the paleogeography of paleozoic and mesozoic storm depositional systems a reply we welcome discussion of our paper on the paleogeography of storm systems but can demonstrate that the major points raised in duke s discussion were considered and discussed already in our original text let s take his points in turn 1 alternative mechanisms for formation of hummocky stratification duke objects to some of the alternative modes of origin which we suggested as possibilities only one of these dealt with stratification within dome dunes readers familiar with mckee s 1979 p 98 102 his figs 57 and 58 work should not experience any difficulty observing similarities between bedding in the upper part of dome dunes and hummocky stratification we did not state that the large angle stratification at the core of the dome dune was hummocky stratification and duke s remarks

fish canyon tuff colorado the problem of two magnetic polarities in a single tuff abstract the fish canyon tuff located in the central san juan mountains southern colorado is currently recognized as one of the largest ash flow tuffs in the world paleomagnetic samples from 21 sites have been obtained from a composite vertical section of the tuff the two uppermost sites from the otherwise normally magnetized tuff exhibit stable but anitpodal reversed remanent magnetic rm directions field relationships and geochemical consistencies indicate that the entire section was emplaced relatively rapidly probably in

the origin of the dead sea rift abstract the dead sea rift is considered to be a plate boundary of the transform type several key questions regarding its structure and evolution are does sea floor spreading activity propagate from the red sea into the dead sea rift did rifting activity start simultaneously along the entire length of the dead sea rift or did it propagate from several centres why did the initial propagation of the red sea into the gulf of suez stop and an opening of the gulf of elat start using crustal structure data from north africa and the eastern mediterranean and approximating the deformation of the lithosphere by a deformation of a multilayer thin sheet that overlies an inviscid half space the regional stress field in this region was calculated using this approach it is possible to take into account variations of lithospheric thickness and the transition from a continental to an oceanic crust by application of a strain dependent visco elastic model of a solid with damage it is possible to describe the process of creation and evolution of narrow zones of strain rate localization corresponding to the high value of the damage parameter i e fault zones mathematical simulation of the plate motion and faulting process suggests that the dead sea rift was created as a result of a simultaneous propagation of two different transforms one propagated from the red sea through the gulf of elat to the north the other transform started at the collision zone in turkey and propagated to the south

pliocene uplift of the eastern iberian margin inferences from quantitative modelling of the valencia trough we present the results of quantitative modelling of subsidence in the valencia trough and associated uplift in the catalan coastal ranges and the balearic islands basement subsidence moho depth and gravity anomalies are calculated for different effective elastic thicknesses eet and durations of rifting along two profiles constrained by recently collected multichannel seismic and gravity data the tectonic evolution of the catalan coastal ranges cannot be explained with traditional two layer lithospheric stretching models for the valencia trough but is consistent with a thermomechanical model invoking lithospheric necking combined with a late stage uplift event the modelling of the topography of the balearic islands suggests an interaction of extension in the valencia trough with extension in the south balearic basin our study supports a rifting model with low values for the flexural rigidity an intermediate level of necking and one finite duration stretching event propagating from the northeast towards the southwestern part of the valencia trough the modelling provides constraints on the magnitude of a large scale pliocene uplift of the eastern margin of the iberian peninsula

thermal effects of normal faulting during rifted basin formation 1 a finite difference model abstract a finite difference model is presented describing the thermal structure of a region undergoing fault controlled extension the model is two dimensional and time dependent we combine temperature changes resulting from conduction advection due to fault movements sediment blanketing and heat production cooling curves can be derived at chosen points in the area this study mainly investigates the effect of the extension rate on the thermal field which turns out to be very important 20 km of extension with extension rates of 0 2 2 0 and 20 mm yr yield maximum deviations in temperatures from the steady slate situation of 2 26 and 84 the sediment blanketing effect can even be reversed by variations in the sedimentation rates fast sedimentation leads to crustal cooling slow sedimentation to crustal heating the turning point value is lithology dependent and corresponds to 2 0 mm yr for shale 2 2 mm yr for clay and 1 4 mm yr for sand a sandstone will cause an increased cooling at both high and low velocities our modelling shows that footwall cooling during or immediately after a rifting event cannot be explained by the downwards movement of the adjacent relatively cold hanging wall consequently in areas where such a cooling is observed other cooling mechanisms must play an important role these can be phenomena like footwall uplift or the fading out of an older thermal anomaly

three dimensional numerical modeling of compressional orogenies thrust geometry and oblique convergence preliminary results from a three dimensional finite element computer model of the dynamics of compressional orogens suggest that the deformation of the brittle crust is the superposition of two modes of deformation a forced mode dictated by preexisting structures or plate boundary geometry and a natural mode resulting from the nonlinear elastoplastic rheology of the crust that may depend on the choice of a particular failure criterion natural modes of deformation exist in both the vertical thrust planes and horizontal shear zones directions and there is a large degree of coupling between them thrust planes accommodate vertical and horizontal shear the obliqueness of convergence affects the geometry of thrusting therefore two dimensional plane strain models may be inappropriate to model orogens in which the direction of compression is oblique to the strike of the orogenic belt

thermal effects of normal faulting during rifted basin formation 2 the lugano val grande normal fault and the role of pre existing thermal anomalies abstract we investigate the thermal consequences of rift related normal faulting and compare the results with a well studied natural example the lugano val grande normal fault southern alps only limited heating of the crust is caused by lithospheric thinning in the simple but realistic situation where heat conduction is substantially faster than heat advection no major thermal disturbance is associated with the downward movement of the hanging wall radiometric ages and fault rocks associated with the lugano val grande normal fault demonstrate that cooling rather than heating affected the crust during normal faulting this pattern is not compatible with such a simple numerical model and is explained by a waning thermal anomaly induced by a magmatic intrusion immediately preceding or overlapping with the first stages of normal faulting the magmatic body must have been emplaced at depths greater than 15 18 km and probably started to cool in the carnian i e few million years before the onset of normal faulting along the lugano val grande fault

stresses in the lithosphere and sedimentary basin formation abstract seven fundamental questions on the relationship of lithospheric stresses and sedimentary basin origin and evolution are posed and elaborated these are 1 1 how is the evolution of sedimentary basins tied to lithospheric dynamics 2 2 what are the sources of stress that lead to formation of sedimentary basins 3 3 how does the interaction between internally and externally applied stresses affect the formation and evolution of sedimentary basins 4 4 how do space and time variations of rheology and pore pressure affect stress transmission strain vertical motions and stratigraphy during basin evolution 5 5 what factors influence spatial and temporal variations of rheology in and around different kinds of sedimentary basins 6 6 what is the role of magmatism in basin formation and what deep seated processes does it reflect 7 7 what is the relationship between basin evolution thermal history and fluid flow these fundamental questions can be investigated by developing fully dynamic models of processes at the basin and sub basin scale models must incorporate or honour basin history and plate reconstructions the intregration of structural and sedimentological data direct observations of the physical state and the synthesis of geological and geophysical data including tectono dynamic stratigraphy petrological and geochemical data and geophysical data gravity magnetics heat flow deep and shallow seismics a number of type basins in passive margin foreland rift and strike slip tectonic settings suitable for modelling studies elucidating the fundamental questions are identified

north sea rift system abstract the mesozoic north sea rift forms an integral part of the arctic north atlantic rift system rifting activity in the north sea commenced during the earliest triassic during the mid jurassic the central north sea underwent a temporary doming stage crustal extension peaked during the late jurassic earliest cretaceous abated subsequently and terminated altogether during the paleocene the saucer shaped cenozoic post rift basin straddles the axis of the viking and central grabens this speaks for the applicability of a depth dependent pure shear model rather than a simple shear model geophysical data indicate an important discrepancy between upper and lower crustal attenuation across the north sea rift suggesting that during rifting processes the mono discontinuity was seriously destabilized this requires a basic modification of currently favoured crustal stretching models

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

density inhomogeneities isostasy and flexural rigidity of the lithosphere in the transcaspian region the 3 d lithospheric density model for the southeastern part of the caspian sea and the transcaspian area practically coinciding with the territory of the turkmen republic has been constructed based on geophysical data and in accordance with the principle of isostasy from the model selected the anomalous density of the subcrustal layer between the moho discontinuity and the 100 km depth level is found to be 100 kg m3 under the tien shan 50 kg m3 under the kopet dag mountain area 80 kg m3 under the central region of the south caspian basin 50 kg m3 under the eastern part of the basin known as the west turkmenian depression and 45 kg m3 under the murgab depression significant disturbances of the local isostasy are determined both in the northern and central areas of the south caspian basin and also in the area of the kara bogaz swell of the turan platform and for the kopet dag foredeep indicating a high level of stresses in the lithosphere the shape of the turan plate determined by the seismic profiling is accounted for by elastic deformation resulting from the forces acting on the southern edge of the plate in the area of the turan plate kopet dag collision the elastic thickness of the turan plate is estimated as 25 5 km the results obtained seem to confirm the idea that the decomposition of the turan plate has taken place in the zone of the plates interaction and the decomposed material is situated under the kopet dag ridge we propose that the kara bogaz swell is supported by the mantle material upwelling whereas the subsidence of the adjacent part of the south caspian basin may be due to the downgoing mantle flow i e a small convection cell is suggested in that area

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

deep explosive focal depths during maar forming magmatic hydrothermal eruption baccano crater central italy we describe the eruptive activity of the pleistocene composite baccano maar crater in the sabatini volcanic complex central italy combining stratigraphy grain size componentry and rare earth element and yttrium rey composition of its eruptive products with the stratigraphy and geothermal data derived from deep wells drilled on the baccano structural high the main lithological characteristics of the basal baccano maar pyroclastic deposit composed of more than 60 wt of non thermometamorphosed lithic clasts from the sedimentary basement show that the first eruption was magmatic hydrothermal in nature the lithology of the sedimentary lithic clasts indicates that the fragmentation level was at a depth of 1 000 to 1 200 m with fragment depth verified by deep well stratigraphy the 15 wt juvenile non vesicular glass components suggest that magma played a minor role in powering the eruption assuming that the high salinity hot hydrothermal fluids 365 1 400 m by the aggressive hydrothermal fluids enriched in acid components hf hcl and h2so4 may have contributed to increased co2 partial pressure that helped to drive the hydrofracturing this could have caused rapid vapour separation and pressure drop allowing the almost simultaneous breaking of the aquifer cover and brecciation of the calcareous units down to 1 000 to 1 200 m depth the relative abundance of calcareous lithics in the basal part of the first baccano eruptive unit representing about the upper 200 m of stratigraphy below the top of the baccano structural high reveals the descent of the piezometric surface during the eruption combining deep well information and maar product stratigraphy using also rey data from maar pyroclastic fragments and the basement rocks we draw an interpretative model for the baccano maar forming eruption concluding that a magmatic hydrothermal eruptions may originate deeper than previously thought and b hydrothermal fluids circulating in limestone aquifers may play an important role in triggering such eruptions

intraplate stresses and dynamical aspects of rifted basins abstract intraplate stresses play a important role during the syn and post rift evolution of sedimentary basins deviations from long term thermal patterns of post rift subsidence as well as the development of unconformities within rifted basins reflect to a large extent the temporal fluctuations of the regional stress fields in the lithosphere it appears that incorporation of intraplate stresses in dynamical models for basin development together with a quantification of the role of petrological changes in the crust lithosphere system can explain some of the observed shortcomings of thermal models of post rift basin subsidence late neogene accelerations in tectonic subsidence in the deeper parts of rifted basins in the northern atlantic and mediterranean regions could reflect a change in the level of intraplate stress over large areas of the globe non thermal contributions to the subsidence record affect estimates of crustal extension and consequently influence the depth at which hydrocarbon source rocks enter the maturity window stress induced changes in basement topography have also important consequences for the budget of fluid flow in the deeper parts of the sediment fill of the rifted basins as well as on the dynamics of diapirism

indication of transpressional tectonics in the gullfaks oil field northern north sea abstract new seismic data have provided important insights into the tectonic evolution of the gullfaks area and the northern north sea extensional tectonics which predominate in the north sea basin were accompanied by subordinate compressional deformation in the gullfaks field involving the formation of thrust and reverse faults and compressional folds the local compression is interpreted as a product of dextral strike slip movement along the ne sw trend of the tampen spur in late jurassic to early cretaceous times in a regional context the strike slip movement may be related to dextral strike slip movements along the more trondelag fault zone further to the northeast

footwall uplift associated with late jurassic normal faulting in the northern north sea tilted fault blocks in the north sea show varying degrees of erosion such footwall erosion is predicted by the domino model of extension in which rigid fault blocks rotate as extension proceeds profiles of three north sea fields snorre brent and heather are analysed to estimate the amount of uplift that accompanied late jurassic normal faulting and these estimates are compared with the footwall uplift predicted by the domino model agreement between observed and predicted uplift is excellent varying from 1 km at snorre to c 100 m at heather for a given amount of extension large fault blocks tend to be bounded by large faults and to show a greater degree of erosion the brent province of the northern north sea is an area of prolific hydrocarbon discoveries almost all of the major oil fields occur in tilted fault blocks which were formed by late jurassic extension and subsequently buried by cretaceous tertiary thermal subsidence badley et al 1988 marsden et al 1990 the crests of the tilted blocks are typically eroded or degraded with varying amounts of middle lower jurassic and upper triassic section removed this footwall erosion is commonly ascribed in the literature to pre rifting doming ziegler 1982 heterogeneous stretching coward 1986 local transpression beach 1985 or even regional compression frost 1987 the purpose of this paper is to show that the uplift and degradation is a direct consequence of the late jurassic normal faulting and fault block rotation the degree of uplift is related in a simple way to the size across strike of

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

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

continental lithosphere folding in central asia part i constraints from geological observations abstract geological observations support the occurrence of long wavelength surface undulations in central asia which can be treated in terms of folding of the whole lithospheric plate the characteristic wavelength of the whole lithosphere folding is 360 km in the western part of the gobi sub plate superimposed on the long wavelength deformation two shorter wavelength scales of spatially periodical deformation are also observed an intermediate wavelength of 30 50 km of the intraplate deformation reflects the response of the mechanically strong upper crustal part of the central asian lithosphere small wavelength 4 9 km deformation corresponds to faulting in the uppermost crustal layer and folding and faulting in the sedimentary cover folding of the lithosphere controls the location and geometry of compressional basin formation in the western part of the gobi sub plate a number of folding and faulting characteristics of continental intraplate deformation in central asia presented in this study are strikingly similar to observed styles of intraplate deformation in oceanic lithosphere in the northeastern indian ocean

relation between salt diapirism and the tectonic history of the sverdrup basin arctic canada the sverdrup basin is a pericratonic sedimentary trough in northern canada containing up to 13 km of carboniferous to tertiary strata the basin formed by late paleozoic continental rifting and was subsequently affected by a series of alternating tectonic settings evaporite diapirs are well exposed at the present erosion level and occur mainly along the basin axis the diapiric source layer consists of about 400 m of anhydrite underlain by salt of unknown stratigraphic thickness deposited during the initial permo carboniferous synrift phase of basin subsidence large salt anhydrite diapirs rose into the sedimentary overburden when the overburden had reached a thickness of several kilometres they grew during a relatively long period of modest horizontal compression from the permo triassic to early cretaceous much smaller tabular anhydrite diapirs were rapidly emplaced during periods of high horizontal compression in the middle cretaceous when large flexural stresses were induced by sedimentary loadi

reply to a discussion of the rio caliente ignimbrite analysis of a compound intraplinian ignimbrite from a major late quaternary mexican eruption by g a mahood the discussion by mahood 1983 concerning my paper on the rio caliente ignimbrites is most welcome even though one would have to admit somewhat tardy and a tittle over vehement in places my work on the rio caliente ignimbrite formed part of a larger study on la primavera from imperial college london during the mid and late 70s and mahood s criticism also concerns other papers and theses published by the group wright 1979 clough 1981 clough et al 1981 walker et al 1981 it is a pity that some of the points raised here could not have been considered in some of mahood s previous papers on la primavera her review paper on the geology and geochemistry of la primavera mahood 1981 would have been an ideal forum but unforhmately in this paper the study carded out by imperial college was ignored this cannot be an oversight because as early as 1976 mahood pers comm was well aware of our work on the volcano which began with a reconnaissance study in 1973

stress evaluation in offshore regions of norway a review has been undertaken of all available information on crustal stresses in norwegian continental shelf and north sea the data have principally come from the results of borehole breakouts supplemented by focal mechanisms a simple model of a soft linear inclusion was developed to explain the rapid changes in stress orientation in the central graben area this example indicates that the stress field is affected locally by the geological or structural environment

neural network analyses of stress induced overpressures in the pannonian basin summary artificial neural networks can learn relationships between sediment characteristics burial depth composition coordinates and thickness of overlying quaternary deposits and overpressures from well data after which they can interpolate and extrapolate to areas and depths not covered by wells we analyse data from the south eastern part of the pannonian basin we use a neural network for analysing fluid overpressures because of the complex interaction of the key variables making it difficult to derive the functional relationships required for a statistical analysis the optimal topology of the network number of hidden layers and neurons is found by minimizing the network s training and testing errors the optimal design of the network resembles the interactions scheme of the key variables the pannonian basin originally formed in an extensional regime has been in a compressive state of stress since late pliocene causing anomalous subsidence patterns numerical forward modelling of compaction driven fluid overpressures shows that due to an increase in the level of compressive interplate stress the fluid overpressures in the deep subbasins have increased substantially since late pliocene giving rise to a very high overpressure up to 45mpa at present the neural network analyses provide an independent estimate of the current amount of overpressuring in this basin complementing the numerical forward modelling results the overpressure profiles obtained by the two modelling approaches are in excellent agreement showing the same magnitude of overpressures a reversal of the overpressure in the deepest parts of the subbasins and a general decrease of the overpressure from sw to ne

sedimentary basins and crustal thickening abstract we consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust well known examples are foreland basins others are ramp basins and buckle basins we have reproduced various styles of compressional basins in experiments properly scaled for gravity a multilayered model lithosphere with brittle and ductile layers floats on a model asthenosphere a computer driven piston provides shortening and thickening synchronous with erosion and sedimentation after a first stage of lithospheric buckling thrust faults appear mainly at inflection points slip on an isolated reverse fault is accompanied by flexure footwall flexure results in a foreland basin and becomes accentuated by sedimentation hangingwall flexure is less marked but may become accentuated by erosion motion on a fault leads to hangingwall collapse at the surface either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin simultaneous slip produces a symmetric ramp basin whereas alternating slip results in a butterfly shaped basin with superposed foredeeps some well developed ramp basins become pushed down until bounding faults meet at the surface and the basin disappears from view at this stage the basin depth is equivalent to 15 km or more slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline between two distinct foredeeps in central asia and in western europe cenozoic crustal thickening is due to continental collision for central asia western china kyrgyzstan uzbekistan tajikistan we have compiled a regional structure contour map on the base of the tertiary as well as 4 regional sections foreland basins and ramp basins are numerous and associated with cenozoic thrusts large basins tarim junggar fergana tajik occur around and between mountain ranges but smaller basins issyk kul naryn occur within them in western europe the alps and pyrenees are surrounded by foreland basins ramp basins or intermediate styles in the andes and its foreland neogene thrusts and compressional basins are due to subduction of oceanic lithosphere in colombia they account for much of the cordillera oriental in nw argentina for the altiplano in west central argentina for the sierras pampeanas compressional basins are also common in other areas of older crustal thickening

radiative heat transfer in lithospheric geodynamics implications for asthenospheric partial melting and basin modelling concepts abstract in geodynamic models describing basin formation by stretching the base of the lithosphere is commonly defined by a fixed temperature an alternative definition is proposed where the base of the lithosphere is determined by the solidus of mantle rocks some consequences of this definition are illustrated in a one dimensional model the model applies the finite difference method and approximates the temperature regime in the upper 400 km of the earth the model uses temperature dependent thermal conductivity and includes effects from heat storage by partial melting in the mantle low velocity layer mlv radiative heat transfer contributes significantly at elevated temperatures giving rise to a steepening of the thermal gradient below the lithosphere this change in gradient allows the perturbed geotherm to exceed the mantle solidus in a limited depth interval which accounts for the mlv and thus defines the base of the lithosphere in the model variations in thickness of and degree of partial melting in the mlv results from thinning of the overlying lithospheric plate thus the thickness of the mlv is roughly inversely proportional to the thickness of the lithosphere whereas the base of the mlv is almost flat temperature dependent variations in the carrying capacity of the mlv where the main isostatic compensation occurs is proposed as an epeirogenetic agent this effect may be significant and is expressed as an enhancement of the initial subsidence if no preexisting mlv is available a secondary effect occurs as solidification of the mlv ceases a long time 300 ma after stretching and causes slow uplift

lithospheric stretching and north sea jurassic clastic sourcelands uniform lithospheric stretching cannot explain three important aspects of north sea basin evolution namely 1 the rapid growth of middle jurassic uplands from which were liberated major fluvial deltaic clastic wedges now forming the main hydrocarbon reservoirs 2 the occurrence of basin centre upwarp and basaltic volcanism and 3 widespread and significant 1 km subsidence outside the area of demonstrable crustal thinning i present here the suggestion that a combination of nonuniform stretching and lithospheric flexuring accounts for these fine tuning effects of the uniform stretching model with magnitudes close to those demanded by independent geological evidence

introduction to special issue on integrated basin studies ibs a european commission dgxii project this issue contains papers issued from the integrated basin studies project ibs which is part of a larger program of the european commission dgxii entitled geosciences ii the ibs project ec contract jou2 ct92 0110 originated in geosciences consultation meetings the first of them being held in strasbourg in june 1989 under the auspices of mr hubert curien former french minister for research and technology and the subsequent meeting being held in brussels these consultation meetings have resulted in the conclusion that a main axis of future research in geosciences should be the study of sedimentary basins upstream of industrial activities mainly the oil and gas industry but also water resources management the mining industry the storage of undesirable products and coastal engineering they also resulted in the conclusion that future research on sedimentary basins should fulfill two requirements integration of disciplines and modelling such a conclusion may now seem trivial but was not so generally accepted at that time in this spirit the ibs project was designed to provide the oil and gas industry with a new generation of models for basin formation basin evolution and basin fill architecture from basin scale to reservoir scale its main objectives are the linkage of crust thermomechanical behaviour to basin formation and deformation mechanics and the corresponding modelling the quantification of tectonic control on the sedimentary record and the analysis of compaction and mass and heat transfer the ibs strategy is to link subsurface data to field data and an important part of the project is devoted to thematic field studies in european sedimentary basins of different tectonic styles taken as natural laboratories basin research has gone through a rapid evolution during the last decade in particular through the development of new acquisition and processing methods for seismic data and advances in drilling technology modelling provides an important tool to analyse different aspects of basin formation processes and find their tectonic expression in the basin fill the need for 3 d modelling techniques and models capable of linking processes operating on basinwide to subbasin scales is increasingly recognized and a significant effort has been made in the ibs project to develop a new generation of basin formation models coping with these needs see also cloetingh et al 1993a van wees and cloetingh 1994 as many data required to test these models reside in industrial companies ibs has promoted a closer link between academic research and basin studies carried out by industry the participation of the petroleum industry has been vital in this project and ibs has been able to establish a well functioning system of cooperation that not only facilitated data access but most important guaranteed the suitability of the end products of the project this has led to an intensive exchange of modelling concepts and data sets between industry and academia the ibs project has also used extensively the international lithosphere program task force origin of sedimentary basins see cloetingh sassi and task force team 1994 cloetingh et al 1993a b 1994 to implement planning and to discuss preliminary results it also builds partly on joint research through the european commission tempus and peco programs on pannonian basin studies horvath 1993 van balen and cloetingh 1995 van balen et al 1995 horvath and cloetingh 1995 a stimulating aspect of the activity of the ibs group is the creation of a research network in the corresponding field of knowledge which comprises now 30 teams belonging to eight european countries england france germany hungary norway spain switzerland and the netherlands this demonstrates the capacity of the procedures used by the dgxii of the european community to formulate a really european research space phd students and young researchers form an important component of the research groups

the neogene evolution of the outer carpathian flysch units poland ukraine and romania kinematics of a foreland fold and thrust belt system abstract various techniques of geometric and kinematic analyses including cross section balancing and forward modeling have been applied successfully to the outer carpathian flysch units along regional geologic profiles crossing the belt from its undeformed foreland to the pieniny klippen belt western carpathians poland or to the central east carpathian allochthon eastern carpathians ukraine and romania the amounts of neogene shortening as well as the mode and the location of basement consumption are discussed on the basis of balanced cross sections compiled along the three most representative regional traverses in addition the complete kinematic evolution of complex carpathian structures such as the fore dukla triangle zone southeastern poland the borislav pokut duplexes ukraine and the marginal folds romania are described in more detail by using a forward modeling approach major results concern the tectonic heritage of laramian inverted structures mesozoic normal faults reactivated as reverse faults during the late cretaceous paleocene compression or transpression in the development of the neogene outer carpathian edifice pre eocene deformation and erosion effectively induce discontinuities in the mid cretaceous blackshale detachment horizon and help to localize the neogene deformations in complex triangle zones the thrust sequence itself evolves alternatively in a piggyback or in an out of sequence mode clearly the critical taper of the outer carpathians is preserved by a constant balance between erosion and thrust reactivations in the inner part of the wedge and sedimentation and tectonic accretion at its front

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

modelling of piggyback basin stratigraphy record of tectonic evolution abstract the tectono stratigraphic evolution of foreland basins is influenced by processes acting on local regional and global scale in this paper the effects of these processes on the basin evolution are investigated using a forward reconstruction model based on the principles of balanced cross section the modelling incorporates the consequences of thrust deformation sedimentation erosion basement subsidence and sealevel changes on the basin stratigraphy basement subsidence due to lithospheric loading is the main cause of foreland basin formation but appears to have minor implications for the occurrence of angular unconformities in the stratigraphie record of piggyback basins thrusting is probably the dominant factor in the control of the stratigraphie evolution of piggyback basins overprinting the effect of eustasy detailed analyses of stratigraphie data enable the reconstruction of the structural evolution of the underlying thrust system modelling of the northern apenninic po basin shows that neogene stratigraphie sequences of the proximal part of the basin can be explained by tectonic activity of the apenninic fold and thrust belt strong basement subsidence accompanied with deep seated mid pliocene thrusting controls the stratigraphie record of the po piggyback basins

seismic stratigraphy and tectonics of the northwestern weddell sea antarctica inferred from marine geophysical surveys abstract marine geophysical studies of the northwestern weddell sea show several interesting stratigraphic and tectonic features synthesis of the odp drilling results on the south orkney microcontinent with the circum antarctic depositional model allows identification of the main reflectors within the cretaceous cenozoic sedimentary cover the age and composition of pre rift syn rift and post rift units probably vary throughout the region the observed seismic pattern of the jane bank suggests a young asymmetric zone of extension a seismic record across the south orkney trough and central scotia basin indicates an association with an accretion zone and spreading centre

the origin of sedimentary basins a status report from the task force of the international lithosphere program abstract better insights into the mechanisms of basin formation have a direct bearing on the predictive power of forward models for basin fill models that link flexure normal faulting and thrusting on a subbasin scale provide important constraints on the tectonic control of the sedimentary record enabling a quantitative comparsion with eustatic contributions to the basin architecture thus of special importance is the need to link processes with different spatial and temporal scales involved in the evolution of extensional and compressional basins by numerical modelling a new generation of basin modelling concepts and techniques is one of the main objectives of the task force origin of sedimentary basins of the international lithosphere program the focus of this work is on the quantification of the effects of the thermomechanical structure of the lithosphere and its interplay with regional and local stress changes other important developments include the step towards three dimensional modelling and the full integration of rapidly expanding fission track and other data sets to constrain vertical motions in basins as natural laboratories for the interactive testing and development of new basin modelling techniques a number of basins with high quality data from industry government and academia have been selected on the basis of specific testable problems

dimensions of the late cretaceous paleocene northeast atlantic rift derived from cenozoic subsidence abstract the distribution of cenozoic subsidence across northeast atlantic volcanic margins have been evaluated to define the width of the rift zone and magnitude of extensional deformation the subsidence profiles are corrected for the effects of lower crustal magmatic bodies emplaced during continental break up the dimensions of the bodies have been derived from the crustal velocity structure the width of the late cretaceous paleocene northeast atlantic rift zone was more than 300 km and the lithospheric extension factor increases gradually towards the line of continental separation a large number of high quality seismic reflection data tied to scientific and commercial wells reveals that the initiation of extensional deformation preceded continental separation by 18 m y on the voring margin off norway these results show that the northeast atlantic volcanic margins commonly considered as typical volcanic margins indeed have similar dimensions as non volcanic margins and as continental rifts thus these margins contrast significantly with previously suggested evolutionary models based on narrow rift zones and formation during rapid lithospheric failure the wide rift is compatible with volume of igneous rocks observed along these margins and with a thermal anomaly similar to that associated with production of northeast atlantic oceanic lithosphere

large wave forms from the fish canyon tuff colorado wave like bedforms of large amplitude and wavelength 6 m 60 m occur in an exposure at the base of the fish canyon tuff colorado an ignimbrite ash flow tuff of great volume 3 000 km 3 the bedforms are located some 30 40 km from the center of the source caldera they are interpreted to be deposits from a series of large pyroclastic surges precursors to the main pyroclastic flows the pyroclastic surge beds have bigger wave forms than those previously recognized such megawaves were perhaps produced by violent explosions related to major unroofing of a magma chamber the depositional sequence of this great ignimbrite forming event is similar to that of much smaller eruptions and implies a similar eruptive sequence

anorogenic granites magmatic underplating and the origin of intracratonic basins in a non extensional setting abstract we present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating a case study of a deeply eroded continental rift belt in south finland shows close association of anorogenic intrusives and remnants of cratonic basins lower crustal thinning took place during anorogenic magmatism whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies combined analyses of field data petrological constraints and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning this model involves the formation of an incipient crustal dome by asthenospheric upwelling associated with basaltic underplating and migration of induced lower crustal partial melts to upper crustal levels in this model light lower crustal diorite is replaced by a denser mafic layer the consequences for the post magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification subsidence occurs in the absence of crustal extension suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin subsidence curves for this model show an incipient nearly linear fast downwarp related to crystallization of gabbro at the moho followed by slow contraction induced by solid state cooling the predicted subsidence is similar to the pattern of the vertical movement induced by passive extension in the proposed model of continental rifting the effects of extension and underplating largely cancel out concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion the model provides a consistent explanation for a number of key aspects of cratonic basins including their circular shape crustal configuration and evolution and their spatial distribution on continents

lithosphere rheology and sedimentary basins introduction the dynamics of sedimentary basins is re flected in the wide range of observational scales inside sedimentary basins the advent of high qu ality data sets have significantly improved the resolution of the record of near surface tectonic processes crustal thinning mechanical and thermal sub sidence sediment filling are involved in exten sional basin evolution and controlled by complex interactions between ductile and brittle modes of deformation of the whole lithosphere the rate of loading extension and sediment filling and the thermal evolution are among the most important controlling parameters these nonlinear interac tions do possess a memory of the evolution his tory and loading quantitative understanding of basin subsidence and the offlap onlap strati graphic patterns of a specific basin obviously benefits from progress in understanding the geo dynamical context horvlth 1993 this issue in compressional settings foreland basins re sult from the flexural lithospheric response to loading most of the models proposed so far have considered the lithosphere as a rheological unit and have been based on classical elastic and viscoelastic plate theory the wavelength and am plitude of the flexural response is not an intrinsic property of the lithosphere but result from the history of the competitive interactions between rate of loading and rate of viscous dissipation thermally activated a clear physical description of this flexural mode has not yet been proposed even though some recent results on the mecha nisms of uplift shoulder formation during rifting evolution may be considered bassi 1991 chery et al 1992 the geodynamical context of those basins is complex and the relationship between the transition of extensional and compressional domains is a topic of vigorous research current models for extensional and compressional basins are strongly based on a bulk rheological descrip tion of the whole lithosphere and on a kinematic description of the deformation mechanisms sawyer 1985 houseman and england 1986 quinlan et al 1993 this issue the basin record does reveal to some extend the memory of the deformation history starting from the construction of the internal structure of the basins i e from the processes leading to the formation of rocks sedimentation diagenesis compaction including possible buildup of fluid overpressures and the pattern of the large de formations by faulting or folding to enhance the understanding of the formation and evolution of sedimentary basins it is important to fully address the physical mechanisms operating at different scale in the continental lithosphere for example faulting forms a most important expression at least at the surface of large scale deformation in the basins how those basins are related to the faulting process and the fault interactions is an unsolved question pull apart basins are certainly

numerical modelling of salt diapirism influence of the tectonic regime abstract in the context of sedimentary basin mechanics salt diapirism has usually been described as a phenomenon initiated and driven by gravity instabilities associated with buoyant forces or differential loading this result holds essentially for an approximation of linear rheologies of both salt and overburden this has been contested by recent analog models and analytical studies buoyancy alone cannot drive salt diapirism through a brittle overburden regional extension or compression could be a necessary mechanism likely to promote salt diapirism numerical models are presented here to describe the relative effects of gravity and lateral forces on the dynamics of salt diapirism the numerical scheme is based on a viscoplastic approximation of deformations and an updated lagrangian description of kinematics which allows a study of large non linear deformations of salt bodies and the evolution of material interfaces this scheme takes into account brittle and ductile deformations of sediments moreover it describes the initiation and evolution of shear bands which play a crucial role in basin mechanics and especially in the dynamics of salt diapirism we show why extension is the most propitious driving mechanism of diapirism necking and faulting induced by extension promote and drive salt diapirism in the case of a brittle overburden this result holds with or without buoyancy forces this demonstrates that buoyancy which alone is not sufficient to break a brittle overburden is not a necessary mechanism either buoyancy however enhances diapirism induced by lateral forces in contrast with extension salt diapirism during compression is strongly constrained by the rheology of the overburden a brittle mode of deformation favours faulting rather than buckling induced thrusting and thickening of the overburden inhibit massive diapirism and favour the confinement of small salt plugs finally the post extension stability of a mature diapir produced by extension is presented

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

finite element modelling of pull apart basin formation abstract we present the results of a finite element modelling study of pull apart basin formation related to left stepping left lateral strike slip faults the modelling quantifies the relationship between fault geometry i e fault overlap and separation and pull apart basin formation two depocentres subbasins separated by a broad zone of relative uplift in between may develop if the strike slip domain is characterized by fault underlap for overlapping faults migration of the subbasins is predicted by the models deep subbasins in a large area of subsidence which spans the entire inner fault zone may form if fault overlap is about three times the fault separation the models suggest that a topographic asymmetry within the fault zone may arise due to a different displacement ratio of the strike slip faults the modelling results show that this asymmetry in topography becomes more pronounced towards the more active fault thus basin deepening occurs progressively towards the fault characterized by the largest amount of lateral displacement moreover the results indicate that the smaller the fault separation less than basin length the less pronounced the topographic asymmetry the models provide quantitative estimates for the effects of changes in elastic material properties the magnitude of the compressive far field stress and the coefficient of friction of the faults on the resulting topography comparison of the modelling results with field observations from the cerro blanco el barranquete cbb subbasin located in the internal zone of the betic cordillera southeastern spain support an interpretation in which the interplay of major faults has formed the cbb subbasin

relationship between deeper lithospheric processes and near surface tectonics of sedimentary basins interactions between upper lithosphere deformation as manifested in the structure and evolution of sedimentary basins and lower lithospheric deformation presumably reflect large scale regional or global tectonic events we review the current state of knowledge regarding these interactions and suggest areas of research which may lead to an improvement in this knowledge understanding the development of sedimentary basins requires insight into the processes responsible both for basin subsidence and also for distribution and preservation of sediments within the basins while some classes of basins are better understood than others there remain significant gaps both in our ability to account for available data and also in the data available to test current models and develop new ones subsidence in some classes of basins can be related to lithospheric scale processes occurring as part of the overall plate tectonic paradigm for example it is widely accepted that passive margin basins subside as a result of lithospheric extension mckenzie 1978 and that foreland basins are flexural consequences of collision between a continent and outboard terranes beaumont 1981 on the other hand models for the origin of intracratonic forearc and trans tensional basins are essentially still at cartoon level involving processes that are difficult to quantify and evaluate in the following we review a number of fundamental questions of vital interest for understanding of the relationship between deeper lithospheric processes and near surface tectonics

jurassic extension estimates for the north sea triple junction from flexural backstripping implications for decompression melting models abstract the rattray volcanics lie at the triple junction between the three arms of the north sea rift system the mildly undersaturated volcanics in this province were erupted during the early syn rift phase of a middle late jurassic rifting event and are thought to be the products of decompression induced partial melting of the mantle resulting from lithospheric attenuation during rifting situated as they are in one of the most intensively studied rift basins in the world the rattray volcanics provide an excellent opportunity to evaluate the relationship between extension and melting described by mckenzie and bickle 1 this study uses a modified version of the backstripping technique which takes into account the flexural strength of the lithosphere to constrain the stretching factor for the jurassic rift phase two dimensional interpreted sections are flexurally backstripped to the base post rift to give an initial estimate of is then further constrained by forward modelling to the base post rift basin geometry generated by backstripping forward modelling uses the flexural cantilever model a combined simple shear pure shear model for continental extension which takes into account the thermal rheological and flexural isostatic consequences of extension this is an approach that is different from that of previous studies 2 5 which assume local rather than regional isostatic compensation to allow the backstripping of individual wells the results obtained in this new study give maximum jurassic factors on each of the three rift arms close to the triple junction of 1 2 1 35 this suggests a maximum jurassic factor in the triple junction itself of 1 35 1 75 this is considerably lower than previous estimates and is due to the breakdown of the assumption used in these previous studies that airy isostasy can approximate the response of a low flexural rigidity lithosphere in subsidence analysis whilst such a factor is insufficient to generate melt by decompression alone an anomalously high mantle potential temperature and the presence of volatiles as well as a complex mesozoic and late palaeozoic rifting history are likely to have played a role in the generation of melt

some remarks on lithospheric forces and decompression magmatism abstract during lithospheric extension the upwelling mantle melts if it crosses the solidus to model this process we have used a dynamic model where strain rate varies with time as a function of a constant tensile force for forces larger than 6 12 tn m there is after a protracted period with a nearly linear increase in the stretching factor a short period with rapidly increasing strain rates and adiabatic conditions are effectively realized the stretching factor at which this is achieved is about 1 1 1 3 increasing with decreasing force the model could explain the observation of late stage voluminous magmatism in some rifts and at divergent margins that have evolved for several tens of millions of years

experimentation in planetary geology the reasons for conducting experimentation are as varied as the topics that are studied whether a specific parameter must be quantified or an attempt is being made to duplicate a peculiar process all experiments are unified by a common ultimate objective to provide a tangible link between theory and observation although a suitable theory is often nonexistent at the time of the experiment experimentation in planetary geology assumes particular importance because of the factors inherent in the field of study itself often enormous spatial scales timescales rivaled only by those of cosmology and the remoteness of the objects of study in addition any geoscientist who has entertained thoughts of developing a realistic mathematical model of even the simplest of processes rapidly becomes acquainted with the intimidating complexity of geological phenomena experimentation is still in many cases the only effective means of learning more about some natural mechanisms

continental rift development in precambrian and phanerozoic europe europrobe and the dnieper donets rift and polish trough basins abstract upper crustal structure and lower crustal and mantle character of the european continental lithosphere differ significantly from west to east the processes of rifting of phanerozoic western europe s hotter thinner crust compared with the more stable thicker 45 vs 30 km proterozoic crust of the east european craton is being investigated by europrobe case studies of western and eastern european sedimentary basins the dnieper donets basin transects the southwestern part of the east european platform in a nw se direction lying between the ukrainian shield and the voronezh massif rifting took place from early frasnian until perhaps the late visean and was accompanied by major volcanic activity the distribution of both was affected by pre existing basement fault systems the syn and post rift sedimentary succession may be as thick as 20 km in the donets trough basin evolution and the present crustal geometry have been much affected by inversion during the permian in association with uralian orogenesis tectonic events of bathonian oxfordian 168 154 ma and turonian santonian 91 84 ma age are also recorded in the basin stratigraphy the polish trough lies along the boundary between the phanerozoic and proterozoic european crustal domains coincident with the trans european suture zone the presence of this rheological boundary may be paramount in structurally controlling the position of the trough tectonic subsidence analysis indicates an initial late rotliegendes early triassic syn rift phase of development subsequently episodes of increased tectonic subsidence rate occurred during the oxfordian tithonian 157 146 ma and beginning in the cenomanian 95 90 ma the oxfordian tithonian episode is likely the signature of a second extensional event correlated with intensification of rifting within the arctic north atlantic rift system while the cenomanian and later is a precursor to mild compressional intraplate orogenic movements present day crustal structure reflects the docking of phanerozoic crust against the proterozoic during palaeozoic orogenies permo carboniferous wrenching and transtensional modifications leading to formation of the polish trough as well as modifications during the late cretaceous early tertiary inversion process

palaeozoic structural development along the tornquist zone kattegat area denmark abstract the interpretation of newly released commercial 2d reflection seismic data in the kattegat area denmark has provided us with a better understanding of the palaeozoic tectonic processes along the tornquist fault zone a base palaeozoic time structure map a lower palaeozoic twt isopach map a true lower palaeozoic twt isopach map an upper carboniferous lower permian syn rift twt isopach map a top pre zechstein time structure map and a zechstein combined twt isopach and palaeogeography map have been generated the uniform lower palaeozoic sequence thickness in the kattegat both inside and outside the tornquist zone indicates only minor lateral movements if any whereas the extensive upper silurian sequence increasing in thickness to the north indicates a relatively fast regional subsidence the base palaeozoic time structure map and the late palaeozoic syn rift isopach map show a clear late palaeozoic extension in the area the syn rift isopach map in combination with the time equivalent opening of the skagerrak graben at right angles to the tornquist zone in the kattegat indicates that this extensional tectonic event had a dextral slip component measurements on internal extensional faults in the tornquist zone give a minimum right lateral displacement of 10 4 km the footwall blocks were deeply eroded during the early permian rifting and at zechstein times the area became a peneplane the tornquist zone was later exposed to several tectonic phases where dextral slip played a role indicated by the push up and pull down structures caused by restraining and releasing bends of the borglum fault the dextral displacement along the borglum fault since the beginning of the permian is in the order of 5 7 km based on the displacement of a lower palaeozoic local depocentre early permian depocentres and faults which gives a total amount of right lateral displacement since the early palaeozoic in the order of 15 20 km the continuously repeated tectonic episodes along the tornquist zone throughout most of the phanerozoic show that the zone was easily reactivated implying deep seated basement faults the tornquist zone can be seen as a buffer zone between continental blocks whenever changes in the regional stress field are induced

on the ability of pyroclastic flows to scale topographic obstacles we present a simple analysis of momentum dissipation in a pyroclastic flow with high initial velocity and show that the deceleration is inversely proportional to the volumetric flow rate consequently for a given mass flow rate dense flows slow down more rapidly than dilute flows the analysis ignores density stratification and flow unsteadiness and assumes that deceleration is due to turbulent dissipation alone but the results have implications for a wide range of flow regimes we apply our analysis to the distribution of the 1800 year old deposits of the taupo pyroclastic flow the velocity of which is constrained by the height of topographic obstacles that it scaled the volumetric flow rate required to maintain velocities high enough to overtop the rugged topography is consistent with a dilute flow but strongly argues against the dense flow hypothesis the same conclusion applies to other pyroclastic flow deposits distributed over rugged terrain

crustal thinning and nature of extension in the northern north sea from deep seismic reflection profiling a regional network of deep seismic reflection profiles recorded in the northern north sea has been used to map mesozoic and cenozoic basin thickness and crustal thickness in the viking graben and adjacent platform areas depth to the reflection moho varies from about 20 km in parts of the viking graben to about 32 km beneath the shetlands and the norwegian margin the shallowing of the reflection moho beneath the viking graben implies crustal stretching factors for mesozoic extension greater than 2 in the center of the north sea basin the axis of crustal thinning is located directly beneath the viking graben the axis of the sedimentary basin steeply dipping basin bounding faults are imaged in the upper crust and several dipping reflectors are observed in the upper mantle but no continuous reflective feature extending from the sedimentary basin to beneath the moho is observed on any of the deep profiles thus these data do not support the existence of lithosphere penetrating low angle detachments zones of simple shear as the cause of extension in the northern north sea some of the mantle reflectors dip to the west and some to the east suggesting that simple shear if it occurs in the upper mantle is not of uniform sense rather these data suggest a complex depth dependent pattern of brittle extensional deformation in the upper crust pervasive ductile extension bulk pure shear in the lower crust which is decoupled from deformation in the mantle and extension accommodated by discrete dipping shear zones in the lithospheric mantle

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

a finite difference technique to incorporate spatial variations in rigidity and planar faults into 3 d models for lithospheric flexure summary we present a finite difference formulation for 3 d elastic flexure of the lithosphere which is solved by a direct matrix method to incorporate the effect of spatial variations in rigidity additional terms for the bi harmonic 3 d flexure equation have been derived from a variational displacement formulation as used in finite element methods additionally planar faults are treated as discontinuities these are implemented by an additional degree of freedom for fault heave and a coupled continuum equation for zero differential tilting across the fault the 3 d finite difference results have been tested for line loads point loads and disc loads by analytical solutions and for spatial variation in effective elastic thickness eet by 2 d finite difference solutions fault related flexure patterns are compared to the 2 d analytical broken plate model developed by vening meinesz 1950 we subsequently apply the 3 d fault model to investigate fault controlled 3 d basement geometries in lake tanganyika east africa we show that our model is capable of predicting 3 d basement geometries characteristically observed in rifted basins the modelling results indicate that fault controlled upper crustal flexure patterns are associated with low values for eet a comparison with regional scale model studies showing a superposition of high eet flexure effects supports a multilayered rheological control on continental rifting

destruction of rocks by low velocity impact and its implications for accretion and fragmentation processes of planetesimals recent experimental results on impact destruction of rocks basalt and tuff are summarized the mode of destruction and some empirical relations between the mass of the largest fragment ml the released kinetic energy divided by target mass ei and the work done during impact w are presented the mass ml can be related to both ei and w log ml mt a log ei and log ml mt b log w an effect due to the difference in impact velocity on the ml versus ei relation can be observed but there seems to be no effect due to impact velocity on the ml versus w relation thus it may be suggested that w is a useful means of describing phenomena associated with the impact destruction of rocks target shape also affects the ml versusei relation but no large differences have been observed between various rocks the size distribution of fragments can be adequately expressed in terms of an inverse power law relation in which slope of the size distribution i e exponent increases with increasing ei 0 90 0 36 log ei for basalt and 1 94 0 24 log ei for tuff based on the empirical ei relations a fragmentation model of the parent bodies of eos and koronis families is presented it is suggested that the parent bodies of eos and koronis families had been differentiated prior to fragmentation and were completely fragmented at ei 2 104 joule kg and 2 103 joule kg

architecture and early evolution of the oslo rift abstract a revised assessment of architecture and pre rift fabric connections of the oslo rift has been undertaken and linked to a new appraisal of observations and data related to the initial phase of the rift evolution in addition to half graben segmentation accommodation zones and transfer faults are readily identified in the linking sectors between the two main grabens and between graben segments axial flexures are proposed between facing half grabens the accommodation zones were generally sites of volcanism during rifting pre rift tectonic structures played an influential role in the rift location and development the deviant n s axis of the vestfold graben segment is viewed as related to pre rift structural control through faults and shear zones this area was probably a site of proterozoic palaeozoic crustal and lithospheric attenuation field evidence suggests that the rift started as a crustal sag with no apparent surface faulting in a flat and low lying land at a time about 305 310 ma volcanism sub surface sill intrusion and faulting started about simultaneously some time after the initial sag 300 305 ma faulting and basaltic volcanism were initially localized to transfer faults along accommodation zones and a nnw sse transtensional zone along the eastern margin of the incipient vestfold graben segment this transtensional zone was probably created by right lateral simple shear tracing pre rift structures in response to a regional stress field with the tensional axis normal and the maximum compressional axis parallel to the nne ssw trending rift axis

european cenozoic rift system abstract the european cenozoic rift system extends from the coast of the north sea to the mediterranean over a distance of some 1100 km it finds its southern prolongation in the valencia trough and a plio pleistocene volcanic chain crossing the atlas ranges development of this mega rift was paralleled by orogenic activity in the alps and pyrenees major rift domes accompanied by subsidence reversal of their axial grabens developed 20 40 ma after beginning of rifting uplift of the rhenish shield is related to progressive thermal lithospheric thinning the vosges black forest and the massif central domes are probably underlain by asthenoliths emplaced at the crust mantle boundary evolution of this rift system is thought to be governed by the interaction of the eurasian and african plates and by early phases of a plate boundary reorganization that may lead to the break up of the present continent assembly

consequences of thrusting and intraplate stress fluctuations for vertical motions in foreland basins and peripheral areas summary we present a conceptual model for short term and long term alternations in subsidence and uplift in foreland basins convergence of lithospheric plates produces forces at the plate boundaries generating stresses in the accretionary wedge the underthrusted crust and the lithosphere when the stresses reach the shear strength of the rocks in these units deformation which results in the redistribution of mass takes place contemporaneously with reduction of the stress level modelling of both the effects of this alternating building up and relaxation of stress and the effects of the resulting mass redistribution on foreland basin geometry employing an elastic rheology of the lithosphere demonstrates that these processes exert an important control on the shape of foreland basins vertical motions take place on different time scales dependent on the scale of thrusting and the style of deformation short term 10 104 yr vertical motions are related to small scale thrust events while long term vertical motions are induced by larger scale deformation in the overriding thrust wedge furthermore the geometry of faults influences the duration of the magnitude of stress fluctuations stress induced alternations of subsidence may lead to the deposition of alternating sequences in foreland basins on scales of centimetres e g cyclothems to scales of several hundreds of metres on lap and off lap sequences in this respect the mechanical coupling between convergent zones and peripheral areas is of particular importance as it induces simultaneous alternations in regional facies patterns the model also provides an explanation for vertical motions in the foreland basin previously attributed to visco elastic relaxation processes

basal layered deposits of the peach springs tuff northwestern arizona usa basal layered deposits of the large volume peach springs tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern arizona usa the basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption the pyroclastic surges traveled a minimum of 100 km from the as yet unknown source changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges some bed forms in the most proximal part of the study area kingman arizona can be interpreted as being shock induced reflecting a blast origin for the surges component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges the eruption apparently began with magmatic blasts which were replaced by hydrovolcanic blasts hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent a single large volume pyroclastic flow immediately followed the blast phase and no evidence has been observed for a plinian eruption column the stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent thus bypassing a plinian fallout phase and causing rapid evolution to a collapsing eruption column similar processes may occur in other large volume ignimbrite eruptions which commonly lack significant plinian fallout deposits

lithospheric necking and regional isostasy at extensional basins 2 stress induced vertical motions and relative sea level changes we investigate long wavelength differential vertical motions at extensional basins which are the result of temporal changes in the intraplate stress field in the lithosphere these stress induced vertical motions are intimately coupled to the state of flexure flexural curvature of the lithosphere we compare a previous model of flexure under the sediment load only fsl model and a recent more consistent model which incorporates regional compensation of other vertical loads induced by lithospheric extension as well these additional loads are controlled by the depth at which the necking of the lithosphere occurs don model tlie fsl model and models assuming a shallow level of necking 15 km are very similar they predict tilting of the basin with the hinge point landward of the shelf edge compression results in uplift of the landward pan of the basin which generally corresponds to the coastal plain or inner to middle shelf and downwarping of the outer shelf and slope the differential vertical motions correspond to relative sea level cycles which are out of phase in different parts of the basin internal architecture of systems tracts can be different from predictions by eustasy by contrast deeper levels of necking 20 km predict a uniform sense of motion landward of the shelf edge type i sequence boundaries can be very easily induced for these conditions and sequence stratigraphie features are probably hard to distinguish from predictions by eustasy these results are for a pure elastic plate rheology stress induced vertical motions for a more realistic depth dependent rheology are more complicated because of the additional effect of stress induced reduction of flexural rigidity for these conditions gradual buildup of tension causes a complete relative sea level cycle for high tensional stress levels total vertical motions become rather similar to vertical motions for compression this may be of importance for the issue of global synchroneity of relative sea level changes

stress distribution over the mozambique ridge abstract the mozambique ridge is an aseismic oceanic plateau in the southwestern indian ocean during the separation of antarctica and south africa in the early cretaceous the mozambique ridge was segmented by fracture zones which were assumed to become inactive during the cenomanian when africa and antarctica were finally separated however the existence of active normal faulting in the central part of the mozambique ridge was demonstrated by single and multichannel seismic surveys numerical modelling of the stress distribution caused by the crustal structure of the mozambique ridge and the adjacent oceanic basins suggests the possible existence of a zone with average horizontal tension up to 70 mpa along the central part of this passive ridge which may cause the modern fault activity these stresses also cause an additional dynamic anomaly which can explain small variations of the geoid anomaly over the ridge

laboratory simulation of planetesimal collision 2 ejecta velocity distribution velocity distributions of fragments produced by low velocity impact against rock were obtained from high frame rate photographs cylindrical projectiles of mild steel s15ck and two kinds of rocks tuff and basalt were impacted against spherical rock targets at velocities from 50 to 400 m s target destruction initiates with longitudinal splitting which is analogous to destructive uniaxial static compression the ejecta velocity component normal to the incident direction lateral component differs for impacts into basalt 20 m s and tuff 5 m s targets the lateral component of kinetic energy of the fragments is shown to be controlled by the total strain energy stored before fracturing begins for equivalent impact energy per unit target mass ejection velocities of larger fragments from low velocity impacts are slightly higher than velocities from high velocity impacts in addition ejection velocities relative to the impact velocity for low velocity impacts are much higher than those for high velocity impacts therefore impact velocity and not the imparted energy density is suggested to be an important parameter for classifying these low velocity collisional events the ejecta velocity distribution is also greatly influenced by differences in relative mechanical properties between the projectile and target ejecta velocities from rock rock collisions are much higher than velocities from steel rock collisions even for equivalent impact velocity this implies that the mechanical properties of planetesimals have played an important role in the early stages of planetary formation

some examples and mechanical aspects of continental lithospheric folding abstract recent theoretical and observational advances in understanding the dynamics of lithosphere processes have yielded strong evidence in favour of the existence of large scale lithosphere folds intraplate compressional stresses generated by plate tectonic forces can reach magnitudes that approach the plastic buckling strength of mechanical models of lithosphere with brittle ductile rheological stratification the models predict lower plastic buckling stresses for continental lithosphere than for oceanic lithosphere intraplate folding of relatively strong oceanic lithosphere documented extensively in the northeastern indian ocean occurs where stresses are concentrated by restricted geometric and dynamic conditions resulting in unusually high stress levels several hundreds mpa on continents where geophysical data suggest that lithosphere folding may have occurred during the development of sedimentary basins in northern canada and central australia buckling stresses can have magnitudes that are of a similar order to those thought typically to occur in the lithosphere several tens mpa folding of continental lithosphere although more difficult to document is likely to be an important mechanism of large scale continental deformation in particular thinned continental lithosphere with a history of superposition of thick sedimentary successions might be the preferential locus of such folding lithosphere folding could also be a controlling factor in the near surface deformation of rifted basins as reflected by tectonic basin inversion

crustal observations beneath the southern north sea and their tectonic and geological implications abstract in 1991 a deep seismic reflection line mpni 9101 was acquired in the southern north sea from the mesozoic broad fourteens basin across the west netherlands basin onto the london brabant massif lbm the resultant section shows a strongly reflective lower crust beneath the area of mesozoic basin development this lower crustal reflectivity continues to be strong beneath the lbm the travel time to the base of the reflective zone increases from approximately 11 0 s beneath the mesozoic basins to 12 5 s beneath the lbm suggesting a southward thickening of the crust rijkers et al 1993 based on these travel times and information from deep wells and refraction surveys moho depth is estimated to increase from about 31 km beneath the mesozoic basins to about 38 km beneath the lbm this difference in depth to the moho can partly be explained by coaxial stretching of the crust beneath the mesozoic basins in comparison with the mesozoic basins the crust beneath the lbm was thickened during the caledonian and variscan orogenies

subcrustal density inhomogeneities of northern eurasia as derived from the gravity data and isostatic models of the lithosphere for the territory of northern eurasia 6 e 165 w 30 75 n the distribution of anomalous masses in the lithosphere has been estimated in accordance with the lithosphere isostatic model the method of model construction is based on the admittance technique the experimental admittance presents a relation between the part of the outer load uncompensated by the moho undulations and the residual gravity field and is used to select the best model the 1 1 averaged values of topography elevations basement and moho depths sedimentary cover density and gravity anomalies have been used as initial data according to the correlation equation relating the outer load and moho depths the mean density contrast between the lower crust and the subcrustal lithosphere is 0 43 g cm3 but the moho undulation can not provide complete isostatic equilibrium in some areas the part of the outer load uncompensated by moho undulations may be as large as 107 kg m2 and the residual gravity field is as intensive as 260 mgal assuming that for loads of wavelength 200 km local isostatic compensation is valid in accordance with the admittance analysis the anomalous masses compensating for the part of the outer load which is not compensated by moho undulations have to be located partly in the lower crust and in the subcrustal layer the regional trend of anomalous compensating masses is negative under western europe the mediterranean eastern asia and adjacent marginal seas and positive under the east european platform and western and central asia the local compensating masses correspond to particular tectonic units the isostatic gravity anomalies of northern eurasia have been determined and the long wave component of the field reflecting anomalous masses under the isostatic compensation level has been evaluated

mantle hotspots plumes and regional tectonics as causes of intraplate magmatism in continental areas it is often difficult to determine the cause of intraplate magmatism large volumes of magma high eruption rates and the presence of a hotspot trace on the adjacent ocean floor are all evidence for the presence of an anomalously hot mantle however in many continental magmas there are chemical variations with time which are inferred to reflect changes from asthenospheric to predominantly lithospheric source regions or vice versa it is argued that these chemical characteristics constrain whether magmatism was triggered by the emplacement of a mantle plume or by lithospheric extension

escarpment evolution on high elevation rifted margins insights derived from a surface processes model that combines diffusion advection and reaction experiments with a surface processes model of large scale 1 1000 km long term 1 100 m y erosional denudation are used to establish the controls on the evolution of a model escarpment that is related to the rifting of a continent the model describes changes in topographic form as a result of simultaneous short and long range mass transport representing hillslope diffusive processes and fluvial transport advection respectively fluvial entrainment is modeled as a first order kinetic reaction which reflects the credibility of the substrate and therefore the fluvial system is not necessarily carrying at capacity one dimensional and planform models demonstrate that the principal controls on the evolution of an initially steep model escarpment are 1 antecedent topography drainage 2 the timescale or equivalently a length scale in the fluvial entrainment reaction 3 the flexural response of the lithosphere to denudation and 4 the relative efficiencies of the short and long range transport processes when rainfall and substrate lithology are uniform a significant amount of discharge draining over the escarpment top causes it to degrade only when the top of the model escarpment coincides with a drainage divide can escarpment retreat occur for these conditions an additional requirement for retreat of a model escarpment without decline is a long reaction time scale for fluvial entrainment this corresponds to a substrate that is hard to detach by fluvial erosion and therefore to fluvial erosion that is not transport limited continuous backtilting of an escarpment due to flexural isostatic uplift in response to denudational unloading helps maintain the scarp top as a divide it is essential if the escarpment gradient is to be preserved during retreat in a uniform lithology low flexural rigidities promote steep and slowly retreating escarpments for given rainfall and substrate conditions the morphology of a retreating model escarpment is determined by the ratio of the short range diffusive and long range advective transport efficiencies a low ratio which is interpreted to correspond to a relatively arid climate and weathering limited conditions promotes steep sharp topped escarpments with straight main slopes and escarpment retreat occurs over a wide range of height scales a high ratio interpreted to correspond to a more humid temperate climate produces a convex upper slope and concave lower slope morphology and only major escarpments are predicted to preserve a high scarp gradient lithological contrasts in the model produce more complex morphologies and predict the formation of scarps crowned by an erosionally resistant caprock however resistant caprocks are not an essential requirement for model scarps to retreat we conclude that the inferred controls and model behavior are both consistent with the present day morphology of rifted continental margins and with modern conceptual models of landscape evolution

an unusually energetic basaltic phreatomagmatic eruption using deposit characteristics to constrain dilute pyroclastic density current dynamics abstract multiple highly erosive base surges of the table rock complex tuff ring trc2 oregon produced dune bedded deposits with crest to crest bedform wavelengths up to 200 m which are amongst the largest ever recognized in the deposits of pyroclastic density currents here we use bedform wavelength surmounted obstacles and a large chute and pool feature to estimate near source velocities 118 233 m s 1 lower bound velocities at radial distances of 1 6 2 and 4 7 km from source 34 29 and 20 m s 1 respectively and corresponding column collapse heights up to 2 8 km this paper represents one of the few studies that attempt to quantify flow characteristics such as emplacement velocities at different distances from source eruption column collapse height and eruptive energy based on deposit characteristics

dynamics of extensional basin formation and inversion introduction this volume contains a cohection of papers on the dynamics of extensional basin formation and inversion presented at the 3rd annual workshop of the international lithosphere program task force origin of sedimentary basins in sundvollen norway 18 23 august 1992 previous workshops of the ilp task force had been in rueil malmaison france 1990 and in matrahaza hungary 19911 on the rim of the pannonian basin the task force also organized a number of sessions during the 1992 european geophysical society and the 1993 european union of geosciences conferences reports of these meetings have been published in a number of special volumes and papers cloetingh et al 1993a b stephenson 1993 cloetingh sassi and task force team 1994 the 4th task force meeting focusing on the interplay of extension and compression was held in benevento southern italy in october 1993 a selection of papers presented at that task force workshop will be published in a forthcoming special volume of tectonophysics the emphasis of the 5th workshop of the task force taking place in the dead sea rift israel is on rifting and strike slip tectonics the 1995 workshop will be organized in the southern pyrenean foreland basin with the aim of examining the relationships between tectonics and eustasy in foreland fold and thrust belts the emphasis of the sundvollen workshop was on modelling studies as well as observational approaches to the dynamics of basin extension and inversion important elements of the meeting were the discussion of extensional tectonics rifting in passive margin settings and volcanism and magmatism in rifts and basins following the task force strategy to link subsurface data to field analogues an important part of the meeting was devoted to a thematic excursion in the oslo rift field guides have been published separately larsen et al 1992a

finite difference modelling of hangingwall deformation abstract modelling of hangingwall deformation during extension or compression may be achieved using a finite difference approach under the assumption of incompressible flow equivalent to area balancing this method enables calculation of a particle velocity field giving the trajectory of any point in the hangingwall during deformation this technique is more flexible than graphical algorithms because it is not restricted to simple shear cases inputs to the algorithm are the bounding fault and the particle displacement directions simple examples are used to illustrate the method

regional patterns of tectonic stress in europe nearly 1500 stress orientation determinations are now available for europe the data come from earthquake focal mechanisms overcoring measurements well bore breakouts hydraulic fracturing measurements and young fault slip studies and sample the stress field from the surface to seismogenic depths three distinct regional patterns of maximum compressive horizontal stress shmax orientation can be defined from these data a consistent nw to nnw shmax stress orientation in western europe a wnw ese shmax orientation in scandinavia similar to western europe but with a larger variability of shmax orientations and a consistent e w shmax orientation and n s extension in the aegean sea and western anatolia the different stress fields can be attributed to plate driving forces acting on the boundaries of the eurasian plate locally modified by lithospheric properties in different regions on average the orientation of maximum stress in western europe is subparallel to the direction of relative plate motion between africa and europe and is rotated 17 clockwise from the direction of absolute plate motion the uniformly oriented stress field in western europe coincides with thin to medium lithospheric thickness approximately 50 90 km and high heat flow values 80 m w m2 in western europe a predominance of strike slip focal mechanisms implies that the intermediate principal stress is vertical the more irregular horizontal stress orientations in scandinavia coincide with thick continental lithosphere 110 170 km and low heat flow 50 m w m2 the cold thick lithosphere in this region may result in lower mean stresses associated with far field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography the stress field of the aegean sea and western anatolia is consistent with n s extension in a back arc setting behind the hellenic trench subduction zone the stress field is influenced in places by regional geologic structures e g in the western alps where shmax directions show a slight tendency toward a radial stress pattern not all major geologic structures however appear to affect the shmax orientation e g in the vicinity of the rhine rift system horizontal stress orientations are continuous

third order middle miocene early pliocene depositional sequences in the prograding delta complex of the pannonian basin abstract few studies exist in the geologic literature that show the distribution of seismic facies and depositional sequences within a lacustrine basin the pannonian basin of central europe offers a unique opportunity to evaluate the influence of the eustatic signal on lacustrine deposition seismic stratigraphie and sedimentological studies indicate that the middle miocene early pliocene infill of the transtensional pannonian basin was formed by large delta systems systematic sequence stratigraphie analysis of 6000 km of reflection seismic data and more than 100 hydrocarbon exploration wells in hungary allowed the identification of twelve third order sequence boundaries in the late neogene sedimentary fill this number of depositional sequences corresponds to that of the published global eustatic curve for this time period furthermore based on magnetostratigraphic and radiometric data the ages of these depositional sequences can be tentatively correlated with the global eustatic curve the pannonian basin became isolated from the world sea at the sarmatian pannonian 11 5 ma boundary and formed a large lake the stratal patterns and sedimentary facies of individual systems tracts within the lacustrine sequences display the same characteristics as marine depositional sequences the relatively low rate of thermal subsidence and the high rate of sediment supply resulted in a good sequence resolution within the third order sequences higher order sequences can be recognized with an average duration of about 0 1 0 5 ma

sustained blasts during large volcanic eruptions we carried out numerical simulations to investigate magma ascent in wide conduits during large explosive eruptions wide conduits allow high discharge rates low frictional pressure losses and shallow levels of explosive fragmentation of the magma within the conduit in contrast with the commonly modeled lower rate eruptions during which magma fragments inside the conduit at depth and feeds a vertical eruptive jet we find that for sufficiently high discharge rates 10 10 kg s 1 the fragmentation level may rise up to the surface gas rich unfragmented magma reaches the surface at high pressure and feeds a sustained volcanic blast geologic evidence for very high discharge rate eruptions wide conduits and shock waves in large pyroclastic flows supports the occurrence of this type of explosive eruption

the influence of conduit geometry on the dynamics of caldera forming eruptions abstract during caldera collapse events new conduits may open and dramatically modify the style of the associated eruption in order to investigate the effect of conduit geometry on the dynamics of caldera forming eruptions we performed numerical simulations of magma ascent in single vent cylindrical conduits and ring fissure conduits the results show that for a given volume of conduit the discharge rate is an order of magnitude smaller in ring fissure conduits due to the higher friction furthermore the transition from a sustained plinian column to a collapsing fountain feeding pyroclastic flows occurs at a discharge rate an order of magnitude higher for ring fissure conduits due to the higher rate of air entrainment in a curtain jet than in a cylindrical jet the production of pyroclastic flows from ring fissure conduits therefore requires a much larger conduit volume than from a cylindrical conduit we argue that this should be correlated with a much larger volume of lithics in the deposit we confront these theoretical considerations with two geological examples the taupo ignimbrite has been emplaced by a high discharge rate pyroclastic flow and contains a small volume of lithics this is consistent with its emplacement during a single vent eruptive phase the bishop tuff has been emplaced by a lower discharge rate pyroclastic flow and yet contains much more lithics this is consistent with an eruption through successive vents migrating along a ring fissure the lithic content of an ignimbrite and the eruption discharge rate can therefore give some insights into the mechanism of caldera collapse and the role of ring fissures as magma conduits a conclusion of our analysis is that perfectly annular ring fissure conduits do not favour the formation of pyroclastic flows the occurrence of pyroclastic flows in virtually all caldera forming eruptions suggests that magma ascent is mainly localised in more restricted conduits a corollary conclusion is that widespread ignimbrites which record high discharge rate eruptions are not necessarily the result of ring fissure opening during caldera collapse

impact experiments i ejecta velocity distributions and related results from regolith targets velocity distributions are determined for ejecta from 14 experimental impacts into regolithlike powders in near vacuum conditions at velocities from 5 to 2321 m sec of the two powders the finer produces slower ejecta ejecta include conical sheets with ray producing jets and in the fastest impacts at vimp 700 m sec high speed vertical plumes of uncertain nature velocities in the conical sheets and jets increase with impact velocity sect 6 ejecta velocities also increase as impact energy and crater size increase a suggested method of estimating ejecta velocity distributions in large scale impacts involves homologous scaling according to r rcrater where r is radial distances from the crater sect 7 the data are consistent with holsapple schmidt scaling relationships sect 8 the fraction of initial total impact energy partitioned into ejecta kinetic energy increases from around 0 1 for the slow impacts to around 10 for the fast impacts with the main increase probably at the onset of the hypervelocity impact regime sect 9 crater shapes are discussed including an example of a possible frozen transient cavity sect 10 ejecta blanket thickness distributions as a function of r vary with target material and impact speed but the results measured for hypervelocity impacts agree with published experimental and theoretical values sect 11 the low ejecta velocities for powder targets relative to rock targets together with the paucity of powder ejecta in low speed impacts 1 projectile mass for vimp 10 m sec enhance early planetary accretion effeciency beyond that in some earlier theoretical models 100 efficient accretion is found for certain primordial conditions sect 12