numerical modelling of lithosphere shortening application to the laramide erogenic province western usa abstract a two dimensional model of lithosphere shortening is presented which quantifies crustal thickening temperature perturbations and flexural isostatic components the model assumes that the upper brittle layer of the crust deforms by thrusting while the lower ductile lithosphere accommodates compressional deformation by a pure shear or squashing mechanism model results suggest that basement uplift foreland basin development and underlying crustal structure are controlled by the amount and configuration of the compressional deformation in the upper and lower lithosphere the perturbation of the temperature field erosion and the flexural rigidity of the lithosphere the model is applicable to regions of thick skinned thrust tectonics and is applied to the laramide orogenic province in the western usa model simulations of structural cross sections across the laramide province show basement uplifts and adjacent foreland basins that are comparable in magnitude with those suggested by geological data the model has also been used to provide insights into the tectonic evolution of the laramide province and an attempt has been made to determine the flexural rigidity of the lithosphere at the time of laramide deformation as well as quantify both the effects of the post shortening re equilibration of the temperature field and thrust uplift erosion the amount of crustal thickening and post shortening cooling of the geotherm predicted by the two dimensional model of lithosphere shortening are used in strength calculations to determine the relative strength or weakness of the lithosphere the results suggest that the lithosphere is relatively strong immediately following shortening due to the cooling of the geotherm following shortening however gradual weakening occurs as the temperature field returns back to its unperturbed state in the presence of an enhanced crustal thickness the results are compatible with the evolution of the laramide province which has experienced a change in tectonic regime to one of extension over the last 20 30 ma

heat flow study of the emeishan large igneous province region implications for the geodynamics of the emeishan mantle plume abstract the emeishan large igneous province elip is widely considered to be a consequence of a mantle plume the supporting evidence includes rapid emplacement voluminous flood basalt eruptions and high mantle potential temperature estimates several studies have suggested that there was surface uplift prior to the eruption of the emeishan flood basalts additionally the plume s lateral extent is hard to constrain and has been variously estimated to be 800 1400 km in diameter in this study we analyzed present day heat flow data and reconstructed the permian paleo heat flow using vitrinite reflectance and zircon u th he thermochronology data in the elip region and discussed implications for the geodynamics of the emeishan mantle plume the present day heat flow is higher in the inner and intermediate zones than in the outer zone with a decrease of average heat flow from 76 mw m2 to 51 mw m2 thermal history modeling results show that an abnormal high paleo heat flow of 90 110 mw m2 was caused by the emeishan mantle plume activity based on the present day heat flow data we can calculate that there is lithospheric thinning in the central elip region which may be due to the destruction of the lithosphere by mantle plume upwelling and magmatic underplating the permian paleo heat flow anomaly implies that there was a temperature anomaly in the mantle the ascending high temperature mantle plume and the thinned lithosphere may have induced the large scale uplift in the elip region according to the range of the surface heat flow anomaly it can be estimated that the diameter of the flattened head of the emeishan mantle plume could have reached 1600 1800 km our research provides new insights into the geodynamics of the emeishan mantle plume through study of heat flow

cenozoic deformation of the tarim plate and the implications for mountain building in the tibetan plateau and the tian shan 1 the tarim basin in nw china developed as a complex foreland basin in the cenozoic in association with mountain building in the tibetan plateau to the south and the tian shan orogen to the north we reconstructed the cenozoic deformation history of the tarim basement by backstripping the sedimentary rocks two dimensional and three dimensional finite element models are then used to simulate the flexural deformation of the tarim basement in response to the sedimentary loads and additional tectonic loads associated with overthrusting of the surrounding mountain belts the results suggest that uplift of both the western kunlun shan and the tian shan orogens started during or before oligocene since the miocene mountain building accelerated in the western kunlun shan but showed segmented development in the tian shan the results also indicate reduced tectonic load along the altyn tagh fault since late miocene consistent with geological and geophysical evidence of the fault cutting the entire lithosphere and migrating southward during its cause of the cenozoic evolution

potential links between surging ice sheets circulation changes and the dansgaard oeschger cycles in the irminger sea 60 18 kyr surface and deepwater paleoclimate records in irminger sea core so82 5 59on 31 ow and icelandic sea core ps2644 68on 22ow exhibit large fluctuations in thermohaline circulation thc from 60 to 18 calendar kyr b p with a dominant periodicity of 1460 years from 46 to 22 calendar kyr b p matching the dansgaard oeschger d o cycles in the greenland ice sheet project 2 gisp2 temperature record grootes and tuiver 1997 during interstadials summer sea surface temperatures sstsu in the irminger sea averaged to 8oc and sea surface salinities sss averaged to 36 5 recording a strong irminger current and atlantic thc during stadials sstsu dropped to 2o 4oc in phase with sss drops by 1 2 they reveal major meltwater injections along with the east greenland current which turned off the north atlantic deepwater convection and hence the heat advection to the north in harmony with various ocean circulation and ice models on the basis of the ird composition icebergs came from iceland east greenland and perhaps svalbard and other northern ice sheets however the southward drifting icebergs were initially jammed in the denmark strait reaching the irminger sea only with a lag of 155 195 years we also conclude that the abrupt stadial terminations the d o warming events were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater covered northwestern north atlantic in the 1 1460 year frequency band benthic 15 80 brine water spikes led the temperature maxima above greenland and in the irminger sea by as little as 95 years thus abundant brine formation which was induced by seasonal freezing of large parts of the northwestern atlantic may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the thc in summary the internal dynamics of the east greenland ice sheet may have formed the ultimate pacemaker of d o cycles

flexural eustatic numerical model for drowning of the eocene perialpine carbonate ramp and implications for alpine geodynamics the paleocene2eocene nummulitic limestone formation of the alpine periphery occupies the lowermost part of a stratigraphic trinity above a major basal unconformity it is thought to have accumulated as a foraminiferal carbonate ramp at the distal featheredge of the underfilled north alpine and southwest alpine foreland basin we present the results of a numerical model that links carbonate sedimentation at the distal featheredge of a peripheral foreland basin to the flexural subsidence of an elastic plate subjected to a distributed load with a superimposed eustatic sea level history carbonate accumulation is treated as depth dependent model parameters are constrained by geological and geophysical observations of the alpine orogen and its peripheral foreland basin and from literature on the ecology of benthic foraminifers the generic alpine model shows that the carbonate ramp accumulates as a number of sedimentary cycles before rapid drowning giving rise to a retrogradational stratigraphic package terminated by a surface of accelerated transgression and backstepping running the model with sets of parameter values appropriate to the paleocene to middle eocene nummulitic limestone in central eastern switzerland and the late eocene nummulitic limestone

the effects of a lateral variation in lithospheric strength on foredeep evolution implications for the east carpathian foredeep abstract lateral variations in lithospheric strength have been adopted often in flexural modeling both 2d and 3d to better fit the observed basement deflections typically supported by gravity data this approach provides essentially a snap shot of the role of lithosphere strength in determining the present day geometry in contrast we investigate and quantify the effects of a lateral change in lithospheric strength on the evolution of the foredeep in front of an advancing orogen transitions in lithospheric strength are common in the foreland of orogens and show large variations in the width of the transition zone and the strength difference former passive margins for instance will display strength changes distributed over several tens to hundreds of kilometers other transitions may originate from juxtaposition or accretion of pieces of lithosphere with different properties and may be characterized by a much smaller width than former passive margins in our modeling a constant load representing an advancing orogenic belt is displaced towards and across a transition from a weak to a strong plate in a 2d elastic thin plate model the effect of different transition widths and strength contrasts on foredeep geometry and bending stress is investigated interference of flexural wavelengths across the transition affects foredeep geometry by causing rapid basin widening oscillation of the bulge and volume increase the bending stresses are found to concentrate and amplify around the strength transition large transition gradients i e large strength contrast or small transition width cause the highest rates of change basin widening caused by the orogenic load advancing towards the transition between the east european craton and the moesian platform appears to control the sarmatian transgression over the east carpathian foreland in romania

petroleum geology of the puguang sour gas field in the sichuan basin sw china abstract the puguang sour gas field in northeastern sichuan basin is the largest gas discovery in the paleozoic marine strata in china with at least 9 trillion cubic feet tcf of proven gas reserves it is one of the many anticlinal structures formed during the indo sinian orogeny with gas being trapped in a composite structure lithology pool the presence of the upper permian coal measures lower silurian and lower cambrian marine shales provided excellent hydrocarbon source rocks the lower triassic and upper permian oolitic dolomites serve as high quality reservoir rocks with 10 25 porosity and 1 1000 md permeability largely due to diagenetically produced secondary porosity the lower and middle triassic evaporate beds have acted as the direct top seal for the large lower triassic upper permian gas accumulation the thick upper triassic and jurassic mudstone beds regionally cap the underlying marine carbonate strata the deep seated basement faults acted as vertical hydrocarbon migration conduits during faulting and became effective flow barriers when the faulting ceased the deep burial of a paleo oil accumulation in reservoirs rich in sulfate resulted in extensive thermochemical sulfate reduction tsr this together with the new gas injection and structural reconfiguration transformed a paleo oil pool to a large sour gas pool

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

dansgaard oeschger and heinrich event imprints in alboran sea paleotemperatures past sea surface temperature sst evolution in the alboran sea western mediterranean during the last 50 000 years has been inferred from the study of c37 alkenones in international marine global change studies md952043 core this record has a time resolution of 200 years allowing the study of millennial scale and even shorter climatic changes the observed sst curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period comparison of this alboran record with 18o from greenland ice greenland ice sheet project 2 core shows a strong parallelism between these sst oscillations and the dansgaard oeschger events five prominent cooling episodes standing out in the sst profile are accompanied by an anomalous high abundance of neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals these features and the isotopic record reflect drastic changes in the surface hydrography of the alboran sea in association with heinrich events hl 5

abrupt climate changes from the past to the future a review a review of climatic variability is given with a focus on abrupt changes during the last glacial evidence from palaeoclimatic archives suggests that these were most likley due to reorganisations of the atmosphere ocean system the mechanisms responsible for these changes are presented their implication for future climate changes is discussed in light of recent climate model simulations

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

gisp2 oxygen isotope ratios the gisp2 oxygen isotope record with its high resolution detail yields crucial information on past climate change the glacial 18o oscillations of the gisp2 core with their very fast onsets are templates of a prototype oscillation of variable duration with an amplitude of 3 9 the halfway mark of the cold warm transition is reached in 2 years the top is reached in 50 years the 18o time gradient of the leading front is about 7 8 per 100 yr after reaching the top 18o slowly declines by 0 14 per 100 yr the duration of 18o decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones the subsequent 18o downturn during the warm cold transition has a 18o time gradient of 3 2 per 100 yr and lasts about 80 yr

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

the rio caliente ignimbrite analysis of a compound intraplinian ignimbrite from a major late quaternary mexican eruption the rio caliente ignimbrite is a multi flow unit orcompound ignimbrite formed during a major late quaternary explosive rhyolitic eruption of la primavera volcano mexico the eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air fall deposits it is therefore anintraplinian ignimbrite air fall layers pyroclastic surge mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units a chaotic near vent facies of the ignimbrite includes co ignimbrite lag breccias segregated from proximal pumice flows the facies locates a central vent but one which could not have been associated with a well defined edifice many of the lithics in the exposed lag breccias and near vent facies of the ignimbrite appear to be fragments of welded rio caliente ignimbrite and indicate considerable vent widening or migration during the eruption nearer vent the ignimbrite is thickest and composed of the largest number of flow units here it is welded and is a simple cooling unit evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded less mobile flows which would be produced by collapse of low eruption columns the analogy of a compound ignimbrite with a compound lava flow is therefore good a compound lava flow forms instead of a simple one when the volumetric discharge rate or intensity is low and in explosive eruptions this predicts lower eruption column heights a corollary is that the ignimbrite has a high aspect ratio

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

permian to late triassic evolution of the longmen shan foreland basin western sichuan model results from both the lithospheric extension and flexure abstract the lithosphere was extended during the permian middle triassic in the yangtze craton where the sichuan basin located and then bent due to thrusting of the longmen shan orogen leading to formation of the longmen shan foreland basin western sichuan during the late triassic indosinian orogeny the lateral variation of the lithospheric strength resulted by former differential extension would inevitably influence the subsequent evolution of the foreland basin in order to investigate this both extensional and flexural models were applied in modeling permian late triassic basin evolution a 2d kinematic extensional model was initially developed along a profile crossing the yangtze craton to simulate the lithospheric thermal evolution during the permian middle triassic based on the thermal results the thermal rheological structure as well as the effective elastic thickness of the lithosphere te was then determined extension model show that the stretching factors decrease gradually from songpan ganzi to the sichuan basin leading to variable thermal rheological structure and increased te from west to east taking into account of the te variation a flexural model was finally constructed to investigate the evolution of the longmen shan foreland basin during the late triassic spanning the time period c 227 206 ma three episodes were divided according to the corresponding tectonostratigraphic units by matching the stratigraphic observations three phase advance distances eastward of the longmen shan along the qingchuan maowen fault turned out to be 18 22 and 18 km it implied a slow and similar thrust advance rate of 3 6 c 227 222 ma 2 2 c 222 212 ma and 3 mm yr c 212 206 ma respectively

introduction to special section on open magmatic systems the idea that magmatic systems are open to intermittent gains and losses of mass and energy is hardly new eruptive conductive and hydrothermally convective losses are obvious and both wall rock assimilation and mixing of discrete magma batches are petrological ideas with long and distinguished conceptual lineages new ideas of the last decade are largely outgrowths of the mounting evidence that mixing and assimilation are ubiquitous that heterogeneity and mixing in mantle source regions are common and that few igneous rock suites are likely to reflect strictly closed system fractionation of single magma batches the evidence has accumulated quickly on many fronts owing in part to the wider availability of high precision analytical tools and the development of sophisticated methods for modeling the wealth of chemical and isotopic data detailed studies of zoned ash flow sheets zoned and layered plutons and macroscopically mixed igneous rocks have made petrologists aware that dynamic and nonequilibrium aspects of magma transport play important roles in the evolution of many magmatic systems technological advances have permitted routine experimentation over a wide range of pressures providing kinetic data and phase equilibrum constraints essential to understanding both mantle source processes and magmatic evolution in crustal reservoirs finally the plate tectonic synthesis and seafloor sampling programs have respectively provided the impetus for a detailed assessment of geochemical heterogeneity in a dynamic mantle and an important avenue for accomplishing that assessment

flexural expression of european continental lithosphere under the western outer carpathians we present the results of an analysis of the flexural down bending of the european lithosphere under the western outer carpathians for a grid of five profiles with a lateral spacing of about 100 km crossing the carpathians and its foreland seismic sections including two deep seismic reflection lines and numerous wells allowed us to quantify the flexural curvature of the lithosphere in addition gravity data are used to obtain independent constraints two evolutionary scenarios involving subduction underthrusting of passive margin and a phase change in the lower crust are investigated in a context of relatively flat moho under the area furthermore recent uplift expressing itself in erosional features observed along the distal margin of the foredeep is quantified the generally modest bouguer gravity anomaly points to nonisostatic processes causing this uplift steeply dipping basement under the carpathians to about 10 km depth at the location of the pieniny klippen belt is explained by relatively weak lithosphere which is expressed in the predicted low values of effective elastic thickness of 16 km decreasing locally to 3 km the low strength of the european lithosphere in this region is probably caused by mechanical weakening controlled by partial yielding due to high flexural curvature the weakest parts of the lithosphere along the profiles corresponding to low effective elastic thicknesses are concentrated in the areas of maximum bending predicted flexural bending stresses as a function of bending moment and effective elastic thickness increase dramatically under the outer zone of the carpathian belt explaining reactivations of older basement structures in these regions

on the coupling between barotropic and baroclinic modes of extratropical atmospheric variability abstractthe baroclinic and barotropic components of atmospheric dynamics are usually viewed as interlinked through the baroclinic life cycle with baroclinic growth of eddies connected to heat fluxes barotropic decay connected to momentum fluxes and the two eddy fluxes connected through the eliassen palm wave activity however recent observational studies have suggested that these two components of the dynamics are largely decoupled in their variability with variations in the zonal mean flow associated mainly with the momentum fluxes variations in the baroclinic wave activity associated mainly with the heat fluxes and essentially no correlation between the two these relationships are examined in a dry dynamical core model under different configurations and in southern hemisphere observations considering different frequency bands to account for the different timescales of atmospheric variability it is shown that at intermediate periods longer than 10 days the decoupling of the baroclinic and barot

a flexural isostatic model of lithosphere shortening and foreland basin formation application to the eastern cordillera and subandean belt of nw argentina a numerical model of lithosphere shortening has been developed to combine flexural isostatic and structural balancing this allows us to quantitatively reconcile lithosphere shortening thrust sheet emplacement and foreland basin formation lithosphere shortening is accommodated by thrusting in the upper crust and by distributed deformation in the lower crust and the lithospheric mantle this leads to loading by thrust sheet emplacement crustal thickening and thermal perturbation additional loading and unloading are generated by sedimentation and erosion respectively the model is kinematically controlled each fault having its position geometry and movement prescribed the model is applied to the subandean late oligocene to recent foreland fold and thrust belt of nw argentina at 22 15 s the subandean belt of nw argentina contains three major stratigraphic units the lower middle and upper terciario subandino formations by modelling these formations we attempt to determine whether the terciario subandino group can be explained as a foredeep fill in response to thrust sheet loading and if so what is the required effective elastic thickness a successful model must predict not only foreland stratigraphy but also exhumation topography and crustal thickness we see a change in the wavelength of sedimentary deposition in the subandean basin through time that can be accounted for by a change in flexural rigidity accommodation space for the lower terciario subandino formation can be generated using an elastic thickness of 70 km for the middle upper terciario an elastic thickness of 15 km produces the correct basin geometries shortening estimates of 110 km have proved sufficient to generate the observed foreland stratigraphy bouguer anomaly and topography across the subandean belt and eastern cordillera however an additional 140 km of shortening is required to generate the observed bouguer anomaly and topography across the western andes this discrepancy could be due to earlier shortening in the western andes or to crustal thickening by magma addition

a new greenland deep ice core the polar ice sheets are rich sources of information on past atmospheric conditions including paleoclimates a new deep ice core has been drilled in south greenland comparison of the oxygen isotopic profile with that from camp century and with a deep sea foraminifera record indicates that the new core reaches back to about 90 000 years before present in a continuous sequence the details in the wisconsin part of the ice core records seem to be climatically significant and the general trends reveal all of the relevant emiliani stages recorded in deep sea cores the redated camp century record suggests a dramatic termination of the eem sangamon interglacial

weak depth and along strike variations in stretching from a multi episodic finite stretching model evidence for uniform pure shear extension in the opening of the south china sea the south china sea is widely believed to have been opened by seafloor spreading during the cenozoic the details of its lithospheric extension are still being debated and it is unknown whether pure simple or conjunct shears are responsible for the opening of the south china sea the depth dependent and along strike extension derived from the single stage finite stretching model or instantaneous stretching model is inconsistent with the observation that the south china sea proto margins have experienced multi episodic extension since the late cretaceous based on the multi episodic finite stretching model we present the amount of lithosphere stretching at the northern continental margin of the south china sea for different depth scales upper crust whole crust and lithosphere and along several transects the stretching factors are estimated by integrating seven deep penetration seismic profiles the moho distribution derived from gravity modeling and the tectonic subsidence data for 41 wells the results demonstrate that the amount of stretching increases rapidly from 1 1 at the continent shelf to over 3 5 at the lower slope but the stretching factors at the crust and lithosphere scales are consistent within error from the uncertainty in paleobathymetry and sea level change furthermore the along strike variation in stretching factor is within the range of 1 11 1 9 in west east direction accompanied by significant west east differences in the thickness of high velocity layers hvls within the lowermost crust this weak along strike variation of the stretching factor is most likely produced by the preexisting contrasts in the composition and thermal structure of the lithosphere the above observations suggest that the continental extension in the opening of the south china sea mainly takes the form of a uniform pure shear rather than depth dependent stretching

on the origin of the southern permian basin central europe abstract a detailed study of the structural and stratigraphic evolution of the southern permian basin during latest carboniferous to early jurassic times supported by quantitative subsidence analyses and forward basin modelling for 25 wells leads us to modify the conventional model for the rotliegend zechstein development of this basin the late permian early jurassic tectonic subsidence curves are typical for a permian to early triassic extensional stage that is followed by thermal subsidence however a purely extensional model is extremely problematic because active faulting during this time is minor and generally hard to document using inverse techniques to model the subsidence curves we quantitatively show that a significant component of late permian and triassic tectonic subsidence can be explained by thermal relaxation of early permian lithospheric thinning and by delayed infilling of paleo topographic depressions that developed during the early permian in this interpretation stephanian autunian wrenching resulted in thermal destabilisation of the lithosphere deep fracturing of the crust disruption and erosion of its sedimentary cover and regional uplift of the area of the future southern permian basin upon termination of wrench tectonics and associated volcanism towards the end of the autunian the southern permian basin began to subside in response to thermal contraction of the lithosphere the evolving basin was isolated from the world oceans and had subsided possibly up to some 700 m below their level at the beginning of upper rotliegend sedimentation after catastrophic flooding of this paleo topographic depression at the beginning of the zechstein changing sea level sedimentation and subsidence rates remained essentially in balance although the effects of triassic rifting overprinted parts of the southern permian basin its overall subsidence pattern persisted well into the jurassic in contrast to the remainder of the southern permian basin permian and triassic crustal extension contributed significantly to the subsidence of the polish trough

mesozoic sedimentary evolution of the northwest sichuan basin implication for continued clockwise rotation of the south china block we present new sedimentary data integrated into a regional mesozoic stratigraphic framework to provide a detailed picture of spatio temporal variations in deposition and depocenter migration of the northwest sichuan basin the mesozoic sedimentary evolution is utilized to interpret basin subsidence history and to unravel coeval basin margin tectonics the northwest sichuan basin together with the songpan ganzi terrane behaved as a passive margin south of the qinling paleo tethys from late paleozoic to early middle triassic times and then evolved into a peripheral foreland basin in response to collision of the north and south china blocks since the late middle triassic coeval with strong north south contraction of the songpan ganzi terrane in the late triassic sinistral transpressional deformation of the longmen shan belt led to flexural subsidence of the adjacent western sichuan basin renewed basin margin fold thrust activity triggered recurrence of flexural subsidence of the northwest sichuan basin since the middle jurassic with the depocenter eventually shifting to the northwestern corner of the basin in the early cretaceous sedimentary evolution of the northwest sichuan basin and the basin margin deformation imply that the south china block had been rotating clockwise relative to the north china block throughout the mesozoic with an interim period of early jurassic tectonic quiescence a model is advanced that invokes clockwise rotation of the south china block as a driver for tectonic evolution of both the basin and adjoining structural belts and provides an explanation for several salient features that are otherwise puzzling

temperature control of continental lithosphere elastic thickness te vs vs article i nfo the elastic thickness of the continental lithosphere is closely related to its total strength and therefore to its susceptibility to tectonic deformation and earthquakes recently it has been questioned whether the lithosphere thickness and strength are dependent on crustal and upper mantle temperatures and compositions as predicted by laboratory data we test this dependence regionally by comparison in northwestern north america of the effective elastic thickness te from topography gravity coherence with upper mantle temperatures mapped by shear wave tomography velocities vs and other temperature indicators the te values are strongly bimodal as found globally less than 20 km for the hot cordillera backarc and over 60 km for the cold stable craton these te correspond to low vs beneath the cordillera and high vs beneath the craton model temperature depth profiles are used to estimate model te for comparison with those observed only limited areas of intermediate thermal regimes i e thermotectonic ages of 300 ma have intermediate te that suggest a weak lower crust over a stronger upper mantle there are large uncertainties in model te associated with composition water content strain rate and decoupling stress threshold however with reasonable parameters model yield stress envelopes correspond to observed te for thermal regimes with 800 900 c at the cordillera moho and 400 500 c for the shield in agreement with temperatures from vs and other estimators our study supports the conclusion that lithosphere elastic thickness and strength are controlled primarily by temperature and that laboratory based rheology generally provides a good estimate of the deformation behaviour of the crust and upper mantle on geological time scales

the mobility of zirconium and other immobile elements during hydrothermal alteration abstract development of zircon and other zr phases in hydrothermal deposits indicates that zr can be highly mobile in these systems mobility is most common in but not restricted to f rich hydrothermal systems related to alkalic f rich igneous suites these suites can range from peralkaline through metaluminous to peraluminous a few examples are neither alkalic nor f rich three locations in the trans pecos magmatic province texas u s a demonstrate this hydrothermal zr mobility all three igneous systems are alkalic and f rich but vary in alkali al ratios peralkaline rhyolites and trachytes in the christmas mountains contain as much as 2100 ppm zr mostly in aegirine or arfvedsonite zircon is rare or absent fluorspar replacement deposits in limestone at contacts with the rhyolites contain as much as 38 000 ppm zr occurring as small disseminated zircons the deposits also are enriched in a variety of incompatible elements including be rare earth elements ree y nb mo hf pb th and u the sierra blanca intrusions a series of mildly peraluminous f rich rhyolite laccoliths contain as much as 1000 ppm zr mostly as zircon hydrothermal zircon occurs as overgrowths on magmatic grains as veinlets connected to overgrowths and in fluorspar replacement bodies in adjacent limestone the highest zr concentrations in fluorspar are 200 ppm metaluminous quartz monzonite from the infiernito caldera contains 400 600 ppm zr mostly as zircon euhedral zircon in quartz fluorite veins in the quartz monzonite indicates mobility of zr zirconium concentrations in the veins are unknown but the paucity of zircon suggests little zr enrichment relative to the host zircon and more rarely zirconolite occur in skarn in the ertsberg district of irian jaya indonesia unlike in texas related igneous rocks are metaluminous and the hydrothermal system was f poor worldwide hydrothermal zircon other zr phases and ti and al bearing phases occur in skarn epithermal precious metal veins volcanogenic massive sulfide deposits and mylonites we propose that differences in zr mineralogy of igneous source rocks is an important factor in determining the availability of zr to hydrothermal fluids although zr concentrations in the sierra blanca and christmas mountains rhyolites are similar zr enrichment in fluorspar was much greater in the christmas mountains we suggest that hydrothermal solutions could easily break down aegirine and arfvedsonite to release zr but that zircon was only moderately attacked therefore far more zr was available for transport and subsequent deposition in the christmas mountains than at sierra blanca availability of other trace elements probably is also governed by their mineral host although zr mobility is most common in f rich hydrothermal systems related to alkalic and f rich igneous systems mobility at ertsberg may have been promoted by sulfate complexing

flexure of the continental lithosphere with multilayered rheology summary in this paper a model of flexure of the continental lithosphere is derived taking into account crustal and mantle rheologies bending of the continental lithosphere is modelled with a double yield stress envelope three layers brittle elastic and ductile for the crust and three analogous layers for the mantle portion the deformation of the layers is controlled by the rheological properties of quartz rich crustal rocks and olivine rich mantle rocks the influence of various factors such as the depth of moho strain rates thermal structure of the lithosphere boundary conditions and topographic load is examined results show that the mechanical strength of the continental lithosphere in the horizontal and vertical directions is primarily controlled by the present thermal structure of the plate boundary forces and moments and the applied topographic load this explains why mountainous regions may be more locally compensated than adjacent regions we also thus are able to explain why many continental plates have apparent effective rigidities much smaller than those predicted on the basis of their geological ages the model is then applied to the tien shan tarim area central asia and original topography and gravity data are used to constrain parameters of the model we found that the model satisfactorily matches the data and is also able to predict the thermal state of the plate and the location of the deep seismicity

chemical and thermal zonation in a mildly alkaline magma system infiernito caldera trans pecos texas postcollapse lavas of the infiernito caldera grade stratigraphically upward from nearly aphyric high silica rhyolite 76 sio2 to highly prophyritic trachyte 62 sio2 plagioclase clinopyroxene orthopyroxene magnetite ilmenite and apatite occur as phenocrysts throughout the sequence sanidine biotite and zircon are present in rocks with more than about 67 sio2 major and trace elements show continuous variations from 62 to 76 sio2 modeling supports fractional crystallization of the observed phenocrysts as the dominant process in generating the chemical variation temperatures calculated from coexisting feldspars pyroxenes and fe ti oxides agree and indicate crystallization from slightly more than 1100 c in the most mafic trachyte to 800 c in high silica rhyolite the compositional zonation probably arose through crystallization against the chilled margin of the magma chamber and consequent rise of more evolved and therefore less dense liquid mineral compositions vary regularly with rock composition but also suggest minor mixing and assimilation of wall rock or fluids derived from wall rock mixing between liquids of slightly different compositions is indicated by different compositions of individual pyroxene phenocrysts in single samples liquid solid mixing is indicated by mineral compositions of glomerocrysts and some phenocrysts that apparently crystallized in generally more evolved liquids at lower temperature and higher oxygen fugacity than represented by the rocks in which they now reside glomerocrysts probably crystallized against the chilled margin of the magma chamber and were torn from the wall as the liquid rose during progressive stages of eruption assimilation is indicated by rise of oxygen fugacity relative to a buffer from more mafic to more silicic rocks calculation of density and viscosity from the compositional and mineralogical data indicates that the magma chamber was stably stratified lower temperature but more evolved thus less dense rhyolite overlay higher temperature less evolved and therefore more dense progressively more mafic liquids the continuity in rock and mineral compositions and calculated temperature viscosity and density indicate that compositional gradation in the magma chamber was smoothly continuous any compositional gaps must have been no greater than about 2 sio2

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

compositional changes in trans pecos texas magmatism coincident with cenozoic stress realignment the composition and inferred sources of cenozoic magmas in trans pecos texas changed abruptly between 32 and 28 ma coincident with a fundamental change in stress regime magmas emplaced between 48 and 32 ma comprise extended differentiation suites beginning with relatively evolved basaltic magmas these rocks have low nb and ta compared to zr hf la ba and k characteristics typical of continental volcanic arcs however the texas rocks are more alkalic have higher concentrations of incompatible elements and have higher ratios of nb and ta to zr hf la ba and k than is typical of rocks from arcs near trenches these differences reflect relative position within the cordilleran arc trace element models involving partial melting in the mantle wedge combined with fractional crystallization and assimilation of upper and lower crust can account for most of the observed trace element concentrations and ratios trans pecos magmatism may have involved smaller degrees of partial melting than is typical of arcs located closer to the subduction zone magmas emplaced after 31 ma were either bimodal alkali basalt rhyolite or exclusively basaltic they have high absolute abundances of incompatible trace elements and high ratios of nb and ta to zr hf la ba and k the mafic rocks are typical of continental rift or ocean island basalts trace element models using nb ta y yb hf and zr reproduce the range of concentrations and ratios observed in these rocks and require very small degrees of partial melting variable amounts of fractional crystallization but little or no assimilation the change in magma compositions and sources is best illustrated by a drop in zr nb from between 18 and 6 25 in the earlier arc rocks to between 6 25 and 2 in the later rift related rocks stress regime appears to play an important role in the process of magma generation and evolution the change in trace element compositions indicative of tectonic setting supports interpretations of paleostress data that activity up to 31 ma occurred in a continental volcanic arc whereas later activity occurred in a setting of intraplate extension the magmatic change may have been simultaneous with the change in stress regime or may have lagged by as much as 3 4 m y

paleoclimatic variability inferred from the spectral analysis of greenland and antarctic ice core data paleoclimate variations occur at various time scales between a few centuries for the heinrich events and several hundreds of millenia for the glacial to interglacial variations the recent ice cores from greenland greenland ice core project and greenland ice sheet project 2 and antarctica vostok span at least one glacial oscillation and provide many opportunities to investigate climate variations with a very fine resolution the joint study of cores from both hemispheres allows us to distinguish between the sources of variability and helps to propose mechanisms of variations for the different time scales involved the climate proxies we analyze are inferred from 18o and d for temperature and chemical species such as calcium for the joint behavior of the major ions in the atmosphere which yield an estimate of the polar circulation index those data provide time series of climatic variables from which we extract the information on the dynamics of the underlying system we used several independent spectral analysis techniques to reduce the possibility of spurious results those methods encompass the multitaper spectral analysis singular spectrum analysis maximum entropy method principal component analysis minimum bias spectral estimates and digital filter reconstructions our results show some differences between the two hemispheres in the slow variability associated with the astronomical forcing common features found in the three ice core records occur on shorter periods between 1 and 7 kyr the holocene also shows recurrent common patterns between greenland and antarctica we propose and discuss mechanisms to explain such behavior

probabilistic tectonic heat flow modeling for basin maturation assessment method and applications tectonic modeling is often neglected in the basin modeling workflow and heat flow is most times considered a user input such heat flows can therefore result in erroneous basin modeling outcomes resulting in false overoptimistic identification of prospective areas or failure to identify prospects this is particularly true for areas with limited data control such as frontier basin areas or deep unexplored plays in mature basins three major factors obstruct routinely use firstly because of the focus of most tectonic models on lithosphere scale processes a large range of models including the mckenzie rift model fail to take into account effects which are of paramount importance for basement heat flow such as transient effects of sediment infill and erosion and changes in crustal heat production over time secondly lithosphere tectonic models often fail to allow inversion of basin data making forward tectonic modeling a cumbersome exercise non vertical sediment movements and 2d and 3d loading effects can play an important role hampering a unique inversion thirdly lithosphere tectonic models generally fail to aid the user to understand the sensitivity of the model results in terms of basin maturation for permissible ranges of tectonic model parameters and for uncertainties in tectonic scenarios such as absence or presence of underplating or two layered stretching vs a mckenzie model for this reason we have developed a multi 1d probabilistic tectonic heat flow model which is capable of calculating tectonic heat flows incorporating a variety of tectonic scenarios the model is capable of inversion of burial histories calibrated to temperature and maturity data calibration and sensitivity analysis is done through monte carlo sampling analysis using an experimental design technique for computational efficiency the tectonic heat flows can easily be used as input for basin modeling in commercially available 3rd party software 2009 elsevier ltd all rights reserved

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

role of the 3 d distributions of load and lithospheric strength in orogenic arcs polystage subsidence in the carpathians foredeep abstract it has been widely documented that the depth of foredeeps does not always reflect the topography of the neighboring orogens in many cases the topographic load is insufficient to explain basin subsidence such is the case of the se carpathians where an anomalously deep almost 13 km foreland basin has evolved since the middle miocene badenian a peculiar feature of this basin is its position relative to the orogen in contrast to typical foredeeps which deepen towards the belt the maximum depth of this basin is 10 20 km out of the orogen the subsidence in the carpathians bend foreland is characterized by two stages the first is middle miocene badenian in age and is related to ne sw extension when fault bounded basins were formed modeling shows that the foreland underwent small pre orogenic uniform thinning the modeling also predicts

heat flow and thermal history of the south china sea abstract a total of 589 heat flow values are available from the south china sea the values are widely scattered ranging from 9 to 181 mw m 2 with a mean of 77 mw m 2 in the northern margin the values are less scattered and the average is about 75 mw m 2 heat flow increases gradually from the northern margin to the central basin in which two high heat flow centers appear one in the east subbasin and the other in the southwest subbasin the southern margin has an average heat flow of 80 mw m 2 similar to the northern margin the western margin where the manila trench is located has an average heat flow as low as 49 mw m 2 in order to understand the high heat flow in the basin in the context of its tectonic history we carried out thermal modeling using a 2d finite element method along a profile across the northern margin a theoretical thermal history has been inferred on the basis of a multistage pure shear extension model the result shows that before miocene the basement heat flow showed relatively low values ranging from 48 to 57 mw m 2 since late miocene the basement heat flow has increased as a result of greater extension the present day high heat flow is primarily the result of pliocene extension

geodynamic models of convergent margin tectonics transition from rifted margin to overthrust belt and consequences for foreland basin development a quantitative geodynamic model for overthrusting of a passive continental margin during attempted continental subduction demonstrates the mechanical and thermal coupling between overthrust loads the lithosphere and the associated foreland basin the model treats the lithosphere as a two dimensional nonuniform elastic plate whose strength is controlled thermally the thermal and flexural evolution of a margin is followed from initial rifting and passive margin development through overthrusting and foreland basin deposition to postdeformational erosion our models indicate the following 1 foreland basin geometry is relatively insensitive to the age of the preexisting passive margin prior to overthrusting sensitivity decreases the farther the overthrust load is empl ced onto the craton 2 foreland basin depth is strongly influenced by the size of the overthrust load maximum depths in excess of 10 km 6 mi can occur given overthrust topography similar to the present day himalayas 3 a few tens of kilometers of postdeformational erosion can occur across topographically high orogenic belts before moderate topography is attained very little of this eroded material is deposited in the foreland basin most must bypass the basin because it also is subjected to erosion and associated flexural uplift 4 thick overthrusts as interpreted from cocorp profiling are consistent with low topographic expression because they were emplaced on preexisting oceanic and stretched continental lithosphere

temporal geographical meltwater influences on the north atlantic conveyor implications for the younger dryas the temporal and geographical roles of meltwater discharge from the laurentide ice sheet on north atlantic deep water nadw production are investigated utilizing a global realistic geometry coupled climate model which does not require the use of flux adjustments model results suggest that preconditioning by meltwater discharge to the mississippi prior to the younger dryas yd is capable of pushing nadw beyond the limit of its sustainability the diversion of meltwater to the st lawrence then merely serves to completely inhibit nadw production the modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the yd and is intimately linked to changes in nadw formation the global thermohaline circulation provides an interhemispheric teleconnection with the southern oceans while changes in the atmospheric heat transport reacting to a global redistribution of oceanic heat transport provide a mechanism for interbasin teleconnection although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model in the absence of additional feedbacks an equilibrium without the presence of nadw is possible the inclusion of the wind stress speed feedback is found to significantly contribute to the resumption of nadw production as suggested by previous studies contrary to these same studies however the coupled model indicates an advective spin up timescale is required for resumption of nadw production and hence the termination of the modeled yd like climate event as opposed to a decadal century timescale the reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies or the duration strength and geographical location of the imposed meltwater applied

lateral variations in mechanical properties of the romanian external carpathians inferences of flexure and gravity modelling we present the results of two dimensional flexure and gravity modelling of the subsidence of the romanian carpathian foreland based on twenty profiles through the southern and eastern carpathians the narrow spacing of the grid of profiles allows us to investigate the lateral tectonic variations transverse to the belt but also along strike the topographic elevation of the romanian carpathian mountain belt is modest and the minimum in the bouguer gravity anomaly characteristic for flexural control of subsidence is located relatively far towards the foreland in respect to the mountain belt as the contribution of topographic loading to the evolution of the romanian carpathian foreland system is small a subduction underplating dominated tectonic regime controlled the nappes emplacement and basin shortening in the external flysch and molasse basins during the late tertiary the modelling results support the existence of important variations in effective elastic thicknesses te and plate boundary forces high flexural bending stresses in the western part of the southern carpathians can explain low te values whereas the increase of te to the east can be explained by changes in rheological properties of the moesian platform field observations indicate that variations in deflection along strike are probably also related to basement unregularities stress field rotations and strike slip movement along lateral ramps during the tertiary in the eastern carpathians the model results indicate a n s trend of lateral back stepping of the subduction system a distinct back step in the vicinity of the seismically active vrancea area supports the involvement of the e w crustal scale trotus fault which forms the transition from the east european platform to the scythian and moesian platforms development of a possible n s migration of plate boundary activity or slab detachment and successive intersection with the trotus fault can explain the large pliocene subsidence in the focsani depression and relative uplife of the east european platform north of the mentioned fault

composition and correlation of volcanic ash beds of silurian age from the eastern baltic sanidine composition and bulk geochemistry of volcanic ash beds from the east baltic indicate the subalkaline nature of the volcanism near the margins of the baltica plate during the silurian several bentonites in the wenlock include a previously unknown sanidine with 48 to 58 mol of the na ca component in contrast to the earlier telychian volcanism sodium rich sanidine occurs in ash beds which originate from relatively moderately evolved dacitic magma the studied material from two drill cores integrated with previous research enables production of a more complete list of 49 volcanic eruption layers for the lower to middle wenlock in the east baltic this updated list of bentonites characterized by their sanidine compositions forms a good basis for future integrated bio and chemostratigraphic correlations in northern europe

apatite liquid partition coefficients for the rare earth elements and strontium abstract sixteen sets of apatite liquid partition coefficients dap liq for the rare earth elements ree la sm dy lu and six values for sr were experimentally determined in natural systems ranging from basanite to granite the apatite melt glass assemblages were obtained from starting glasses artificially enriched in ree sr and fluorapatite components these were run under dry and hydrous conditions of 7 5 20 kbar and 950 1120 c in a solid media piston cylinder apparatus an sem equipped electron microprobe was used for subsequent measurement of ree and sr concentrations in coexisting apatites and quenched glasses the resulting partition coefficient patterns resemble previously determined apatite phenocryst groundmass concentration ratios in the following respects 1 the rare earth patterns are uniformly concave downward i e the middle ree are more compatible in apatite than the light and heavy ree 2 dreeap liq is much higher for silicic melts than for basic ones and 3 strontium and therefore eu2 is less concentrated by apatite than are the trivalent ree the effects of both temperature and melt composition on dreeap liq are systematic and pronounced at 950 c for example a change in melt sio2 content from 50 to 68 wt causes the average ree partition coefficient to increase from 7 to 30 a 130 c increase in temperature on the other hand results in a two fold decrease in dreeap liq partitioning of sr is insenstitive to changes in melt composition and temperature and neither the sr nor the ree partition coefficients appear to be affected by variations in pressure or h2o content of the melt the experimentally determined partition coefficients can be used not only in trace element modelling but also to distinguish apatite phenocrysts from xenocrysts in rocks reported apatite megacryst host basalt ree concentration ratios 12 for example are considerably higher than the equilibrium partition coefficients which suggest that in this particular case the apatite is actually xenocrystic a reversal experiment incorporated in our study yielded diffusion profiles of ree in apatite from which we extracted a ree ca interdiffusion coefficient of 2 4 10 14 cm2 s at 1120 c extrapolated downward to crustal temperatures this low value suggests that complete ree equilibrium between felsic partial melts and residual apatite is rarely established

the evolution of aqueous vapor from silicate melts effect on oxygen fugacity abstract the simplest possible aqueous vapor evolved from an iron bearing magma under normal conditions is an h 2 o h 2 mixture hydrogen is produced by the oxidation of fe ii to fe iii by water in the magma yielding an increase in f o 2 equations are derived which model this mass transfer process the resulting differential equation expresses the change in magmatic f o 2 as a function of a change in n 1 h 2 o the vapor evolution progress variable the calculations demonstrate that in this simple scheme very iron poor magmas h 2 o feo are required to produce significant increases in magmatic f o 2 the incremental increase in o 2 during vapor evolution becomes progressively smaller as o 2 increases with this process becoming highly inefficient as the hematite stability field is approached other more complete processes involving meteoric water assimilation of altered rocks and subsolidus processes may produce the observed increase in f o 2 inferred to have occurred in systems of normal iron contents in low iron systems such as the high silica rhyolitic magmas associated with climax type porphyry molybdenum systems high oxygen fugacities may be obtained by this mechanism

mineralogy and geothermometry of high temperature rhyolites from the central and western snake river plain voluminous mid miocene rhyolitic ash flow tuffs and lava flows are exposed along the northern and southern margins of the central and western snake river plain these rhyolites are essentially anhydrous with the general mineral assemblage of plagioclase sanidine quartz augite pigeonite hypersthene fayalitic olivine fe ti oxides apatite zircon which provides an opportunity to compare feldspar pyroxene and fe ti oxide equilibration temperatures for the same rocks estimated pyroxene equilibration temperatures based on the geothermometers of lindsley and coworkers range from 850 to 1000 c and these are well correlated with whole rock compositions with the exception of one sample agreement between the two pyroxene thermometers tested is well within 50 c fe ti oxide geothermometers applied to fresh magnetite and ilmenite generally yield temperatures about 50 to 100 c lower than the pyroxene temperatures and erratic results are obtained if these minerals exhibit effects of subsolidus oxidation and exsolution results of feldspar thermometry are more complicated and reflect uncertainties in the thermometer calibrations as well as in the degree of attainment of equilibrium between plagioclase and sanidine in general temperatures obtained using the ghiorso 1984 and green and usdansky 1986 feldspar thermometers agree with the pyroxene temperatures within the respective uncertainties however uncertainties in the feldspar temperatures are the larger of the two and exceed 60 c for many samples the feldspar thermometer of fuhrman and lindsley 1988 produces systematically lower temperatures for many of the samples studied the estimated pyroxene temperatures are considered most representative of actual magmatic temperatures for these rhyolites this range of temperatures is significantly higher than those for rhyolites from many other suites and is consistent with the hypothesis that the snake river plain rhyolitic magmas formed by partial fusion of relatively dry e g granulitic crustal lithologies

heat flow and thermal modeling of the yinggehai basin south china sea abstract geothermal gradients are estimated to vary from 31 to 43 c km in the yinggehai basin based on 99 temperature data sets compiled from oil well data thirty seven thermal conductivity measurements on core samples were made and the effects of porosity and water saturation were corrected thermal conductivities of mudstone and sandstone range from 1 2 to 2 7 w m k with a mean of 2 0 0 5 w m k after approximate correction heat flow at six sites in the yinggehai basin range from 69 to 86 mw m2 with a mean value of 79 7 mw m2 thick sediments and high sedimentation rates resulted in a considerable radiogenic contribution but also depressed the heat flow measurements indicate the radiogenic heat production in the sediment is 1 28 w m3 which contributes 20 to the surface heat flow after subtracting radiogenic heat contribution of the sediment and sedimentation correction the average basal heat flow from basement is about 86 mw m2 three stages of extension are recognized in the subsidence history and a kinematic model is used to study the thermal evolution of the basin since the cenozoic era model results show that the peak value of basal heat flow was getting higher and higher through the cenozoic the maximum basal heat flow increased from 65 mw m2 in the first stage to 75 mw m2 in the second stage and then 90 mw m2 in the third stage the present temperature field of the lithosphere of the yinggehai basin which is still transient is the result of the multistage extension but was primarily associated with the pliocene extension

the deglaciation of the northern hemisphere a global perspective orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming ice albedo sealevel and greenhouse gas feedbacks together with tropical warming from weakening winds in response to polar amplification of warming caused regionalto global near synchronization of deglaciation effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly ice sheet linked changes in freshwater delivery to the north atlantic and possibly free oscillations in the climate system forced millennial climate oscillations associated with changes in north atlantic deep water nadw flow the north atlantic typically operates in one of three modes modern glacial and heinrich deglaciation occurred from a glacial mode ocean that in comparison to modern had shallower depth of penetration of nadw formed further south causing strong northern cooling and the widespread cold dry and windy conditions associated with the glacial maximum and the cold phases of the millennial dansgaard oeschger oscillations the glacial mode was punctuated by meltwater forced heinrich conditions that caused only small additional cooling at high northern latitudes but greatly reduced the formation of nadw and triggered an oceanic seesaw that warmed some high latitude southern regions centered in the south atlantic

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

thermal history and tectonic subsidence of the bohai basin northern china a cenozoic rifted and local pull apart basin the bohai basin is a part of the larger bohai bay basin a cenozoic rifted intraplate basin heat flow measurements show that the bohai basin is characterized by present day heat flow varying between 53 and 74 mw m 2 with a mean of 63 mw m 2 however thermal history analyses derived from vitrinite reflectance vr and apatite fission track aft data indicate that tertiary cooling took place following a period of much higher paleo heat flow 70 90 mw m 2 prior to 25 million years to the present furthermore tectonic subsidence analysis reveals that the bohai basin experienced episodic sub rifting processes from the eocene to the end of the oligocene the post rift thermal subsidence was superimposed by intensified subsidence from the late miocene 12 million years in the bozhong depression and from the early quaternary 2 4 million years in the east liao bay indicating a rejuvenation of sedimentation related likely to transpressional structures developed locally on strike slip faults the present results indicate that there is good agreement between the reconstructed thermal history and the timing of the tectonic subsidence stages within the bohai basin the bohai basin is an early cenozoic rifted and local pull apart sub basin of the larger bohai bay basin 2001 elsevier science b v all rights reserved

structural and thermal characters of the longmen shan sichuan china article i nfo the longmen shan sichuan china is characterised by an unusual morphology which results from a triassic prism tectonics and a more recent neogene thick skin thrusting among its specific features isthe high elevation of the internal zones incontinuity with the songpan garze fold and thrust belt sg which is associated with an abrupt 20 km moho offset between the sichuan basin and the tibetan plateau as revealed by the analysis of teleseismic data acquired by a dense seismic network the jump in crustal thickness is located at the apex of the wenchuan shear zone wsz and marks the western boundary of the metamorphosed units of sg characterized by temperatures varying from 590 c down to 300 c as commonly observed in mature accretionary wedges both the structural style marked by intense shortening in tight kink folds and geophysical data suggest the presence of an horizontal discontinuity at 15 km depth over a thick crust 63 km the longmen shan east of the wsz is characterized by thick skin detachment which thrusts the internal sedimentary unitsandtheproterozoicbasementoverthe seriesof thesichuanbasindepositedona thinnercrust of 44 km thick the majorfront isthebeichuanfault zone bfz which broughtthe internalzones onto triassic and jurassic series with lower temperatures less than 400 c locally temperatures of 425 c are found just below the klippes these results are in agreement with an original contact of the sg zone represented by an accretionary wedge of sediments thrusted over the margin of the continental crust of the yangtze craton in the early stage indosinian of the evolution the present day slow e w component of the convergence added to the difference in crustal thickness caused the yangtze crust to indent the songpan tibetan crust which was softened by a high thermal regime asaresponsetheedgeofthetibetcrustwasinflatedtothebottom upto70 km whereastothetop the crystalline massif were exhumed and pushed the deformation eastward as emerging and blind thrusts this configuration reflects a moderate shortening of the crust which behaves as a soft thick material abutting the

millennial variability in an idealized ocean model predicting the amoc regime shifts abstracta salient feature of paleorecords of the last glacial interval in the north atlantic is pronounced millennial variability commonly known as dansgaard oeschger events it is believed that these events are related to variations in the atlantic meridional overturning circulation and heat transport here the authors formulate a new low order model based on the howard malkus loop representation of ocean circulation capable of reproducing millennial variability and its chaotic dynamics realistically it is shown that even in this chaotic model changes in the state of the meridional overturning circulation are predictable accordingly the authors define two predictive indices which give accurate predictions for the time the circulation should remain in the on phase and then stay in the subsequent off phase these indices depend mainly on ocean stratification and describe the linear growth of small perturbations in the system thus monitoring particular indices of the ocean state could help predict

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

initiation of asymmetric extension in continental lithosphere govers r and wortel m r 1993 initiation of asymmetric extension in continental lithosphere in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 x 96 the physical conditions are investigated under which the lithospheric scale style of extension is pure shear or simple shear we focus on the initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions continental collision magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension we use a finite element method to model the thermo mechanical evolution of continental lithosphere experimental flow laws are used to model the elastic hutle power law creep or diffusion creep rheology of lithospheric rocks our results indicate that if in plane forces change from compressive to tensile immediately after a rapid mountain building phase initiation of a lithosphere scale detachment fault is possible we find a strong dependence of the extensional style on the distribution with depth of residual stresses from the collision phase this result is consistent with observations of gravitational collapse in regions like the aegean and the basin and range province where detachment faults have exhumed lower crustal rocks the predicted dip direction of the fault also agrees with observations in these areas intrusion of magma into continental lithosphere which is subject io in plane tensile forces will cause localization of pure shear deformation the style of deformation resulting from mantle plumes impinging to the base of the lithosphere is symmetric delamination of lithospheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase when in plane forces change sign from compressive to tenstle this result also strongly depends on the assumed residual stress distribution if delamination occurs during the mountam huilding phase the style of deformation will be pure shear another interesting outcome from our modeling is that dramatic strain weakening as a result of a deformation mechanism change from dislocation creep to diffusion creep reduces the tendency to strain localization a large volume of the published work on continental extension has been concentrated on quantifying the stretching factor p of the pure shear model of mckenzie 1978a in some basins it was found that the kinematics of deformation could be more accurately described by depth dependent stretching factors royden and keen 19801 which introduced asymmetry near basin edges in the simple shear model of wemicke

hydrostatic vertical velocity and incompressibility in the northern hemisphere the vertical velocity w is evaluated for the northern hemisphere from reanalysis data and two forms of the richardson equation this equation is based on the hydrostatic assumption and the thermodynamic energy equation the standard form of the richardson equation allows one to quantify the contributions to the vertical velocity of the horizontal divergence

stress orientations during oligocene volcanism in trans pecos texas timing the transition from laramide compression to basin and range tension orientations of dikes and hydrothermal veins indicate that east northeast compression occurred during the main pulse of volcanism in west texas 39 to 32 m y ago direction of compression was essentially the same as that during eocene laramide deformation the observations support a continental arc tectonic setting the beginning of extension between 32 and 30 m y ago coincides with a regional change in style of magmatism from intrusions and eruptions spanning a wide range in chemical composition to basaltic volcanism in texas and to bimodal but dominantly rhyolitic volcanism in northern mexico

temporal and spatial variations in tectonic subsidence in the iberian basin eastern spain inferences from automated forward modelling of high resolution stratigraphy permian mesozoic by subsidence analysis on eighteen surface sections and 6 wells which cover large part of the iberian basin e spain and which are marked by high resolution stratigraphy of the permian triassic jurassic and cretaceous we quantify the complex permian and mesozoic tectonic subsidence history of the basin backstripping analysis of the available high resolution and high surface density of the database allows to quantify spatial and temporal patterns of tectonically driven subsidence to a much higher degree than previous studies the sections and wells have also been forward modelled with a new automated modelling technique with unlimited number of stretching phases in order to quantify variations in timing and magnitude of rifting it is demonstrated that the tectonic subsidence history in the iberian basin is characterized by pulsating periods of stretching intermitted by periods of relative tectonic quiescence and thermal subsidence the number of stretching phases appears to be much larger than found by earlier studies showing a close match with stretching phases found in other parts of the iberian peninsula and allowing a clear correlation with discrete phases in the opening of the tethys and atlantic

the evolution of the suez rift a combination of stretching and secondary convection abstract different physical mechanisms have been put forward in order to explain the lithospheric evolution beneath a continental rift models of stretching of the lithosphere though giving good values for the trough subsidence could not satisfactorily account for other features of the rifts such as the rapid uplift of the rift shoulders in some cases however the idea that the thinning of the lithosphere is also accomplished by secondary convection phenomena is now receiving growing attention using a thermomechanical model dealing with a non newtonian crust and mantle moretti and froidevaux have suggested that the evolution of numerous continental rifts is the consequence of the combination of the upward propagation of an asthenospheric thermal anomaly and of an extensional regional stress the suez rift evolution examined by tectonic subsidence studies may be regarded as the result of an asthenospheric upwelling and of a short active initial lithospheric stretching phase

oxygen 18 16 variability in greenland snow and ice with 10 3 to 105 year time resolution the 3 km long greenland ice sheet project 2 gisp2 ice core presents a 100 000 year detailed oxygen isotope profile covering almost a full glacial interglacial cycle measuranents of isotopic fluctuations in snow frost and atmospheric water vapor samples collected during summer field seasons up to 20 0 are compatible with the large and abrupt 80 160 changes observed in accumulated tim snow pit 15 80 profiles from the gisp2 summit area however show rapid smoothing of the 180 160 signal near the surface beyond about 2 m depth the smoothedi5180 signal is fairly well preserved and can be interpreted in terms of average local weather conditions and climate the longer climate fluctuations also have regional and often global significance in the older part of the record corresponding to marine isotope stages mis 5a to 5d the effect of orbital climate forcing via the 19 and 23 kyr precession cycles and the 41 kyr obliquity cycle is obvious from the end ofmis 5a at about 75 000 years b p till the end of the glacial at the younger dryas preboreal transition at 11 650 years b p the o180 160 record shows frequent rapid switches between intermediate interstadial and low stadial values fourier spectra of the oscillations that are superimposed on the orbitally induced changes contain a strong periodicity at 1 5 kyr a broad peak at 4 0 kyr and additional shorter periods detailed comparison of the gisp2 180 160 record with the vostok antarctica 15d record pacific ocean foraminiferal 180 160 grande pile france tree pollen and insolation indicates that a counterpart to many of the rapid 180 160 fluctuations of gisp2 can be found in the other records and that the gisp2 isotopic changes clearly are the local expression of climate changes of worldwide extent correlation of events on the independent gisp2 and specmap time scales for the interval 10 000 50 000 years b p shows excellent chronometric agreement except possibly for the event labeled 3 1 the glacial to interglacial transition evidently started simultaneously in the arctic and the antarctic but its development and its expression in greenland isotopes was later suppressed by the influence of meltwater especially from the barents sea ice sheet on deep water formation and ocean circulation meltwaters from different ice sheets bordering the north atlantic also influenced ocean circulation during the bolling allerod interstadial complex and the younger dryas and led to a distinct development of european climate and greenland 180 160 values the holocene interval with long term stable mean isotopic values contains several fluauations with periods from years to millennia dominant is a 6 3 year oscillation with amplitude up to 3 to 4 0 periodicities of 11 and 210 years also found in the solar modulated records of the cosmogenic isotopes 1obe and 14c suggest solar processes as the cause of these cycles depression of180 160 values cooling by volcanic eruptions is observed in stacked gisp215 80 records but the effect is small and not likely to trigger major climate changes

a zonally averaged ocean model for the thermohaline circulation part ii interocean circulation in the pacific atlantic basin system abstract the zonally averaged latitude depth ocean model developed in part i is extended to a two basin system representing the atlantic and pacific steady states are calculated under two different surface boundary conditions to study a possible global thermohaline circulation linking the pacific and the atlantic if the surface temperature and salinity profiles are identical functions of latitude in both the pacific and the atlantic the steady state under restoring boundary conditions consists in spite of the different basin extensions of a two cell structure in each basin upon switching to mixed boundary conditions the state is unstable and a one cell circulation with downwelling at high northern latitudes develops in both basins for a realistic surface salinity profile that is fresher in the pacific the steady state under restoring boundary conditions is completely different it exhibits a global thermohaline circulation with strong interocean man exchange deep water is formed primarily in

stochastic resonance in the north atlantic large abrupt changes between warm and cold modes of north atlantic climate exhibit spectral power at 1500 years yet some climatic changes were linked to outburst floods and other events that are unlikely to have been truly periodic we hypothesize that a weak periodic forcing has combined with noise from ice sheet related events to cause the observed mode switches this stochastic resonance hypothesis predicts a recurrence pattern between warmings that is distinct from the predictions of simple periodic and stochastic models and at least some other models the ice isotopic data from central greenland ice cores are consistent with the stochastic resonance hypothesis but not with other models we have tested we thus support arguments for the existence of a periodicity of 1500 years in the north atlantic climate system and for the importance of ice sheet events in forcing north atlantic changes

episodicity in foredeep basins the stratigraphic record contained in several foredeep basins is incompatible with the formation of these basins by flexure of a homogeneously strong foreland lithosphere for example the stratigraphy of the early proterozoic kilohigok foredeep basin of northern canada indicates that the inner part of the basin deepened in time without migration of the flexural bulge toward the foreland and that the flexural bulge remained fixed in time relative to the downgoing plate one mechanism that may be responsible for the localization of the flexural bulge in the kilohigok and similar basins is that of lateral strength variations in the flexing lithosphere our results show that lateral strength variations within the foreland lithosphere can produce temporal variations in basin geometry and also serve to localize the position of the flexural bulge relative to the foreland for extended periods of time this produces basin episodicity on a time scale of millions to tens of millions of years perfectly elastic models require a large strength contrast about 50 in elastic plate thickness but a brittle elastic ductile lithosphere rheology reduces the magnitude of the strength contrast required for episodicity to about 10 15 of the initial effective elastic plate thickness we suggest that basin episodicity may induce comparable episodicity in the coeval thrust belt because theoretical studies of thrust mechanics suggest that the deformation in thrust belts is highly sensitive to the dip of the basal thrust angle through mechanisms such as this the laterally heterogeneous continental lithosphere may provide a natural filter that transforms the presumably continuous and smoothly varying processes of the mantle into the spatially irregular and temporally episodic deformation observed in the crust at time scales of tens of millions of years

the last glacial maximum climate over europe and western siberia a pmip comparison between models and data under the framework of the palaeoclimate modelling intercomparison project pmip 17 climate models 16 of which are atmospheric general circulation models have been run to simulate the climate of the last glacial maximum 21 000 years ago using the same set of boundary conditions parallel to these numerical experiments new consistent data bases have been developed on a continental scale the present work compares the range of the model responses to the large perturbation corresponding to the conditions of the last glacial maximum with consistently derived climate reconstructions from pollen records over europe and western siberia it accounts for the differences in the model results due to the models themselves and directly compares this error bar due to the models to the uncertainties in the climate reconstructions from the pollen records overall the last glacial maximum climate simulated by the models over western europe is warmer especially in winter and wetter than the one depicted by the reconstructions this is the region where the reconstructed increase in temperature precipitation and moisture index from the last glacial maximum to the present conditions is largest the same disagreement but of smaller amplitude is found over central europe and the eastern mediterranean basin while models and data are in broad agreement over western siberia the numerous modelling results allow a study of the link between the changes in atmospheric circulation and those in temperature and an interpretation of the discrepancies in precipitation in terms of those in temperature

simulation of an abrupt change in saharan vegetation in the mid holocene climate variability during the present inter glacial the holocene has been rather smooth in compar ison with the last glacial nevertheless there were some rather abrupt climate changes one of these changes the desertication of the saharan and arabian region some 4 6 thousand years ago was presumably quite important for human society it could have been the stimulus leading to the foundation of civilizations along the nile euphrat and tigris rivers here we argue that saharan and arabian de sertication was triggered by subtle variations in the earth s orbit which were strongly amplied by atmosphere vegeta tion feedbacks in the subtropics the timing of this tran sition however was mainly governed by a global interplay between atmosphere ocean sea ice and vegetation

the role of the deep ocean in north atlantic climate change between 70 and 130 kyr ago the suggestion1 that changes in north atlantic deep water nadw production are linked through surface heat flux to the atmospheric temperature over greenland is supported by earlier indications2 3 that nadw production decreased during glacial times and by the subsequent finding4 6 that it declined during the younger dryas cool period at the end of the last glaciation changes in north atlantic surface temperatures have been found7 to mirror high frequency temperature changes recorded in greenland ice cores over the past 80 kyr but the connection to abyssal circulation has yet to be established except for one or two isolated oscillations8 9 here we present carbon and oxygen isotope analyses of benthic foraminifera in a high resolution north atlantic deep sea sediment core for the period 70 130 kyr ago these data allow us to reconstruct the history of nadw production which shows a close correlation with greenland climate variability for much of this time interval suggesting that the climate influence of nadw variability was widespread we see no evidence however for changes in nadw production during substage 5e the eemian interglacial period in contrast with recent ice core data10 which suggest severe climate instability in greenland during this time period our results may support suggestions based on data from a second ice core that this apparent instability is an artefact caused by ice flow11 alternatively the eemian climate instability may have had a different origin from the subsequent climate events

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

new geochronology constraints on timing and depth of the ancient earthquakes along the longmen shan fault belt eastern tibet pseudotachylyte is an ideal target to directly date ancient earthquake associated with regional faulting here we perform step heating 40ar 39ar clay mineral k ar and zircon fission track zft analyses on the pseudotachylyte samples collected from the yingxiu beichuan co seismic rupture of the longmen shan fault belt lsfb to provide time constraints for the tectonic evolution of the lsfb during the indosinian orogeny 40ar 39ar results from the matrix show that the frictional melting occurred 226 235 ma ago combined with mylonite dating of the host rock the age of the ancient earthquakes is constrained at 231 238 ma with a formation depth of 10 14 km as a response to the earthquakes a series of soft sediment deformation structures are widely preserved in the middle and late triassic strata along the yingxiu beichuan fault ybf indicating that the lsfb was seismically active since the late anisian and persisted episodically until the end of the indosinian orogeny both clay mineral k ar and zft analyses record a younger deformation or alteration with an age of ca 195 ma corresponding to a post collisional orogeny these new data represents the first direct evidence of the regional thrusting ybf in the central lsfb during the indosinian orogeny concurrent with the initial ductile deformation of the western boundary fault tectonic inheritance then strongly influenced the evolution of the lsfb as most of the mesozoic faults are reactivated by major tertiary tectonic deformations

compositional layers in the zoned magma chamber of the grizzly peak tuff the 34 ma grizzly peak tuff in west central colorado preserves evidence for distinct compositional layers in a high level magma chamber intracaldera tuff consists of a single cooling unit at least 2 7 km thick zoned from a high silica rhyolite base to an eroded low silica rhyolite top two horizons of heterogeneous tuff beneath wedges of caldera collapse breccia contain fiamme of dacite to mafic latite along with the rhyolitic fiamme that make up the rest of the tuff compositional zoning defined by fiamme is a step function rather than a continuous gradient seven petrographic groups of fiamme each have distinct compositions separated by compositional gaps as the same clusters and gaps are found in collections of fiamme from widely separated stratigraphic levels step function zoning must be intrinsic to the chamber rather than a consequence of the tapping process we interpret the seven fiamme groups as seven separately converting layers in a density stratified magma reservoir tapped by eruption of the grizzly peak tuff excluding the thin heterogeneous tuff horizons 3 km of caldera fill is drawn from the three most silicic layers suggesting that the three rhyolitic layers were each at least 1 km thick in the magma chamber

flexure of lithosphere beneath apennine and carpathian foredeep basins evidence for an insufficient topographic load the pliocene foredeep basin external to the apennine thrust belt and the late miocene foredeep basin external to the outer east carpathians are considered typical molassic foredeeps that formed contemporaneously with thrusting in the adjacent mountain belts a simple mechanical model in which the response of the lithosphere to loading was approximately by a semi infinite broken elastic plate overlying a weak fluid was used to analyze the deflection of the foreland lithosphere beneath the apennine and outer east carpathian foredeep basins along three transects perpendicular to the thrust belts comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the foreland lithosphere during mountain building failed to pr duce a sufficient load to create the observed foredeep basins and observed topography regardless of the position of the slab end or the flexural rigidity assumed for the foreland lithosphere we infer that an additional subsurface load must be present and that its magnitude must be roughly equal to the load created by that part of the thrust belt that is above sea level for the two apennine transects an acceptable fit to the shape and size of the observed foredeep basin was generated by assuming effective elastic plate thicknesses of 8 and 15 km 4 9 and 9 3 mi and applied vertical forces of 2 1015 dynes cm and 1 3 1015 dynes cm at the slab end for the carpathian transect an effective plate thickness of 30 km 18 6 mi and an applied force of 1 5 215 1015 dynes cm generated an acceptable fit this study suggests a method whereby an estimate of the depth to autochthonous basement rocks beneath overthrust belts can be made in some areas where this information is not available from seismic or drilling data such an approach may enable exploration geologists to determine if crystalline rocks encountered in drilling overthrust terranes are a part of the autochthonous basement or if they are part of an overthrust sheet in the latter situation oil bearing sequences of sedimentary rocks might lie below crystalline rocks contained in the allochthonous sheet