age and cooling history of the manaslu granite implications for himalayan tectonics abstract the manaslu granite is one of the high himalayan leucogranites which are believed to be derived by partial melting of quartzofeldspathic gneissess of the high himalayan crystallines hhc and emplaced at the contact between the hhc and the tethyan sedimentary series we have analyzed a suite of muscovite 15 biotite 3 and alkali feldspar 10 by the 40 ar 39 ar method to help constrain the age and cooling history of the granite 40 ar 39 ar ages ranges are 18 4 to 13 3 ma 17 0 to 14 7 and 16 4 to 3 4 for muscovite biotite and k feldspar respectively based on the muscovite analyses and a re interpretation of a previously published u pb monazite age we conclude that the crystallization age of the manaslu granite is everywhere 20 ma the postcrystallization cooling history given by the micas and the feldspars suggests the presently exposed parts of the pluton were emplaced at depths of 8 to 15 km these data place a minimum age of movement on the mct in this area of 20 ma we estimate the time of magma segregation and transport to be no more than 5 ma

crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

a p t t d discontinuity in east central nepal implications for the evolution of the himalayan mid crust abstract rock specimens collected from lower middle and upper portions of the exhumed himalayan mid crust in the upper tama kosi region of east central nepal yield pressure temperature time deformation p t t d paths that demonstrate the presence of a structural metamorphic and geochronologic discontinuity a p t t d path from below the discontinuity defines a loading path with deformation at staurolite grade metamorphic conditions occurring at 10 8 ma p t t d paths from above the discontinuity in contrast record an earlier and protracted metamorphic history that includes decompressional heating monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be 21 ma with decompression related garnet breakdown initiating at c 19 ma and likely continuing until c 15 ma these contrasting p t t d histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen these results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen

identifying marine incursions into the paleogene bohai bay basin lake system in northeastern china abstract the extent of marine influence on the paleogene bohai bay basin in northeastern china which accumulated mainly continental e g fluvial and lacustrine facies remains under debate evidence from geochemical paleontological and lithological data have documented several marine incursions but no systematic study has been conducted to investigate their timing and frequency and their significance for understanding the paleogene tectonic evolution of northeastern china in order to address the marine influence on sediment accumulation and to identify variations in paleosalinity during deposition of the mid eocene lower es3 es3l member of the shahejie formation this study analyzed multiple paleosalinity proxies b ga sr ba and s toc in fine grained siliciclastic argillaceous sediments of the luo 69 drillcore from the dongying depression all three proxies yielded similar paleosalinity interpretations with peak salinity and marine influence i e maximum b ga sr ba and s toc values occurring at the base of the es3l member and progressively declining salinities representing a shift toward brackish conditions upsection over an interval of 185 5 m representing 2 2 myr of sedimentation this paleosalinity record suggests gradual long term variations that were probably due to changes in tectonically controlled gateways linking the bohai bay basin to the yellow sea in addition to analysis of the mid eocene es3l member published data on the stratigraphic distribution of marine fossils glauconite and elemental paleosalinity proxies are applied to evaluate the history of marine incursions into the bohai bay lake system throughout the mid to late paleogene 50 24 ma this analysis revealed four intervals of probable major marine incursions dating to the early eocene the middle eocene the early oligocene and the late oligocene

an early pliocene thermal disturbance of the main central thrust central nepal implications for himalayan tectonics since the beginning of the collision between india and asia at about 50 ma the convergence in the himalaya has largely been taken up along major thrust zones in this study samples of the lesser himalaya formations up to 10 km below the main central thrust mct and the greater himalaya sequence up to 12 km above the mct have been analyzed by the 40ar 39ar and u pb methods to investigate the thermal history of the mct the ages can be summarized as follows 1 the youngest ages from muscovites 3 1 ma biotites 3 4 ma and hornblendes 4 1 ma all come from within 1 km of the mct 2 there is a marked asymmetry of ages between the footwall and the hanging wall of the mct the maximum mica age in the hanging wall greater himalaya sequence is 13 ma whereas a muscovite 5 km below the mct in the footwall has an age spectrum with a gradient from 400 to 1400 ma 3 five k feldspars from 6 2 to 11 9 km above the mct all give minimum ages in the range 3 0 6 4 ma and 4 a 206pb 238u age on a brannerite from the greater himalayan sequence is 4 8 ma structural and petrologic observations preclude the possibility that this age distance distribution reflects faulting within the greater himalaya sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses infiltration of hot fluids through the mct zone appears to be the best hypothesis to explain these data simple numerical simulations which account for heat transfer by advection within the fluid flow zone and by conduction outside it indicate that the observed age distribution could have been produced by infiltration of hot fluids through the mct zone at circa 5 to 4 ma within the following range of conditions the fluids heated rocks to temperatures in the range 470 to 510 c for less than 1 million years in a region narrower than the entire mct zone the temperature of the thermal disturbance inferred from the 40ar 39ar data is consistent with petrologic data from albitic alteration assemblages in the greater himalaya sequence and the u pb age of the brannerite is similar to the youngest 40ar 39ar hornblende age this scenario hydrothermal heating culminating at about 4 ma is similar to the model of le fort 1981 for the generation of the high himalayan leucogranites in the late oligocene early miocene by dehydration of the footwall rocks of an active thrust in the younger instance we interpret that fluids derived from the footwall rocks of the main boundary thrust migrated upward through the lesser himalaya formations and were subsequently channeled along the mct producing the thermal disturbance

contrasting accessory mineral behavior in minimum temperature melts empirical constraints from the himalayan metamorphic core abstract medium grained leucogranite in the tama kosi region of the nepalese himalayan metamorphic core yields a relatively narrow range of monazite 208pb 232th dates with a dominant population at 21 0 ma inferred to represent crystallization of an early plutonic phase in contrast the pegmatitic portion of the same intrusive complex that cross cuts the medium grained leucogranite contains zircon monazite and xenotime that each display near identical age spectra recording semi continuous re crystallization from 27 5 ma to 21 0 ma followed by a 2 m y hiatus then further re crystallization between 19 4 and 18 6 ma the gap in pegmatite dates corresponds well to the crystallization age of the older leucogranite whereas the end of accessory phase growth in the pegmatite coincides with the onset of regional scale cooling detailed textural trace element and thermochronologic data indicate that the range of zircon monazite and xenotime dates recorded in the pegmatite reflect inherited components that underwent semi continuous re crystallization during metamorphism and or anatexis in the source region s whereas dates younger than the hiatus indicate accessory phase recrystallization related to both fluid influx and a concomitant increase in temperature in contrast the lack of an inherited component s in the medium grained leucogranite phase is inferred to be a result of complete dissolution during partial melting a model is proposed in which influx of heat and h2o rich fluids associated with early leucogranite emplacement temporarily delayed zircon and monazite and xenotime crystallization respectively these data highlight the importance of measuring spatially resolved dates trace elements and textural patterns from multiple accessory minerals combined with model constraints to better understand the often complex crystallization history of anatectic melts in collisional orogens

the tectonostratigraphy and structure of the eastern nepal himalaya six years of geological research in eastern nepal has resulted in a new geological map of the eastern nepal himalaya which includes the region stretching from the sikkhim border in the east to the kathmandu valley in the west and from the summits of the higher himalaya in the north to the ganges plain in the south this research has permitted the determination of the tectonostratigraphy and structure of one section of the central himalayan arc south of the tibetan plateau the eastern nepal himalaya can be divided into three distinct thrust bound tectonic packages 1 the higher himalayan thrust sheet composed of the higher himalayan crystallines 2 the lesser himalayan thrust sheet composed of the lesser himalayan series and 3 the sub himalayan imbricate zone composed of sedimentary rocks belonging to the siwalik group the higher himalayan thrust sheet of eastern nepal has been thrust over the lesser himalayan metasediments a minimum of 140 km and possibly as much as 175 210 km along the main central thrust mct the lesser himalayan thrust sheet is overlain by the mct and is underlain by the main detachment fault mdf and the main boundary thrust mbt out of sequence thrust faults in the hanging wall of the mbt have breached and offset the presently inactive mct the sub himalayan imbricate zone is an emergent imbricate fan bounded by the mbt to the north and the main frontal thrust mft to the south and is underlain by the mdf which lies at a depth of between 5 km and 7 km a balanced cross section constructed across the higher lesser and sub himalaya of eastern nepal shows that the eastern nepal himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal north south tectonic shortening since the initiation of the mct the lesser and sub himalaya have absorbed 70 km of north south shortening by thrusting along the basal mdf of which the sub himalayan imbricate zone has accommodated 25 km the sun kosi thrust has accommodated about 10 km and the mbt has accommodated the remaining 35 km of shortening since the initiation of the mct between 15 ma and 25 ma shortening across the eastern nepal himalaya has occurred at an average rate of 8 4 18 6 mm per year the structural geometry of the eastern nepal himalaya suggests an overall piggyback sequence of thrusting with motion transferred from the mct to the underlying mdf and its emergent splay thrust the mbt and with the mbt rotated to its present steep orientation by imbricate thrusting within the sub himalaya

24 norcholestanes as age sensitive molecular fossils biomarkers molecular fossils are organic compounds in holocene to precambrian sedimentary deposits that can be related to specific chemical compounds produced in the biosphere we demonstrate here that 24 norcholestane biomarkers i e c26 steranes saturated hydrocarbons having a steroid skeleton can be useful to constrain the age and paleolatitude of geologic samples the biological precursors of 24 norcholestanes remain unclear but samples from more than 100 basins provide evidence that 24 norcholestanes may be derived directly or indirectly from diatoms ratios of 24 norcholestanes to 27 norcholestanes show an initial increase above background in jurassic oils but they increase dramatically in cretaceous oils coincident with diatom evolution the highest ratios are found in oils and rock extracts from oligocene or younger marine siliceous source rocks in which the sources were deposited at paleolatitudes 30 n

viscosity of himalayan leucogranites implications for mechanisms of granitic magma ascent the viscosity of the gangotri himalayan leucogranite has been experimentally determined between 80 o and 1100oc 300 and 800 mpa for meltwater contents of 3 98 and 6 66 wt the melt viscosity is independent of pressure and shows an arrhenian behavior relative to temperature within the range of conditions investigated we present an empirical relation that can be used to determine leucogranite magma viscosities knowing their meltwater content and temperature this relation together with phase equilibria experiments constrain the viscosity of the itimalayan leucogranit s to be around 104 5 pas during their eraplacement these viscosities and the widths of dikes belonging to the feeder system are consistent with the theoretical relationship relating these two parameters and show that the precursor magma of the leucogranite was at near liquidus conditions when emplaced within host rocks with preintmsion temperatures around 350oc calculated terminal asa nt rates for the magma in the dikes are around 1 m s magma cmber assembly time is on ths basis estimated to be less than 100 years for a volume of 150 icm3 in addition the dynamical regime of the magma flow in the dikes was essentially laminar thus allowing preservation of any chemical heterogeneity acquired in the source these results constrain the viscosity of melts f rmed during the first steps of crustal anatexis those involving muscovite breakdown to be alsoround 104 5 pas thus compaction may not be the only mechanism of melt segregation in partially melted crustal rocks in view of the very short timescale inferred for magma ascent and emplacement

source oil charging history and filling pathways of the ordovician carbonate reservoir in the halahatang oilfield tarim basin nw china abstract the ordovician reservoir in the halahatang depression is one of the most commercially prolific oil reservoirs in the tabei uplift of the tarim basin nw china for this study a total of 94 oil samples were collected from 88 exploration production wells in this oilfield all of the oils possess similar organic molecular compositions and belong to the same oil population oil to source correlation results indicate that oil accumulations discovered in halahatang oilfield were derived from the upper ordovician source rocks the homogenization temperatures of fluid inclusion in the ordovician reservoir in the halahatang oilfield range from 90 c to 100 c and from 140 c to 160 c the bimodal distribution pattern shown in a histogram of the homogenization temperatures indicates that the ordovician oil reservoir in the halahatang oilfield has been charged twice coupling this information with the reconstructed burial and geothermal histories by 1 d basin modeling the homogenization temperatures can be related to the relative geological ages of 419 ma to 410 ma and 16 ma to 8 ma namely the middle to late silurian and the middle miocene respectively representing the two phases of oil charging and entrapment the oil migration orientation and charging pathways can be traced using molecular indicators such as ts ts tm and tmnr parameters the overall oil migration orientation in the oilfield is generally from south to north therefore the source kitchen for the halahatang oilfield is predicted to be to the south side of the depression the southern part of the halahatang depression along the oil charging pathways is identified as the most promising region for oil exploration

biodegradation and possible source of silurian and carboniferous reservoir bitumens from the halahatang sub depression tarim basin nw china abstract five core samples comprising three silurian and two carboniferous bituminous sandstones from the halahatang sub depression tarim basin nw china were studied to determine the nature and extent of microbial alteration and possible source s of the reservoir bitumens a total of twelve ordovician crude oil samples from this sub depression were also examined for comparative purposes co occurrence of a series of 25 norhopanes nhs c 28 c 34 variable amounts of chromatographically unresolved complex mixture ucm and diminished concentrations of n alkanes below ca c 20 in saturated hydrocarbon fractions of the silurian and carboniferous bitumens indicates that they are mixtures of heavily biodegraded oil from a former charge and slightly biodegraded oil from a later charge the presence of c 28 c 34 25 nhs suggests a biodegradation level of at least pm 6 for these bitumen samples nevertheless minor differences in terpane and sterane distributions between the silurian and carboniferous bitumen samples were observed which can be attributed to small variations in biodegradation levels values for the 25 nh ratio peters et al 1996 gammacerane gammacerane c 30 hopane c 28 25 nh c 29 hopane and c 29 25 nh c 30 hopane ratios indicate slightly higher levels of microbial alteration for the silurian bitumens compared to the carboniferous bitumens although caution must be exercised when comparing the biodegradation intensities using molecular parameters the ordovician crude oil samples appear to be more severely biodegraded than the silurian and carboniferous bitumens as evidenced by higher concentrations of tricyclic terpanes relative to hopanes and pregnanes and diasteranes relative to the regular steranes although thermal maturity effects cannot be ruled out geochemical correlation studies were successfully applied based on carbon isotopes and biomarkers including tricyclic terpanes and triaromatic steroids which are highly resistant to severe biodegradation pm 6 bulk isotopic compositions distribution patterns and certain ratios of these biomarkers indicate that the ordovician crude oils and the silurian and carboniferous bitumens originated from a single source they were probably derived from middle upper ordovician source rocks

uncommon steranes in brazilian marginal crude oils dinoflagellate molecular fossils in the sergipe alagoas basin brazil comprehensive two dimensional gas chromatography with time of flight mass spectrometry gc gc tofms was used to identify unusual steranes in branched cyclic hydrocarbon fractions of crude oils from sergipe alagoas basin brazil and infer the geochemical settings of their cretaceous source rocks hopane sterane and sterane ratios and the presence of gammacerane and carotane in the oil samples indicates a saline source rock depositional environment and predominantly marine organic matter om input steroidal hydrocarbons were represented by alkyl steranes and several sterane series including abundant short chain steranes c21 and c22 regular steranes c27 c29 and diasteranes c21 c23 c27 c29 the ring a methyl sterane series contained 3 methyl steranes c22 and c23 c28 c30 abundant 3 methyldiginane c22 and 3 methyldiacholestanes c28 including 4 methyl steranes with abundant c30 4 23 24 trimethylcholestanes in the branched cyclic fraction of these crude oils the c29 sterane isomers 23 24 dimethylcholestane and 24 ethylcholestanes were resolved by gc gc tofms and a c29 dinosterane derivative was identified for the first time in brazilian crude oils the 23 24 dimethyl and 4 23 24 trimethyl steroids can be related to dinoflagellate om and abundant dinosteranes suggests dinoflagellate blooms during source rock deposition in this brazilian marginal basin additionally the detection of c28 c32 3 alkyl cholestanes c29 c33 3 alkyl 24 methylcholestanes and c30 c34 3 alkyl 24 ethylcholestane with ethyl to pentyl groups at c 3 resulted in a complex sterane fingerprint as expected for petroleum generated from source rocks deposited under predominantly marine saline conditions with high microalgal input these biomarker results provide additional insight into the character and geologic processes responsible for the source rocks of crude oils in the brazilian marginal basins this information will be a valuable exploration tool that can be used to constrain the distribution of different oil types in these basins

gravity anomalies flexure and the elastic thickness structure of the india eurasia collisional system wehaveusedbouguergravityanomalyandtopographydatatodeterminetheequivalentelasticthicknessofthelithosphere te in the region of the india eurasia collisional system comparison of observed and modelled gravity anomalies along 1dimensional profiles suggest there are significant variations in te along strike of the himalaya foreland estimates decrease from a high of 70 km in the central region to 30 50 km in the east and west we have verified these inferences of spatial variations using a 2 dimensional non spectral interative flexure and gravity anomaly modelling technique the himalaya foreland forms a high te 40bteb100 km rigid block with a well defined edge as shown by the localisation of faulting and deformation along its northern margin other highteblocks occur to the north beneath the qaidam and sichuan basins the tibetan plateau forms a low te 0bteb20 km weak region that extends from the central part of the plateau into south western china tectonic styles in the india eurasia collisional system therefore involve both drigidt and dnon rigidt blocks where high te rigid blocks are present the stylesdominatedbyunderthrustingofthemorerigidblock wherethecollisionalzoneisnotconstrainedbyrigidblocks however the style appears to be dominated by lower crustal flow and a more continuous style of deformation d 2005 elsevier b v all rights reserved

cooling exhumation and kinematics of the kanchenjunga himal far east nepal new single crystal 40ar 39ar and apatite fission track ages from the kanchenjunga region of far east nepal yield insight into the timing of assembly of the himalayan mid crust and the mechanisms that controlled its exhumation the 40ar 39ar data are compared with new u th pb zircon and monazite intrusive crystallization ages and existing metamorphic monazite ages from across the study area to test for internal consistency and potential excess ar contributions this new dataset which significantly enhances the density and spatial coverage available from the region shows that inferred thrust sense discontinuities within the now exhumed former midcrustal rocks exposed therein must have ceased motion by ca 12 ma furthermore the spatial distribution of ages across the kanchenjunga region older ages ca 12 16 ma to the south and north and younger ages ca 8 ma in the middle portion of the transect is compatible with simulations of tectonic enhanced exhumation above a developing duplex system in nearby bhutan

recognition of a novel precambrian petroleum system based on isotopic and biomarker evidence in yangtze platform south china abstract the precambrian doushantuo formation is regarded as a possible petroleum source rock horizon with toc values of 0 85 1 85 and type i kerogen in northwestern sichuan basin yangtze platform south china while showing high to post mature thermal maturity a set of outcropping bitumen and oil sands from various reservoirs display overall depleted values of 13 c from 36 to 32 average 35 typical of a precambrian source in addition the sterane distributions in these bitumens and oils are unusual compared to marine phanerozoic oils with a predominance of c 29 members as well as their triaromatic counterparts high amounts of 20 n alkylpregnane with very low diasteranes indicate restricted sulfur rich conditions typical of an anoxic carbonate source rock this interpretation is supported by some other specific biomarkers such as abundant 30 norhopane and c 24 tetracyclicterpane and elevated c 35 hopane furthermore the presence of gammacerane indicates a saline stratified water column hydrocarbons extracted from the solid bitumen and oil sands have many compositional characteristics in common with other oils derived from precambrian carbonate source rocks from elsewhere this implies the existence of a precambrian marine petroleum system in the yangtze platform with the doushantuo carbonate as the likely source it is supposed that the reduction of temperature and pressure following basin uplift resulted in petroleum fractionation deasphalting in lower paleozoic reservoirs with the fractionated oils having migrated along the faults becoming trapped in stratigraphically younger reservoirs where the petroleum underwent heavy biodegradation

muscovite 40ar 39ar ages help reveal the neogene tectonic evolution of the southern annapurna range central nepal we present new muscovite 40 ar 39 ar ages from thirteen greater himalayan rocks and one lesser himalayan rock collected from four north trending transects across the southern annapurna range combining the new data with previously published ages leads to the following new insight into the tectonic development of the southern annapurna muscovite cooling ages from greater himalayan rocks are c 16 10 ma in the western annapurna and c 6 2 ma in the eastern annapurna revealing a decrease of 4 14 ma from west to east similarly the muscovite cooling age from one lesser himalayan rock in the west is c 7 ma and ages from several samples in the east are c 5 2 ma indicating a decline of 2 5 ma towards the east earlier cooling in the western annapurna can be explained by along strike differences in the geometry of the frontal ramp on the underlying thrust that carries these greater and lesser himalayan rocks and or by a ne striking fault that cut these rocks in greater himalayan rocks from the modi river valley one sample yielded muscovite 40 ar 39 ar ages of 18 0 0 7 and 16 2 0 5 ma for grain sizes of approximately 750 and 200 m respectively in contrast a sample collected 200 m structurally lower produced ages of 12 6 0 2 and 9 9 0 1 ma for these two grain sizes the north dipping bhanuwa fault has been proposed between these samples with different authors arguing for normal or thrust sense motion our newfound pattern of an older muscovite 40 ar 39 ar age pair in the hanging wall supports arguments for the existence of the bhanuwa fault and suggests normal sense motion

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

an intercalibration study of the fish canyon sanidine and biotite 40ar 39ar standards and some comments on the age of the fish canyon tuff the age of the fish canyon tuff fct has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the k ar and u pb isotopic systems minerals from the fct notably sanidine and biotite are in widespread use as 40ar 39ar fluence monitors standards and have been employed as both primary and intercalibrated standards in attempting to constrain the true age of fct the distinction between primary vs intercalibrated ages is important as only the primary age determinations are valid an investigation of both fish canyon sanidine fcs and fish canyon biotite fct 3 was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain to grain scale electron microprobe results show that fcs is chemically homogeneous verifying previous studies although there are some very small zones of minor k depletion fct 3 biotite commonly contains numerous and small mineral inclusions 40ar 39ar experiments further show that fcs is remarkably homogeneous confirming previous observations whereas fct 3 shows much greater isotopic variability due to contamination from the mineral inclusions by setting the age of fcs equal to 27 98 0 15 ma 2 as intercalibrated to the u pb system by villeneuve et al 2000 the resultant intercalibration factor between fcs and fct 3 is 1 005 0 017 yielding an age of 28 13 0 47 ma 2 for fct 3 although the significant intragrain chemical and isotopic variability precludes its use as an effective 40ar 39ar fluence monitor the true age of the fct is problematic a survey of all primary ages for fct shows that mean k ar ages for plagioclase hornblende biotite and sanidine are consistently younger than mean u pb ages from titanite and zircon in conjunction with recently published petrological results which indicate that the fct contains material from multiple crystallization events this significant discrepancy between the k ar and u pb data sets demonstrates that the fish canyon tuff cannot serve as a golden standard between the k ar and u pb systems

orogenic pulses in the alberta rocky mountains radiometric dating of major faults and comparison with the regional tectono stratigraphic record radiometric ages from regionally distributed thrust fault gouge show that the eastward propagation of the southern rocky mountain fold and thrust belt in canada occurred in four orogenic pulses that correlate with tectonic events of the cordilleran interior and with depositional patterns in the adjacent foreland in the main ranges the pyramid 163 0 ma simpson pass 161 7 ma and johnson creek 145 7 ma thrusts were related to the initiation of thin skinned deformation from jurassic terrane accretion and were partly contemporaneous with development of the first clastic wedge in the foreland basin in the front ranges the emplacement of the greenock thrust 103 1 ma and broadview snake indian thrust 99 2 ma was contemporaneous with development of cenomanian deltaic deposits in the immediate foreland three thrusts in the front ranges rocky pass 74 8 ma sulfur mountain 75 6 ma and clearwater 74 2 ma thrusts define a campanian phase of tectonic loading that led to the last major transgression in the southern portion of the alberta foreland along the eastern margin of the front ranges the mcconnell thrust 54 0 ma together with the muskeg 52 4 ma brule 53 9 ma and nikanassin 52 1 ma thrusts in the foothills recorded the last phase of regional contraction the late jurassic mid cretaceous late cretaceous and early eocene deformation pulses are separated by relatively long periods 40 m y 20 m y and 10 m y of tectonic quiescence these spatially and temporally restricted fault motion pulses contrast with gradual forward fault propagation while regional eastward progression of deformation is preserved

influence of thrust belt geometry and shortening rate on thermochronometer cooling ages insights from thermokinematic and erosion modeling of the bhutan himalaya advancements in thermochronology and numerical modeling offer the potential to associate the age of thermochronometric samples to both exhumational and deformational processes however understanding how these components are related in compressional systems requires linking the geometry and magnitude of fault slip to the distribution and amount of erosion to address this we apply a 2 d thermokinematic model to a forward modeled balanced cross section to quantify the cooling history in fold thrust belt settings the restored cross section provides a kinematic path of rocks and structures necessary to reproduce the surface geology by assigning ages to displacement amounts we produced a range of potential velocity vectors used to calculate heat transport erosion and rock cooling we test the predicted ages against a suite of previously published thermochronometric data from the bhutan himalaya to explore the utility of the data to constrain the timing rate and geometry of fault motion as well as variations in the exhumation rate we evaluate the cooling history associated with a constant rate of shortening of 18 mm yr rates that are 2 0 1 5 0 75 and 0 5 times the constant rate and rates that vary with time to determine which kinematic history best matches the measured cooling ages the combination of relatively old apatite fission track and zircon u th he measured ages and younger 15 9 ma 40ar 39ar ages from white mica is best matched with faster rates relative to constant rates between 11 5 and 8 ma and slower than constant rates from 17 to 11 5 ma and 8 ma to present

rb sr ages of micas from the kathmandu complex central nepalese himalaya implications for the evolution of the main central thrust the kathmandu complex comprising the bhimphedi and phulchauki groups which range in age from precambrian to lower palaeozoic is situated in the frontal part of the main central thrust sheet although it is likely that a tectonothermal event antedated the intrusion of early ordovician granites rb sr dating of muscovites in deformed pegmatites suggests that the high grade metamorphism in the bhimphedi group is of tertiary age these pegmatites cooled through c 500 c at 18 14 ma biotite rb sr data indicate cooling through c 300 c at 7 5 ma thermobarometric and radiometric data from central nepal suggest an evolutionary model for the emplacement history of the main central thrust sheet involving cooling from a peak temperature c 660 c to c 500 c in 7 ma and loss by erosion or tectonic denudation of c 4 11 km of section

identification and significance of 3β ethyl steranes in sediments and petroleum abstract 3 ethyl steranes 5a c occur in ancient sediments and petroleum particularly those of paleozoic and proterozoic age this newly recognised modification to the sterane ring system has no known direct counterpart in the steroid metabolism of extant organisms when they are present in a sample the pattern of side chain substitution and some aspects of the isomer distributions of co occurring 3 ethyl steranes 5a c 3 methyl steranes 3a c 2 methyl steranes 2a c and desmethyl steranes 1a c are similar this suggests that these ring a alkylated steranes are not synthesised de novo by their source organism s but arise via microbial modifications to sedimentary sterenes during the early stages of diagenesis

argon behaviour in an inverted barrovian sequence sikkim himalaya the consequences of temperature and timescale on 40ar 39ar mica geochronology abstract 40 ar 39 ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle single grain fusion and step heating data were obtained for rocks sampled through a major thrust sense shear zone the main central thrust and the associated inverted metamorphic zone in the sikkim region of the eastern himalaya this transect provides a natural laboratory to explore factors influencing apparent 40 ar 39 ar ages in similar lithologies at a variety of metamorphic pressure and temperature p t conditions the 40 ar 39 ar dataset records progressively younger apparent age populations and a decrease in within sample dispersion with increasing temperature through the sequence the white mica populations span 2 9 ma within each sample in the structurally lower levels garnet grade but only 0 3 ma at structurally higher levels kyanite sillimanite grade mean white mica single grain fusion population ages vary from 16 2 3 9 ma 2 to 13 2 1 3 ma 2 from lowest to highest levels white mica step heating data from the same samples yields plateau ages from 14 27 0 13 ma to 12 96 0 05 ma biotite yield older apparent age populations with mean single grain fusion dates varying from 74 7 11 8 ma 2 at the lowest structural levels to 18 6 4 7 ma 2 at the highest structural levels the step heating plateaux are commonly disturbed temperatures 600 c at pressures of 0 4 0 8 gpa sustained over 5 ma appear to be required for white mica and biotite ages to be consistent with diffusive open system cooling at lower temperatures and or over shorter metamorphic timescales more 40 ar is retained than results from simple diffusion models suggest diffusion modelling of ar in white mica from the highest structural levels suggests that the high temperature rocks cooled at a rate of 50 80 c ma 1 consistent with rapid thrusting extrusion and exhumation along the main central thrust during the mid miocene

fault fracture mesh petroleum plays in the zhanhua depression bohai bay basin part 2 oil source correlation and secondary migration mechanisms abstract the chemical compositions of crude oils from over 20 oilfields in the zhanhua depression bohai bay basin were studied in order to understand their genetic relationships thermal maturity and biodegradation ranking molecular geochemical parameters indicate the presence of three possible oil families in reservoirs ranging from paleozoic basement to neogene a number of biomarker features support deposition of the source rocks for these oils under a wide range of depositional environments the majority of the oils discovered in the depression belong to the fl family and display characteristics consistent with a dominantly freshwater to slightly brackish water lacustrine source rock in the es 3 member of the shahejie formation the somewhat geographically restricted sl family oils are apparently linked to saline lacustrine carbonate evaporite source rocks distributed mostly in the upper es 4 member of the shahejie formation from the bonan gubei sag areas the thermally less mature bl family oils occur mostly within the es 1 es 2 or dongying formation reservoirs near the central sag areas and show close affinities to the interbedded or intercalating es 1 calcareous mudstones and oil shales deposited in a slightly saline to dominantly brackish water lacustrine setting the sl and fl family oils are generally mature whereas most of the bl family oils usually show low maturity oil source correlation demonstrates strong genetic relationships between the mature es 4 and es 3 source rocks of the eocene oligocene shahejie formation in the sag areas and the oils accumulated in the shallow neogene petroleum plays the geochemical data provide strong support for two sharply contrasting hydrocarbon migration scenarios 1 short distance lateral migration is dominant in the central sag areas and 2 the fault fracture mesh facilitated vertical migration and coupled with the regional porous guantao formation sand sheets makes the structural stratigraphic and lithological features over basement high blocks the focal points for petroleum accumulation therefore further exploration in this highly mature petroleum province should focus on heavy oils in the shallow neogene strata and light oils in the tight sandstones of the deep sag areas

dinoflagellate cysts from the upper campanian upper cretaceous of hornby island british columbia canada with implications for nanaimo group biostratigraphy and paleoenvironmental reconstructions abstract thirty mudstone samples from coastal exposures of the northumberland formation on hornby island british columbia canada yielded diverse dinoflagellate cyst and terrestrial sporomorph assemblages a late campanian age for the formation has been well defined through magnetostratigraphy macrofossil biostratigraphy and geochemical studies of the hornby island section dinoflagellate cyst taxa corresponding to 61 genera are exceptionally well preserved and include specimens referable to at least 68 formally established species the earliest occurrences of canninginopsis maastrichtiensis druggidium cf discretum phanerodinium belgicum renidinium cf vitilaire senegalinium simplex spiniferella cornuta and xenicodinium delicatum sensu slimani et al are reported along with the most recent occurrence of senoniasphaera protrusa extending their stratigraphic ranges into the upper campanian the palynomorph assemblages support a late campanian age two dinoflagellate cyst ecozones are recognized ecozone 1 and ecozone 2 assemblages suggest sediment deposition in an inner shelf environment with moderate to high nutrient input and primary productivity

investigation of hydrocarbon biodegradation from a downhole profile in bohai bay basin implications for the origin of 25 norhopanes abstract a suite of reservoir cores oil sands from a single well in bohai bay basin east china displayed a progressive increase in petroleum biodegradation extent on the basis of bulk composition and 25 norhopane content this fits with the proposal that subsurface petroleum biodegradation is dominantly an anaerobic process and usually occurs at the oil water contact it is likely that sequential microbial degradation of hydrocarbons under anoxic conditions does not occur in a true stepwise fashion but is controlled by various factors such as concentration and solubility of hydrocarbons and their diffusion rate to the oil water contact in fact 25 norhopanes were formed prior to the complete elimination of the acyclic and mono and bicyclic alkanes an inverse response of the 22 s 22 s 22 r ratio between each extended 17 h hopane and its corresponding 25 norhopane was observed as severe biodegradation occurred supporting the proposal that the 25 norhopanes originate from demethylation of hopanes field observation revealed that biomarkers without extended alkyl side chains such as oleanane gammacerane and carotane have significant resistance to biodegradation and can be used as naturally occurring internal standards to evaluate variations in other biomarkers the results suggest that the quantity of 25 norhopanes showed a minor increase as the hopanes decreased significantly i e only partial hopane conversion to the corresponding 25 norhopanes alternative degradation pathways for hopanes might occur in reservoirs in addition to c 25 demethylation

a new method for attributing climate variations over the atlantic hurricane basin s main development region 1 we propose a new approach to decompose observed climate variations over the atlantic hurricane basin s main development region mdr into components attributable to radiative forcing changes and to internal oceanic variability our attribution suggests that the observed multidecadal anomalies of vertical shear uz and a simple index of maximum potential intensity simpi for tropical cyclones are both dominated by internal variability consistent with multidecadal variations of atlantic hurricane activity changes in radiative forcing led to increasing uz and decreasing simpi since the late 50 s unfavorable for atlantic hurricane activity physically at least for the gfdl model sea surface temperature sst anomalies induced by ocean heat transport variations are more efficient in producing negative uz anomalies than that induced by altered radiative forcing

tropical sea surface temperature vertical wind shear and hurricane development 1 the anomalously strong hurricane activity in the atlantic sector during the recent years led to a controversy about the impact of global warming on hurricane activity in the atlantic sector here we show that the temperature difference between the tropical north atlantic and the tropical indian and pacific oceans indo pacific is a key parameter in controlling the vertical wind shear over the atlantic an important quantity for hurricane activity the stronger warming of the tropical north atlantic relative to that of the indo pacific during the most recent years drove reduced vertical wind shear over the atlantic and is thus responsible for the strong hurricane activity observed in 2006 however the temperature difference between the tropical north atlantic and the tropical indian and pacific oceans is much reduced which explains the relatively weak hurricane season

petroleum migration and accumulation in the bozhong sub basin bohai bay basin china significance of preferential petroleum migration pathways ppmp for the formation of large oilfields in lacustrine fault basins abstract this paper discusses migration styles and primary controls of petroleum migration pathways in heterogeneous carrier beds in bozhong sub basin bohai bay basin modeling of secondary petroleum migration pathways has been conducted using a simple three dimensional model which assumes that the positions of petroleum migration pathways are controlled by the morphology of the sealing surfaces the modeling results have accurately predicted the petroleum occurrences and have been supported by petroleum geochemical studies most commercial petroleum accumulations are on the predicted preferential petroleum migration pathways ppmp formed by focusing of numerous small petroleum streams close to the kitchens and most large fields petroleum reserves greater than 1 10 8 ton have more than one preferential petroleum migration pathway to convey petroleum to the traps the focusing of petroleum originated from a large area of the generative kitchens into restricted channels is essential for the formation of large oilfields the strong porosity and permeability heterogeneities of the carrier beds and the relatively high prediction accuracy by a model that does not take into consideration of the effect of heterogeneity suggest that the positions of petroleum migration pathways in heterogeneous carrier beds in the lacustrine fault basins are determined primarily by the morphology of the sealing surfaces at regional scales

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

new insight into the south tibetan detachment system not a single progressive deformation 1 low angle normal faults lanf typically regarded as accommodating crustal or lithospheric extension may also form during lithospheric shortening the best studied system of syn contractional lanfs is the south tibetan detachment system a network of low angle normal sense faults and shear zones that formed coevally with and parallel to south vergent thrusts during lithospheric shortening accompanying development of the himalayan orogen in the eastern himalaya there are several across strike exposures of the south tibetan detachment system we present new structural and thermometry data from the eastern himalaya that demonstrate that the south tibetan detachment system cannot have formed as a single progressive structure we characterize and distinguish two distinct structural and tectonic components within the currently recognized system 1 an extensive diffuse sheared layer that formed the boundary between strong upper crust and weak southward flowing middle crust and 2 a network of brittle ductile lanfs that locally exhume partly excise and overprint the earlier mylonite zone at the topographic break between the himalayan orogen and the tibetan plateau the sheared layer not a lanf formed the boundary between upper and middle crust during ductile flow of the middle crust and is extensively exposed in the himalaya at the base of klippen of upper crustal rocks preserved in bhutan along the crest of the himalaya where it has been excised and exhumed by the brittle ductile extrusion lanfs and bounding the cores of the north himalayan gneiss domes citation kellett d a and d grujic 2012 new insight into the south tibetan detachment system not a single progressive deformation tectonics 31 tc2007 doi 10 1029 2011tc002957

application of 24 norcholestanes for constraining source age of petroleum abstract two c26 sterane ratios the 24 nordiacholestane ratio ndr and the 24 norcholestane ratio ncr are useful for assessing petroleum source age values for these parameters generally increase in the cretaceous when diatoms were rapidly radiating and further in the oligo miocene due to widespread high latitude deposition of diatomaceous sediments sources with shale lithologies tend to exhibit the highest ratios while carbonate and terrigenous rich lithologies generally show lower ratios on the north slope of alaska elevated ratios are observed for cretaceous hrz oils and mixed oils containing cretaceous input compared to older triassic and jurassic derived oils tertiary sourced oils from romania and bulgaria have much higher ratios relative to jurassic sourced oils in the black sea region oleanane was observed in comparable levels in some black sea tertiary and jurassic oils and source rocks ndr and ncr coupled with other age diagnostic indicators such as oleanane and dinosterane provide powerful tools for differentiating age of source and organic facies for oils from the black sea and sub andean basins of peru and ecuador respectively oils derived from sources deposited in low tropical latitudes tend to have little diatom input and as a consequence do not display high values regardless of age

extended 3β alkyl steranes and 3 alkyl triaromatic steroids in crude oils and rock extracts abstract in oils and precambrian to miocene age source rocks from varying depositional environments we have conclusively identified several novel 3 alkyl sterane and triaromatic steroid series including 1 3 n pentyl steranes 2 3 isopentyl steranes 3 3 n hexyl steranes 4 3 n heptyl steranes 5 3 4 dimethyl steranes 6 3 butyl 4 methyl steranes 7 triaromatic 3 n pentyl steroids and 8 triaromatic 3 isopentyl steroids we have also tentatively identified additional homologs with 3 alkyl substituents as large as c11 the relative abundances of these compounds vary substantially between samples as indicated by 1 the ratio of 3 n pentyl steranes to 3 isopentyl steranes and 2 the ratio of 3 n pentyl triaromatic steroids to 3 isopentyl triaromatic steroids these data suggest possible utility for these parameters as tools for oil source rock correlations and reconstruction of depositional environments although no 3 alkyl steroid natural products are currently known several lines of evidence suggest that 3 alkyl steroids result from bacterial side chain additions to diagenetic 2 sterenes

chemostratigraphic reconstruction of biofacies molecular evidence linking cyst forming dinoflagellates with pre triassic ancestors new data from numerous detailed mass spectrometric studies have detected triaromatic dinosteroids in precambrian to cenozoic rock samples triaromatic dinosteroids are organic geochemicals derived from dinosterols compounds known in modern organisms to be the nearly exclusive widely occurring products of dinoflagellates we observed the ubiquitous occurrence of these dinosteroids in 49 late triassic through cretaceous marine source rocks and the absence of them in 13 permian carboniferous source rocks synergistic with the dinoflagellate cyst record however finding dinosteroids in lower paleozoic and precambrian strata presents challenging results for molecular paleontologists evolutionary biologists palynologists and especially for those concerned with the food web at various times of biological crisis other than the few species known as parasites and symbionts many other dinoflagellate species are important as primary producers the presence of precambrian to devonian triaromatic dinosteroids gives chemostratigraphic evidence of dinoflagellates or other organisms with similar chemosynthetic capabilities in rocks significantly older than the oldest undisputed dinoflagellate fossils dinoflagellate cysts from the middle triassic 240 ma and older than the putative silurian 420 ma dinocyst arpylorus antiquus calandra sargent from tunisia this systematic chemostratigraphic approach can shed light not only on lineages of dinoflagellates and their precursors but potentially on many other lineages especially bacteria algae plants and possibly some metazoans

pregnanes as molecular indicators for depositional environments of sediments and petroleum source rocks steroids with unconventional side chains have increasingly been applied as diagnostic markers for geological source and age assessments however one of the most distinctive characteristics the abnormal abundance of pregnane and homopregnane in ancient sediments and petroleum remains unresolved higher pregnane and homopregnane as well as c23 c26 20 n alkylpregnanes relative to the regular steranes were observed in samples collected from different petroleum basins in china these included precambrian marine carbonate derived petroleum nw sichuan basin lower paleozoic marine marl derived crude oils tarim basin and eocene hypersaline lacustrine carbonate source rocks and associated petroleum bohai bay basin however all of the samples have many common biomarker characteristics such as pristane phytane ratios 1 low amounts of diasteranes and high c29 c30 hopane 0 6 1 c35 c34 hopane mostly p 1 and dibenzothiophene phenanthrene dbt phen mostly 0 5 1 ratios revealing a contribution from anoxic carbonate marl source rocks deposited in restricted clastic starved settings we suggest that 5a l4b l7b pregnane and homopregnane as well as their higher c23 c26 homologues are geological products derived from steroids bound to the kerogen by a sulfurized side chain carbon or carbonate minerals are considered to be natural catalysts for this cracking reaction via preferential cleavage of the bond between c 20 and c 22 similar distributions occur in the short chain analogues of 4 methylsterane triaromatic steroid and methyltriaromatic steroid hydrocarbons providing circumstantial evidence for this proposal the ratio of pregnane and homopregnane to the total regular steranes and the ratio of c27 diasteranes to cholestanes can be sensitive indicators of sedimentary environments and facies in general high diasteranes and low pregnanes with homologues indicate an oxic water column or significant input of terrigenous organic matter in clay rich source rocks and some organic lean carbonate rocks low diasteranes with high pregnanes implies restricted sulfur rich conditions typical of anoxic carbonate source rocks furthermore the two ratios may be useful to assess the variation of mineralogy and openness of source rock depositional settings

dinoflagellate origin for sedimentary 4α methylsteroids and 5α h stanols biological marker compounds provide useful tools for evaluating the depositional environments of recent and ancient sediments and petroleums1 largely through established relationships between the organic matter of sediments and source organisms1 2 such relationships are best tested and explored by investigating the lipid components of natural populations of autochthonous biota that can be clearly recognized as contributors to underlying bottom sediments in this context we have studied the lipids of the freshwater dinoflagellate peridinlum lomnickii woloszynska order peridiniales class dinophyceae collected from the waters of priest pot3 a eutrophic lake in the english lake district its distributions of both 4 methylsterols and 4 methylstanones closely resemble those of the underlying bottom sediments demonstrating that dinoflagellates are important contributors of these sedimentary compounds the 4 methylsteroidal hydrocarbons found in ancient sediments and petroleums4 6 are presumably diagenetic products of such 4 methylsteroids and therefore reflect dino flagellate inputs to the original depositional environments furthermore the prominence of 5 h cholestan 3 ol in p lomnickii suggests that dinoflagellates may be the long sought direct biological source of sedimentary 5 h stanols

paleozoic oil source rock correlations in the tarim basin nw china we studied a suite of 40 oils and extracts of purported source rocks from the tarim basin in nw china the main group of oils comes from tazhong and tabei wells which sample the largest known petroleum accumulations in the basin these oils can be statistically correlated with extracts of ordovician rocks based upon high relative concentrations of 24 isopropylcholestanes and low relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes in contrast extracts from cambrian rocks have low relative concentrations of 24 isopropylcholestanes with high relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes although some tarim basin cambrian rocks yield high total organic carbon contents we see little evidence in the analyzed oil samples to suggest that they came from cambrian source rocks

seismotectonics of the nepal himalaya from a local seismic network the national seismological network of nepal consists of 17 short period seismic stations operated since 1994 it provides an exceptional view of the microseismic activity over nearly one third of the himalayan arc including the only segment between longitudes 78 e and 85 e that has not produced any m 8 earthquakes over the last century it shows a belt of seismicity that follows approximately the front of the higher himalaya with most of the seismic moment being released at depths between 10 and 20 km this belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the higher himalaya the seismic activity is more intense around 82 e in far western nepal and around 87 e in eastern nepal western nepal between 82 5 and 85 e is characterized by a particularly low level of seismic activity we propose that these lateral variations are related to segmentation of the main himalayan thrust fault the major junctions between the different segments would thus lie at about 87 e and 82 e with possibly an intermediate one at about 85 e these junctions seem to coincide with some of the active normal faults in southern tibet lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long lived feature we infer four 250 400 km long segments that could produce earthquakes comparable to the m 8 4 bihar nepal earthquake that struck eastern nepal in 1934 assuming the model of the characteristic earthquake the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years

monazite geochronology unravels the timing of crustal thickening in nw himalaya abstract greenschist to amphibolite grade haimanta metasediments of the nw himalaya preserve much of the prograde metamorphic history of eohimalayan crustal thickening which has been erased by oligo miocene migmatization elsewhere in the himalaya our zircon and monazite u th pb data unravel a multi stage prograde metamorphic evolution the earliest evidence of prograde barrovian metamorphic monazite growth is 41 ma peak metamorphic conditions 8 8 5 kbar 600 700 c were attained at 37 36 ma and followed by a prolonged evolution at high temperatures with at least three distinct episodes of monazite growth which may be related to the formation of the northern himalayan nappes e g shikar beh nappe nyimaling nappe rapid exhumation of the crystalline started at 26 ma and resulted in cooling through the muscovite 40 ar 39 ar closure temperature by 21 8 ma although a local continuation of the south tibetan detachment is not unambiguously identified in central himachal pradesh extrusion was likely facilitated by a system of several minor late oligocene early miocene top to the n to ne shear zones in contrast to the crystalline of zanskar and eastern himachal pradesh extrusion was not accompanied by widespread decompression melting

probing the depths of the india asia collision u th pb monazite chronology of granulites from nw bhutan 1 rocks metamorphosed to high temperatures and or high pressures are rare across the himalayan orogen where peak metamorphic conditions recorded in the exposed metamorphic core the greater himalayan sequence ghs are generally at middle to upper amphibolite facies however mafic garnet clinopyroxene assemblages exposed at the highest structural levels in bhutan eastern himalaya preserve patchy textural evidence for early eclogite facies conditions overprinted by granulite facies conditions monazite hosted within the leucosome of neighboring granulite facies orthopyroxene bearing felsic gneiss yields la mc icp ms u th pb ages of 13 9 0 3 ma monazite associated with sillimanite grade metamorphism in granulite hosting migmatitic gneisses yields u th pb rim ages between 15 4 0 8 ma and 13 4 0 5 ma monazite associated with sillimanite grade metamorphism in gneiss at structurally lower levels yields u pb rim ages of 21 17 ma these data are consistent with miocene exhumation of ghs material from a variety of crustal depths at different times along the himalayan orogen we propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh pressure eclogites in the western himalaya their formation and exhumation in the miocene therefore do not require diachroneity in the timing of the initial india asia collision

pliocene episodic exhumation and the significance of the munsiari thrust in the northwestern himalaya abstract the himalayan thrust belt comprises three in sequence foreland propagating orogen scale faults the main central thrust the main boundary thrust and the main frontal thrust recently the munsiari ramgarh shumar thrust system has been recognized as an additional potentially orogen scale shear zone in the proximal footwall of the main central thrust the timing of the munsiari ramgarh and shumar thrusts and their role in himalayan tectonics are disputed we present 31 new zircon u th he ages from a profile across the central himachal himalaya in the beas river area within a 40 km wide belt northeast of the kullu larji rampur window ages ranging from 2 4 0 4 ma to 5 4 0 9 ma constrain a distinct episode of rapid pliocene to present exhumation north and south of this belt zircon u th he ages are older 7 0 0 7 ma to 42 2 2 1 ma we attribute the pliocene rapid exhumation episode to basal accretion to the himalayan thrust belt and duplex formation in the lesser himalayan sequence including initiation of the munsiari thrust pecube thermokinematic modelling suggests exhumation rates of 2 3 mm yr from 4 7 to 0 ma above the duplex contrasting with lower

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

sulfur rich petroleum derived from lacustrine carbonate source rocks in bohai bay basin east china abstract crude oils produced from the zhanhua depression bohai bay basin are high in sulfur and differ from most lacustrine oils in this basin and elsewhere physical properties biomarker ratios and aromatic hydrocarbon assemblages reveal that they are derived from carbonate source rock horizons representative source rock samples with abundant organic matter toc 1 and s 2 5 mg hc g rock were selected for further correlation and their high bitumen contents indicated their indigenousness to saline or carbonate depositional environments the results of oil source rock correlation suggest that these sulfur rich heavy oils originate from es 3 4 carbonates and marls limited to the bonan sag maturity assessments with burial history showed that they were expelled at the early oil generation window high levels of 20s 5 14 17 and 5 14 17 steranes together with low values for the ts ts tm ratio and two methylated aromatic ratios mdr and mpi 1 were observed in these oils sulfur associated with evaporite and carbonate environments is a significant factor not only in controlling the occurrence of biomarker isomers but also for petroleum formation at abnormally low thermal stress levels

a double cycle lake basin formed in extensional to transtensional setting the paleogene nanpu sag bohai bay basin china abstract it has been known that both extensional and transtensional tectonics commonly trigger a one cycle evolution of lake sediments but lake cycle development co controlled by extensional and transtensional tectonics still need identification here we report a double cycle of lake sediments formed in extensional to transtensional phases in the paleogene nanpu sag of the bohai bay basin china the sag successively experienced five phases of lake type evolution characterized by 1 overfilled 2 balanced fill 3 overfilled 4 balanced fill and 5 overfilled extensional tectonics was responsible for the opening of the basin and the initial creation of accommodation 1st through 3rd phase next subsidence increased again through transtensional tectonics resulting in the creation of new accommodation 4th 5th phases investigations show this double cycle lake type evolution is also present in other lake basins with similar tectonic settings e g both extensional and transtensional tectonics a different exploration and exploitation strategy should be devoted to the double cycle evolution of lake basins controlled by extensional to transtensional tectonics in comparison to the single cycle evolution in extensional or transtensional lake basins

the kumaun and garwhal lesser himalaya india part 2 thermal and deformation histories an integrated petrologic thermochronologic and numerical modeling study constrains the thermal evolution of the lesser himalayan sequence in the kumaun and garwhal regions of india south of the tons thrust peak metamorphic temperatures do not exceed 330 c on the northern side a transition toward higher temperatures adjacent to the main central thrust hanging wall is documented in the immediate footwall of the main central thrust peak temperatures average 550 c an inverted thermal field gradient of 30 c km beneath the vaikrita thrust is calculated an inverted thermal field gradient of 20 c km is also documented beneath a klippe of the main central thrust hanging wall 25 km south of the vaikrita thrust and a peak footwall temperature of 530 c is recorded close to the shear zone at the base of the klippe the mechanism for thermal metamorphism in the lesser himalayan sequence is interpreted to be conduction of heat from the main central thrust fault and or hanging wall to the footwall a localized thermal field gradient is also associated with the tons thrust the 40 ar 39 ar thermochronology reveals that the exposed rock in the kumaun and garwhal lesser himalaya underwent cooling below the closure temperature tc for white mica at different times in the late tertiary the proximal footwall of the munsiari thrust experienced cooling below the white mica tc at ca 8 5 ma south of the munsiari thrust in the proximal footwalls of the chaukori and askot klippen muscovites ages are ca 11 5 ma 3 ma older than samples to the north the age differences in samples from northern and southern exposures are interpreted to document cooling related to thermal relaxation following passage of the main central thrust hanging wall atop the lesser himalayan sequence results of thermal modeling achieve good fits to the data when scenarios involve an early miocene phase of overthrusting of a hot hanging wall over a downgoing footwall followed by the initiation of a duplex within lesser himalayan sequence rocks

midcrustal discontinuities and the assembly of the himalayan midcrust detailed quartz lattice preferred orientation lpo data define two structural discontinuities in the exhumed high grade metamorphic core of the himalaya exposed in the upper tama kosi region of east central nepal the structures are marked by abrupt breaks in a general trend of up structural section increasing quartz lpo defined deformation temperatures deformation associated with the upper structural discontinuity which occurs within sillimanite grade rocks is postpeak metamorphism in both the hanging wall and the footwall new geochronologic data constrain the timing of metamorphism in the hanging wall of the upper discontinuity to between 24 and 16 ma indistinguishable from previously published ages for the footwall movement across this structure represents early miocene strain localization and thickening in the himalayan midcrust movement across the lower discontinuity which occurs between staurolite and kyanite grade rocks appears to be synmetamorphic with material in its footwall at approximately 10 ma but postpeak metamorphism for material in its hanging wall this movement is interpreted to reflect the underplating and incorporation of material into the metamorphic core the recognition of two thrust sense discontinuities in the exhumed himalayan core in the tama kosi region is consistent with other similar structures recognized along the himalaya the widespread nature of these structures reinforces that they are important to our understanding of the evolution of the kinematics of large hot orogens

high temperatures in the late cretaceous arctic ocean to understand the climate dynamics of the warm equable greenhouse world of the late cretaceous period it is important to determine polar palaeotemperatures the early palaeoceanographic history of the arctic ocean has however remained largely unknown because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice a shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated upper cretaceous organic rich sediment from alpha ridge a submarine elevated feature of probable oceanic origin a lack of carbonate in the sediments from this core has prevented the use of traditional oxygen isotope palaeothermometry here we determine arctic palaeotemperatures from these upper cretaceous deposits using tex86 a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton crenarchaeota from these analyses we infer an average sea surface temperature of approximately 15 degrees c for the arctic ocean about 70 million years ago this calibration point implies an equator to pole gradient in sea surface temperatures of approximately 15 degrees c during this interval and by extrapolation we suggest that polar waters were generally warmer than 20 degrees c during the middle cretaceous approximately 90 million years ago

a short duration pulse of ductile normal shear on the outer south tibetan detachment in bhutan alternating channel flow and critical taper mechanics of the eastern himalaya in easternmost bhutan the south tibetan detachment std is a ductile shear zone that juxtaposes the radi or sakteng klippe of the tethyan sedimentary series from underlying high grade greater himalayan rocks in situ la icpms u th pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the std within the shear zone at the base of the klippe constrains north vergent normal shear to between 25 and 20 ma coeval thrusting on the main central thrust during this time supports a phase of channel flow viscous wedge model activity lasting only ca 3 ma geochronologic data from the eastern himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the late oligocene through to the late miocene switching from channel flow viscous wedge behavior to critical taper frictional wedge behavior each phase lasting approximately only 2 to 5 ma the tectonic evolution of the eastern himalaya is comparable to central and western himalayan tectonics during the early miocene but during the middle miocene metamorphism and magmatism in the eastern himalaya migrated towards the orogenic hinterland a process not widely documented elsewhere in the himalaya thus highlighting the need for an orogenic model in three spatial dimensions

secular and environmental constraints on the occurrence of dinosterane in sediments abstract the distribution patterns of sedimentary a ring methylated steranes have changed markedly over geological time although dinosterane and its isomer 24 ethyl 4 methylcholestane have been tentatively identified in three proterozoic rock units they are either not detectable or occur in low abundance relative to 3 methyl steranes throughout most of the palaeozoic between permian and middle triassic times 260 220 ma ago 4 methyl sterane abundances in marine sediments increased markedly the presence of dinosterane in some middle triassic marine sediments is contemporaneous with the appearance of fossil cysts of uncontested dinoflagellate affinity 4 methyl steranes including dinosterane or their precursor sterenes and sterols then show a continuous presence often in high abundance in marine sediments from the late triassic through to the present day assemblages of 4 methyl steranes and their precursors but with dinosterane absent or in low relative abundance are often the predominant steroids in lacustrine sediments in the cainozoic dinosterane appears to arise predominantly from marine dinoflagellates and as a consequence is a useful biological marker for mesozoic and cainozoic marine organic matter the isomer 24 ethyl 4 methylcholestane is likely to have multiple origins although its very high abundance in tertiary lacustrine sediments and oils compared to older materials suggests that dinoflagellates could also be the source in these cases

the distribution of molecular fossils derived from dinoflagellates in paleogene lacustrine sediments bohai bay basin china steroids with unconventional side chains in ancient sediments and petroleum have been increasingly used as age diagnostic markers triaromatic dinosteroids and dinosteranes occur widely throughout the eocene oligocene shahejie formation in the southeastern bohai bay basin china consistent with its abundant fossil dinoflagellate assemblages in addition 24 norcholestane biomarkers which commonly appear in marine oils have been found in this lacustrine formation although a firm link has not been established between 24 norcholestane and specific modern taxa both diatoms and dinoflagellates are shown to be likely sources of 24 norcholestanes although the shahejie strata lack intact diatom frustules they do exhibit a good relationship between 24 norcholestane other molecular fossils of dinoflagellates and dinocysts dinoflagellates are important precursor taxa of 24 norcholestanes and the various species identified in eocene oligocene lacustrine shales and mudstones of the shahejie formation are all potential sources of the 24 norcholestanes

reconciling himalayan midcrustal discontinuities the main central thrust system the occurrence of thrust sense tectonometamorphic discontinuities within the exhumed himalayan metamorphic core can be explained as part of the main central thrust system this imbricate thrust structure which significantly thickened the orogenic midcrustal core comprises a series of thrust sense faults that all merge into a single detachment the existence of these various structures and their potential for complex overprinting along the main detachment may help explain the contention surrounding the definition mapping and interpretation of the main central thrust the unique evolution of specific segments of the main central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position and structural level of exposure of the mid crust this helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt

a biomarker study of immature crude oils from the shengli oilfield people s republic of china abstract a suite of crude oils from the shengli oilfield in northwestern china was subjected to detailed chemical analysis crude oils in this suite believed to be derived from source rocks deposited under fresh brackish and saline environments have chemical compositions and biological marker compounds indicative of immature source rocks in this respect the crude oils are unusual apparently having been formed and expelled from source rocks that have been subjected only to mild thermal stress prior to the main zone of petroleum generation

tectonometamorphic discontinuities within the greater himalayan sequence in western nepal central himalaya insights on the exhumation of crystalline rocks abstract the core of the greater himalayan sequence in the mugu karnali area western nepal is affected by a thick shear zone with development of nearly 4 km of mylonites mangri shear zone it is a contractional shear zone showing a top to the sw and wsw sense of shear the shear zone developed during the decompression in the sillimanite stability field of rocks that previously underwent relatively high pressure metamorphism deformed under the kyanite stability field p t conditions indicate that the footwall experienced higher pressure 1 0 0 9 gpa than the hanging wall 0 7 gpa and similar temperatures 675 700 c u pb in situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 ma samples from the lower part of the greater himalayan sequence underwent similar ductile shearing at 17 13 ma these ages and the associated p t t paths revealed that peak metamorphic conditions were reached 5 7 ma later in the footwall of the shear zone with respect to the hanging wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself mangri shear zone with the other recently documented tectonic and metamorphic discontinuities within the greater himalayan sequence point to the occurrence of a regional tectonic feature the high himalayan discontinuity running for more than 500 km along the strike of the central himalayas it was responsible of the exhumation of the upper part of the greater himalayan sequence starting from 28 ma well before the activation of the main central thrust and the south tibetan detachment our data point out that exhumation of the greater himalayan sequence was partitioned in space and time and different slices were exhumed in different times starting from the older in the upper part to the younger in the lower one

biomarker geochemistry of crude oils and lower paleozoic source rocks in the tarim basin western china an oil source rock correlation study abstract in order to select biomarker parameters that have potential to quantitatively distinguish between the cambrian lower ordovician and upper ordovician source rocks in the tarim basin with a total organic carbon content 0 5 42 lower paleozoic shale samples were collected from a number of wells and outcrops in the tarim basin northwestern china the biomarkers in the extracts of the source rocks were analyzed by gas chromatography mass spectrometry the results show that c23 and c30 were dominant in the tricyclic terpanes and pentacyclic triterpanes respectively and c27 and c29 were both dominant in c27 29 steranes of the source rock samples the distribution of n alkanes isoprenoids and terpanes indicate that the source rock samples have been slightly to moderately biodegraded and they originated from a reducing depositional environment with considerable marine algal input in comparatively upper ordovician source rock samples presented lower values of c28 c29 regular steranes and higher values of c29 20s 20s 20r and c29 abnormally lower values of c29 20s 20s 20r and c29 were observed in the cambrian lower ordovician strata the set of data in this study show that homohopane index including c31homohopane c30hopane c31 35homohopanes c30hopane and the ratios related to the triaromatic steroids including c26 20s c28 20s tas c27 20r c28 20r c26 20r c27 20s c28 20s and c28 c26 c27 tas could serve as potential biomarker parameters to distinguish between the two groups of source rocks in the tarim basin the values of those parameters in above order in cambrian lower ordovician source rocks were 0 3 0 5 0 85 and 0 8 1 9 1 2 respectively the correlation between the source rock samples and 19 oils obtained from the tadong tabei and tazhong uplifts in the tarim basin show that most of the oils in tabei uplift are probably from upper ordovician source rocks the oils in tadong uplift are more likely from cambrian lower ordovician source rocks and the tazhong uplift is more like a mixed oil region with oils from both of the two groups of source rocks

40ar 39ar age constraints on deformation and metamorphism in the main central thrust zone and tibetan slab eastern nepal himalaya in eastern nepal structural and petrologic observations suggest that movement on the main central thrust mct the development of an inverted metamorphic sequence and leucogranite plutonism were genetically related samples from across the mct zone and its hanging wall the tibetan slab were collected from four locations in the everest region for 40ar 39ar studies 1 the mct zone 2 the lower tibetan slab 3 the upper tibetan slab away from intrusive rocks and 4 the upper tibetan slab adjacent to and including leucogranitic material within the mct zone muscovite and hornblende yield cooling ages of 12 0 0 2 ma and 20 9 0 2 ma respectively k feldspar gives a minimum age of 8 0 0 2 ma closure temperature tc 220 15 c in the lower tibetan slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7 7 0 4 ma and 7 5 0 6 ma and a k feldspar minimum age of 6 4 0 8 ma tc 225 20 c an adjacent gneiss yields a 9 1 0 2 ma biotite isochron and a k feldspar minimum age of 3 6 0 2 ma tc 210 50 c in the upper tibetan slab samples collected 200 m from visible intrusives yield a biotite isochron age of 20 2 0 2 ma and a complex hornblende age spectrum suggestive of excess argon at the fourth location an augen gneiss a pegmatite and a tourmaline bearing leucogranite all in mutual contact yield indistinguishable mineral ages the average biotite and muscovite isochron ages for these samples are 17 0 1 4 and 16 6 0 2 ma respectively the minimum age for k feldspar from the leucogranite is 15 5 1 8 ma as thermobarometric data from the mct zone in this area suggest synmetamorphic deformation at temperatures of 500 550 c the mct hornblende tc 500 c dates this event at 21 ma diffusion experiments on hornblendes from the mct zone provide data which support a maximum duration of peak metamorphic temperatures of 2 ma biotite ages as old as 20 ma from the upper tibetan slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the mct zone very young ages in a ductile shear zone in the lower tibetan slab suggest that there has been deformation within the tibetan slab that postdates major movement on the mct

the geneses of sedimentary organic matter with anomalous 13c enriched isotopic composition in saline and freshwater lakes a case study of lacustrine source rocks from dongpu and qikou sags bohai bay basin eastern china abstract the carbon isotopic composition of sedimentary organic matter 13com is notably important for the genetic identification of source rock and its products the sapropelic organic matter mostly plankton with high hydrogen index is generally depleted in 13c as compared to humic organic matter mainly higher plants with lower hydrogen index however some lacustrine sediments contain sapropelic source rocks with anomalous 13c enriched isotopic ratios especially reported in freshwater lakes the purpose of this study is to discuss the geneses of lacustrine sedimentary organic matter characterized by anomalous 13c enriched ratios in saline and freshwater sediments to this end we integrate the detailed organic geochemical data including rock eval analysis data total organic carbon contents total organic carbon isotopic compositions biomarkers and trace element concentrations of 64 saline lake sediments and 39 freshwater lake sediments obtained from the paleogene shahejie formation of dongpu sag and qikou sag separately in bohai bay basin of eastern china both sags exist samples with high hydrogen index values hi 400 mg g and high total organic carbon contents toc 2 but anomalous 13c enriched isotopic ratios up to 21 1 in qikou sag the 13com values of sapropelic kerogen correlate positively with toc contents the anomalous enrichment of 13c in organic matter is associated with extremely high productivity of algae such as algae bloom in dongpu sag the enrichment of 13c in organic matter is attributed to the significant increase of drought and salinity high salinity and aridity climate may lead to the abundance of c4 plants and chlorobium as well as the enrichment of 13c in plankton

norcholestane in miocene onnagawa siliceous sediments japan abstract a significant amount of 24 norcholestane with 20r 5 h 14 h 17 h stereochemistry is often found in marine siliceous sediments of the middle miocene onnagawa formation japan some onnagawa siliceous sediments are abnormally abundant in 24 norcholestane the primary production in the northeastern japan sea during the middle miocene was markedly increased due to the invasion of nutrient rich cold seawater from the north accompanied by the tectonic opening of the japan sea 24 norcholestane in high relative abundance is derived mainly from the marine diatoms which were enhanced during this event the drastic ecological change caused by rapid tectonic opening of the japan sea and global climatic deterioration during middle miocene time is suggested to be influential in the formation of 24 norcholestane rich onnagawa siliceous sediments

refining the structural framework of the khimti khola region east central nepal himalaya using quartz textures and c axis fabrics abstract new quartz texture and c axis fabric data from across the paleoproterozoic ulleri phaplu melung orthogneiss in the khimti khola region of east central nepal provide new constraints on the internal structural framework of the himalaya that help shed light on the convergence accommodation processes active in the upper portion of the crust during orogenesis these data outline a strain history that varies across the unit deformation near the base of the unit occurred at 605 50 c with evidence of significant static recrystallization and recovery preserved in quartz whereas deformation near the top of the unit occurred at 540 50 c with quartz characterized by dynamic recrystallization mechanisms the strength of the quartz c axis fabrics follows a similar spatial pattern with those from near the top of the unit recording stronger fabrics than those measured from lower in the unit together these data are interpreted to indicate strain localization possibly at progressively lower temperature near the top of the ulleri phaplu melung orthogneiss this interpretation is consistent with cooling ages that indicate the upper boundary of the unit coincides with an out of sequence shear zone this study not only provides a structural characterization of the shear zone helping to refine the kinematic framework of this portion of the himalaya but also confirms the utility of fabric strength analysis in deciphering strain localization within pervasively deformed rocks

k ar and ar ar geochronology of the himalayan collision in nw pakistan constraints on the timing of suturing deformation metamorphism and uplift mica and hornblende k ar and ar ar data are presented from each of the three crustal components of the himalayan collision zone in north pakistan the asian plate the kohistan island arc and the indian plate together with u pb and rb sr data published elsewhere these new data 1 date the age of suturing along the northern suture which separates kohistan from asia at 102 85 ma 2 establish that the basic magmatism in kohistan which postdates collision along the northern suture predates 60 ma and that the later granite magmatism spanned a range of 60 25 ma 3 show that uplift amounts within kohistan are greater toward the nanga parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 ma to a current figure of about 5 5 mm a year 4 show that much of southern kohistan had cooled to below 500 c by 80 ma and that the major deformation which imbricated kohistan probably predated 80 ma and certainly predated 60 ma and was related to the kohistan asia collision rather than the kohistan india one 5 imply that uplift along the hunza shear in the asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 ma and was associated with breakback thrusting in the hanging wall of the main mantle thrust 6 suggest that the indian plate has a lengthy pre himalayan history with an early metamorphism at about 1900 ma major magmatism at 500 550 ma and early jurassic lithospheric extension or inversion and 7 show that the indian plate rocks were metamorphosed shortly after the collision within kohistan which occurred at circa 50 ma and subsequently cooled back through 500 c at circa 38 ma and 300 c at 30 35 ma with ages of cooling through 200 and 100 c as determined by fission track data locally controlled by nanga parbat related uplift tectonics

the significance of 24 norcholestanes triaromatic steroids and dinosteroids in oils and cambrian ordovician source rocks from the cratonic region of the tarim basin nw china abstract two oil families in ordovician reservoirs from the cratonic region of the tarim basin are distinguished by the distribution of regular steranes triaromatic steroids norcholestanes and dinosteroids oils with relatively lower contents of c 28 regular steranes c 26 20s c 26 20r c 27 20s and c 27 20r regular triaromatic steroids dinosteranes 24 norcholestanes and triaromatic dinosteroids originated from middle upper ordovician source rocks in contrast oils with abnormally high abundances of the above compounds are derived from cambrian and lower ordovician source rocks only a few oils have previously been reported to be of cambrian and lower ordovician origin especially in the east region of the tarim basin this study further reports the discovery of oil accumulations of cambrian and lower ordovician origin in the tabei and tazhong uplifts which indicates a potential for further discoveries involving cambrian and lower ordovician sourced oils in the tarim basin dinosteroids in petroleum and ancient sediments are generally thought to be biomarkers for dinoflagellates and 24 norcholestanes for dinoflagellates and diatoms therefore the abnormally high abundance of these compounds in extracts from the organic rich sediments in the cambrian and lower ordovician and related oils in the cratonic region of the tarim basin suggests that phytoplankton algae related to dinoflagellates have appeared and might have flourished in the tarim basin during the cambrian period steroids with less common structural configurations are underutilized and can expand understanding of the early development history of organisms as well as define petroleum systems

the tethyan himalayan detrital record shows that india asia terminal collision occurred by 54 ma in the western himalaya the himalayan orogen is a type example of continent continent collision knowledge of the timing of india asia collision is critical to the calculation of the amount of convergence that must have been accommodated and thus to models of crustal deformation sedimentary rocks on the indian plate near the suture zone can be used to constrain the time of collision by determining first evidence of asian derived material deposited on the indian plate however in the himalaya for this approach to be applied successfully it is necessary to be able to distinguish between asian detritus and detritus from oceanic island arcs that may have collided with india prior to india asia collision zircons from the indian plate asian plate and kohistan ladakh island arc can be distinguished based on their u pb ages combined with hf signatures we undertook a provenance study of the youngest detrital sedimentary rocks of the tethyan himalaya of the indian plate in the western himalaya we show that zircons of asian affinity were deposited on the indian plate at 54 ma we thus constrain terminal india asia collision when both sutures north and south of the kohistan ladakh island arc were closed to have occurred in the western himalaya by 54 ma

variable shortening rates in the eastern himalayan thrust belt bhutan insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions 1 we present data on the burial displacement and exhumation history of the himalayan fold thrust belt in eastern bhutan these data document the magnitude and timing of displacement of large discrete structures and highlight temporal variability in shortening rates eight new40ar 39ar ages from white mica 32 new zircon u th he ages 7 new apatite fission track ages and 1 new u pb zircon la mc icp ms metamorphic rim growth age are combined with published cooling ages and deformation temperatures and incremental shortening magnitudes from restorations of two published balanced cross sections to illustrate the kinematic and temporal development of the bhutan thrust belt integrating these data from 23 ma to the present illustrates rapid horizontal shortening rates 28 35 mm yr between 23 20 ma and 15 10 ma separated by more moderate rates 10 23 mm yr shortening rates decrease significantly to 7 10 mm yr and possibly as low as 3 4 mm yr from 10 to 0 ma this decrease is interpreted to represent the onset of strain partitioning in the eastern part of the himalayan tibetan orogenic system between shortening in the bhutan thrust belt uplift of the shillong plateau and deformation and outward growth of the northern and eastern tibetan plateau within estimated error horizontal shortening rates during emplacement of the main central thrust sheet and during construction of the upper lesser himalayan duplex approached india asia tectonic velocities thus for periods of time between 23 20 ma and 15 10 ma the bhutan thrust belt may have absorbed nearly all india asian convergence at this longitude

kinematic implications of regional 40ar 39ar ages east central nepal abstract this study presents new 40ar 39ar ages from across the himalaya in east central nepal 40ar 39ar ages from the mahabharat region the southernmost and structurally lowest part of the study area range between 27 5 and 17 3 ma while those from the high himalaya the northernmost and structurally highest part of the study area range from 17 7 to 17 4 ma these two areas bracket a middle portion characterized by younger ages that range between 8 6 and 14 3 ma and cluster around 10 ma these new dates outline a u shaped pattern across the region that is interpreted to reflect cooling facilitated by out of sequence motion across the upper tama kosi thrust in agreement with previous work in the area the results presented herein are also consistent with published model cooling predictions and highlight the potential importance of late lower temperature deformation on the present geometries of the exhumed former metamorphic core exposed mountain belts around the globe

the occurrence of isorenieratane and 24 n propylcholestanes in paleogene lacustrine source rocks from the dongying depression bohai bay basin implications for bacterial sulfate reduction photic zone euxinia and seawater incursions abstract lacustrine organic rich source rocks shales and mudstones were examined to assess the depositional environment and organic input sources of the third and fourth members of the eocene shahejie formation bohai bay basin china major organic contributions to the shahejie formation from dinoflagellates diatoms botryococcus and marine pelagophyte chrysophyte algae were determined using n alkane distributions gammacerane index and diagnostic steranes traces of higher plant biomarkers suggest that terrigenous organic inputs were limited the pervasive presence of carotane aryl isoprenoids paleorenieratane renieratane isorenieratane 2 methylhopanes and crocetane indicate significant organic contributions from bacterial communities including cyanobacteria purple sulfur bacteria chromatiaceae green sulfur bacteria chlorobiaceae and sulfate reducing bacteria the extent of h2s content bacterial sulfate reduction and photic zone euxinia pze were evaluated by determining the aryl isoprenoids ratio air 0 7 9 0 the presence of isorenieratene derivatives and the fate of sulfur in the water column there was a decrease of pze extent and duration from the northeast to the southwest of the palaeo lake and there was a decrease of water body salinity and bottom water anoxia from the fourth to the third member of the shahejie formation abrupt environmental perturbations in the third member of the shahejie formation are indicated not only by significant changes in parameters such as the pristane phytane and carotane n c20 alkane ratios the gammacerane index the isorenieratane c18 aryl isoprenoid and the air but also by the time lag in the expansion of cyanobacterial lineages after the expansion of purple green sulfur bacteria as well as seawater incursions the unexpected occurrence of intact c30 4 desmethylsteranes 24 n propylcholestanes in the shahejie formation is first reported here after very careful checking and provides unambiguous biological evidence for at least two marine transgression events reaching the eocene dongying palaeo lake the organic enrichment of the petroleum source rocks in the shahejie formation is a consequence of combined indigenous lacustrine algal production the constrained and favourable preservation conditions and significant bacterial contributions and marine organic matter inputs to the lake

fault fracture mesh petroleum plays in the zhanhua depression bohai bay basin part 1 source rock characterization and quantitative assessment abstract in the prolific zhanhua depression of the bohai bay basin eastern china over 76 of the proven petroleum reserves occur in the neogene guantao and minhuazhen formations detailed geological and geochemical mapping of the phanerozoic strata in this area reveals that oil shales and dark mudstones in the es4 es3 and es1 members of the eocene oligocene shahejie formation have excellent potential as petroleum source rocks regional distributions of total organic carbon content kerogen type and thermal maturation indicate that abundant mature source rocks are present in several discrete sag areas bounded by extensive basement related fault networks in contrast source rocks along the paleo topographic highs are currently immature with respect to the generation of conventional oils this study examined the key molecular characteristics for the common isoprenoid hopanoid and steroid biomarkers in relation to source facies variations in lacustrine sediments the hydrocarbon source potential and expulsion efficiency for each of the source rock units were evaluated which integrated the source rock characteristics with regional thermal subsidence and sediment burial an important observation made during this study is the striking contrast between the source potential and proven oil reserves within each stratigraphic section illustrating how significant the migration of hydrocarbon fluids derived from deep mature source kitchens through over 2000 m of fine grained formations must have been to form several giant oil accumulations in the shallow strata these results support the concept of the neogene fault fracture mesh petroleum plays proposed by zhang et al 2004 marine petroleum geology 21 651 668 and the vital role that the faults and fracture systems have in controlling hydrocarbon migration conduits and accumulation habitat in this rift basin

unprecedented occurrence of novel c26 c28 21 norcholestanes and related triaromatic series in evaporitic lacustrine sediments a novel series of c26 c28 21 norcholestanes and their triaromatic analogs have been tentatively identified as major components in eogene evaporitic sediments in jinxian sag of the bohai bay basin northern china these compounds may be viewed as representatives of a new class of molecules whose biological or diagenetic origin remains to be determined their unprecedented high abundance in the evaporitic lacustrine sediments and related oils show important source and or depositional environment controls in addition to thermal maturity

fission track and numerical thermal modeling of differential exhumation of the glarus thrust plane switzerland abstract geometry and differences in metamorphic grade along the glarus thrust external european alps can be explained by a restoration of the hanging wall some 10 km toward the south and back rotation of the thrust plane into a ramp flat ramp geometry the apatite fission track ft age pattern from the area suggests that isotherms were parallel to the thrust plane during cooling of the rocks along the thrust plane below the apatite closure temperature two forward modeling programs that both produce time temperature paths are used to constrain the thermal and structural evolution of differential exhumation along the glarus thrust a a two dimensional finite difference thermal model is used to examine differential exhumation of the thrust plane starting from an assumed horizontal position of the middle part of the glarus thrust at some 10 ma when parallel to the paleo isotherms five locations in the crustal section corresponding to locations on a final horizontal and vertical sampling traverse record their time temperature t t history during exhumation along particle paths the near surface temperature distribution is strongly influenced by the development of a simplified step like topographic relief which causes all modeled samples to reach the surface at the end of the modeled time period b the calculated t t paths of these five modeled samples were used to forward model apatite ft data based on an existing annealing model by comparison between synthetically generated ft data and actual measured ft samples the exhumation history of the glarus alps was evaluated with respect to its thermal and temporal brackets several parameters such as the location of the rotation point the thermal gradient and the topography have significant influence on model results best fit models produce results that are consistent with independent geological and geophysical information

extremely high sea surface temperatures at low latitudes during the middle cretaceous as revealed by archaeal membrane lipids the middle cretaceous 125 88 ma greenhouse world was characterized by high atmospheric co 2 levels the general absence of polar ice caps and much higher global temperatures than at present both 1 8 o based and model based temperature reconstructions indicate extremely high sea surface temperatures ssts at high latitudes however there are a number of uncertainties with sst reconstructions based on 1 8 o isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the 1 8 o value of cretaceous seawater the paleoecology of middle cretaceous marine organisms and seawater ph here we applied a novel sst proxy i e text tetraether index of 86 carbon atoms based on the membrane lipids of marine crenarchaeota derived from middle cretaceous sedimentary rocks deposited at low latitudes the text proxy indicates that tropical ssts in the proto north atlantic were at 32 36 c during the early albian and late cenomanian early turonian this finding agrees with sst estimates based on 1 8 o paleothermometry of well preserved foraminifera as well as global circulation model calculations the text proxy indicates cooler ssts 27 32 c for the equatorial pacific during the early aptian which is in agreement with sst estimates based on 1 8 o paleothermometry

interaction of metamorphism deformation and exhumation in large convergent orogens coupled thermal mechanical models are used to investigate interactions between metamorphism deformation and exhumation in large convergent orogens and the implications of coupling and feedback between these processes for observed structural and metamorphic styles the models involve subduction of suborogenic mantle lithosphere large amounts of convergence 450 km at 1 cm yr 1 and a slope dependent erosion rate the model crust is layered with respect to thermal and rheological properties the upper crust 0 20 km follows a wet quartzite flow law with heat production of 2 0 w m 3 and the lower crust 20 35 km follows a modified dry diabase flow law with heat production of 0 75 w m 3 after 45 myr the model orogens develop crustal thicknesses of the order of 60 km with lower crustal temperatures in excess of 700 c in some models an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 pa s at 700 c in the upper crust and 900 c in the lower crust in these models a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where t 600 c the channel flow zone is characterised by a reversal in velocity direction on the pro side of the system and is driven by a depth dependent pressure gradient that is facilitated by the development of a temperature dependent low viscosity horizon in the mid crust different exhumation styles produce contrasting effects on models with channel flow zones post convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust related exhumation on the flanks velocities in the pro side channel flow zone are enhanced but the channel itself is not exhumed in contrast exhumation resulting from erosion that is focused on the pro side flank of the plateau leads to ductile extrusion of the channel flow zone the exhumed channel displays apparent normal sense offset at its upper boundary reverse sense offset at its lower boundary and an inverted metamorphic sequence across the zone the different styles of exhumation produce contrasting peak grade profiles across the model surfaces however p t t paths in both cases are loops where pmax precedes tmax typical of regional metamorphism individual paths are not diagnostic of either the thickening or the exhumation mechanism possible natural examples of the channel flow zones produced in these models include the main central thrust zone of the himalayas and the muskoka domain of the western grenville orogen

fault fracture mesh petroleum plays in the jiyang superdepression of the bohai bay basin eastern china abstract based on the results of laboratory simulation experiments and field observations the concept of fault fracture mesh petroleum play is proposed to explain the secondary petroleum accumulations in the neogene reservoirs of the jiyang superdepression bohai bay basin eastern china a well defined fault fracture mesh petroleum play usually contains three basic elements an oil source network at the base a transient storage for migrating hydrocarbons in the middle and a petroleum accumulation network at the top the oil source network provides fairways for hydrocarbons generated in a deep source to migrate vertically upward to shallow strata often consisting of primary faults and fracture zones and or regional unconformities in direct contact with mature petroleum source beds and secondary faults intercepting lenticular fluvial sandstones the transient storages for migrating hydrocarbons include the thick porous mat or sheet like sandstones deposited under the fluvial braided stream setting and occurring over relatively large geographic areas these sandstones serve as temporary storage spaces through which deep sourced petroleum fluids migrate further either laterally or vertically into shallower reservoirs the upper petroleum accumulation network provides suitable habitats for oil and gas accumulation comprising structural lithologic stratigraphic and combination traps the jiyang superdepression of the bohai bay basin hosts several excellent examples of the fault fracture mesh petroleum play where hydrocarbons in the neogene reservoirs were derived either directly from eocene oligocene source rocks or from the redistribution of pre existing oil accumulations in the deeper reservoirs reactivation of basement involved faults and regional unconformities provided conduits for channeling the pre neogene hydrocarbons into the shallow strata the magnitudes and frequencies of the faulting activities and their relative timings to those of peak oil generation and expulsion were critical factors for controlling the quantities of hydrocarbons transmitted to the neogene strata the thick sandstones and conglomerates in the lower guantao formation acted as excellent transient storages for the deep sourced oils which were then trapped to form accumulations in the shallow structural lithologic stratigraphic and combination traps of the guantao ng minhuazhen nm formations with the recent discovery of several giant oilfields in the shallow neogene reservoirs both onshore and offshore bohai bay basin the proposed fault fracture mesh petroleum play concept helps the exploration geologists to modify and clarify long held traditional concepts of petroleum play types in this basin and expand upon the new exploration horizons of the shallow subtle petroleum traps

dinosteranes 4 23 24 trimethylsteranes and other biological markers in dinoglagellate rich marine sediments of rhaetian age abstract alkane biological marker distributions and concentrations have been determined in 18 samples from a section of rhaetian age from the bristol trough and a quantitative micropalaeontological analysis carried out taking into account previous geological investigations of the sequence three end member depositional settings have been recognised from changes in total organic carbon content the biological markers and the micropalaeontological composition these depositional environments are attributed as follows the first which occurs in the lower part of the section corresponds to deposition in a supratidal sabkha type setting with the major organic input being of allochthonous origin the second also in the lower part of the section indicates the occurrence of periods of marine incursion with enhanced salinity resulting from evaporation in the upper part of the section marginal marine conditions are apparent with higher organic carbon biological marker and amorphous organic matter contents at the top suggesting the establishment of more stable conditions of oxygen depletion some sediments in the sequence are notable for containing an abundance of cysts of rhaetogonyaulax rhaetica one of the oldest marine dinoflagellates comparison of microfossil abundances with hydrocarbon abundances extends the circumstantial evidence for the dinophyceae being the major biological source of dinosteranes 4 23 24 trimethylcholestanes and some other 4 methyl steranes in marine sediments the presence of dinosteranes and their 24 ethyl counterparts extends the stratigraphic range of occurrence of such components back to the rhaetian c 31 methyl steranes in the sediments from the second depositional setting are tentatively assigned a 4 22 23 24 tetramethyl sterane skeleton and a dinoflagellate origin

early diagenetic pyrite cementation influences molecular composition of sedimentary organic matter in the dongying depression china abstract micro variations of lipid hydrocarbons and biomarkers in well laminated shales have not been extensively documented especially within different rock textures fabrics and mineralogies comparison of a pyrite cemented zone and its surrounding sediment may offer a new perspective on mineral control on preservation of hydrocarbons and organic carbon a black well laminated shale comprising two adjacent and contrasting calcareous light and pyrite dark laminae at the millimetre scale was selected from the eocene shahejie formation dongying depression bohai bay basin it was horizontally sliced to separate a pyrite cemented concretion from the unaltered laminated shale and investigated using geochemical techniques to assess the heterogeneity of elements minerals microfossils and hydrocarbons within the two slices micro scale changes in source inputs were noted for instance the 4 methylsteranes dinosteranes and the triaromatic dinosteranes show varying distributions between the two slices attributed to slight differences in dinoflagellate contributions similarly variation in the amount of bacterial sulfate reduction is indicated by variations in the aryl isoprenoid ratio relative amount of isorenieratane long chain n alkane carbon isotopic composition and other molecular indicators including the relative amounts of dibenzothiophene dibenzofuran methyldibenzothiophene and methyltrimethyltridecylchromans the primary control on these variations appears to be different euxinic water conditions but diagenesis has also influenced the distributions differential anaerobic bacterial reworking of organic matter and pyrite concretion protection are responsible for organic geochemical variations on a millimetre scale in the shale the pyrite concretion has an altered record of its thermal history but has a more accurate record of the organic geochemistry of microfossils and water column conditions this study shows the potential for future investigations of closely spaced vertical variation in organic geochemical parameters that may shed light on rapid temporal variation in organic matter input and depositional environment

weddell polynya transport mechanisms in the abyssal ocean abstractweddell polynya transport mechanisms in the deep and abyssal oceans are examined in the noaa geophysical fluid dynamics laboratory s gfdl coupled climate model cm2g during an 1820 yr long integration of the model polynyas are forced every 29 years in the weddell sea via an increase in the diapycnal diffusivity composites of the events are used to examine the mechanisms responsible for transporting polynya signals away from the weddell sea polynya signal transport is governed by two dynamical mechanisms that act on different time scales and spread at different rates large scale waves such as kelvin and planetary and topographic rossby waves propagate the polynya signal rapidly on interannual to decadal time scales while advection transports the signal more slowly on decadal to centennial time scales despite their different spreading rates these mechanisms can act contemporaneously and it is often their combined effect that governs the property changes in the global deep and abyssal o

assessing the suitability of holocene environments along the central belize coast central america for the reconstruction of hurricane records since the belize coast was repeatedly affected by hurricanes and the paleohurricane record for this region is poor sediment cores from coastal lagoon environments along the central belize coast have been examined in order to identify storm deposits the paleohurricane record presented in this study spans the past 8000 years and exhibits three periods with increased evidences of hurricane strikes occurring at 6000 4900 4200 3600 and 2200 1500 cal yr bp two earlier events around 7100 and 7900 cal yr bp and more recent events around 180 cal yr bp and during modern times have been detected sand layers redeposited corals and lagoon shell concentrations have been used as proxies for storm deposition additionally hiatuses and reversed ages may indicate storm influence while sand layers and corals represent overwash deposits the lagoon shell concentrations which mainly comprise the bivalve anomalocardia cuneimeris and cerithid gastropods have been deposited due to changes in lagoon salinity during and after storm landfalls comparison with other studies reveals similarities with one record from belize but hardly any matches with other published records the potential for paleotempestology reconstructions of the barrier lagoon complexes along the central belize coast differs depending on geomorphology and deposition of washovers in the lagoon basins is limited probably due to the interplay of biological geological and geomorphological processes

metamorphic history of the central himalaya annapurna region nepal and implications for tectonic models pressure temperature time p t t conditions of metamorphism have been determined in the annapurna region of central nepal that place new constraints on the structural and tectonic evolution of the himalayan orogenic wedge peak p t conditions increase structurally upward 525 c and 8 kbar in the lesser himalayan sequence 650 c and 12 kbar at the base of the greater himalayan sequence across the main central thrust 750 c and 12 kbar in the middle of the greater himalayan sequence and 775 c and 13 kbar near the top of the greater himalayan sequence metamorphic monazite ages in the greater himalayan sequence also increase structurally upward 16 21 ma for subsolidus growth at the base of the greater himalayan sequence to 25 ma for peak t metamorphism and anatexis near the top of the greater himalayan sequence these ages are several million years older than at equivalent structural levels at langtang 200 km to the east the p t t data recommend reinterpretation of the bhanuwa fault within the greater himalayan sequence as a thrust and the presence of a different thrust structurally above the bhanuwa thrust here named the sinuwa thrust the new data are consistent with progressive stacking of tectonic slices with calculated overthrust rates that are consistent with some but not all models that presume 2 cm yr convergence across the himalaya since 25 ma despite differences in absolute ages similarities among the chemical systematics of monazite peak p t conditions and overthrust rates calculated for annapurna when compared to langtang imply that the broad geodynamics in one part of an orogen can be realistically extrapolated within a few hundred kilometers although the timing and duration of movement on discrete thrust surfaces may differ