marine crenarchaeotal membrane lipids in decapods implications for the tex86 paleothermometer pelagic crenarchaeota produce glycerol dibiphytanyl glycerol tetraethers gdgts as membrane lipids and the gdgt composition changes according to growth temperature this forms the basis of the tex86 paleotemperature proxy this ratio correlates with sea surface temperature sst despite the fact that crenarchaeota are distributed through the water column therefore there must be mechanisms that transport the surface signal to sediments such as repackaging in fecal pellets marine snow mass falls after phytoplankton blooms or daily migration to study gdgt transport we analyzed stomachs and intestines of atlantic and mediterranean decapods as they are one of the major megafaunal groups are easy to sample and occur in both pelagic and benthic environments gdgts were found in most decapods guts gdgt abundances are significantly lower in intestines but tex86 derived temperatures are not significantly different between stomachs and intestines 1oc suggesting that tex86 values are not altered during gut transit atlantic decapods show no difference in tex86 values between benthic detritivors and pelagic predators however mediterranean decapods show a substantial difference between macroplankton feeders and bentho pelagic predators this is probably related to the freshness of the material consumed tex86 derived temperatures in atlantic decapods are close to the sst around the time of sampling in agreement with stomach content analysis that shows fresh organic matter being ingested for mediterranean decapods tex86 temperatures are significantly higher than sst around the time of sampling this can be partly attributed to the large variability between decapod specimens and the low amounts of fresh material found in their stomachs

volcanic plutonic parity and the differentiation of the continental crust the continental crust is central to the biological and geological history of earth however crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks volcanic and plutonic rocks and the processes by which they differentiate to felsic compositions our analysis of a comprehensive global data set of volcanic and plutonic whole rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics in both tectonic settings our results indicate that fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas emphasizing the role of mafic cumulates as a residue of crustal differentiation a global geochemical data set of volcanic and plutonic rocks indicates that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but are divergent in continental rifts brenhin keller and co authors present a global geochemical dataset of the two fundamental building blocks of the continental crust volcanic externally erupted and plutonic internally solidified rocks their results indicate that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction zone settings but divergent in continental rifts offsets in major and trace element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics this work indicates that in both tectonic settings fractional crystallization rather than crustal melting is predominantly responsible for the production of intermediate and felsic magmas

archaea mediate anaerobic oxidation of methane in deep euxinic waters of the black sea we evaluate anaerobic oxidation of methane aom in the black sea water column by determining distributions of archaea specific glyceryl dialkyl glyceryl tetraethers gdgts and 13c isotopic compositions of their constituent biphytanes in suspended particulate matter spm sinking particulate matter collected in sediment traps and surface sediments we also determined isotopic compositions of fatty acids specific to sulfate reducing bacteria to test for biomarker and isotopic evidence of a syntrophic relationship between archaea and sulfate reducing bacteria in carrying out aom bicyclic and tricyclic gdgts and their constituent 13c depleted monocyclic and bicyclic biphytanes down to 67 indicative of archaea involved in aom were present in spm in the anoxic zone below 700 m depth in contrast gdgt 0 and crenarchaeol derived from planktonic crenarchaeota dominated the gdgt distributions in the oxic surface and shallow anoxic waters fatty acids indicative of sulfate reducing bacteria i e iso and anteiso c15 were not strongly isotopically depleted e g 32 to 25 although anteiso c15 was 5 more depleted in 13c than iso c15 our results suggest that either aom is carried out by archaea independent of sulfate reducing bacteria or those sulfate reducing bacteria involved in a syntrophy with methane oxidizing archaea constitute a small enough fraction of the total sulfate reducing bacterial community that an isotope depletion in their fatty acids is not readily detected sinking particulate material collected in sediment traps and the underlying sediments in the anoxic zone contained the biomarker and isotope signature of upper water column archaea aom specific gdgts and 13c depleted biphytanes characteristic of the spm in the deep anoxic zone are not incorporated into sinking particles and are not efficiently transported to the sediments this observation suggests that sediments may not always record aom in overlying euxinic water columns and helps explain the absence of aom derived biomarkers in sediments deposited during past periods of elevated levels of methane in the ocean

spatial gap between lhasa and qiangtang blocks inferred from middle jurassic to cretaceous paleomagnetic data abstract we present paleomagnetic results from middle jurassic red sandstones of the sangba formation orientated samples were collected at 14 sites on north side of the basu 30 1 n 96 9 e northeastern border of the lhasa block after stepwise thermal demagnetization high temperature component with unblocking level of 680 c is isolated from 7 sites primary nature of this magnetization is ascertained through positive fold and reversal tests at 95 confidence level combined with previously reported paleomagnetic results tilt corrected mean direction of d 353 4 i 15 1 95 14 1 n 8 is calculated indicating that during middle jurassic the study area was located at 7 7 n 7 4 comparison with the jurassic paleomagnetic data reported from eastern tibet indicates that the lhasa block was located south of the qiangtang block at a latitudinal difference of 31 11 compilation of paleomagnetic data further suggest that until the cretaceous large spatial gap of more than 1200 km remained open between the lhasa block and the qiangtang block of eastern himalayan syntaxis and eventually vanished during tertiary cretaceous paleolatitude corresponding to this gap 13 0 n 31 0 n is almost identical to that reported from shan thai block 25 4 n 5 2 furthermore the permo carboniferous sequences of gondwanic affinity are distributed in western part of the shan thai block as well as in the lhasa and qiangtang blocks keeping in view these similarities we propose a new pre collision india vs asia reconstruction model in which the lhasa shan thai and qiangtang blocks are shown in their respective order from south to north along southern margin of the asian continent the aftermath of gigantic india asia collision however introduced north south compressional regime in the asian continent that eventually played a role in eastward extrusion of the shan thai block from an area between the qiangtang and lhasa blocks

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

instability in tropical pacific sea surface temperatures during the early aptian the cretaceous has long been recognized as a time when greenhouse conditions were fueled by elevated atmospheric co2 and accompanied by perturbations of the global carbon cycle described as oceanic anoxic events oaes yet the magnitude and frequency of temperature change during this interval of warm and equable climate are poorly constrained here we present a ighresolution record of sea surface temperatures ssts reconstructed using the tex86 paleothermometer for a sequence of early aptian organic rich sediments deposited during the first cretaceous oae oae1a at shatsky rise in the tropical pacific ssts range from 30 to 36 0c and include two prominent cooling episodes of 4 0c the cooler temperatures reflect significant temperature instability in the tropics likely triggered by changes in carbon cycling induced by enhanced burial of organic matter sst instability recorded during the early aptian in the pacific is comparable to that reported for the late albian early cenomanian in the atlantic suggesting that such climate perturbations may have recurred during the cretaceous with concomitant consequences for biota and the marine environment

structure and present day compression in the offshore area between alicante and ibiza island eastern iberian margin abstract this study deals with the structure and recent deformation of the eastern iberian margin extending from basement to seafloor and including the south western margin of the valencia basin the alicante shelf the ibiza channel and its southern margin descending into the algerian basin this area underwent a complex tectonic evolution linked to the back arc opening of the north western mediterranean and the concomitant contraction of the betic belt due to the collision with blocks located between africa and europe this oligo miocene structural heritage gave rise to a complex and continuous deformation through times including late miocene post orogenic extension and pliocene to quaternary compression in the western balearic area and coeval extension in the valencia basin this study presents maps of the depth to basement and base of the pliocene as well as bathymetry data and seismic lines which provide a precise integrated 3d study of the offshore domain it reveals a major reactivation of the area represented by the n80 to n60 trending structures small discontinuous folds and thrusts in the ibiza channel and a large flexure on the alicante shelf the structures are picked out by erosion surfaces or deposits linked to the messinian salinity crisis msc these markers are ubiquitous in the seismic sedimentary sequences and record the lateral and vertical deformation active from the messinian salinity crisis to the present the contraction in the western iberian margin and concomitant extension in the southern valencia basin are consistent with the regional stress field as determined from the focal mechanisms of offshore earthquakes or recent gps measurements the tectonic compression of the studied area casts doubt on the eventual propagation of the present day compressive stress from the algerian margin to the western balearic promontory

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

archaeal tetraether membrane lipid fluxes in the northeastern pacific and the arabian sea implications for tex86 paleothermometry the newly introduced temperature proxy the tetraether index of archaeal lipids with 86 carbon atoms tex86 is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraether gdgt lipids of marine crenarchaeota the composition of sedimentary gdgts used for tex86 paleothermometry is thought to reflect sea surface temperature sst however marine crenarchaeota occur ubiquitously in the world oceans over the entire depth range and not just in surface waters we analyzed the gdgt distribution in settling particulate organic matter collected in sediment traps from the northeastern pacific ocean and the arabian sea to investigate the seasonal and spatial distribution of the fluxes of crenarchaeotal gdgts and the origin of the tex86 signal transported to the sediment in both settings the tex86 measured at all trap deployment depths reflects sst in the arabian sea analysis of an annual time series showed that the sst estimate based on tex86 in the shallowest trap at 500 m followed the in situ sst with a 1 to 3 week time delay likely caused by the relatively low settling speed of sinking particles this revealed that the gdgt signal that reaches deeper water is derived from the upper water column rather than in situ production of gdgts the gdgt temperature signal in deeper traps at 1500 m and 3000 m did not show a seasonal cyclicity observed in the 500 m trap but rather reflected the annual mean sst this is probably due to a homogenization of the tex86 sst signal carried by particles as they ultimately reach the interior of the ocean our data confirm the use of tex86 as a temperature proxy of surface ocean waters

late cretaceous high mg granitoids in southern tibet implications for the early crustal thickening and tectonic evolution of the tibetan plateau abstract this study presents new major and trace element plus sr nd and zircon u pb isotope data for the zhongcang granitic plutons which are located to the south of the yongzhu asuo ophiolite belt within the northwestern part of the central lhasa subterrane tibetan plateau these data provide new insights into the late cretaceous tectonic evolution of southern tibet the zhongcang plutons are dominated by granodiorites and granites that yield zircon u pb emplacement ages of 94 88 ma they can be further divided into metaluminous and peraluminous subtypes the metaluminous rocks have adakite like geochemical signatures including high sio 2 al 2 o 3 and sr concentrations and low yb and y concentrations and high sr y and la yb n ratios these rocks also have negative end t values 3 17 to 0 17 variable initial 87 sr 86 sr i ratios 0 705927 0 707668 and high k 2 o and th concentrations suggesting that they were not derived from the partial melting of subducted oceanic crust in an arc setting the zhongcang adakitic rocks have higher mgo and cr concentrations and mg values than do contemporaneous intrusive rocks derived from a region of thickened lower crust within the central lhasa subterrane these data suggest that the zhongcang adakitic rocks were generated by the partial melting of a delaminated thickened lower crust within a late cretaceous continental setting in comparison with the zhongcang adakitic rocks the peraluminous rocks have significant negative eu and sr anomalies and lower end t values 4 06 to 6 64 this combined with their high mg values and cr concentrations suggests that the peraluminous units formed from primitive magmas similar to those that formed the zhongcang adakitic rocks but modified by contamination with ancient crustal material and by fractional crystallization of plagioclase and apatite during uprising and or emplacement the zhongcang high mg granitoids provide robust evidence for late cretaceous crustal thickening prior to india asia collision

temporal and spatial variation in tetraether membrane lipids of marine crenarchaeota in particulate organic matter implications for tex86 paleothermometry the tex86 is a new temperature proxy which is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraether gdgt lipids of the membranes of crenarchaeota that occur ubiquitously in oceans and shelf seas this proxy was calibrated by core top sediments but it is as yet not clear during which season and at which depth in the water column the gdgt signal used for tex86 paleothermometry is biosynthesized here we analyzed 200 particulate organic matter pom samples from 11 different marine settings for tex86 this revealed that the gdgts occur seasonally in surface waters and occur in higher abundances during the winter and spring months the depth distribution showed that gdgts generally appeared in higher amounts below 100 m depth in the water column however the tex86 values for waters below the photic zone 150 1500 m did not correlate with the in situ temperature but rather correlated linearly with surface temperature the tex86 for pom from the upper 100 m showed a linear correlation with in situ temperature which was nearly identical to the previously reported core top equation the correlation of all pom samples with surface temperature was also strikingly similar to the core top correlation these findings demonstrate that the gdgt signal which reaches the sediment is mainly derived from the upper 100 m of the water column this may be caused by the fact that gdgts from the photic zone are much more effectively transported to the sediment by grazing and repackaging in large particles than gdgts from deeper waters

coherent millennial scale patterns in u37k and tex86h temperature records during the penultimate interglacial to glacial cycle in the western mediterranean 1 the tex86h temperature proxy is a relatively new proxy based on crenarchaeotal lipids and has rarely been applied together with other temperature proxies in this study we applied the tex86h on a sediment core from the alboran sea western mediterranean core odp 977a covering the penultimate climate cycle that is from 244 to 130 ka and compared this with previously published sea surface temperatures derived from the u37k of alkenones of haptophyta and mg ca records of planktonic foraminifera the tex86h temperature record shows remarkably similar stadial interstadial patterns and abrupt temperature changes to those observed with the u37k palaeothermometer absolute tex86h temperature estimates are generally higher than those of u37k though this difference 3 c in 81 of the data points is mainly within the temperature calibration error for both proxies suggesting that crenarchaeota and haptophyta experienced similar temperature variations during occasional events 5 of the analyzed time span however the tex86h exhibits considerably higher absolute temperature estimates than the u37k comparison with mg ca records of planktonic foraminifera as well as other mediterranean tex86 and u37k records suggests that part of this divergence may be attributed to seasonal differences that is with tex86h reflecting mainly the warm summer season while u37k would show annual mean biases in the global calibration of both proxies or specific biases in the mediterranean are an alternative though less likely explanation despite differences between absolute tex86h and u37k temperatures the correlation between the two proxies r2 0 59 95 significance provides support for the occurrence of abrupt temperature variations in the western mediterranean during the penultimate interglacial to glacial cycle

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

record of the messinian salinity crisis in the sw mallorca area balearic promontory spain the current interpretation of the messinian salinity crisis msc involves the deposition of the so called peripheral or marginal evaporites in onshore basins as well as the erosion of the margins and the formation of thick evaporites in the deep parts of the basins the present study focuses on intermediate depth basins i e located between the onshore outcrops and the deep basins indeed the balearic promontory shows small stepped basins filled with msc deposits between its western extremity on the alicante shelf and its eastern end around the menorca block new and already available seismic reflection profiles and onshore data allow us to investigate the nature and geometrical relationships of these deposits in the area between the islands of ibiza and mallorca our observations suggest the existence of three msc related units in this area the lowermost transparent seismic unit can be interpreted as a 100 to 200 m thick salt layer deposited in the deepest part of the central mallorca depression these evaporites pass laterally and upward into a thin bedded seismic unit which could be lithologically equivalent to the upper evaporites of the deep basins all around the borders of the central mallorca depression this bedded seismic unit onlaps a slope unit which shows bedded to chaotic and rafted facies the messinian continental shelf is eroded and incised by a valley extending from the palma onshore area to the mallorca downslope domain our results suggest the presence offshore of at least two generations of diachronous msc deposits the observation of salt on a continental high and the widespread msc deposits on the margins of the central mallorca depression call into question the location of halite preferentially in the deep abyssal basins and the significance of the deep basin msc units that onlap onto the margin erosional surface mes as observed all around the passive margins of the northwest mediterranean basin the dual terminology used for peripheral deep msc evaporites cannot be applied to any of the msc units found in the study area

the messinian salinity crisis in mallorca new insights for a western mediterranean stratigraphic scenario abstract the messinian salinity crisis msc was an extraordinary geological event that affected the whole mediterranean region as well as global marine circulation between 5 97 and 5 33 ma in the mediterranean the crisis was mainly characterized by isolation from the atlantic ocean resulting in a new hydrological budget leading to the accumulation of large amounts of evaporites and large scale erosion of its margins the nature of the majority of the evaporites and erosional surfaces is still unknown thus representing a challenge which forms the subject of debate in the scientific community the neogene mallorca basins balearic islands are well situated to provide a stratigraphic record which expresses the sequence of events related to the msc being especially suitable for testing possible geological models or scenarios new research on the island of mallorca provides data about the msc and zanclean reflooding in the central part of the western mediterranean balearic basin with obvious repercussions for both academia and industry the main aim of this study is to describe the features which characterize sedimentation during the miocene pliocene transition on the island of mallorca to establish how this stratigraphic record relates to the different proposed msc scenarios for the whole of the mediterranean in this regard a stratigraphic analysis was carried out on a total of 100 exposures and boreholes corresponding to the neogene basins of mallorca the results indicate that on the island of mallorca the development of the msc is most consistent with models that represent the main phase of desiccation occurring after the accumulation of marginal evaporites primary lower gypsum the terminal carbonate complex and the lago mare deposits

membrane tetraether lipids of planktonic crenarchaeota in pliocene sapropels of the eastern mediterranean sea abstract the distribution of glycerol dibiphytanyl glycerol tetraethers gdgts predominantly derived from planktonic crenarchaeota were studied in three age equivalent pliocene sapropels as well as in the homogeneous intervals below and above the sapropel in both the homogeneous intervals and sapropels the dominance of gdgt 0 and crenarchaeol among the gdgts indicated their origin from planktonic crenarchaeota as it was found in many other marine sediments in the homogeneous intervals highest gdgt abundances normalized to toc were measured at the easternmost study site 967 of the eastern mediterranean sea and decreased at the central and western sites 969 and 964 within the three studied sapropels large variations in gdgt abundance were observed the newly established sea surface temperature sst proxy the tex 86 index derived from gdgts of planktonic crenarchaeota was used to estimate past sst comparison with previous obtained sst data using the alkenones from haptophyte algae revealed substantial differences whereas the u 37 k based ssts were almost constant at ca 25 c in both the homogeneous intervals and the sapropels the tex 86 based ssts were 26 29 c in the homogeneous intervals and decreased to 15 17 c in the sapropels the tex 86 based sst values outside the sapropel probably reflect summer sst based on a comparison with the present day mediterranean sea the surprisingly low tex 86 based sst estimates for the sapropels showed similarities with those obtained for the contemporary euxinic black sea the distribution of marine crenarchaeota in the modern black sea i e thriving at the deeper and colder chemocline suggests that this leads to a reduction in tex 86 resulting in artificially low sst estimates a similar situation is inferred for the pliocene eastern mediterranean during sapropel deposition since the sst anomaly co occurs with the build up of a shallow chemocline

dense shelf water cascading and messinian canyons a new scenario for the mediterranean salinity crisis the salt giant beneath the deep mediterranean seafloor is the impressive record of the messinian salinity crisis a dramatic event that occurred about 6 ma ago following the reduction of the connections with the atlantic ocean according to the shallow water deep basin model developed for these deposits hsu and others 1973a 1973b 1978a 1978b the messinian evaporites formed in a deep but desiccated mediterranean while shelves and slopes underwent subaerial erosion due to fluvial rejuvenation triggered by a 1500 m sea level drawdown deeply incised messinian canyons in the continental slopes surrounding the mediterranean are the main argument supporting this scenario using a state of the art model and idealized but realistic numerical simulations here we demonstrate that the activation of downslope flows of hypersaline dense waters in a process similar to present day dense shelf water cascading but much more energetic may account for both slope erosion and progressive salinity rise leading to the formation of deep seated supersaturated brines our findings support a deep water deep basin model schmalz 1969 1991 de benedetti 1976 1982 dietz and woodhouse 1988 thus implying that evaporite deposition may have occurred in a non desiccated basin with strongly reduced ocean connections

sources and distributions of tetraether lipids in surface sediments across a large river dominated continental margin glycerol dialkyl glycerol tetraether gdgt based proxies are increasingly used in modern carbon cycling and palaeoenvironmental investigations it is therefore crucial to examine the robustness sources transport and degradation of all gdgt based proxies in continental margins where sedimentation rates and extent of carbon cycling are high we have analyzed the distributions of gdgts in surface sediments from the lower yangtze river and east china sea ecs shelf the results revealed multiple sources and complex shelf processes that govern the distributions the isoprenoid gdgt inferred sea surface temperatures ssts are robust and reflect the satellite derived annual mean ssts on the shallow ecs shelf confirming an origin from surface water column dwelling crenarchaeota the input from methanogen sourced isoprenoid gdgts is significant in the river surface sediments but they are almost absent from the ecs shelf branched gdgts are also abundant in the river sediments but ca 95 are degraded in the yangtze estuary a much greater extent than observed for other terrigenous organic matter om proxies there is also evidence for production of branched gdgts in the oxic ecs shelf water column and the anoxic sediments waters of the lower yangtze river as a result branched gdgt based proxies in the lower river and ecs surface sediments do not reflect the catchment environmental conditions the effective degradation in the estuary and widespread aquatic contributions of branched gdgts improves our understanding of how to use branched gdgt based proxies in marginal seas

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

westward younging high mg adakitic magmatism in central tibet record of a westward migrating lithospheric foundering beneath the lhasa qiangtang collision zone during the late cretaceous abstract magmatic records with high mg and low y signatures provide important insights into the deep mantle dynamics that dictate the evolution of ancient orogenic belts this paper reports the presence of felsic intrusive dikes with high mg and low y signatures intruding the baingoin batholith central tibet at ca 94 ma these dikes show adakitic signatures including high sr and low y and yb contents and high sr y and la yb n ratios the adakitic dacitic dikes can be grouped into high mg mg 48 52 with positive zircon e hf t 8 3 12 4 and whole rock e nd t 0 27 and low mg mg 37 45 with positive zircon e hf t 7 2 13 5 varieties these distinct geochemical features along with geological observations show that the baingoin high mg adakitic dacitic dikes were most likely derived from the partial melting of a foundering juvenile lower crust and subsequent melt mantle reaction whereas the low mg adakitic dacitic dikes can be interpreted as resulting from the partial melting of a thickened lower crust that did not delaminate the generation of these adakitic rhyolitic dikes can be attributed to hornblende dominated fractional crystallization of the adakitic dacitic melts a decrease in y and heavy rare earth element contents of magmatic records from 120 100 ma to 95 80 ma within the lhasa qiangtang collision zone suggest an increase in crustal thickness with time the volumetrically minor westward migrating high mg adakitic magmatism can readily be interpreted as the consequences of the westward migrating foundering of the lithosphere triggered by rayleigh taylor instabilities along the lhasa qiangtang collision zone during the period 94 80 ma

silicic magma formation in overthickened crust melting of charnockite and leucogranite at 15 20 and 25 kbar 50 60 vol accompanied by settling of residualof crustal source rocks at depths 50 km 2 production of syenite crystals would lead to a magma of restricted compositional rangemagma by partial melting of quartzofeldspathic rocks at pressures with a heavy rare earth element depleted pattern overall the granitic 15 kbar melting experiments at 15 20 and 25 kbar were melt composition obtained in the experiments divers from that ofperformed on archaean biotite bearing charnockite of opx bearing a type granite however at degrees of melting below

climate sensitivity to arctic seaway restriction during the early paleogene abstract the opening and closing of ocean gateways affects the global distribution of heat salt and moisture potentially driving climatic change on regional to global scales between 65 and 45 million years ago ma during the early paleogene exchange between the arctic and global oceans occurred through two narrow and shallow seaways the greenland norway seaway and the turgai strait sediments from the arctic ocean suggest that during this interval the surface ocean was warm brackish and episodically enabled the freshwater fern azolla to bloom the precise mechanisms responsible for the development of these conditions in the paleogene arctic remain uncertain here we show results from an isotope enabled atmosphere ocean general circulation model which indicate that northern hemisphere climate would have been very sensitive to the degree of oceanic exchange through the arctic seaways we also present modelled estimates of seawater and calcite 18o for the paleogene by restricting these seaways we simulate freshening of the surface arctic ocean to 6 psu and warming of sea surface temperatures by 2 c in the north atlantic and 5 10 c in the labrador sea our results may help explain the occurrence of low salinity tolerant taxa in the arctic ocean during the eocene and provide a mechanism for enhanced warmth in the north western atlantic we propose that the formation of a volcanic land bridge between greenland and europe could have caused increased ocean convection and warming of intermediate waters in the atlantic if true this result is consistent with the theory that bathymetry changes may have caused thermal destabilisation of methane clathrates and supports a tectonic trigger hypothesis for the paleocene eocene thermal maximum petm

the messinian salinity crisis past and future of a great challenge for marine sciences forty years after the image of the mediterranean transformed into a giant salty lake was first conceived the fascinating history of the messinian salinity crisis msc still arouses great interest across a large and diverse scientific community early outcrop studies which identified severe palaeoenvironmental changes affecting the circum mediterranean at the end of the miocene were followed by investigations of the marine geology during the 1950s to 1970s these were fundamental to understanding the true scale and importance of the messinian event now after a long period of debate over several entrenched but largely untested hypotheses a unifying stratigraphic framework of msc events has been constructed this scenario is derived mainly from onshore data and observations but incorporates different perspectives for the offshore and provides hypotheses that can be tested by drilling the deep mediterranean basins the msc was an ecological crisis induced by a powerful combination of geodynamic and climatic drivers which had a great impact on the subsequent geological history of the mediterranean area and on the salinity of the global oceans these changed the mediterranean s connections with both the atlantic ocean and the freshwater paratethyan basins causing high amplitude fluctuations in the hydrology of the mediterranean the msc developed in three main stages each of them characterized by different palaeoenvironmental conditions during the first stage evaporites precipitated in shallow sub basins the msc peaked in the second stage when evaporite precipitation shifted to the deepest depocentres and the third stage was characterized by large scale environmental fluctuations in a mediterranean transformed into a brackish water lake the very high resolution timescale available for some late miocene intervals in the mediterranean makes it possible to consider environmental variability on extremely short time scales including in some places annual changes despite this fundamental questions remain some of which could be answered through new cores from the deepest mediterranean basins improvements in seismic imaging and drilling techniques over the last few decades make it possible to plan to core the entire basinal messinian succession for the first time the resulting data would allow us to decipher the causes of this extreme environmental change and its global scale consequences

late cretaceous k rich magmatism in central tibet evidence for early elevation of the tibetan plateau abstract major and trace element sr nd pb and zircon u pb and hf isotope data are presented for the newly discovered abushan volcanic rocks in the southern qiangtang terrane central tibet these results offer new insights into the evolution of the tibetan plateau during cretaceous the abushan volcanic rocks are composed mainly of andesites and trachy andesites zircon u pb dating constrains the timing of emplacement as late cretaceous 79 9 2 7 75 9 0 49 ma major element geochemistry shows that the abushan volcanic rocks belong to high k calc alkaline series all the andesites and trachy andesites are enriched in large ion lithophile elements lile and light rare earth elements lree and depleted in high field strength elements hfse and heavy rare earth elements hree the geochemical features and ages of the abushan volcanic rocks relate them with post collisional setting furthermore the volcanic rocks display negative end t 3 1 to 2 5 relatively constant 87sr 86sr ratios 0 70614 0 70735 and negative ehf t values 5 8 to 2 1 the sr nd pb and hf isotope signature suggests that the andesites and trachy andesites were derived from the anatexis of mafic lower crust by intrusion or underplating of mantle derived basaltic magma the petrogenesis of abushan andesites provides robust evidence for the crustal thickening delamination and early uplifting of central tibet during late cretaceous

early cretaceous foreland like northeastern qaidam basin tibetan plateau and its tectonic implications insights from sedimentary investigations detrital zircon u pb analyses and seismic profiling abstract the early cretaceous collision between the lhasa and qiangtang terranes is one of the critical tectonic events that shaped the southern margin of the eurasian continent and that impacted the formation of the cenozoic tibetan plateau however how far north this collision affected the structural and sedimentary geology of the early cretaceous remains uncertain to address this issue we present results from an integrated study of lower cretaceous strata in the northeastern qaidam basin neqb northern tibetan plateau based on field observations detrital zircon u pb analyses and seismic profile interpretations the lower cretaceous mainly consists of deltaic fluvial and alluvial sandstone and conglomerate with a pattern of orogeny related molasse deposits and a provenance in the nearby south qilian shan to the north a number of south directed reverse faults developed and formed piggy back basins in the early cretaceous these reverse faults remained active and affected erosion during the late cretaceous leaving an unconformity above the cretaceous strata these observations indicate a foreland like neqb due to the uplift and southward thrusting of the south qilian shan in the early cretaceous we further interpreted the neqb and the south qilian shan to be the northern edge of the compressional regime associated with the lhasa qiangtang collision in the early cretaceous which contrasts with the contemporaneous extensional setting in the north qilian shan and further north we also observed that some early cretaceous faults were rejuvenated as major thrusts in the south qilian shan thrust belt in the cenozoic for the first time this study maps the northern boundary of the late mesozoic qiangtang plateau and highlights a protracted influence on tibetan plateau tectonism since the early cretaceous

chronology causes and progression of the messinian salinity crisis the messinian salinity crisis is widely regarded as one of the most dramatic episodes of oceanic change of the past 20 or so million years 1 3 earliest explanations were that extremely thick evaporites were deposited in a deep and desiccated mediterranean basin that had been repeatedly isolated from the atlantic ocean1 2 but elucidation of the causes of the isolation whether driven largely by glacio eustatic or tectonic processes have been hampered by the absence of an accurate time frame here we present an astronomically calibrated chronology for the mediterranean messinian age based on an integrated high resolution stratigraphy and tuning of sedimentary cycle patterns to variations in the earth s orbital parameters we show that the onset of the messinian salinity crisis is synchronous over the entire mediterranean basin dated at 5 96 0 02 million years ago isolation from the atlantic ocean was established between 5 59 and 5 33 million years ago causing a large fall in mediterranean water level followed by erosion 5 59 5 50 million years ago and deposition 5 50 5 33 million years ago of non marine sediments in a large lago mare lake sea basin cyclic evaporite deposition is almost entirely related to circum mediterranean climate changes driven by changes in the earth s precession and not to obliquity induced glacio eustatic sea level changes we argue in favour of a dominantly tectonic origin for the messinian salinity crisis although its exact timing may well have been controlled by the 400 kyr component of the earth s eccentricity cycle

middle miocene paleoaltimetry of southern tibet implications for the role of mantle thickening and delamination in the himalayan orogen the stable isotope composition of pedogenic and early diagenetic carbonates from the oiyug basin of southern tibet allows model estimates of the paleoaltimetry of the tibetan plateau for the middle miocene pedogenic calcium carbonate nodules have average d 18 occ values of 219 6 whereas nodular lacustrine dolomites range in composition from 27 6 to 25 5 the most negative of the carbonate isotope values can be used to constrain the oxygen isotope composition of paleoprecipitation from which model estimates of paleoaltimetry can be made model results indicate that the southern tibetan plateau achieved elevations of 5200 11370 2605 m by at least 15 ma our results are identical within uncertainty to previous workers paleoelevation estimates based on oiyug basin fossil floral physiognomy this is the first time that two paleoaltimeters have been directly compared and are in accord collectively these data strongly support tectonic models in which thickening of mantle lithosphere beneath the domain of crustal thickening and subsequent detachment of the mantle lithosphere plays an indiscernible role in the elevation history of this part of the himalaya tibet orogenic system

did the mediterranean sea dry out during the miocene a reassessment of the evaporite evidence from dsdp legs 13 and 42a cores abstract the idea that the mediterranean sea dried out completely during the late miocene messinian 5 6 ma is now widely accepted this idea first published in 1972 hsu 1972a hsu 1972b grew out of the interpretation by deep sea drilling project dsdp scientists that the evaporites recovered from beneath the floor of the mediterranean by drilling on dsdp legs 13 1970 and 42a 1975 were of shallow water origin since that time there have been a number of advances in understanding of evaporite depositional processes and with this in mind we have reexamined the evidence provided by the dsdp evaporite cores we show that the case for deposition of the miocene evaporites of the sub mediterranean under shallow water conditions is based on interpretations that are either equivocal or incorrect several features of these evaporites used by the dsdp workers as evidence of shallow water conditions are more compatible with deposition under deep water below wave base conditions and others can only be considered as of uncertain origin in addition coring has sampled only the upper few tens of meters of the evaporites a factor that seriously limits any interpretation of the origin of the deposit as a whole in view of these findings we suggest that the question of the origin of the miocene evaporites beneath the floor of the mediterranean sea be reexamined by the geologic and oceanographic communities

cretaceous to cenozoic evolution of the northern lhasa terrane and the early paleogene development of peneplains at nam co tibetan plateau abstract highly elevated and well preserved peneplains are characteristic geomorphic features of the tibetan plateau in the northern lhasa terrane north northwest of nam co the peneplains were carved in granitoids and in their metasedimentary host formations we use multi method geochronology zircon u pb and u th he dating and apatite fission track and u th he dating to constrain the post emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas la icp ms u pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 ma and 85 ma and a third group records paleocene volcanic activity 63 58 ma in the nam co area the low temperature thermochronometers indicate common age groups for the entire nam co area zircon u th he ages cluster around 75 ma apatite fission track ages around 60 ma and apatite u th he ages around 50 ma modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest cretaceous to earliest paleogene time by middle eocene time the relief was already flat documented by a thin alluvial sediment sequence covering a part of the planated area the present day horst and graben structure of the peneplains is a late cenozoic feature triggered by e w extension of the tibetan plateau the new thermochronological data precisely bracket the age of the planation to early eocene i e between ca 55 and 45 ma the erosional base level can be deduced from the presence of early cretaceous zircon grains in eocene strata of bengal basin the sediment generated during exhumation of the nam co area was transported by an early cenozoic river system into the ocean suggesting that planation occurred at low elevation

tropical warming and intermittent cooling during the cenomanian turonian oceanic anoxic event 2 sea surface temperature records from the equatorial atlantic oceanic anoxic event 2 oae 2 occurring during the cenomanian turonian c t transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 myr however the impact of this major perturbation on and interaction with global climate remains unclear here we report new high resolution records of sea surface temperature sst based on tex86 and 18o of excellently preserved planktic foraminifera and stable organic carbon isotopes across the c t transition from black shales located offshore suriname french guiana demerara rise ocean drilling program leg 207 site 1260 and offshore senegal cape verde basin deep sea drilling project leg 41 site 367 at site 1260 where both sst proxy records can be determined a good match between conservative sst estimates from tex86 and 18o is observed we find that late cenomanian ssts in the equatorial atlantic ocean 33 c were substantially warmer than today 27 29 c and that the onset of oae 2 coincided with a rapid shift to an even warmer 35 36 c regime within the early stages of the oae a marked 4 c cooling to temperatures lower than pre oae conditions is observed however well before the termination of oae 2 the warm regime was reestablished and persisted into the turonian our findings corroborate the view that the c t transition represents the onset of the interval of peak cretaceous warmth more importantly they are consistent with the hypotheses that mid cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide co2 and that major oaes were capable of triggering global cooling through the negative feedback effect of organic carbon burial led co2 sequestration evidently however the factors that gave rise to the observed shift to a warmer climate regime at the onset of oae 2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 myr

comparison of eastern tropical pacific tex 86 and globigerinoides ruber mg ca derived sea surface temperatures insights from the holocene and last glacial maximum abstract the use of the tex 86 temperature proxy has thus far come to differing results as to whether tex 86 temperatures are representative of surface or subsurface conditions in addition although tex 86 temperatures might reflect sea surface temperatures based on core top holocene values this relationship might not hold further back in time here we investigate the tex 86 temperature proxy by comparing tex 86 temperatures to mg ca temperatures of multiple species of planktonic foraminifera for two sites in the eastern tropical pacific on the cocos and carnegie ridges across the holocene and last glacial maximum core top and holocene tex 86 h temperatures at both study regions agree well within error with the mg ca temperatures of globigerinoides ruber a surface dwelling planktonic foraminifera however during the last glacial maximum tex 86 h temperatures are more representative of upper thermocline temperatures and are offset from g ruber mg ca temperatures by 5 8 c and 2 9 c on the cocos ridge and carnegie ridge respectively this offset between proxies cannot be reconciled by using different tex 86 temperature calibrations and instead we suggest that the offset is due to a deeper export depth of gdgts at the lgm we also compare the degree of glacial cooling at both sites based on both temperature proxies and find that tex 86 h temperatures greatly overestimate glacial cooling especially on the cocos ridge this study has important implications for applying the tex 86 paleothermometer in the eastern tropical pacific

sources and distributions of branched and isoprenoid tetraether lipids on the amazon shelf and fan implications for the use of gdgt based proxies in marine sediments branched glycerol dialkyl glycerol tetraethers brgdgts in river fan sediments have been used successfully to reconstruct mean annual air temperature maat and soil ph of the congo river drainage basin however in a previous study of amazon deep sea fan sediments the reconstructed maats were ca 10 c colder than the actual maat of the amazon basin in this study we investigated this apparent offset by comparing the concentrations and distributions of brgdgts in amazon river suspended particulate matter spm and sediments to those in marine spm and surface sediments the riverine brgdgt input was evident from the elevated brgdgt concentrations in marine spm and surface sediments close to the river mouth the distributions of brgdgts in marine spm and sediments varied widely but generally showed a higher relative abundance of methylated and cyclic brgdgts than those in the river since this difference in brgdgt distribution was also found in intact polar lipid ipl derived brgdgts which were more recently produced the change in the marine brgdgt distribution was most likely due to marine in situ production consequently the maats calculated based on the methylation of branched tetraethers mbt and the cyclisation of branched tetraethers cbt were lower and the cbt derived ph values were higher than those of the amazon basin however spm and sediments from stations close to the river mouth still showed mbt cbt values that were similar to those of the river therefore we recommend caution when applying the mbt cbt proxy it should only be used in sediment cores that were under high river influence the influence of riverine derived isoprenoid gdgt isogdgt on the isogdgt based tex86 temperature proxy was also examined in marine spm and sediments an input of riverine isogdgts from the amazon river was apparent but its influence on the marine tex86 was minor since the tex86 of spm in the amazon river was similar to that in the marine spm and sediments

a study of the alkenone tex86 and planktonic foraminifera in the benguela upwelling system implications for past sea surface temperature estimates suspended particulate matter from seawater and core top sediments were collected during 2003 meteor cruise m57 1 in january february from the continental margin off western south africa for analysis of alkenones glycerol dibiphytanyl glycerol tetraether gdgt lipids and planktonic foraminifera alkenone analysis of suspended particulates in seawater and core top sediments indicates that u37k temperatures were representative of annual mean sea surface temperature in contrast gdgt analysis suggests that tex86 temperatures are cold biased due to upward transports of gdgts produced below the mixed layer the analysis of plankton tow samples revealed that the 18o of globorotalia inflata in core top sediments could be biased toward lower temperatures due to subsurface calcification accordingly our study shows that each paleotemperature proxy may record different temperature signals in the benguela upwelling system emphasizing the general need to constrain potential biases in each proxy for better interpreting paleoclimate records

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

synchronous onset of the messinian evaporite precipitation first mediterranean offshore evidence the messinian salinity crisis msc was a major ecological crisis affecting shallow and deep water settings over the entire mediterranean basin however the evolution of the msc and its ecological impacts have mainly been explained on the basis of sediments from onshore outcrops lack of complete and physically connected records from onshore and offshore settings has inhibited comprehensive understanding of basin behaviour during the msc herein we present a continuous record from an intermediate depth basin on the balearic promontory that comprises late tortonian messinian marls and evaporitic beds from the first msc phase i e primary lower gypsum plg stage well log and biostratigraphic data allow us establishing a large scale calibration to the astronomical solutions and to correlate pre msc sediments with classical rhythmic successions outcropping onshore thickness and characteristic sedimentary patterns observed in the offshore evaporitic records resemble those from marginal plg sequences furthermore seismic reflectors from a bedded unit bu which corresponds to an evaporitic interval according to well to seismic ties are correlated with the onshore plg sequences this correlation constitutes the first attempt to link well known marginal sequences with intermediate depth offshore settings which have previously only been studied through seismic imaging our time calibration provides direct evidence supporting a synchronous onset of the plg phase between onshore and offshore settings along the southwestern balearic promontory margin those bu reflectors which were positively correlated to the plg were likely precipitated offshore the continental shelf at messinian times these results suggest that gypsum precipitation and or preservation was not always limited to 200 m water depths and could occur in non silled basins finally we only found a major erosion at the top of the plg sequences implying that the msc drawdown occurred after the precipitation of the onshore lower evaporites studied sequences provide new insights into the plg precipitation preservation settings as well as into the land sea correlations of msc units and thus could potentially help refine current msc models

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

age and petrogenesis of a type granites in the middle segment of the bangonghu nujiang suture tibetan plateau abstract during geological investigations conducted over the past several years the authors found a group of a type granites in the middle segment of the bangonghu nujiang suture on the tibetan plateau these granitic plutons occur as stocks with exposed areas of less than 1 km 2 and lithologically include biotite granite and granodiorite porphyry which had intruded into cretaceous sedimentary rocks petrogeochemically the a type granites contain high sio 2 levels varying from 68 62 75 36 wt moreover they contain relatively high fe 2 o 3 t 0 86 5 39 wt and low al 2 o 3 12 76 15 54 wt levels and are weak peraluminous to metaluminous a trace element n morb normalized spider diagram showed apparent enrichment of the large ion lithophile elements lile rb th u k and pb with marked depletions of ba and sr the high field strength elements hfse nb ta and ti were sharply depleted with relative enrichment of zr and hf these signatures are characteristic of a type granite the granite samples had high rare earth element ree contents with ree 122 37 291 19 ppm light ree were rich relative to heavy ree with lree hree 4 89 9 58 and negative eu anomalies were sharp with eu varying from 0 14 0 5 and a mean value of 0 34 their chondrite normalized rare earth element distribution patterns appear as gently right dipping v types zircon u pb laicpms dating for three granitic plutons yielded weighted mean ages of 109 6 1 4 ma mswd 3 7 112 3 0 51 ma mswd 0 21 and 113 73 0 5 ma mswd 0 21 indicating that they formed in the late stage of the early cretaceous epoch the nd sr and pb isotopic results showed that the granites were rich in radiogenic isotopes the 87 sr 86 sr and i sr ratios varied from 0 719842 0 786395 and 0 706900 0 710378 respectively the 143 nd 144 nd ratios were from 0 512123 0 512392 with relatively large negative e nd values 3 37 to 10 34 the 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios ranged from 18 703 19 070 15 680 15 732 and 39 121 39 576 respectively exhibiting isotopic signatures typical of crustal sources all of the data demonstrated unanimously that these granites were derived from partial melting of arc basaltic rocks underplated in the lower crust during oceanic crust subduction the high temperature heat source for melting of anhydrous basaltic rocks of the lower crust was from upwelling asthenosphere through slab windows formed as a result of slab break off during post collisional extension of the orogenic zone

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

age refinement of the messinian salinity crisis onset in the mediterranean we propose a revised age calibration of the messinian salinity crisis onset in the mediterranean at 5 971 ma based on the recognition of an extra gypsum cycle in the transitional interval of the perales section sorbas basin spain and the revision of the magnetostratigraphy of the monticino section vena del gesso basin italy this age re calibration allows to state more accurately that i the interval encompassing the msc onset is continuous thus ruling out any erosional feature or stratigraphic hiatus related to a major sea level fall affecting the mediterranean ii the first gypsum was deposited during the summer insolation peak at 5 969 ma associated with an eccentricity minimum and roughly coincident with glacial stage tg32 iii the msc onset was preconditioned by the tectonically driven reduction of the hydrological exchanges with the atlantic ocean and finally triggered by glacial conditions in the northern hemisphere and by arid conditions in northern africa

process of lithospheric delamination beneath the lhasa qiangtang collision orogen constraints from the geochronology and geochemistry of late cretaceous volcanic rocks in the lhasa terrane central tibet abstract the geochemistry of post collisional granitoids can be used to reconstruct crustal thickness and constrain deep geodynamic processes two series of volcanic rocks of the jiangba formation in the xiongmei area central tibet record the process of lithospheric delamination in the lhasa qiangtang collisional orogen the andesites and dacites crystallized at c 87 and 85 ma respectively and have positive zircon ehf t values 2 7 5 4 and 2 7 7 1 respectively they have calc alkaline compositions with relatively high mg 100 molar mg2 mg2 fe2 39 66 sr contents and sr y and la yb n ratios subscript n signifies normalization to chondrite low y and yb contents and no clear negative eu sr or ti anomalies similar to adakites the andesite dacite series likely formed by partial melting of foundering juvenile mafic lower crust with residual garnet followed by the interaction of this melt with peridotite the rhyolite series was emplaced at c 81 ma and has positive zircon ehf t values 1 0 3 5 the rhyolites are characterized by non adakitic signatures and have high k calc alkaline to shoshonite compositions with low mgo contents mg 21 34 and sr y and la yb n ratios coupled with flat hree patterns and marked negative eu sr and ti anomalies geochemical data indicate that these rhyolites were generated by partial melting of a much shallower crustal root with a plagioclase rich residue zircon u pb ages and different compositional trends for the two series indicate that the lithospheric mantle keel was partially removed by small scale mantle convection that triggered the late cretaceous high mg adakitic magmatism in the lhasa qiangtang collisional orogen partial lithospheric delamination increased heat flow at the base of the crust and caused the thinned crust to melt producing the low mg non adakitic rocks

the deep record of the messinian salinity crisis evidence of a non desiccated mediterranean sea abstract this research is focused on a complete reexamination of the evaporite facies present in all the cores that cut through the topmost deposits of the messinian salinity crisis lying below the floor of the mediterranean sea dsdp legs 13 and 42a odp legs 107 and 161 this review suggests that the uppermost evaporite units in both western and eastern deep mediterranean basins consist mainly of clastic gypsrudite gypsarenite and gypsiltite and fully subaqueous deposits laminar gypsum selenite and cumulate halite that are partially affected by burial anhydritization and tectonic induced recrystallization no unequivocal evidence of shallow water or even supratidal sabkha deposition is in evidence suggesting that at the very last phase of the salinity crisis the mediterranean sea did not experience desiccation but that deposition took place under permanent subaqueous conditions

geochemistry of limestones deposited in various plate tectonic settings abstract limestone a major part of the global sedimentary succession susceptible to post depositional diagenesis studies of limestone geochemistry are essential in the discrimination of tectonic settings of basins in which the limestones were deposited six late mesozoic and one tertiary limestone successions of tibet western china that were deposited in oceanic plateau passive continental margin active continental margin fore arc basin back arc basin and foreland basin and continental inland freshwater basins were analyzed for their major trace and rare earth element ree composition this geochemical dataset in combination with the deep sea drilling project and ocean drilling program dsdp and odp literature geochemical data regarding limestones deposited in open ocean environments permitted delineation of the geochemical characteristics of limestones accumulated in these various plate tectonic settings major elements e g fe2o3 and mno except for cao of these limestone successions show large variations but are positively correlated with al2o3 the ree and trace element abundances for the inland and margin limestones show a distinct positive correlation with al2o3 whereas rees and trace elements of the open ocean limestones are positively correlated with mno there is a systematic increase in the magnitude of ce anomalies of open ocean floor limestones away from spreading ridges to open ocean highs to passive margins and to active margins and inland freshwater basins open ocean limestones display a narrow range of la sm n 0 46 0 96 sm yb n 0 25 1 96 and la yb n 0 23 1 38 but high la ce n 1 5 whereas the inland margins limestones display a much larger range la sm n 0 43 2 18 sm yb n 0 6 2 98 and la yb n 0 7 2 25 but low la ce n

thickened juvenile lower crust derived 90 ma adakitic rocks in the central lhasa terrane tibet abstract the questions of why the late cretaceous magmatism generated and how the nature of the lower crust evolves in central tibet remain poorly constrained in this paper we report the presence of early late cretaceous adakitic rocks from the azhang area northern edge of the central lhasa subterrane central tibet these rocks are rhyodacites dacites in composition and have geochemical characteristics of adakitic rocks e g high sr 554 836 ppm sr y 66 100 and la yb n 20 21 low y 7 96 8 96 ppm and heavy rare earth elements hree in situ zircon u pb dating for two samples yields an early late cretaceous age 90 1 ma and 87 1 ma the low mgo 1 4 1 9 wt contents and compatible element abundances e g cr 22 30 ppm ni 19 25 ppm indicate that these rocks were most likely derived from the partial melting of a garnet bearing amphibolite under a thickened lower crust condition the positive whole rock e nd t 2 5 to 5 6 and zircon e hf t 8 9 to 16 0 values suggest that this thickened lower crust was juvenile the crust beneath the central lhasa subterrane may have been significantly thickened due to tectonic shortening in response to the lhasa qiangtang collision and magma underplating before the emplacement of azhang adakitic rocks 90 ma we argue that regional lithospheric delamination at 90 ma triggered the partial melting of the lowermost garnet bearing crust that is still attached to the middle crust to generate the azhang adakitic rocks the presence of the 90 ma azhang adakitic rocks provides valuable constraints on the origin of the early late cretaceous magmatism in the central lhasa subterrane and on the crustal evolution beneath the lhasa qiangtang collisional zone prior to the cenozoic india asia collision

distributional variations in marine crenarchaeotal membrane lipids a new tool for reconstructing ancient sea water temperatures it has recently been shown that membrane lipids of marine crenarchaeota a ubiquitous and abundant component of plankton occur in relatively high concentrations in recent and ancient sediments in this study we investigated the environmental controls on the relative distribution of these lipids in surface sediments we especially focussed on temperature as it is known from the thermophilic genetic relatives of marine crenarchaetoa that the composition of their membrane strongly depends on growth temperature indeed a significant linear correlation r2 0 92 is found between the number of cyclopentane rings in sedimentary membrane lipids derived from marine crenarchaeota and the annual mean sea surface temperatures this suggests that the mechanism of physical adaptation of their membrane compositions to temperature is identical to that of their thermophilic relatives in turn archaeal lipid distributions in sediments may thus allow the reconstruction of sea water temperatures of ancient marine environments

temperature dependent variation in the distribution of tetraether membrane lipids of marine crenarchaeota implications for tex86 paleothermometry 1 recently a new geochemical temperature proxy the tex86 was introduced this proxy is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraethers gdgts of the membrane lipids of marine crenarchaeota which changes as a response to temperature however until now only sediment data have been used to establish this proxy and experimental work is missing we performed mesocosm studies with marine crenarchaeota incubated at temperatures ranging from 5 to 35 c and salinities of 27 and 35 to test the validity of the tex86 proxy growth of marine crenarchaeota in these mesocosms was evident from the substantial increase in the concentration of marine crenarchaeotal membrane lipids with amounts up to 3400 ng l with increasing temperature an increase in the number of cyclopentane moieties in the crenarchaeotal membrane lipids was observed different salinities did not show any effect on the gdgt distribution the tex86 showed a significant linear correlation to incubation temperature tex86 0 015 t 0 10 r2 0 79 this equation has a similar slope to the correlation obtained from core tops but differs in the intersection tex86 0 015 t 0 28 r2 0 92 this difference is mainly determined by the smaller amount of the regioisomer of crenarchaeol in the incubation series compared to core top samples these incubation experiments indicates that water temperature is indeed the major controlling factor for the membrane distribution of marine crenarchaeota and confirms that the tex86 proxy depends on a physiological response to regulate membrane fluidity

large temperature variability in the southern african tropics since the last glacial maximum 1 the role of the tropics in global climate change is actively debated particularly in regard to the timing and magnitude of thermal and hydrological response continuous high resolution temperature records through the last glacial maximum lgm from tropical oceans have provided much insight but surface temperature reconstructions do not exist from tropical continental environments here we used the tex86 paleotemperature proxy to reconstruct mean annual lake surface temperatures through the last glacial maximum lgm in lake malawi east africa 9 14 s we find a 3 5 c overall warming since the lgm with temperature reversals of 2 c during the younger dryas 12 5 ka bp and at 8 2 ka bp maximum holocene temperatures of 29 c were found at 5 ka bp a period preceding severe drought in africa these results suggest a substantial thermal response of southeastern tropical africa to deglaciation and to varying conditions during the holocene

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

influence of andean uplift on climate and paleoaltimetry estimates article i nfo article history recent elevation reconstructions of the andean plateau suggest a rapid 2 5 1 0 km rise of the central andes between 10 and 6 ma this rapid rise has been attributed to a catastrophic removal of a dense lithospheric mantle root beneath the andes however these findings are based on the assumption that climate did not change during deposition of paleoaltimetry proxies here we evaluate south american climate change due to andean uplift and its influence on interpretations of plateau elevation from climate sensitive paleoaltimetry data a series of experiments are presented using the regcm3 regional general circulation model rcm to characterize changes in andean precipitation amount surface temperature and wind direction vapor source as a function of changing plateau elevation results indicate that south american and andean climate changed significantly in response to plateau growth more specifically rising of the plateau results in up to a 900 mm increase in rainy season december january february precipitation over the plateau plateau uplift also results in a decrease in non adiabatic surface temperature of up to 6 5 c in addition to adiabatic cooling directly related to elevation change through the lapse rate finally the prevailing wind direction and the vapor source for precipitation switches from the south pacific ocean to the equatorial atlantic as plateau elevation increases above of its present day elevation taken together these changes in paleoclimate would have substantially depleted the oxygen isotopic concentration of paleoprecipitation through the cenozoic unless this climatic effect is taken into consideration paleoaltimetry reconstructions based on stable isotope methods may overestimate the rapid rise of the andes by up to several kilometers we conclude that some or all of the apparent rapid rise of the andean plateau from paleoaltimetry data could be an artifact of large changes in paleoclimate

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

early cretaceous palaeogeographic evolution of the coqen basin in the lhasa terrane southern tibetan plateau abstract the lower cretaceous sedimentary strata in the lhasa terrane record the palaeogeographic and tectonic history of the tibetan plateau prior to the india asia collision the lower cretaceous strata in the coqen basin of the northern lhasa subterrane include the duoni and langshan formations the duoni formation is composed of conglomerates sandstones siltstones and mudstones in the south deposited in a fluvial environment and quartzose lithic sandstones siltstones and mudstones in the north deposited in a shelf coastal environment the overlying langshan formation is characterized by abundant orbitolina and rudist bearing wackestones and packstones which were deposited on a low energy carbonate ramp the langshan formation in the coqen basin was deposited between 119 115 ma and 98 ma based on its large benthic foraminiferal assemblages age constraints based on the interbedded tuff and conformably overlying langshan formation indicate that the duoni formation was deposited between 123 and 115 ma sandstone detrital modal compositions further indicate that the duoni formation was primarily derived from magmatic arc and recycled orogen sources detrital zircons from the duoni formation yield a primary age population of 160 110 ma peaking at 130 ma with negative e hf t values 18 7 to 2 7 and additional age ranges of 550 500 1200 900 and 1600 1500 ma thereby indicating that the zenong volcanic rocks and palaeozoic sedimentary bedrocks in the northern lhasa subterrane are the most likely sources the palaeogeography of the northern lhasa subterrane can be approximately described in two stages during the aptian 123 115 ma with the eruption of the zenong volcanic rocks the duoni formation accumulated in fluvial and shelf coastal environments and was sourced from the zenong volcanic rocks and basement rocks from the southern region of the northern lhasa subterrane the enhanced magmatic activities in the northern lhasa subterrane may have influenced the duoni formation deposition during the late aptian early cenomanian 115 98 ma a north south transgression occurred causing the majority of the northern lhasa region to be dominated by shallow marine limestones of the langshan formation

messinian salinity crisis deposits widespread over the balearic promontory insights from new high resolution seismic data abstract the current interpretation of the messinian salinity crisis msc involves the deposition of peripheral or marginal evaporites in onshore basins as well as the erosion of the margin and the deposition of thick evaporites in deep basins the so called intermediate basins are formed in domains between the onland outcrops and the deep basins the balearic promontory is a bathymetric high located between the deep algerian and liguro provencal basins and the onland spanish basin the simbad project aims to investigate the spatial variability of the msc related deposits and to assess the extent of post msc reactivation over the balearic promontory we present here the first results of the simbad high resolution seismic survey january 2013 which imaged for the first time a thin msc related unit widely distributed in small sub basins over the balearic promontory borehole analyses have shown that this unit could be correlated with primary gypsum formations linked to the peripheral evaporites locally in the central depression between mallorca and ibiza islands a thicker msc unit is observed whose lowermost transparent part could correspond to a salt layer geometrical relationships suggest that the msc in the central depression could postdate the primary gypsum the occurrence of a halite layer in the central depression at depths of 1000 to 1500 m favours the hypothesis that the evaporites precipitated passively in closed or partially closed perched sub basins possibly as a result of evaporative drawdown at different depths and possibly diachronously at least with respect to the deep basin evaporites

secular geochemical variations of the lower cretaceous siliciclastic rocks from central tibet china indicate a tectonic transition from continental collision to back arc rifting abstract fifty five lower cretaceous siliciclastic rocks from central tibet were analyzed for major and trace elements in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited the spatial temporal variation of both sediment maturity indices sio 2 al 2 o 3 la v and zr tio 2 the ratios of immobile trace elements cr th th sc th co la sc and la co and sio 2 k 2 o na 2 o eu eu gd yb n and ferromagnesian contents displays that the berriasian valanginian bv sediment could have been derived from northern orogenic source areas whereas the hauterivian lower barremian hb sediment could have been closely related to the syndepositional volcanism in the northern lhasa block itself the na 2 o al 2 o 3 k 2 o al 2 o 3 relations also place approximately half of hb samples in the field of volcanic rocks of back arc rifting origin whereas little volcanic clasts exist in the bv rocks there are distinct signatures that the mafic ultramafic materials in the bv rocks were derived from oceanic island basalts ophiolites in the bangong nujiang suture therefore the collisional orogen due to the qiangtang lhasa collision should have played a key role in supply of the sediments during bv time bimodal sedimentary basaltic and rhyolitic tuffs with respective distinguished geochemical features exist in the hb sequence therefore a distinct tectonic transition from continental collision to back arc rifting in central tibet is believed to have occurred during early cretaceous time extreme depletion of na of most lower cretaceous samples compared to the upper crust and their high average chemical index alteration cia values and th u ratios point out the source rocks formed in tropical climatic regions with little tectonic uplift although the weathering degree of the source rocks for hb samples could likely have been affected by the syndepositional volcanism during middle cretaceous time the entire southern eurasian margin could have been characterized by a series of back arc rift basins including those in kohistan and ladakh