organic chemistry in a co 2 rich early earth atmosphere the emergence of life on the earth has required a prior organic chemistry leading to the formation of prebiotic molecules the origin and the evolution of the organic matter on the early earth is not yet firmly understood several hypothesis possibly complementary are considered they can be divided in two categories endogenous and exogenous sources in this work we investigate the contribution of a specific endogenous source the organic chemistry occurring in the ionosphere of the early earth where the significant vuv contribution of the young sun involved an efficient formation of reactive species we address the issue whether this chemistry can lead to the formation of complex organic compounds with co2 as only source of carbon in an early atmosphere made of n2 co2 and h2 by mimicking experimentally this type of chemistry using a low pressure plasma reactor by analyzing the gaseous phase composition we strictly identified the formation of h2o nh3 n2o and c2n2 the formation of a solid organic phase is also observed confirming the possibility to trigger organic chemistry in the upper atmosphere of the early earth the identification of nitrogen bearing chemical functions in the solid highlights the possibility for an efficient ionospheric chemistry to provide prebiotic material on the early earth

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

δ2h of water from fluid inclusions in proterozoic halite evidence for a deuterium depleted hydrosphere abstract 2h were measured in water present in tiny primary fluid inclusions trapped in mesoproterozoic 1 4 ga and neoproterozoic 0 8 ga halite crystals while the hydrogen concentrations range from 300 to 1500 ppm for the whole sample collection 2h values range from 74 to 54 vsmow for the mesoproterozoic halite sampled from the sibley group ontario canada and from 89 to 38 for the neoproterozoic halite from the browne formation officer basin australia the amount of evaporation required to precipitate halite is accompanied by a deuterium enrichment of 30 to 90 it means that the original aqueous solutions had 2h values comprised between 140 and 90 considering the estimated tropical paleolatitudes for both depositional environments surface waters were most likely significantly deuterium depleted relative to modern ones if such interpretation is partly at variance with the geological record it could be reconciliated with some modelling outputs of the long term water cycle indeed several mechanisms produce a progressive deuterium enrichment of the oceans those identified so far are the addition of deuterium rich extraterrestrial water cometary dust the loss of h during water photodissociation by ultraviolet radiation in the upper atmosphere and the isotopic fractionation that takes place during the hydroxylation of nominally anhydrous mantle rock forming minerals we also propose that the major process able to shift through time the 2 of the hydrosphere is the reduction of h2o into h2 during the serpentinization of mantle rocks at mid ocean ridges

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

crustal structure in the tengchong volcanic area and position of the magma chambers abstract in this paper we extract 1500 p receiver functions in the tengchong volcanic area from 211 teleseismic events recorded at nine digital seismic stations a common conversion point stacking technique is used to improve the signal to noise ratio and to get the time delays of the ps ppps psps ppss phases within grids of 10 km 10 km finally the crustal thickness and poisson s ratio are calculated the results show that the crustal thickness ranges from 28 to 40 km and the poisson s ratio ranges from 0 28 to 0 36 there exist two mantle uplifting sites each with a horizontal scale of about 30 km 30 km one in mazhan tengchong maanshan and the other in wuhe longjiang tuantian the high poisson s ratio is consistently located within these two sites recorded shocks with ms 2 0 reveal that most of the shocks are distributed around the two sites and few are located at the centers the shocks the geothermal distribution and the crustal structure suggest that the magma is still active and the two mantle uplifting sites detected may be the positions of two magma chambers in the crust

high archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3 5 ga swaziland supergroup south africa new and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3 5 3 2 ga swaziland supergroup barberton greenstone belt south africa the low 1 8 o values of these cherts indicate extremely high ocean temperatures of 55 85 c previously the large depletion in 1 8 o shown by all barberton cherts relative to their phanerozoic counterparts has been attributed to low 1 8 o in archean oceans chert formation during late diagenesis wholesale loss of 1 8 o during alteration and or regional silicification of sediments around hot springs these alternative explanations are not compatible with the new results cherts in the onverwacht group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying fig tree and moodies groups demonstrating that the chert 8 0 o values were fixed prior to archean uplift and erosion which started at 3 26 ga the maximum 1 8 o value in barberton cherts 22 is lower than the minimum values 23 in phanerozoic bedded cherts precluding late diagenesis as the explanation of the overall low 1 8 o values regional metamorphic hydrothermal or long term resetting of original 1 8 o values is also precluded by preservation of 1 8 o values across different metamorphic grades and by systematic 1 8 o differences among interbedded chert types stratigraphic units and conglomerate clasts the 7 1 8 o variation of these archean cherts is similar to that of phanerozoic deep sea cherts formed when opal converted to microquartz during burial but the actual archean values are 10 lower marine opal was apparently converted to microquartz during burial to depths of 1 km cherts with 1 8 o 15 reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and or hydrothermal activity cherts with higher 1 8 o values formed during early diagenesis and indicate an extremely hot archean ocean and surface environment

seismic signature of the mantle transition zone beneath eastern tibet and sichuan basin abstract the strong interaction between the eastward flow escaping from tibet and the rigid sichuan basin resulted in the rise of the longmenshan however the detailed dynamics in the mantle remains controversial in this study the structure of the mantle transition zone mtz beneath eastern tibet and sichuan basin is investigated using 5080 receiver functions from 51 broadband stations the depth of the 410 km discontinuity is close to the global average except for the longmenshan where the 410 and 660 km discontinuities are found to be depressed by up to 10 25 km and 5 10 km respectively the observed simultaneous depressions of the 410 and 660 km discontinuities distributed along the lms together with proofs from tomography and regional tectonics suggest that asthenospheric flow sinks into the mtz resulting in a high velocity zone as well as variation in the mtz thickness the depressions are not from the traditional clapeyron slopes or temperature variation also the depression of the 410 km discontinuity and the dehydration of wadsleyite are syngenetic both of which originate from the dry mantle flow traveling across the old 410 km interface

mauna loa volcano is not a methane source implications for mars 1 thirteen years of continuous atmospheric carbon dioxide and methane measurements at the mauna loa observatory in hawaii are used to determine the methane emission rate from the summit of mauna loa volcano we find no measurable methane emissions coming from the summit area with a 95 confidence upper limit of 9 t ch4 yr 1 recent studies have detected 10 ppb ch4 in the martian atmosphere requiring emissions of about 300 t ch4 yr 1 volcanic activity has been suggested as a source of abiogenic ch4 on mars either by magmatic degassing or reactions in hydrothermal fluids heated by a magma intrusion the most recent lava flows on mars 2 my ago are on the tharsis shield volcanoes which may still be active if mauna loa is a valid terrestrial analog our findings suggest that volcanic activity is not a significant source of methane to the martian atmosphere

cenomanian early turonian carbonate platform of northern oman stratigraphy and palaeo environments abstract the jebel akhdar and adam foothills outcrops in northern oman arabian peninsula offer the opportunity to study in detail the lateral and vertical changes of a cenomanian early turonian carbonate platform formed by the natih formation wasia group eleven sections with less than 5 km offset were logged to express metre scale lithological and biostratigraphical vertical changes the first rudist zonation is established complementing classical benthonic foraminifer and ammonite biozones typical cenomanian rudist association caprinids and radiolitids occur in the natih e c d a lower part whereas lower turonian hippuritid rudists are represented in the upper part of the natih a a sedimentological and palaeoecological approach leads to identification and interpretation the facies and discontinuities of the natih carbonate platform and to a reconstruction of its spatial organization application of the concepts of sequence stratigraphy allows interpretation of the units of the natih in terms of systems tracts an attempt is made to correlate the sequence stratigraphical model to eustatic sea level changes and regional tectonic events three palaeoenvironmental models taking into account bio and lithofacies are proposed

crustal structure across longmenshan fault belt from passive source seismic profiling 1 we analyse receiver functions from 29 broad band seismographs along a 380 km profile across the longmenshan lms fault belt to determine crustal structure beneath the east tibetan margin and sichuan basin the moho deepens from about 50 km under songpan ganzi in east tibet to about 60 km beneath the lms and then shallows to about 35 km under the western sichuan basin the average crustal vp vs ratios vary in the range 1 75 1 88 under songpan ganzi in east tibet 1 8 2 0 under the lms and decrease systematically across the nw part of the sichuan basin to less than 1 70 a negative phase arrival above the moho under songpan ganzi and sichuan basin is interpreted as a ps conversion from the top of a low velocity layer in the lower crust the very high crustal vp vs ratio and negative polarity ps conversion at the top of lower crust in east tibet are inferred to be seismic signatures of a low viscosity channel in the eastern margin of the tibetan plateau the lateral variation of moho topography crustal vp vs ratio and negative polarity ps conversion at the top of the lower crust along the profile seem consistent with a model of lower crust flow or tectonic escape

structure of the crust beneath the southeastern tibetan plateau from teleseismic receiver functions abstract southeastern tibet marks the site of presumed clockwise rotation of the crust due to the india eurasian collision and abutment against the stable sichuan basin and south china block knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region here we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method we find that the crustal thickness decreases gradually from the tibetan plateau proper to the sichuan basin and yangtze platform but that significant intra crustal heterogeneity exists on shorter lateral scales 0 30 are detected beneath the southeastern margin of tibet but in the sichuan basin and southeastern yangtze platform the values are close to the global average combined with high regional heat flow and independent evidence for mid crustal layers of high electric conductivity the large intra crustal s wave velocity reduction 12 19 and the intermediate to high average crustal poisson s ratios are consistent with partial melt in the crust beneath parts of southeastern tibet these results could be used in support of deformation models involving intra crustal flow with the caveat that significant lateral variation in location and strength of this flow may occur

early cretaceous platform margin configuration and evolution in the central oman mountains arabian peninsula the hajar supergroup middle permian lower cretaceous of northeastern oman records rifting and development of a passive margin along the edge of the arabian platform facing neo tethys the jurassic and lower cretaceous part comprising the sahtan kahmah and wasia groups was deposited during the maximum extent of the broad epicontinental sea landward of this margin these limestone units reach a total of 1500 m in thickness and they correlate with the principal hydrocarbon reservoirs of the arabian peninsula the trace of the jurassic and cretaceous margin in northeastern oman followed a zigzag series of rift segments resulting in promontories and reentrants that changed in position through time in response to the configuration and differential motion of underlying rift blocks in an overall extensional regime synsedimentary normal faulting with 600 m of throw occurred locally during the middle jurassic whereas during the late jurassic the margin was subjected to erosion from variable uplift of up to 300 m before subsiding to below storm wave base this uplift may have been caused by compression from oceanic crust that obducted along the southeastern side of the platform subsequently lower cretaceous strata document varying rates of block subsidence the lower cretaceous succession in the central oman mountains and adjacent subsurface began with regional drowning around the jurassic cretaceous boundary the succession in the east saih hatat records a single regressive sequence culminating in the progradation of the shallow water carbonate platform by the cenomanian however the succession in the west jebel akhdar and interior is dominated by shallow water carbonate facies but punctuated by a second regional drowning in the late aptian a third late cretaceous drowning terminated deposition of the wasia group in the turonian and was caused by convergence of oceanic crust and foreland basin formation although previously ascribed to eustatic sea level rises the two early cretaceous drowning events can be explained as consequences of the tectonic behavior of the platform the earlier event rayda formation occurred when first uplift tilted the eastern side of the platform and induced a relative sea level stillstand and widespread evaporative conditions in the interior hith anhydrite and then the eastern side subsided rapidly to below storm wave base forming a broad embayment starved of periplatform sediment by contrast the second drowning nahr umr formation was due to detrital influx generated by uplift on the west side of the platform both tectonic episodes led to a reduction in the calcium carbonate generating capacity of the epicontinental sea causing shallow water fa ies to be abruptly overlain by deeper water facies the effects from these two drownings eventually diminished and shallow water environments reestablished themselves the overall succession appears independent of eustatic sea level change

anisotropic h k stacking and revisited crustal structure in the southeastern margin of tibet abstract p receiver functions prfs and the h k stacking technique are tools often used to constrain the thickness of the crust and the ratio between the velocities of p and s waves both the crustal depth and the poisson s ratio are very important parameters to study the tectonic evolution of the lithosphere but they can be significantly affected by the presence of crustal anisotropy in order to address this problem we have extended the isotropic stacking approach to include six converted p to s phases that are generated in the anisotropic case instead of only three phases considered in the isotropic case and we have examined the feasibility of the stacking technique using synthetic and real data based on prfs acquired by 108 permanent broadband seismic stations deployed in the southeastern margin of tibet including the sichuan and yunnan areas we have applied the anisotropic h k stacking scheme to investigate the crustal thickness and poisson s ratio the results reveal that the crustal thickness varies from 60 km in the songpan ganzi fold system and the northern part of the sichuan yunnan diamond shaped block near the eastern himalayan syntaxis to 33 km in southern yunnan and that the poisson s ratio varies mostly from 0 24 to 0 32 the highest values of 0 28 0 30 are found along the axis formed by the longmenshan fault the lijiang jinhe fault and the xiaojiang fault and are attributed to the accumulation of lower crustal flow in front of the sichuan basin the highest value of 0 32 is observed at the tengchong volcano area to the north of the indochina block and is attributed to the upwelling of hot mantle associated to the eastward subduction of the indian plate rather than to the expansion of east tibet comparing with the results provided by the isotropic stacking scheme it is appreciated that the crustal depth determined by the anisotropic stacking method well adjusts the observed pattern of gravity anomaly we conclude that the results obtained by anisotropic stacking yield a much better constraint on the estimation of the crustal depth and poisson s ratio when compared to those achieved by isotropic stacking

geodetic measurement of crustal motion in southwest china global positioning system measurements performed over a 2 4 yr period confirm the xianshuihe xiaojiang fault system as one of the primary active structures in southwest china stations southwest of these faults show southerly motions of 5 15 mm yr relative to the western sichuan basin and stations in a 200 km geodetic network located northwest of the basin move at only 0 5 mm yr these results are consistent with clockwise rotation of southwestern sichuan and western yunnan about the eastern tibetan syntaxis accommodated by left lateral slip of 12 4 mm yr on the xianshuihe xiaojiang fault system the results imply that if eastward extrusion of crustal material from the plateau occurs at present it is not accommodated by east west shortening along the margin of the plateau and must involve wholesale eastward motion of low lying regions to the east

abiogenic methanogenesis during experimental komatiite serpentinization implications for the evolution of the early precambrian atmosphere abstract hydrothermal alteration of oceanic ultramafic rocks can produce significant abiotic methane as a byproduct however the abundance of peridotites is limited in the modern basaltic oceanic crust abiogenic methanogenesis was likely more productive in the early precambrian when komatiitic volcanism delivered a much larger proportion of ultramafic material to the crust it is therefore possible that hydrothermal alteration in the hadean and archean could have led to a higher methane flux to the atmosphere than today potentially influencing the average surface temperature of the early earth this hypothesis was tested in a series of experiments in which natural and synthetic komatiite powders were reacted with an aqueous solution containing isotopically labeled formic acid at 300 c and 350 bars over 2 months experiments containing the natural powder generated labeled methane 13 ch 4 at a rate 5 8 faster than concurrent experiments containing the synthetic komatiite a similar difference was also observed between the natural komatiite and previous experiments on olivine serpentinization at the same conditions faster rates of methanogenesis in the natural komatiite experiments could be due to catalysis by magnesiochromite which was present only in the natural komatiites however this was ruled out because parallel experiments containing additional magnesiochromite failed to increase methane yields the most likely catalysts of the remaining candidate minerals were deduced to be ni sulfides common accessory minerals in komatiites that have previously been shown to accelerate organic reactions because ni sulfides were particularly abundant in archean komatiites the experiments imply that methane yields during seafloor alteration were higher on the early earth than today this inference in combination with the assumptions of faster crustal production rates and greater volumetric proportions of ultramafic lithologies leads to the conclusion that methane flux from serpentinization of an archean komatiite bearing oceanic crust was likely higher than that from metamorphism of the modern oceanic crust incorporation of this result into existing geodynamic and atmospheric models suggests that alteration of the archean seafloor may have contributed to the regulation of temperature of the paleoatmosphere however additional sources of greenhouse gases are required to solve the faint young sun paradox for the early archean

constraining methane release due to serpentinization by the observed d h ratio on mars it has been suggested that mars atmospheric ch4 could be produced by crustal hydrothermal systems the two most plausible mechanisms proposed so far not exclusive from each other are homogeneous formation by fluid rock interaction during magmatic events and serpentinization of ultramafic rocks the first goal of the present paper is to provide an upper limit on the release rate of serpentinization derived ch4 due to the release of numerous h2 molecules together with one ch4 molecule followed by thermal escape of all released h atoms to space and subsequent h isotopic fractionation even a relatively modest serpentinization derived ch4 release acting over geological time scales may result in a significant enrichment of d wrt h in mars cryo hydrosphere including atmosphere polar caps and subsurface reservoirs by assuming that the ch4 release rate has been proportional to the volcanic extrusion rate during the last 4 billion years we calculate the present d h ratio resulting from the crustal oxidation due to serpentinization including the additional effect of sulfur oxidation we show that this rate doesn t exceed 20 within a factor of 2 of the estimated present value of the ch4 release rate if not the present d h ratio on mars would be larger than observed 5 smow this result suggests that either the production of ch4 is sporadic with a present release rate larger than the average rate or there are other significant sources of ch4 like homogeneous formation from mantle carbon degassing or bacterial activity second assuming further that most of the h isotopic fractionation observed today is due to serpentinization we show that a 400 m thick global equivalent layer of water may have been stored in serpentine since the late noachian this result doesn t depend on the chemical form of the released hydrogen h2 or ch4 such a quantity is generally considered as the amount required for explaining the formation of valley networks on mars serpentinization therefore appears as a potentially efficient sink of water on mars much more efficient than o escape for having removed large amounts of water from the hydrosphere

tectonic evolution of the eastern margin of the tibetan plateau insight from crustal structures using p wave receiver functions abstract the eastern margin of the tibetan plateau is under ongoing expansion rise and growth of the longmenshan are results of the interaction between the songpan ganzi terrane and the sichuan basin in order to understand the block interactions in the eastern plateau margin we studied the crustal structures using p wave receiver functions including h k staking ccp stacking and velocity inversion with the waveforms recorded by 115 portable stations the moho indicated by clear ps phases is generally deeper in the songpan ganzi terrane than in the sichuan basin while a step is notable under the longmenshan fault zone we found a low velocity anomaly in the mid lower crust below the songpan ganzi terrane and longmenshan fault zone which may imply the accumulated partial melting materials in the deep crust under the eastern plateau margin clear p to s converted phases are found within the crust of the sichuan basin they look as apparent huge anticline structures which may be caused by ancient crustal deformation under the stable craton

crustal structure variation along 30 n in the eastern tibetan plateau and its tectonic implications abstract we determined crustal structure along the latitude 30 n through the eastern tibetan plateau using a teleseismic receiver function analysis the data came mostly from seismic stations deployed in eastern tibet and western sichuan region from 2004 to 2006 crustal thickness and vp vs ratio at each station were estimated by the h k stacking method on the profile the mean crustal thickness and vp vs ratio were found to be 62 3 km and 1 74 in the lhasa block 71 2 km and 1 79 near the bangong nujiang suture 66 3 km and 1 80 in the qiangtang block 59 8 km and 1 81 in the songpan garze block and 42 9 km and 1 76 in the yangtze block respectively the estimated crustal thicknesses are consistent with predictions based on the topography and the airy isostasy except near the bangong nujiang suture and in the qiangtang block where the crust is 5 10 km thicker than predicted indicating that the crust may be denser possibly due to mafic underplating we also inverted receiver functions for crustal velocity structure along the profile which reveals a low s wave velocity zone in the lower crust beneath the eastern tibetan plateau although the extent of the low velocity zone varies considerably the low velocity zone together with previous results suggests limited partial melting and localized crustal flow in the lower crust of the eastern tibetan plateau

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

a review on the analysis of the crustal and upper mantle structure using receiver functions abstract the discontinuities in the earth such as moho lithosphere asthenosphere boundary lab 410 and 660 km discontinuities are characterized with an abrupt jump in velocities of p and s waves the depths of these discontinuities are an important parameter to investigate tectonic evolution in the lithosphere receiver functions technique with teleseismic events is very suitable for studying the crust and upper mantle structure beneath stations thus becoming one of the standard tools for such study the principle of receiver functions is to separate the converted ps or sp phases generated at the discontinuities beneath stations in the case that the direct p or s is a delta function in this paper the methods of receiver function analysis are collected from literatures we introduce the coordinate transform technique for the separation of ps or sp waves the deconvolution algorithm to extract p and s receiver functions the waveform fitting method to invert for s wave velocity structure the stacking technique to improve signals and the migration from time series to depth domain with some illustrative examples the care that should be taken in study of the crustal and upper mantle structure using receiver functions are summarized

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

geologic evidence for archean atmospheric and climatic evolution fluctuating levels of co2 ch4 and o2 with an overriding tectonic control the 3 5 3 2 ga sedimentary record shows evidence of surface temperatures of 70 15 c nahcolite nahco 3 as a primary evaporitic mineral and an aggressive weathering regime even in the absence of land vegetation these features are best explained by a mixed ch 4 and co 2 atmospheric greenhouse in which ch 4 co 2 ratios were 1 and pco 2 was at least 100 1000 times the present value and perhaps as high as several bars the formation of large areas of continental crust at 3 2 3 0 ga including the kaapvaal and pilbara cratons resulted in the gradual depletion of atmospheric co 2 through weathering by 2 9 2 7 ga declining pco 2 was associated with climatic cooling and siderite free soils transitory ch 4 co 2 ratios of 1 may have resulted in the sporadic formation of organic haze from atmospheric ch 4 reflected in one or more isotopic excursions involving global deposition of abnormally 1 3 c depleted organic c surface temperatures of 60 c after 2 9 ga may have also increased the distribution and productivity of oxygenic photosynthetic microbes eventual lowering of new continental blocks by erosion reduced loss of atmospheric co 2 due to weathering and continued long term tectonic recycling of co 2 resulted in rising pco 2 and decreasing ch 4 co 2 ratios in the later archean and eventual reestablishment of a mainly co 2 greenhouse similar events may have been repeated in the latest archean and earliest proterozoic but gradually rising production of o 2 effectively kept ch 4 co 2 ratios to 1 at this time

analysis of the crustal thickness and poisson s ratio in eastern tibet from teleseismic receiver functions summary crustal thickness and poisson s ratio are two key parameters for investigating tectonic setting and evolution the h k technique has become popular to determinate their values in recent years however ifacomplexstructureexistsinthecrust thereverberatedphasesfromdifferent depths may interfere with each other resulting in reduction or even absence of the ppps phase from the moho such that the h k algorithm may not reveal unambiguous estimates of these two parameters in this paper the h k technique is applied to process a synthetic receiver function for a test case the result of which is compared with that from direct picking of the time delays of the converted and reverberated phases then we process the observational data recorded at two stations in eastern tibet determine the crustal thickness and the velocity ratio using the two methods compare the result with that obtained by xu et al a t the same location and analyse the potential causes for discrepancy as well as the reliability of the two methods finally the crustal thickness and the poisson s ratio in eastern tibet and sichuan basin are determined from the receiver functions recorded at 51 broad band stations using the two different methods the results indicate that the crustal thickness from the time delays is thicker than that from the h k algorithm near the eastern himalayan syntaxis and the crustal poisson s ratio in the rigid sichuan basin from the time delays ranges from 0 26 to 0 28 which are more reasonable than that from the h k 0 28 to 0 34 additionally the poisson s ratio under the faults from the time delays is found to be higher than those on the two sides which is consistent with the tectonic background and previous study

abiotic methane on earth 1 over the last 30 years geochemical research has demonstrated that abiotic methane ch4 formed by chemical reactions which do not directly involve organic matter occurs on earth in several specific geologic environments it can be produced by either high temperature magmatic processes in volcanic and geothermal areas or via low temperature 100 c gas water rock reactions in continental settings even at shallow depths the isotopic composition of c and h is a first step in distinguishing abiotic from biotic including either microbial or thermogenic ch4 herein we demonstrate that integrated geochemical diagnostic techniques based on molecular composition of associated gases noble gas isotopes mixing models and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic ch4 in natural gases which are frequently mixtures of multiple sources although it has been traditionally assumed that abiotic ch4 is mainly related to mantle derived or magmatic processes a new generation of data is showing that low temperature synthesis related to gas water rock reactions is more common than previously thought this paper reviews the major sources of abiotic ch4 and the primary approaches for differentiating abiotic from biotic ch4 including novel potential tools such as clumped isotope geochemistry a diagnostic approach for differentiation is proposed

crustal structure beneath the eastern margin of the tibetan plateau and its tectonic implications 1 two crustal cross sections through the eastern margin of the tibetan plateau are jointly determined from deep seismic sounding the e w trending line aa passes through the western sichuan plateau including the songpan garze terrane and the longmenshan fault belt and ends in the sichuan basin a part of the yangtze craton line bb has a trend of nne and crosses the songpan garze terrane two dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data our seismic velocity models indicate that the western sichuan plateau and the sichuan basin have crustal thicknesses of 62 and 43 km average crustal p wave velocities of 6 27 and 6 45 km s and lower crustal vp 6 5 km s thicknesses of 27 and 15 km respectively density models constructed from the seismic velocity models are consistent with observed bouguer gravity anomalies we infer that collision between the tibetan plateau and the yangtze craton has caused thickening of the lower crust and uplift of the western sichuan plateau we detect a low velocity layer in the upper crust of the western sichuan plateau but observe no equivalence in the sichuan basin west dipping thrusts may detach into this low velocity layer the seismic phase pmp in the western sichuan plateau has low amplitude suggesting high attenuation in the lower crust qp of 100 300 we suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western sichuan plateau

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

the cenomanian turonian oceanic anoxic event ii palaeoceanographic controls on organic matter production and preservation correlation of the 13c spike with the well dated occurrences of strata rich in organic carbon detailed in schlanger et al this volume indicates that a global episode of intense organic carbon orgc burial took place during the latest cenomanian earliest turonian oceanic anoxic event oae a plenus through i labiatus macrofossil zones and upper r cushmani trz through w archecretacea prz foraminiferal zones over a period of no more than 1 million years m y the shape of the 13c curve indicates that rates of orgc burial gradually increased in the early part of the late cenomanian increased more rapidly in the later cenomanian and levelled off at peak values in latest cenomanian early turonian time during the maximum rate of orgc burial the 13c values decreased nearly to pre late cenomanian levels in the early to middle turonian the decrease in 13c reflects decreasing rates of orgc burial following the cenomanian turonian oceanic anoxic event as well as the probable oxidation and return of significant amounts of orgc to the oceans following regression and re oxygenation of much of the deeper water masses in contact with the seafloor

microbial utilization of abiogenic carbon and hydrogen in a serpentinite hosted system abstract mantle rocks exposed on the seafloor constitute a highly reactive chemical and thermal system in which interaction with seawater to produce serpentinite has major consequences for lithospheric cooling global geochemical cycles and microbial activity serpentinite hosted hydrothermal activity is exemplified by the lost city hydrothermal field 30 n mid atlantic ridge where fluid rock reactions in the underlying ultramafic rocks result in high concentrations of abiotic hydrogen methane c 2 alkanes and formate such systems have been proposed as possible analogs to the early earth environments that gave rise to the first biochemical pathways thus characterizing the local microbial communities and their potential link with abiogenic compounds is of particular significance here we demonstrate that in active carbonate chimneys where microbial sulfate reduction is important up to 50 of the microbial biomass is synthesized from mantle carbon conversely mantle carbon contributes only 10 of the biomass in areas with minimal sulfate reduction we attribute this difference to greater incorporation of formate or methane by the dominant microbial species the lost city methanosarcinales in locations where sulfate reducers are able to facilitate this assimilation the ability of autotrophic communities at lost city to capitalize on the steady stream of chemical products resulting from serpentinization reactions and to utilize abiogenic mantle carbon lend credence to the hypothesis that early biosynthetic pathways could have developed in similar environments

global rates of mantle serpentinization and h2 production at oceanic transform faults in 3 d geodynamic models previous studies have estimated that mantle serpentinization reactions generate h2 at a rate of 1010 1012mol per year along the global mid ocean ridge mor system here we present results of 3 d geodynamic simulations that predict rates of additional mantleserpentinization and h2 production at oceanic transform faults otf we find that the extent and rate of mantle serpentinization increases with otf length and is maximum at intermediate slip rates of 5 to 10 cm per year the additional global otf relatedproduction of h2 is found to be between 6 1 and 10 7 x 1011mol per year which is comparable to the predicted background mor rateof 4 1 15 0 x 1011mol h2 yr this points tooceanic transform faults as potential sites of intense fluid rock interaction where chemosynthetic life could be sustained by serpentinization reactions

reduction of co2 during serpentinization of olivine at 300 c and 500 bar co 2 reduction processes occurring during experimental serpentinization of olivine at 300 c and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas data reveal that conversion of fe ii in olivine to fe iii in magnetite during serpentinization leads to production of h 2 and conversion of dissolved co 2 to reduced c species including methane ethane propane and an amorphous carbonaceous phase hydrocarbon gases generated in the process fit a schulz flory distribution consistent with catalysis by mineral reactants or products magnetite is inferred to be the catalyst for methanization during serpentinization because it has been previously shown to accelerate fischer tropsch synthesis of methane in industrial applications involving mixtures of h 2 and co 2 the carbonaceous phase was predominantly aliphatic but had a significant aromatic component although this phase should ultimately be converted to hydrocarbon gases and graphite if full thermodynamic equilibrium were established its formation in these experiments indicates that the pathway for reduction of co 2 during serpentinization processes is complex and involves a series of metastable intermediates

upper mantle beneath southeast asia from s velocity tomography 1 we present a three dimensional s velocity model of the se asian western pacific upper mantle with 400 km lateral resolution using the novel automated multimode inversion technique we processed 4038 vertical component seismograms and extracted 22 708 linear equations with uncorrelated uncertainties that constrain upper mantle structure we used time frequency windows to select signal with negligible proportion of scattered energy the windows included the fundamental rayleigh mode and s and multiple s waves the observed range of s velocity variations is the widest 17 18 in the upper 150 km of the mantle high velocity continental roots can reach beyond the present extent of the overlying archean proterozoic crust by 500 km beneath some precambrian units the roots are absent which can be attributed to deformation and gradual destruction of the ancient lithosphere at 120 150 km s velocity beneath some cratons reaches 4 8 km s this can be accounted for by thermal and compositional effects beneath the hainan island area a low velocity anomaly is observed from near the surface to the bottom of our model the hot spot type volcanism here may be caused by the deep mantle hainan plume a low velocity mantle domain underlies the south central sea of japan surrounded on the surface by intraplate volcanoes a deep seismicity gap is present near 40 n in the pacific slab subducting below and may result from a plume slab interaction a high velocity anomaly is present in the transition zone beneath the northern boundary of the yangtze craton we propose that the anomaly corresponds to subducted continental lithosphere stagnant atop the 660 km discontinuity

fine crustal and uppermost mantle s wave velocity structure beneath the tengchong volcanic area inferred from receiver function and surface wave dispersion constraints on magma chamber distribution the tengchong volcanic area is located in the southeastern margin of the collision zone between the indian and eurasian plates it is one of the youngest intraplate volcano groups in mainland china imaging the s wave velocity structure of the crustal and uppermost mantle beneath the tengchong volcanic area is an important means of improving our understanding of its volcanic activity and seismicity in this study we analyze teleseismic data from nine broadband seismic stations in the tengchong earthquake monitoring network we then image the crustal and uppermost mantle s wave velocity structure by joint analysis of receiver functions and surface wave dispersion the results reveal widely distributed low velocity zones we find four possible magma chambers in the upper to middle crust and one in the uppermost mantle the chamber in the uppermost mantle locates in the depth range from 55 to 70 km the four magma chambers in the crust occur at different depths ranging from the depth of 7 to 25 km in general they may be the heat sources for the high geothermal activity at the surface based on the fine crustal and uppermost mantle s wave velocity structure we propose a model for the distribution of the magma chambers

a note on the fluxes of abiogenic methane and hydrogen from mid ocean ridges the concentrations of methane and hydrogen in hydrothermal vent fluids issuing from mid ocean ridges tend to fall into two groups one with high concentrations of these gases in ultramafic hosted vent fields and a second group with relatively lower concentrations in basalt hosted vent fluids ultramafic hosted systems however appear to be restricted to slow spreading ridges and constitute only a fraction of the hydrothermal systems found there in this note the hydrothermal fluxes of methane and hydrogen have been calculated by estimating the percentages of the total subsurface hydrothermal circulation that circulate through each type of host rock even though the percentage of the total subsurface flow that is affected by serpentinization appears to be rather small 8 it still appears that this process produces about 70 of the total mid ocean flux of these gases the total production of methane and hydrogen is calculated to be about 20 x 10 9 mol yr 1 and 190 x 10 9 mol yr 1 respectively the hydrogen flux is comparable to that most recently calculated on the basis of the rate of hydration of mantle rock in newly formed crust and the stoichiometry of the serpentinization reaction this suggests that except for the production of methane a major portion of the hydrogen produced in the subsurface is not consumed before venting

comprehensive crustal structure and seismological evidence for lower crustal flow in the southeastern margin of tibet revealed by receiver functions abstract the lower crustal flow model has become an interesting proposal to describe the growth and expansion of eastern tibet to date however the geophysical results are not definitive due to the non uniqueness of the solutions found through data inversion and the resolution limitations in this study the polarity of p to s converted phases at intra crustal interfaces are analyzed in order to reveal evidence for low seismic velocity and lower crustal flow in southeastern tibet we are interested in a joint study that is both a mapping of the crustal structure moho topography and poisson s ratio and an attempt to constrain an iclvz we have determined the crustal thickness and poisson s ratio from p prfs and s srfs receiver functions acquired in 108 permanent seismic stations deployed in the sichuan and yunnan areas also we have obtained 98 measurements of pms phase splitting parameters at the array stations the crustal thickness provided by prfs is consistent with that given by srfs it varies from 60 km in the songpan ganzi sg fold system and the northern part of the sichuan yunnan diamond shaped block sydsb near the eastern himalayan syntaxis to 33 km in southern yunnan the poisson s ratio varies mostly from 0 24 to 0 32 the highest values 0 28 0 32 are found along the axis formed by the longmenshan fault the lijiang jinhe fault and in the tengchong volcano area the value of 0 28 observed in sydsb and the sg fold system is attributed to the presence of a low velocity zone in the lower crust and partly to the great sediment thickness in sichuan basin the crust is clearly anisotropic in the southeastern margin of tibet the fast wave polarization direction exhibits a nw se regional bias while the time delay varies between 0 07 s and 1 27 s being the average 0 54 s 0 12 s implying the existence a strong shear zone in the lower crust based on the negative polarity observed after the direct p wave in teleseismic receiver functions we have detected an intra crustal low velocity zone iclvz in a wide region including east and southeast of tibet and southern yunnan so both the shear zone and iclvz suggest the existence of lower crustal flow the reverberation ppps and psps ppss phases at the moho are clearly weaker in sydsb and the sg fold system than in the indochina block and eastern yunnan implying that sydsb has a structure and tectonic history different from indochina on its west side and eastern yunnan on its east side the above results provide seismological evidence for a lower crustal flow spreading from the eastern tibet and that in its advance southward has invaded sydsb widely crossed the lijiang jinhe fault and reached southern yunnan

seismic upper mantle discontinuities beneath southeast tibet and geodynamic implications abstract in this study a total of 13 080 p receiver functions and 2800 s receiver functions recorded by 116 stations in southeast tibet are used to determine the depths of the moho the lithosphere asthenosphere boundary lab and the 410 km and 660 km discontinuities the results show that the lab depth increases from 80 100 km beneath southern yunnan to 140 180 km beneath sichuan most of the 410 km discontinuity is flat and at 410 km depth with a slight depression of 10 20 km beneath the longmenshan lms fault the central sichuan yunnan sy diamond shaped block and southeastern yunnan for the 660 km discontinuity the greatest depth occurs beneath the lms and southern yunnan reaching 670 680 km the deepening of 660 km discontinuity is attributed to compositional heterogeneity within the mtz or dynamic pressure on the discontinuity additionally the velocity of movement in the crust and upper mantle beneath sy is different in yunnan crustal movement is southeastward whereas across the red river fault part of the crustal motion is redirected southwest by the westward rollback of the mantle beneath myanmar the interaction between the asthenosphere moving northeastward from myanmar and southeastward from tibet near latitude 26 n causes the crust and lithospheric mantle beneath southern yunnan to have different movement directions

estimating the viscosity and prandtl number of the tso morari crystalline gneiss dome indian western himalaya the tso morari crystalline tmc gneiss dome in the indian himalaya extruded from a depth of 120 km through an inclined subduction channel of sub elliptical cross section at the leading edge of the indian plate the velocity profile of this gneiss dome is derived after 1 presuming its incompressible newtonian rheology 2 finding the best fit of the outcrop of the gneiss dome to an ellipse 3 taking into account different lithologies to have existed at the top of the extruding gneiss body 4 considering the extrusion to have been driven by the buoyant push of the denser mantle beneath the lighter gneiss and 5 assigning a range of plausible densities for different litho units fitting the known rates of extrusion from a few centimetres up to about one hundredth of a millimetre per year from 53 ma onwards of this gneiss dome to its velocity profile constrains its maximum possible viscosity to 7 5 1022 pa s this magnitude is 102 104 times higher than previous estimates for gneisses and granites alternative explanations of our data are the following 1 there was a fall in extrusion rates of the tmc gneiss from 53 to 30 ma because of an increase in the estimated maximum viscosity from 6 2 1020 to 7 5 1022 pa s possibly indicating a fall in temperature and or compositional change of the tmc gneiss 2 lower the extrusion rates higher are the estimated viscosities 3 the tmc gneiss was more viscous probably due to its eclogite content 4 the estimated maximum viscosity is 102 times higher than that in collision zones and 102 104 times than that in the tibetan lower crust but broadly conforms to that for the crustal channel and average lithospheric and asthenospheric values the high magnitude of maximum possible prandtl number of 1028 of the tmc gneiss might be related to isothermal decompression of the gneiss during its extrusion

moho topography of the tibetan plateau using multi scale gravity analysis and its tectonic implications abstract determining the moho topography of the tibetan plateau is crucial to understand the tectonic development over the past decades seismic explorations have obtained profound results about the moho topography except in regions where seismic station coverage is poor especially in the central and western tibetan plateau in comparison gravity data have the advantage of global homogeneous coverage which can thus be used to determine the moho structure beneath the entire tibetan plateau in this paper a novel approach the multi scale gravity analysis method is developed to extract the gravity signals originated from the moho undulations and to determine the moho topography beneath the whole tibetan plateau the inverted moho topography for the tibetan plateau is consistent with that derived from the previous works in addition a rich geophysical structure and tectonic development can be revealed from the inverted moho topography 1 the moho depth in the west is deeper than that in the east and the deepest moho which is approximately 77 km is located beneath the western qiangtang block 2 there is an obvious moho offset of approximately 5 km beneath the yarlung zangbo suture at the juncture between the himalayan and lhasa blocks 3 the moho fold and low density channel flow the directions of which are in agreement with the results of surface movement velocities estimated from global positioning system can be observed from the moho topography

methane and climate during the precambrian era abstract the sun was substantially less bright in the distant past yet earth s surface temperature remained above freezing higher concentrations of the greenhouse gases co 2 and ch 4 were likely responsible for keeping the early climate warm ch 4 concentrations of 1000 ppm or higher are predicted for the late archean paleoproterozoic atmosphere prior to the rise of o 2 photolysis of this ch 4 may have created an optically thin organic haze during much of this time the rise of o 2 at 2 3 ga eliminated most of the methane and probably triggered the paleoproterozoic glaciations ch 4 concentrations could have remained elevated throughout much of the proterozoic however as a consequence of low concentrations of dissolved o 2 and sulfate in the deep oceans and a corresponding increase in organic matter recycling by fermentation and methanogenesis

upper cretaceous muti formation transition of a mesozoic nate platform to a foreland basin in the oman mountains the upper cretaceous turonian campanian muti formation sayja member documents the transition from a passive continental margin to a foreland basin related to overthrusting of continental margin and ophiolitic nappes derived from the tethys ocean upper cretaceous northeastward subduction culminated in collision of a trench with the arabian margin as the trench docked with the margin the lithosphere was flexed forming a peripheral bulge that migrated cratonward with time the platform edge was initially uplifted turonian and deeply eroded creating the wasia aruma break after passage of the peripheral bulge subsidence began with accumulation first of ferruginous crusts on hardgrounds lime muds were then deposited on a deepening unstable sea floor along with phosphatic nodules and crusts turonian coniacian passage of the overthrust load over the arabian continental margin edge downflexed the lithosphere santonian campanian resulting in drastic foundering of the old shelf edge to form a foredeep upper platform horizons collapsed as slump sheets and debris flows limestone blocks and lithoclastic debris flows were shed by mass wasting of the already deeply eroded old platform edge mud and silt were derived from the uplifted arabian continent and deposited by mainly gravitational processes in a foredeep below the c c d subsidence of the arabian platform edge allowed the semail ophiolite nappe finally to override the muti basin late campanian with little internal deformation submarine emplacement is suggested by the absence of ophiolitic detritus in the muti formation the stratigraphic evolution of the muti formation is in good general agreement with a model of the transition of an old thermally mature passive continental margin to a foreland basin where the emplaced load is submerged

causes and consequences of the relation between area and age of the ocean floor the distribution of area of the ocean floor with age t is approximately described by da dt c0 1 t tm where c0 is the rate of crustal generation and tm the maximum age a linear differential area versus age relation can be explained by a balance between crustal generation and consumption where consumption is uniformly distributed with age the present distribution of consumption with age was estimated from the isochron map used to derive the area age relation and a recently published set of angular velocity vectors describing present plate motions the rate of consumption per unit age shows considerable scatter about a mean value equal to c0 tm 1 7 10 8 km2 yr2 but consumption is sufficiently close to being uniformly distributed with age to explain the observed degree of approximation to a linear differential area age relation the trenches appear to be distributed randomly with respect to age provinces in the oceans it was not possible to discern any trend of mean subduction velocities with age apart from the smaller velocities observed for the two youngest age provinces 0 9 ma changes in the rate of plate generation and the distribution of consumption with age result in shifts in the area age distribution in turn these shifts produce changes in the plate driving forces which act to restore the rate of plate generation and distribution of consumption to their initial states the ridge pushing force for example decreases as the rate of crustal generation increases this coupling between driving forces and the area age distribution provides a feedback mechanism limiting the extent of any changes a measure of the magnitude of shifts in the area age distribution is given by global changes in sea level if long term changes in sea level are less than 150 m as suggested by recent estimates then only small changes 12 in the rate of plate generation and distribution of consumption with age are required sea level changes as large as 300 m require about a 50 change in the rate of crustal generation the above area age relation can be combined with simple expressions for depth and heat flow versus age to obtain an empirical hypsometric distribution parameterized in terms of age and exact expressions for the heat loss from the ocean floor the latter is estimated to be 2 9 1013 w with 25 of this heat being lost by seafloor younger than 5 ma of the total heat loss from the mantle 26 is estimated to be transported by small scale convection

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

abiotic hydrogen h2 sources and sinks near the mid ocean ridge mor with implications for the subseafloor biosphere free hydrogen h2 is a basal energy source underlying chemosynthetic activity within igneous ocean crust in an attempt to systematically account for all h2 within young oceanic lithosphere

late cenozoic to holocene deformation in southwestern sichuan and adjacent yunnan china and its role in formation of the southeastern part of the tibetan plateau from at least 2 4 ma to present crust in the southeastern part of the tibetan plateau west of the convex east xianshuihe xiaojiang fault system has deformed internally and rotated clockwise around the eastern himalayan syntaxis the northwest striking ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78 100 km of which 60 km is transferred to the xianshuihe fault zone across a diffuse transfer zone and 22 40 km is absorbed by bending of older structures and crustal shortening crustal shortening is expressed along and east of the eastern end of the ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the tibetan plateau a similar transfer of left slip into shortening occurs farther south across the xianshuihe fault in the high mountains around and east of gongga shan 7556 m the northwest striking convex east left lateral litang fault zone lies southwest of the ganzi xianshuihe xiaojiang fault zone and appears to be less well developed but otherwise similar to the ganzi fault zone the batang chenzhi and other northeast striking right lateral faults of small displacement occur within the rotating crustal fragment together with the left slip faults they accommodate east west shortening northeast of the eastern himalayan syntaxis south of this region of shortening the crust is extending to form grabens within the dali and southern xiaojiang fault systems and in the tengchong volcanic province the progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern himalayan syntaxis the assemblage of structures in southwestern sichuan geometrically resembles structures of eocene to miocene age in southern yunnan that were positioned northeast of the eastern himalayan syntaxis similar to present day southwestern sichuan at the time of their development the similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past 50 m y structures in sichuan are less fully developed than older structures in southwestern yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis deformation in these areas indicates that plateau formation has been complex inhomogeneous and diachronous at scales from 1000 km to less than 100 km

sequence architecture and carbonate platform configuration late cenomanian santonian sinai egypt relative sea level changes on the mixed carbonate siliciclastic platform of sinai are manifested in shifts of distinct facies belts deep water facies highenergy subtidal shallow subtidal lagoon shallow shoreface siliciclastics supratidal and are interpreted in terms of sequence stratigraphy eight sedimentary sequences are recognized for the upper cenomanian to santonian their correlation along a north south transect reveals distinct changes in lithofacies and progradation retrogradation patterns within the individual systems tracts the number and stratigraphy of the sequence boundaries of sinai correlate well with those from adjacent areas patterns of increased subsidence are documented for the central sinai basin since the late cenomanian by increased thickness of the stratal packages post cesin 7 hst post tusin 1 lst and hst post tusin 2 lst and are balanced by varying accumulation rates based on new sedimentological and biostratigraphic data large scale palaeogeographic maps and cross sections show the 1 temporal and spatial evolution of the central sinai basin e g its latest cenomanian initial formation lower turonian deep water facies middle turonian to coniacian synsedimentary subsidence 2 drowning of the cenomanian platform coinciding with the latest cenomanian to early turonian relative sea level rise 3 re establishment of the platform in middle late turonian times and 4 a coniacian basin and swell morphology

gaia s breath global methane exhalations abstract methane ch 4 is the most abundant organic compound in the earth s atmosphere where it acts as a greenhouse gas and thus has implications for global climate change the current atmospheric ch 4 budget however does not take into account geologically sourced ch 4 seepage geological sources of ch 4 include natural macro and micro seeps mud volcanoes and other miscellaneous sources such as gas hydrates magmatic volcanoes geothermal regions and mid ocean ridges macro seeps contribute 25 tg teragrams ch 4 yr to the atmosphere whereas micro seepage contributes perhaps 7 tg ch 4 yr mud volcanoes emit 5 tg ch 4 yr and miscellaneous sources emit 8 tg ch 4 yr to the atmosphere thus the total contribution to the atmosphere from geological sources is estimated to be 45 tg ch 4 yr which is significant to the atmospheric organic carbon cycle and should be included in any global inventory of atmospheric ch 4 we argue that the atmospheric ch 4 global inventory of the interplanetary panel on climate change must be adjusted in order to incorporate geologically sourced ch 4 from naturally occurring seepage

implications of present day abiogenic methane fluxes for the early archean atmosphere 1 during earth s early history greenhouse warming by atmospheric methane helped to maintain elevated surface temperatures here we estimate the present day abiogenic ch4 flux generated by mineral alteration serpentinization at mid ocean ridges volcanic emissions and geothermal sources in addition we assess the impact that abiogenic methane may have had on greenhouse warming during the early prebiotic archean based on estimates of the rate of seafloor spreading and the degree of serpentinization within the oceanic crust the flux of methane generated by serpentinized lithosphere is calculated to be 1 35 mt ch4 y 1 while volcanic and geothermal sources are estimated to contribute 0 1 and 0 9 mt ch4 y 1 respectively furthermore it is shown that if atmospheric co2 partial pressures were above 0 01 bar the present day level of abiogenic methane production could have been sufficient to maintain above freezing surface temperatures during the archean the very high temperatures 70 c that have been suggested for the early archean however would have required extremely high methane fluxes or more likely greatly elevated atmospheric co2 levels