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

the kinematics of movements along the insubric line and the emplacement of the ivrea zone schmid sm zingg a and handy m 1987 the kinematics of movements along the insubric line and the emplacement of the ivrea zone in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tecta a hys 135 47 66 the insubric line west of locamo is characterized by a 1 km thick greenschist facies mylonite belt east of locamo these mylonites are overprinted by a discrete brittle fault the mylonites are derived from basement units of the central alps sesia zone and the southern alps ivrea zone as well as from the permo mesozoic cover of the southern alps canavese sense of shear criteria indicate that the mylonites accomm ated backt s g followed by dextral strike slip motion mylonitization during backtbrusting was synchronous with backfolding of the central alpine nappes under higher metamorphic conditions a horizontal temperature gradient resulted from the rapid juxtaposition of the warm central alpine block against the cold southern alpine block mylonites formed during the later dextral strike slip event are related to large transcurrent displacements in the central alps deduced from regional kinematic considerations thus both mylo t ation events are contempor eous with deformation to the north and south of the insubric line insubric phase extensively modifying the pre insubric crustal configuration of the alps the insubric phase post dates the bergell intrusion 30 m y the emplacement of the geophysical ivrea body is a combined effect of vertical uplift due to e w directed crustal thinning during the early jurassic and underplating by continental crust associated with late cretaceous compression a deep crustal normal fault pogallo line subsequently rotated during tertiary alpine orogenesis separates deeper parts of the southern alpine crust ivrea zone from intermediate crustal levels strona ceneri zone the rigid ivrea body localir ed large strains within the insubric mylonite belt and is responsible for the present curvature of the insubric line in uetion

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

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

geodynamic models of crustal scale episodic tectonic accretion and underplating in subduction zones we investigate the dynamics of crustal scale episodic accretion at convergent margins using a finite element model of subduction the numerical model incorporates effects from flexure subduction loading and large scale variations in material properties the model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction the primary controls investigated are spatial variations in density material strength and thickness of material layers these controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere giving rise to areas of thermally activated ductile creep attempted subduction of small crustal blocks density frictional and or ductile strength variations interaction between well attached basement blocks and overlying detachable sediments and spatial changes in thickness of sediment and crust in the experiments accretion of small continental terranes within a model subduction zone can cause crustal scale fold nappes and shear zones to develop with accompanying tectonic underplating and or frontal accretion the model results may provide insight into real cases of episodic accretion for example the emplacement of the upper penninic nappe stack of the central alps

suspended sediment transport across a tidal flat abstract many investigations have shown that most of the suspended sediment supplied to the wadden sea originates from the north sea based on measurements of current velocity depths and suspended sediment concentrations in the danish part of the wadden sea computations indicate however that suspended sediment is in fact exported to the north sea the reason for this discrepancy is twofold firstly measurements have been carried out mainly in tidal channels where the largest amounts of water and suspended sediment are in the main transported during ebb periods secondly computation of net transport in an estuarine environment such as the wadden sea always suffers from the difficulty that net transport within each single tidal period is of the same order as the uncertainty of the transport rates during ebb and flood periods respectively the first problem can be dealt with if measurements are carried out in both tidal channels and over the tidal flats the second problem can be solved by collecting time series instead of discrete measurements because accumulation of the transport rates found for successive tidal periods will neutralise the relatively large random error in this paper time series of suspended sediment flux across a tidal flat covering a full spring neap tidal cycle are presented they showed net transport of suspended sediment in a landward direction which is mainly caused by flocculation of fine grained sediment during periods of high concentration level during such periods the field settling velocities of suspended sediment are so large that the settling and scour lag processes can be effective

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

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

mechanical model for subduction collision tectonics of alpine type compressional orogens alpine type orogens are characterized by three distinct convergent tectonic phases subduction with deformation that has primarily single vergence a transition from subduction to collision and continental collision with double vergence although the cenozoic history of the european alps has additional complexities a mechanical explanation for these three phases would provide the necessary crustal scale framework in which to develop an understanding of the smaller scale processes we present results from a simple numerical model which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen the development and exhumation of a subduction complex suture zone and basement nappe stack piemont suture penninic nappes the presence of a crustal scale back fold and thrust insubric line and uplift of basement on the pro european side of the orogen external basement massifs may be explained as a simple consequence of changing dynamics during the transition from subduction to collision

frontal accretion an internal clock for bivergent wedge deformation and surface uplift 1 we propose a conceptual model of frontal accretion within bivergent wedges which is based on two dimensional sandbox simulations and the analysis of particle displacement fields each frontal accretion cycle consists of a thrust initiation an underthrusting phase and a reactivation phase the location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable therefore the frontal accretion cycle can be considered as an internal clock for wedge scaled deformation and surface uplift we further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle we estimate cycle duration to range between 104 and 105 years which is thus similar to the time frame of climatic cycles the proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic paleoseismologic and geologic estimates of fault slip

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

eoalpine and mesoalpine tectonics in the southern alps according to structural and stratigraphic data eoalpine and mesoalpine tectonics can be clearly recognized in the southern alps and distinguished from the better known neoalpine deformation

problems in the extrapolation of laboratory rheological data abstract the many types of variables and deformation regimes that need to be taken into account in extrapolating rheological behaviour from the laboratory to the earth are reviewed the problems of extrapolation are then illustrated with two particular cases in the case of divine rich rocks recent experimental work indicates that within present uncertainties of extrapolation the flow in the upper mantle could be either grain size dependent and near newtonian or grain size independent and distinctly non newtonian both types of behaviour would be influenced by the present of trace amounts of water in the case of quartz rich rocks the uncertainties are even greater and it is still premature to attempt any extrapolation to geological conditions except as an upper bound the fugacity and the scale of dispersion of the water are probably two important variables but the quantitative laws governing their influence are not yet clear

dynamics of sediment subduction accretion at convergent margins short term modes long term deformation and tectonic implications a conceptual model is proposed for the short term operation of subduction accretion systems at convergent margins the operation is described in terms of the mass fluxes among the four system components namely the accretionary or prowedge wedge p an uplifted plug u a retrowedge r and the subduction zone the latter consists of a conduit c of slowly moving and deforming material and an underlying subduction channel where material moves and deforms more rapidly a broad range of convergent margin types can be accommodated within the conceptual model framework with differing short term modes that depend on which of the p u r c components that are active finite element geodynamical model experiments are used to investigate whether these modes are dynamically feasible these experiments determine excitation of modes under controls that include model entry and exit mass fluxes flexural loading by accreted material and slab pull forces boundary velocities corresponding to subduction and subduction zone advance retreat distribution thickness and density of accreted sediment and internal frictional properties of the coulomb sediments that all modes with the exception of p in which only a prowedge is created are seen in the models provides support for the concepts and allows conditions that favor particular modes to be recognized in addition the finite deformation predicted by the models is used to infer the long term tectonic styles associated with both single mode evolutions and those that exhibit mode switching a steady state mass balance can occur for modes 0 pure subduction p c i e where components p and c are active or other modes when the surface mass denudation plus the subduction flux balances accretion plus tectonic underplating the p c mode is favored by subduction zone retreat and or an increasing subduction load and creates an apparent landward dipping backstop p u c is predicted to be the most common mode and it creates an apparent seaward dipping backstop p u r c is an inefficient mode but is favored by easily detached retrocrust or sediment and or a decreasing subduction load

tidal circulation pattern on a tidal flat minas basin canada abstract although tidal flats have been studied extensively from the geomorphologic sedimentologic and stratigraphic standpoints there is little information related to the actual water circulation over them and its consequences on sediment transport and the corresponding changes in the geomorphology and sediment distribution the circulation induced by tidal forcing over a macrotidal flat in the minas basin bay of fundy canada is addressed while the geomorphologic and wind influences on the currents are also assessed tidal currents are reversible although the main axis orientation depends on the local morphology since a linear ridge runs along the main axis of the flat differences among sites are evidently associated with their position in relation to the ridge and also to the length of the inundation time forcing for the observed minor rotation to the right may originate in the surface water slope developed by the differential water filling of the area since wind was mostly calm and rarely exceeded 10 m s the observed circulation was the result of the interaction of the tidal wave and the general morphology of the intertidal flat

brittle precursors of plastic deformation in a granite an example from the mont blanc massif helvetic western alps abstract in the mont blanc helvetic massif granites record mesoscale alpine structures which include joints veins cataclastic to mylonitic shear zones and foliated granites a detailed structural analysis indicates that brittle deformation predates plastic strain joints never pass through and veins are offset by cataclastic shear zones and mylonites the mylonites progressively develop by plastic reactivation of cataclastic shear zones during greenschist facies metamorphic conditions plastic deformation is first localized in the brittle discontinuities and the fine grained matrix of cataclasites then it involves the granite within brittle shear zones and this is initially accomplished mainly by flow of reaction softened aggregates of sericite widely replacing the strain supporting magmatic plagioclase the brittle to plastic evolution has resulted in highly localized discontinuous plastic shear zones with high lateral continuity and these characteristics are derived from reactivation of and focusing along pre existing brittle discontinuities in addition mylonites may inherit high angles of intersection and may contain granite porphyroclasts these features may allow the inference of a precursor brittle deformation where the plastic overprint has completely erased the initial brittle fabrics

mechanical modeling of compressional basins origin and interaction of faults erosion and subsidence in the ventura basin california using geophysical observations and some basic geological ideas on the present structure of the ventura basin we model its possible mechanical evolution during the last 4 million years we use a two dimensional finite element model with frictional and elastoviscoplastic rheologies that allow strain localization this allows us to study the mechanical origin of the san cayetano thrust fault scf and its interaction with the oak ridge fault orf the results of our modeling indicate that the geometry and activity of a shear zone which mimics the san cayetano fault are conditioned both by the preexistence of the oak ridge fault and by the shape of the brittle ductile transition furthermore the presence of a decollement level is shown to favor the development of the san cayetano fault shear zone as a low angle structure the value of the effective friction on the oak ridge fault is evaluated from this model possible values of the viscosity of the sediments are also estimated it is shown that low sediment viscosity may completely hinder the activity of such a fault at the surface while slip at depth continues the stratigraphic profile of the basin is also shown to depend on the relative strengths of the orf and scf which also provide an explanation for the shortening and rapid subsidence of the basin

rheological dependence of extension in wedge models of convergent orogens abstract although several mechanical models of extension in contractional orogens have been proposed in many cases these models have been based on viscous constitutive laws lack of more complicated rheological models has primarily been due to the technical difficulty of solving for deformation with complex rheological behavior and boundary conditions a finite element model is presented that allows for solution of the wedge mechanical problem with viscous non linear viscous and coulomb plastic rheologies this model is used to investigate extension in a doubly vergent wedge model in which deformation is primarily contractional driven by a reversal of basal traction proposed models showing extension in the crust at shallow crustal levels in spite of overall contraction and contraction at depth are confirmed but only for high argand number low crustal viscosity at low argand number little or no extension is observed non linear stress dependence power law flow also suppresses extension but not entirely some extension is observed at high effective argand number however with a coulomb plastic rheology no extension is observed for a wide range of parameter values no extension is observed for models with viscous deformation at depth and plastic deformation near the surface this suggests that the extension is a function of the rheologic model not the effective strength of the crust coeval extension and contraction in convergent orogens with steady external forcing therefore requires viscous deformation to very shallow depths

subduction of continental margins and the uplift of high pressure metamorphic rocks abstract the mechanism by which high pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis we suggest that the emplacement results from partial subduction of the continental margin which because of its high flexural rigidity produces a rapid change in the trajectory of the descending slab we assume a two fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity we assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate the wedges overlying the subduction zones and the subducting slab itself experience substantial extra compressional stress at depths of around 100 km and extensional stress at shallower depths as the continental margin passes through the zone of maximum curvature the magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and or the descending plate and are experienced in a time interval of less than 5 m y for typical subduction rates the spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab which is the weakest region in the deeper parts of the subduction zone this may result in rapid upward migration of the crust of the partially subducted continental margin against the flow of subduction high pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above movement on the faults would have been coeval and would have resulted in rapid unroofing of the high pressure terranes synchronous with arrival of the continental margin at the subduction zone and therefore relatively early in the history of a collisional orogen

three dimensional interface modelling with two dimensional seismic data the alpine crust mantle boundary summary we present a new approach to determine the 3 d topography and lateral continuity of seismic interfaces using 2 d derived controlled source seismic reflector data the aim of the approach is to give the simplest possible structure consistent with all reflector data and error estimates we define simplicity of seismic interfaces by the degree of interface continuity i e shortest length of ovsets and by the degree of interface roughness least surface roughness the method is applied to structural information of the crust mantle boundary moho obtained from over 250 controlled source seismic reflection and refraction profiles in the greater alpine region the reflected and refracted phases from the moho interface and their interpretation regarding crustal thickness are reviewed and their reliability weighted weights assigned to each reflector element are transformed to depth errors considering fresnel volumes the 2 d derived reflector elements are relocated in space 3 d migration and interpolation is performed between the observed reflector elements to obtain continuity of model parameters interface ovsets are introduced only where required according to the principle of simplicity the resulting 3 d model of the alpine crust mantle boundary shows two ovsets that divide the interface into a european an adriatic and a ligurian moho with the european moho subducting below the adriatic moho and with the adriatic moho underthrusting the ligurian moho each sub interface depicts the smoothest possible i e simplest surface fitting the reflector data within their assigned errors the results are consistent with previous studies for those regions with dense and reliable controlledsource seismic data the newly derived alpine moho interface however surpasses earlier studies by its lateral extent over an area of about 600 km by 600 km by quantifying reliability estimates along the interface and by obeying the principle of being consistently as simple as possible

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

basement cover relationships in the tambo nappe central alps switzerland geometry structure and kinematics in the pennine zone of the central alps the tambo nappe forms a thin crystalline sliver embedded in the mesozoic cover four tertiary alpine deformations are observed the d1 ductile deformation is linked to the progressive eocene stacking of the nappes towards the northwest during d1 basement deformation is governed by imbricate tectonics whereas cover is thin skinned and intensively folded these different structures reflect the original strong rheological contrast between basement and cover during the heterogeneous and ductile d2 deformation the behaviour of the basement and cover became similar the strong vertical d2 shortening associated with a top to the east shear led to the folding of the prior se dipping structures developing se vergent folds with axes close to the e w l2 stretching lineation d2 corresponds to post collisional crustal thinning following d2 crustal thickening the d3 and d4 deformations occurred under retrograde conditions and can be correlated with uplift and late dextral movement on the insubric line respectively

physical characteristics of the bahia blanca estuary argentina abstract based on temperature salinity and current velocity and direction data the physical characteristics of the bahia blanca estuary are described data were gathered in vertical profiles made in longitudinal as well as in hourly surveys freshwater runoff averages 2 m3 s 1 however peak floods may reach 10 50 m3 s 1 the temperature distribution is quite homogeneous in the estuary based on the salinity distribution the estuary can be divided into two sectors an inner one showing partially mixed characteristics with a strong tendency to become sectionally homogeneous during runoff conditions similar to the historical averages and an outer sector which is sectionally homogeneous salinity values in the inner sector may be larger than those observed in the inner continental shelf this is initiated by the restricted circulation in the inner estuary and added to by the tidal washing of back estuary salt flats and by evaporation processes analysis of the residual circulation shows a marked difference in the direction of mass transport in the deeper regions of the sections northern flank the flow reverses with depth being headward near the bottom however net transport is landward in the shallower parts

crustal structure beneath the eastern swiss alps derived from seismic refraction data abstract the eastern part of the swiss central alps is densely covered by a network of seismic refraction and wide angle reflection profiles recorded mostly parallel to the tectonic strike of the alps and along the newly acquired n s oriented european geotraverse egt to obtain a well constrained crustal transect along the egt an initial model was constructed from information based mainly on the coincident reflection seismic profile of the swiss national research program nfp20 and short range observations of the egt data for the shallow structure and on the along strike wide angle profiles for the deep structure the refinement of the initial model by 2 d raytracing during the subsequent interpretation of the egt data leads to a detailed p wave velocity distribution of the crustal cross section beneath the central swiss alps and its adjacent areas in general the distinctly layered crustal structure below the alpine foreland thickens considerably as the alps are approached reaching a maximum thickness of nearly 60 km below the insubric line the upper and middle crust has velocities between 6 0 and 6 2 km s except for the area below the southern part of the molasse basin and the helvetic nappes a distinct lower crust with a relatively low velocity of 6 5 6 6 km s is found below the penninic nappes the lower crust thickens remarkably merging probably with the high velocity zone of 6 6 km s at a depth of about 21 km which has been interpreted as the top of the indenting lower crust of the adriatic promontory of the african plate a clear vertical offset between the smoothly south dipping european and the more rapidly rising adriatic crust mantle boundaries is found the complex structures of the upper crust beneath the alps caused by the nappe tectonics can only be partly resolved by the refraction seismic data a south dipping high velocity zone within the penninic nappe pile and a reflector beneath the northern front of the aar massif can possibly be interpreted as incomplete images of the shallow heterogeneous 3 d structure

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

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

styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere abstract crustal scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere the plane strain finite element model results provide indications of the styles of deformation to be expected in small compressional orogens in particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other the crust is modelled using coulomb plastic frictional and thermally activated power law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary surface denudation partial and total subduction of the crust and compositional layering are examined the results show the development of inclined step up shear zones which are a consequence of conjugate thrusting in regions of coulomb controlled rheology these zones link to sub horizontal shear zones which occur where composition and temperature render the crust viscously weak the model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation finally we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

physical modelling of lithosphere subduction in collision zones abstract physical mechanical modeling of continental collision is performed with a 4 layered mantle model compositionally distinct layers of hydrocarbons are used to model a lithosphere which consists of two crustal layers the upper one quasi brittle the lower one plastic and a plastic mantle layer the lower crustal layer the crustal asthenosphere has a low yield limit a low viscosity liquid water represents the asthenosphere under the lithosphere lateral compression of the model plate by a piston is the driving mechanism of collision similarity criteria are satisfied in the modeling the collisional process was investigated with various boundary conditions and values of the model parameters the yield limit and the thickness of the crustal asthenosphere were varied as well as the action of the piston on the plate the force was applied either throughout the plate section or on its mantle layer only in the latter case this layer slipped relative to the upper crust hence different results were obtained a more realistic pattern seems to occur when the yield limit of the crustal asthenosphere is not too small and free sliding of the crustal layers over the mantle lithosphere is not possible the upper crustal layer then experiences failure in the collision zone and thrusting develops there intensive thickening of the lower crust was observed the mantle layer being subducted like oceanic lithosphere in a subduction zone

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

observations from the floor of a granitoid pluton inferences on the driving force of final emplacement an east west profile across the tilted bergell pluton exposes a 10 km thick interval in terms of crustal depth consequently the floor as well as the root and side of the main intrusive body of the pluton crop out at the surface and a tentative three dimensional geometry is constructed at the highest crustal level the geometry and deformation features at the margin of the pluton indicate ballooning whereas the folded floor of the main intrusive body indicates synmagmatic shortening related to regional deformation these contrasting features are best explained by shortening of the base of the pluton which caused an expansion at a higher crustal level final emplacement of the pluton into higher crustal levels was therefore not driven primarily by buoyancy but rather by regional deformation within deeper levels of the crust