seismic investigations along the european geotraverse and its surroundings in central europe since 1975 several high resolution seismic refraction and reflection surveys have been carried out in western germany to investigate the structure of the earth s crust and uppermost mantle the investigation culminated in the seismic refraction survey along the 825 km long central part of the european geotraverse egt in 1986 this contribution summarizes the main results of the more recent crustal investigations along and around the egt the internal crustal structure throughout the area of the variscides is very complex and changes laterally considerably distinct crustal blocks differing in their internal structure can be assigned to geologically defined units of the variscan and caledonian orogeny in spite of local deviations in general a more or less transparent and low velocity upper crust contrasts with a highly reflective lower crust a subdivision of upper and lower crust by a well defined boundary conrad discontinuity is not always seen towards the alps the average velocity of the lower crust is as low as 6 2 km s 1 in contrast to the area north of the swabian jura where the velocities above moho vary between 6 8 and 7 2 km s 1 in northern germany the elbe line separates the lower crust into two regions with 6 4 km s 1 average velocity in the south and 6 9 km s 1 in the north the total crustal thickness under the variscan part of germany is fairly constant between 28 and 30 km except under the rhine graben area with 25 26 km and beneath the central part of the rhenish massif where an anomalous crustal thickening to 37 km is observed under northern germany the moho rises to about 26 km depth and the data indicate at least one fault like step of 1 km before the crust thickens toward the ringkobing fyn basement high the synthesis of seismic velocity structure and petrological information from xenolith studies allows us to propose a mafic composition for the deeper levels of the crust and uppermost mantle which may be valid at least for the central part of the variscan crust along the european geotraverse in central europe

a finite amplitude necking model of rifting in brittle lithosphere we formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic plastic layer overlying a fluid substrate a perfectly plastic rheology is a continuum description of faulting in rift zones two important aspects of rift evolution are illustrated by this model the evolution of the rift width as extension proceeds and the finite strain that occurs a region at yield initially develops with a width determined by the thickness of the brittle layer and the internal deformation within this yield zone is proportional to the topographic slope as extension proceeds the surface within the rift subsides and the width of the subsiding yield zone decreases at any stage of rifting material in regions just outside the yield zone is deformed but no longer deforming the width of these deformed regions increases with increasing extension vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone creating flanking uplifts the external force required to maintain active rifting increases with the amount of lithospheric stretching indicating that rifting is a quasi static stable process because the yield zone will revert to elastic behavior if the external force causing extension is removed the model predicts that the rift depression and flanking uplifts will be preserved after extension stops our simple mechanical model demonstrates the inherent relationship among graben formation lithospheric thinning and rift shoulder uplift in rift zones

cadomian terranes wrench faulting and thrusting in the central europe variscides geophysical and geological evidence sixty five per cent of the paleozoic basement of western and central europe is hidden by a sedimen tary cover and or sea this work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the variscan or ogeny it is based on the interpretation of various forms of data a published gravity maps corrected for the ef fects of the crust mantle boundary topography and light sedimentary basins b aeromagnetic maps c measurements of densities and d induced and rema nent magnetizations on rocks from paleozoic outcrops of the upper rhenish area from the northern b ohem ian massif to the eastern paris basin the saxothurin gian is characterized by a 500 km long belt of gravity highs the most important being the kraichgau high most of the corresponding heavy bodies are buried un der a post early visdan cover they are interpreted as relics of late proterozoic terranes overlain by an early to middle paleozoic sequence equivalent to the bo hemian t errane in the bohemian massif the most probable continuation of these dense bohemian ter ranes toward the west is the southern channel north ern brittany cadomian terrane the gravity lows are correlated with variscan granites and pre and early variscan metagranites gravity and magnetic maps demonstrate large scale displacement in devonian early carboniferous times along the parallel and equidistant nw se striking vistula elbe bavarian bray and south armorican dextral wrench faults in the vosges schwarzwald and central massif the faults continue with the east west striking lalaye lubine ba den baden and marche faults and with south vergent thrusts the bavarian faults shift the kraichgau terrane by 150 km relative to the bohemian terrane whereas the offset of the north ern brittany cadomian relative to the northern vosges kraichgau terranes is estimated at 400km along the bray fault sinistral wrench faults are the ne sw striking sillon houiller rheingraben rodl vitis and diendorf faults the southern vosges schwarzwald devonian dinanti an basin is interpreted as a pull apart basin at the south easterly extremity of the bray fault the bohemian and kraichgau body form allochthonous terranes which were thrust over the saxothuringian crust thrusting to the north west was accompanied by back thrusting and led to the forma tion of pop up structures contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision the zonal structure of the varis cides in the sense of kossmat 1927 is relevant only to the rhenohercynian foreland belt kossmat 1927 al ready spoke of a moldanubian region because it dis plays no real zonal structure the saxothuringian zone was formed by terrane accretion their apparent zonal structure is not a pre collisional feature but only the result of accretion and collision

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

on the relationship between isostatic elevation and the wavelengths of tectonic surface features on venus abstract regional isostatic elevation d and the widths and spacings wavelengths of tectonic features formed due to horizontal extension and compression are observed quantities on venus that are sensitive to the thermal and compositional structure of the lithosphere to investigate the relationships between these parameters and the implications for venus lithosphere structure we formulate models in which d and are functions of surface thermal gradient dt dz 0 and crustal thickness c in models that relate to dt dz 0 and c we impose a requirement that for tectonic features with multiple wavelengths of deformation to develop the growth rates of instabilities that control the dominant wavelengths must be similar in magnitude this constrains the maximum values of c and dt dz 0 that can result in multiple tectonic wavelengths on venus to be less than or equal to those determined in our previous study c 30 km and dt dz 0 25 k km m t zuber 1987 j geophys res 92 e541 e551 we also estimate the strength of venus upper crust in areas with multiple wavelengths and obtain limits that are most consistent with a compressional origin for ridge belts which are enigmatic features that have been purported to be of both extensional and compressional origin in models that relate d dt dz 0 and c we show that in areas of venus where the upper mantle is stronger than the upper crust the spacings of short wavelength 30 km features should increase with increasing d if the change in d is due to increasing c but should decrease with increasing d if the change in d is due to increasing dt dz 0 long wavelength 30 km features are not strongly correlated to variations in d due to the effect of crustal thickness on the depth distribution of lithospheric strength which influences wavelength selection in a complex manner in areas of venus where the upper mantle is weaker than the upper crust the spacings of short wavelength features should remain constant with increasing d if the change in d is due to a change in c but should decrease with increasing d if the change in d is due to increasing dt dz 0 the results demonstrate that in shallowly compensated areas of venus that contain regularly spaced tectonic features it may be possible to distinguish whether isostatic elevation differences are a consequence of variations in crustal thickness or thermal gradient we interpret the observed relationship of with d for short wavelength tectonic features in several tessera regions to reflect crustal thickness variations

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

age relations among oslo rift magmatic rocks implications for tectonic and magmatic modelling abstract this paper presents a compilation of 67 published and unpublished radiometric age determinations by the rb sr method on magmatic rocks in the continental oslo rift in southeast norway the data include most major rock types and structural units in the subaerial part of the rift the implications of these data with respect to the tectonomagmatic evolution of the rift and processes in the mantle and crust are discussed the oslo rift was active for about 60 million years the earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre rift lower paleozoic sediments at about 300 ma extrusion of plateau lavas of basaltic and intermediate composition rhomb porphyries started about 295 ma and lasted to about 275 ma with largest magma volumes and highest extrusion rate from 295 to 285 ma in the vestfold graben segment gs one flow per 250 000 years and 0 30 km3 per 103 years the main faulting activity and graben formation occurred during this period the plateau volcanism was succeeded by a stage of bimodal central volcanoes most of which later underwent caldera collapses ring dykes and central intrusions in these calderas give ages in the range 268 266 ma in the vestfold and 266 243 ma in the akershus gs large composite intrusive complexes were emplaced in the periods 27 268 and 273 241 ma in the vestfold and akershus gs respectively a rate of propagation of the rift from ssw towards the nne has been estimated to 1 2 cm y 1 on the basis of 87 sr 86 sr initial ratios and type of magmatic products we estimate magmas to have been emplaced into the crust only during the first 20 25 ma of the magmatic period some of the magmatism appears to be initiated by movement along precambrian fault zones which were reactivated during the oslo rifting event

from symmetric necking to localized asymmetric shearing the role of mechanical layering we study the formation of localized shear zones during the layer parallel extension of viscous multi layers using two dimensional numerical simulations based on the finite difference method for power law viscous layers and a linear viscous embedding medium the extended multi layer develops boudins due to necking for power law viscous layers embedded in a power law viscous medium the extended multi layer develops first distributed necks and subsequently a localized shear zone with a vertical offset with a size of several layer thicknesses along the multi layer during the extension the deformation style switches from distributed and symmetric necking to localized and asymmetric shearing a localized shear zone develops in the viscous multi layer although the rheology is everywhere strain rate hardening power law stress exponent 1 and no material softening and or energy feedback mechanism e g shear heating combined with a temperature dependent viscosity is applied the shear localization is caused by structural softening because the formation of a localized shear zone decreases the bulk resistance and hence the work required to deform the multi layer a localized shear zone forms in the multi layer when the distance between the stiff layers is approximately equal to or less than the layer thickness the shear localization was observed in multi layers with nine and with only three stiff layers

the rift to break up evolution of the gulf of aden insights from 3d numerical lithospheric scale modelling abstract the gulf of aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore recent surveys showed that the spatio temporal evolution of the gulf of aden rift system is dominated by three fault orientations displacement orthogonal wsw rift parallel wnw and an intermediate e w trend the oldest parts of the rift that are exposed onshore feature displacement orthogonal and intermediate directions whereas the subsequently active necking zone involves mainly rift parallel faults the final rift phase recorded at the distal margin is characterised by displacement orthogonal and intermediate fault orientations we investigate the evolution of the gulf of aden from rift initiation to break up by means of 3d numerical experiments on lithospheric scale we apply the finite element model slim3d which includes realistic elasto visco plastic rheology and a free surface despite recent advances 3d numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved we address this issue by proposing a simple post processing method that uses the surface stress tensor to evaluate stress regime extensional strike slip compressional and preferred fault azimuth the described method is applicable to any geodynamic model and easy to introduce our model reproduces the observed fault pattern of the gulf of aden and illustrates how multiple fault directions arise from the interaction of local and far field tectonic stresses in an evolving rift system the numerical simulations robustly feature intermediate faults during the initial rift phase followed by rift parallel normal faulting at the rift flanks and strike slip faults in the central part of the rift system upon break up displacement orthogonal as well as intermediate faults occur this study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new insights in the timing of fault successions of the gulf of aden and continental rifts in general

fission track analysis of basement apatites at the western margin of the gulf of suez rift egypt evidence for synchroneity of uplift and subsidence fifty six apatite fission track ages and 52 horizontal confined track length measurements are reported from precambrian crystalline rocks along the western margin of the gulf of suez egypt ages fall in the range of ca 11 385 m y and older ages often occur within very close geographic proximity to younger ones indicating non uniform uplift the wide range in ages is accompanied by a systematic variation in the distribution of horizontal confined fission track lengths on the basis of apatite fission track ages and their length distributions data fall into three distinct groups group i ages ranging from 43 to 385 m y length distributions are all positively skewed and with decreasing age become progressively broader with shorter mean track length group ii ages ranging from 23 to 31 m y length distributions are negatively skewed with either a distinct tail or a small peak of short tracks group iii ages ranging from 11 to 20 5 m y length distributions are al unimodal narrow negatively skewed and have the longest mean lengths among samples studied apatite ages from groups i and ii are interpreted as mixed ages as a result of cooling during uplift from different levels within the apatite partial track annealing zone ages from group iii are interpreted as cooling ages due to uplift from the apatite total track annealing zone with minor partial annealing correcting the ages of the two oldest samples in this group for track length reduction yields ages of21 2 2and23 1 5m y it is proposed that the onset of rift flank uplift in the gulf of suez northern red sea area occurred between 21 and 23 m y ago fission track analysis in combination with subsidence data from the gulf of suez basin indicate that commencement of basement uplift postdate the start of rifting and is interpreted as evidence for passive rifting at the gulf of suez furthermore this uplift is contemporaneous with and is directly related to the process of extension and subsidence at the gulf of suez

deep structure and evolution of the harz mountains results of three dimensional gravity and finite element modeling a new bouguer anomaly map is presented for the region of the entire harz mountains based on more than 60 000 gravity values the various gravity anomalies are discussed and interpretation is carried out by high resolution 3 d gravity modeling one of the main subjects of interest in the investigation is the northern boundary fault zone of the harz mountains separating the mesozoic sediments in the north from the palaeozoic rocks of the harz in the south dip and vertical displacement are determined for this fault zone mean values are 3400 m and 70 respectively gravity modeling shows that the brocken and the ramberg granites are distinctly different the brocken granite is shallow whereas the ramberg granite has a maximum depth of 8 5 km and a n s dimension of 37 km the prominent benneckenstein gravity high is explained by two different models one based on a granodioritic intrusion 2900 kg m3 with a center depth of 14 km and the other based on phyllites 2740 kg m3 on a depth of 3 4 km studies on the geodynamic evolution of the harz mountains are carried out using the finite element method on the basis of a 3 d model vertical displacements that can be related to horizontal forces are computed for the period of the variscan orogeny an uplift of 600 m in the harz area is calculated for late cretaceous and tertiary 400 m are determined the total amount of 1000 m is about 12 of the vertical displacement of the northern boundary fault zone of the harz mountains shown by the gravity modeling these results do not contradict geological ideas

some remarks on the relation between vertical motions of the lithosphere during extension and the necking depth parameter inferred from kinematic modeling studies a well established fact of continental stretching is that during the extension phase the lithosphere retains a finite flexural strength vertical loads caused by the redistribution of material with crustal and mantle densities during lithospheric thinning can therefore be supported by the lithosphere whether these loads cause flexural uplift or subsidence depends on the strength distribution within the continental lithosphere in kinematic model studies stratigraphic and other data acquired in the vicinity of sedimentary basins and passive margins are inverted for these loads in these models vertical rebound loads are parameterized in terms of a necking depth parameter we use thermal mechanical finite element simulations to investigate whether rheological information about the continental lithosphere can be extracted from the necking depth parameter derived from kinematic models we conclude that spatial and temporal variations in necking depths near basin edges and passive margins as derived from our dynamic simulations are not consistent with the uniform and stationary necking depth that is usually assumed in kinematic models this is very important since typically the stratigraphic data used in kinematic model studies are acquired in these regions we therefore conclude that there is no one to one relation between the necking depth parameter and the strength distribution in the lithosphere

transform faults orthogonal to rifts insights from fully gravitational physical experiments abstract on the one hand it is not yet understood why transform faults commonly form orthogonal to rifts because this cannot be explained by simple theories of fracture mechanics on the other hand rock experimental data have shown that a very strong layer elastic core should exist between brittle and ductile layers of the lithosphere therefore we investigated the influence of an elastic core on the generation of a transform fault rift orthogonal configuration we used fully gravitational scaled physical experiments of brittle viscous models and the results indicate that the elastic core and localized heat flow can be responsible for the observed common pattern of transforms orthogonal to rifts

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

the shallow structure of the martian lithosphere in the vicinity of the ridged plains wrinkle ridges in the smooth plains of the coprates and lunae planum regions of mars form linear or concentric patterns with a regular spacing of 25 50 km the authors test the hypothesis that the periodic development of deformation was a consequence of unstable horizontal compression that occurred prior to probable ridge related faulting of a strength stratified lithosphere which consists of a mechanically strong surface plains unit that successively overlies a weak megaregolith and a strong lithospheric basement results show that a range of models with both rigid and deformable megaregolith basement interface conditions yield solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials in models that incorporate viscous and plastic rheologies uniform and exponentially varying vertical strength distributions and the presence or absence of interfacial slip at the base of the surface plains unit the ridge spacing is primarily controlled by the power law exponent of the lithosphere the megaregolith plains unit thickness ratio and the plains unit megaregolith strength contrast deformable basement models with a viscous rheology 1 le n approx le 3 can explain the ridge spacing if the megaregolith was thicker than and approximately 1 3 orders of magnitude weaker than themore plains unit similar models with a perfectly plastic n yields infinity rheology require plains unit thicknesses greater than 5 km which exceeds most observational estimates ridge spacing in the combined viscous and plastic models can be consistent with either a dry water rich or ice rich megaregolith at the time of ridge formation less

asthenospheric and lower lithospheric mantle contributions to continental extensional magmatism an example from the british tertiary province abstract the compositions of palaeocene basic magmas in skye and mull british tertiary igneous province btip shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension after the effects of crustal contamination in some magmas have been detected and the samples showing it eliminated from consideration the remainder show a significant diversity of trace element compositions basic lavas from skye and mull which lack crustal contamination have contents of compatible and moderately compatible elements e g ni cr y yb and also of moderately incompatible elements e g ti p zr hf sr mree which are within the range shown by the tholeiitic and mildly alkalic ocean island basalts oib as from iceland in contrast their contents of strongly and very strongly incompatible elements e g k rb ba u th nb ta lree are within the range shown by normal mid ocean ridge basalts these btip magmas are interpreted as second stage melts from mantle which had already lost a very small melt fraction the latter is thought to be represented by the strongly alkalic permo carboniferous dyke swarms associated with rifting at that time in nw scotland residual uppermost asthenosphere from the late palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath nw scotland and thus to have remained coupled to it throughout the mesozoic until reactivated by palaeocene stretching occasional hawaiites in the top third of the skye lava pile remnant and also basalt dykes of the swarm cutting the granites of the central complex the last significant magmatic event at this centre have the elemental characteristics of typical north atlantic oib notably la ta 10 12 these compositions are thought to demonstrate input from asthenospheric source basic magmas throughout the lithospheric source dominated skye and mull magmatism it is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources but that the larger magma batches became contaminated during uprise by melting and assimilating the most fusible parts of overlying lithospheric mantle followed by lower crust in many cases smaller magma batches traverse the overlying lithosphere with little or no contamination perhaps the difference between the two lies in their flow behaviour laminar or turbulent during uprise

a back of the envelope approach to boudinage mechanics abstract smith has shown from linear hydrodynamic stability that pinch and swell boudinage is possible only in a non newtonian material by assuming uniaxial stress and strain and large viscosity contrast between the competent layer and the incompetent matrix smith s result can be derived in a very simple way the one dimensional force balance also predicts growth rates although not dominant wavelengths however we question the ability of even linear stability analysis to accurately predict boudin widths

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

the effect of rift shoulder erosion on stratal patterns at passive margins implications for sequence stratigraphy abstract numerical modelling indicates that the erosion of uplifted rift flanks at passive margins has a profound effect on offshore stratigraphic patterns flexural uplift due to isostatic rebound in response to erosion extends far into the basin and causes uplift of the shelf as a result the contemporaneously deposited sedimentary wedge displays a characteristic offlap pattern when the rift shoulder is largely eroded onlap promoting mechanisms related to cooling of the lithosphere enable sediments to onlap onto the basin margin the initial offlapping and subsequent onlapping strata form one complete second order depositional sequence comprising a shelf margin transgressive and highstand systems tract the modelling inferences are in broad agreement with stratal patterns and basin geometries observed on the u s east coast the southeastern brazilian and southeastern australian passive margins and the transantarctic mountains ross sea shelf system the initial offlaps caused by erosion of rift shoulders have important implications for the deriviation of eustatic signals from coastal onlap patterns

structure of the southern rhine graben from gravity and reflection seismic data ecors dekorp program the southern rhine graben was investigated by the seismic reflection ecors dekorp program gravity data available in this region have been reprocessed in order to develop a coherent map along the seismic line the new map has been corrected for the gravity effects of tertiary sediment and crust mantle discontinuity the strong negative gradient towards the alps may be explained by a density contrast of 300 500 kg m3 at the crust mantle interface the residual map shows a negative long wavelength anomaly superimposed on the moldanubian domain of the variscan belt along a vosges black forest swabian jura axis this anomaly cannot be explained by the properties of the variscan crust but may result from a low density body within the upper mantle however its ene wsw direction rules out any connection with the formation of the tertiary grabens which trend north south the recent uplift of topography and the volcanic activity 20 ma of the southern rhine graben may partly be a consequence of an asthenospheric upwelling beneath this area the analysis of the residual short wavelength anomalies shows a fundamental difference between the light variscan terranes of the moldanubian zone and the dense barrandian saverne kraighau structures of the saxothuringian domain a density cross section along the ecors dekorp line confirms that the moho uplift extends beyond the graben whereas thinning of the lower crust affects the graben area only we interpret this crustal structure as an initially thin hercynian crust later deformed by asymmetric extension during oligocene rifting

venus volcanism and rift formation in beta regio a new high resolution radar image of beta regio a venus highland area confirms the presence of a major tectonic rift system and associated volcanic activity the lack of identifiable impact craters together with the apparent superposition of the theia mons volcanic structure on the rift system suggest that at least some of the volcanic activity occurred in relatively recent geologic time the presence of topographically similar highland areas elsewhere on venus aphrodite terra dali chasma and diana chasma suggests that rifting and volcanism are significant processes on venus

glacial advance and stagnation caused by rock avalanches abstract new model results show that a glacier advance caused by deposition of a rock avalanche on the ice will be followed by stagnation of the advanced ice lobe producing distributed hummocky deposits quite different from the single moraine ridges typically dated in paleoclimatic reconstructions we obtain these results using a numerical flow line glacier model with superimposed rock debris in particular our results imply that the single moraine ridge known as the waiho loop of the franz josef glacier new zealand is not the result of an advance triggered by a rock avalanche else there would be a widespread hummocky glacial stagnation deposit associated with it

evolution of pinch and swell structures in a power law layer analytical and finite element fem solutions for the necking of a single power law layer up to large finite amplitude are obtained continuous necking of the layer produces pinch and swell structures the layer is either a free plate or embedded in a homogeneous medium an analytical solution for finite amplitude necking based on the assumption that plane sections remain plane psrp agrees well with the fem result for a layer power law stress exponent n 5 and for a ratio of layer to medium effective viscosities m 100 fem simulations for embedded layers verify that psrp for m 20 the presented numerical experiments generate localized necking and pinch and swell structures similar to natural ones for n 5 and m 20 additional weakening mechanisms such as strain softening although likely to be operative in nature are not required to generate natural pinch and swell structures fem experiments with random perturbation of the layer interfaces show that even with strong necking instability the layer is thinned at the swell as well as at the necks affecting strain estimation from pinch and swell geometry 2008 elsevier ltd all rights reserved

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

venus summary and review of science research activities 1983 87 in the past four years the first high resolution synoptic images of the surface of venus have been obtained and we have learned more about the nature of its surface and atmosphere the major accomplishments of the u s s r in venus exploration have been observed with great interest by u s investigators some of whom have begun joint research efforts with soviet scientists venera 15 and 16 radar imaging experiments and new data obtained at the arecibo observatory have provided answers to some important questions conjectured volcanic features have been resolved into unmistakable calderas or distinct lava flow morphologies the question of whether venus like the earth may have high erosion rates is answered and from the observed number of presumed impact craters it appears that resurfacing rates on venus are much smaller than on earth no evidence of water or wind activity has been seen

flexural rift flank uplift at the rio grande rift new mexico like other cenozoic continental rifts the rio grande rift in colorado and new mexico exhibits prominent flanking uplifts several driving stresses and thermal mechanical processes have been proposed to explain the origin of rift flank relief which can be modeled to infer lithospheric structure although we have identified multiple uplift styles at the rio grande rift only one range the sacramento mountains is attributable to flexural upwarping of the lithosphere the process most suitable for geodynamic modeling and interpretation we demonstrate that two common assumptions in such modeling potentially introduce serious errors first presuming only one mechanism acts to uplift the flanks is inappropriate various forces influence flank topography at different depths and wavelengths and no single one is dominant second the end member boundary conditions of complete mechanical continuity or discontinuity broken plate at the range bounding normal fault are in general not applicable at rift flanks we examine alternative analytic plate flexure solutions by comparing them to finite element models of footwall flexure at a normal fault in a two dimensional elastic plate undergoing extension these simulations indicate that broken plate fits to rift flanks underestimate the plate thickness unless the uplift is large at least 1 km which promotes decoupling between the hanging wall and footwall if denudation dominates the flank unloading as may commonly be the case the best fit broken plate thickness error can be even greater our flexural analysis of the sacramento mountains suggests that the pecos river valley originated as a flexural down warp adjacent to the rift flank sensitivity tests of least squares fits to the sacramento mountains imply typical plate thickness errors of 20 although in extreme cases the combined errors may be 50 the average effective elastic lithosphere thickness is 23 km we find that elastic plastic models of rift flank flexure are unable to provide meaningful constraints on the thermal structure of continental lithosphere

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

the upper rhenish shield basement vosges upper rhinegraben and schwarzwald main structural features deduced from magnetic gravimetric and geological data abstract interpretation of magnetic and gravimetric data and correlations with geological observations on outcrops and in boreholes have enabled the construction of a geological sketch map of the pre westphalian basement in the area of the upper rhenish shield the map demonstrates the contrast between the heavy and magnetic saxothuringian characterized by palaeozoic units metamorphosed to varying degrees and intruded by early carboniferous basites and the lighter moldanubian consisting mainly of gneisses and granites geological structures and tectonic features are recorded beneath the mesozoic and cenozoic cover the present picture of the basement is mainly the result of late visean early namurian structural features represented by a linear arrangement of magmatic bodies and the distribution of strike slip and thrust faults the major structural features are the e w vittel lalaye lubine baden baden fault that marks the border between saxothuringian and moldanubian the n35 left lateral wrench fault system running along the present rhinegraben the n60 grain of the northern vosges and the saar basin and the s verging crustal thrusts of moldanubian

a new multilayered model for intraplate stress induced differential subsidence of faulted lithosphere applied to rifted basins in plane horizontal stresses acting on predeformed lithosphere induce differential flexural vertical motions a high precision record of these motions can be found in the sedimentary record of rifted basins originally it was proposed that rifted basins experience flank uplift and basin center subsidence in response to a compressive change of in plane stress which agrees well with observed differential motions subsequently published models predicted that the vertical motions may be opposite because of the flexural state of the lithosphere induced by necking during extension however the total flexural and permanent geometry of the lithosphere underlying the rifted basin is the controlling parameter for the in plane stress caused vertical motions the largest part of this preexisting geometry is caused by faulting in the uppermost brittle part of the crust and ductile deformation in the underlying parts of the lithosphere we present a new multilayered model for stress induced differential subsidence taking into account the technically induced preexisting geometry of the lithosphere including faults in the upper crust as continental lithosphere may exhibit flexural decoupling due to a weak lower crustal layer the new multilayer in plane stress model discriminates the geometries of the separate competent layers at a basin wide scale the new model predicts that a compressive change of in plane force results in basin center subsidence and flank uplift confirming the original hypothesis compared to all previous models the new model requires a lower horizontal stress level change to explain observed differential vertical motions

crustal structure along the central segment of the egt from seismic refraction studies abstract the seismic refraction line between the gulf of genova and the baltic sea was carried out in 1986 as part of the european geotraverse egt 86 the central segment of the egt also known as eugemi european geotraverse middle part has a total length of 825 km and runs from the northern margin of the alps to the baltic sea traversing mainly the variscan mountain area of western germany the p wave data of nine shotpoints were interpreted in detail and a two dimensional p velocity model of the crust was developed between the molasse basin and the southern end of the north german basin for the north german basin the data quality justifies only the construction of a simplified model the internal crustal structure is quite complex in the upper crust of the moldanubian and saxothuringian zones of the variscan orogen the velocities are lower than in the rhenohercynian area the velocity of 6 0 km s representing consolidated seismic basement is reached at depths of as much as 10 12 km northern margin of alps and north german basin and of only 3 km under the mid german crystalline rise mid crustal velocities range from 5 8 to 6 1 km s forming weak velocity reversals in some parts in the lower crust velocities as low as 6 2 km s are seen near the alps in contrast to velocities between 6 8 and 7 2 km s in the exposed variscan orogen north of shotpoint e between hannover and the river elbe the velocity of the lower crust is about 6 4 km s north of the elbe line it increases abruptly to 6 9 km s the crust mantle boundary rises from 34 km depth under the northern margin of the alps to 29 km under the swabian jura under the variscan orogen to the north the crustal thickness varies only slightly around an average depth of about 29 1 k but drops to about 30 km under the north german basin between the elbe line and the ringkobing fyn high the moho rises to a depth of about 26 km and the data indicate at least one fault like step of 1 km before the crust thickens toward the basement high the velocities in the uppermost mantle show considerable variation the highest velocity of 8 4 km s is observed beneath the saxothuringian zone and the lowest values of 7 8 7 9 km s are recorded under the hessen depression

evolution and structure of the upper rhine graben insights from three dimensional thermomechanical modelling the evolution and geometry of the tertiary upper rhine graben were controlled by a continually changing stress field and the reactivation of pre existing crustal discontinuities a period of wnw ese extension in the late eocene and oligocene was followed by lateral translation from the early miocene onwards this study utilizes 3d finite element techniques to simulate extension and lateral translation on a lithospheric scale brittle and creep behaviour of lithospheric rocks are represented by elastoplasticity and thermally activated power law viscoplasticity respectively contact elements allocated with cohesion and frictional coefficients are used to describe pre existing zones of weakness in the elastic brittle field our results suggest that 1 extension is accommodated along listric border faults to midcrustal depth of 15 16 km beneath pure shear stretching occurs without a need for localized shear zones in lower crust and upper mantle 2 ductile flow at midcrustal depth across the graben accounts for the pronounced halfgraben morphology thereby the shape of the border faults their frictional coefficients and sedimentary loads have profound effects on the rate of ductile flow across the graben 3 horizontal extension of 8 8 5 km and sinistral displacement across the rift of 3 4 km are needed to accommodate the observed sediment thickness

spacing of faults at the scale of the lithosphere and localization instability 1 theory 1 large scale tectonic structures such as orogens and rifts commonly display regularly spaced faults and or localized shear zones to understand how fault sets organize with a characteristic spacing we present a semianalytical instability analysis of an idealized lithosphere composed of a brittle layer over a ductile half space undergoing horizontal shortening or extension the rheology of the layer is characterized by an effective stress exponent ne the layer is pseudoplastic if 1 ne 0 and forms localized structures if 1 ne 0 two instabilities grow simultaneously in this model the buckling necking instability that produces broad undulations of the brittle layer as a whole and the localization instability that produces a spatially periodic pattern of faulting the latter appears only if the material in the brittle layer weakens in response to a local perturbation of strain rate as indicated by 1 ne 0 fault spacing scales with the thickness of the brittle layer and depends on the efficiency of localization localization is more efficient for more negative 1 ne leading to more widely spaced faults the fault spacing is related to the wavelength at which different deformation modes within the layer enter a resonance that exists only if 1 ne 0 depth dependent viscosity and the model density offset the instability wavelengths by an amount al that we determine empirically the wave number of the localization instability is kjl j al 1 ne 1 2 h with h the thickness of the brittle layer j an integer and 1 4 al 1 2 if the strength of the layer increases with depth and the strength of the substrate decreases with depth

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

effect of lithospheric geometry on rift valley relief 1 continental and mid oceanic rifts commonly show a rift valley bounded by normal faults and uplifted shoulders in this paper we numerically model the evolution of rift valley relief in a notched brittle lithosphere results show that the rift valley has a maximum valley relief which is controlled by the competition between isostasy and lithospheric geometry if the axial lithosphere is much thinner than the flanking lithosphere the isostatic force will break outward dipping faults on the axis and stop the deepening of the rift valley we call this the axial failure mode on the other hand if the thickness of axial lithosphere is close to that of the flanks flexural bending will break new faults on the flanks and stop the valley deepening we call this the flanking failure mode triggering of either mode gives the maximum valley depth and the selection is controlled by the interplay between isostasy and lithospheric geometry on the basis of these mechanisms we propose a scaling equation that relates the maximum valley depth to the geometry flanking thickness axial thickness and notch width and strength friction angle and cohesion of the lithosphere the model explains along axis variation of valley relief at mid oceanic ridges by changes of the axial lithosphere thickness

stratigraphic and sedimentary records of the rift to drift evolution of the northern north china craton at the paleo to mesoproterozoic transition article i nfo this paper presents a stratigraphic and sedimentary study of late paleoproterozoic to early mesoproterozoic successions of the northern margin of the north china craton including the changcheng and nankou groups a reconstruction of the tectono sedimentary history of these two groups demonstrates that the northern north china craton experienced a two stage evolution as recorded by the changcheng group early synrift subsidence and sedimentation were controlled by normal faults and accompanied by volcanism in contrast a late postrift stage was characterized by broad subsidence and widespread occurrence of shallow marine clastic and carbonate rocks a transgressive unconformity is identified and together with its time equivalent conformable contact clearly separates the synrift and postrift sequences the diachronous unconformity can be related to continental rifting and the separation of the north china craton from the adjacent india craton at roughly 1600 ma and is thus interpreted as a breakup unconformity this age of ca 1600 ma is constrained by newly reported precise shrimp u pb zircon ages of transgressive deposits in the overlying mesoproterozoic dahongyu and gaoyuzhuang formations just coincident with the paleoproterozoic to mesoproterozoic transition the 1600 ma time estimate for the tectonic departure of the north china craton from the india craton is comparable with the timing for the breakup of columbia a pre rodinia supercontinent containing almost all of the earth s continental blocks

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

physical characteristics and evolutionary trends of continental rifts rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension they are widespread and occur in a variety of tectonic settings and range up to 2 600 m y in age the object of this review is to highlight characteristic features of modern and ancient rifts to emphasize differences and similarities in order to help characterize evolutionary trends to identify physical conditions favorable for initiation as well as termination of rifting and to provide constraints for future modeling studies of rifting rifts are characterized on the basis of their structural geomorphic magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts and it is concluded that while simple models can give valuable insight into specific processes of rifting individual rifts can rarely if ever be characterized by well defined trends predicted by these models more data are required to clearly define evolutionary trends and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories

the gravity field along the central segment of the egt abstract we give an overview of the gravity field of central europe based on a compilation of bouguer maps available by mid 1989 a generalized gravity map corrected for moho effects is presented as well as several filtered maps of the second derivative additionally sediment stripped bouguer maps are used in the case of the northwest german basin and for northeastern franconia the north german line the erzgebirge minimum the kraichgau high and the regensburg gottingen low are treated in detail and a number of geologic interpretations are discussed

deformational models of rifting and folding on venus tectonic features on venus with characteristic widths and or spacings can be used to place quantitative constraints on the structure of the mechanical lithosphere through geophysical modeling we develop theoretical models for both compressional buckling and extensional rifting deformation based on classical elastic plate models with characteristic spacings determined by the flexural parameter modified to account for the effects of a realistic rheology this rheology consists of a relatively weak crust overlying a more creep resistant mantle in both layers the upper portions deform by frictional yielding on preexisting faults with a yield stress that increases linearly with depth and the lower portions deform by non newtonian temperature dependent creep for extensional deformation a graben model is used in which the primary structure of the rift is formed in the strong mantle layer and secondary faulting occurs in the brittle crustal layer the introduction of a weak zone in the lower crust that acts to decouple these two layers allows the formation of rifts with the large widths and depths observed on venus the compressional folding model utilizes an elastic buckling approach in which the non elastic yielding of the upper and lower portions of the layer modifies the elastic folding wave length this approach avoids the unreasonably high stresses that are required for conventional buckling models the two scales of deformation observed in radar images 15 25 km and 100 300 km can be related to the elastic flexural properties of the crust and mantle respectively which are separated by an intermediate weak zone in the lower crust these results imply that for regions in which both scales of deformation are present the crust is 5 15 km thick and the thermal gradient is 10 15 km l in regions such as ishtar terra where only the smaller features are observed the crust may be considerably thicker the apparent structure seen in high resolution images of the beta regio rift with the major faults located at the edges of an extended linear central depression lends support to the assumption of a graben mechanism of rift formation

the egt bouguer gravity compilation abstract two colour plates showing the bouguer gravity anomalies along the european geotraverse egt were produced by compiling all available bouguer gravity data and where possible by applying a standard density of 2 67 mg m3 for the topographic correction the data points used to create the contours are printed on the reverse side of the maps so that an analyst using a light table can quickly identify artifacts produced by uneven data distribution the data are available on magnetic tape for those interested in working on interpretation along the egt

the dynamics of rifting deformation of the lithosphere by active and passive driving forces summary the thermal and mechanical behaviour of the lithosphere during rifting is investigated using a two layer model for its rheology and different driving forces for the rift process the uppermost crustal layer is assumed to deform by brittle failure while the lower lithosphere including the lower crust is assumed to behave as a newtonian viscous fluid results of numerical modelling of the deformation flow and heat transport in the viscous layer are presented in which viscosity is a strong function of depth and temperature deformation of the upper brittle layer is complex and difficult to quantify thus deformation of this upper layer was approximated by a simple kinematic model in which the layer is thinned by an amount proportional to the amount of extension it experiences two fundamentally different driving forces were investigated they correspond to active rifting in which the litho sphere is thinned by an upwelling limb of a convection cell or mantle plume and to passive rifting or stretching of the lithosphere the results show that for zones of stretching which are wider than the thickness of the viscous layer thinning at all levels in the lithosphere can be described by a single parameter as in earlier stretching models this is not always true when the rift zone is narrow or near the edges of the rift zone initial subsidence due to isostatic and thermal changes during stretching slows as stretching progresses giving an exponential character to the rift stage subsidence active rifting is associated with thermal uplift in this case lithospheric deformation is confined to a narrow zone near the base of the lithosphere and thinning of the crust does not occur until the base of the lithosphere approaches that level the rate of lithospheric thinning and width of the rift zone depend on the convection velocities in the underlying asthenosphere and on the viscosity depth gradient near the lithosphere asthenosphere boundary also contributing to the elevation changes during rifting are dynamic stresses which result from the flow these dynamic affects may significantly modify the uplift or subsidence of the rift zone another important process which results from viscous flow is the generation of secondary convection in the upwelling asthenosphere ths will further thin the lithosphere and can cause depth dependent stretching in which the lower lithosphere is thinned more than the upper lithosphere both of these properties of the flow dynamic stresses and secondary convection critically depend on absolute values of viscosity and so are not well constrained at present the model results presented here exhibit the degree of complexity which results from viscous flow in the lower lithosphere and which is matched by the wide variety of observations on rifting

spatio temporal evolution of strain accumulation derived from multi scale observations of late jurassic rifting in the northern north sea a critical test of models for lithospheric extension abstract we integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern north sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire viking graben and its flanking platform areas these data show how syn rift accommodation within the basin varied in space and time with sub kilometer scale spatial resolution and a temporal resolution of 2 3 myr regional interpretations of 2d seismic reflection refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust these image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio temporal evolution of strain accumulation throughout the lithosphere during the 40 myr 170 130 ma period of late jurassic extension across this basin for the first 25 30 myr strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined as strain rates across the whole basin were consistently very low 16 s l thermally induced strength loss cannot explain this phenomenon the strain localisation is manifest in the near surface by a systematic migration of fault activity the pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control especially when estimates of flexural rigidity for this area are considered the best explanation for what is observed in this time period is a coupling between near surface strain localisation driven by brittle or plastic failure and the evolving thermal structure of the lithosphere we demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate dependence of frictional strength observed in laboratory studies during the final 10 myr of basin formation strain accumulation was focused within the axis and strain rates declined rapidly replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline

dynamic topography in rift zones implications for lithospheric heating extended abstract extension of the lithosphere in rift zones results in heating of the crust and mantle beneath the rift based on simple stretching models mckenzie 1978 the subsidence history of rifts has been attributed to crustal thinning during extension and subsequent conductive cooling of the lithosphere beneath the rift the amount of subsidence caused by cooling should provide an estimate of the amount of heat added to a column of lithosphere as it extends models of this type which assume local isostatic compensation and uniform extension over the whole thickness of the lithosphere underpredict the amount of post rift thermal subsidence royden keen 1980 and do not explain the flanking uplifts associated with rifts predictions of central subsidence from models that do not explain flanking uplifts should be suspect since the same mechanisms producing the uplift may also affect the central subsidence the origin of flanking uplifts may have important implications for the amount and horizontal distribution of lithospheric heating beneath a rift in a study of the gulf of suez rift steckler 1985 proposes that flanking uplifts are isostatically supported by thermal expansion due to heating because horizontal heat conduction occurs too slowly to explain the amplitude and width of flanking uplifts convection due to stretching induced horizontal temperature variations beneath the rift buck 1986 has been suggested as a heat transfer mechanism

finite element models of continental extension abstract numerical models of the initial deformation of extending continental lithosphere have been computed to investigate the control of pre existing thermal and mechanical heterogeneities on the style of deformation the finite element technique was used to calculate deformation with a viscoelastic plastic model for the lithosphere initial heterogeneities tested included a variety of thermal anomalies anomalous fracturing in the upper crust and anomalous crustal thickening deformation is strongly localized by thermal mechanical or compositional heterogeneities the style of initial deformation is controlled by the depth and width of the heterogeneities shallow weakening of the lithosphere causes broad uparching of the lithosphere in the early stages of deformation due to elastic bending stresses caused by non uniform extensional deformation deeper weakening of the lithosphere has the opposite effect the width of the zone of necking is closely controlled by the width of the initial heterogeneity with a minimum width controlled by the layer structure of the model the continuum mechanics formulation of the models prevent the reproduction of fault details in the models but general features of extensional style are indicated thermal heterogeneities are most effective in controlling deformational style broad zones of high strain are predicted by wide thermal anomalies penetrating up to the moho or shallower brittle low strain extension is predicted by models with deeper narrower thermal anomalies deep penetrating low angle faults are not required to reproduce the broad lithospheric deformational styles observed in lithospheric extension

transform margins development controls and petroleum systems an introduction this paper provides an overview of the existing knowledge of transform margins including their dynamic development kinematic development structural architecture and thermal regime together with the factors controlling these this systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock seal rock and reservoir rock distribution expulsion timing trapping style and timing and migration patterns the paper then introduces individual contributions to this volume and their focus supplementary material location table and map of specific transform examples structural elements of the romanche transform margin and glossary of terms used in this article is available at https doi org 10 6084 m9 figshare c 3276407 gold open access this article is published under the terms of the cc by 3 0 license this volume shows dynamic linkage between new transform margin research and increasing transform margin exploration it offers a critical set of predictive tools via an understanding of the mechanisms involved in the development of play concept elements at transform margins and their controlling factors our introduction to this volume reviews the current knowledge of the transform margins focusing on their dynamic development kinematic development structural architecture thermal regimes and petroleum systems the last part of this paper introduces the contributions to this volume

effects of a temperature dependent rheology on large scale continental extension the influence of a temperature dependent rheology on large scale continental extension is investigated using a thin viscous sheet model a vertically averaged rheology is used that is consistent with laboratory experiments on power law creep of olivine and that depends exponentially on temperature the behavior of the model depends principally on two parameters the peclet number which describes the relative rates of advection and diffusion of heat and a dimensionless activation energy which controls the temperature dependence of the rheology numerical calculations of such a thin sheet subjected to an extensional boundary condition show the following results at short times following the beginning of extension deformation occurs with negligible change in temperature so that only small changes in lithospheric strength occur due to attenuation of the lithosphere and maximum rates of deformation are located close to the extensional boundary however after a certain time interval thermal diffusion results in lowered temperatures in the lithosphere strongly increasing lithospheric strength and slowing the rate of extension this critical time depends principally on the peclet number and is short compared with the thermal time constant of the lithosphere the changes in strength cause the locus of high extensional strain rates to shift with time from regions of high strain to regions of low strain results of the calculations are compared with observations from the aegean where maximum extensional strains are found in the south near crete but the largest present day extensional strain rates occur about 300 km further north the comparisons support the hypothesis that the observed separation of the loci of maximum strain and maximum present day strain rates in the aegean may be the consequence of changes in lithospheric strength arising from the temperature dependent mechanical properties of lithospheric materials

quantifying the thermo mechanical impact of plume arrival on continental break up abstract the arrival of a plume head at earth s continental lithosphere is often considered to be an important factor for continental break up however the impact of plume impingement on strength and duration of a rift remains unclear in this study we quantify the mechanical and thermal influence of a plume i e lithosphere erosion on continental break up to do that we apply the three dimensional numerical code slim3d that features realistic elasto visco plastic rheology we model the thermo mechanical response of a segment of earth s lithosphere that is affected both by extension as well as plume related lithosphere erosion in order to evaluate the influence on the overall force budget we find that lithosphere erosion leads to a moderate lithospheric strength reduction of several tn m in a force limited environment however this strength reduction may have strong influence on the timing of continental break up or it may even control whether continental break up takes place at all additional reduction of the lithospheric strength is likely due to the massive emplacement of dikes that follows intensive melting within the plume head

subsurface granites in the franconian basin as the source of enhanced geothermal gradients a key study from gravity and thermal modeling of the bayreuth granite the franconian basin in ne bavaria is a region of gravity low between the bohemian massif in the east and the kraichgau terrane in the west borehole measurements have identified the northern part of the franconian basin as a regional geothermal anomaly and new heat flow calculations give values of 100 mw m2 distinct negative bouguer anomalies observed in this basin are modeled as granitic intrusions in the saxothuringian basement that underlies the permo mesozoic units the interpretation of gravity gradients in combination with the filtering of gravity data gives the possible depth constraints of the intrusive bodies the resulting depths were cross checked using microstructural studies of quartz veins in the basement rocks at 1341 m depth the quartz shows structures typical for low temperature plasticity and we infer a deformation temperature of ca 300 c this indicates a considerable pre permian uplift of at least 7 km for parts of the saxothuringian basement and supports depth estimates from gravity data the heat supply of granitic intrusions by radiogenic decay is modeled considering several scenarios for the geological conditions recovered by the obernsees borehole model for conductive heat transfer the 1390 m deep drillhole is in a marginal position to the most pronounced negative bouguer anomaly it could be shown that the saxothuringian basement including heat producing granites heat production rates 4 6 w m3 covered by the insulating sedimentary rocks 1 35 km of permian to lower jurassic units can account for the enhanced geothermal gradient 38 c km that were measured in the borehole

on the interpretation of geoid anomalies in europe with special regard to the egt profiles abstract we have studied medium wavelength geoid anomalies over europe in particular along three main egt profiles fennolora profile central profile through west germany alpine profile across the alps and the po basin in relation to the lithospheric structure for a general interpretation of the whole european area we used a simple model prescribing topography moho depth undulations and appropriate densities and applying the concept of pratt isostasy with this simple model we could explain most of the larger features in the geoid while in northern europe the crustal thickness is crucial for central and southern europe it is the combination of both topography and moho depth we found a geoid gradient of 4 06 m km for topography and 0 58 m km for moho undulations with a compensation depth of 60 km to be the best to explain the observations however also a compensation depth of 27 km for topography and 80 km for crustal thickness undulations fits the data the negative anomalies over the north sea and the po basin become even more pronounced if crustal effects are considered which means they can not be explained adequately by crustal thickness of constant density but need additional thermal or lithological effects to locally reduce the densities a remaining negative anomaly in fennoscandia of about 4 m might reflect the effect of post glacial return flow but can also be explained by increased crustal density compared to central europe along the fennolora profile the geoid undulations can be explained by the combined effect of the increased thickness of the crust and lithosphere lateral density variations in each layer are not necessary to explain the geoid observation and upper and lower crustal density is high the density decrease at the lithospheric base is small along the central profile the effect of topography together with the low density sediments in the northern german lowland already explains most of the geoid anomaly the existence of a large low velocity body in the middle crust which might be a trapped granitic intrusion is weakly supported by the geoid observation a clear decrease in density at the transition to the asthenosphere fits the geoid observation the short but structurally most complicated alpine profile is difficult to interpret geoid observations can well be explained by the effect of low compensated topography and sedimentary layers this means that isostasy in the alps demands considerable lateral intra crustal density variations and can not be mainly attributed to the moho deflection however isostasy can well be incomplete across the alps two seismically detected crustal bodies penetrating into the upper mantle are best compatible with the geoid observation if their density is around 3 g cm 3 which agrees with an eclogitic composition of these bodies

venus banded terrain tectonic models for band formation and their relationship to lithospheric thermal structure recent radar images of the mountains of ishtar terra venus at approximately 3 km radar resolution show a series of linear bands of alternatively greater and lesser backscatter the bands are generally aligned with the topographic strike of the mountain ranges we test the hypothesis that these band features are tectonic in origin the product either of folding during lithospheric compression or of block faulting during lithospheric extension tested models for folding include uniform elastic and viscous plates overlying inviscid substrates layered plates and a viscous half space in which viscosity decreases exponentially with depth the uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on venus about 15 to 20 km if the elastic or high viscosity layer is at most a few kilometers in thickness though the required compressive stresses are several kilobars in magnitude for the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness and similar results hold layered elastic or viscous plates however can fold at the required wavelength with subkilobar stresses and therefore are favored over uniform plate models tested extensional models include graben and horst formation in a surface elastic brittle layer imbricate normal faulting and necking of a surficial layer that is everywhere in a state of extensional failure the spacing between adjacent extensional features can match the spacing between bands on venus if the surficial brittle layer is no more than a few kilometers in thickness both compressional and extensional models for band formation are therefore consistent with radar images of venus mountain ranges at the image resolution presently available additional evidence however including the linearity and continuity of individual bands the relationship between band trends and topographic contours in some regions near the ends of mountain ranges and the very different manifestations of lithospheric extension in other highland areas of venus leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of ishtar terra as a result of horizontal compression of the venus lithosphere independent arguments on the thermal structure of the venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of venus is approximately 1 10 km thick we therefore suggest that the surficial layer of elastic brittle or high viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the venus highlands at the time of band formation

thermal history of the eastern margin of the gulf of suez i reconstruction from borehole temperature and organic maturity measurements abstract the gulf of suez is a tertiary continental rift associated with prominent flank uplift despite numerous studies which focused mainly on the western and central parts of the graben the thermo mechanics controlling the tectonic evolution of the gulf of suez is still enigmatic we have integrated borehole temperatures and organic maturity measurements in the eastern margin of the gulf of suez in order to study rift related paleothermometry and the present day thermal regime the data obtained suggest that the present thermal regime represents the maximum heat flow and temperatures for the sedimentary section in the basin furthermore lateral distributions of geothermal gradient and heat flow in the gulf of suez do not correlate mainly because of extensive variability in lithology and thermal conductivity rift related heat flow increases systematically and subparallel to the rift axis from about 60 mw m2 in the darag subbasin in the north to about 80 mw m2 in the ras garra area in the south both values are higher than 45 mw m2 the average heat flow assumed for the pre rift stage and the characteristic level for the present day heat flow away from the rift the north to south increase in heat flow probably reflects the southward increase of extension as well as lateral transfer of heat flow from the red sea this latter conclusion is supported by the fact that heat flow in the southern gulf of suez recorded by the paleothermometric reconstructions and borehole temperature data is somewhat higher than that estimated by model calculations for the extension derived from structural and subsidence reconstruction

lithospheric necking a dynamic model for rift morphology abstract rifting is examined in terms of the growth of a necking instability in a lithosphere consisting of a strong plastic or viscous surface layer of uniform strength overlying a weaker viscous substrate in which strength is either uniform or decreases exponentially with depth as the lithosphere extends deformation localizes about a small imposed initial perturbation in the strong layer thickness for a narrow perturbation the resulting surface topography consists of a central depression and uplifted flanks the layer thins beneath the central depression the width of the rift zone is related to the dominant wavelength of the necking instability which in turn is controlled by the layer thickness and the mechanical properties of the lithosphere for an initial thickness perturbation with a width less than the dominant wavelength deformation concentrates into a zone comparable to the dominant wavelength if the initial perturbation is wider than the dominant wavelength then the width of the zone of deformation is controlled by the width of the initial perturbation deformation concentrates in the region of enhanced thinning and develops periodically at the dominant wavelength a surface layer with limiting plastic stress exponent n behavior produces a rift like structure with a width typical of continental rifts for a strong layer thickness consistent with various estimates of the maximum depth of brittle deformation in the continental lithosphere the width of the rift is essentially independent of the layer substrate strength ratio for a power law viscous surface layer n 3 the dominant wavelength varies with layer substrate strength ratio to the one third power and is always larger than for a plastic surface layer of the same thickness the great widths of rift zones on venus may be explained by unstable extension of a strong viscous surface layer

extension of continental lithosphere a model for two scales of basin and range deformation we develop a model for deformation in an extending continental lithosphere that is stratified in density and strength assuming a rheology consistent with seismic focal depths and experimental flow laws the model demonstrates that necking instabilities at two wavelengths will arise due to the presence of a strong upper crust and upper mantle separated by a weak lower crust the magnitudes of the instabilities are directly related to strength contrasts within the lithosphere while the dominant wavelengths of necking are controlled mainly by the thicknesses of the strong layers the results are applied to the basin and range province of the western united states where two scales of deformation can be recognized one corresponding to the spacing of ranges and the other to the width of tilt domains a bouguer gravity anomaly and associated regional topography with a wavelength comparable to the width of tilt domains has also been recognized for plausible density and strength stratifications our results show that the horizontal scale of short wavelength necking is consistent with the spacing of individual basins and ranges while that of the longer wavelength necking is consistent with the width of tilt domains we thus suggest that basin and range deformation may be controlled by two scales of extensional instability extension in the weak lower crust in this model is laterally displaced from regions of upper crustal extension the resultant horizontal shearing in the lower crust may be a mechanism for the initiation of low angle extensional detachments

deep seismic a tool in the recognition and interpretation of large geological elements the starting point for deterministic basin modelling deep seismic became increasingly important for the interpretation of large geologic structures and the discussion of the geologic history of such complexes within the last decades deep reflection seismic work first started in the fifties has reached a high standard which enables the geoscientist to recognize details of the earth s crust down to the moho