comparative stratigraphy and subsidence history of mesozoic rift basins of north atlantic mesozoic rift basins around the north atlantic all possess many of the following upper jurassic to lower cretaceous oxfordian to albian stratigraphic elements 1 an early oxfordian hiatus 2 backfill deposits in upper jurassic paleovalleys 3 multiple tithonian transgressive regressive sequences 4 early berriasian regressive sequences 5 a sub valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates 6 valanginian to hauterivian fining upward sequences 7 condensed barremian sequences and 8 prominent aptian unconformities more local features include abrupt kimmeridgian deepening lusitanian basin portugal albian deepening cantabrian and aquitaine basins northern spain and southern france and late albian early cenom nian source rejuvenation cantabrian basin northern iberia decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence relatively high rates 50 100 m m y characterize onset of rift phases in the triassic and the kimmeridgian locally in the lusitanian basin of portugal kimmeridgian rates reach about 250 m m y in the cantabrian and aquitaine basins a third phase of rapid subsidence characterizes the aptian albian interval after the main phase of continental breakup around iberia this phase may be due to transtensional stresses as iberia continued to rotate relative to europe tensional intraplate stresses or gravity driven partial collapse of the new continental marg n the north atlantic mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of europe and north america pulses of extension generally are reflected in the stratigraphy of all of the basins superimposed on this record are the effects of eustatic sea level variations the main difference between the canadian jeanne d arc basin and iberian basins is the significantly greater early and middle jurassic basement subsidence rate in the former about 40 m m y vs 0 10 m m y this difference may result from prolonged extension on the canadian side perhaps due to northward transfer of shear stress from left lateral transform motion between the grand banks and northwestern africa

aptian albian palaeogeography of neo tethyan domain abstract the aptian albian interval 124 5 97 0 ma was a critical time both globally and for the tethyan domain in the tethyan domain it was the time when a united neo tethyan subduction zone became established between the future site of the alps and southeast asia and greatly accelerated the rate of north south convergence throughout the tethyan region a consequent widespread transgression on both laurasian and gondwanian margins deposited orbitolina limestones and associated clastics on wide continental shelf areas on the sea floor of the neo tethys ocean organic rich siliceous black shales radiolarites and turbidites with minor amounts of carbonates were deposited siliceous black shales accumulated under euxinic conditions of restricted small ocean basins of the neo tethyan realm and along their margins these areas received large amounts of organic debris from deltaic areas and coastal plains of the neighbouring continental shelves following the establishment of the north dipping subduction zone the laurasian passive margin was largely converted to an active one fringed by a large number of magmatic arcs that greatly influenced the sedimentation on the margin land areas and continental basins were widespread on this margin in northern tibet china and indo china this was a consequence of the late triassic to early jurassic continental collissions and accompanied intracontinental convergence that lasted well into the cretaceous a detailed aptian albian facies map and a crude palinspastic palaeogeographic map of the entire tethyan realm are prepared in order to express the contrasts in sedimentation on laurasian and gondwanian margins and to show the compatibility of the presented data with the most recent and reliable reconstructions of the major tectonic elements in tethysides

cyclogenesis in frontal zones abstract observational evidence has demonstrated that localized baroclinic zones in the atmosphere are unstable to disturbances with wavelengths of approximately 1000 km examples of such disturbances include polar front cyclones baiu cyclones polar lows and comma clouds in an attempt to understand the dynamical processes responsible for the generation of these diverse but apparently related phenomena a two dimensional baroclinic zone is tested for stability against small amplitude three dimensional perturbations the baroclinic zone employed in this analysis is the hoskins bretherton semigeostrophic model of a synoptic scale front the fronts am found to be highly unstable and the fastest growing disturbances compatible with an origin through secondary instability in the longwave are those with horizontal length scales on the order of 1000 km the energetics of these disturbances demonstrate that they grow by a new cyclone scale mode of baroclinic instability the structure of the most unstable cyc

formation and deformation of hyperextended rift systems insights from rift domain mapping in the bay of biscay pyrenees the bay of biscay and the pyrenees correspond to a lower cretaceous rift system including both oceanic and hyperextended rift domains the transition from preserved oceanic and rift domains in the west to their complete inversion in the east enables us to study the progressive reactivation of a hyperextended rift system we use seismic interpretation gravity inversion and field mapping to identify and map former rift domains and their subsequent reactivation we propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen this study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation two spatially and temporally distinct rift systems can be distinguished the bay of biscay parentis and the pyrenean basque cantabrian rifts while the offshore bay of biscay represent a former mature oceanic domain the fossil remnants of hyperextended domains preserved onshore in the pyrenean cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain both domains accommodated most of the shortening the final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes the complex 3 d architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system these results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the bay of biscay and pyrenean domains

oxygen isotope calibration of the onset of ice rafting and history of glaciation in the north atlantic region we report here that dsdp site 552a cored with the hydraulic piston corer on the west flank of rockall bank recovered an undisturbed sequence of alternating white deep sea carbonate oozes and dark coloured layers that are rich in glacial debris oxygen isotope analysis of the sequence together with detailed nannofossil and palaeomagnetic stratigraphy shows that the first major horizon of ice rafting occurred at about 2 4 myr and was preceded by a minor pulse of ice rafting at about 2 5 myr the carbon isotope record shows that the site has been bathed by a water mass of similar characteristics to present day north atlantic deep water at least since 3 5 myr

on the skewness of vorticity in the upper ocean conservation of potential vorticity suggests that when the magnitude of relative vorticity approaches the planetary vorticity cyclonic vorticity should be preferred small scale 3 km relative vorticity is examined using shipboard acoustic doppler current profiler adcp velocity measurements and p code global positioning system gps navigation from a cruise in the north pacific the skewness of relative vorticity in the mixed layer is positive consistent with theory the probability density function of vorticity reveals that positive skewness occurs because large vorticities are more likely to be cyclonic than anticyclonic the skewness is shown to be significant relative to measurement and random errors in contrast skewness in the thermocline has no preferred sign the skewness peaks at the surface and decays to zero at the base of the mixed layer

a new conceptual model for the deposition process of homogenite application to a cretaceous megaturbidite of the western pyrenees basque region sw france abstract the north pyrenean megaturbidite is a 19 63 m exceptional thick bed deposited during the late turonian and extending over more than 90 km in the basque region country sw france its thickness varies from more than 63 m to about 19 m from east mauleon area to west basque coast it represents a total compacted volume of about 90 km3 of carbonates on the field rare sedimentary structures are visible in the turbidite bed they consist of laminar planar lamination and rare cross laminations in the eastern region and antidune like structures in the western region five sites have been sampled with a vertical step of 50 cm to 1 m thin sections have been quantitatively analyzed for counting the terrigeneous fraction the quartz grain size and the mineral orientation the deposits fine westward which suggests a source located in the east of the mauleon basin this is consistent with the quartz grain orientation in the lower part of the megaturbidite the deposits fine upward from medium sand to clayey silt this is consistent with the classical definition of homogenites and suggests that these kinds of deposits are turbidites these observations also suggest that the term megaturbidite is appropriated for these deposits the sedimentary analysis indicates that the deposit results from a single event the volume of sediment involved in the process as well as the quartz grain orientation indicating flow motion in the opposite direction of the initial flow and the antidune like structure suggest the formation of reflected flows and the formation of standing waves over the complete water column in the basque flysch sub basins corresponding to a seiche effect the origin of the megaturbidite is probably an earthquake generated collapse on the carbonate platform this example allows providing a new conceptual model for the process of homogenite deposition this model explains the deposition of thick fine grained crudely graded megaturbidites

last interglacial and devensian deposits of the river great ouse at woolpack farm fenstanton cambridgeshire uk abstract this paper describes pleistocene fluvial deposits of the river great ouse at woolpack farm fenstanton cambridgeshire uk these sediments consist of a basal gravel fossiliferous fluviatile muds sands and gravels later disturbed and formed into a diamicton and overlying gravels the regional climate inferred from palaeobotany mollusca coleoptera and vertebrates from the diamicton indicates temperate conditions coleopteran evidence suggests a mean july temperature of ca 21 c 4 c warmer than today in eastern england and winter temperatures a little colder than at present molluscan assemblages indicate a slight brackish influence during deposition of the muds which form the diamicton the gravel succession is represented by three members which have a broad distribution in the great ouse valley and which were laid down in a braided river under periglacial conditions the occurrence of permafrost is indicated by the presence of ice wedge casts in the gravels the pollen and macrofossil evidence from the diamicton suggests correlation with ipswichian substage ipii pinus quercetum mixtum corylus phase the basal gravel is of pre ipswichian age a devensian age is proposed for the overlying gravels and their attendant periglacial phenomena

late pleistocene early holocene paraglacial and fluvial sediment history in the turbio valley semiarid chilean andes abstract the transitional character of climatic conditions confers great relevance to paleoclimate studies in the semiarid region where glacial and holocene geomorphologic records are scarce here we present the paraglacial and fluvial evolution of the turbio valley 30 s using both field observations and 14 c ams chronology two key sites at the uppermost turbio valley show glacial margins which likely formed during the 17 12 ka central andean pluvial event and earlier 37 27 ka episodes associated with glacial advances reported elsewhere in the semiarid andes likewise two episodes of subsequent paraglacial response are identified a first episode corresponds to early holocene fine grained deposits 11 500 7800 cal yr bp extending far downstream 40 km from the glacial margins these deposits and coeval debris cones 11 000 5500 cal yr bp are the result of arid conditions with occasional runoffs that were unable to export sediments along the trunk valley the second episode corresponds to disconformably overlying fluvial gravels extending 70 km downstream from the glacial margin indicative of an increase in the fluvial transport capacity occurring not long after 5500 cal yr bp fluvial transport increase resulted from a late holocene shift to wetter climate conditions representing a forcing factor which enhanced the paraglacial response

the north pyrenean aquitaine basin france evolution and hydrocarbons the mesozoic tertiary aquitaine basin overlaps the aquitaine block and the northern edge of the iberian margin both units are situated in the southwesternmost part of the european continent the aquitaine shelf is a stable platform overlying a relatively thin crustal segment it underwent extensional block faulting and many of its folded structures are related to salt tectonism the iberian block is a thicker lithospheric unit that acted as a buttress at the junction of these two structural domains the south aquitaine or north pyrenean area developed where crustal stretching rifting wrenching and orogenic compression were maximal from the mesozoic to the tertiary the history of the basin has been a suite of rifting attempts in the context of the progressive opening of the bay of biscay which never completely succeeded the relative plate motions and the later convergence led nevertheless to the formation of the aquitaine basin and to the emergence of the pyrenean fold belt the mesozoic tertiary sedimentary infill of the basin is mostly marine with thick evaporites stable platform carbonates subsiding platform shaly calcareous deposits and a characteristic diachronous turbiditic flysch and molasse complex events during the basin forming extensional and translational periods contributed more to the generation of the hydrocarbon accumulations than did the effects of the subsequent compressive structural regime the latter however may have enhanced thermal flow and thus maturation of hydrocarbon source rocks

a rhaetian 40 ar 39 ar age for the rochechouart impact structure france and implications for the latest triassic sedimentary record ar 39 ar dating of potassium feldspar primary spherulitic blocky and secondary idiomorphic k feldspar separated from impact metamorphosed gneiss found near videix in the western central part of the rochechouart impact structure nw massif central france yielded a rhaetian combined age of 201 2 ma 2r indistinguishable within uncertainty from the age of the triassic jurassic boundary ballen quartz intergrown with the primary k feldspar indicates post shock temperatures exceeding approximately 1000 c that affected the precursor gneiss geochemically both feldspar types represent essentially pure potassium end members apart from the approximately 15 km diameter impact deposit area the youngest crystallization age known for basement rocks in this part of the massif central is approximately 300 ma no endogenic magmatic thermal events are known to have occurred later in this region the k feldspar recrystallized from local feldspar melts and superimposed post shock hydrothermal crystallization probably within some thousands of years after the impact it is therefore suggested that the 40 ar 39 ar age for the videix gneiss as a potassic impact metasomatite dates the rochechouart impact in consistence with evidence for k metasomatism in the rochechouart impactites the new age value is distinctly younger than the previously obtained karnian norian age for rochechouart and thus contradicts the late triassic multiple impact theory postulated some years ago in agreement with the paleogeographic conditions in the western tethys domain around the triassic jurassic boundary the near coastal to shallow marine rochechouart impact is compatible with the formation of seismites and tsunami deposits in the latest triassic of the british isles and possible related deposits in other parts of europe

intensification of the asian winter monsoon after 7 4 ma grain size evidence from the linxia basin northeastern tibetan plateau 13 1 ma to 4 3 ma abstract grain size studies of eolian deposits loess and red clay on the chinese loess plateau have produced a detailed history of the asian winter monsoon covering the last 8 ma this paper extends the gain size record back to 13 1 ma in the linxia basin at the northeast edge of the tibetan plateau by comparing the grain size distribution of the linxia basin sediment with known eolian sediments in the region we argue that the 10 70 m fraction is mostly eolian this fraction becomes increasingly dominant in the record after 7 4 ma with almost all sediments eolian in origin after 6 2 ma given that the 10 70 m fraction in the linxia basin are well sorted and have grain size spectra that are very similar to sediments transported by the asian winter monsoon we suggest that the continuous presence of this fraction throughout our record indicates that the asian winter monsoon has been present in some form since 13 1 ma if the assumption that variation in grain size spectra on the million year time scale is primarily controlled by the character of eolian input to the basin is valid the long term pattern of the eolian fraction reveals two abrupt intensifications of the asian winter monsoon at 7 4 ma and 5 3 ma these are superimposed on a gradual intensification of the asian winter monsoon beginning at 8 0 7 4 ma we note that the two abrupt intensifications may be related to arctic ice volume increase and that the gradual intensification of the asian winter monsoon after 7 4 ma is accompanied by central asia desertification and may therefore be related to increase in the height or extent of the tibetan plateau

evolution of salt structures during extension and inversion of the offshore parentis basin eastern bay of biscay the late jurassic cretaceous parentis basin eastern bay of biscay illustrates a complex geological interplay between crustal tectonics and salt tectonics salt structures are mainly near the edges of the basin where jurassic lower cretaceous overburden is thinner than in the basin centre and allowed salt anticlines and diapirs to form salt diapirs and walls began to rise reactively during the late jurassic as the north atlantic ocean and the bay of biscay opened some salt cored drape folds formed above basement faults from the upper jurassic to albian during albian late cretaceous times passive salt diapirs rose in chains of massive salt walls many salt diapirs stopped growing in the mid cretaceous when their source layer depleted during the pyrenean orogeny late cretaceous cenozoic the basin was mildly shortened salt structures absorbed almost all the shortening and were rejuvenated to form squeezed diapirs salt glaciers and probably subvertical welds some of which were later reactivated as reverse faults no new diapirs formed during the pyrenean compression and salt tectonics ended with the close of the pyrenean orogeny in the middle miocene using reprocessed industrial seismic surveys we document how salt tectonics affected the structural evolution of this offshore basin largely unknown to the international audience during the last decade or two the important role of salt in facilitating shortening in sedimentary basins has been highlighted with examples around the world see reviews by jackson 1995 letouzey et al 1995 rowan et al 2004 hudec jackson

mechanisms of anabranch avulsion within gravel bed braided rivers observations from a scaled physical model the physical modelling of gravel bed braided rivers using a 1 20 froude scale model permits analysis of the types and relative occurrence of different avulsion mechanisms anabranch avulsion within braided rivers involves three main mechanisms choking avulsion caused by blockage of one channel by a sediment lobe constriction avulsion produced by deflection confinement and subsequent diversion of the flow by a barform and apex avulsion following erosion at the outside of sinuous thalwegs and confined meander bends each avulsion mechanism is described and analysis of the abundance of each type illustrates choking avulsion to be predominant without braided rivers three factors are found to control anabranch avulsions namely flow discharge sediment flux and floodplain topography these are briefly discussed together with the implications of avulsion type for the alluvial architecture of gravelly braided alluvium

mesozoic diapirism in the pyrenean orogen salt tectonics on a transform plate boundary the mesozoic history of the pyrenean region was dominated by the opening of the north atlantic ocean and the left lateral movement of iberia relative to europe most deformation was tensional or transtensional but this style was interrupted by several short compressional events perhaps related to bends in the transform plate boundary or minor changes in plate movement directions these compressional events were recorded by deformation adjacent to and above salt structures which were the weakest parts of the system and so served as especially sensitive strain barometers the alternation of extension and shortening controlled the location and style of salt structures evaporites were initially deformed during the latest jurassic early cretaceous neocimmerian transpression which formed salt cored anticlines above west northwest east southeast trending basement faults transtension in the aptian albian caused salt domes to pierce to the surface at the intersections of these anticlines with reactivated northwest southeast and northeast southwest trending basement structures transtension was at least locally interrupted by brief periods of transpression at the aptian albian and albian cenomanian boundaries transpression shortened the diapirs causing rotation uplift and erosion of beds near the salt most diapirs were buried during the late cretaceous in some cases postburial hydrothermal circulation dissolved much of the halite causing the diapirs to sag and the basins to form in roof strata finally the entire region was shortened during the late cretaceous tertiary pyrenean orogeny which greatly distorted many of the preexisting geometries

scale selection of baroclinic instability effects of stratification and nongeostrophy abstract a simple two dimensional linear stability analysis is presented to study the characteristics of the scale selection and structure of baroclinic cyclogenesis effects of a realistic vertical thermal structure of the environment and the related nongeostrophic wind are investigated in detail both short mesoscale waves trapped in the lower layer with a small static stability and long waves extending throughout the column are unstable which supports blumen s results the integral constraints imply that the vertical structure of the stratification can provide an internal lid to confine the wave into the mixed layer nongeostrophic effects become important as richardson number decreases as pointed out by stone the scale of the most unstable eady s mode becomes sensitive to the change of shear unlike the geostrophic case the measure of the horizontal scale of the disturbance is the modified rossby radius of deformationwhere is the vertical shear and provides o ri correction to the conventio

destruction of potential vorticity by winds the destruction of potential vorticity pv at ocean fronts by wind stress driven frictional forces is examined using pv flux formalism and numerical simulations when a front is forced by downfront winds that is winds blowing in the direction of the frontal jet a nonadvective frictional pv flux that is upward at the sea surface is induced the flux extracts pv out of the ocean leading to the formation of a boundary layer thicker than the ekman layer with nearly zero pv and nonzero stratification the pv reduction is not only active in the ekman layer but is transmitted through the boundary layer via secondary circulations that exchange low pv from the ekman layer with high pv from the pycnocline extraction of pv from the pycnocline by the secondary circulations results in an upward advective pv flux at the base of the boundary layer that scales with the surface nonadvective frictional pv flux and that leads to the deepening of the layer at fronts forced by both downfront winds and a destabilizing atmospheric buoyancy flux f b the critical parameter that determines whether the wind or the buoyancy flux is the dominant cause for pv

thermohaline variability in the upper ocean the main goal of this exploratory study is to determine how the temperature salinity relationship changes with horizontal length scale and depth in the ocean temperature and salinity were measured on a range of scales from 4 m to 1000 km towing a seasoar along isobars and isopycnals in the subtropical gyre of the north pacific during the winter of 1997 the wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed that is horizontal temperature and salinity gradients tend to cancel each other in their effect on density thermohaline variability at small scales is intermittent and clusters around large scale thermohaline anomalies below the base of the mixed layer horizontal gradients of temperature are only partially opposed by salinity and the density ratio is close to 2 in the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals density compensated variability ubiquitous in the mixed layer is reduced along deeper isopycnals compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density gradients

comparison between the galicia and aquitaine margins abstract there is a great similarity between the galicia margin and the aquitaine margin i e the aquitaine basin and the north pyrenean zone in particular the timing of the rifting as well as the resulting structures exposure of perodotite tilted fault blocks normal and transverse faults half grabens are practically the same this comparison leads to the interpretation that the northern side of the pyrenean fold belt is inherited from the mesozoic deep passive margin of europe the cenozoic thrust sheets and reverse faults of the northern pyrenees zone are tentatively interpreted as former rift structures tilted blocks and listric faults which were removed during the convergence of the iberian and european plates during the paleocene and eocene

geostrophic adjustment and restratification of a mixed layer with horizontal gradients above a stratified layer abstract the restratification of a mixed layer with horizontal density gradients above a stratified layer is considered solutions are obtained on the assumption that the width across this front is much larger than the local radius of deformation bh f based on the buoyancy change across the front b mean mixed layer depth rdh and the coriolis parameter f where b is defined as rd g 0 0 but the fractional change in the mixed layer depth is not required to be small for an initially quiescent mixed layer created by homogenizing a fluid of constant stratification to a depth that varies horizontally the isopycnals in the mixed layer tilt about their intersections with the top surface in the adjusted state and the base of the mixed layer flattens slightly in the frontal region other cases considered include mixed layer fronts with initial momentum out of geostrophic balance created by vertical mixing of a layer with horizontal gradients previously in thermal wind balance for a wide front the

successive post variscan stress fields in the french massif central and its borders western european plate comparison with geodynamic data abstract structural analysis and particularly microtectonic methods have made it possible to define the different stages of brittle deformation of the massif central basement and the surrounding sedimentary cover from the end of the hercynian orogeny to the end of the tertiary during the stephanian a compressional tectonic regime prevailed regional faults appeared or were react vated reactivation as a result of initial n s compression becoming nw then e w these regional strike slip faults caused local extension which led to the formation of small coal bearing basins this compressional regime which marked the end of the formation of pangea was followed by a series of extension episodes permian triassic extension oscillating around n s e w to nw se extension in the early and middle jurassic and finally n s to ne sw extension in the late jurassic to cretaceous the normal faults formed during these episodes strongly influenced the distribution of emerging continents and sedimentary basins from the campanian 75 ma to the present the convergence of africa and eurasia involved the distribut on of stresses in the west european plate several tectonic episodes are distinguished in the massif central during the eocene approximately n s compression predominated general e w extension in the late eocene oligocene resulted in grabens with general northerly strike mainly in the centre of the massif central and on its east and southeast borders lastly compression varying from nw se to e w in the north and south of the massif central prevailed during the alpine orogenic phase at the end of the miocene these successive stages of brittle deformation are interpreted in the context of the evolution of the west european plate and its displacement in relation to the african plate the correspondences between the major geodynamic periods and the distribution of stresses over the west european continent are noted as well as the problems which remair to be resolved

structure and evolution of the galicia interior basin atlantic western iberian continental margin abstract an industrial multichannel seismic reflection data set is presented on the galicia interior basin g i b a sedimentary basin that has been poorly studied until now seismic stratigraphic units defined in other areas of the western iberian continental margin as well as some deeper reflections can also be identified within this basin although they show some particular characteristics the g i b is bounded to the east by a major fault identified beneath the continental shelf and to the west by an alignment of highs western banks the structural framework of the area consists of nnw sse trending normal faults cross cut by ne sw transfer faults the inception of the g i b probably ocurred during triassic times within the late jurassic early cretaceous rifting phase it probably experienced several extensional episodes the most intense one being during valanginian times evidence for compressional tectonic activity in this basin during cenozoic times has also been found when comparing the g i b with the adjacent deep galicia margin a gap in the timing of the main extensional events as well as some differences in the structural styles have been found in a wider regional context an integration of data about the timing of the processes of continental rifting and oceanic spreading along the whole western iberian margin is consistent with a northward propagating rift for the late jurassic early cretaceous phase

mixed layer instabilities and restratification abstract the restratification of the oceanic surface mixed layer that results from lateral gradients in the surface density field is studied the lateral gradients are shown to be unstable to ageostrophic baroclinic instabilities and slump from the horizontal to the vertical these instabilities which are referred to as mixed layer instabilities mlis differ from instabilities in the ocean interior because of the weak surface stratification spatial scales are o 1 10 km and growth time scales are on the order of a day linear stability analysis and fully nonlinear simulations are used to study mlis and their impact on mixed layer restratification the main result is that mlis are a leading order process in the ml heat budget acting to constantly restratify the surface ocean climate and regional ocean models do not resolve the scales associated with mlis and are likely to underestimate the rate of ml restratification and consequently suffer from a bias in sea surface temperatures and ml depths in a

sedimentary facies changes and climatic tectonic controls in a foreland basin the urumqi river tian shan northwest china abstract a sedimentological study was conducted on an interbedded fine gravel and silt succession up to 70 m thick at cangfang gou in northern chaiwopu basin tian shan nw china the succession informally termed urumqi gravel is part of the gravelly basin infill that commenced around 1 87 ma bp in the early pleistocene its upper boundary determined by a magnetostratigraphical study is slightly above the brunhes matuyama b m at 780 ka bp detailed facies analysis indicates that the river underwent multiple cyclic changes from a bed load dominated braided to a suspension load dominated sinuous regime the latter probably adopted a single thread meandering pattern confined within its course and was predominated by overbank accretion of fines without development of distinct point bars the river probably represents the proto urumqi river it appears that the cyclic sequences are climatically controlled their preservation in the succession is largely attributed to the continued aggradation resulted from the subsidence of the chaiwopu basin in the early pleistocene

impact of geodynamics on fluid circulation and diagenesis of carbonate reservoirs in a foreland basin example of the upper lacq reservoir aquitaine basin sw france abstract orogeny driven fluids that circulate in foreland basins can have strong impacts on petroleum systems and reservoir properties this applies to the upper cretaceous lacq reservoir of the aquitaine basin in southwestern france located north of the pyrenean mountains we study a drillcore from a 650 m deep oil reservoir to document how the evolution of a foreland basin which formed after a hyperextension phase affected fluid circulation and eventually reservoir diagenesis using drillcore samples petrographic observations fluid inclusion studies coupled with thermodynamic modeling isotopic and rare earth element geochemistry as well as basin modeling were performed to investigate and describe the fluid types and sources the pressure temperature evolution and the timing of diagenetic fluid flow early diagenesis involves some bacterial activity represented by micrite rims and framboidal pyrites as well as early dolomitization involving a mixture of meteoric and formation fluids during burial dolomite overgrowth saddle dolomites coarse blocky calcites and anhydrites precipitate the last products consist of iron oxide precipitation and meteoric recharge of the reservoir with groundwater our results indicate that after the deposition of the paleocene flysch formation mg rich low salinity hot and relatively deep clay released fluids migrated along thrust faults from deeper parts of the basin during the climax of the compressional pyrenean phase eocene they acted as dolomitizing hydrothermal fluids with a thermal disequilibrium in the order of 30 40 c hotter than the ambient host rocks another orogeny driven hydrothermal and mg poor fluid mixed with oil generated by jurassic barremian source rocks and precipitated calcites and anhydrites finally during uplift oxidizing fluids were laterally introduced as the present day groundwater hence by combining different petrographic geochemical and modeling proxies we document how the eocene orogenic phase played a crucial role in basin scale fluid flow and carbonate diagenesis

loess stratigraphy in central china abstract the loess deposits in central china record world wide climate changes of the last 2 5 ma numerous climatic oscillations are marked by alternating loess and soil units with continuous coverage of the matuyama and brunhes epochs magnetic susceptibility of the deposits correlates closely with the oxygen isotope record of the deep sea sediments and provides an independent measure of climate and time the key marker bed of the chinese loess sequence is the paleosol s5 the time equivalent of the mid brunhes oxygen isotope stages 13 14 and 15 it marks a prolonged interval of warm humid climate lasting from approximately 615 to 470 thousand years ago several episodes of river downcutting coincide with deposition of the exceptionally thick loess units l1 oceanic oxygen isotope stages 2 to 4 l2 stage 6 l5 stage 12 l6 stage 16 l9 stage 22 l15 stage 38 about 1 15 ma and ws4 about 2 3 ma these erosional events are interpreted as a result of episodic uplift of the loess plateau the deposition of the earliest loess layers between 2 5 and 2 3 ma ago marks a first order shift from warm and humid environments toward harsh continental steppes comparable to those of the middle and late pleistocene little if any lithologic or paleontologic changes were noted within or above the olduvai magnetozone so that the proposed plio pleistocene boundary at approximately 1 65 ma has no lithostratigraphic and biostratigraphic representation in the chinese loess series comparison with the deep sea drilling program core 552a in the north atlantic and with the santerno river section near bologna shows that the occurrence of the earliest loess in china coincides with the timing of the first significant ice rafting in the north atlantic and with the appearance of cold water foraminifers in the marine deposits of northern italy an extension of the stage system of the oxygen isotope signal extended back to the gauss matuyama boundary is proposed

the mylonite zones in the pyrenees and their place in the alpine tectonic evolution abstract in the pyrenees the development of mylonites zones is one of the most striking structural features two sets of mylonites of regional extent have been recognized large longitudinal e w to n110 e trending zones e g merens fault and north pyrenean fault and oblique nw se trending zones cross cutting both the hercynian and the post hercynian terrains the longitudinal zones limit the major structural zones of the pyrenees and are associated with nw se en echelons folds in the mesozoic terrains and rotations of rootless plutonic or gneissic massifs acting as competent inclusions in a more ductile matrix in the hercynian basement the oblique mylonite zones limit map scale fold bands and appear as the sheared limbs of these folds the age of the oblique zones and of the major movements along the longitudinal zones is clearly alpine and the en echelons folds seem to have controlled the sedimentation during the upper albian and possibly during the upper cretaceous early movements along the longitudinal zones may have been hercynian the analysis of the structures at all scales leads us to interpret these mylonite zones and associated structures as the ultimate result of a transcurrent simple shear acting during the whole mesozoic period this strike slip shearing was probably associated with an extension perpendicular to it from the permian to the upper cretaceous and then to a shortening component also perpendicular to it from the late cretaceous to the eocene the development of the mylonite zones appears to have predated the major alpine thrusting but to have been reactivated during this thrusting acting as initiation sites for the thrusts or as oblique ramps in the case of the oblique mylonite zones

the mesozoic tertiary evolution of the aquitaine basin the aquitaine basin situated in southwest france with an area of about 60 000 km 2 has the form of a triangle which opens towards the atlantic bay of biscay and is limited to the north by the hercynian basement of brittany and the massif central and to the south by the pyrenean tertiary orogenic belt beneath the tertiary sequence 2 km thick and which outcrops over much of the basin a mesozoic series up to 10 km thick rests generally on a tectonized hercynian basement but locally it covers narrow nw se trending post orogenic trenches of stephano permian age the mesozoic history can be subdivided into four major structural sedimentary episodes 1 during a triassic taphrogenic phase a continental evaporitic complex developed with associated basic magmatism 2 throughout the jurassic a vast lagoonal platform developed initially lower lias as a thick evaporitic sequence followed by a uniform shale carbonate unit indicating a relative structural stability 3 the end of the jurassic and the lower cretaceous saw a fragmentation of this platform due to an interplay between the iberian and european tectonic plates resulting in an ensemble of strongly subsident sub basins 4 during the upper cretaceous and until the end of the neogene the evolution of the aquitaine basin was influenced by the pyrenean orogenic phase with the development towards the south of a trench infilled by flysch which from the upper eocene is succeeded by a thick post orogenic molasse complex the main hydrocarbon objectives in the basin are situated in the jurassic platform e g the lacq giant gas field and the cretaceous sub basins e g the cazaux and parentis oil fields to date production has been about 4 times 10 7 m 3 of oil and about 15 times 10 10 m 3 of gas since the first gas discovery st marcet in 1939

vorticity and vertical circulation at an ocean front abstract density and velocity data with 4 km horizontal resolution from a survey of a front during fasinex frontal air sea interaction experiment are combined to describe the structure of the top 300 m of the ocean the geostrophic velocity field is derived and is used to examine the relative importance of stratification relative vorticity and twisting terms in ertel s potential vorticity q tenfold isopycnic changes in q are found across a horizontal scale of only 10 km these changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer the ageostrophic velocity field is quantified by solution of the omega equation and vertical velocities of up to 40 m day 1 are found small 40 km surface trapped top 200 m eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer

isopycnal mixing in ocean circulation models abstract a subgrid scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non eddy resolving ocean circulation models the mixing is applied in isopycnal coordinates to isopycnal layer thickness or inverse density gradient as well as to passive scalars temperature and salinity the transformation of these mixing forms to physical coordinates is also presented

eddy induced mixed layer shallowing and mixed layer thermocline exchange the effects of eddies on the mixed layer and on the transfer of properties between the mixed layer and thermocline are studied in a high resolution model of an unstable front a spring scenario is considered with a mixed layer maintained against surface warming by wind mixing tilting of the buoyancy gradients by ageostrophic flow is found to shallow the mixed layer over the front giving a mixed layer as shallow as 10 m in some places compared with the equilibrium monin obukhov depth of 30 m away from the front variations of mixed layer depth with time and lateral advection lead to a subduction field that is substantially different from the vertical velocity field vertical velocities associated with the growing instabilities push tongues of a ventilation tracer initially only present within the surface layer down to depths of up to 100 m after 24 days a quarter of this ventilation tracer has been deposited in the thermocline conversely the vertical motions pull nutrients from the thermocline up into the mixed layer giving enhanced surface values along the front in the basin average the geostrophic eddies give vertical eddy diffusivities for a ventilation tracer of o 3 10 10 4m2s 1

acceleration of continental rifting due to a thermomechanical instability abstract we have made a numerical experiment on continental rifting based on the rheology of rocks constituting continental lithosphere the model assumes homogeneous pure shear stretching of a three layered lithosphere we also assume constant extensional force in the model to explore acceleration or deceleration of rifting the main parameters which affect the evolution of continental rifting for an initial geotherm are the applied constant extensional force and the thermal diffusivity whereas continental extension elevates the isotherms in continental lithosphere leading to its softening the cooling of the extended continental lithosphere by thermal diffusion leads to its strengthing therefore the applied force and the thermal diffusivity play counteracting roles which cause a stretching instability to separate the final states of rifts i e breakup or failed the transition from the breakup to a failed rift in continental lithosphere is caused by a subtle change in the constant extensional force or in the thermal diffusivity similar to a phase transformation we have shown that a non dimensional number that is modified form the peclet number determines the final state

the opening of the atlantic the mesozoic new england igneous province and mechanisms of continental breakup abstract plume and tensionally induced rifting models have been proposed as causes of continental rifting and breakup mantle plumes should produce early lithospheric doming events associated with alkaline magmatism however the triassic alkaline rocks on the east north american e n a margin are structurally localized the uplifts are mostly jurassic in age and voluminous plateau magmatism post dates the initiation of taphrogenesis by 10 to 20 ma in addition there are no chains of anorogenic igneous rocks leading to any of the postulated triassic plumes these facts argue against the plume generated rifting model for the e n a margin but are consistent with the predicted effects of the tensionally induced rifting model the genesis of linear chains of alkaline rocks has been attributed to passage of a lithospheric plate over a mantle plume this explanation has been proposed for the cretaceous monteregian and white mountain intrusive series and the kelvin seamount chain however these rocks are distributed along structurally controlled e w trending lineaments the monteregians are localized by the presence of a paleorift ottawa bonnechere graben in the underlying grenvillian basement and the kelvin seamounts are emplaced along the early jurassic kelvin fracture zone the structures localizing the southern white mountain lineament are less well defined the spatial temporal and geochemical relationships along and between these lineaments are consistent with an origin as a southeastward propagating tensional event the intra plate stresses are probably the result of shifts in the pole of rotation of the north american plate due to irregular northward extensions of seafloor spreading during the cretaceous

late quaternary alluvial sequence and uplift driven incision of the urumqi river in the north front of the tian shan northwestern china abstract this work focuses on the driving force behind the late quaternary river incision and terrace formation of the urumqi river in the north piedmont of the tian shan northwestern china field investigations on geomorphic surfaces terrace deposits and its underlying bedrock identify four most significant features which creates a local applicable framework for subdivision of the late quaternary terrace sequence in the study area nine stepped river terraces are defined and designated as t 1 to t 9 increasing systematically in elevation morphologically the highest t 9 correlates with the oldest alluvial fan f 1 of the urumqi river river incision and the resultant abandonment of fan f 1 are chronologically constrained at ca 550 ka the stratigraphic geometry of the saerqiaoke anticline a structure developing at the fan end of f 1 reveals the existence of growth strata implying continuous growth of this fold when the f 1 alluvial sediments were deposited in the range front of the urumqi river growth of the saerqiaoke anticline has derived tectonically from uplift and basinward thrusting of the tian shan range such thrusting and basinward extension of the range are expected to force rock uplift of the headwater of the urumqi river with respect to the chaiwopu basin to the north and thus river incision occurring at ca 550 ka during the subsequent period several younger terraces have been formed in response to the further uplift of the saerqiaoke anticline as well as climate changes during glacial interglacial transitions in the present study area the total incision during quaternary comes close to 400 m with about 85 contribution likely attributed to rock uplift of the saerqiaoke anticline

deep structure of the aquitaine shelf constraints from expanding spread profiles on the ecors bay of biscay transect abstract this paper describes the analysis and interpretation of six expanding spread profiles esp which were shot approximately perpendicular to a 300 km long vertical reflection profile along the eastern continental margin of the bay of biscay aquitaine shelf by the french ecors program in association with hispanoil this transect crosses various tectonic features of different ages the armorican shelf the parentis basin and the cantabria shelf velocity depth models have been derived from the esps by the combination of two complementary methods using time distance and intercept slowness domains they provide important constraints for the analysis of the vertical reflection data the velocities allow definition of crustal layering with a 5 8 6 2 km s 1 upper crust and a 6 5 7 1 km s 1 lower crust this layering matches the change of reflectivity observed on cdp data with a relatively transparent upper crust and upper mantle in opposition to a highly layered lower crust important variations of the thickness of these two layers are revealed by this study the most important one occurs beneath the parentis basin with a 15 km shallowing of the upper mantle the velocity distribution suggesting that major crustal thinning has taken place at the cost of a large part of the lower crust

magnetochronology of the upper cenozoic strata in the southwestern chinese tian shan rates of pleistocene folding and thrusting abstract the southwestern chinese tian shan of central asia is an actively deforming part of the indian asian collision system paleomagnetic investigations of two plio pleistocene terrestrial successions provide the first detailed magnetostratigraphy for the upper cenozoic strata in this region paleomagnetic samples were collected from 358 sites within the atushi formation and xiyu formation across the atushi talanghe anticline near atushi thermal demagnetization behavior reversal and fold tests of paleomagnetic stability indicate that the characteristic remanence directions were acquired before tilting and folding of the strata a composite magnetostratigraphic section for this sequence correlates with the mammoth subchron to the jaramillo subchron between 3 3 and 1 07 myr of the geomagnetic polarity timescale of cande and kent j geophys res 100 1995 6093 6095 the mean declination 349 3 in the boguzihe section indicates a counterclockwise vertical axis rotation 11 2 during the past 1 4 myr the conglomeratic xiyu formation is time transgressive along its progradational contact with the underlying atushi formation across a map distance of 6 km the basal contact ranges from less than 1 0 myr in the ganhangou section to 1 9 myr in the boguzihe section to the southwest and to 2 8 myr on the northwest limb of the anticline northeastward lateral propagation and growth of the atushi talanghe anticline initiated at 1 4 myr in the boguzihe and 1 2 myr in the ganhangou the cross section in the boguzihe provides a conservative maximum estimate of shortening rate of 3 3 4 4 mm yr and uplift rate of 3 mm yr this rate represents about half of the geodetically determined shortening rate between the northern tarim basin and the kyrgyz tian shan wang et al acta seismol sin 22 2000 263 270 reiger et al earth planet sci lett 191 2001 157 165 during fold growth over the past 1 4 myr the crest of the atushi talanghe anticline eroded at an average rate of 2 6 2 7 mm yr

paleoconstraints during rifting of the northeast atlantic passive margins the northeast atlantic passive margins formed during the early cretaceous phase of rifting from goban spur to galicia bank these margins are characterized by a tilted fault block geometry well defined by numerous single channel and multichannel seismic data stress directions cannot be deduced from these structural data but recently withjack and jamison 1986 have described the strain states and fault patterns produced by oblique rifting using experimental and analytical techniques we have applied these techniques to the northeast atlantic continental margins all types of extensional margins are represented there with pure extensional rifting goban spur and west galicia margins oblique rifting north spanish and celtic margins and close to pure wrenching armorican margin the rifting directions are parallel to the adjacent oceanic fracture zones or flow lines defined in the northeast atlantic and in the bay of biscay except for the west galicia margin where a 20 difference exists this difference could be within the error range of the method or could be due to a slight relative motion between galicia bank and iberia during rifting globally the coherence between rifting and spreading directions is an argument in favor of the validity of the method on the basis of kinematic arguments the motion of iberia with respect to eurasia has an eastward component related to the west to east motion of africa with respect to eurasia while iberia was a small plate whose motion was roughly coupled to africa this is confirmed by our analysis we conclude that during both the rifting and spreading episodes in the bay of biscay the motion of iberia with respect to eurasia was in the southeastward direction recently boillot 1984 suggested that iberia moved about 150 km southwestward with respect to eurasia during the rifting phase and then moved in the southeastward direction during the spreading phase we suggest that the boillot hypothesis could be wrong

late pleistocene fluvial sedimentary facies the dhadhar river basin western india abstract the late pleistocene fluvial succession is exposed as 18 20 m high incised vertical cliffs all along the dhadhar river basin in western india the major fluvial sedimentary facies of the late pleistocene deposits in the dhadhar river basin have preserved evidence of palaeodrainage and could provide an important link between the sub humid narmada basin in the south and the semi arid mahi basin in the north the sedimentary facies documented include overbank fines which are associated with crevasse splays fine grained overbank sediments are interpreted as having formed by sheet flow of sediments over the banks of minor distributary channels during the flood stage the overlying thinly stratified fluvial sands and silts at the top of the exposed sediment succession show a thin cap of aeolian sediments suggesting less intense aeolian activity than that observed in sabarmati mahi and orsang basins though a significant reduction in fluvial activity is suggested during the arid phase of the lgm however the river may still have been perennial assuming that it retained the larger part of the catchment

fluvial response to rapid climate change during the devensian weichselian lateglacial in the river great ouse southern england uk few lateglacial 15 200 11 500 cal bp fluvial deposits are known from southern england uk this paper outlines a sedimentological investigation of a lateglacial site near the village of roxton in the middle reach of the river great ouse below the holocene argillaceous alluvium several large gravelly lithofacies units representing long term fluvial processes are recognised based on their lower bounding surfaces and sedimentary features in line with palaeontological evidence and radiocarbon dating detailed facies analysis indicates that the river changed from a braided to a wandering meandering regime in response to the warming induced reduction in nival peak discharge and increase in vegetation cover consisting of birch woodland and grassland during the lateglacial interstadial 15 200 12 700 cal bp this was followed by a change back to a braided river in response to climatic deterioration during the younger dryas chronozone 12 700 11 500 cal bp the interstadial floodplain consisted of both overbank fines and channel lateral or point bar sands and gravels the two suites of contemporaneous sediments in the geological succession may be easily mis interpreted as independent climato stratigraphical units due to their cross cutting relationship and marked difference in facies assemblage lateral channel movement in such a system also implied that the floodplain underwent constant erosion resulting in underdeveloped overbank sequences insufficient to withstand post depositional removal by erosion this probably accounts for the rare preservation of fine textured overbank deposits of the lateglacial interstadial in southern england

the nappe des marbres unit of the basque cantabrian basin the tectono thermal evolution of a fossil hyperextended rift basin the architecture of the pyrenean cantabrian belt results from the inversion of a series of former cretaceous rift basins a ht lp metamorphic event dated at 105 to 85 ma ago is commonly associated with an albo cenomanian episode of hyperextension of the continental crust this metamorphism is well known in the eastern basque cantabrian basin within the nappe des marbres unit nmu that is preserved from intense compressional deformation during the pyrenean orogeny based on a structural study at the scale of the eastern basque cantabrian basin bcb and on a dense sampling for trscm estimates with the rscm method raman spectrometry on carbonaceous material we show following results 1 the nmu has recorded two major phases of deformation related to a localized extensional ductile foliation s1 during the cretaceous rifting and later by the regional n s shortening recorded by a regional cleavage s2 observed only in shale sediments 2 the nmu is affected by early salt tectonics related to several diapirs and salt walls fed by the upper triassic evaporitic layer 3 the nmu recorded maximum temperatures exceeding 550 c which represent some of the highest temperatures along the pyrenees these new data demonstrate that there is no e w lateral metamorphic gradient across the belt and that hyperextension rifting occurred consistently within a high thermal regime 4 metamorphic isograds are oblique in respect of the main structures suggesting that the metamorphic event postdates most of the early salt related deformations prior to or coeval with early stages of rifting as it represents one of the best preserved example of pre orogenic hyperextended basin the eastern bcb record may be used regionally to better understand the rift to orogen evolution of the pyrenean internal metamorphic zone but also more generally as an analog of salt bearing hyper extended rift record

crustal structure and orogenic material budget in the west central pyrenees surface and subsurface data are combined to construct a crustal scale cross section of the western central pyrenees france and spain at the boundary between the european and iberian plates the position of moho reflections in the ecors arzacq reflection profile suggests a tectonic wedge of european crust and upper mantle had indented the iberian plate at lower crustal levels the european wedge is overlain by an upper iberian wedge thus constituting a double stacked wedge geometry the upper wedge was delaminated and deformed giving rise to the pyrenean orogenic prism manifested as a bivergent fan in upper crustal levels the underthrust lower iberian plate has been imaged to depths of 55 60 km but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km total orogenic contraction calculated from surface structures is about 75 80 km which was accomplished between the latest cretaceous and the early miocene at an averaged rate of 1 2 mm yr consistent with these moderate values exhumation of the orogen is much less than in the more shortened eastern parts of the range paleozoic basement and preorogenic mesozoic rocks are little eroded and much of the exhumation involved cannibalization of early foreland basins which once covered the entire poorly emergent orogen at this transect this resulted in a continuous process of sediment recycling and coupled with a considerable lateral arrival of material a bulk negative erosion sedimentation budget