constraints on the deep structure and dynamic processes beneath the alps and adjacent regions from an analysis of gravity anomalies summary gravity anomalies including terrain corrections over the western alps and the molasse basin are within 30 mgal of those calculated assuming local airy isostatic equilibrium but isostatic anomalies over regions nw and se of the alps exceed 50 mgal therefore mass excesses and deficits that must be supported by the strength of the lithosphere or by dynamically induced stress in the asthenosphere appear to be greater beneath the areas surrounding the alps than beneath the alpine chain itself attempts to account for the gravity gradient over the molasse basin and sub alpine chains and the small bouguer anomalies measured over the elevated areas of the vosges and black forest by the flexure of an elastic plate require an absurd set of parameters the high elevations of the vosges and the black forest are presumably due to high temperature in the underlying uppermost mantle but the deviations from isostatic equilibrium as well as the high temperature probably result from active upwelling in the asthenosphere beneath this area the absence of large isostatic anomalies over the alps and the failure of an elastic model to account for the gravity anomalies over the molasse basin suggest that the dynamic processes that flexed the european plate down to form the molasse basin and that built the alpine chain have waned the late cenozoic uplift of the molasse basin and the alps might be a consequence of a diminution or termination of downwelling of mantle material beneath the alps

marine oxygenation lithistid sponges and the early history of paleozoic skeletal reefs abstract microbial carbonates were major components of early paleozoic reefs until coral stromatoporoid bryozoan reefs appeared in the mid ordovician microbial reefs were augmented by archaeocyath sponges for 15 myr in the early cambrian by lithistid sponges for the remaining 25 myr of the cambrian and then by lithistid calathiid and pulchrilaminid sponges for the first 25 myr of the ordovician the factors responsible for mid late cambrian microbial lithistid sponge reef dominance remain unclear although oxygen increase appears to have significantly contributed to the early cambrian explosion of marine animal life it was followed by a prolonged period dominated by greenhouse conditions as sea level rose and co2 increased the mid late cambrian was unusually warm and these elevated temperatures can be expected to have lowered oxygen solubility and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia greenhouse conditions would also have stimulated carbonate platform development locally further limiting shallow water circulation low marine oxygenation has been linked to episodic extinctions of phytoplankton trilobites and other metazoans during the mid late cambrian we propose that this tendency to dysoxia hypoxia in shallow marine environments also limited many metazoan reef builders in contrast during the mid late cambrian the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs these conditions ameliorated during the ordovician as temperature decline promoted ocean ventilation the prolonged time gap occupied by low diversity reefs between the cambrian explosion and the great ordovician biodiversification event reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments

numerical models of lithospheric deformation forming the southern alps of new zealand compression of the entire continental lithosphere is considered using two dimensional numerical models in order to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages the models are based on the central section of new zealand southern alps where continental collision has occurred along the alpine fault since about 7 ma they incorporate brittle elastic ductile rheology heat transfer surface processes and fault localisation the models are compared to the surface relief the gps convergence velocity the measured electrical conductivity and the geometry of the crustal root imaged from seismic velocity measurements the crustal deformation is characterized by localized uplift at the plate boundary alpine fault and by two secondary zones of faulting one is located about 60 to 80 km east of the alpine fault at the start of upper crust bending or tilting and the other is located about 100 to 130 km east of the alpine fault as a result from shear strain propagating to the surface through the ductile lower crust the observed asymmetric shape of the crustal root is best reproduced for a mantle lithosphere strength of the order of 200 mpa and an intermediate rate of strain softening a lower strength of the mantle lithosphere can produce symmetric thickening but provides an amplitude of the crustal root too small when compared to observations the observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced this offset is most likely due to the three dimensionality of oblique collision in new zealand southern alps

geophysical geological transect and tectonic evolution of the swiss italian alps a complete alpine cross section integrates numerous seismic reflection and refraction profiles across and along strike with published and new field data the deepest parts of the profile are constrained by geophysical data only while structural features at intermediate levels are largely depicted according to the results of three dimensional models making use of seismic and field geological data the geometry of the highest structural levels is constrained by classical along strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units because the transect is placed close to the western erosional margin of the austroalpine nappes of the eastern alps it contains all the major tectonic units of the alps a model for the tectonic evolution along the transect is proposed in the form of scaled and area balanced profile sketches shortening within the austroalpine nappes is testimony of a separate cretaceous age orogenic event west directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the meliata hallstatt ocean further to the east considerable amounts of oceanic and continental crustal material were subducted during tertiary orogeny which involved some 500 km of n s convergence between europe and apulia consequently only a very small percentage of this crustal material is preserved within the nappes depicted in the transect postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments both detachments nucleating at the interface between lower and upper crust large scale wedging of the adriatic or apulian lower crust into a gap opening between the subduced european lower crust and the pile of thin upper crustal flakes alpine nappes indicates a relatively strong lower crust and detachment between upper and lower crust

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

palinspastic reconstruction and topographic evolution of the eastern alps during late tertiary tectonic extrusion this paper presents a new palinspastic restoration of the eastern alps for neogene time and an attempt to reconstruct the neogene palaeogeology palaeotopography and palaeohydrography in connection with the structural evolution the eastern alps underwent radical horizontal displacement during the neogene due to large strike slip systems and formation of structural windows our palinspastic reconstruction considers a the rearrangement of tectonic units dismembered during tectonic extrusion b the tectonic denudation driven by displacement of the crystalline blocks c geochronological arguments and d the sedimentary record of the syn extrusion basins the rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre miocene unstretched pattern of the eastern alps reduced to 65 of its present day e w elongation using this structural frame and considering the sedimentary record a set of palaeogeologic and palaeotopographic sketch maps with the palaeo river systems is presented for three time slices pre syn and post extension situation in late oligocene and early miocene times the western eastern alps were already mountainous whereas the eastern part of the orogen formed lowlands or hilly areas enhanced block movement in the course of the extrusion process around the early middle miocene boundary led to the formation of intramontane sedimentary basins and a fault induced reorientation of the drainage pattern which forms the basis of the modern river system in the area east of the tauern window this region where pre miocene land surfaces are preserved probably became a mountainous area not before late miocene time and never reached the elevations of the areas further west

new probable cnidarian fossils from the lower cambrian of the three gorges area south china and their ecological implications abstract lower cambrian assemblages of phosphatic and chitinous tubular fossils of probable cnidarian affinities were recovered from the yanjiahe shuijingtuo and shipai formations in the three gorges area south china terreneuvian cambrorhytium cf c major co occur with an unnamed taxon in the yanjiahe formation they can be compared with specimens from the ediacaran miaohe fauna in the three gorges area the most abundant and diversified cnidarian assemblages including cambrorhytium gracilis n sp cambrorhytium cf major cambrorhytium minor sphenothallus songlinensis sphenothallus taijiangensis sphenothallus kozaki sphenothallus sp byronia sp and mackenzia sp are reported from the overlying cambrian stage 3 4 shuijingtuo and shipai formations these new fossils show previously unknown evidence for cnidarian affinities such as an attachment disk clonal clusters and probable budding structures these diversified assemblages of probable cnidarian fossils further fill major gaps in the cambrian record of stem metazoans and document associated early evolutionary patterns in addition our discovery suggests that along with sponges these tubular fossils represent another important component of benthic ecosystems that colonized often dysoxic deep settings during the cambrian explosion the observed increases of average body size and biomineralization among these probable cnidarians are tentatively linked to oxygenation of deep oceanic settings and appearance of predatory pressure during the ediacaran cambrian transition respectively

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

lower and middle cambrian acritarchs from northeastern poland abstract acid resistant organic walled microfossils acritarchs are reported from lower and middle cambrian rocks penetrated by borehole okuniew ig 1 in the western podlasie depression the study showed the presence of 11 acritarch species previously known from coeval rocks in northeast poland the assemblage from the okuniew borehole is compared with characteristic assemblages from other parts of the east european platform

biodiversity patterns of ordovician marine microphytoplankton from baltica comparison with other fossil groups and sea level changes abstract based on the extensive literature on ordovician acritarchs biodiversity curves of marine microphytoplankton of the palaeocontinent baltica have been compiled the dataset is derived from more than 100 publications and includes over 600 species whose ranges can be used in diversity analysis stratigraphically well constrained data from the rapla and mannamaa boreholes northern estonia are analysed separately in order to provide additional information on the middle to late ordovician microphytoplankton evolution on shallow shelf settings the total species diversity and normalised diversity curves based on range through data show a slight decrease from the tremadocian to floian which may partly be attributed to limited data available a continuous increase in diversity from the base of the dapingian to late darriwilian early katian can be observed with highest total diversity approaching 250 species in the keila regional stage the highest appearance rates are recorded in the dapingian and darriwilian diversity progressively diminished after the early katian with a more pronounced decrease in the upper ordovician porkuni regional stage corresponding to the hirnantian a significant faunal turnover took place in the uppermost ordovician pirgu and porkuni stages with high extinction rates as well as the appearance of many acritarch taxa that are typical of the silurian floras the phytoplankton diversity curves match rather well with those of several other fossil groups in baltica notably brachiopods and ostracods comparison with other palynomorphs reveals both similarities and differences the chitinozoans show the highest diversities in the upper darriwilian but their later decline predates that of acritarchs a rapid diversification of scolecodonts jawed polychaetes can also be observed in the upper darriwilian however unlike acritarchs or chitinozoans their diversity continues to increase until the uppermost katian the acritarch diversity changes are analysed and discussed in the context of palaeogeographic northwards drifting of baltica and palaeoclimatologic changes rising sea levels up to the middle part of the upper ordovician the increasing diversity of the phytoplankton not only roughly correlates with the baltic and global sea level changes but also with the diversification of marine invertebrate groups

deep reflection surveying in central tibet lower crustal layering and crustal flow summary in the summer of 1998 a series of multichannel deep seismic reflection tests were conducted in central tibet around 89 e as part of the third phase of project indepth a multidisciplinary study of the structure of the core of the tibetan plateau short seismic reflection sections from four locations spanning the jurassic banggong nujiang suture show several features thin 2 km sedimentary cover sequences an unreflective upper crust down to approximately 25 km depth strongly reflective lower crust down to 65 km depth 22 s traveltime and a distinct change in seismic character from layered reflectivity to unreflective that we interpret as marking the reflection moho these profiles are unlike those in parts of the yadong gulu rift to the south where few deep reflections exist and reflection profiles are dominated by highamplitude reflection bright spots at 6 s which have been interpreted as fluid bodies either water or magma analyses of seismic amplitude decay on individual shot records confirm sourcegenerated energy at traveltimes beyond 22 s thus indicating that the change in reflection character at the moho is not the result of a loss of signal penetration the depths to the top and bottom of the lower crustal reflectivity match both a mid crustal velocity increase and the moho determined for this area from refraction profiling lower crustal reflectivity is consistent with current ductile deformation as suggested by models for middle and lower crustal flow in tibet however cretaceous age magmatic intrusions may explain at least part of the observed reflectivity

eastern alpine tectono metamorphic evolution constraints from two dimensional p t t modeling we use two dimensional 2 d p t t modeling to constrain the thermal and rheological aspects of different scenarios for the late mesozoic and cenozoic tectonic evolution of the eastern alps inferred from excellent data sets from the tauern window tw models invoking subduction of the south penninic sp oceanic lithosphere during thrusting and subsequent erosion of the austro alpine aa upper plate nappe stack are inconsistent with the observed thermal evolution within the aa and penninic units in these models predictions for the aa peak thermal conditions are lower than observed after exhumation and cooling to midcrustal levels and subduction of the continental middle penninic mp block the aa undergoes a phase of renewed heating to almost the previous peak temperatures simultaneously the penninic units experience a phase of heating upon subduction followed by cooling after onset of subduction of the north penninic np basin the model predictions are inconsistent with the observed nearly isothermal uplift path of the sp after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper aa amphibolite facies down to the higher penninic unit greenschist facies a model with the beginning of subduction of the sp occurring after crustal thickening of the aa and subsequent return to normal crustal thicknesses is compatible with the p t t data in this model peak temperature conditions are higher in the aa followed by a phase of strong cooling in the aa upper plate with the onset of underthrusting this model also explains successfully nearly isothermal exhumation of the mp and inverted metamorphic peak conditions in the deeper aa material accreted to the hanging wall from the oceanic crust sp experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heat up to its thermal climax after the subduction of trailing continental lithosphere the subsequent pt path of the penninic units strongly depends on the timing and rates of underthrusting by the foreland observed pt paths in the mp within the tw require continuous subduction of the np and the trailing european foreland under the exhuming mp block documented rapid cooling in the final uplift phase of the penninic units in the tw requires exhumation rates up to approximately 4 mm yr predictions of slightly elevated present day geothermal gradients in the tw area are consistent with available heat flow data as a result of mesozoic rifting followed by late mesozoic crustal thickening of the aa paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the penninic units can explain observed late cretaceous crustal extension in the aa units in terms of gravitational spreading ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric mantle beneath the central orogen further enhanced by rapid late stage uplift predictions for the present day rheological structure of the eastern alps support the existence of a strong upper crustal layer two wedge shaped strong upper mantle layers to the north and the south of the orogen and a weak upper mantle underlying the central orogen

the strength of the continental crust detachment zones and the development of plastic instabilities abstract the maximum strength of the continental crust is constrained by the geothermal gradient the lithological make up of the crust and whether or not the byerlee relation holds to the base of the continental crust if this linear byerlee relation holds then maximum shear stress levels sustained at low geothermal gradients 10 20 c km 1 could be as high as 300 mpa in the upper part of the mantle for strike slip regimes and up to 700 mpa in thrust regimes towards the base of the crust however if byerlee s relation breaks down at moderate temperatures and pressures and if such breakdown is associated with the transition from unstable to stable sliding on pre existing faults then the maximum stress levels in the crust being set by the shear stress at the breakdown depth are much lower ca 300 mpa for low geothermal gradients and thrust regimes the lithosphere is quite weak below the crust for geothermal gradients greater than 20 c km 1 so attempts to treat the lithosphere as an elastic slab for time scales for which plastic flow contributes significantly to the deformation should be questioned for a geothermal gradient of 10 c km 1 the entire continental crust and part of the upper mantle behave as an elastic brittle slab at higher geothermal gradients the distribution of strength is variable vertically according to the magnitude of the geothermal gradient and the lithological make up large contrasts in strength occur across rheologically defined boundaries as well as across lithological boundaries at these higher geothermal gradients such contrasts in strength could act as nuclei for detachment zones within the continental crust and upper mantle detachment at or near the moho is to be expected only for low geothermal gradients lateral variations in the geothermal gradient produce shallowly dipping zones of highly contrasting strength throughout the continental crust and uppermost mantle possibly resulting in flat lying thrusts and normal faults which may ramp upwards through the crust instabilities leading to seismic events which occur solely during plastic shearing should be common in thrust terranes where the stresses are relatively high ca 100 200 mpa for geothermal gradients ranging from 10 to 50 c km 1 and for quartz rich felsic mafic and peridotitic rock types thrust terranes are therefore predicted to be characterized by coseismic plastic instabilities on all detachment zones throughout the crust and into the uppermost mantle but embryonic rift zones should be aseismic at depth ca 10 km

lateral extrusion in the eastern alps part 1 boundary conditions and experiments scaled for gravity lateral extrusion encompasses extensional collapse gravitational spreading away from a topographic high in an orogenic belt and tectonic escape plane strain horizontal motion of wedges driven by forces applied to their boundaries in the eastern alps it resulted from 1 an overall northerly compression apulia against eurasia 2 a strong foreland bohemian massif 3 lack of constraint along a lateral boundary carpathian region and 4 a previously thickened gravitationally unstable thermally weakened crust eastern alpine orogenic belt six indentation experiments reproduce lateral extrusion at lithospheric scale the models have two to four lithospheric layers with a mohr coulomb rheology for the upper and a viscous rheology for the lower crust the lithosphere rests upon a low viscosity asthenosphere a broad indenter a narrow deformable area and a weakly constrained eastern margin fullfill as closely as possible conditions in the eastern alps indentation produces both thickening in front of the indenter and escape of triangular wedges lateral variations in crustal thickness become attenuated by gravitational spreading the overall fault pattern includes domains of reverse strike slip oblique normal and pure normal faults strike slip faults in conjugate sets develop serially the narrow width of the deformable area and the strength of the foreland determine the angles between the sets gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin opposite the unconstrained margin the indenter front shows thrusts and folds intersecting with the conjugate strike slip sets a triangular indenter favors spreading high velocity of indentation favors escape high confinement limits lateral motion inhibits spreading and favors thickening lateral extrusion in the eastern alps is best modeled by 1 a weak lateral confinement 2 a broad and straight indenter 3 a narrow width of the deformable area and 4 a rigid foreland crustal thickening lateral escape and gravitational spreading all contribute to the overall deformation

lithospheric scale folding numerical modelling and application to the himalayan syntaxes we describe the eastern and western himalayan syntaxes which are large scale antiforms situated at geodynamically similar locations and the metamorphic evolution of which is coeval in the india asia collisional history to understand the mechanical plausibility of the structural interpretation we present two dimensional finite element modelling of lithospheric folding the models reveal the coeval development of adjacent synformal basins analogous to the peshawar and kashmir basins on both sides of the western syntaxis similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large scale continental shortening and an efficient mountain building process

do faults trigger folding in the lithosphere a number of observations reveal large periodic undulations within the oceanic and continental lithospheres the question if these observations are the result of large scale compressive instabilities i e buckling remains open in this study we support the buckling hypothesis by direct numerical modeling we compare our results with the data on three most proeminent cases of the oceanic and continental folding like deformation indian ocean western gobi central asia and central australia we demonstrate that under reasonable tectonic stresses folds can develop from brittle faults cutting through the brittle parts of a lithosphere the predicted wavelengths and finite growth rates are in agreement with observations we also show that within a continental lithosphere with thermal age greater than 400 my either a bi harmonic mode two superimposed wavelengths crustal and mantle one or a coupled mode mono layer deformation of inelastic folding can develop depending on the strength and thickness of the lower crust

exhumation in a convergent orogen the western tauern window unroofing of the western tauern window involved both low angle detachment faulting brenner fault and enhanced footwall erosion contemporaneous with upright antiformal folding this combination reflects orogen parallel e w extension during continued n s alpine convergence new fission track ages establish the relative chronology of folding and faulting and demonstrate that displacement was not always accommodated on the same surface during exhumation some units migrated from the footwall to the hanging wall of the main detachment fault due to the interplay between folding and faulting the region can effectively be divided into 3 distinct domains 1 the penninic units of the western tauern window were always in the footwall to the fault with maximum exhumation in the core of the dome due to folding and erosion 2 the lower austroalpine unit north of the tauern window was first part of the hanging wall to the brenner fault at a later stage this unit was exhumed by a further 4 5 km as part of the footwall to a more discrete through going fault the silltal fault 3 the middle and upper austroalpine units west of the tauern window were always within the hanging wall exhumation of the footwall from an initial depth of 25 km led to a transition in mechanical behaviour the curviplanar folded ductile shear zone marking the boundary to the tauern window was eventually transected by a more planar discrete brittle fault silltal fault with unit 2 now in the footwall along which the pre existing mylonites were passively exhumed to the surface

structural and rheological implications of lower crustal earthquakes below northern switzerland abstract a detailed study of the recent seismicity in the northern alpine foreland of switzerland reveals that earthquakes with magnitudes between 0 9 and 4 2 occur not only in the upper part of the crust but contrary to observations in most other intracontinental settings seismicity extends down to depths of about 30 km in the lower crust observed p wave velocities in the focal region of even the deepest earthquakes are less than 6 3 km s 1 the average value of poisson s ratio for the entire focal depth range 6 30 km lies between 0 23 and 0 24 focal mechanisms are mostly a combination of strike slip and normal faults with consistent orientations of p and t axes but without any systematic dependence on focal depth from geological observations regional strain rates are of the order of 10 13 s 1 two dimensional temperature modelling indicates that temperatures in the lower crustal seismogenic zone are well above 450 c under the assumption of a dry environment the observed p wave velocities and poisson s ratios imply either a granitic or quartz rich gneissic composition or possibly a gabbroic quartz free granulite the deformation would be governed either by the rheology of quartz or of plagioclase the onset of ductility observed in laboratory studies lies around 300 c for quartz and around 450 c for plagioclase thus conditions in the lower crust beneath northern switzerland appear to be incompatible with the occurrence of either brittle failure or plastic instabilities as is implied by the existence of earthquakes alternatively lower crustal earthquakes could be a manifestation of the presence of fluids in the lower crust fluids under near lithostatic pressure either evolved in situ from metamorphic reactions or from infiltration from the mantle would tend to decrease the effective stress on pre existing faults and thus allow brittle friction failure to occur at depths where under dry conditions or low fluid pressures only ductile deformation would be expected

continental lithosphere folding in central asia part ii constraints from gravity and topography abstract periodical sub parallel fold like structures observed in the western gobi have two characteristic dominant wavelengths of approximately 50 and 300 360 km we explain these observations in terms of independent quasi viscous folding of the crustal and upper mantle parts of the lithosphere due to horizontal transpressional stresses in central asia induced by the collision with the indian plate we derive a model for the mechanical response of the continental lithosphere to horizontal stresses for a rheologically layered plate with non newtonian power law rheology overlying a low viscosity asthenosphere the differentiation in the effective viscosity and thickness of the strong upper crust and upper mantle lithosphere along with the presence of a low viscosity lower crust between them leads to their partial decoupling during compressional deformation these features result in the appearance of different wavelengths of the folds in the western gobi region we derive simple semi analytical estimates for the dominant wavelengths and rates of growth of surface undulations constrained by data from experimental rock mechanics topography and gravity

extrusion vs accretion at the frictional viscous decollement transition in experimental thrust wedges the role of convergence velocity in many cases thrust wedges accreted at shallow crustal levels show an across strike rheological variability along the basal decollement notably from brittle to ductile behaviour in this paper we illustrate the results of sandbox analogue modelling research devoted to studying the influence of convergence velocity on wedge architecture when laterally juxtaposed frictional and viscous materials occur along the basal decolle ment of accreting thrust wedges our results show that slow convergence favours a near symmetrical distribution of thrust vergence within wedge sectors accreted above viscous decoll ement material whereas fast convergence favours vergence asymmetry in particular at fast convergence rates the hinter landward extrusion of viscous decollement material at the toe of the frictional wedge is favoured and contributed to accommodate a significant amount of the total contraction

prediction of faulting from the theories of elasticity and plasticity what are the limits elasticity rigid plasticity and elasto plasticity are the simplest constitutive models used to describe the initiation and evolution of faulting however in practice the limits of their application are not always clear in this paper we test the behaviour of these different models using as examples tectonic problems of indentation of a die compression with basal shear bending of a plate and normal faulting around a dike by comparing the results of these tests we formulate some guidelines that may be useful for the selection of an appropriate constitutive model of faulting the theory of elasticity reasonably predicts the initiation of the fault pattern but gives erroneous results for large strains the theory of rigid plasticity is more appropriate for large deformations where the geometry of faults can be found by the method of characteristics this method works well for zones of failure that are not severely constrained by elastic material outside e g when faults are connected to the free surface a viscous substratum or a zone of weakness non associated elasto plasticity is the most complete theory among those considered in this paper it describes the evolution of faults from the initiation of localized deformations to the formation of a complicated fault network

coordinated biotic and abiotic change during the great ordovician biodiversification event darriwilian assembly of early paleozoic building blocks abstract the ordovician period records an extraordinary biodiversity increase known as the great ordovician biodiversification event gobe which coincided with a series of environmental changes to the earth system notably a cooling global ocean increased oxygenation and increased nutrient supply from volcanism and continental weathering the co evolution of earth and its biota during this interval has been studied in various contexts on multiple paleocontinents emerging patterns depend on the lens of investigation here we summarize the current state of understanding by reviewing and synthesizing the fossil and sedimentary records recent paleontological studies mainly focused on rhynchonelliform articulated brachiopods bryozoa cephalopods trilobites graptolites echinoderms and reef organisms have documented details of diversification body size increase development of ecosystem complexity and intensification of inter continental dispersal from the late cambrian through late ordovician biomass increased markedly between the early and middle ordovician furthermore diversification rates increase statistically during the mid darriwilian age both globally and regionally coincident with these biotic changes geochemical proxies record significant changes to earth s physical system oceanic temperatures decreased and atmospheric oxygen levels increased to near modern levels in concert with the middle ordovician diversification of shelly fauna anoxic pulses ceased and evidence for deep ocean ventilation prevails in middle ordovician strata furthermore a major middle late ordovician change in oceanic strontium isotopic composition indicates increased weathering of juvenile volcanic rocks and delivery of nutrients to marine settings this multi proxy dataset records near simultaneous changes in fossil rich shallow marine environments during exactly the interval of greatest diversification by integrating biotic and geochemical datasets a clear picture of the co evolution of earth and its biota emerges indicating that the darriwilian was the critical interval facilitating increased capacity of ecosystems we therefore recommend restricting the term gobe to indicate this short interval of rapid diversification and ecosystem change and using ordovician radiation when referencing the sum of diversifications that occurred throughout the ordovician period

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

syn and post collisional heat flow in the cenozoic eastern alps the heat flow evolution of a continental collisional zone is exemplified by the eastern alps heat flow maps for the syn collision oligocene syn extrusion early middle miocene and post extrusion late miocene recent stages are presented and are discussed in relation to the orogenic evolution continental collision during paleogene time was characterized by extremely low heat flow 150 mw m2 a few hundred kilometers south of it the former was a result of crustal thickening and of thermal blanketing due to rapid sedimentation and nappe stacking the latter was caused by slab break off and magmatic activity the early middle miocene syn extrusion stage was characterized by rapid exhumation of metamorphic core complexes tauern and rechnitz windows and by magmatic activity styrian basin both mechanisms caused extremely high heat flow 200 mw m2 in contrast the orogenetic front remained cold thereafter magmatic activity ended and uplift rates decreased thus late miocene heat flow is characterized by low to moderately high values heat flow values 75 mw m2 were restricted to the transition zone of the pannonian basin characterized by thinned crust and to the tauern window area recent temperature data indicate a subtle post miocene increase in heat flow

quartzose mafic spectral feature space model a methodology for extracting felsic rocks with aster thermal infrared radiance data abstract the original spectral features of felsic rocks are often intermingled with other surface objects which results in difficulty of detecting felsic rocks using remote sensing techniques few felsic rock indices were proposed and visual interpretation with rgb false color composition is widely used to detect felsic rocks this paper aims to construct a two dimensional spectral feature space model to extract felsic rocks using aster thermal infrared radiance data the study area is located in northern qinghai province western china with average altitude of approximately 4200 m a large number of training pixels of mafic ultramafic rock quartz rich rock felsic rock carbonate rock and vegetation were selected from the aster images as samples of these surface objects then we used a quartz rich rock index qi qi band14 0 844 band12 1 897 and a mafic ultramafic rock index mi mi 0 915 band10 band13 1 437 to generate a two dimensional scatter plot the plot was named after quartzose mafic spectral feature space qmfs the samples show an approximate triangular shape in the qmfs mafic ultramafic rock quartz rich rock and carbonate rock are located in separate locations in the three vertex regions respectively while felsic rock is located in the central region of the triangle next we calculated a linear belt of silicate rocks in which silicate rocks vary regularly by using a linear regression analysis in the qmfs statistical characteristics of the felsic rock samples are analyzed afterwards a polygon which delineates the distribution of felsic rock samples was constructed from the linear belt of silicate rocks then we generated a system of inequalities based on the equations of the edges of the polygon the application of the inequalities to the aser images shows a good performance of the qmfs for extracting felsic rocks

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

at what stress level is the central indian ocean lithosphere buckling internal contrasts in strength are responsible for lithospheric buckling these are quantified by comparing the indian ocean data to two dimensional visco elasto plastic numerical models where the material properties depend on temperature and pressure the central indian basin is known for its intraplate seismicity and long wavelength undulations of the sea floor and associated gravity signatures to simulate the amplitudes of undulations that reach 1 km within about 11 ma of compression and 60 km of shortening the required mean yield strength of the lithosphere is 400 mpa if either a hydrostatic fluid pressure is considered in the crust or a mechanical decoupling at the depth of the moho small crustal wavelengths are superimposed on the long wavelength deformation in agreement with observations it is then possible to match the alternative indication that buckling commenced only 4 ma ago with a total amount of shortening of 30 km and with a required yield strength of 200 mpa about 10 of homogeneous thickening accompanies buckling taking into account variable thermal conductivity demonstrates that a change in the geotherm is sufficient to increase buckling amplitudes by 10 within 5 ma fl 2000 elsevier science b v all rights reserved

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

continuity and episodicity in the early alpine tectonic evolution of the western carpathians how large scale processes are expressed by the orogenic architecture and rock record data

1 3 billion years of acritarch history an empirical morphospace approach abstract acritarchs are a group of organic walled vesicular microfossils interpreted as protists and are among the first eukaryotes preserved in the fossil record taxonomic inconsistencies amongst acritarch workers have made it difficult to address the evolutionary history of this group through more traditional methods i e biodiversity through species counts we have constructed an empirical morphospace to examine the first 1 3 billion years of acritarch evolution we show that protist morphologic evolution is broadly correlated with major environmental and biologic revolutions in earth history such as late neoproterozoic global glaciations the first appearance of the ediacaran metazoans and the cambrian explosion our results also show that protist morphologic expansion precedes their taxonomic diversification this pattern similar to that seen in phanerozoic animal clades suggests that early morphospace saturation and convergence are common occurrences in eukaryote macroevolution in addition our data do not support a monotonic increase in maximum diameter of acritarch vesicles through the proterozoic instead maximum vesicle diameter appears to fluctuate in the proterozoic before decreasing significantly in the early cambrian

the great ordovician biodiversification event gobe the palaeoecological dimension abstract the great ordovician biodiversification event gobe saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so called cambrian explosion the diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them the present paper reviews the palaeoecological dimension of the gobe it involved major increases in s and biodiversity largely associated with the rise of the paleozoic evolutionary fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the ecological evolutionary units p1 and p2 in the benthos these include more complex food webs than those of the cambrian greater tiering especially above the sediment water interface and the development of guild structures indicating increased competition between taxa for particular resources the ordovician is characterized by a profound change in reef composition with a switch from microbial dominated reefs in the early and middle ordovician to metazoan dominated reefs in the late ordovician increases in complexity of deep water trace fossil assemblages began in the early ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities within the water column the gobe involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton including planktotrophic larvae from a range of invertebrate clades this revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates molluscs and arthropods and promoted the rise of suspension feeders in the benthos an escalation amongst predators and thus community evolution may also have been a major driver of biodiversification

acritarchs from the maclean brook formation southeastern cape breton island nova scotia canada new data on middle cambrian lower furongian acritarch zonation abstract we present here the results of a pilot study on cambrian acritarchs from cape breton island nova scotia based on a material from the maclean brook formation in the mira river valley an assemblage from the base of the formation contains 8 species including eliasum llaniscum cristallinium dubium and symplassosphaeridium cambriense and correlates with a position close to the paradoxides davidis p forchhammeri zone boundary an assemblage from the upper part of the formation contains 16 species of which 3 are new including cristallinium aciculatum petaloferidium lacrimiferum n sp pirea orbicularis stelliferidium pingiculum s magnum n sp s albanii n sp and timofeevia microretis correlated to the olenus zone the upper assemblage is particularly noteworthy as it provides the first direct evidence for the olenus zone in the maclean brook formation furthermore it contains stratigraphically significant taxa known from sections on baltica and northern gondwana but which have not been reported previously on avalonia the distinction between stelliferidium and timofeevia is discussed in part based on new data on timofeevia lancarae from its type area in northern spain it is suggested that many reports of furongian timofeevia phosphoritica and t lancarae are better referred to stelliferidium

palynology of uppermost proterozoic and lowermost cambrian formations central mackenzie mountains northwestern canada the acritarch assemblages of strata from the base of the upper proterozoic sheepbed formation to the base of the lower cambrian atdabanian sekwi formation are described the sections sampled are in southwestern internal structural units where erosion beneath the sub cambrian unconformity is least evident problems of lithostratigraphic correlation of post sheepbed pre backbone ranges formations remain acritarchs indicate the age of the sheepbed formation and the blue flower formation above it is latest proterozoic vendian whereas that of the vampire formation is early cambrian atdabanian the backbone ranges formation has not yielded datable acritarchs but it is for the most part cambrian in age based on other fossil evidence comparisons are made with the russian platform and southern canadian rocky mountains successions the total number of acritarch genera increases markedly across the precambrian cambrian transition

the spatial nearshore offshore distribution of latest permian phytoplankton from the yangtze block south china abstract in order to analyze the spatial paleoecological distribution of the organic walled microphytoplankton in the late permian five sections from the yangtze block south china displaying different sediment facies types from neritic to offshore paleoenvironments including basinal facies have been investigated palynologically based on the diversity and relative abundance of acritarch species and genera the new data from the permian provide similar patterns as those described from other geological periods 1 low diversities with 2 to 4 acritarch species occur in nearshore environments whereas the higher diversities more than ten acritarch species appear in the offshore environments 2 at the generic level the genera leiosphaeridia reduviasporonites and micrhystridium are distributed widely from nearshore facies corresponding to shallow water environments to offshore facies corresponding to deeper water settings whereas some genera such as dictyotidium and veryhachium have a narrower distribution occurring on the continental shelf and towards the basin indicating open marine environments 3 the genus schizosporis only occurs around the permian triassic boundary ptb when the sea level declined probably indicating nearshore environments with shallow water settings 4 at the specific level the species micrhystridium breve displaying short spines and leiosphaeridia minutissima are indicative of neritic facies whereas the other species of micrhystridium and veryhachium with longer spines e g micrhystridium stellatum and veryhachium hyalodermum and leiosphaeridia microgranifera indicate more open marine environments big spherical acritarch species over 80 m in diameter such as dictyotidium reticulatum indicate shallow water environments and 5 in the ptb strata the relative abundance of the enigmatic reduviasporonites interpreted by some authors as a fungal spore is not higher than 14 in every sample indicating that a spike of reduviasporonites does not occur in the yangtze area reduviasporonites chalastus 40 m in length obviously dominates in shelf environments of shallow water whereas the smaller reduviasporonites catenulatus 15 m in length are more common in deeper water

biodiversity patterns of silurian radiolaria abstract data analysis of all published and well dated silurian radiolarian localities was conducted with the goal of revealing long term patterns in silurian radiolarian biological diversity the chronostratigraphic distribution of 161 species was compiled from 25 publications each selected because of their independent age control the pattern and dynamics of changes in biodiversity are described using indices that allow for the assessment of changes in taxonomic richness through time following the hirnantian end ordivician mass extinction the rhuddanian stage records very low levels of diversity which then increase gradually throughout the llandovery series and reach a maximum by the sheinwoodian stage before decreasing during the homerian and finally rebounding during the gorstian stage the early silurian llandovery appears to be an interval during which few species went extinct although the number of last occurrences increases progressively to reach a peak in the ludfordian at the same time first occurrences progressively decrease and are relatively low during the silurian only the gorstian is marked by a sudden increase in origination the early silurian is an interval of faunal recovery from the end ordovician mass extinction event similarly the gorstian stage is characterized by a recovery following the homerian low in radiolarian diversity that may be correlated with the c lundgreni extinction event which affected a number of other pelagic groups e g graptolites acritarchs conodonts and chitinozoans and with the mulde event which coincides with a 4 6 positive excursion of the 13 c curve

crustal shortening root spreading isostasy and the growth of orogenic belts a dimensional analysis scaling laws that describe the growth of mountain belts and the change in their regional topographic profiles with time t are obtained by physical arguments that account for crustal shortening isostasy and creeping flow at the mountain roots an average power law rheology characterized by an exponent n is assumed and different values of n are considered the model predicts the formation of a localized range characterized by a height h and a width a as crust on the sides of a range is compressed the root broadens due to buoyancy driven creep this process in conjunction with isostasy causes h to increase with time as t1 4 t1 3 and a as t3 4 t2 3 as n is varied from 1 newtonian rheology to plastic rheology accordingly the aspect ratio a h increases as t1 2 t1 3 the scaling laws depend on the rheology but rather weakly the aspect ratio increases with age but is nearly independent of the rate of shortening the deformation extends gradually inland with increasing time so that the age of tectonic deformation decreases with distance from the continental border uplift takes place inland from the crest while subsidence occurs at the opposite side the ratio a h of the andes and tibet is consistent with this model

thermomechanical structure of european continental lithosphere constraints from rheological profiles and eet estimates summary the eet equivalent elastic thickness of the lithosphere is a measure of the integrated lithospheric strength it is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere we present a comparison of eet estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the european and eurasian continental lithosphere we discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for precambrian segments of europe s lithosphere variscan europe and the alpine collision belt this analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western europe and asian parts of eurasia the eet estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of eet derived from flexural studies of sedimentary basins forelands and orogenic belts these rheological profiles suggest weakening of the major parts of the european and eurasian continental lithosphere by decoupling of the crustal and upper mantle parts a comparison with the seismicity depth distribution for some selected sites suggests that the intra plate seismicity is essentially restricted to the upper crustal parts of europe s lithosphere providing additional support to the notion of the decoupled lithosphere the presence of intra plate stress fields can explain a significant part of the observed variations in eet estimates within individual thermotectonic age groups a comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of eet inferred from the rheological response of basins and orogens at more moderate levels of intra plate stress

a two level concept of plate tectonics application to geodynamics abstract a two level plate tectonics concept is developed on the basis of data on lithosphere rheological stratification this approach differentiates between crust and subcrust plate ensembles separated by a lower crust viscoplastic asthenolayer similarly to classical plate tectonics three types of boundaries are distinguished in the lower layer which do not always coincide with crust plate boundaries especially for continents applications of this concept to geodynamics are considered and a corresponding quantitative analysis for several important processes is carried out a quantitative model of mountain formation and collision plateau origins is proposed also a geodynamic model of the evolution of passive margins taking into account a lower crust viscous flow is considered and its geological consequences are discussed a mechanism of rifting taking into consideration rheological lithospheric layering and its vertical movements caused by extension is developed both a qualitative scheme and quantitative analysis of the slow evolution of intracraton structures of shield basin type taking into account erosion and sedimentation processes are worked through also historical aspects of plate tectonics are discussed from the point of view of the proposed concept

present day lithospheric strength of the eastern alps and its relationship to neotectonics 1 we calculate the present day lithospheric strength of the eastern alps along the new reflection seismic profile transalp to examine vertical and lateral strength variations and their implications on neotectonic activity of the eastern alps the large scale geometry of the eastern alps and the spatial distribution of upper and lower crustal layers and the lithospheric mantle is constrained by the deep seismic line two rheological models coupled to a kinematic thermal model that accounts for the thermal evolution of the eastern alps for the last 30 myr are investigated for the present day lithospheric configuration in the eastern alps models with strong model a and weak model b crustal rheologies predict the european and the adriatic plates to be stronger than the central zone of the orogen comprising the region between the inntal fault and the periadriatic fault model a is characterized by a brittle ductile boundary between 14 and 9 km depth and strong coupling of the mechanically strong lower crust to the upper mantle whereas model b suggests the presence of a thick decoupling zone between the upper crust and the upper mantle and a shallower brittle ductile boundary 7 10 km of these end member scenarios model a is in better agreement with neotectonic data including seismicity down to the upper lower crust boundary within the adriatic plate uplift of the central zone of the eastern alps and the southern alps and eastward escape of fault bound blocks such deformation pattern is best explained by lateral extrusion upon north south compression supporting a strong weak strong configuration of tectonic units along the transalp line

a lithospheric cross section through the swiss alps ii constraints on the mechanical structure of a continent continent collision zone summary the calculation of strength profiles along the european geotraverse egt through the swiss alps yields constraints on the large scale vertical and lateral mechanical structure through the alpine continent continent collision zone strength profiles are evaluated for different assumptions on petrological stratification and strain rate and are based on temperature depth profiles derived from transient thermo kinematic modelling of the neoalpine orogeny the main contribution to the total strength results from the mantle lithosphere which is strongly controlled by temperature in contrast the crustal contribution is mainly determined by variations in petrological stratification a direct correlation between surface heat flow and the total strength of the crust the mantle lithosphere and the entire lithosphere crust and mantle lithosphere is not observed our results demonstrate that in tectonically active areas a transient thermal model along with detailed knowledge of the deep structure and petrology is necessary to evaluate lithospheric strength envelopes inside the collision zone strain rate has a strong control on the bottom of the mechanically strong crust whereas outside the collision zone the effect is less significant the cut off depth of seismicity along the profile which correlates largely with the bottom of the mechanically strong crust deviates from the 300 400 c isotherm the inferred effective elastic thickness for the molasse basin north of the alps is in agreement with flexural modelling results whereas for the southern alps the predictions deviate

lower cambrian acritarchs from northern spain the precambrian cambrian boundary and biostratigraphic implications acritarchs are reported from basal cambrian rock units inthe cantabrian region of northern spain that are known to contain archaeocyathan and trilobite faunas biostratigraphic correlation of the iberian sequences with other regions has been hampered by the strong provincialism of these faunas however this report of evidently cosmopolitan acritarch taxaestablishes the time equivalence of early cambrian trilobite faunas from iberia baltoscandia and the east european platform eep our data suggest that the detrital deposition of the lower cambrian herreria formation embraces at least three and possibly four lower cambrian acritarch zones previously identified in the eep eastern siberia baltoscandia scotland greenland svalbard and western north america the early cambrian transgression in northern spain was probably initiated in talsy times schmidtiellus mickwitzi trilobite zone in baltoscandia and the eep in part corresponding to the dokidocyathus regularis archaeocyathian zone of the middle tommotian in siberia

structural evolution of the accretional continental margin of the paleoproterozoic svecofennian orogen in southern sweden abstract volcanic and sedimentary rocks of the vastervik and valdemarsvik groups along the southern margin of the 1 90 1 85 ga south svecofennian crustal province in sweden have been migmatised and intruded by granitoids and gabbros during extension deformation phase d1 at a depth of at least 10 km peak metamorphic conditions were reached around 1825 ma extension switched to compression before cooling the partially molten middle crust facilitated shortening and thickening by isoclinal and sheath folds on scales from a decimetre to 10 km d2 a pervasive mylonitic s2 foliation developed these structures were refolded into eastward plunging crustal scale upright folds with e se w nw trending axial planes d3 d2 d3 deformation structures are colinear and developed during top to the wnw slip along s2 locally zones of high ductile strain in garnet bearing migmatite assemblages record considerable vertical displacement from 850 to 300 mpa possibly in nappe roots some major d2 sheath folds are embraced by d3 like but nnw trending megafolds d4 apparently resulting from shortening around the former d2 d4 deformations are interpreted to be broadly coeval and related to transpressive crustal shortening against the northern svecofennian continental back stop after cooling to high greenschist facies conditions early structures underwent renewed transpression in north south direction expressed in localized retrogressive shear zones phase d5 much d5 strain was accommodated by the 10 km wide nw se striking right lateral loftahammar linkoping deformation zone lldz operative around 1800 1780 ma regional strain partitioning caused transverse shortening in 15 20 km wide border zones of the lldz later overprints d6 involve narrow subparallel nw se striking and steeply ne dipping low greenschist facies mylonite zones some of which merge into the regional polyphase asbro norrkoping deformation zone d6 was transpressive oblique left lateral ne block up slip suggests a w sw e ne orientation of 1 peak metamorphism and partial melting in the supracrustal rocks was approximately coeval with 1800 1835 ma old subduction related magmatism in the oskarshamn jonkoping belt ojb 100 km to the south west seismic reflection profiling off the baltic coast babel line b suggests northward polarity of subduction beneath the ojb and a back arc environment of the vastervik and valdemarsvik groups between the ojb arc and the margin of the 1850 ma old svecofennian orogen we suggest that d1 d5 deformation was caused by intracontinental back arc extension and subsequent closure of the back arc basin by oblique accretion of the ojb marginal arc onto the northern continent accretion involved northward indentation by mid crustal wedges one of which coincides with and possibly caused the nucleation of the d5 stage lldz 40ar 39ar data from hornblende and white mica identify a prolonged cooling history of the area 1810 1490 ma influenced by the nearby late and postorogenic transscandinavian igneous belt and possibly by largely hidden c 1530 ma old anorogenic rapakivi type intrusions

the continental collision zone south island new zealand comparison of geodynamical models and observations the south island zone of oblique continent continent convergence occurs along a 400 km long section of the modern australia pacific plate boundary zone across which about 50 km of shortening has been accommodated since about 10 ma the orogen comprises a central mountain range southern alps flanked on both sides by what are interpreted to be foreland basins two essential features that characterize the orogen are 1 the degree of denudation that accompanied deformation and 2 a fundamental structural asymmetry the architectural asymmetry of the orogen can be explained by plane strain finite element models of continental convergence incorporating mantle subduction comparison of model and orogen polarity implies that pacific plate mantle subducts the models predict two crustal scale dipping shear zones that form above the point where the pacific mantle subducts the localized one more distant from the incoming plate retro step up shear zone corresponds to the alpine fault whereas its conjugate pro step up shear zone corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt parameters that modify the model boundary conditions top surface degree of denudation basal zone subduction load crust mantle velocity discontinuity subduction of lower crust mantle retreat and distributed decrease in mantle velocity and the internal strength of the crust two layer crust with moderate coupling temperature distribution strain weakening are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone in turn these are compared with observations about the south island orogen the calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses north to south along strike differences in the width and two dimensional architecture of the orogen simulated in the experiments by varying the model parameters can be explained by a combination of southward increases in preconvergent crustal thickness geothermal gradient convergence and potentially subduction retreat with the added possibility of a southward decrease in the component of lower crustal subduction

collision tectonics in the swiss alps insight from geodynamic modeling this paper compares results from two dimensional finite element dynamic modeling with the kinematic evolution of the swiss alps during the collision phase in particular we investigate the role of inherited lateral strength heterogeneities on orogenesis a number of first order characteristics are directly comparable at crustal scales in the models the entry of continental crust into the convergent margin marks the end of near perfect subduction accretion of material of the subducting plate to the upper plate creates an orogenic wedge on the incoming pro side and initiates a retroshear zone or model backthrust the addition of material to the upper plate builds a bivergent orogen heterogeneities in the pro crust focus shear and lead to the development of nappe structures the combined action of pro shear nappe stacking and retroshear backthrusting uplifts a plug between the two shear zones subsequent focusing of shear along the retroshear zone results in rotation of the plug and overlying units leading to crustal scale backfolds as observed in the swiss alps the model experiments predict features relevant to alpine dynamics including 1 similar crustal thicknesses and exhumation patterns to those observed in the swiss alps today for erosion rates comparable to natural ones 1 mm yr 1 2 continued accretion and subduction of upper crustal fragments allowing high pressure metamorphic conditions 3 tilting and exhumation of lower crust when a midcrustal weak zone is present and 4 shunting of material across the strong lower crustal wedge of the upper plate

the onset of the ordovician plankton revolution in the late cambrian abstract the great ordovician biodiversification event comprises the rapid diversification of marine organisms during the ordovician period it is now clear that this adaptive radiation started for some organisms already in the cambrian and continued for others beyond the end of the ordovician making the great ordovician biodiversification event part of a long term late proterozoic and early palaeozoic radiation that in part is expressed by the fossil record as the cambrian explosion a significant diversification of different groups of the plankton is observed in the late cambrian early ordovician interval leading to the subsequent ordovician plankton revolution the possible causes of this plankton revolution are currently debated they include changes in palaeoclimate palaeogeography or tectonic and volcanic activity as well as a modified nutrient supply in this context the steptoean positive carbon isotope excursion 13 c carb spice event in the late cambrian paibian stage furongian series has been related to a major increase in atmospheric o 2 from 10 18 to some 20 29 and to increased oceanic nutrient availability here we analyze the diversification of the planktonic groups during the late cambrian and early ordovician in particular in relation to the spice event our analyses include the changing diversities of the phytoplankton acritarchs diverse groups of zooplankton e g radiolarians graptolites chitinozoans and the switch to a planktonic mode of life of fossil groups e g arthropods molluscs that were part of the cambrian benthos and that later occupied pelagic niches in addition we focus also on data indicating evidence for a late cambrian to ordovician origin of planktotrophy in invertebrate larvae it can be concluded that none of the diversifications of the different planktonic organisms can be related directly to the spice event however a long term 10 20 million years oxygenation pulse related to the spice event might have fuelled the explosion of phytoplankton diversity observed in the latest cambrian early ordovician that led to completely modified trophic structures permitting an increase in diversity and abundance of plankton feeding groups during the ordovician

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

decoupling during continental collision and intra plate deformation abstract fundamental aspects of mountain building such as the dominant deformation mechanism strain localisation and topography development in response to the presence of decoupling zones either along boundaries of weak and strong lithospheres or within weak domains of continental lithosphere are studied by analogue lithospheric models scenarios are investigated where a weak part of the lithosphere is bordered on either side by stronger lithospheres geometries of the weak zone vary from box to wedge to graben shaped representing lateral strength variations as a consequence of earlier deformation phases the rheologic and geometric configuration at the onset of shortening is therefore considered as representative for continental collision and intra plate settings the modelling results show that the geometric and topographic evolution of mountain belts through time is sensitive to the presence of decoupling horizons along plate or tectonic boundaries this is because the decoupled boundaries localise strain control the dominant deformation mechanism and hence the locus of uplift and subsidence accordingly decoupled boundaries favour deformation by thrusting and particularly in the case of an initially wedge shaped weak zone the development of mountain belts with overall triangular shape in contrast lithospheres with coupled boundaries dominantly deform by buckling irrespective of the initial geometry of the weak zone as this deformation mechanism affects all model domains it produces a distinguishable pattern of uplift and subsidence controlled by buckling strong decoupling at mid crustal levels within the weak zone favours vertical strain partitioning and the formation of plateau shaped mountains the modelling results suggest that natural collisional systems should respond to an increase in plate coupling through time by widespread uplift of the mountain belt including its foreland basins which were formerly created under decoupled conditions due to the absence of decoupling zones like plate contacts weak domains in intra plate settings are predicted to deform dominantly by buckling

phytoplankton dynamics from the cambrian explosion to the onset of the great ordovician biodiversification event a review of cambrian acritarch diversity most early palaeozoic acritarchs are thought to represent a part of the marine phytoplankton and so constituted a significant element at the base of the marine trophic chain during the cambrian explosion and the subsequent great ordovician biodiversification event cambrian acritarch occurrences have been recorded in a great number of studies in this paper published data on cambrian acritarchs are assembled in order to reconstruct taxonomic diversity trends that can be compared with the biodiversity of marine invertebrates we compile a database and calculate various diversity indices at global and regional i e gondwana or baltica scales the stratigraphic bins applied are at the level of the ten cambrian stages or of fourteen commonly used biozones in a somewhat higher resolved scheme our results show marked differences between palaeogeographical regions they also indicate limitations of the data and a potential sampling bias as the taxonomic diversity indices of species are significantly correlated with the number of studies per stratigraphic bin the total and normalized diversities of genera are not affected in the same way the normalized genus diversity curves show a slow but irregular rise over the course of the cambrian these also are the least biased a radiation of species and to a lesser extent of genera in the lower cambrian series 2 appears to mirror the cambrian explosion of metazoans this radiation not evident on gondwana is followed by a prominent low in species diversity in the upper series 3 and lower furongian highest diversities are reached globally and on both baltica and gondwana in the uppermost cambrian stage 10 more precisely in the peltura trilobite zone preceding a substantial phase of acritarch species extinction below and at the cambrian ordovician boundary nearly all the genera present in stage 10 survived into the ordovician the forms that emerged during the cambrian therefore became the foundation for the more rapid radiation of acritarchs during the great ordovician biodiversification event

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

revisiting the great ordovician diversification of land plants recent data and perspectives abstract recent molecular clock data suggest with high probability a cambrian origin of embryophyta also called land plants indicating that their terrestrialization most probably started about 500 ma the fossil record of the cambrian explosion was limited to marine organisms and not visible in the plant fossil record the most significant changes in early land plant evolution occurred during the ordovician for instance the earliest bryophyte like cryptospores and the oldest fragments of the earliest land plants are from the middle and late ordovician respectively organic geochemistry studies on biomarker compositions hint at a transition from green algae to land plants during the great ordovician biodiversification event gobe the colonization of the terrestrial realms by land plants clearly had an impact on marine ecosystems interactions between the terrestrial and marine biospheres have been proposed and the radiation of land plants potentially impacted on co2 and o2 concentrations and on global climate in addition the shift of strontium isotopes during the ordovician is probably linked to changing terrestrial landscapes affected by the first massive land invasion of eukaryotic terrestrial life the land plants seem unaffected by the first global mass extinction at the end of the ordovician that eliminated many marine invertebrate taxa

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

a thermomechanical model of exhumation of high pressure hp and ultra high pressure uhp metamorphic rocks in alpine type collision belts using a fully coupled numerical thermomechanical model handling strain localization surface processes and ultra high viscosity contrasts 11 orders of magnitude we test a number of possible mechanisms of high pressure hp low temperature lt high temperature ht exhumation in continental collision zones the model considers two end member cases low or high buoyancy of the downgoing crust the first model case predicts three levels of exhumation in the same collisional context the classical corner flow lp lt low pressure low temperature exhumation in the accretionary prism deeper 70 km hp ht exhumation for the thickened subducting crustal sedimentary wedge and ultra hp ht exhumation from the lower crustal chamber forming at the depth of 100 120 km and separated from the upper one by a narrow crustal channel the width of this channel can oscillate in the process of shortening thus controlling the quantity of the crustal material exchanged between the crustal wedge and the lower crustal chamber although both zones of crustal accumulation and the narrow channel between them resemble a vortex shaped nozzle this nozzle appears to be too soft to produce any significant overpressures from the upper crustal wedge the material is exhumed following the ascending shear flow created by the overriding plate assisted by positive buoyancy of the heated crustal material from the lower crustal chamber the material is transported upward to the upper crustal wedge by a flow induced by the asthenospheric traction and a small scale convective instability forming in the lower crustal chamber due to its heating by the overriding asthenosphere in the second modelled case of high buoyancy the latter mechanisms become dominant resulting in hyper fast exhumation of the crust to the surface accelerated or slowed subduction in case of full or partial crustal decoupling respectively and upper plate extension d 2001 elsevier science b v all rights reserved

lower crust indentation or horizontal ductile flow during continental collision conditions for indentation and channelised flow are investigated with two dimensional thermomechanical models of alpine type continental collision the models mimic the development of an orogen at an initial central portion of weakened lithosphere 150 km wide coherent with several geological reconstructions we study in particular the role of lower crustal strength in developing peculiar geometries after 20 ma of shortening at 1 cm year crustal layers produce geometries of imbricate layers which result from two contrasted mechanisms of either channelised ductile lateral flow or horizontal rigid like indentation channelised lateral flow develops when the lateral lower crust has a viscosity less than 1021 pa s exhibiting velocities opposite to the direction of convergence this mechanism of deformation produces subhorizontal shear zones at the boundaries between the lower crust and the more competent upper crust and lithospheric mantle it is also associated with a topographic plateau that equilibrates with a wide about 200 km but quasi constant crustal root about 50 km deep in contrast indentation occurs with lateral lower crust layers that have a viscosity greater than about 1023 pa s producing significant shortening and thickening of the central crust in this case topography develops steep and narrow around 100 km wide associated with a thickened crust exceeding 60 km depth a crustal scale pop up forms bounded by subvertical shear zones that root into the mantle lithosphere

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

evolution of orogenic wedges and continental plateaux insights from crustal thermal mechanical models overlying subducting mantle lithosphere the links between an early phase of orogenesis when orogens are commonly wedge shaped and a later phase with a plateau geometry are investigated using coupled thermal mechanical models with uniform velocity subductionboundary conditions applied to the base of the crust and simple frictional plastic and viscous rheologies models in which rheological properties do not change with depth or temperature are characterized by growth of back to back wedges above the subduction zone wedge taper is inversely dependent on the ramberg number rm gravity stress basal traction increasing convergence velocity or crustal strength produces narrower and thicker wedges models that are characterized by a decrease in crustal viscosity from c to b with depth or temperature leading to partial or full basal decoupling of the crust from the mantle display more complex behaviour for models with a moderate viscosity ratio b c 10 1 the crustal wedges have dual tapers with a lower taper in the central region and a higher taper at the edges of the deformed crust a redaction in the viscosity ratio b c 10 2 is sufficient to cause a transition of the central wedge region to a plateau this transition depends on the basal traction therefore the thickness of the weak basal layer also affects the transition further reduction of the viscosity ratio b c 10 4 leads to full basal decoupling and the development of plateaux in all cases considered in most models the plateaux grow laterally at constant thickness between characteristic edge peaks associated with the transitions from coupled to decoupled lower crust where the crust is fully decoupled large scale model geometries for both depth and temperature dependent rheologies are similar with gravity driven flow concentrated in the low viscosity region however strong lateral temperature gradients within these models controlled by the interaction of horizontal and vertical thermal advection diffusion and heterogeneous thickening of the radioactive crustal layer lead to differences in the velocity and deformation fields between the two cases particularly at the plateau margins the results suggest that simple depth dependent viscosity models may be reasonable approximations for describing the large scale geometry of fully developed plateaux but that they are not appropriate for describing the internal features of large orogenic systems or the transition from wedge to plateau geometry

the arc of the western alps in the light of geophysical data on deep crustal structure recently two international deep seismic campaigns in the western and central alps etude continentale et oceanique par reflexion et refraction sismique progetto strategico crosta profonda ecors crop roure et al 1996b and national research program 20 nrp 20 pfiffner et al 1997 have been completed here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the alps collected during the past 40 years including the two above mentioned geophysical projects the tectonic reinterpretation of the ecors crop high resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology combined with previously published interpretations regarding the central alps schmid et al 1996 this study reveals substantial differences in geometry and kinematics between transects across the western and central alps respectively at depth the transition between the central and western alps is marked by the western limits of an adriatic lower crustal wedge shaped structure and the northeastern limits of a similar structure made of european lower crustal material at the surface it coincides with a corridor of dextral strike slip along the tonale and simplon lines in addition the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the penninic realm of the western alps to less than 20 km in the central alps the formation of the western alpine arc was initiated during convergence and collision before 35 myr ago when the adriatic micro plate moving northward with respect to the european foreland caused sinistral transpression in the western alps while the central and eastern alps underwent head on convergence and collision during the post collisional stage i e after 35 myr ago the arcuate shape of the western alps was accentuated by wnw directed movement and anticlockwise rotation of the adriatic microplate decoupled from the central and eastern alps along the tonale simplon dextral shear zone this led to wedging of lower crustal slices both in the western alps and in the central alps the new tectonic interpretation of the ecors crop transect allows a kinematic model to be established for crustal shortening in the western alps during the past 35 myr the ivrea mantle plays the role of a backstop in our tectonic model we distinguish three episodes of post collisional crustal shortening along the ecors crop transect from 35 to 30 ma the brianconnais basement was back thrusted over the gran paradiso units leading to 30 km of crustal shortening first episode in the early miocene movements concentrated mainly along the penninic frontal thrust and resulted in about 60 km shortening second episode post 12 ma shortening within the external massifs is associated with folding in the jura mountains when the crust was shortened by an additional 30 km third episode

grain size sensitive flow and shear stress enhancement at the brittle ductile transition of the continental crust localized shear zones along low angle normal faults have been identified in regions of extension at the brittle ductile transition of the continental crust the possibility of the strain localizing at a depth of 10 km is interpreted here as a consequence of an increase in the equivalent shear stress applied to the flow of the lower crust this enhancement of the flow stress is seen as a prerequisite for the triggering of brittle deformation mechanisms leading to strain localization the lower crust rheology used to examine this stress increase is strain rate temperature and grain size dependent due to the coupling of dislocation and diffusion creep the model structure proposed consists of a top layer the upper crust gliding rigidly above a bottom layer the lower crust which deforms in simple shear during a short time interval 1400 years the equivalent shear stress is found to increase by a factor of up to 3 67 mpa for anorthite and 17 mpa for quartz for anorthite this stress could explain the activation of a mohr coulomb failure with a friction coefficient of 0 2 which is reasonable at the depth of 10 km dislocation creep is activated during a rapid change in the prescribed velocity whereas diffusion creep dominates if the velocity is held constant highlighting the importance of grain size sensitivity for lower crustal rheology

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

stress balance above subduction application to the andes the frequent changes in tectonic and morphological evolutions of active plate margins suggest that similar processes drive apparently antagonistic tectonics we address the problem by a thin viscous sheet approach where the governing stresses are restricted to interplate traction buoyancy and viscous stresses various cases are explored from compression to extension leading to dismiss corner flow driven models and to favor interplate traction driven models from the topography of the andes we determine the magnitude and profiles of the current stresses beneath the andes at various locations the total transmitted force is about 9 1012 n m 1 shear stresses at the boundaries of the deforming lithosphere range between 25 and 110 mpa and almost balance the buoyancy stresses due to crustal thickening the remaining difference being the viscous stresses the deviatoric stress is less than 10 mpa and the effective viscosity of the andean lithosphere is as low as 3 1021 pa s maximum compression is located in the eastern side of the andes matching the geological observations but we find that the current stress regime is insufficient to explain the present day elevation this suggests that interplate traction was slightly stronger in the past when the neogene plate convergence was faster and this decrease in interplate traction implies that high and steep parts of the andes cannot be sustained anymore the shape of the altiplano can be explained by a weak lateral variation of the viscosity which allows the high plateau to be constructed in the characteristic time span of andean building

acritarchs from the duolbagaisa formation cambrian series 2 miaolingian on the digermulen peninsula finnmark arctic norway towards a high resolution cambrian chronostratigraphy

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

biostratigraphy taxonomic diversity and patterns of morphological evolution of ordovician acritarchs organic walled microphytoplankton from the northern gondwana margin in relation to palaeoclimatic and palaeogeographic changes abstract acritarchs the fossilizable resting cysts of phytoplanktonic algal protists are the dominant component of marine organic walled microfossils in the palaeozoic the majority of acritarchs show strong similarities with dinoflagellate cysts in morphological and biogeochemical features as well as distributional patterns in the sediments the production of these organic walled microfossils and their distribution and survivorship in the sediments were controlled by differences in ecological tolerances and life cycle autecology of the planktonic parent organisms calculation of evolutionary rates and development of a detailed diversity curve at specific level form the basis for discussing the influence of global palaeoenvironmental perturbations on the evolution of organic walled microphytoplankton in northern gondwana during latest cambrian through ordovician times the potential of acritarchs for biostratigraphic correlation at the regional scale northern gondwana domain is much improved by our detailed revision of distributional patterns of 245 acritarch taxa the most important cambro ordovician acritarch bio events are short periods of diversification which also correspond to introduction of morphological innovations observed in latest cambrian and earliest tremadoc late tremadoc early arenig basal llanvirn and latest ashgill and an important extinction phase in the early caradoc overall acritarch diversity increased from the basal ordovician up to the middle llanvirn then declined in the early and middle caradoc during ashgill times the assemblages are poorly diversified at the generic level as a result of a combined effect of sea level drawdown and onset of glacial conditions but no major extinction event is observed in connection with the end ordovician biotic crisis the peak in acritarch diversity during middle ordovician times appears to be correlated to maximum spread of palaeogeographical assembly acritarch dynamics appear largely uncorrelated to second order sea level oscillations the primary abiotic controls on acritarch evolution were palaeogeographical and the associated palaeoceanographic changes especially during middle ordovician and the end ordovician palaeoclimatic shift the acritarch fossil record provides important information on the evolution of oceanic primary producers however the relationships between acritarch diversity oceanic productivity and evolution of invertebrate animals are proving much more complex than previously thought in particular the hypothesis of a causal relationship between changes in acritarch diversity and metazoan evolution in the palaeozoic is not supported by our data

marine biodiversification in response to evolving phytoplankton stoichiometry abstract diversification of the marine biosphere is intimately linked to the evolution of the biogeochemical cycles of carbon nutrients and primary productivity a meta analysis of the ratio of carbon to phosphorus buried in sedimentary rocks during the past 3 billion years indicates that both food quantity and critically food quality increased through time as a result of the evolving stoichiometry nutrient content of eukaryotic phytoplankton evolving food quantity and quality was primarily a function of broad tectonic cycles that influenced not just carbon burial but also nutrient availability and primary productivity increasing nutrient availability during the middle to late proterozoic culminated in the production of food phytoplankton biomass and fresh dead organic matter with c p redfield ratios sufficient to finally promote geologically rapid biodiversification during the proterozoic phanerozoic transition this resulted in further massive nutrient sequestration into biomass that triggered positive feedback via nutrient recycling bioturbation mesozooplankton grazing on phytoplankton productivity increasing rates and depths of bioturbation through the phanerozoic suggest that nutrient recycling continued to increase increasing bioturbation and nutrient cycling appear to have been necessary to sustain the primary productivity and energetics biomass metabolic rates and physical activity such as predation of the marine biosphere because of the geologically slow input of macronutrients like phosphorus from land and the continued sequestration of nutrients into marine and terrestrial biomass

first deep seismic reflection images of the eastern alps reveal giant crustal wedges and transcrustal ramps 1 the alps are considered as a classical example for an orogen created by continental plate collision in this study we present new images obtained from deep seismic reflection profiling in the eastern alps between munich and venice which give rise to examine and revise existing concepts the seismic sections exhibit a prominent bi verging reflection pattern at crustal scale a major ramp like structure outcropping at the inn valley fault can be traced southward over 80 km into the mountain root where relics of the subducted penninic ocean are expected new models of the evolution of the eastern alps show an upper lower crustal decoupling along transcrustal thrust faults with opposite thrust directions of both the european and the adriatic african continents

subduction initiation and continental crust recycling the roles of rheology and eclogitization abstract a mechanical model with viscous ductile and plastic brittle rheologies is used to investigate the effect of eclogitization on the dynamics of convergence density increases by 300 600 km m3 during eclogitization of crustal rocks and continental lower crust and oceanic crust reach higher density than mantle we explore cases of intracontinental deformation subduction and continental collision we consider a wide range of parameters for friction activation energy and initial thermal state and cases with or without eclogitization the style of deformation appears to be primarily controlled by the presence or absence of weak zones simulations are run with a constant convergence velocity 1 5 or 4 cm year and the evolution of the compressive force through time is thus a critical test of the model viability for intracontinental deformation when the brittle crust is decoupled from the mantle the mantle deforms by symmetrical bending or subduction and a variable amount of lowermost crust is entrained with the sinking mantle in these cases the compressive force remains in the 1012 3 1013 n m acceptable range oceanic subduction only occurs if a low friction shear zone is specified in the brittle realm during the transition from oceanic subduction to collision the oceanic crust in this shear zone remains trapped between colliding crusts eclogitization has no influence on the initial mode of deformation thus it is probably not an important factor in the initiation of subduction in all simulations the influence of eclogitization increases progressively with time in proportion of the amount of lower continental crust and oceanic crust entrained into the mantle the structural evolution of the orogen model depends on eclogitization if initial decoupling occurs at mid crustal level the simulations thus indicate that eclogitization promotes convergence and also in some cases enables the recycling of the whole continental lower crust into the mantle we also suggest that eclogitization could regulate the long term convergence rate in orogens

phytoplankton acritarch community changes during the permian triassic transition in south china abstract the permian triassic boundary ptb at 252 ma coincided with the largest mass extinction of the phanerozoic previous research on diversity and abundance changes during this event has focused mainly on the terrestrial vertebrate and marine invertebrate records with little attention to date given to the phytoplankton that form the base of the marine trophic web although the fossil record of permian triassic phytoplankton is relatively poor owing to preservational factors sufficient material is now available to evaluate secular changes in acritarch communities through the mass extinction interval in this contribution we evaluate diversity and abundance changes among 8 genera and 25 species of acritarchs including large spherical small spherical long spined and short spined forms ranging from the upper permian clarkina yini to the lower triassic isarcicella isarcica zones in eight sections representing different sedimentary facies of the south china craton acritarchs declined sharply from the latest permian c meishanensis zone to the earliest triassic i staeschei zone with extinctions and abundance changes concentrated at two horizons the first in the latest permian c meishanensis zone bed 25 at meishan d and the second in the earliest triassic i staeschei zone bed 28 at meishan d mirroring the pattern of mortality among marine invertebrates differences in the nature and intensity of these two extinction episodes the second having a relatively larger effect on acritarch community composition suggest that these events may have resulted from different types of environmental perturbations

timing of metamorphism in the tauern window eastern alps rb sr ages and fabric formation the peripheral schieferhulle of the tauern window of the eastern alps represents post hercynian penninic cover sequences and preserves a record of metamorphism in the alpine orogeny without the inherited remnants of hercynian events that are retained in basement rocks the temperature time deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whose p t evolution and structural setting are already well understood the eclogite zone of the central tauern formed from protoliths with penninic cover affinities and suffered early alpine eclogite facies metamorphism before tectonic interposition between basement and cover it then shared a common metamorphic history with these units experiencing blueschist facies and subsequent greenschist facies conditions in the alpine orogeny the greenschist facies phase associated with penetrative deformation in the cover and the influx of aqueous fluids reset sr isotopes in metasediments throughout the eclogite zone and cover schists recording deformation and peak metamorphism at 28 30 ma the peripheral schieferhulle of the south east tauern window yields rb sr white mica ages which can be tied to the structural evolution of the metamorphic pile early prograde fabrics pre date 31 ma and were reworked by the formation of the large north east vergent sonnblick fold structure at 28 ma peak metamorphism post dated this deformation but by contrast to the equivalent levels in the central tauern peak metamorphic conditions did not lead to widespread homogenization of the sr isotopes localized deformation continued into the cooling path until at least 23 ma partially or wholly resetting sr white mica ages in some samples these isotopic ages may be integrated with structural data in regional tectonic models and may constrain changes in the style of crustal deformation and plate interaction however such interpretations must accommodate the demonstrable variation in thermal histories over small distances

modeling the behavior of the continental mantle lithosphere during plate convergence in studying orogenic processes the mechanism of viscous rayleigh taylor type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere using numerical models we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence the numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere asthenosphere system our results suggest that there are a number of deformational modes of the model mantle lithosphere 1 a dripping or rayleigh taylor type instability 2 an asymmetric underthrusting or subduction 3 symmetric ablative plate consumption 4 slab breakoff the failure and detachment of the strong lithosphere and 5 mixed modes with combinations of these processes the development of the modes is controlled by the rate of convergence associated with the background tectonic regime the density field and the rheology of the mantle lithosphere it is important to determine whether these modes occur in the earth beneath collisional orogens

thermomechanical evolution of a thinned continental lithosphere under compression implications for the pyrenees in order to demonstrate the interactions between plate boundary forces and preexisting zones of weakness and how those control the localization of continental deformation inside the lithosphere the thermomechanical evolution of an initially thinned and thermally perturbed continental lithosphere in compression was analyzed within the framework of the pyrenees while a concensus exists as to the shallow structures the extent and geometry of deformation at depth have been the subject of great controversy the thermomechanical implications of recent geological hypotheses on the initial state prior to the compression recently derived from balanced cross section reconstructions are here quantified and compared to the recent ecors deep seismic reflection profile across the pyrenees a thermomechanical model based on finite elastoplastic viscoplastic deformations has therefore been formulated using phenomenological constitutive equations derived from experimental studies the thermally coupled problem is solved by an appropriate vertical two dimensional plane strain finite element approximation in those models most of the shortening is accommodated inside a local zone of intense plastic shearing that creates a sharp wedge shaped root progressively sinking deep into the mantle the main thickening takes place inside the lower crust where it can reach a factor 2 this indentation and crustal imbrication process is controlled by a buckling instability occuring inside the uppermost part of the mantle the evolution of the topography can be related to these phenomena lateral conductive heat transfer overtakes the advective transfer in the vicinity of the inflow of colder crustal material inside a warmer mantle the deformation pattern can be explained by a three layer mechanical lithosphere with a soft ductile internal layer of potential decoupling between two more competent layers characterized by a significant elastic stored energy the overall response of this composite plate is not much influenced by smaller scale variations of crustal rheology but is very much dependent on the contrast in rates of viscous dissipation between the lower crust and the upper mantle and on the possibility of singling out a layer of significant elastic stored energy by the balance between rate of loading and rate of dissipation inside the uppermost mantle predicted deformation pattern is found to agree with the seismic reflection and gravity data even though extrapolation to other rates of shortening must be verified by appropriate numerical modelling these physical mechanisms may be regarded as a paradigm for other crustal imbrication phenomena found in the himalayas tibet or the alps