost the whole wedge however a slight increase in porosity and at depth with respect to the normal compaction curve indicates that the turbiditic sequence might be overpressured landward of the deformation front

constraints on pleistocene pluvial climates through stable isotope analysis of fossil spring tufas and associated gastropods kharga oasis egypt abstract stable isotope analyses of fossil spring carbonates tufas and freshwater gastropods melanoides tuberculata from the currently hyperarid western desert of egypt indicate that this region received enough precipitation to support a small perennial lake during the height of the oxygen isotope stage 6 5e pluvial event and a substantial volume of spring discharge during prior pluvial phases tufa and gastropod oxygen isotope ratios are generally low relative to modern precipitation in the region confirming that the majority of pleistocene pluvial precipitation came from an atlantic source as determined by previous workers mean tufa 13c 2 pdb suggests a significant proportion of c4 vegetation in the recharge area of the tufa depositing springs while the paucity of tufas with 13c greater than 0 suggests that equilibrium between spring water and atmospheric co2 represented an endpoint for the evolution of spring water carbon though spring carbonate deposition requires a substantial increase in humidity in central egypt relative to modern conditions high variation in tufa 18o up to 4 within individual stratigraphic units as well as positive covariance between tufa carbon and oxygen isotope ratios reflect significant effects of evaporation on spring water during tufa formation similarity in 18o and 13c values of tufas from different stratigraphic units indicates that climatic conditions did not vary significantly among pluvial phases recorded by spring carbonate deposition the lack of substantial carbonate formation at tufa localities during the holocene pluvial phase however suggests that tufa deposition represents substantially wetter conditions than have existed during holocene time

latest cretaceous himalayan tectonics obduction collision or deccan related uplift abstract diverging interpretations and incompatible scenarios have been proposed for the early stages of himalayan history numerous researchers have postulated that northern india was involved in ophiolite obduction arc continent or continent continent collision during the late cretaceous or early paleocene but firm geological evidence was never produced in this article we argue against orogenic events predating the late paleocene when the neotethys ocean was still open the tethys himalayan sedimentary record testifies to anorogenic evolution primarily controlled by dynamic uplift of the passive margin prior to the massive outburst of deccan lavas and eventually followed by thermal subsidence major stratigraphic gaps in pelagic sediments suggest that such tectono magmatic episode started to affect the base of the indian plate in the campanian or possibly even in the santonian 10 to 20 ma before the climax of deccan flood basalt eruptions the abrupt increase in siliciclastic supply and accumulation rates recorded in sedimentary basins all around the indian subcontinent during the maastrichtian was followed by progradation of coastal quartzarenites along the northern indian margin in the early paleocene sandstones derived from the rejuvenated craton and uplifted inner continental margin in the south are dominantly but not exclusively quartzose felsitic volcanic rock fragments and cr spinels many of which with the same geochemical fingerprint as deccan spinels and newly found throughout the maastrichtian to danian succession resisted the combined effect of subequatorial weathering and subsequent diagenesis and testify that detritus from deccan basalts reached the indian passive margin as far as south tibet at the close of the early paleocene india drifted away from the seychelles block and thermal subsidence led to widespread carbonate deposition along the tethys himalaya this article illustrates how mega events of magmatic upwelling followed by lithospheric cooling may control passive margin sedimentation and stratigraphic patterns as occurred in northern india first in the early cretaceous and next in the latest cretaceous paleocene

the transhimalaya gangdese plutonism in the ladakh region a upb and rbsr study abstract the age and origin of the transhimalaya gangdese plutonic belt in the ladakh area has been studied by high resolution u pb analyses of accessory minerals zircon and monazite allanite and rb sr measurements on whole rock samples the ages determined of 101 2m y for a granodiorite and of 60 7 0 4m y for a granite substantiate that the transhimalaya plutonism was active at least in mid cretaceous albian and earliest tertiary palaeocene times a rb sr isochron of 73 4 2 4m y might be fortuitous because the rb sr systematics shows important heterogeneities on the whole rock scale the inherited radiogenic lead in zircon demonstrates that anatexis of continental crust was involved in magma genesis the thus recycled continental material was heterogeneous with respect to its primary ages and or metamorphic history the approximate minimum ages of the continental sources range from 350 to 590 m y the 101 2m y old transhimalaya granodiorite which intrudes a series of mainly basaltic island arc rocks dras series shows that this island arc was attached to the continental margin at that time this is consistent with the hypothesis that the ladakh tethys closed through two subduction regimes 1 an early cretaceous subduction which formed the dras island arc and 2 a late cretaceous to palaeocene subduction at the continental margin which caused the emplacement of the transhimalaya plutonic belt

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

u pb age of the baltoro granite northwest himalaya and implications for monazite u pb systematics the baltoro granite is a major late to post tectonic plutonic phase of the karakoram batholith of the northwest himalaya in northern pakistan u pb zircon analyses indicate both emplacement at 21 0 0 5 ma and significant precambrian zircon inheritance dates on monazite are 17 19 ma and are interpreted to have remained near their closure temperature of about 700 c for several million years after emplacement resulting in pb loss by diffusion we suggest that the granite was emplaced into rocks which were at high temperature and that they remained so until late miocene northeast tilting rapid uplift and or tectonic denudation and cooling

a giant catastrophic mud and debris flow in the miocene makran we challenge the former interpretation of the sedimentary me lange of the makran accretionary complex as a tectonic me lange diapirically emplaced from below and provide evidence for its sedimentary gravitational emplacement from the north during tortonianmessinian times between 11 8 and 5 8 ma it is an olistostrome that includes blocks of ophiolites and oceanic sediments derived from the coloured me lange to the north and reworked chunks of the turbidites on which it rests with an erosional truncation the chaotic scattering of blocks of any size and lithology and the weak soft sediment deformation of the matrix argue against a tectonic emplacement of the chaotic formation its size and internal structure together with the size of the individual blocks make the olistostrome a fossil equivalent of the large gravitationally emplaced debris flows observed along present day continental margins and unstable volcanic edifices

plio pleistocene marine continental correlation using habitat indicators from olduvai gorge tanzania abstract reconstructions of plio pleistocene habitats and lake levels from a brief interval 200 000 yr of hominid bearing sediments from bed i and lower bed ii olduvai gorge tanzania are for the first time integrated with climatic data from the deep sea in order to examine the effect of largescale climatic events in east africa the period of densely vegetated habitats and wet conditions at olduval at 1 82 myr shows a best fit with a period of northern hemisphere warming as evidenced by oxygen isotopes from deep sea core v28 239 from the western pacific this environmental response is similar to that witnessed in east africa during the period of warming climates associated with deglaciation at 10 000 8 000 yr

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

jurassic rifting at the eurasian tethys margin geochemical and geochronological constraints from granitoids of north makran southeastern iran this study focuses on an east west trending belt of granitic to intermediate intrusions and their volcanic cover in the northern dur kan complex a continental slice outcropping to the north of the exposed makran accretionary wedge in southeastern iran field observations petrographic descriptions trace element and isotope analyses combined with u pb zircon geochronology are presented to determine the time frame of magmatism and tectonic setting during the formation of these rocks results document three magmatic episodes with different melt sources for 1 granites 2 a diorite trondhjemite plagiogranite sequence and 3 diabases and lavas granites dated at 170 175 ma represent crystallized melt with a strong continental isotopic contribution the diorite trondhjemite plagiogranite sequence is 165 153 ma old and derives from a mantle magma source with minor continental contribution east west trending diabase dikes and bodies intruded the granitoids which were eroded and then covered by valanginian 140 133 ma alkaline lavas and sediments alkaline dikes and lavas have a mantle isotopic composition temporal correlation with plutonites of the sanandaj sirjan zone to the northwest defines a narrow nw se striking and nearly 2000 km long belt of jurassic intrusions the increasing mantle influence in the magma sources is explained by thinning of continental lithosphere and related mantle upwelling decompression melting accordingly the formation of the studied igneous rocks is related to the extension of the iranian continental margin which ultimately led to the formation of the tethys related north makran ophiolites

u pb zircon ages from the spontang ophiolite ladakh himalaya the spontang ophiolite in the ladakh himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of india during the late cretaceous precise isotopic ages of himalayan ophiolites are presently unavailable u pb dating of zircons from a dioritic segregation in the high level gabbros of the spontang ophiolite shows that the complex formed at 177 1 ma mid jurassic and we suggest that it represents a fragment of neo tethyan oceanic crust this jurassic age contrasts with late cretaceous ages 91 95 ma previously reported for the formation of the troodos and oman ophiolite complexes further west along the neo tethyan suture zone and also predates the early to mid cretaceous masirah ophiolite c 150 ma and its alkaline volcanic seamounts 115 125 ma an andesitic rock from an arc sequence overlying the spontang ophiolite was also dated using as 88 5 ma this late cretaceous age constrains the minimum age of initiation of subduction beneath the spontang ophiolite the complex was emplaced during the turonian maastrictian some 80 ma after its formation when the indian continental margin collided with a cretaceous arc about 25 ma before the india asia collision the lack of a sub ophiolite metamorphic sheet beneath the spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction

constraints on the collision and the pre collision tectonic configuration between india and asia from detrital geochronology thermochronology and geochemistry studies in the lower indus basin pakistan abstract knowledge of the timing of india asia collision is a fundamental prerequisite for understanding the evolution of the himalayan tibetan orogen and its role in global climate oceanic chemistry and ecological evolution despite much active research the basic pre collision tectonic configuration and the timing of terminal india asia suturing remain debated for example debates regarding when and how the intervening kohistan ladakh arc was sutured with india and asia still remain elusive some models propose the arc collided with asia at about 100 ma with india asia collision at ca 55 ma whilst a newer model proposed the arc s collision with india at 50 ma and subsequently with asia at 40 ma another example is the recent proposition that an oceanic greater india basin separated the tethyan himalaya microcontinent from the remaining indian plate until 20 25 ma with the consumption of this oceanic basin marking the final collision at this time these controversies relate to whether the commonly documented 50 ma contact represents the terminal india asia suturing or the amalgamation between various arcs or microcontinents with india or asia here we present an integrated provenance study of geochronology thermochronology and geochemistry on the late cretaceous pleistocene sediments from the lower indus basin on the indian plate the detrital zircon u pb and fission track data show a reversal in sediment source from a pure indian signature to increasing inputs from the suture zone and the asian plate between the middle paleocene and early oligocene the nd and sr isotopes narrow down this change to 50 ma by revealing input of asian detritus and the establishment of a nd sr isotopic pattern similar to the present day indus fan by 50 ma with no significant variations up section contrary to what might be expected if later major collisions had occurred our isotopic data indicate that greater india was occupied by a fluvial deltaic system analogous to the present day indus and named as the paleo indus which has been transporting asian detritus southward across the suture zone and kohistan ladakh arc since 50 ma suggesting no other ocean basins intervened between india and asia after this time in this region our data require that in the west the india asia collision were accomplished by 50 ma

an isotopic study of a fluvial lacustrine sequence the plio pleistocene koobi fora sequence east africa stable isotopic analyses of plio pleistocene and modern sediments in the fluvial lacustrine system occupying the turkana basin east africa provide constraints on the paleoenvironmental and diagenetic histories of the pliocene through the recent sediments in the basin the 13c values for carbonates in lacustrine sediments range from 15 to 22 relative to pdb depending on the varying proportions of co2 from the atmospheric reservoir and from various metabolic sources the 18o values of carbonates in lacustrine sediments indicate that the isotopic composition of paleolake water varied by over 10 from the pliocene to the present the 13c values for pedogenic carbonates record paleoccologic variations and suggest that c4 plants did not become well established in the preserved depositional parts of the basin until about 1 8 myr ago the 18o values pedogenic carbonates suggest a range of over 10 for the isotopic composition of soil water during this interval they also suggest a period of major climatic instability from about 3 4 to 3 1 myr and at about 1 8 myr together the 13c and 18o values of pedogenic carbonates indicate that the present conditions are as arid and hot as any that had prevailed during deposition of these plio pleistocene sediments

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

morphotectonics and mechanics of the central makran accretionary wedge off pakistan abstract swath bathymetry images and 3 5 khz data acquired during rv sonne cruise 123 in autumn 1997 are the base for analysing the morphology and deformation styles of the huge makran accretionary wedge the continental slope off pakistan is characterised by three morphological provinces a the lower slope is built of a sequence of imbricate thrust slices which form long and narrow accretionary ridges with flanks locally as steep as 20 and between 10 and 20 on average while the regional slope is only 1 2 b the nearly flat mid slope terrace is present in the central east part of the margin narrows significantly towards the east and is absent east of 63 45 and c the upper slope up to the shelf has a regionally uniform slope as steep as about 8 and comprises a very rough topography with many gullies and canyons and evidence for mass wasting two meandering canyons crossing the entire margin down to the abyssal plain at about 63 15 63 30 can be correlated with onshore rivers they have been formed by episodic erosion through turbidity flows and are characterised by erosional and depositional portions sinuosity of the canyons is exclusively caused by the accretionary ridges functioning as obstacles for flow to be directly downhill application of coulomb rheology to the frontal part of the makran wedge implies that the mid level decollement is intrinsically extraordinary weak with a great strength contrast to the overlying sediments comparison with nankai cascadia and the western mediterranean ridge reveals new insights in the parameters controlling the shape of an accretionary wedge and the role of a mid level decollement

palaeoclimates at lake turkana kenya from oxygen isotope ratios of gastropod shells climatic changes in the plio pleistocene of east africa during the evolutionary development of early man have been identified only in their broadest outlines a variety of palaeontological1 3 and geochemical4 techniques have already been applied to the study of palaeoclimates in this area but the use of stable oxygen isotope ratios so helpful in marine palaeoclimatology has not been fully exploited5 6 the frequent occurrence of well preserved mollusc shells in the sedimentary column at lake turkana provides a sensitive oxygen isotope record varying over a range of 7 which can be interpreted in terms of evaporative concentration of the heavier oxygen isotope in waters of the closed lake alternating with isotopically lighter freshwater when the lake has been open i report here the results of a study of the 18o 16o ratios of three species of gastropods from the sediments of the koobi fora region of lake turkana and evaluate how the technique might be applied to the study of the palaeoclimates of other tropical lakes

slip vectors and fault mechanics in the makran accretionary wedge southwest pakistan the makran is a broadly east west trending arcuate accretionary wedge currently forming where the oceanic floor of the gulf of oman is being subducted north northeastward beneath the south asian margin two traverses across the makran coast ranges extending about 80 km inland from the coastline which lies about 100 km north of the wedge front show that the structure of this region is dominated by large back rotated south directed thrust faults and associated folds related structures include duplexes fault gouge containing reidel shears and melange along fault zones these structures are cut by later north directed backthrusts out of sequence south directed thrusts and apparently conjugate sets of nne trending sinistral and nw trending dextral strike slip faults slip vectors were determined from slickenlines fiber lineations gouge fabrics and riedel shears vectors from early thrusts and bedding parallel slip surfaces corrected for tectonic rotation form a broad south directed concentration lying between the normal to the regional strike varying from 160 to 180 and the present plate convergence vector 200 locally the concentration is bimodal this pattern may represent the interaction between plate forces and body forces in the accretionary wedge the late thrusts have widely variable slip vectors and stair step off the early thrusts they overlap in orientation and slip vector with gently dipping sinistral strike slip faults suggesting that they also stair step in plan off the latter the steep strike slip faults represent the final tectonic stage involving minor n s shortening and longitudinal extension these three stages probably occurred in sequence in any one area but migrated diachronously southward as the wedge grew

the initiation of northern hemisphere glaciation for well over a century scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases such as carbon dioxide could control the earth s climate e g arrhenius 1 896 recent observations of significant glacial interglacial variation in atmo spheric co2 in ice cores put this idea on a firm observational as well as theoretical footing e g barnola et ai1 987 in this century as atmospheric co2 levels creep steadily upwards in response to the burning of fossil fuels and forests the specter of manmade global climate change looms before us ipcc 1990 a possibility that has heightened interest in the natural variability of climate in the past in particular the 3 c warming pre dicted for the next century has stimulated interest in past warm climates such as the middle pliocene around 3 0 million years ma ago an interval that has often been invoked as a greenhouse world budyko et al 1985 in this paper the climate transition from the warm mid pliocene around 3 2 ma to the onset of northern hemisphere ice ages around 2 4 ma is examined evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe while the cause of individual glacial interglacial oscillations is tied to milankovitch variations in the earth s orbit around the sun e g imbrie et ai 1 992 these insolation changes cannot account for the long term cooling trend which culminated in northern hemisphere glaciation in the final section of this paper mechanisms of long term climate change

himalayan detrital chromian spinels and timing of indus yarlung ophiolite erosion abstract the geochemistry of detrital chromian spinels is commonly used to discriminate provenance from different tectonic settings of mafic and ultramafic igneous rocks detrital spinels in cenozoic foreland basin successions fed from the himalaya orogen were assertively interpreted as sourced from the ophiolitic rocks of the indus yarlung suture zone this study compares the geochemistry of detrital cr spinels from the tethys himalaya passive margin and cretaceous xigaze forearc successions with those from the indus yarlung ophiolites cr spinels in the indus yarlung ophiolites have low tio2 mostly 0 2 and low al2o3 mainly 6 23 indicating a rift related basaltic origin detrital cr spinels from the xigaze forearc basin have either low tio2 mostly 1 0 indicating intra plate basaltic origin compositional fingerprints of detrital cr spinels from lower eocene foreland basin strata in the central eastern himalaya indicate provenance from the lhasa block without input from the indus yarlung ophiolites only cr spinels from the lower eocene foreland basin strata in the north western himalaya and the upper eocene lower miocene remnant ocean turbidites of the bengal basin are mostly ophiolite derived the indus yarlung ophiolites were thus emplaced and exposed to erosion since the early eocene 50 ma in the nw himalaya but only subsequently 50 38 ma in the eastern himalaya

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

stratigraphy and isotope records of middle and late pleistocene mollusks from a continuous corehole in the hula basin northern jordan valley israel morphological changes in melanopsis shells and the sudden disappearance of viviparus were observed in a continuous core representing about 700 000 yr taken from the l 12 corehole in the hula basin northern jordan valley israel two major events in 18o values are recorded the first at about 435 000 yr b p is accompanied by a shift in 13c values and indicates a shallowing phase with slightly higher evaporation this event is concurrent with the abundant occurrence of a monotypic distorted melanopsis suggesting an ecophenotypic event during deposition of the lake s pleistocene lower lacustrine beds the second major isotopic event is evidenced in the lower part of the section as a gradual depletion in 18o toward the bottom of the corehole minor shifts of 1 20 in the oxygen isotope content along the core may represent changes in the 18o values of the water from glacial to interglacial periods representing the major climatic cycles of the pleistocene five of these cycles were observed along the core

a tectonic pulse in the makran accretionary prism recorded in iranian coastal sediments many recent studies of syndeformational sediments in a wide range of tectonic environments have improved our understanding of the development of individual or regional deformation structures here we report studies of growth strata that imply an unexpected tectonic history within part of the makran accretionary prism currently the largest on earth we interpret the lithostratigraphy and ichnofacies of plio pleistocene marine deposits logged along part of the makran coast in terms of their palaeoenvironments lateral trends in thickness and depositional facies of rock units and erosional surfaces among them are correlated and then related to displacements along syndepositional faults that are distinguishable from changes in global sea level past studies of accretionary prisms have emphasized thrusting at the deformation front offshore however at the shoreline we find episodes of deposition during subsidence as a result of oblique slip normal faulting alternating with episodes of erosion during uplift driven by oblique slip thrusting the absolute timing of these displacements is not yet known however faults above a deep zone of subcretion or underplating well behind the surficial deformation front have clearly undergone episodic reversals in throw that imply alternations in lateral shortening and extension

detrital zircon and provenance analysis of late cretaceous miocene onshore iranian makran strata implications for the tectonic setting a multidisciplinary provenance study including sandstone framework heavy mineral analysis in situ u pb dating of detrital zircon and hf isotopic ratio analysis of dated zircons was undertaken on late cretaceous miocene deep marine turbiditic and deltaic sandstones of makran accretionary wedge se iran to determine their sedimentary provenance and tectonic setting sandstone framework modes reveal both magmatic arc rocks as a source of late cretaceous oligocene detritus and recycling of miocene sandstones heavy mineral assemblages cr spinel and blue amphibole indicate ophiolite and high pressure low temperature metamorphic rocks blueschists as a supplementary provenance in total 2931 laser ablation inductively coupled plasma mass spectrometry icp ms u pb detrital zircon ages on 21 sandstone samples yielded three major age peaks at ca 167 ma 88 7 ma and 48 9 ma also 241 in situ hf isotope analyses of dated zircons provide evidence for dominantly igneous source rocks two main detrital zircon ages are identified 1 abundant middle jurassic grains with hf isotopic compositions of continental crust suggesting a rifting related magmatic provenance and 2 late cretaceous eocene grains with hf isotopic compositions of continental crust and nondepleted mantle suggesting a continental magmatic arc provenance this change in provenance is attributed to the late cretaceous convergence between arabia and eurasia

molluscan biostratigraphy of the koobi fora hominid bearing deposits since the discovery1 of late cenozoic hominid bearing deposits in the koobi fora area north kenya a variety of studies2 9 have been undertaken to provide a chronostratigraphical and palaeoenvironmental context for the important palaeoanthropological and early archaeological discoveries1 10 unfortunately although the exposures are areally extensive 1 000 km2 they are broken up into subareas fig 2 these local sections are often difficult to correlate with one another due to intervening bush cover and faulting here i summarize a molluscan biostratigraphical analysis which demonstrates serious stratigraphical miscorrelations between certain local sections including some previously indicated by vertebrate evidence these miscorrelations require substantial revisions to current stratigraphical interpretation of the koobi fora deposits in particular both the suregei and tulu bor tuff horizons actually represent different levels although the tulu bor is considered to overlie the suregei some exposures currently attributed to the tulu bor are actually older than some units previously mapped as the suregei fortunately the stratigraphical miscorrelations reported here do not significantly affect the relative placement of the mollusc faunas reported in a previous evolutionary analysis15 superpositional relationships in the areas from which these faunas were collected mainly collecting areas 102 103 110 123 202 and 203 are unaffected by the miscorrelations reported here

granites of western karakorum and northern kohistan pakistan a composite mid cretaceous to upper cenozoic magmatism abstract the petrography the chemical mineralogical typology and the ages of six plutonic units four from the karakorum axial batholith darkot pass ghamu bar batura and hunza units and two of northern kohistan gindai and nomal plutons are defined and compared based on field data and analyses of up to 78 samples major elements ree rb s sbnd sr and k ar isotopes the karakorum axial batholith is a composite body three major intrusive stages occurred 1 around mid cretaceous times ca 110 95 ma with the emplacement of subalkaline i e monzonitic darkot pass and calc alkaline hunza ghamu bar units 2 during palaeogene maybe up to 43 ma batura subalkaline unit a strong tectonommetamorphic event recorded in the gneissification of the cretaceous intrusives occurred between these two stages it may be of palaeocene age p t estimates of the highest metamorphic grade rocks of the hunza unit have yielded values of 580 640 c and 5 0 5 kbar and 3 during upper miocene ca 9 ma baltoro subalkaline unit debon et al 1986c in addition a conspicuous network of aplo pegmatitic dykes emplaced into the hunza area possibly from the eocene up to the upper cenozoic with a maximum during the middle miocene ca 15 ma most of these major magmatic stages are met again among the acidic intrusives and dykes of northern kohistan the first one as blastomylonitic tholeiitic plutons nomal ca 102 ma petterson and windley 1985 the second one as subalkaline plutons gindai ca 59 ma and a third one as leucocratic dykes of oligocene age ca 30 ma petterson and windley 1985 these data may be related to the geodynamic evolution of the nw part of the india eurasia suture zone thus allowing better constraints on the major steps of this evolution the partly synchronous closures by n dipping subduction of the two tethys branches which are assumed to have encircled the kohistan arc in upper mesozoic times may have generated both the karakorum and the kohistan intrusives of cretaceous and palaeogene ages the northern branch very likely closed before the southern one at the time of the second intrusive stage palaeogene a kohistan and karakorum had already collided were welded and had suffered the same major tectonometamorphic event b subduction of the southern tethys floor beneath the welded kohistan karakorum was still active c however collision between india and kohistan karakorum may have already begun particularly at the level of the nanga parbat promontory finally it is emphasized that the intrusive processes continued in the karakorum long after the collision e g baltoro granite

correlation of fluvial terraces and temporal steady state incision on the onshore makran accretionary wedge in southeastern iran insight from channel profiles and 10be exposure dating of strath terraces we describe and summarize the geomorphology of fluvial terraces along the four major rivers draining the central onshore makran accretionary wedge in iran and describe uplifted marine terraces on the coast of this region thirty five strath terraces at different sites were dated using in situ produced cosmogenic 10 be concentrations from surfaces and depth profiles these new measurements reveal abandonment ages between ca 15 and 380 ka the age distribution allows determining the chronology of terrace levels and establishing regional correlations between two major regional levels the geographically widespread correspondence suggests that these two levels result from a regional climatically driven force systematic dating also provides evidence for time and spatial variations in incision rate which enables distinguishing between a regionally uniform incision rate of 0 3 0 4 mm a and higher local incision and or uplift 0 8 mm a rates these spatial changes are consistent with where localized tectonic activity is recognized in the field we also dated using 14 c in shells four uplifted marine terraces on the coast of the study area comparing incision rates derived from strath terraces 0 3 mm a with published uplift estimates from marine terraces 0 2 mm a reveals that fluvial rivers responded to a regional long term incision and surface uplift pattern this rate reflects the tectonically steady state of the wedge on a regional scale locally high incision rates delineate active faults and folds indicating that perfect steady state is unlikely on short length scales

oxygen and carbon isotopic records of mollusks in the kuokang shell bed taiwan implications and applications abstract oxygen and carbon isotopic compositions have been determined in fossil molluscan shells from the kuokang shell bed and in modern shells from coastal areas around taiwan fossil mollusks have been well preserved based on x ray diffraction stable isotopic data and their occurrence in the field the stable isotopic data of fossil shells indicate that they lived in an open sea environment heavier oxygen and lighter carbon isotopic ratios of these fossils relative to modern counterparts around taiwan indicate that these fossil mollusks grew in a glacial period as supported by the 14c dating results the calculated mean uplift rate for the kuokang shell bed shows a lower rate compared to the eastern and southern parts of taiwan and reflects a more stable away from collision nature for western taiwan

stable carbon and oxygen isotopic composition of early holocene gastropods from wadi mansurab north central sudan the 8500 to 7000 year old shell bearing sediments in shallow claypans near wadi mansurab in north central sudan appear to reflect a widespread and much wetter period in northeast africa stable carbon and oxygen isotope analysis of the wadi mansurab gastropods reveals that the shell carbonate is highly depleted in 13c and 18o the highly depleted oxygen isotopic composition is indicative of rainfall derived from a distant oceanic source the highly negative nature of the oxygen isotopes indicates that the region had significantly less evaporation than today while the extreme variability in isotopic composition up to 6 7 pdb is consist ent with a seasonal rainfall regime characterized by a high degree of inter annual variability the dominance of semi aquatic and swamp dwelling gastropods over truly aquatic species implies that the region was probably a seasonally flooded grassplain similar to the toich lands of south central sudan today our data support the inference that towards 8500 7000 bp there