palaeomagnetism of late cretaceous sediments from southern tibet evidence for the consistent palaeolatitudes of the southern margin of eurasia prior to the collision with india abstract twenty sites were drilled in the late cretaceous shexing formation for palaeomagnetic studies in the lhasa terrane near the locality of maxiang 29 9 n 90 7 e the stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions the tilt corrected mean direction is d i 350 8 32 1 with 95 8 1 and n 20 sites corresponding to a paleopole at 75 0 n 306 7 e with a 95 6 8 positive fold tests indicate a primary origin for the characteristic remanence based on previous cretaceous data mainly from the takena formation and paleocene data from the linzizong volcanic rocks near the city of lhasa the latitude of the southern margin of asia is located at about 15 n and yields a stable position of the lhasa terrane during cretaceous and paleocene compared with expected paleomagnetic directions from the stable india and eurasia blocks the collision palaeolatitude further implies the total latitudinal convergence was accommodated by 1700 800 km 16 2 7 6 between southern tibet and stable eurasia and 1500 830 km 14 4 7 9 between southern tibet and stable india since the collision of india and eurasia a collision age between c 54 and 47 ma was determined using the results for the southern margin of eurasia according to our new data and the extent of greater india

paleogene island arc collision related conglomerates yarlung tsangpo suture zone tibet abstract coarse clastic rocks in the liuqu conglomerate formed in both terrestrial and subaqueous settings record a paleogene phase in the tectonic evolution of tibet facies changes are commonly abrupt with rapid changes in clast types grain size and stratal patterns sediments were derived from the leading northern edge of the indian margin and a late jurassic cretaceous intraoceanic island arc that lay within tethys the coarse clastic sedimentary rocks of the liuqu conglomerates are extremely proximal but are locally offset relative to their original source terranes they record aspects of the history of collision between these terranes and are interpreted to have been deposited in oblique slip basins that developed along the zone of collision the absence of clasts derived from terranes to the north of the yarlung tsangpo suture suggests that basins associated with deposition of the liuqu conglomerate developed prior to the final collision between india and asia

miocene burial and exhumation of the india asia collision zone in southern tibet response to slab dynamics and erosion the india asia collision zone in southern tibet preserves a record of geodynamic and erosional processes following intercontinental collision apatite fission track and zircon and apatite u th he data from the oligocene miocene kailas formation within the india asia collision zone show a synchronous cooling signal at 17 1 ma which is younger than the ca 26 21 ma depositional age of the kailas formation constrained by u pb and 40 ar 39 ar geochronology and requires heating burial after ca 21 ma and subsequent rapid exhumation data from the gangdese batholith underlying the kailas formation also indicate miocene exhumation the thermal history of the kailas formation is consistent with rapid subsidence during a short lived phase of early miocene extension followed by uplift and exhumation driven by rollback and northward underthrusting of the indian plate respectively significant removal of material from the india asia collision zone was likely facilitated by efficient incision of the paleo indus river and paleo yarlung river in response to drainage reorganization and or intensification of the asian monsoon

propagation of the deformation and growth of the tibetan himalayan orogen a review abstract long standing problems in the geological evolution of the tibetan himalayan orogen include where the india asia convergence was accommodated and how the plateau grew to clarify these problems we review the deformations and their role in the plateau s growth our results show that 1630 km of shortening occurred across the tibetan himalayan orogen since 55 ma with more than 1400 km accommodated by large scale thrust belts these thrust belts display an outward expansion from central tibet and couple with the surficial uplift the development of the tibetan plateau involved three significant steps primitive plateau 90 55 ma proto plateau 55 40 ma and neoteric plateau 40 0 ma several processes have collaborated to produce the proto plateau including the pre existing primitive plateau the india asia collision and subductions of greater india and songpan ganzi beneath the lhasa qiangtang terrane since 40 ma the proto plateau which was dominated by a topographic gradient lower crustal flow and continuous india asia convergence experienced three periods of rapid outward growth 40 23 23 10 and 10 0 ma in general the n s trending rifts were caused by the eastward growth of the plateau dominated by thrusting and crust flow in central tibet while they were the results of intense n s shortening in himalaya

the xigaze forearc basin evolution and facies architecture cretaceous tibet abstract the mid cretaceous to eocene flysch deposits of the xigaze forearc basin in southern tibet were investigated in a 120 km segment along the indus yarlung suture zone the basin evolved south of the magmatic arc gangdise belt of the lhasa block on top of trapped oceanic or transitional crust remnants of shelf carbonates are preserved along the northern edge of the basin the mapped segment was shortened by about 65 metamorphism reached low grade conditions along the northern margin of the basin the flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic andesitic to dacitic material shed from the magmatic arc gangdise belt particularly in the western part of the study area plutonic and sedimentary rocks from deep erosion levels and or more distal sources contributed to the basin fill rivers from the lhasa block acted as point sources and fed five major deep sea channel systems turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing the forearc flysch is subdivided into at least three megasequences which begin with wide up to several km incised 10 to 50 m coarse grained channel fills and their associated fan deposits the upper parts of the megasequences contain hemipelagic dark shales and marls deposited above the calcite compensation depth lateral channel migration channel lobe switching but also volcanic pulses generated a predominantly fining upward high frequency cyclicity after continental collision the marine sedimentation in the forearc basin was replaced by fluvial deposits of the eocene oligocene qiuwu formation which is time equivalent to the kailas and indus molasses farther west and rich in coarse gravel derived from the gangdise belt both forearc flysch and qiuwu formation were deformed simultaneously during the miocene we assume that the molasse type qiuwu formation represents the final continental facies of the forearc basin filling

constraining the age of liuqu conglomerate southern tibet implications for evolution of the india asia collision zone abstract controversy over the depositional age and provenance of the liuqu conglomerate along the major structural indus yarlung suture zone in south tibet clouds our understanding of the process of the india asia collision here we report low temperature thermochronometric data apatite fission track apatite and zircon u th he for the liuqu conglomerate in the xigaze area our new data constrain its depositional age to latest oligocene early miocene time indicating that rather than having formed immediately following paleogene india asia collision or collision between india and an intra oceanic arc as previously proposed the conglomerate was probably deposited in an intermontane basin at a slightly later time than the gangrinboche group to the north the liuqu conglomerate should therefore not be used as a key horizon in models constraining the early stages of india asia collision our data together with previous studies suggest that the liuqu conglomerate was sourced from the xigaze forearc basin indus yarlung suture zone as well as the tethyan himalaya furthermore our data indicate that exhumation of the liuqu conglomerate commenced at

the b isotopic composition of arc lavas from martinique lesser antilles island arc samples from martinique lesser antilles are enriched in b relative to oceanic crust or the mantle with b contents varying between 5 and 54 ppm and have 11b falling in a range between 1 8 and 5 3 less than the 11b of altered oceanic crust the combination of these results indicates that both sediments and altered oceanic crust must have been involved in the generation of those magmas since the abundance of b in the mantle is too low to produce such low 11b values with such high b concentrations b content and 11b are not correlated showing that the amount of b added to the arc magma was decoupled from its isotopic composition by the mixing of at least three distinct components boron concentrations considered in conjunction with 87sr 86sr 143nd 144nd 18o and 11b data show that variable amounts of b were added by a b enriched fluid to different batches of mantle derived magma the b sr and nd isotopic compositions of these samples are consistent with a two stage process in which enriched magma is produced in the mantle wedge by the addition of an aqueous fluid derived from dehydrating subducting lithosphere and then modified during ascent by assimilation of variable amounts of sediment and altered oceanic crust most of the b contained in the fluid was probably derived from altered oceanic crust plus as little as 3 sediment while the material involved in assimilation was apparently largely continentally derived sediment a two stage process like this also explains the distribution of 18o within the context of the b sr and nd isotopic compositions of martinique arc lavas

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

carbonate oxygen isotope paleoaltimetry evaluating the effect of diagenesis on paleoelevation estimates for the tibetan plateau carbonate oxygen isotope paleoaltimetry is based on analysis of the d 18 o value of carbonate precipitated from surface water deciphering the diagenetic history is important for establishing whether particular carbonates are accurate recorders of paleosurface waters which reflect paleoelevation this study provides examples from southern east central and northeastern tibet of approaches aimed at evaluating the diagenetic history of lacustrine micrites and pedogenic carbonates the most desirable technique for avoiding erroneous interpretations related to diagenetic overprinting is to analyze carbonates that are known to be primary such as aragonitic shell material in rocks that do not contain shell material we have compared lacustrine micrites and pedogenic carbonates to diagenetic carbonate phases to determine the effects of diagenesis on the isotopic composition of primary carbonates where the potential effects of diagenesis are subtle or ambiguous we have evaluated the fidelity of the carbonate record from systematic trends in c and o isotopes that agree with other interpretations of paleoenvironment such as high frequency covariance in c and o that corresponds with changes in the mg concentration of carbonates using these strategies we have determined that diagenesis has not affected the isotopic composition of carbonates in the late miocene pliocene thakkhola graben in southern tibet and the oligocene to pliocene linxia basin in northeastern tibet in the case of paleogene basins in east central tibet however ambiguity in data precludes the determination of diagenetic effects d 2004 elsevier b v all rights reserved

latest marine horizon north of qomolangma mt everest implications for closure of tethys seaway and collision tectonics the newly discovered marine horizon at zhepure syncline north of qomolangma revises timing of the closure of the eastern neo tethys seaway in the central himalayas the marine pengqu formation conformably overlies a lutetian shallow water carbonate platform and comprises shales interbedded with sandstones deposited in a neritic shelf environment the strata are dated by nannofossils and foraminifera as late early lutetian to late priabonian age np15 np20 deposited 47 34 ma indicating that the final closure of the tethys seaway in this region occurred at 34 ma the newly discovered strata provide evidence about emplacement of a major thrust sheet south of southern tibet prior to 37 ma which affected the regional climate

stable warm tropical climate through the eocene epoch earth s climate cooled from a period of extreme warmth in the early eocene epoch ca 50 ma to the early oligocene ca 33 ma when a large ice cap first appeared on antarctica evidence from the planktonic foraminifer oxygen isotope record in deep sea cores has suggested that tropical sea surface temperatures declined by 5 10 degrees over this interval eventually becoming much cooler than modern temperatures here we present paleotemperature estimates from foraminifer isotopes and the membrane lipids of marine crenarcheota from new drill cores in tanzania that indicate a warm and generally stable tropical climate over this period we reinterpret the previously published isotope records in the light of comparative textural analysis of the deep sea foraminifer shells which shows that in contrast to the tanzanian material they have been diagenetically recrystallized we suggest that increasingly severe alteration of the deep sea plankton shells through the eocene produced a diagenetic overprint on their oxygen isotope ratios that imparts the false appearance of a tropical sea surface cooling trend this implies that the long term eocene climatic cooling trend occurred mainly at the poles and had little effect at lower latitudes

the andean type gangdese mountains paleoelevation record from the paleocene eocene linzhou basin abstract paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the cenozoic uplift of the himalayas and the tibetan plateau this paper presents new oxygen and carbon isotopic compositions from well dated tertiary paleosols lacustrine calcareous carbonates and marls from the nianbo 60 54 ma and upper pana formations 51 48 ma of the linzizong group in the linzhou penbo basin the sediments of the nianbo formation which are 180 m thick were deposited in alluvial fans braided rivers fan deltas and on nearshore to offshore lacustrine settings whereas those of the upper pana formation are 100 m thick and are comprised predominantly of proximal alluvial fan and braided river deposits correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment it is confirmed that the 18 o c and 13 c c values from nianbo and pana formations have not yet been reset by late stage diagenesis based on petrographic examination oxygen isotope of the fossil ostracodes and tectonic deformation of strata the paleoelevations are reconstructed using the corrected most negative paleosurface water 18 o psw values these imply that the linzhou area had attained an elevation of 4500 400 m during the period of the indo asian collision i e achieved a near present elevation and may form an andean type mountain range stretching the gangdese arc before collision the gangdese mountains probably maintained high elevations since at least the paleocene and could play a crucial role in the climate change in the interior of the tibetan plateau during the early cenozoic the paleogeomorphic scenario of the eocene tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin

the june 2000 mw 7 9 earthquakes south of sumatra deformation in the india australia plate 1 two large mw 7 9 earthquakes occurred on 4 and 18 june 2000 south of sumatra beneath the indian ocean both earthquakes were predominantly left lateral strike slip on vertical n s trending faults that we interpret to be reactivated fracture zones the 4 june enggano earthquake occurred at the edge of the rupture area of the 1833 subduction earthquake the first strike slip subevent within the subducting plate triggered a thrust subevent on the plate interface which comprised at least 35 of the total moment and ruptured se away from the 1833 earthquake the 18 june earthquake in the wharton basin is one of the largest shallow strike slip faulting earthquakes ever recorded a small second subevent with reverse slip is required to fit the body waves the orientation of both subevents in our preferred model is consistent with the current stress field in the region both the june 2000 earthquakes are consistent with recent models of distributed deformation in the india australia composite plate the occurrence of the enggano earthquake implies that the stress field within the indian plate continues to a depth of 50 km in the subducting slab the purely strike slip source model of the wharton basin earthquake obtained by robinson et al 2001 matches the p waves very poorly and fits the s waves no better than our preferred model the strike slip subevents of both earthquakes had few aftershocks and higher stress drops than the subduction thrust subevent of the enggano earthquake this difference is consistent with previous observations of oceanic and subduction earthquakes

carbonate clumped isotope bond reordering and geospeedometry carbonate clumped isotope thermometry is based on the preference of 13c and 18o to form bonds with each other at elevated temperatures such bond ordering is susceptible to resetting by diffusion of c and o through the solid mineral lattice this type of bond reordering has the potential to obscure primary paleoclimate information but could also provide a basis for reconstructing shallow crustal temperatures and cooling rates we determined arrhenius parameters for solid state reordering of c o bonds in two different calcites through a series of laboratory heating experiments we find that the calcites have different susceptibilities to solid state reordering reaction progress follows a first order rate law in both calcites but only after an initial period of non first order reaction that we suggest relates to annealing of nonequilibrium defects when the calcites are first heated to experimental temperature we show that the apparent equilibrium temperature equations or closure temperature equations for carbonate clumped isotope reordering are analogous dodson s equations for first order loss of daughter isotopes for each calcite the sensitivity of apparent equilibrium temperature to cooling rate is sufficiently high for inference of cooling rates within a factor of 5 or better for cooling rates ranging from tens of degrees per day to a few degrees per million years however because the calcites have different susceptibilities to reordering each calcite defines its own cooling rate apparent equilibrium temperature relationship the cooling rates of carrara marble inferred from carbonate clumped isotope geospeedometry are 10 6 10 3 degrees per annum and are in broad agreement with rates inferred from thermochronometric methods cooling rates for 13c depleted calcites from the late neoproterozoic doushantou cap carbonates in south china are on the order of 102 104 degrees per annum consistent with rapid cooling following formation of these calcites by a short lived hydrothermal event bristow et al 2011 nature v 474 p 68 71 most of the uncertainty in these estimates relates to uncertainty in arrhenius parameters for different calcites thus while the carbonate clumped isotope geospeedometer shows promise for recording cooling rates in settings and lithologies where other geospeedometers may not be applicable the uncertainty in cooling rate will be large without independent knowledge of the reordering kinetics of each study material thus the full potential of the method will only be realized if reordering kinetics can be accurately determined for each study material or predicted on the basis of mineral composition texture or other observable parameters

the clumped isotope geothermometer in soil and paleosol carbonate we studied both modern soils and buried paleosols in order to understand the relationship of temperature t c 47 estimated from clumped isotope compositions 47 of soil carbonates to actual surface and burial temperatures carbonates from modern soils with differing rainfall seasonality were sampled from arizona nevada tibet pakistan and india t c 47 obtained from these soils shows that soil carbonate forms in the warmest months of the year in the late morning to afternoon and probably in response to intense soil dewatering t c 47 obtained from modern soil carbonate ranges from 10 8 to 39 5 c on average t c 47 exceeds mean annual temperature by 10 15 c due to summertime bias in soil carbonate formation and to summertime ground heating by incident solar radiation secondary controls on t c 47 are soil depth and shading site mean annual air temperature maat across a broad range 0 30 c of site temperatures is highly correlated with t c 47 from soils following the equation maat c 1 20 t c 47 0 21 72 r 2 0 92 where t c 47 0 is the effective air temperature at the site estimated from t c 47 the effective air temperature represents the air temperature required to account for the t c 47 at each site after consideration of variations in t c 47 with soil depth and ground heating the highly correlated relationship in this equation should now permit mean annual temperature in the past to be reconstructed from t c 47 in paleosol carbonate assuming one is studying paleosols that formed in environments generally similar in seasonality and ground cover to our calibration sites t c 47 0 decreases systematically with elevation gain in the himalaya following the equation elevation m 229 t c 47 0 9300 r 2 0 95 assuming that temperature varied similarly with elevation in the past this equation can be used to reconstruct paleoelevation from clumped isotope analysis of ancient soil carbonates we also measured t c 47 from long sequences of deeply buried 5 km paleosol carbonate in the himalayan foreland in order to evaluate potential diagenetic resetting of clumped isotope composition we found that paleosol carbonate faithfully records plausible soil t c 47 down to 2 5 4 km burial depth or 90 125 c deeper than this and above this temperature t c 47 in paleosol carbonate is reset to temperatures 40 c we observe 40 c as the upper limit for t c 47 in modern soils from soil depths 25 cm and therefore that t c 47 40 c obtained from ancient soil carbonate indicates substantially warmer climate regimes compared to the present or non primary temperatures produced by resetting during diagenesis if representative this limits the use of t c 47 to reconstruct ancient surface temperature to modestly buried 3 4 km paleosol carbonates despite diagenetic resetting of 47 values 18 o and 13 c values of the same deeply buried paleosol carbonate appear unaltered we conclude that solid state reordering or recrystallization of clumping of carbon and oxygen isotopes can occur in the absence of open system exchange of paleosol carbonate with significant quantities of water or other phases

the youngest marine deposits preserved in southern tibet and disappearance of the tethyan ocean abstract fossil ages as young as priabonian 38 34 ma are reported for the last marine sedimentary rocks in southern tibet correlation is based on examination of foraminifers and nannofossil biostratigraphy of youngest preserved sediments in sections at gamba zongpu tingri qumiba as well as a previously unreported section at yadong our results demonstrate that a marine seaway remained in existence south of the yarlung tsangpo suture zone until at least priabonian time notably this remains a maximum age estimate in this area as all sections are truncated by erosion or faulting we compare our results with sections throughout the himalaya region to demonstrate that shallow marine conditions existed widely during the eocene period in fact it seems likely that the marine conditions in the tethyan himalaya did not entirely disappear by the end of priabonian especially in the eastern himalaya the data presented in this study place direct constraints on the elimination of the tethyan ocean and thus have important implications for timing of the india eurasia collision

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

remnants of a cretaceous intra oceanic subduction system within the yarlung zangbo suture southern tibet extensive field investigations along the yarlung zangbo suture zone in southern tibet reveal the presence of now fragmented remnants of a south facing intra oceanic subduction system this system developed within tethys during the cretaceous the associated arc forearc ophiolite and subduction complex were emplaced onto the leading edge of india at the end of the cretaceous rapid sedimentation in oblique slip basins and disruption of water saturated sediments into melange was widespread and concomitant with ophiolite emplacement we describe the tectonic entities that developed during this previously unrecognized phase of tethys tibet evolution and present a new model for the evolution of this portion of tibet

stable isotopic results from paleosol carbonate in south asia paleoenvironmental reconstructions and selective alteration article i nfo we measured 18 o and 13 c values of paleosol carbonate nodules from a variety of sedimentary units in southern asia in order to reconstruct paleoenvironmental conditions and to evaluate for possible diagenetic alteration paleosol carbonate nodules were collected from lower to upper cretaceous strata exposed in two locations in southern tibet and from lower to middle miocene strata in northern india and nepal additional samples were collected from stratigraphically adjacent marine carbonate units to test for possible resetting of stable isotope values based on the implausibly low 18 o values 13 of most of the marine carbonate samples we interpret the 18 o values of the associated paleosol carbonate nodules as being reset and therefore unreliable for paleoenvironmental or paleoelevation reconstruction importantly the 18 o values of 13 c values of marine carbonate rocks fall within the expected primary

cool equatorial terrestrial temperatures and the south asian monsoon in the early eocene evidence from the gurha mine rajasthan india abstract early eocene 55 52 ma laminated lacustrine sediments overlying lignites in the gurha mine 27 87398 n 72 86709 e rajasthan india yield a diversity of fossil leaves flowers fruits seeds and insects clamp multivariate foliar physiognomic analysis of two horizons separated by an estimated several tens of thousands of years of deposition indicates cool equatorial 10 n temperatures and a pronounced monsoon signature a lower assemblage consisting of 54 leaf morphotypes and an upper assemblage of 57 leaf forms yielded mean annual temperatures mat of 24 7 and 23 9 c respectively the uncertainty 2 82 c means these temperature regimes are identical despite few similarities in the morphotypes between the two assemblages the mean annual range of temperature mart was approximately 9 7 c for both assemblages when corrected for evapotranspirational cooling these temperature regimes are similar to those experienced today at 10 n on the west coast of india and surprisingly cool for the tropics at a time of extreme global warmth growth was year round the tropical to paratropical fossil floras also suggest a moist regime 80 annual relative humidity and high mean annual precipitation of 1800 mm for both assemblages but with a pronounced wet dry seasonality indicative of a pronounced monsoonal regime the lower assemblage has a stronger monsoon index 11 8 than the upper assemblage 8 8 the two assemblages seem to have been deposited less than 100 ka apart this suggests that not only a pronounced south asian monsoon existed when india and asia first made contact but also a variation in monsoon strength existed that cannot be ascribed to tectonic drivers

provenance of the liuqu conglomerate in southern tibet a paleogene erosional record of the himalayan tibetan orogen abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone in southern tibet has been interpreted as a paleogene molasse deposited during india asia collision or the product of collision between india and an intra oceanic arc detailed petrographic studies and analyses of u pb and hf isotopes of detrital zircon indicate multiple sources for the conglomerate including the sedimentary succession within the xigaze forearc basin to the north of the suture the yarlung zangbo ophiolite within the suture itself and the langjiexue group and tethyan himalayan sedimentary rocks to the south of the suture the occurrence of clasts of both indian and asian origin suggests that the conglomerate was deposited after india asia collision the present data suggest that the onset of himalayan tibetan orogenic erosion occurred as early as the middle eocene when the yarlung zangbo suture zone and xigaze forearc succession were uplifted and became sources for the liuqu conglomerate thus the results are inconsistent with the proposal that the provenance and age of the liuqu conglomerate support the hypothesis that india asia collision did not occur until the oligocene it is further suggested that eocene neotethyan slab breakoff which may result in accelerated exhumation and uplift of the orogen provided a deep level dynamic control on the deposition of the liuqu conglomerate

micromorphological fabrics and developmental stages of carbonate nodular forms related to soil characteristics abstract carbonate nodules often constitute the first stage of calcrete formation micromorphological analysis of carbonate nodules and nodular fabrics in different soil materials shows that their development is a function of several factors nature of the host matrix texture porosity carbonate and non carbonate clay distribution bulk density and the interactions among them different micromorphological stages therefore exist in different soil materials in calcareous medium textured soil materials the stages of nodule formation are related to the increase of the density of the nodule caused by the accumulation of microcalcites in such a soil material the following stages occur 1 microcalcites within the low to moderate density matrix 2 microcalcites of moderate to high density and moderately dense diffuse nodules parallel calcans may occur 3 microcalcites of high density with dense microcalcitic nodules during these stages the amount of non carbonate clay decreases but is homogeneously dispersed and disseminated with the microcalcites in general the size of microcalcites 1 8 m is inversely related to the clay content in non calcareous medium to fine textured soil materials secondary carbonate precipitates in intrapedal and or interpedal voids as large sparry crystals due to pedoturbation processes the nodules thus formed are subjected to recrystallization resulting in a microsparry pattern the development stages are 1 few calcans and crystal chambers 2 crystic nodules mainly of sparry crystals and an increase of calcans and crystic chambers 3 recrystallized nodules composed mainly of microspars in coarse grained materials secondary carbonates precipitate directly in voids as sparry crystals and the resulting nodules are of the crystic type the large crystals are often subjected to recrystallization in partly calcareous coarse grained materials with additions of calcareous dust particular micromorphological carbonate forms occur in the c horizon cutanic calciasepic fabric forms besides zones of crystic fabric which gradually become calciasepic in the b horizon and include the typical nodular forms of the calcareous medium textured soil material together with appreciable amounts of glaebular halos the processes leading to the formation of these stages occur essentially without a significant biogenic influence where carbonate segregation occurs as rhizomorphs surrounding active roots the fabric is always micritic even in coarse textural soil material

new constraints on the india asia collision the lower miocene gangrinboche conglomerates yarlung tsangpo suture zone se tibet abstract lower miocene conglomerates crop out along the length of the yarlung tsangpo suture zone on the southern margin of the lhasa terrane these conglomerates known by various local names are correlated herein as the gangrinboche conglomerates all units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the lhasa terrane is ubiquitous at most localities the tops of sections are either removed by erosion or truncated by north directed thrusts these conglomeratic molasse units developed in response to the india asia collision and record aspects of its development in all units initial clast derivation was from the lhasa terrane on the northern margin of the yarlung tsangpo suture zone up section the first appearance of clasts derived from terranes within the suture zone and the northern margin of india all of which lie to the south of any outcrops of gangrinboche conglomerates is observed although these units were previously thought to be eocene analysis of fossil and structural constraints indicates early miocene deposition as development of the gangrinboche conglomerates records a significant phase in the evolution of the india asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models

tectonostratigraphy and provenance of an accretionary complex within the yarlung zangpo suture zone southern tibet insights into subduction accretion processes in the neo tethys abstract accretionary complexes record critical information about the history of subduction and accretion along the southern margin of asia prior to the india asia collision this paper presents detailed field mapping petrographic and detrital zircon u pb data from an accretionary complex within the yalung zangpo suture zone southern tibet from structurally higher to lower levels the accretionary complex consists of a serpentinite melange the chert matrix tangga melange the mud matrix pomunong melange and the coherent rongmawa formation the tangga melange consists of late triassic early cretaceous abyssal chert and siliceous shale with blocks of chert and mafic to ultra mafic rocks the tangga melange was accreted beneath ophiolitic rocks during the aptian the pomunong melange consists of a late jurassic early cretaceous hemipelagic siliceous shale and chert matrix with blocks from early permian seamounts and from late cretaceous trench fill sandstones it was accreted beneath the tangga melange after the aptian but prior to 71 ma structurally beneath these melanges to the south the uppermost cretaceous rongmawa formation consists of turbiditic sandstone pelagic chert and siliceous shale and records a transition in depositional setting from lower abyssal plain to upper trench detrital zircons from sandstone blocks of the pomunong melange and the coherent rongmawa formation display similar u pb age spectra and are dominated by peaks at 71 231 ma 481 693 ma and 701 1372 ma these age peaks overlap with igneous crystallization ages and detrital zircon ages from sedimentary strata in the lhasa terrane our data indicate that the serpentinite melange tangga melange pomunong melange and rongmawa formation comprise a southward younging accretionary complex that developed during the northward subduction of neo tethyan oceanic lithosphere beneath the south margin of the lhasa terrane and that all exposed ophiolitic and accretionary complex assemblages were accreted to the asian margin prior to the india asia continental collision

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

the burial and exhumation history of the liuqu conglomerate in the yarlung zangbo suture zone southern tibet insights from clumped isotope thermometry abstract the liuqu conglomerate exposed along the yarlung zangbo suture zone yzsz in southern tibet archives not only the process of the india asia collision but also the exhumation history of the india asia collision zone the burial and exhumation history of the liuqu conglomerate thus could potentially provide clues to the india asia collision and orogeny low temperature thermochronometric data indicate that maximum burial temperatures of the liuqu conglomerate ranged from 80 to 110 c however the exact burial history is not well constrained here we examine the burial conditions of the liuqu conglomerate by applying clumped isotope thermometry to paleosol carbonates extensive microcrystalline recrystallization indicates that the samples have been diagenetically altered during burial moreover the clumped isotope temperatures ranging from 48 to 97 c are clearly above earth surface conditions indicating the original climatic information was overwritten by alteration at higher burial temperatures nevertheless the calculated diagenetic water 18o values represent maximum meteoric water values that can be used to broadly constrain the paleoelevation the highest clumped isotope temperature implies that the maximum burial temperature of the liuqu conglomerate was at least 97 c we conclude that the liuqu conglomerate was buried as deeply as 3 7 4 3 km during the latest paleocene to eocene and then exhumed during the miocene 10 12 ma incision of the paleo yarlung river was likely responsible for erosion and exhumation of the liuqu conglomerate which suggests that the drainage system of paleo yarlung river was similar to that of today and supports the model of outward growth of the tibetan plateau

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

variations in the strontium isotopic composition of seawater during the neogene the authors report 261 strontium isotopic analyses of well preserved planktonic foraminifers from three deep sea drilling project sites 519 588 and 607 these samples cover the period from 24 ma to present with an average of approximately one sample per 100 ka the combination of high sample density and uniformity of analytical procedures has produced a well defined record of changes in the sup 87 sr sup 86 sr of seawater during the neogene the record can be viewed as a series of essentially linear segments with slopes ranging from as high as 6 times 10 sup minus 5 m y to as low as 0 m y the times associated with major inflections in the curve do not appear to correspond to simple geologic phenomena such as eustatic cycles but are probably controlled by a combination of tectonic and climatic factors that influenced the abundance and isotopic composition of terrestrial strontium input to the oceans the strontium isotopic data are consistent with a progressive increase in the chemical weathering rates of the continents during the neogene probably related to repeated glaciations increased exposure of continents by lowered sea level and increased continental relief resulting from high rates of tectonic uplift

climatic and diagenetic signals in the stable isotope geochemistry of dolomitic paleosols spanning the paleocene eocene boundary abstract the precipitation of primary dolomite in soil is rare and controversial and its occurrence indicates highly unusual geochemical and climatic conditions we utilize isotopic and petrographic techniques including clumped isotope thermometry to evaluate diagenetic alteration and preservation of primary climatic signal in dolomitic paleosol nodules from a section spanning the paleocene eocene boundary in central utah isotopic measurements differentiate samples into groups that may be interpreted in terms of alteration however moderate burial depths and extreme warmth during the interval of soil formation make it difficult to reach definitive conclusions from isotope values alone with the addition of petrographic analysis including cathodoluminescence and scanning electron microscopy we are able to identify textural differences between samples suggesting differing amounts of recrystallization we conclude that while some nodules have experienced significant diagenetic alteration the original nodule mineralogy is dolomite and the majority of sampled nodules retain a primary paleoclimate signal early paleogene temperatures recorded at this site are considerably warmer than those suggested for the continental interior from alternate proxies a result that may be partially the result of the clumped isotope temperatures recording warm season soil temperatures rather than mean annual air temperature soil water isotopic composition is suggestive of heightened aridity particularly through the paleocene eocene thermal maximum petm

the paleoaltimetry of tibet an isotopic perspective stable isotopes provide a valuable perspective on the timing of elevation change of the tibetan plateau we begin our paper by looking in depth at isotopic patterns in modern tibet we show that the 18 o value of surface waters decreases systematically up the himalayan front in central nepal by about 2 8 km in agreement with the patterns documented and modeled by previous research on the tibetan plateau itself there is no apparent correlation between elevation and the 18 o value of flowing surface waters both surface waters and soil carbonates display a northward increase in 18 o values of about 1 5 north of the himalayan crest even though elevation increases modestly the isotopic increase with latitude reduces the isotope elevation gradient for water in the northernmost plateau to 1 to 2 km carbonates in both soils and lakes form at higher temperatures than assumed by previous studies on the plateau temperature estimates from clumped isotope 47 analyses of modern soil carbonates significantly exceed mean annual air t and modeled maximum summer soil temperatures by 15 8 2 8 and 9 7 2 5 c respectively similarly elevated temperatures best account for the 18 o values observed in modern soil and lake carbonates we recalculated paleoelevations from previous studies on the plateau using both higher formation temperatures and latitude corrected isotopic values with one notable exception our revised model produces paleoelevation estimates very close to previous estimates the exception is the reconstruction from late eocene age deposits at xoh xil for which we calculate elevations that are higher and much closer to the current elevation than previously reconstructed therefore there is no evidence for northward progression through time of tibetan elevation change instead the available but admittedly very scanty evidence suggests that much of tibet attained its modern elevation by the mid eocene a truly robust test of the various geodynamic models of uplift await expansion and replication of isotopic records all across tibet especially in the center and north and for 15 ma