characteristic reflection patterns in the southeast canadian cordillera northern appalachians and swiss alps abstract there are some striking similarities and some differences between the seismic reflection sections recorded across the fold and thrust belts of the southeast canadian cordillera quebec maine appalachians and swiss alps in the fold and thrust belts of all three mountain ranges seismic reflection surveys have yielded high quality images of 1 nappes thin thrust sheets stacked on top of ancient continental margins 2 ramp anticlines in the hanging walls of faults that have ramp flat or listric geometries 3 back thrusts and back folds that developed during the terminal phases of orogeny and 4 tectonic wedges and regional decollements a principal result of the cordilleran and appalachian deep crustal studies has been the recognition of master decollements along which continental margin strata have been transported long distances whereas a principal result of the swiss alpine deep crustal program has been the identification of the adriatic indenter a crustal scale wedge that caused delamination of the european lithosphere significant crustal roots are observed beneath the fold and thrust belts of the alps southeast canadian cordillera and parts of the southern appalachians but such structures beneath the northern appalachians have probably been removed by post orogenic collapse and or crustal attenuation associated with the mesozoic opening of the atlantic ocean

gravity anomalies and dynamics of the swiss alps an interpretation of the gravity observations in switzerland is presented the gravity anomalies are mainly caused by crustal effects taking 0 5 g cm3 as the average density contrast between crust and upper mantle the remaining positive residual field of 50 mgal can be explained as the effect of the so called lithospheric root underneath the swiss alps this root must be considered as a relatively cold body with increased density in order to calculate the thermally induced density distribution in the upper mantle a kinematic and geothermal model has been constructed simulating the alpine history for a time span covering the last 40 m y at the same time the moving lithospheric body crustal uplift and subsidence of the lower lithosphere is analysed from a dynamical point of view in order to test the kinematic conditions the calculation leads to a mantle subsidence rate of 1 mm y for the present period

geotraverses across the swiss alps the alpine chain marks the collision zone between the european and african lithospheric plates the earth s crust below the alps has been thickened by tectonic activity to form a root of crustal material which extends to a depth of approximately 60 km here we present the initial results of three seismic reflection traverses across the alps which show large scale crustal indentations resulting from collision between continental plates as well as the nappeforming style of deformation produced by compression of attenuated ocean crust the data suggest that the gross structural deformation of the collision zone results from large scale crustal indentations the descending european crust is delaminated and deformed by wedges of the african crust protruding northwards below the alps the nappe forming deformation seems to be restricted to the upper 10 20 km of the penninic domain in the core of the alps

recent crustal movements in switzerland and their geophysical interpretation abstract in this paper a review is presented on the most recent results of geodetically determined recent crustal movements in switzerland these data are compared with the new bouguer gravity map of switzerland and an attempt is made to discuss the geodetic and gravity results in connection with the crustal structure of the swiss alps in terms of seismic compressional wave velocities and depths to the crust mantle boundary

fold axis parallel extension in an arcuate fold and thrust belt the case of the helvetic nappes abstract the helvetic nappes of western switzerland are discussed as an example of an arcuate foreland fold and thrust belt in which active fold axis parallel stretching occurred fold axis parallel extension is recorded by 1 1 incremental strain data from pressure shadow fibres the significance of pressure shadow fibres for the determination of the deformation history of a region is discussed pressure shadows are used to quantify the amount of and to describe the distribution of fold axis parallel extension occurring in the helvetic nappes 2 2 the extension directions of conjugate systems of en echelon veins it is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern the relative age of the conjugate en echelon vein systems in the helvetic deformation history can be assessed the geometry of the conjugate sets relative to the local anisotropy plane is described and the significance of the preferred orientation of their extension directions is discussed 3 3 fold axis parallel sections a comparison of the regional distribution of the fold axis parallel strain with the shape of the helvetic nappes in fold axis parallel sections shows that the fold axis parallel strain cannot be related to the footwall topography of the nappes it is concluded that the fold axis parallel extension in the helvetic nappes was induced by a change of direction of overthrust shear this change occurred late in the deformation history and was superposed on the already formed nappes the changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting alpine nappe pile relative to the european plate a rotation which lead to the arcuate shape of the western alps

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

delineation of the north central italian upper mantle anomaly the italian peninsula is the focus of intense deformational tectonic activity it is underlain by mantle material whose inferred relatively anomalous properties1 2 are not inconsistent with the seismic volcanic and thermal activity that is manifested in this region the most effective geophysical tool for mapping the structure of the uppermost 200 300 km of the earth is the observation and analysis of seismic surface wave dispersion on a regional scale here we synthesize the interpretations of rayleigh wave dispersion measurements made by several authors each for different parts of north central italy3 8 to delineate the lateral extent of the upper mantle anomaly in this region

long lasting tectonic activities of the lepontine dome new evidence from low temperature thermochronology abstract to investigate the neogene exhumation history of the central european alps we apply low temperature thermochronology in combination with thermal history modelling fission track and u th sm he ages on apatites from the central lepontine dome ticino switzerland indicate higher exhumation rates in the centre of the dome and rather moderate exhumation at the northern and southern boundaries since neogene times we present a model for explaining the latest stage exhumation of the central lepontine dome and show that i both episodic and continuous exhumations are found on small scale throughout the neogene ii compressional tectonics control the exhumation until the late neogene iii the exhumation regime changes between 6 and 4 ma and iv increasing hinterland exhumation rates at the mio pliocene boundary cannot be related to tectonic structures of the dome and they are thus explained by climatic changes

results of nfp 20 seismic reflection profiling along the alpine section of the european geotraverse egt summary recent deep crustal seismic reflection profiling across the central alps of eastern and southern switzerland has provided a detailed image of this continental collision zone as part of the swiss national science project nfp 20 a series of reflection profiles was recorded during two phases along n s transects crossing the alpine edifice the eastern and southern profiles coincide with the alpine segment of the european geotraverse egt and in part with the earlier swiss geotraverse these data together with other geophysical and extensive geologic information form a unique and comprehensive volume of crustal information for this region of the alps on a larger scale the new reflection data also add to a growing set of profiles crossing the alpine fold belt which includes an additional traverse carried out by nfp 20 across the western swiss alps and the ecors crop profile across the western alps the initial results of the reflection data across the central alps outline a crustal framework involving northward indentation of the adriatic hinterland into the subducting european foreland this occurs beneath the collapsed oceanic basins of the penninic allochthon which is defined by its highly reflective crystalline nappes the present crustal thickness within the alpine collision zone has doubled to about 60 km by both vertical and horizontal displacements along an inferred complex detachment system controlled by the adriatic wedge the south plunging european foreland implies that portions of the thickened crust have been subducted into the upper mantle this general framework of flake tectonics is laterally consistent with the ecors crop results from the western alps however the western alps show a higher degree of crustal imbrication which may be related to the presence of the rigid ivrea body and an overall greater amount of crustal shortening compared to the central alps

tectonic trigger for the formation of late miocene cu rich breccia pipes in the andes of central chile during the late miocene the decreasing angle of subduction below the andes of central chile caused a reduction in the influx of new magma and heat into the base of magmatic systems that had been active throughout the miocene this led to their cooling and solidification triggering the release of large volumes of metal rich magmatic fluids from middle and upper crustal magma chambers and generating a group of economically significant cu rich breccia pipes the sr and nd isotopic compositions of the magmatic fluids that formed these brecccas determined from breccia matrix minerals were variable implying that these fluids were not exsolved from a specific cu porphyry magma as is often invoked to explain andean cu deposits instead the formation of the late miocene deposits in central chile was tectonically triggered by cooling of a variety of magma types during the last stages of existence of long lived andean magmatic systems as both subduction angle and as a result subarc magma supply decreased

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

time dependent crustal temperature modeling central alps abstract the conventional modeling of crustal temperatures is based on the steady state approach which is reliable in tectonically stable regions only the present work focuses on the assessment of the influence of the transient effects on the thermal field at regions of ongoing continental collision in the first part of this study the thermal effects of most evident processes operating in collision zones thickening and flaking of the crust frictional heating on the surfaces of flakes bulk replacement of mantle lithosphere by hotter rocks and also uplift and erosion are estimated on the basis of 1 d time dependent synthetic models the results show that surface heat flow caused by the most of investigated transient processes may differ from the steady state one by quantities of the order of magnitude larger than the uncertainties of the heat flow measurements these differences may exist over tens of millions of years in the second part three 2 d time dependent temperature sections gradually including the above assessed effects have been calculated for the central alps the results show that time dependent models give better agreement of the calculated and measured heat flow patterns than previously developed for the same region steady state model cermak et al 1990 best agreement is achieved when only the transient effects of thickening and flaking and increased heat inflow from the mantle are taken into account a poorer coherence of the other versions with the measured heat flow is probably attributable to uncertain knowledge and consequently great simplification of spacing and time dependence of erosion and or frictional heating

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

towards a mechanical model for the formation of the pannonian basin abstract new maps showing crustal and lithospheric thickness variations in the pannonian basin and the surrounding east alpine carpathian and dinaric mountains have been prepared on the basis of recent seismic seismologic and electromagnetic data a map presenting miocene faults of regional significance has been also constructed for the same region by using a wealth of recently available national and or more local studies it is suggested that observed crustal and lithospheric structural contrasts have been controlled primarily by neogene kinematic history of the region kinematic history is characterized by the following main patterns 1 1 indentation by adria and large scale backthrusting in the southern alps 2 2 eastward escape and extension of the eastern alps 3 3 extensional collapse in the pannonian basin area 4 4 formation of broad wrench fault systems along strike in the dinarides and southern carpathians and the western carpathians with dextral and sinistral shear respectively 5 5 compression and accretion of the external thrust and fold belt in the eastern carpathians these kinematic patterns are thought to be interrelated and all are manifestations of the late stage evolution of an overthickened orogenic wedge in order to arrive at a better understanding of the mechanism of extension which formed the pannonian basin deep crustal seismic profiles hydrocarbon exploration reflection lines and borehole data have been analyzed in the little hungarian plain this plain represents the transition zone between the alps and the pannonian lowlands and the results are illustrated by eight interpreted cross sections one result of regional importance is the clear recognition that the transdanubian central range at the southeastern flank of the little hungarian plain is composed of alpine pre senonian thrust sheets this finding marks the end of a century of debate the allochthony of the substrata of the pannonian basin can be now considered proven another result of more general interest is that these cross sections document the mode of lithospheric extension preexisting compressional detachment planes reactivate as low angle normal faults and lead to tectonic unroofing of deeply buried metamorphic terranes characterized by ductile flow along subhorizontal lineation

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

gravity interpretation of a unified 2 d acoustic image of the central alpine collision zone summary four individual deep seismic reflection profiles across the eastern and southern swiss alps have been combined along the trend of the bouguer gravity anomalies onto the european geotraverse the profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide angle profiles running parallel to the strike of the alpine arc the contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles the resulting acoustic image depicts the subduction of parts of the lower crust of the european plate beneath the adriatic promontory of the african plate at a low angle of 15 based on the interpretation of the seismic data orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the european plate and wedging of the european and adriatic middle and lower crusts the latter being ill constrained by the seismic evidence alone the south vergent thrusting of the southern alps can be accounted for by the observed downbending of the adriatic moho and the lower crust in conjunction with the inferred wedging at mid crustal levels using the geometric constraints provided by seismic data gravity modelling of the alpine lithosphere asthenosphere system relative to stable central european platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth such a gently dipping subduction zone is at odds with prominent models on alpine geodynamics which favour a near vertical orientation of the subducting lithosphere verschluckung in agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen both seismic and gravimetric evidence therefore suggest that late orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper middle and lower parts of the crust the amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab

building reliable local models of the venus gravity field from the cycles 5 and 6 of the magellan los gravity data we present a new way to build reliable local models of the venus gravity field from the cycles 5 and 6 of the magellan probe by directly considering the doppler residual profiles instead of the derived residual acceleration profiles we give an example over a test area

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

gravity constraints on the structure of the northern margin of tunisia implications on the nature of the northern african plate boundary summary bouguer gravity data were analysed to determine the general crustal and upper mantle structure in northern tunisia residual gravity anomalies were determined by removing the gravitational effect of crustal thickness variations imaged by regional seismic experiments residual gravity anomalies contain short wavelength anomalies superimposed on a long wavelength component that decreases in amplitude northward towards the tunisian coastline an edge enhancement analysis e g enhanced analytic signals of the short wavelength anomalies suggests a previously unknown east west trending gravity anomaly south of 37 n with source depths of between 3 and 7 km modelling of residual and bouguer gravity anomalies indicate that there are two possible solutions for the residual gravity decrease in northern tunisia 1 thickening of cenozoic and mesozoic sediments north of a strike slip fault or 2 a crustal and uppermantle low density zone interpreted as being crustal material of the remnant subducted african plate the latter result is favoured based on seismic tomographic images of the mediterranean region which implies that subducting material exists under the african coast geological interpretations suggesting that the tell atlas may be a thrust wedge accreted by underplating of the african continental crust and seismic refraction models indicating a thinning of sediments in northern tunisia the east west trending gravity anomalies south of 37 n correspond to an important structural feature that may be related to either a structural boundary e g a transform fault or subduction beneath the african plate

the deep structure of the central alps inferred from both geophysical and geological data gravity surveys of the past century established that mountains have roots seismic refraction lines shot in the second half of this century confirmed the downbulge of the moho under the alps and recent reflection traverses provided new details on the behaviour of crustal layers in the deep part of the alps however geophysical data are ambiguous geologically for models of the root in terms of rock distribution to be tectonophysically acceptable they must be the retrodeformable result of kinematic sequence that fits the geological surface data for a cross section through the swiss alps based on refraction data and somewhat modified by the recent reflection traverses a kinematic model compatible with large scale geological data may be obtained by the superposition of three neogene phases with alternating vergence although alpine collision is largely dextrally compressive in the central alps the n s component may be discussed in a cross section particularly puzzling geophysical features include a high velocity body in the middle crust and the disappearance of the layered foreland crust in the root in order to account for these phenomena it is proposed that the crustal root is interpreted as the result of complex reshuffling of middle and lower crustal masses as well as large scale phase transformations the mid crustal highvelocity body is interpreted as a delaminated section of the lower crust of the adria plate that was wedged into the middle crust of the alps in the middle miocene the disappearance of the foreland lower crust is attributed to eclogitization attendant on the subduction of continental crust material balance estimates suggest that during alpine collision large volumes of continental crust have disappeared through subduction

a high resolution gravity model for venus gvm 1 a spherical harmonic model of the gravitational field of venus complete to degree and order 50 has been developed using the s band doppler tracking data of the pioneer venus orbiter pvo collected between 1979 and 1982 the short wavelengths of this model could only be resolved near the pvo periapse location 14 n latitude therefore a priori constraints were applied to the model to bias poorly observed coefficients towards zero the resulting model has a half wavelength resolution of 400 km near the pvo periapse location but the resolution degrades to greater than 1000 km near the poles this gravity model correlates well with a degree 50 spherical harmonic expansion of the venus topography derived from a combination of magellan and pvo data new tracking data from magellan s gravity mission should provide some improvement to this model although a complete model of the venusian gravity field will depend on tracking of magellan after the circularization of it s orbit using aerobraking

crustal structure beneath the northern margin of the swiss alps abstract the structure of the earth s crust under the northern margin of the swiss alps was determined by means of seismic refraction and reflection measurements the profile runs northeast from montreux on lake geneva along the helvetic nappes on the southern edge of the molasse basin to the rhine valley south of lake constance with two shotpoints 165 km apart we derived a two dimensional p wave velocity model from these data using data from two fans perpendicular to the profile at a distance of 110 km from the shotpoints we migrated the overcritical moho reflections laterally to their proper position the complicated sedimentary structure was mainly taken from various exploration surveys and earlier shallow research main features of the crust are the 3 km thick upper basement has an average velocity of 6 0 km s which is followed by 2 km with a reduced velocity of 5 6 km s at 10 km depth the velocity increases to 6 15 km s no significant structural changes occur in the depth range between 10 and 26 km with an average velocity of 6 2 km s at a depth of 26 km in the west and 24 km in the east the velocity increases to 6 6 km s for about 3 km followed by a low velocity zone of 6 4 km s with an average thickness of 6 km the crust mantle boundary is reached at 34 km depth under shotpoint jaunpass and deepens to 35 km near shotpoint saentis the southeastward downdip of the moho observed on the p m p fans migrates the derived deeper crustal structure laterally to the northwest by about 9 km from the surface position of the profile the velocity depth structure derived from the refraction and wide angle reflection data agrees well with a short near vertical reflection profile near shotpoint jaunpass

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

average strain rate in the italian crust inferred from a permanent gps network ii strain rate versus seismicity and structural geology summary high quality weekly solutions for the coordinates of 45 permanent gps stations spanning 1 6 years and covering the alpine mediterranean area are used to estimate a horizontal velocity field aligned with the itrf2000 velocity datum after subtracting model velocities for the eurasian plate we show that the residual velocity field north of the apennines in the adriatic dinaric region describes a nne trending tectonic flow with a negative velocity gradient from 6 mm yr 1 ne of the apennines to virtually zero ne of the dinarids in the calabria region we observe a divergent pattern with nnw orientation which turns to west as one moves north towards tuscany very small residual velocities are observed for stations in the sardinia corsica block spain southern france and north of the alps this regional pattern of velocities defines areas of intraplate strain on the basis of good station coverage and of a strain rate value above a threshold we define five provinces calabria central apennines both in the extensional regime and north adriatic eastern alps and south adriatic under compression we show that the orientations of the principal strain rate axes agree with the stress pattern that is inferred from local seismicity a comparison of the geodetically determined strain rate with the strain rate associated with the seismic moment of earthquakes in the past 30 yr shows that each province may differ considerably from the others in the amount of seismic moment that is accommodated by surface strain for example the umbria seismic sequence of 1997 seems to have released a seismic moment that was not exceptionally large but concentrated over a short time interval so that the equivalent coseismic strain rate was larger than the value observed geodetically on average depending on the assumptions on the seismogenic volume and time span 70 100 per cent of the surface strain seems to be accommodated by the release of seismic moment in the upper italian crust

the 3d crustal structure of the alps of eastern switzerland and western austria interpreted from a network of deep seismic profiles abstract a 3d crustal model of the alps of eastern switzerland and western austria was constructed a total of twenty two dimensional deep seismic reflection profiles recorded between 1986 and 1991 were two dimensionally interpreted based on depth migrated line drawings and using correlation with surface geology refraction derived velocity depth models and correlation with intersecting or neighboring profiles the network of crustal cross sections was then unified into 3d surfaces this analysis was restricted to four crustal discontinuities 1 the top of the european basement first european basement unit including the foreland encountered going downward from the topographic surface 2 the top of the lower crust 3 the top of the european moho transition zone and 4 the european moho simulation of the seismic profiles using 3d normal incidence and shot ray tracing enabled the quantification and correction of distortions caused by out of plane reflected energy the inferred 3d crustal configuration is likely the result of the latest phase of alpine shortening during the late tertiary it depicts a smooth sse dipping european moho overlain by a concordant lower crust in the external alpine domain and a presumably wedge shaped body of lower crust in the internal alpine domain in contrast to the lower crustal discontinuities the top of the european basement reveals a highly structured 3d geometry dominated by thrust tectonics the pronounced structural disharmony with respect to the lower crust implies an intracrustal decoupling mechanism during the formation of the external basement uplifts with the zone of decoupling possibly located near the top of the lower crust comparison of the top basement topography with the mapped surface geology indicates a close relation between the basement geometry and the shape of the austroalpine basal thrust delineating the prattigau halfwindow in the west and the engadine window in the east

three dimensional numerical simulations of crustal deformation and subcontinental mantle convection abstract 3 d simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large scale mantle flow simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 1 1 cartesian domain convective stresses cause crust to thicken above a sheet like mantle downwelling for mild convective vigor an initial crustal thickness variation is required to induce 3 d lithospheric instability below the zone of crustal convergence the amplitude of the required variation decreases with increasing convective vigor morphologically instability is manifest in formation of drip like thermals that exist within the large scale roll associated with initial crustal thickening a strong surface signature of the drips is their ability to cause deviations from local airy compensation of topography after the initial thickening phase the crustal accumulation that forms serves as a model analog to a continent its presence leads to mantle flow patterns distinctly different from the steady state roll that results in its absence large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small scale thermal instabilities eventually these instabilities induce a restructuring of large scale mantle flow with the roll pattern being replaced by a square cell although preliminary and idealized the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2 d mountain chain by the formation of the chain itself the ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low order despite the seemingly trivial volume of crust relative to mantle

analysis of the los gravity data set from cycle 4 of the magellan probe around venus abstract the new line of sight gravity data set from cycle 4 of the magellan spacecraft around venus is analyzed and its usefulness for geodetic and geophysical purposes is assessed these data contain large low frequency biases which are estimated by comparison with synthetic data computed from a harmonic model of the gravity field with the corrected data set the noncentral part of the radial gravity field at the surface over eistla regio and the west part of aphrodite terra with a 1 1 resolution is calculated

regional gravity fields on venus from tracking of magellan cycles 5 and 6 from august 1993 to october 1994 magellan was at heights 180 to 550 km products of the deep space network dsn x band tracking are line of sight los doppler frequencies and accelerations residual to a 40th degree gravity field the intrinsic accuracy and abundance of these data may be good enough to push the resolution half wavelength to less than spacecraft height the observation equation to infer gravity from los accelerations is simple however noise and non uniform geometry necessitate a singular value analysis cutoff or an a priori weighting the latter was chosen for computational economy further the non gaussian character of the noise necessitates a reject limit choices made were 1 surface element size 100 km 2 region size 1600 km 3 buffer zone width 600 km 4 reject limit 1 mgal 10 5 m s and 5 criterion for a priori weighting recovering a known solution i e a field transferred from earth to venus probably the most significant technical advance this criterion was optimized by a priori increments to the normal main diagonal about 10 of the minimum main diagonal element averaging about 0 5 of the maximum best resolutions defined by spectral coherence of 0 7 for the known field obtained were 110 km near the equator from 51 285 points over eisila and 180 km at high latitudes from 120 231 points over maxwell and 162 000 over akna freyja the limitation on resolution near the equator is the cubic polynomial fitting over 335 km lengths to determine accelerations however environmental effects were the limiters elsewhere root mean square rms residuals to solutions had negative correlations with spacecraft altitude and the angle at the earth between venus and the sum and positive correlations with the earth venus distance and latitude on venus indicating venus s influence on its electromagnetic surroundings out to several 100 km especially on its side toward the sum correlations with dsn zenith angles and the kp index of solar activity were positive but slight the main characteristic of the enhanced resolution of regional gravity fields is greater correlation with the topography at shorter wavelengths the main improvement yet to be made is to generate a full a priori matrix of the regional gravity field

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

tomographic images of the upper mantle below central europe and the mediterranean results from delay time tomography of the european mediterranean upper mantle are discussed and where possible interpreted in terms of geodynamic processes slab like positive velocity anomalies of which the locations correlate well with deeper seismicity are found beneath spain the tyrrhenian basin and the aegean these structures are interpreted as images of subducted slabs large aseismic regions with positive velocity anomalies are found beneath the western mediterranean italy the alps dinarides the pannonian basin northern greece and the aegean these anomalies can also be linked to subducted lithosphere from the anomaly patterns it is deduced that subduction occurred below the western mediterranean and along both sides of the adriatic micro plate beneath the dinarides and northern greece the velocity structures suggest detachment of the slab from the surface

the spectra of the topography of the earth venus and mars the global topography of the terrestrial bodies has been more or less determined from past planetary missions and from ground measurements and the behaviour of their spectra studied over the low frequencies only except the earth s case we here perform new spherical harmonic analysis of the topography of the earth venus and mars by using existing sometimes recently made available data sets to the highest possible degree and order so as to establish the decay of these spectra over a larger range of frequencies than before study the limit of validity of the used data sets and provide some new inputs to further geophysical analysis

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

deep structure of the alps what do we really know abstract in the last five decades the deep structure of the alps has been probed by every geophysical method applicable and the resulting amount of data is unmatched for any other orogen in this study an attempt is made to review the data and the proposed structural models with the aim of separating the certain from the probable and from the speculative information this can be achieved by first reviewing the theoretical resolving power and ambiguity of the applied interpretation methods and then analysing the proposed models the methods reviewed are inversion of surface wave data teleseismic and local earthquake seismic tomography near vertical reflection seismology wide angle reflection and refraction seismology and gravity modelling all information about the moho rated as certain is combined to give a moho map of the alpine area the information rated as certain and probable and additional qualitative arguments are used to discuss a crustal model of the western and central alps represented by two cross sections major structural elements in this crustal model are a thick overthrust penninic nappe system wedging at mid to lower crustal levels a discontinuous moho and strong variations along the strike of the orogen whereas the structures of the european upper crust and of the penninic nappe system are well constrained only few and isolated lower crustal structural elements are rated as certain finally the shape of the lower lithosphere in the alps is discussed by review and comparison of the results from surface wave teleseismic travel time residual and tomographic studies qualitative arguments suggest the existence of a lithospheric root or slab beneath the alps probable tomographic information suggests a south vergent european lithospheric slab beneath the southern alps and the po plain despite the considerable number of studies aimed at resolving the deepest part of alpine orogeny the available quantitative information on the sub moho structure cannot be rated as certain

some observations on basin evolution and dynamics sedimentary basin evolution is intimately bound in with continental lithosphere dynamics deep seismic reflection profiling has revealed images of the crust and upper mantle that suggest linkages between the shape and structure of sedimentary basins as they evolve with the rheological responses to tectonic stress of upper and lower crust and upper mantle components of the lithosphere this is well illustrated by the european geotraverse lithosphere profile from north cape norway to tunisia the alpine mountain belt has a lithospheric root as well as a crustal root by virtue of its asymmetric disposition this may be the cause of compressive and tensile forces in close association in the upper crust which have created mountain uplift and basin subsidence extension and thrusting all in juxtaposition evidence for an elastic upper layer of crust above a plastic substratum can be used as a basis for modelling basin geometry structure and stratigraphy built upon the flexural nature of the upper crust such models go a long way towards simulating the evolution of basins such as the north sea viking graben in more detail analogue structural models can reveal the geometry of basin fill in relationship to crustal movement and the relationships between structure and sedimentation patterns however sedimentary basin modelling needs to begin with the quantitative simulation of depositional environments in relation to such factors as tectonics sea level variations sediment supply and erosion in order to match sedimentary architecture only then can the range of deposits be placed in space and time within a basin environment as building blocks within a dynamically evolving structural framework to create predictive models for comparison with geological reality