isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

continental subductions beneath tibet and the himalayan orogeny a review argand 1924 and later holmes 1965 proposed that the himalayan orogeny and tibet uplift were the result of large underthrusting of the indian continental crust beneath tibet since then plate motion studies have yielded new constraints on the relative displacements of india and asia and a large number of geological and geophysical investigations have helped to determine the structure of the himalayas and tibet the best natural laboratory for understanding mountain belts the aim of this paper is to review briefly the various models of himalayan mountain building in light of recent geological and geophysical data and to estimate the role of continental subductions

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

early himalayan exhumation isotopic constraints from the indian foreland basin nd and sr isotopic compositions of palaeogene foreland basin sediments are used to provide insights into early himalayan evolution particularly the timing of exposure of high 87sr 86sr units erosion of which may have caused the late tertiary increase in oceanic sr isotopic ratios during the late palaeocene early eocene erosion was from mixed sources including suture zone rocks exhumation of the high himalaya was occurring by the time of deposition of alluvial sediments after mid oligocene times and this source has dominated himalayan sediments from at least this time until the present day the transition is interpreted to reflect exhumation of basement rocks of the indian plate when the high himalaya became a sufficient topographic barrier to separate suture zone rocks from the foreland basin the marked rise in seawater 87sr 86sr from 40 ma is consistent with the erosion of a himalayan source with a high 87sr 86sr ratio

stratigraphic and sedimentological constraints on the age and tectonic evolution of the neotethyan ophiolites along the yarlung tsangpo suture zone tibet

the yarlung zangbo paleo ophiolite southern tibet implications for the dynamic evolution of the yarlung zangbo suture zone the yarlung zangbo suture zone a major geological structure in tibet is well known as the locus of tectonic emplacement of the tethyan ophiolites current models propose that most of the east tethyan oceanic lithosphere was subducted within a single subduction zone active during the middle or late cretaceous which was completed during the paleogene collision between india and asia the early cretaceous sedimentary giabulin formation in southern tibet includes conglomeratic members that contain ultramafic and mafic plutonic pebbles as well as radiolarian chert clasts that record the erosion of oceanic lithosphere involved in a subduction event which occurred earlier than previously believed geochemical analyses mineral chemistry stratigraphic chronology and sedimentary analysis including source provenance suggest that the pebbly conglomerate was formed through erosion of an unknown ophiolitic source that was geochemically distinguishable from the xigaze ophiolites within the yarlung zangbo suture zone southern tibet we infer the existence of an older ophiolitic source termed the yarlung zangbo paleo ophiolite that was dismembered and eroded during an earlier subduction stage not taken into account in current models q 2000 elsevier science ltd all rights reserved

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

evolution of the himalaya abstract the compression and attendant deformation of a thick and vast sedimentary prism formed since early riphean times on the northern continental margin of the indian craton gave rise to the himalaya mountains as a result of convergence and collision of the indian and asian plates the oceanic trench sediments tectonically implanted with sea floor material and intimately associated with calc al kaline volcanics in the narrow sindhu tsangpo belt extending from kohistan through dras leh darchen mansarovar to shigatse and beyond represent the subduction island arc complex which developed south of the dynamic southern margin of the asian continent and was welded to the colliding indian plate during the late eocene to oligocene period this complex is fringed to the north by a wide zone of andean type granitic bodies the evolution of the himalayan orogen is closely connected with the development of the present day andaman nicobar indonesia island arc subduction system in the southeast and the makran ranges oman trench in the southwest the evolution of the himalaya was accomplished in four major phases of tectonic upheaval during the late cretaceous to palaeocene karakoram phase late eocene to oligocene malla johar phase middle miocene to pontian sirmurian phase and late pliocene to middle pleistocene siwalik phase while the karakoram phase marks the convergence of continents and the malla johar phase represents the collision and subduction it was during the sirmurian upheaval that the main tectonic features developed and the himalaya acquired its distinctive structural complexion

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

on the observed relationships between wintertime variability in kuroshio oyashio extension sea surface temperatures and the atmospheric circulation over the north pacific abstractobservational analyses reveal that wintertime variations in sea surface temperatures sst in the kuroshio oyashio extension koe region of the north pacific are associated with two distinct and robust patterns of atmospheric variability 1 a pattern that peaks in amplitude approximately 2 3 weeks prior to large koe sst anomalies and is consistent with atmospheric forcing of the sst field and 2 a very different pattern that lags sst anomalies in the koe region by approximately a month the latter pattern is dominated by low sea level pressure anomalies and turbulent heat fluxes directed into the atmosphere over warm sst anomalies and is interpreted as the transient atmospheric response to sst anomalies over the koe region the results contribute to a growing body of evidence that suggests variations in ssts in the midlatitude oceans are capable of significantly influencing the large scale atmospheric circulation especially near western boundary currents

precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the dazhuqu terrane yarlung tsangpo suture zone tibet well preserved abundant radiolarians provide high precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the yarlung tsangpo suture zone in southern tibet dazhuqu terrane ophiolites were generated in an intra oceanic supra subduction zone setting within a relatively short

the first stage in the formation of the uranium deposit of arlit niger role of a new non continental organic matter abstract the uranium deposits of arlit are located in fluvial sandstone rich in organic matter of continental origin type iii deposited in palaeochannels previous studies on arlit deposits documented that organic matter of continental origin type iii was the main contributor to the reduction of uranium from the oxidation state of uranium vi u vi to uranium iv u iv this study shows that migrated oils of non continental origin strongly contributed to the genesis of uranium mineralization the petrographic analyses showed that i solid bitumen resulting from the polymerization of migrated oils is present in abundance in the porosity of the sandstone and fractures at the level of the channels of the tarat formation tamou takriza southern taza and guezouman formation mine sector of akouta and ii uranium mineralization is mainly associated with this migrated organic matter currently expressed in the form of solid bitumen the association occurs at an infra micrometric scale in the solid bitumen the geochemical analysis rock eval py gc ms and gc ms and the analysis of different indicators and biomarkers n alkanes pr ph steranes indicate the presence of a second organic matter with a different signature to type iii the oils present in the porosity of the sandstone of the guezouman and tarat formations confirm that channels of these two formations are the traps of the oil migration

role of organic matter in the proterozoic oklo natural fission reactors gabon africa of the sixteen known oklo and the bangombe natural fission reactors hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals reactors 1 to 6 at oklo contain only traces of organic matter but the others are rich in organic substances reactors 7 to 9 are the subjects of this study these organic rich reactors may serve as time tested analogues for anthropogenic nuclear waste containment strategies organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality the bitumen soon became a solid consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite which enclosed and immobilized uraninite and the fission generated isotopes entrapped in uraninite this mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1 2 b y 24 refs 4 figs

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

paleoproterozoic high δ13c dolomites from the lastoursville and franceville basins se gabon stratigraphic and synsedimentary subsidence implications abstract new data for the isotopic composition of carbon in shallow marine sedimentary carbonates in southeastern gabon franceville and lastoursville sub basins indicate that the seawater value of 13 c in these sediments underwent a large positive excursion around 2 1 ga these carbonate rocks belong to the fc formation of the francevillian group deposited along the margin of the archean chaillu craton in open marine shelf and evaporite settings and experienced locally lower greenschist facies metamorphism four microfacies mf1 mf4 are recognized from subtidal cyanobacterial dolobindstones to inter and supratidal collapse breccias mf4 the dolomites exhibit elevated 13 c values most values are above 0 50 the highest value is 9 74 and the average value is near 4 vs vpdb consistent with deposition during the large worldwide positive excursion between 2 22 and 2 06 ga these positive 13 c values are independent of the microfacies type such high 13 c values are also recorded in reworked carbonates dolomites embedded in coarse grained sandstones and conglomerates interstratified in the black shales of the underlying fb formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits interaction between early structural development and sediment fill lead to the formation of several sub basins with distinct differences in terms of amount and timing of subsidence the 13 c enriched dolomites are probably linked to high burial rate of organic carbon starting with the deposition of the fb sediments in this extensional geotectonic context as a consequence the cherty and dolomitic fc formation cannot longer be considered as a marker interval the end of the positive excursion is located near the top of the francevillian series i e fe and could be used in future studies as a potential chemostratigraphic tool in the paleoproterozoic

field evidence for active normal faulting in tibet crustal thickening does not now occur in southern tibet field observations made during the chinese french expedition of 1980 support an earlier hypothesis based on a combined analysis of satellite images and fault plane solutions of earthquakes immediately north of the highest peaks of the himalayas the tectonic regime is dominated by east west extension we have mapped a large number of north south normal faults which sharply cut the glacial and post glacial morphology and followed several recent earthquake breaks along faults detected on satellite and aerial photographs microtectonic measurements in the quaternary suggest that the maximum principal stress is vertical

the origin distribution and hydrocarbon generation potential of organic rich facies in the nonesuch formation central north american rift system a regional study abstract precambrian unmetamorphosed organic rich clastic rocks whilst uncommon in the geologic record are of interest with respect to their potential for hydrocarbon generation the 1 1 billion year old nonesuch formation is primarily a lacustrine deposit which is part of a thick sequence of volcanic and clastic sedimentary rocks that filled an aborted rift in central north america previous organic geochemical investigations of the nonesuch formation have been limited to isolated samples from the basal portion of the nonesuch formation at a copper mine white pine in northern michigan in the present study 183 outcrop and core samples of the nonesuch formation from northern wisconsin and michigan upper peninsula were collected for sedimentologic and organic geochemical analyses total organic carbon toc values for the samples ranged from 0 0 to 2 5 and showed a strong correlation between organic richness and depositional environment detailed petrographic analysis using incident white light and reflected blue light fluorescence revealed two major organic petrographies 1 and 2 which could be further distinguished using combined pyrolysis gas chromatography mass spectrometry py gc ms whereas the composition of organic petrography 1 kerogens is primarily aliphatic the composition of organic petrography 2 kerogens is dominated by aromatic and or phenolic constituents the geographic and stratigraphic distribution of rocks bearing the two organic petrographies indicate that variable source input or preservation superimposed over maturity differences is responsible for both organic petrographic and kerogen pyrolysate compositions

sedimentary fractionations between al ti and zr and the genesis of strongly peraluminous granites sedimentary sorting produces alternating shales and sandstones with complementary chemistries the preferential partitioning of quartz and zircon in the coarse grained fraction of the sediments results in a strong fractionation between sio 2 zr and other components al 2 o 3 tio 2 as a consequence shales may be defined chemically as having higher al 2 o 3 sio 2 and tio 2 zr ratios than their source rocks sandstones have the opposite characteristics and vary in composition according to the maturity of their source and the intensity of sorting processes most sedimentary suites display a good correlation between al 2 o 3 sio 2 and tio 2 zr this result is used to evaluate an average bulk composition for recent clastic materials and to model their composition on a ternary al ti zr diagram because the al 2 o 3 tio 2 ratio of igneous rocks is highly sensitive to magmatic differentiation igneous and sedimentary trends crosscut in the al ti zr di

saturated hydrocarbon biomarkers in oils of late precambrian age from eastern siberia abstract large quantities of petroleum derived from source rocks of late precambrian early cambrian age have been discovered in the lena tunguska region of eastern siberia over the last twenty five years we have examined three oils with presumed late precambrian vendian source rocks from this region for c 15 saturated hydrocarbon biomarkers using gas chromatography gc and gas chromatography mass spectrometry gc ms our results show these oils to have many characteristics in common with other oils of similar age from eastern siberia and oman amongst the compounds detected in the three oils analyzed were 1 n alkanes which show a marine derived distribution 2 centrally branched monomethyl alkanes 3 acyclic isoprenoids pr ph 1 4 tricyclic and tetracyclic terpanes 5 hopanes 6 c 29 unrearranged steranes and 7 c 30 nuclear steranes all of these compounds with the exception of the 4 methylsteranes which on current biochemical knowledge are unexpected in oils of this age can be assigned a prokaryotic source this and other evidence such as the lack of diasteranes suggest microorganism were the major contributors to organic matter deposited as part of a carbonate source rock

precambrian oil inclusions in late veins and the role of hydrocarbons in copper mineralization at white pine michigan liquid oil was trapped as primary fluid inclusions in calcite crystals in late cu fe sulfide bearing veins that crosscut and offset the cupriferous shale deposit at white pine the age of the calcite 1047 35 ma is an entrapment age and thus a minimum age for the oil close temporal and spatial associations of oil and metallic sulfides in the late veins suggest that liquid and solid hydrocarbons may have been dominant controls of the main stage white pine copper mineralization

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

natural fission reactors of oklo fourteen natural fission reactors are known to date in three different ore deposits located in the franceville basin the most famous and the first discovered are those in the oklo uranium ore deposit petrographic geochemical and oxygen isotope studies have been performed in an attempt to answer these questions 1 what are the main characteristics of the uranium ores in which fission reactions took place 2 what are the processes that created such exceptionally high grade ores and 3 what are the effects of such fission reactions on the host rocks the ore which hosts the fission reactions at oklo is characterized by a gangue of clay minerals whereas the normal ore of oklo consists of sandstones these clay minerals are the result of hydrothermal alteration of sandstone hydrothermal circulation was induced by the heat produced by fission reactions the reactor ores are further characterized by very high uranium contents 20 60 and high uranium accumulation by means of silica depletion and by uranium enrichment of the normal ore without uranium depletion hydrothermal convective circulation induced by the fission reactions created very high grade ore next to the reactors hydrothermal processes are thus critical to the propagation of the fission reactions

lead isotopic study of the xigaze ophiolite tibet the problem of the relationship between magmatites gabbros dolerites lavas and tectonites harzburgites abstract the pb isotopic study of the xigaze ophiolite tibet shows that the tectonic harzburgites and magmatic gabbros dolerites lavas units bear different isotopic signatures the harzburgitic rocks show much higher207pb 204pb values than the samples from the magmatic unit analyses of primary magmatic clinopyroxenes separated from the harzburgites confirmed that this difference is real and not due to secondary alteration processes u pb analysis allowed determination of the crystallization age of the magmatites at 120 10 m y this age is compatible with paleontological age determinations and thus demonstrates that u pb behaved more or less as a closed system since formation of the ophiolite if an exchange with seawater has occurred it must necessarily have happened shortly after crystallization of the xigaze oceanic crust dolerite dykes crossing the harzburgites show similar pb isotopic composition as the magmatites indicating that they were emplaced when the ultramafic rocks were already brittle however the time difference of the emplacement was probably not larger than 280 m y this makes it evident that two sources with different isotopic signatures one similar to normal morb the other with much higher207pb 204pb ratios were active in the immediate neighborhood one possible environment which can account for this association is a propagating ridge

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

stratigraphic evidence for an early collision between northwest india and asia the collision of india with asia1 had a profound influence on late cretaceous and cenozoic oceanography2 climate3 fauna extinctions4 and the motion of at least some of the earth s litho spheric plates5 as the collision ended a period of rapid indo asian convergence6 a precise knowledge of its timing when the crust of the neo tethys ocean was completely subducted7 at some point along the plate boundary is important for understanding its wider consequences but current estimates of the collision age range from 65 to 38 myr before present6 8 11 here we report the results of extensive biostratigraphic analyses from waziristan and kurram in northwest pakistan which show that accretionary prism and trench strata were first thrust onto the northwest indian passive margin after 66 myr but before 55 5 myr after this time volcanic arc fragments the accretionary prism trench material and imbricates of the north indian slope were raised to shallow water depths and overlapped by upper palaeocene shallow water carbonates and shales12 14 indicative of post collision thrusting in this region finally both the suture and the indian craton were overlapped by continuous unconformable upper lower eocene shallow marine strata demonstrating that suturing was complete by 49 myr

oil and its biomarkers associated with the palaeoproterozoic oklo natural fission reactors gabon abstract the 2 1 ga fa formation sandstone of the franceville basin in gabon that hosts the oklo natural fission reactors has been discovered to contain abundant palaeoproterozoic oil bearing fluid inclusions inside which syngenetic biomarkers are preserved the oil occurs within h2o and co2 dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and co2 with liquid hydrocarbons prior to trapping textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca 115 to 200 c soon after deposition of the sequence at ca 2 1 ga and secondly during a hydrothermal event at 200 to 285 c linked to the operation of the reactors ca 1 98 ga a series of sequential outside rinse blanks of framework grains analysed by gas chromatography mass spectrometry gc ms prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co occurring solid bitumens which have a different geochemical distribution compared to the fluid inclusion oil the crush leach extract containing oil from fluid inclusions is markedly richer than the final outside rinse blank displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen the fluid inclusion oil has a high maturity is non biodegraded and lacks clear effects of radiolysis it is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor heated fluids rather than by the criticality event itself biomarkers include hopanes 2 methylhopanes terpanes and steranes indicative of bacteria including cyanobacteria and of eukaryotes the relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution trace amounts of c30n propylcholestanes indicate a contribution from marine algae the oil was likely derived from conformably overlying marine black shales that were deposited during the great oxidation event during which the earth s atmosphere became oxygenated the biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2 7 and 1 4 ga it is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period despite apparent fluctuations of atmospheric o2

trace elements as source indicators in cratonic sediments a case study from the early proterozoic libby creek group southeastern wyoming to understand how grain size and sorting effect trace element distributions in sediments we compare the compositions of closely associated quartzites pelites and glacial diamictites from the 2 ga libby creek group in se wyoming we found similar ree distributions lree enriched negative eu anomalies and that most compositional differences can be explained by variable quartz dilution trace element distributions from mineral separates in a fuchsite quartzite show that fuchsite contains most of the th u ree sc co and ni and controls the whole rock ree distributions zircon contains most of the zr and hf but 5 of the hree in the pelites and 20 in the quartzites other than zr and hf most trace elements in libby creek sediments are chiefly in micas and were probably in clays in the original sediments increased eu anomalies in upper libby creek sediments signal a sudden increase in eu depleted granitoids in sediment sources which reflects late tectonic uplift mixing models favor granite ba

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

petrography and paragenesis of organic matter associated with the natural fission reactors at oklo republic of gabon a preliminary report sixteen known uranium rich pockets in the sediment hosted uranium ore deposits in the oklo area became nuclear fission reactors 1968 50 ma ago and operated as such for up to 1 ma nuclear criticality was caused by unique and fortuitous geological environments and events these included the localized high concentrations of uranium in small pockets in the ore bodies the fact that the relative abundance of the fissile 235u isotope was five times greater in uranium 2 ga ago than present the presence of water which acted as a moderator in the natural reactors and the absence of neutron poisons which are elements that can prevent nuclear chain reactions organic matter is present in all of the oklo natural reactors but it is abundant only in reactors 7 to 16 organic matter has been studied in natural reactors 7 to 9 and at locations at various distances from these reactors bitumen in the oklo reactors is the predominant organic phase it is now solid bitumen containing dispersed cryptocrystalline graphite in crystal domains smaller than optical microscopic size its immediate precursor was liquid bitumen generated from syngenetic kerogen in organic rich sedimentary rocks in the francevillian basin of gabon and in the natural reactors themselves by hydrous reaction mechanisms prior to during and after nuclear criticality a preliminary paragenesis of organic matter at oklo may be defined through the following sequence of events a protokerogen and kerogen evolve from abundant cyanobacteria in the tidal and deltaic sediments in the basal fa formation of the francevillian series during sedimentation and burial respectively b during subsequent subsidence of the basin the first generation of bitumen and petroleum occurred c during uplift the sedimentary strata were fractured and bitumen and petroleum migrated through fractures following this oxidizing aqueous solutions carrying uranyl ions migrated through the highly fractured rocks met and were reduced by organic matter and precipitated pitchblende or uraninite at the present sites of the oklo uranium ore deposits and natural reactors the natural reactors reached nuclear criticality they were heated and produced the second generation of bitumen through hydrous reactions from the by now solid first generation bitumen and remaining kerogen finally hydrothermal effects associated with the dolerite dike swarm intrusion 750 150 ma ago produced yet another localized generation of liquid bitumen from solid organic matter possibly other bitumen generations also occurred during and following this time interval the organic matter in the natural reactors soon became a solid and was able to prevent the loss of uraninite grains enclosed in it these uraninite grains trapped and then immobilized fission products until and to an extent even after a major tectonic igneous event the intrusion of the dolerite dike swarm

the indigenous origin of witwatersrand carbon in the witwatersrand approximately 40 of the gold is intimately associated with so called carbon in carbon seam reefs which occur in over a dozen paleoplacers many of them concentrated at two stratigraphic levels in the 7000 m thick succession of archean siliciclastic sedimentary rocks this is reduced carbon present as kerogen admixed in various proportions with derivative now solid bitumen s oil generation and migration were active geological processes during early earth history numerous possible source rocks for oil generation including the carbon seams themselves occur within the witwatersrand basin in the witwatersrand ore oil bearing fluid inclusions are also present derived like the bitumen by thermal maturation of the kerogen the presence of kerogen and bitumen in the witwatersrand sedimentary rocks together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested paleochannels which truncated auriferous carbon seams early in the depositional history are of widespread occurrence and micro synsedimentary faults offset carbon seams the carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development the various features highlighting the nature and spatial distribution of witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

paleomagnetism of the xigaze ophiolite and flysch yarlung zangbo suture zone southern tibet latitude and direction of spreading abstract the xigaze ophiolite 29 2 n 89 5 e which outcrops in the yarlung zangbo suture zone represents the remnants of an oceanic lithosphere formed in middle cretaceous times between the lhasa block to the north and the indian plate to the south in an attempt to define the paleo orientation and latitude of the spreading center at which it has been created a paleomagnetic study has been done on three sites in volcanics and overlying or interbedded radiolarites forming the upper part of the ophiolite sequence and also on seven sites in the xigaze group flysch which stratigraphically overlies the volcanics to the north in each site hand blocks carefully oriented both with sun and magnetic compass have been sampled the paleomagnetism data combined with structural data on the ophiolite dolerite intrusives allow a partial reconstruction of south eurasia at the time of formation of the xigaze ophiolite the paleolatitude of accretion and deposition of the xigaze ophiolite and overlying sediments is found to be 10 20 n both ophiolite and basin have encountered a 85 20 anti clockwise rotation the corresponding ridge was close to the southern margin of the lhasa block and was oriented n175 25

geochemistry and provenance of metasedimentary rocks from the archean golden pond sequence casa berardi mining district abitibi subprovince the archean golden pond sequence is made up of deformed and metamorphosed conglomerates greywackes and mafic volcanic rocks and is overlain by ferrugineous metasedimentary rocks of the north iron formation the clastic rocks were derived mainly from a volcanic source that had undergone weak chemical weathering their source area was dominated by the presence of 60 80 high al2o3 felsic volcanics having strongly fractionated la sm n 3 7 0 3 and very low ta th ratios 0 09 0 02 with lesser proportions of basaltic 10 30 and ultramafic volcanic rocks 1 10 the ferrugineous metasedimentary rocks can be modelled by mixing 20 40 siliciclastic material of the composition of the average golden pond greywacke with an fe and si rich precipitate molecular fe si 0 6 0 2 the high al2o3 felsic source rocks were most likely produced by subduction processes within an oceanic arc environment but the mafic and ultramafic volcanic rocks were derived by different processes from an asthenosphe

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

geochemistry of sedimentary rocks from melange and flysch units south of the yarlung zangbo suture zone southern tibet abstract the mesozoic yamdrock melange and the triassic flysch lie immediately south of the yarlung zangbo suture zone yzsz ophiolitic belt in southern tibet the whole rock chemical signature of sandstones mostly greywackes red shales and black shales of the yamdrock melange and flysch units do not present significant variations between the different rock types and geological units and is concordant with a continental passive margin source despite fairly important chemical weathering the geochemical signature was not affected by significant sedimentary recycling nor significant heavy mineral accumulation ree patterns show an enrichment in lree typical of shales and indicate that the neo tethyan turbidites mostly derive from a felsic old upper continental crust source slight enrichments in ti and sc indicate a mafic contribution that could originate from mafic blocks of enriched intraplate geochemical affinity found in the sedimentary units sedimentary rocks from both units were thus derived from the old indian continental crust including granitic and intraplate mafic blocks but were deposited in different tectonic settings in the neo tethys ocean the homogeneity of the yzsz data which cover the whole neo tethyan basin and correlation with himalayan data support the formation of all himalayan lithotectonic units along a continuous passive margin of northern india

bainang terrane yarlung tsangpo suture southern tibet xizang china a record of intra neotethyan subduction accretion processes preserved on the roof of the world the bainang terrane an intra oceanic island arc subduction complex into which tethyan oceanic rocks were accreted during the cretaceous is preserved within the yarlung tsangpo suture zone of tibet the lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central tethyan domain from late triassic to mid cretaceous time these rocks are preserved within a south verging imbricate thrust stack of thin 1 km thick northward younging tectonic slices five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts the northern tract which accumulated on the north side of neotethys was probably separated from its southern counterpart by a mid ocean ridge detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract oceanic plate stratigraphy of the northern tract records its northward travel and mid cretaceous late aptian approach towards a south facing intra oceanic subduction zone rocks in the southern tract developed closer to the indian subcontinent and experienced thermotectonic subsidence and mid jurassic basic alkaline intraplate magmatism they were probably accreted late in the cretaceous variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion the overall tectonostratigraphy of the terrane reflects its development in a remote intra oceanic setting

various types of olistostromes in a closing ocean basin tethyan himalaya cretaceous tibet abstract the cretaceous rocks of the central tethyan himalayas display different types of deep sea olistostromes and chaotic deposits which are accompanied by turbidites the deposits were incorporated into an accretionary prism and more or less deformed according to the composition and provenance of their clasts we distinguish four types of olistostromes 1 p type derived from the passive indian continental margin 2 po type derived from both the passive indian margin and ocean floor pelagic sediments and oceanic crust 3 poa type with clasts from the same sources as the po type but also containing material from the active margin along the lhasa block and 4 a and oa type mainly derived from the active margin accretionary prism and magmatic arc the occurrence of these different types of deep sea chaotic deposits in space and time is closely related to the evolution of the neo tethyan basin from a wide open ocean basin to a narrowing remnant and trench basin prior to collision whereas pure p type olistostromes formed when the basin was still wide predominating a type deposits characterize a late stage of basin evolution prior to collision mixed type olistostromes record an intermediate stage of basin evolution and require specific syn sedimentary tectonic activity

carbonaceous substances associated with the paleoproterozoic natural nuclear fission reactors of oklo gabon paragenesis thermal maturation and carbon isotopic and trace element compositions field relationships textural and reflectance characteristics and carbon isotopic compositions indicate that at least eight different categories of carbonaceous substances cs occur in the black shales uranium ores and fossil nuclear fission reactors of oklo the categories include kerogens of the mikouloungou coal in the fa formation the c1 pelites of the fa formation the black shale of the fb formation and bitumens present in veinlets in the basal black shale of the fb formation in the fa formation in normal ore and in hydraulic fractures in and near the fossil reactors and in late dolerite dikes bitumen migration occurred at oklo during at least three intervals 1 ca 2 0 ga ago when petroleum liquids derivatives of which are preserved as solid bitumen were generated by thermal maturation of black shale kerogen 2 ca 1 968 50 ma ago when 15 or more pockets of uranium ore achieved criticality and 3 977 to 981 27 ma ago when dolerite dikes intruded radionuclide migration occurred during intervals 2 and 3 bitumenization trends established for organic matter which has undergone thermal maturation fail to discriminate between heat affected and normally matured cs at oklo in and near oklo reactors where several types of solid bitumen commonly occur in close association carbon of the youngest low reflectance bitumen is isotopically heavier than that of older high reflectance bitumen due to thermal cracking this process together with enhanced radiolysis of bitumen in and near the reactors have combined to increase its 13c avg 25 8 compared to that of bitumen samples in normal ore avg 35 5 distant from the reactors graphitized bitumen inhibited migration of 235u and some fission products in uraninite of the natural reactors however alkaline earth elements such as ba sr and mo and alkaline elements rb and cs not compatible in the uo2 crystalline structure have migrated in hydrothermal solutions and in the once fluid oklo bitumens in situ analysis of solid bitumens by laser ablation icp ms detects differences in trace element composition developed as a result of their varying geological histories and evolutionary trends

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

further palaeomagnetic data from chitral eastern hindukush evidence for an early india asia contact the eastern hindukush forms part of an elongate belt central domain collage of cimmerian microcontinents that encircles the northern part of the indian subcontinent a gondwanan origin is commonly assumed for this belt but a laurasian origin for the chitral region has been argued on palaeontological talent and mawson 1979 and palaeomagnetic klootwijk and conaghan 1979 grounds the laurasian view was based on a pilot study we undertook of upper devonian pisolitic ironstones from a thrust sheet at kur gh spur in chitral preliminary results showed a characteristic magnetization component d 318 i 6 5 n 7 block samples k 14 95 16 5 indicating an equatorial palaeoposition this component was thought to be of primary origin and was interpreted in terms of a late devonian laurasian affinity of the kur gh spur rocks this controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the resh n kur gh b ni region of chitral and the primary origin of the characteristic magnetization component refuted thermal demagnetization of 333 block samples from middle to upper devonian variegated sediments permian quartz flysch permo triassic carbonates and mid cretaceous redbeds showed two interpretable components a softer component of recent origin a and a harder characteristic component b of both normal and reverse polarity whose mean direction d 314 1 i 6 0 n 4 thrust sheets k 198 2 95 6 5 is comparable to the characteristic component observed in our preliminary study however the universal presence of this component throughout the thrust pile proves its overprint origin which we attribute to initial india asia contact palaeomagnetic information pertinent to the controversy of a laurasian versus a gondwanan origin of the chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt hence we retract herewith our original conclusion of a late devonian laurasian affinity of the chitral region on the basis of the palaeomagnetic evidence the secondary component b comprises a suite of secondary magnetizations acquired at equatorial to low northern palaeolatitudes and is attributed to initial contact between greater india and southern asia component b has been observed previously in the himalayan tibetan region both north and south of the indus tsangpo suture zone identification is herein extended to the hindukush region north of the northern kohistan or shyok suture zone which is a western continuation of the indus tsangpo suture comparison of this suite of collision at tributed equatorial palaeolatitude data from the india asia convergence zone with new palaeolatitude constraints from the ninetyeast ridge on the northward movement of the indian plate constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of greater india indicates that initial contact between northwestern greater india and southern asia was established at or before the cretaceous tertiary boundary the overprint origin of component b at about this time is further supported by observations by zeitler 1985 on rocks from the sampled area in chitral of partially reset zircon fission track ages around 68 55 ma the nw se declination axis of component b indicates a 60 70 counterclockwise rotation of the sampled thrust pile with respect to eurasia and a counterclockwise rotation between 10 and 30 with respect to india some of the recent field components a show a comparable rotation and indicate that the tectonic activity that led to the formation of the hindukush pamir karakorum syntaxial zone has continued into recent times

structural and metamorphic evolution of the gabon orogenic belt collision tectonics in the lower proterozoic abstract the archaean granite gneiss terrain of central gabon comprises medium to high grade gneisses metamorphosed at 3 15 ga forming the monts cristal and underlying the mitzic region intruded by and enclosed as enclaves in granitoids 3 0 2 6 ga old that form the extensive chaillu and north gabon blocks these rocks are unconformably overlain in the east of gabon by early proterozoic francevillian sedimentary rocks between 2 3 and 2 0 ga old forming the franceville and booue basins the francevillian is overthrust on its west side by the ogooue metamorphics themselves considered to be of early proterozoic age these rocks crop out in a broad synclinorium consisting essentially of paragneisses with orthogneisses appearing locally at the base the overall tectonic setting is discussed in terms of the evolution of the various domains from west to east 1 1 on the west flank of the synclinorium is a migmatitic granulitic dome the abamie dome in which structures diverge away from the core metamorphism is of high temperature intermediate to low pressure type 2 2 on the east flank of the ogooue synclinorium the medium grade ogooue metamorphics sillimanite biotite kyanite staurolite staurolite muscovite biotite have been thrust eastwards on to the low grade kyanite sericite francevillian 3 3 the francevillian is parautochthonous being widely detached from its archaean basement owing to the effects of the same eastward directed tangential tectonics 4 4 locally slices of archaean basement have been tectonically incorporated the structural cross section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts the suture marking the collision is interpreted to be located in the abamie migmatite dome where the nappes are rooted the gabon orogenic belt is correlated with the collision belts of about the same age on the eastern margin of the congo craton and with the trans amazonian orogenic belt of the guyana shield

characterization of insoluble organic matter from the lower proterozoic huronian supergroup elliot lake ontario abstract elliot lake organic matter falls into two main groups 1 stratiform kerogens with reflectance from 2 63 to 7 31 ro max exhibit a high level of aromaticity and low atomic h c ratios 0 41 0 60 2 less aromatic disseminated globules most resembling solid bitumen are of lower maturity with ro max from 0 72 to 1 32 and atomic h c ratios 0 71 0 81 stratiform kerogens originated from cyanobacteria have 13c values from 15 to 25 one exception with 13c 31 8 may have originated from methanogenic source biota thus two genetically distinct populations of kerogen are tentatively recognized in elliot lake metasediments globular bitumens remobilized by hydrous pyrolysis from kerogen exhibit 13c values ranging from 25 tp 33 0 the overall isotopic bimodality among elliot lake organic material is explained by analogy to petroleum generation from particulate kerogen that matured under conditions of catagenesis to metagenesis and was subjected to radiation over a long time barium u th au pt pd and ree contents of elliot lake stratiform kerogen greatly exceed of average black shales except for as pt pd and au all metals ti v cr mn fe co ni zn se rb sr ag sb cs ba hf ta w hg th and u are more concentrated in stratiform kerogens than in the derivative bitumen this signals a significant role for a low temperature transfer of noble metals to migrating bitumen

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

geochemical discrimination of clastic sedimentary rock sources abstract factors controlling the geochemistry of a clastic sedimentary rock can include 1 composition of source terrain 2 chemical weathering 3 hydraulic sorting 4 diagenesis 5 metamorphism and 6 hydrothermal alteration a linear solution inferring source terrain composition from geochemistry of the sediment is impossible in this multivariate system as several unknowns will commonly be present the use of graphical analysis of element pairs circumvents the problem chemically immobile elements will maintain invariant ratios during rock mass change caused by either addition or depletion of mobile elements this results in chemically immobile element scattergrams exhibiting linear trends along radians extending from the origin if the major mineral phases of the immobile elements have behaved in a hydrodynamically similar manner as chemically mobile element plots will produce a scatter of points this relationship can be used to test chemical immobility and similarity in hydrodynamic sorting history the constraint on analysis is that the source area must be compositionally uniform or the sediment well mixed prior to delivery to the basin a second technique using sio2 plots has also been developed to investigate element mobility and hydrodynamic behaviour of the mineral phases containing the elements sio2 immobile element plots result in a linear arrangement of points extending towards either 0 or 100 sio2 the 0 intercept position occurs for elements with major mineral phases concentrated in sand the 100 intercept for those concentrated in clays plotting chemically mobile elements produces different patterns and this can be used to gain information on alteration and sorting history elemental ratios for chemically immobile elements with similar hydrodynamic behaviour will be the same as those for the weighted average composition of the source material this provides a powerful tool for deducing source terrain from sediment geochemistry techniques outlined above were tested on archaean metasandstones from superior province canada immobile element ratio diagrams for nb al ti and zr al ti indicate that a calc alkaline extrusive intrusive suite lying to the north of the study area was the source and not five other volcanic suites in the region this conclusion agrees with previous clast lithology studies and accentuates the applicability of the geochemical techniques

the himalayan passive margin from precambrian to cretaceous times abstract the himalaya passive margin should be separated into the indian himalaya and the pakistan himalaya divided by the nanga parbat haramosh basement uplift in the northwestern syntaxes the indian passive margin shows a complex history involving splitting of microcontinents off the northern gondwana margin from early paleozoic times until the jurassic in contrast the pakistan himalaya formed part of the stable indian shelf until separation from africa started in the jurassic each margin has a distinct stratigraphic history until the mid mesozoic after which they show similar histories until the tertiary the himalaya mountains in india consist of three distinct tectonic units juxtaposed during the neogene by southward thrusting of the northern indian precambrian to mesozoic passive margin the lesser himalaya on the south forms a late precambrian passive margin usually directly overlain by late cretaceous to tertiary clastics its stratigraphy is closest to that of the indian shield in contrast the high himalaya has a thin ordovician to carboniferous shelf sequence deposited after early ordovician deformation and granite intrusion above this thick permian and mesozoic shelf sequences mark the separation of continental blocks off the northern indian margin and the opening of the neotethys ocean the north himalaya formed the slope and basin of this ocean and consists of reactivated paleozoic gneiss domes overlain by thick mesozoic sediments which pass in the northeastern himalaya into an enormously thick sedimentary sequence resting on oceanic crust paleomagnetic and structural evidence indicates at least 500 km of southward thrusting along the main central thrust between the lesser and high himalaya this thrusting together with a further 250 km of erosion at the front of the nappes has removed the entire inner shelf of the himalaya passive margin and can explain the startling contrast between the high and lesser himalaya stratigraphic sequences but there are still discrepancies particularly in the eastern himalaya where precambrian basement is juxtaposed with the north himalaya mesozoic slope these discrepancies can be partly resolved by strike slip movements roughly parallel to the himalaya which have removed parts of the northern indian passive margin such faults of the requisite orientation displacement and age occur in southeast asia where their cumulative displacements can add up to several thousand kilometres similar faults of undoubted pre tertiary age occur in the pakistan himalaya here the passive margin shows a much simpler history of jurassic to miocene subsidence

review and update of the applications of organic petrology part 2 geological and multidisciplinary applications abstract the present paper is focused on organic petrology applied to unconventional and multidisciplinary investigations and is the second part of a two part review that describes the geological applications and uses of this branch of earth sciences therefore this paper reviews the use of organic petrology in investigations of i ore genesis when organic matter occurs associated with mineralization ii the behavior of organic matter in coal fires self heating and self combustion iii environmental and anthropogenic impacts associated with the management and industrial utilization of coal iv archeology and the nature and geographical provenance of objects of organic nature such as jet amber other artifacts and coal from archeological sites and v forensic science connected with criminal behavior or disasters this second part of the review outlines the most recent research and applications of organic petrology in those fields

propagation of the thrust system and erosion in the lesser himalaya geochemical and sedimentological evidence centre de recherches pe trographiques et ge ochimiques bp 20 54501 vandoeuvre les nancy franceabstractsedimentological and nd isotope data of two sections of the sub himalaya of westernnepal are used as new constraints for understanding the erosion history of the himalaya throughout the deposition of the middle and upper members of the siwalik group thelesser himalaya contribution to the total detrital input progressively increased from lessthan 20 to 40 the increasing proportion of lesser himalaya sediments started at ca 10 8 ma and is associated with a coarsening of the maximum grain size at both micro scopic and macroscopic scales thin skinned tectonics of the lesser himalaya thrust sys tem would have controlled the exhumation of the lesser himalaya rocks and would havebegun at 12 10 ma taking into account the delay for denudation together with otherstudies these data restrict the onset of movement on the lesser himalaya thrust systemto less than 3 m y along more than 1750 km this short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust thus we suggest instead the simultaneous initiation of several thrusts aheadof the main central thrust at ca 12 ma we suggest that a rapid rise of the tibetanplateau at this time has transformed the himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state this regional rising could be the primecause of the increase of sediment in ux at ca 11 ma around the himalaya keywords siwalik formations neodymium grain size erosion thrust sheets himalaya

sr and nd isotopic evidence for punctuated clay diagenesis texas gulf coast we report rb sr sm nd and mineralogical data for leached whole rock 1200 m independent of the duration of burial the exchangeable sr in the clay has remained an open system and has undergone isotopic exchange since that time minimum fluid rock ratios of 0 2 to 2 are modeled from sr isotopic data consistent with rock dominated open system diagenesis leachates and residues of the 0 1 zm fraction below the smectite illite transition appear to have achieved nd isotopic equilibrium during clay diagenesis and show no signs of subsequent exchange the clay residues have low 147sm 144nd 0 085 indicating that authigenic illite has a lower sm nd ratio than its bulk rock protolith conversely the coexisting exchangeable ree inventory has elevated sm nd as reflected by leachate 14vsm 144nd as high as 0 188 this sm nd fractionation in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid rock interaction in older rocks

structural evolution and sequence of thrusting in the high himalayan tibetan tethys and indus suture zones of zanskar and ladakh western himalaya reply abstract the timing of motion on major thrusts in the western himalaya shows an extremely complex sequence that spans approximately 70 ma from the latest cretaceous throughout the tertiary three major phases of thrusting can be distinguished the earliest phase t1 is associated with emplacement of tethyan basin thrust sheets lamayuru sediments and spontang ophiolite south and south westwards onto the submerged northern passive margin of india 75 60 ma collision between india and asia occurred at 50 36 ma and was followed immediately by the major phase t2 of crustal shortening involving large scale south and south westward directed thrusting of the complete palaeozoic mesozoic and late tertiary tibetan tethys zone rocks preliminary balanced cross sections show a minimum shortening of 126 km of these rocks across the zanskar range the late collision phase t3 involved re thrusting of the previously stacked pile breaching or leap frog thrusting reversing the earlier stacking order in places and widespread steepening overturning and backthrusting along the whole northern margin of the tibetan tethys zone and throughout the indus suture zone

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

jurassic cretaceous radiolarian biostratigraphy and sedimentary environments of the ceno tethys records from the xialu chert in the yarlung zangbo suture zone southern tibet radiolarian biostratigraphic research has been carried out along two continuous sections through the xialu chert one of the accreted sheets included in the yarlung zangbo suture zone six radiolarian zones have been identified as follows laxtorum jurassicum zone aalenian tricolocapsa plicarum zone bajocian lower bathonian stylocapsa spiralis zone upper callovian oxfordian hsuum maxwelli zone kimmeridgian pseudodictyomitra carpatica zone upper tithonian lower valanginian and turbocapsula costata zone aptian a reconstructed stratigraphy of the xialu chert based on lithological succession and radiolarian dating indicates that the chert exhibits a long depositional history at least from early middle jurassic aalenian to late early cretaceous aptian the separation of the lhasa block from the northern gondwana margin must therefore be dated before the aalenian the absence of calcareous sediments in the xialu chert indicates that the oceanic basin was deeper than the ccd throughout the depositional history the transition from chert to siliceous mudstone is recorded sometime in early cretaceous most probably around the barremian aptian boundary this means that the oceanic plate had already started being consumed at a trench by that time the accretion of the xialu chert occurred after the aptian time

nanoscale relationships between uranium and carbonaceous material in alteration halos around unconformity related uranium deposits of the kiggavik camp paleoproterozoic thelon basin nunavut canada concentrations of 7 u and 1 cu were identified in massive brecciated and amorphous carbonaceous materials cm characterized by strongly negative values of carbon stable isotopes 13c 39 1 relative to pdb the anomalies are restricted to clay alteration halos developed in neoarchean woodburn lake group metagreywacke that is the predominant host of unconformity related uranium u deposits in the kiggavik exploration camp petrographic and microstructural analyses by sem x ray diffraction hr tem and raman spectroscopy identified carbon veils best described as graphene like carbon upon which nano scale uraninite crystals are distributed cm are common in u systems such as the classic cretaceous roll front deposits and the world class paleoproterozoic unconformity related deposits however the unusual spatial and textural association of u minerals and cm described herein raises questions on mechanisms that may have been responsible for the precipitation of the cm followed by crystallization of u oxides on its surfaces based on the characteristics presented herein the cms at kiggavik are interpreted as hydrothermal in origin furthermore the nanoscale organization and properties of these graphene like layers that host u oxide crystallites clearly localized u oxide nucleation and growth

geochemistry of uraniferous bitumen in the southwest athabasca basin saskatchewan canada the sediment hosted unconformity type southwest athabasca uranium mineralization in northern saskatchewan canada contains uraniferous bitumen as observed elsewhere in the athabasca basin uraninite is present either as micron sized grains dispersed in bitumen uraniferous bitumen or as larger grains situated in the pore space between bitumen globules the spatial and textural relationships between uraninite and bitumen suggest that they were co precipitated la icp ms u pb dating of uraniferous bitumen yielded an age of 1575 53 ma similar to that of other unconformity type uranium mineralization in the athabasca basin indicating that the bitumen was formed early in the history of the basin nitrogen carbon and sulfur isotope analyses of uraniferous bitumen indicate that its source material was probably kerogen produced by cyanobacteria in a clastic environment

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

solid bitumens an assessment of their characteristics genesis and role in geological processes familiar since antiquity and subject in contemporary times to various characterization schemes the exact nature of solid bitumen is not yet fully known bitumens have random polymer like molecular structures are mobile as highly viscous fluids or were once fluids but have since turned into solids solid bitumens consist mainly of large moieties of polyclyclic aromatic hydrocarbons occasionally with finely admixed fine grained cryptocrystalline graphite solid bitumens are distinguished from kerogen which is the syngenetic and generally finely dispersed particulate organic matter in sedimentary rock that virtually does not migrate following its deposition occurrences of solid bitumens are relevant to petroleum exploration as well as the search for and evaluation of a variety of metallic mineral deposits genesis of bitumen is in many cases linked to the thermal and hydrothermal history of organic matter in sedimentary rock apparently bitumen or more specifically organic acids generated along with bitumen during diagenesis may alter porosity of reservoir rocks or otherwise prepare the ground for ore deposition bitumen is also relatively sensitive to alteration processes some of which such as oxidative weathering water leaching biodegradation contact metamorphism and ionizing radiation may likewise affect its nature elemental composition of bitumen commonly reflects the nature of mineral deposits is is possible that in petroleum exploration trace metal abundances of bitumen may eventually allow prediction of crude oil types and volumes anticipated from a given source rock beside transition elements notably ni and v highly anomalous concentrations of u pt and au occur in some solid bitumens during the generation of petroleum from kerogen the trend in 13c is toward lighter values the opposite seems to occur when liquid petroleum is subjected to thermal cracking and or related processes yielding solid bitumen enriched in 13c and isotopically light methane in fact except for deasphalting and possibly some irradiation processes the result of thermal cracking oxidation water leaching inspissation drying and bacterial degradation of crude oil is that lower molecular weight hydrocarbons are removed leaving bitumen residues enriched in aromatic hydrocarbons heteroatomic compounds nso and 13c such phenomena are relevant to bitumen paragenesis in petroleum reservoir rocks to certain phanerozoic occurrences of multiple generations of bitumens and to bitumens in mineral deposits

testing for fullerenes in geologic materials oklo carbonaceous substances karelian shungites sudbury black tuff fullerenes have been reported from diverse geologic environments since their discovery in shungite from karelian russia our investigation is prompted by the presence of onionskin like structures in some carbonaceous substances associated with the fossil nuclear fission reactors of oklo gabon the same series of extractions and the same instrumental techniques laser desorption ionization and high resolution mass spectroscopy electron impact mass spectroscopy were employed to test for fullerenes in samples from three different localities two sites containing putative fullerenes sudbury basin and russian karelia and one new location oklo gabon we confirm the presence of fullerenes c 6 0 and c 7 0 in the black tuff of the onaping formation impact breccia in the sudbury basin but we find no evidence of fullerenes in shungite samples from various locations in russian karelia analysis of carbonaceous substances associated with the natural nuclear fission reactors of oklo yields no definitive signals for fullerenes if fullerenes were produced during sustained nuclear fission at oklo then they are present below the detection limit 100 fmol or they have destabilized since formation contrary to some expectations geologic occurrences of fullerenes are not commonplace

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

the late oligocene early miocene himalayan belt constraints deduced from isotopic compositions of early miocene turbidites in the bengal fan abstract we present a mineralogical and isotopic study of early miocene turbidites from the bengal fan which are the oldest sediments recovered during odp leg 116 clay fractions are dominated by illite and chlorite and have low 18o 11 16 the clays are dominantly derived from a metamorphic precursor by physical erosion and have undergone only minor low t water rock exchange and chemical weathering sr and nd isotope analysis of silt and clay fractions and coarse muscovite biotite and feldspar fractions are remarkably similar to those of the presently exposed high himalaya crystalline hhc sequence the data show that a close analogue of the hhc was already subaerially exposed to active erosion during early miocene rb sr data on the biotite separates has been used to evaluate the time elapsed between cooling through the closure temperature for sr in biotite in the hhc ca 325 c and final sedimentation in the bengal fan despite the large uncertainties in the initial 87 sr 86 sr values of sedimentary mixtures our estimates of the elapsed time between cooling and deposition are short 4 14 ma assuming a transport time scale 1 ma deduced exhumation rates are in the same range of modern values 1 3 mm yr since ca 30 ma given that a the rocks exposed at the surface b the style of weathering and c rates of exhumation were similar to the present we infer that the factors controlling erosion of the early miocene himalayan belt were also similar to the present the data suggest that the early miocene himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range it appears that the thrusting along the main central thrust mct or similar structures has uplifted metamorphic basement analogous to the present hhc at sufficiently high rate to maintain steep relief throughout miocene time despite high average erosion rates

structure and metamorphism of a tectonically thickened continental crust the yalu tsangpo suture zone tibet three tectonic units the active palaeomargin the palaeo oceanic domain and the passive palaeomargin are now juxtaposed and constitute the yalu tsangpo suture zone between india and eurasia in southern tibet we examine the deformation metamorphic and uplift histories of each of these units our observations are accounted for in terms of the processes whereby the crust was thickened to about 70 km we finally discuss the implications for ancient continental collision orogens

characterization of organic matter associated with uranium deposits in the francevillian formation of gabon lower proterozoic abstract elemental analysis organic petrography and high resolution transmission electron microscopy were used to study organic matter in lower proterozoic rocks of the francevillian series in africa results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil and deposition of uraninite ores evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones moreover the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization

paleontological view of the ages of the deccan traps the cretaceous tertiary boundary and the india asia collision volcanism in the deccan traps of india occurred over at most three magnetic chrons centered on a main reversed chron paleontological data indicate that this reversed chron must coincide with 29r which contains the cretaceous tertiary k t boundary recent 40 ar 39 ar data may therefore provide one of the best estimates of the absolute age of the k t boundary 65 7 2 0 ma moreover the eurasian character of k t boundary terrestrial faunas in india suggests that this was also close to the time of india asia collision this is significantly earlier than generally recognized and implies about 4000 km of continental shortening in the collision zone probably absorbed jointly by crustal thickening and strike slip extrusion

the occurence of unusual c27 and c29 sterane predominances in two types of oman crude oil abstract two types of oman crude oils reveal unusual sterane distributions type a which is the more common 74 examples is characterised by a predominance of c 29 iso and normal steranes and generally none or only very low relative concentrations of rearranged steranes the triterpanes are characterised by the predominance of the c 29 17 h 21 h norhopane over the c 30 17 h 21 h hopane and non predominant c 20 c 30 tricyclic terpanes the c 29 steranes of this type of crude were not derived from the c 29 sterols of land plant origin frequently proposed as the source of c 29 steranes since there is good geological evidence that these crudes were generated from a pre cambrian source rock a geological period when land plants did not exist the type b crude oil 11 known examples is characterised by a strong predominance of c 27 iso normal and rearranged steranes relatively lower concentrations of 17 h 21 h hopanes and relatively high concentrations of c 20 c 30 tricyclic terpanes the remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different

the neodymium isotope system as a tool for petroleum exploration abstract mineralogical petrological geochemical and neodymium isotopic investigations strongly suggest isotopic and chemical exchange between authigenic silicate phases and petroleum alteration of organic matter sulfur reduction generation of aqueous acidic solutions with the resultant dissolution of k feldspar leads to in situ crystallization of illite and chlorite as well as neodymium isotopic equilibration between the silicate phases and petroleum these findings are of significance not only to understand the mechanisms leading to neodymium isotopic equilibration between authigenic clay minerals in detrital sediments but also to source rock petroleum in reservoir correlations in particularly when biomarker molecules i e isoprenoid hydrocarbons have been obliterated by prolonged severe diagenesis