lithospheric structure and along strike segmentation of the central andean plateau seismic q magmatism flexure topography and tectonics the central andean plateau is a 300 km wide nearly 4 km high plateau which is situated above a 30 e dipping segment of the subducted nazca plate systematic changes in the topography the upper mantle seismic attenuation q structure the lithospheric flexural rigidity and the distribution and chemistry of back arc lavas indicate that the upper mantle structure changes along strike beneath the plateau south of 23 s the upper mantle beneath the plateau becomes hotter and the lithosphere becomes thinner and weaker this lateral variation in upper mantle structure coincides with variations in the tectonic style and timing of deformation in two physiographically distinct segments of the plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south the segments different mantle lid thicknesses result in an altiplano supported mainly by crustal thickening and a puna supported by a combination of a crustal root and a thermal mantle root a thicker stronger foreland lithosphere in the north favors thin skinned underthrusting of the foreland beneath the plateau while to the south the thinner weaker lithosphere allows more basement involved shortening within the foreland and less underthrusting beneath the plateau a weaker lithosphere results in a longer period of compressional deformation and greater local and structural relief in the puna than in the relatively stable altiplano to the north ultimately this observed n s directed segmentation on the plateau and in the foreland may reflect the different responses of strong and weak continental lithosphere to an andean type subduction margin

no flat wadati benioff zone in the central and southern central andes abstract the wadati benioff zone wbz is an approximate plane defined by earthquakes hypocentres observed in convergent plate boundaries and that usually dips at angles greater than 30 in some areas of the andes where there are gaps in volcanic activity and where heat flow is abnormally low this plane in most studies has nearly horizontal dip at a depth of about 75 100 km and it has been associated to flat subduction of the oceanic lithosphere this situation has been taken as the present day analogue of the laramide orogeny of western north america for which a flat slab episode has been proposed in the past years in this work the observed low heat flow in areas of the andes is assumed to be due to low radiogenic heat generation in geologically old and allochthonous terranes constituting large regions of western south america on the basis of geotherms obtained for areas of ecuador peru chile and argentina and of rheological results describing the partition between brittle and ductile regimes the seismic activity observed both in the lower crust and at depths of about 75 100 km is thoroughly explained at these depths earthquakes occur within the subcontinental upper mantle and then there is no flat wbz associated to subduction of the oceanic lithosphere there is evidence from recent seismological observations that the real wbz lies not horizontally and deeper in the tectonosphere

dependence of the seismotectonic regime on the thermal state in the northern italian apennines abstract in order to study the relationships between recent seismotectonic processes and thermal conditions of the lithosphere we analyzed the lateral variations both in surface heat flow density and in seismic activity of the northern italian apennines the focal depths have been related to the mechanical properties of the lithosphere the hypocentre distribution indicates that the thickness of the seismogenic crustal layer increases towards the outer part of the range rheological models confirm this result showing that in the western areas marked by heat flow density greater than 60 mw m 2 and by positive gravity anomalies the depth of the crustal brittle ductile transition is rather shallow being about 10 km in the cold external parts of the chain this depth increases ranging from 15 to 25 km brittle seismogenic levels can be also found in the upper mantle below an essentially aseismic ductile crustal layer the estimated limiting temperatures of the brittle strength zones are about 320 380 c and 550 600 c for crustal and upper mantle materials respectively

thermal regime rheology and seismicity in central japan abstract in the present study geothermal models of the crust for a number of individual structural units of honshu island japan are constructed the depth distribution of crustal seismicity is used as a tool to assess the reliability of the calculated thermal models it is demonstrated that where reliable information is available on cutoffs in seismicity at certain depths the temperature at those depths can be estimated from theological considerations to the accuracy of not worse than from geothermal modelling such temperatures can then be used as reference values in checking the validity of a geothermal model for a region this approach may be especially useful in constructing geothermal models for tectonically disturbed areas with non steady state thermal conditions

electromagnetic study of the active continental margin in northern chile magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern chile between 19 5 and 22 s to study the electrical conductivity structures of this active continental margin the instruments used covered a very broad period range from 10 4 s to approx 2 104 s and thus enabled a resolution of deep as well as shallow structures in this paper we focus on the interpretation of data from an east west profile crossing chile from the pacific coast to the western cordillera at 20 5 s a decomposition of the impedance tensors using the groom bailey decomposition scheme shows that a two dimensional interpretation is possible the resulting regional strike direction is n9 w two dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the andes the final model shows a generally high resistivity in the forearc and a very good conductor below the precordillera unlike earlier models from areas further south a good conductor is not observed below the magmatic arc itself this correlates with the so called pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas

mountain building in the central andes the central andes is the middle part of the andean chain between about 13 s and 27 s characterized by the parallel running high mountain chains the western and eastern cordilleras at the edges of high plateaus with a height of about 4000 m and a width of 200 to 450 km the altiplano puna from the examination of geophysical and geological data in this area including earthquakes deformation gravity anomaly volcanism uplift history and plate motion we conclude that the continued plate subduction with domination of compressive stress over the entire arc system is the main cause of the tectonic style of the central andes we propose that the present cycle of mountain building has continued in the cenozoic with the most active phase since the miocene and that the present subduction angle 30 is not typical in that period but that subduction with more shallowly dipping oceanic lithosphere has prevailed at least since the miocene because of the young and buoyant slab involved this situation is responsible for the production of a broad zone of partial melt in the mantle above the descending slab addition of volcanic materials was not restricted to the western edge where active volcanoes of the western cordillera exist but extended to the western and central portion of the altiplano puna the western half of the central andes is essentially isostatic because the heat transferred with the volcanic activities softened the crust there in the eastern edge the thermal effect is small and the crust is strongly pushed by the westward moving south american plate this caused the shortening of crustal blocks due to reverse faulting and folding in the eastern cordillera and amazonian foreland the magmatism and crustal accretion are dominant at the western end of the mountain system and decrease eastward while the compression and consequent crustal shortening are strongest at the eastern end and wane toward west these two processes are superposed between the two mountain chains and form high plateaus there the altiplano of bolivia and peru and the puna of argentina this interpretation is supported by the observation that 1 neogene sedimentary formations have been uplifted to high elevations without heavy distortion in the altiplano and the western cordillera 2 no significant reverse fault systems are observed on the altiplano 3 neogene volcanic rocks and volcanic centers since the miocene are not restricted to the western cordillera but are widely distributed over most of the altiplano 4 most of the altiplano is in a zone of high heat flow values 5 thick paleozoic rocks are strongly folded and faulted in the eastern cordillera with little volcanism and no large scale plutonism in the cenozoic age 6 crustal earthquakes with reverse fault mechanisms are concentrated on the eastern flank of the eastern cordillera and amazonian foreland and 7 the crustal thickness suddenly decreases at the junction of the eastern cordillera and the amazon basin exactly at the place of reverse earthquakes

gravity anomalies and flexure of the brazilian shield beneath the bolivian andes abstract seismological and geological data suggest that the brazilian shield is now underthrusting beneath the sub andes in order to constrain the minimum amount of shortening that results from this underthrusting we analyzed bouguer gravity anomalies over the shield and the bolivian andes assuming that they are due primarily to variations in crustal thickness that in turn are due to the flexure of an effectively elastic plate brazilian shield loaded by the weight of the andean mountain range gravity anomalies show that airy type isostatic equilibrium does not exist the sub andes are overcompensated and the chaco plain to the east is undercompensated the flexure of the plate can help explain both the overcompensation and the undercompensation if the brazilian shield behaves as an essentially elastic plate that extends at least 150 km and probably 200 km beneath the sub andes and the cordillera oriental and if the flexural rigidity of the plate is between 0 1 and 2 10 24 n m the fit of observed and calculated anomalies however is far from perfect and additional sources of short wavelength anomalies appear to be present assuming that the sediments and sedimentary rocks in the sub andes and cordillera oriental were deposited on the brazilian shield and that later they were overthrust eastward onto one another causing the present topography and the deflection of the plate we computed a family of estimates for the minimum amount of crustal shortening these estimates depend strongly on the amount of sedimentary rock deposited before deformation which does not seem to be well constrained by published geologic mapping nevertheless the calculations suggest that at least 100 km of crustal shortening have occurred and probably more implying that the andes may have been built mostly by crustal shortening

attenuation of high frequency seismic waves beneath the central andean plateau observed patterns of high frequency seismic wave attenuation suggest that near 22 s the upper mantle structure beneath the central andean plateau changes along strike to the south contrasting regions of high and low upper mantle seismic wave attenuation beneath the plateau are identified based on striking azimuthal variations in the character and frequency content of p and s waves propagating beneath the plateau to a portable seismic network deployed in jujuy province argentina 24 s 65 w ray paths from intermediate depth earthquakes located north and northwest of the network transmit seismic waves with a higher frequency content than ray paths from earthquakes at similar depths and distances but located west and south of the network the estimated apparent q values fall into two categories qp 500 and qs 350 for the high q paths and qp 350 and qs 200 for the low q paths in addition sn phases from regional crustal earthquakes in the subandean foreland fold thrust belt to the north propagate efficiently to the jujuy network while sn is not observed from foreland earthquakes located at similar distances to the south of the network these observations combined with data from previously reported wave propagation studies suggest that south of about 22 s the upper mantle beneath the plateau and its adjacent foreland thrust belt is more highly attenuating than the upper mantle farther north forward modeling of the q measurements made at jujuy indicates that the observations can be explained either by a variable thickness high q upper plate beneath the plateau or by a thin variable width very low q zone in the asthenospheric wedge above the subducted slab this lateral variation in upper mantle structure coincides with two physiographically distinct segments of the central andean plateau and its adjacent foreland thrust belt to the east the bolivian altiplano and subandean ranges in the north and the argentine puna and santa barbara system in the south we interpret the north south change in upper mantle attenuation and the corresponding changes in physiography topography and tectonic style at the surface to be due to a mantle lid that is thicker beneath the altiplano and the subandean belt than beneath the puna and the santa barbara ranges

models of passive margin inversion implications for the rhenohercynian fold and thrust belt belgium and germany abstract positive tectonic inversion is related to the transmission of compressional stresses along a decollement into the foreland of an orogenic zone this stress and strain concentration in regions remote from the main orogenic front is commonly related to the presence of pre existing rheological heterogeneities such as normal syn depositional faults during inversion these pre existing normal faults are reactivated as reverse faults tectonic inversion in the rhenohercynian fold and thrust belt during the variscan orogeny shows that inversion is likely synchronous with the onset of collision in the hinterland here we present the results of a simplified thermo mechanical model stm which allows one to study strain partitioning between two orogenic zones we show that if the two orogenic zones have the same mechanical properties the viscosity of the decollement which links them controls the initial strain partitioning during subsequent finite shortening erosional processes determine the partitioning of strain rate the presence of a weak structure in the inverted zone and of a low viscosity decollement leads to initial strain concentration in the inverted track rather than in the collision zone and a progressive decrease in strain partitioning between the two orogenic zones the stm results are in good agreement with results of a 2d finite element model we conclude that in the western part of the rhenohercynian massif simultaneous uplift and deformation within the mid german crystalline rise the main collision zone and the ardenne anticlinorium the inverted zone lead to interpreting this orogenic event as a case of vice tectonic rather than the propagation of a wave of folding towards the variscan front as suggested by previous authors

electrical conductivity of the continental crust geophysical measurements indicate that the earth s continental lower crust has a high electrical conductivity for which no simple cause has been found explanation usually relies on either saline fluids saturating the pores or interconnected highly conducting minerals such as graphite fe ti oxides and sulphides providing conducting pathways attempts in the laboratory to clarify the problem have hitherto been unable to recreate conditions likely to be present at depth by controlling the confining pressure and pore fluid pressure applied to a rock saturated with saline fluids at temperatures between 270 c and 1000 c here we report conductivity data obtained using a cell designed to make such measurements on rocks saturated with saline fluids our results show that the conductivity of saturated samples of acidic rocks is explicable entirely in terms of conduction through the pore fluid whereas the conductivity of saturated basic rocks requires the presence of an additional conduction mechanism s we have used the experimental data to construct electrical conductivity depth profiles for the continental crust which when compared with profiles obtained from magnetotelluric observations demonstrate that a mid to lower crust composed of amphibolite saturated with 0 5 m nacl shows electrical conductivities sufficient to explain conductivity depth profiles for the continental crust inferred from geophysical measurements

a comparative study of results obtained in magnetotelluric deep soundings in villarrica active volcano zone chile with gravity investigations distribution of earthquake foci heat flow empirical relationships isotopic geochemistry 87sr 86sr and sb systematics abstract the first magnetotelluric deep soundings in chile were carried out during 1986 in the villarrica active volcano zone 39 25 s 71 57 w in the tm mode of polarization the curves show a distorted segment with dispersion a static distortion at long periods is observed in curves in the te mode of polarization the segment was shifted vertically to fit the geomagnetic global model values at daily periods this modified curve was used for 1d modelling to determine the electrical structure in the study area the upper level of the intermediate conducting layer of resistivity 20 60 m is found to be at 35 50 km depth a higher resistivity layer 600 m starting at 100 km depth may be resolved in the intermediate conducting layer a sharp decrease in the resistivity is shown by the model at 500 km large heterogeneities at the level of the conducting layer encountered in the 1d modelling and increased resistivity of the ultimate layer may account for distortion observed at long periods two dimensional test models show that the conducting layer in the area of villarrica volcano may be an anomalous heated layer surrounded by rocks of higher resistivity of about 2 10 3 m these features correspond to the interaction with a subsiding oceanic lithosphere resulting in a complex thermal structure and perturbed resistivity distribution in transition zones of the pacific type such as chile and to the existence of a megafault and a system of fractures in the sounding area these facts make it difficult to determine the conductance of the electrical asthenosphere the parameters of the model structure correlate well with geophysical and geochemical results obtained in the area by other workers gravity studies indicate a maximum crustal thickness of about 37 km which implies a non full compensation according to the airy hypothesis the morphology of the wadati benioff zone clearly shows a sharp decrease of earthquake foci at 50 km depth and a reinitiation of seismic activity from a depth of 100 km down to 160 km kaufman and keller and levi and lysak empirical relationships between heat flow and thickness of the lithosphere are somewhat consistent when the parameters of the model structure are assumed the strontium isotope ratios are indicative of minimal crustal contamination of mantle derived magmas thus allowing a relatively thin crust under the area furthermore the sb index of partial melting of mantle peridotite may suggest the contribution of uprising material from the deep asthenosphere through fractures extending to depth

finite element analyses of thrust tectonics computer simulation of detachment phase and development of thrust faults abstract the mechanics of thrust fault formation in rectangular and wedge shaped homogeneous thrust blocks are examined using finite element techniques incorporating elasto plastic material behaviour compression is simulated by applying an incremental end displacement parallel to the top of the horizontal or inclined basement shear zones or faults can form where the material experiences plastic strain softening these models simulate that thrusts initiate in sequences that are pushed from behind by some tectonic force over a decollement horizon in all models the formation of thrusts is preceded by the formation of a basal slip plane near the base of the block or in a weak substratum the front of this slip plane does not control the location of thrusts formed subsequently in the rectangular block a thrust forms at the pushed end in the tapered wedge drained as well as undrained a fault system composed of a thrust and a backthrust forms it is argued that the geometry of the block is the main controlling feature for the formation of thrusts within the tapered wedge the shape of reverse faults depends on the magnitude of shear stresses acting along the basal slip plane the higher the shear stress the more pronounced the curvature in the drained tapered wedge the intersection of the backthrust with the basal slip plane appears to control the location of the thrust under undrained conditions the backthrust forms subsequent to the thrust and acts only as an accommodation feature

combined control of decollement layer thickness and cover rock cohesion on structural styles and evolution of fold belts a discrete element modelling study abstract a series of numerical experiments based on the discrete element method were performed to simulate the formation of fold belts with varied decollement layer thickness and cover rock cohesion to investigate their controls on structural styles of fold belts each model consists of a weak unbonded basal decollement layer with a relatively low to high thickness and a bonded cover with a relatively low to high cohesive strength horizontal shortening of the particle assemblage was achieved by horizontal motion of a vertical boundary wall resulting in deformations in the system the results show that shortening was mainly accommodated by detachment folds sinusoidal and box folds fault related folds and opening mode fractures in the models the combination of a lower cohesion and a thinner decollement resulted in more distributed strain and a larger number of folds whilst the combination of a higher cohesion and a thicker decollement led to more pronounced strain localisations in fewer folds surface uplift and fold amplitude diapir height are mainly positively affected by the decollement thickness i e the thicker the decollement is the greater the surface uplift and fold amplitude are the propagation rate of deformation is predominately controlled by the cover rock cohesion a lower cohesion led to a higher propagation rate of deformation the model with a relatively low cohesion and decollement thickness produced regularly spaced box folds which are comparable to those in the southern zagros fold and thrust belt the models presented demonstrate that the combined effect of decollement layer thickness and cover rock cohesion can play a critical role in the structural styles and kinematic evolution of fold belts

possible orogeny parallel lower crustal flow and thickening in the central andes the relatively low elevation and thick crust in the altiplano in comparison to the higher elevation but thinner crust in the puna plateau together with geophysical data suggests that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the altiplano excess density in the altiplano mantle could create differential horizontal stress in the order of 25 mpa between both lithospheric columns numerical models accounting for pressure and temperature dependent rheology show that such stress can induce horizontal ductile flow in the lower crust from the puna towards the altiplano with a minimum viscosity of 1019 pa s this flow reaches 1 cm year displacing more than 50 km of material within 5 ma if the lower crust viscosity is smaller the amount of orogeny parallel lower crustal flow can be even greater such a mechanism of channel flow may explain that different amounts of crustal material have been accommodated by shortening in the altiplano and in the puna because of the strength of the elastic brittle upper crust this channel flow does not necessitate large amounts of surface deformation except vertical uplift making it difficult to detect from the geology

the irish variscides a fold belt developed within a major surge zone an arcuate fold belt characterizes the irish variscides south of the dingle dungarvan line the folds are cross cut by numerous faults which tend to be sinistral in the west and dextral in the east clockwise transection of major fold axes by minor folds and by cleavage has been recorded in the east while anticlockwise transections are common in the west these and other features suggest that north south dextral and sinistral shearing occurred in the east and west respectively this is considered to be a consequence of the morphology of the munster basin which allowed the development of a surge zone in the centre of the fold belt the variscan sole thrust is believed to lie at the base of or within the old red sandstone however to the east thinning of the upper palaeozoic cover sequence forces the sole thrust to cut down laterally stratigraphic section into lower palaeozoic basement in the depocentre the sole thrust surged ahead by propagating along bedding surfaces or possibly a devonian carboniferous extensional detachment beneath the basin while to the east and offshore to the west its propagation was inhibited by the absence of easy decollement horizons within the basement consequently the tip line became strongly arcuate and the thrust developed into a surge zone the sole thrust propagated northwards by stick slip processes much of the deformation within the cover sequence occurred during sticking events particularly fold tightening and accommodation faulting

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

fabric development and metamorphic evolution of lower palaeozoic slaty rocks from the rocroi massif french belgian ardennes new constraints from magnetic fabrics phyllosilicate preferred orientation and illite crystallinity data abstract this paper presents new results of a petrofabric study from cambrian slates cropping out within the anchi epimetamorphic core of the rocroi massif ardennes variscan fold thrust belt of northern france and southern belgium this study includes measurements of the anisotropy of the isothermal remanent magnetisation airm and the phyllosilicate preferred orientation it completes the previous results of anisotropy of magnetic susceptibility ams and of magnetic mineralogy which were obtained in 47 sites situated in and around this massif these new data allow the characterisation of the different subfabrics carried by the ferromagnetic minerals metamorphic pyrrhotite and magnetite and the paramagnetic matrix fe rich phyllosilicates combined with some illite crystallinity data measured throughout the massif the petrofabric measurements evidence a polyphase tectonometamorphic evolution a major variscan compressional event is evidenced inducing the growth of highly anisotropic pyrrhotite coarse grained hematite and phyllosilicates by contrast the magnetite grains display a less well organised fabric due to the existence of an inherited orientation pattern in agreement with recent metamorphic studies it argues for an early diastathermal metamorphic event developed during a devonian crustal scale extension

dependency of slab geometry on absolute velocities and conditions for cyclicity insights from numerical modelling summary the aim of this study is to quantify the relationship between the kinematics of subduction deformation in the overriding plate and the evolution of slab geometry a 2 d finite element numerical code is used and a first objective consists in benchmarking previously published analogue models far field plate velocities are applied and once the subducting plate reaches the 660 km discontinuity modelled as a rigid base we obtain two different forms or styles of subductionthatdependontheoverridingplatevelocity vop ifvop 0 theslabliesforwardson the 660km discontinuity style 1 and if vop 0 the slab lies backwards on the discontinuity style 2 we also obtain a cyclic pattern with the slab folding on itself repeatedly when vsp 0 and 2vop vsp 0 where vsp is the subducting plate velocity these conditions result from the analysis of several simulations in which the subduction velocities and plate viscosities are varied when the slab periodically folds on the 660km discontinuity periods of shallow slab dip and compression in the overriding plate are followed by periods of slab steepening and relative extension in the overriding plate folding periodicity is controlled by the slab viscosity and subduction velocity when a low viscosity zone is included in the overriding plate the trench motion is effectively decoupled from the overriding plate velocity therefore allowing it to be directly controlled by the deep dynamics of the slab for the cyclic style 2 corresponding to forward folding of the slab the low viscosity region in the overriding plate increases the stress amplitudes oscillations the trench motion and the folding periodicity with time therefore the strength of the entire overriding plate is shown to directly control the dynamics of subduction using the nazca and south american plate velocities we produce models of cyclic folding with a period of ca 22ma and a minimal dip angle of ca 10 episodic folding of the slab on the 660km discontinuity would produce the necessary changes in slab dip and overriding plate deformation that explain episodes of volcanic quiescence alternating with greater rates of shortening along the andes

metamorphism isotopic ages and composition of lower crustal granulite xenoliths from the cretaceous salta rift argentina crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the cretaceous salta rift at quebrada de las conchas provincia salta argentina were investigated to get information about the age and the chemical composition of the lower crust most of the xenoliths have a granitoid composition with quartz plagioclase garnet rutile k feldspar as major minerals the exceedingly rare mafic xenoliths consist of plagioclase clinopyroxene garnet hornblende all xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca 900 c and pressures of ca 10 kbar the sm nd mineral isochron ages are 95 1 10 4 ma 91 5 13 0 ma 89 0 4 2 ma granitoid xenoliths and 110 7 23 6 ma mafic xenolith these ages are in agreement with the age of basanitic volcanism ca 130 100 and 80 75 ma and are interpreted as minimum ages of metamorphism lower crustal temperature at the time given by the isochrons was above the closure temperature of the sm nd system 600 700 c the sm nd and rb sr isotopic signatures 147sm 144nd 0 1225 0 1608 143nd 144ndt0 0 512000 0 512324 87rb 86sr 0 099 0 172 87sr 86srt0 0 708188 0 7143161 and common lead isotopic signatures 206pb 204pb 18 43 18 48 207pb 204pb 15 62 15 70 208pb 204pb 38 22 38 97 of the granitoid xenoliths are indistinguishable from the isotopic composition of the early paleozoic metamorphic basement from nw argentina apart from the lower 208pb 204pb ratio of the basement the sm nd depleted mantle model ages of ca 1 8 ga from granitoid xenoliths and early paleozoic basement point to a similar proterozoic protolith time constraints the well equilibrated granulite fabric p t conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca 90 ma and to a prominent thermal anomaly in the lithosphere the composition of the xenoliths is similar to the composition of the early paleozoic basement in the andes of nw argentina and northern chile a thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths

finite element modelling of the competition between shear bands in the early stages of thrusting strain localization analysis and constitutive law influence finite element simulation of a sandbox model of thrusting is performed using large strain analysis and two different non associated elastoplastic constitutive laws namely druckerprager and van eekelen criteria the analysis of strain localization using the rice bifurcation criterion coupled with a kinematic indicator shows that in the early stages of imbricated thrusting the development of major shear bands can be influenced by some competition with second order bands the influence of the vertical horizontal stress ratio is checked as well as the influence of lode angle in the constitutive law significant differences are found between a classical plasticity criterion drucker prager and a more realistic one van eekelen regarding the resistance of the model and the stress paths these differences may result in erroneous fault type prediction sandbox experiments have been used extensively for many years to assist structural interpretation and validation of tectonic models main interests of this method corne from the very large range of initial geometries which can be investigated and from the large amount of strain which can be applied physical parameters obtained are mainly displacement fields at different stages especially since non destructive methods are used e g x ray tomography colletta et al 1991 the finite element method is also an attractive method for geological modelling as it enables computation in several parameters examples of which are displacement strain and stress fields and also more recently strain localization analysis moreover it allows the explicit choice of mechanical characteristics constitutive laws and boundary conditions abilities of the finite element method for the investigation of strain localisation along shear bands and particularly thrusting mechanism have already been shown see for example miikel walters 1993 the present paper aims to show the relative influence of the constitutive law in the results and also to analyse information obtained from localization analysis in the first part some theoretical considerations are presented basics of the large strain formulation used and principle of the finite element method are given the choice of a representative constitutive law for sand between mohr coulomb druckerprager and van eekelen models is discussed a short literature overview of the strain localization analysis performed in finite element method is presented eventually the localisation analysis used in the present paper is detailed the second part is mainly concerned with the simulation of a sandbox model of thrusting carried out with the lagamine finite element code early stages of thrust development are analysed in a first simulation in the light of the rice bifurcation criterion and of a kinematic indicator of localization a non associated drucker prager yield criterion is used a remeshing procedure is also performed in this first simulation to increase the applied loading value the influence of the vertical horizontal stress ratio is checked in a second simulation the last simulation investigates the influence of the third stress invariant in the plasticity surface definition using a more sophisticated yield criterion namely the van eekelen in the last part finite element simulation results obtained are compared with some experiments performed on sandbox models

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

k ar dating of magmatic sericite crystallites for determination of cooling paths of metamorphic overprints a combined optical and x ray characterisation of fine grained micas in granodioritic to tonalitic dikes and intrusions of the eastardennes eifel nappe linksrheinisches schiefergebirge germany belgium was completed with k ar dating of separated size fractions from these rocks in each sample the k ar ages decrease with decreasing size fraction but the same size fractions of the different samples yield different ages application of the closure temperature concept to the k ar ages of the various size fractions indicates that the crystallite size expressed as cis fwhmser001ad and coherent scattering domain csd thickness acts as the effective radius of ar diffusion a linear model may describe the correlation between k ar ages and crystallite thickness the ages younger than the model prediction are caused by the occurrence of microcline as an accessory k bearing mineral phase the obtained k ar ages were interpreted as representative of the cooling path of the magmatic components in the eastardennes eifel nappe below 330 c the k ar age of the largest size fraction of the tonalitic dike v11 313 6 7 2 ma is the closest value for the age of the variscan cleavage development the oldest age of the hydrothermally altered tonalitic dike 300 2 8 4 ma might represent an episode of hydrothermal fluid migration after cleavage development it represents at least a minimum age the microcline age 242 8 6 0 ma of the hill intrusion may represent the closure temperature of this mineral at about 140 c

gravity anomalies flexure and deformation of the converging indian lithosphere in nepal and sikkim darjeeling himalayas researchers ubiquitously noted that the common processes of partitioning oblique convergence in response to drag from the trench hanging plate simultaneously produce radial slips along strike translation and extension parallel to the deformation front here we focus on the area between nepal and sikkim darjeeling himalayas and carry out gravity and finite element stress modeling of the strike orthogonal converging indian lithosphere we delineate the geometries of different layers and their interfaces through gravity modeling the optimum model parameters along with rheological parameters of different layers are used for finite element modeling finite element modeling is done with boundary conditions of keeping the upper surface free and rigidly fixing the section of the northern boundary below the main himalayan thrust we impart on its frontal section an amount of 6 1012 n m force equivalent to resistive force of the himalayan tibet system and analyze the maximum and minimum compressive stress fields evolved in the lithosphere we testify our observations with earthquake database and other geophysical and geological studies we note that an increasing flexing of the indian lithosphere beyond the main boundary thrust becomes maxima between the main central thrust and south tibetan detachment in both the areas however more steepening of the moho boundary is identified in the sikkim darjeeling himalaya this abrupt change in lithospheric geometry beneath the greater himalaya is likely correlated with the sharp elevation changes in the topography although the highest seismicity concentration is dominant in this zone the lesser and the tethys himalayas in sikkim darjeeling area also record relatively fair seismic activity more compressive stress field in different layers right within the sharp bending zone supports this observation we thus propose that the sharp bending zone beneath the greater himalaya is suffering maximum deformation and the deformation is continued in the mantle too we also identify both right lateral shear and radial vergence slip which are presumably associated with the general dynamics and kinematics of the himalaya

neotectonic fault analysis by 2d finite element modeling for studying the himalayan fold and thrust belt in nepal abstract this paper examines the neotectonic stress field and faulting in the fold and thrust belt of the nepal himalaya using the 2d finite element technique incorporating elastic material behavior under plane strain conditions three structural cross sections eastern central and western nepal where the main himalayan thrust mht has different geometries are used for the simulation because each profile is characterized by different seismicity and neotectonic deformation a series of numerical models are presented in order to understand the influence of a mid crustal ramp on the stress field and on neotectonic faulting results show that compressive and tensional stress fields are induced to the north and south of the mid crustal ramp and consequently normal faults are developed in the thrust sheets moving on the mid crustal ramp since the shear stress accumulation along the northern flat of the mht is entirely caused by the mid crustal ramp this suggests that as in the past the mht will be reactivated in a future large mw 8 earthquake the simulated fault pattern explains the occurrence of several active faults in the nepal himalaya in all models the distribution of the horizontal 1 maximum principal stress is consistent with the sequence of thrusting observed in the fold and thrust belt of the himalaya failure elements around the flat ramp flat coincide with the microseismic events in the area which are believed to release elastic stress partly during interseismic periods

role of faults and layer interfaces on the spatial variation of stress regimes in basins inferences from numerical modelling abstract the spatial variation of the computed state of stress is studied in a series of numerically solved boundary value problems of basin scale deformation these numerical experiments illustrate simple situations of decollement tectonism under compression the study focuses on the local variation of the state of stress in a sedimentary overburden where major discontinuities such as large scale faults and layer interfaces are present the states of stress are computed using either the distinct element method or the finite element method classic elasto plasticity laws are used for the behaviour of the continuum and the discontinuities are modelled as coulomb contact surfaces a geologic interpretation of the numerical experiments is proposed to highlight the role played by major structural discontinuities in the local variations of the stress regime the models are two dimensional in space and assume plane strain but the full three dimensional stress strain constitutive relation is used therefore the stress regime variations can be represented by constructing the contour map of the wallace bott stress ratio this simple technique together with the construction of diagrams of principal stress magnitude across the model enables us to rapidly examine the stress field pattern and further discuss the deformation modes of faulting and fracturing which may take place at a small scale we have found that the local variations of the stress tensor are in good agreement with common geologic interpretation of brittle deformations in foreland and thrust systems we demonstrate how the presence of layer interface and the difference in mechanical properties of formation lithology may strongly affect the variation of stress in space this modelling approach has important issues when interpreting well tests to gain access to in situ stresses

the deep structure of the ardennes variscan thrust belt from structural and ecors seismic data abstract a new crustal tectonic model is proposed for the ardennes external variscides of northern france belgium on the basis of original microtectonic data further complemented by revised ecors deep seismic information this model is illustrated by both restored and balanced crustal sections that clearly emphasize the direct control exerted by pre existing extensional structures on the collision related processes of variscan deformation indeed the deep structure of the ardennes belt appears as a continental accretionary prism that thickens south towards the hinterland above a staircase type basal thrust innermost crustal shortening is here assigned to a deeply rooted foreland directed ramp thrust system soled at depth by a thick ductile decoupling shear zone actually imaged at about 20 km depth on the ecors section this thrust system is bounded upwards by a shallow infra devonian roof thrust to account for the present day high structural position of the rocroi cambrian inlier as well as for its internal superimposed ductile fabrics major uplift of a previously sheared crust is assumed to have occurred along a frontal ramp which nucleated on an early s dipping extensional fault the rocroi fault according to the ecors data the hanging wall of the rocroi fault has been translated about 40 km northwards along the dinant infra devonian shallow decollement this estimate is quite consistent with the 40 stratal shortening undergone at its front by the devonian detached cover whose thin skinned fold thrust structures also reflect the direct influence of early devonian normal faulting basin inversion tectonics are clearly expressed along the frontal thrust zone where the thin condroz basement sheet is now interpreted as resulting from slicing of the dinant decollement through the footwall of a major lower devonian extensional fault scarp that separated the namur shallow shelf from the dinant downthrowing basin the present thrust model also implies that the dinant decollement extends as a blind thrust beneath the slightly detached namur carboniferous coalfield and that the midi frontal ramp only represents a minor out of sequence thrust reactivating a rotated basement cover contact

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

predictive modelling of structure evolution in sandbox experiments in this paper a computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement the approach adopts the lagrangian method complemented by robust and efficient automated adaptive meshing techniques a constitutive model based on critical state concepts and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution the efficacy of the approach is benchmarked by forward simulation of two extensional analogue experiments that exhibit the development of a roll over anticline with a series of superimposed crestal collapse graben systems these sandbox experiments are excellent benchmarks for computational models as both intersecting localisations with different rates of slip and large relative movement on localisations must be represented while other complex phenomena associated with structural evolution over geological timeframes are not present

detection of an ionospheric current for the preliminary impulse of the geomagnetic sudden commencement magsat observed a geomagnetic sudden commencement at an altitude of 550 km on its dawnside orbit above the south atlantic ocean the h component of the magnetic field increased monotonically by 46 nt in about 4 minutes while the d component decreased westward by 10 nt in the first 1 2 minutes and then increased gradually there was no significant deviation in the z component the comparison with ground based magnetograms showed that the negative impulse of the d component provided the first experimental evidence of the global ionospheric current system for the preliminary impulse of the geomagnetic sudden commencement

accretion of a rifted passive margin the late paleozoic rhenohercynian fold and thrust belt middle european variscides in the western rhenish massif the rhenohercynian fold and thrust belt of the middle european varsicides exposes a telescoped complete devonian to early carboniferous passive margin this permits the analysis of geometry and kinematic processes of passive margin accretion to an erogenic wedge during variscan collision 330 300 ma the sedimentary cover of the passive margin was shortened by some 50 or 180 km crustal scale balancing and restoration reveals a wide symmetric rift with a central graben a marginal plateau separated this failed lower devonian rift from an emsian middle devonian oceanic basin in the south remnants of which are preserved in the southernmost imbricates and the giessen harz nappes the seismically well imaged aachen midi detachment faille du midi acted as the basal decollement of this thin to thick skinned orogenic wedge it shows a ramp and flat geometry from the blind tip down to middle crustal levels owing to its position below the base of the basin fill the thick synrift sequence and structure controlled structural evolution during contraction by localizing thrust branch lines and by inversion of rift structures synthetic to the subduction direction with formation of basement footwall shortcuts moreover the three dimensional detachment geometry shows large scale oblique ramp flat features which control the architecture of the belt and the distribution of metamorphic grade rocks and fabrics from the detachment show that the latter was located at the transition into the ductile field at the fossil 300 400 c isotherm in the restored section the detachment trajectory displays a saucershaped geometry rising to the surface at the rear and at the front this suggests that the basal detachment propagated into the passive margin by ductile failure during lithospheric flexure under the load of an advancing upper plate the regional pattern of synkinematic metamorphic grade shows varying modes of margin accretion basal underplating into the orogenic wedge by the shelf slope and marginal plateau with formation of duplex stacks and elongate thrust sheets and frontal accretion of the remaining thicker failed rift in the north into the propagating wedge with formation of several imbricate fans cyclical changes in accretion history and or critical taper are suggested by spaced domains of out of sequence thickening these are also related to internal domainal kinematic partitioning of the wedge controlled by oblique collision

seismotectonics of thin and thick skinned deformation in the andean foreland from local network data evidence for a seismogenic lower crust local network data from san juan argentina provides new information about crustal seismicity in the andean foreland above a horizontal segment of the subducted nazca plate the authors find two areas of foreland seismicity one associated with the sierras pampeanas basement uplifts and the other beneath but not within the precordillera foreland fold thrust belt the precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation in both areas events are concentrated between 15 and 35 km depth and have volumetric rather than planar fault like distributions the depth distribution is usually deep for intraplate earthquakes and suggests a brittle ductile transition near 30 35 km it also suggests the upper plate may have a geotherm similar to stable cratonic regions rather than a steeper one that may be expected due to a lithospheric thinning implied by the location of the subducted plate this can be understood as a transient effect of a neogene flattening of the dip of the subducted plate where a rapid thinning of the upper plate was followed with underplating by the cool subducted plate leaving the upper plate geothermmore unchanged while producing a thinned mechanically weaker lithosphere finally a spatial correlation of upper plate seismicity and structures with subducted plate seismicity raises the possibility that a major lithospheric structure cutting across the strike of the foreland near 31 degree s may be related to subduction of the juan fernandez ridge less

the variscan belt of northern france southern belgium geodynamic implications of new palaeomagnetic data abstract palaeomagnetic investigations were carried out in devonian early carboniferous rocks of the variscan foreland chain of northern france southern belgium in order to reveal the origin of its arcuate shape the brabant parautochthon was sampled in the boulonnais near calais and near tournai while the ardenne allochthon was sampled near maubeuge and in the givet area all the sampled localities yielded characteristic remanent magnetization as a result of stepwise demagnetization and component analysis fold or tilt tests were possible for three localities with negative results indicating pervasive remagnetization the tectonic position was sub horizontal at two localities while the tilt was monoclinal for the rest therefore the acquisition time of the magnetic signals was estimated by comparing the palaeolatitude computed from each magnetic component to the palaeolatitudes of variscan europe calculated after van der voo 1993 three components showing a a southern b a near equatorial and c a northern palaeolatitude are recognized from our data since a pre variscan age of component a observed only in boulonnais at 10 sites is not supported by data it is assigned to an early phase of deformation component b 16 sites was acquired during the peak of the variscan tectonics late westphalian while component c five sites originated during permian times regardless of the palaeolatitudes declinations fall between 190 and 210 thus being conformable with the declinations expected for variscan europe the declinations show no correlation with the arcuate shape of the belt neither are they different in the paraauthochthon and in the allochthon nor in the different components arc formation by moulding of the allochthon on the brabant parautochthon is therefore not supported by these data since this mechanism requires substantial opposed rotations on both wings of the arc the available palaeomagnetic data are conformable with a pre formed arc simply docking to the brabant obstacle of similar shape variable offsets along a main thrust make possible a third model which slightly unfolds the former passive arc

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

orogen scale decollements the continental lithosphere responds to stress by deforming as a generally layered medium deep seismic reflection data coupled with a variety of ancillary geological and geophysical data are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers and flat detachments in mechanically weak layers this geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen scale decollements in a manner that is analogous to so called thin skin deformation in sedimentary rocks decollements may occur within the crust for example near the base of a sedimentary section or in the middle crust near the moho in the subcrustal lithosphere or in the asthenosphere even intracratonic basement cored thick skin uplifts that occasionally occur in foreland regions such as the wyoming province are probably large crustal scale versions of ramp flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tectonic processes

magnetotelluric image of the crust and upper mantle in the backarc of the northwestern argentinean andes magnetotelluric data from the backarc of the central andes in nw argentinawere re examined by employing impedance tensor decomposition and 2 d inversion and modelling techniques the data in the period range of 50 15 000 s were collected on a profile of 220 km length reaching from the eastern cordillera across the santa barbara system to the andean foreland of the argentinean chaco after a dimensionality analysis data from most sites were treated as regional 2 d the exception was the eastern section of the profile where the magnetotelluric transfer functions for periods 1000 s reflect a 3 d earth application of two tensor decomposition schemes yielded a regional strike direction of n s which is the azimuth of the central andean mountain chains several 2 d models were obtained by pseudo and full 2 d occam inversion schemes special emphasis was placed on the inversion of phase data to reduce the influence of static shifts in the apparent resistivity data the smooth inversion models all show a good conductor at depth a final model was then calculated using a finite element forward algorithm the most prominent feature of the resulting model is a conductor which rises from depths of 180 km below the chaco region to 80 km beneath the santa barbara system and the eastern cordillera its interpretation as a rise of the electrical asthenosphere is supported by seismic attenuation studies magnetotelluric results surface heat flow distribution in the area and the electrical properties of crustal and mantle rocks suggest that the upper mantle is predominantly ductile beneath the eastern cordillera and the western santa barbara system this generally agrees with anelastic seismic attenuation models of the area and is useful in discriminating between models of q quality factor distribution

an application of two dimensional finite element modelling for studying the deformation of the variscan fold and thrust belt belgium abstract this paper presents 2d finite element simulations of fold and thrust belts with a focus on the variscan foreland thrust belt in belgium the numerical models are limited to the deformation of a 5 km thick sedimentary layer separated from a backstop by a deformable zone the sedimentary layer obeys an elastic plastic law and can include one detachment horizon with some ramps our study deals with the effects of rheological and geometrical parameters on the structural characteristics of the belt for regular basin geometries rectangular and wedge shapes results of numerical simulations show that deformation continuously propagates and decreases towards the foreland in agreement with sandbox experiments the basal friction appears to be a major parameter controlling the tectonic style of a fold and thrust belt our analysis has also shown that the strain propagation in time and space is strongly controlled by the presence of a weak intra detachment and of ramps which initiate shear bands for the rhenohercynian fold and thrust belt a major decollement is usually considered within the palaeozoic sediments and the existence of steps in the basin geometry is proven by numerous geological data showing the activity of pre orogenic normal faulting during the variscan orogeny this irregular detachment horizon influenced the regional tectonics simulation results show that ramps create strongly deformed anticlinioria separated by weakly deformed synclinoria which is the basic structure observed in the western and northern rhenohercynian belt

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

resistivity cross section through the southern central andes as inferred from magnetotelluric and geomagnetic deep soundings to investigate the upper lithospheric structure and subduction induced processes across an active continental margin magnetotelluric mt and geomagnetic deep sounding gds experiments were done in the southern central andes 21 25 s of northern chile southern bolivia and northwestern argentina using two dimensional modeling we have constructed an e w resistivity cross section at latitude 21 45 s that involves all the andean units from the pacific coast to the lowland plains the crust of the coastal cordillera has high resistivities of more than 5000 ohm m with some conductive structures embedded increased electrical conductivities between depths of 8 and 40 km may be related to the rise of fluids released from the subducted nazca plate the electrical resistivity of the normal andean crust seems to be low having values of only 200 ohm m a wide zone of extremely high electrical conductivity with an e w extent of more than 330 km was detected this high conductivity zone hcz is found at a depth of about 25 km under the western cordillera and has a total conductance of more than 23 000 s here it may be caused by partially melted crust an interpretation which would correlate with those from gravity and seismic data the hcz can be followed in the lower crust eastward below the altiplano where it has a total conductance of about 15 000 s in the western part of the eastern cordillera total conductance first increases and then drops abruptly at about longitude 65 10 w the latter hczs can be explained in terms of thrust tectonics where detachment zones may involve fluids as well as increased mineralizations the lowlands of the sub andean and the chaco are characterized by a cover of anisotropic low resistivity the resistivity of the crust and uppermost mantle increases from west to east to more than 1000 ohm m while the deeper mantle gets unexpectedly much more conductive

combined sr o isotope relationships and petrogenesis of andean volcanics of south america active andesitic type volcanism in the southern andes lat 42 36 s is characterized by low sr and o isotopic compositions which fall in the range 87sr 86sr 0 7036 0 7043 18o 5 2 6 8 in contrast volcanic centres in the central andes lat 24 21 s are significantly enriched in both 87sr and 18o we present the first combined sr and o isotope study of the southern volcanic zone svz in chile crustal thickness 40 km 1 and of the southern part of the central volcanic zone cvz in the borderlands of bolivia chile and argentina crustal thickness 60 70 km 1 only one of 13 volcanic centres studied lascar exhibits a correlated increase in both 87sr and 18o parallel with the basalt andesite rhyolite compositional change indicative of contamination by upper crust through a fractional crystallization assimilation mechanism the other centres of the cvz for which there is an overall sr o correlation document the variable contamination of mantle derived magmas at depth in the lower crust which had been thickened by mesozoic and earlier plutonism

influence of heterogeneous lithostructural layering on orogenic deformation in the variscan front zone eastern belgium the ramp flat thrust model and other classical models based on rocky ountains or appalachians structures allow a better understanding of ault and fold interaction in a sequence dominated by competent rocks n which in most cases thrusts propagate forward in sequence this attern however fails to explain the deformation history of rock uccessions with low proportions of competent rocks in such ithologies out of sequence faults are more common features the ariscan front zone in eastern belgium serves as a case study for this on classical deformation model cross sections are well constrained by urface data covering a large area the devono carboniferous sequence esting unconformably on caledonian basement has shortened drastically nd the tectonic history can best be explained by a composite structural odel continuous and discontinuous deformations took place in uccessive steps a lame anticline developed first it evolved ubsequently into either break thrust or translated fault propagation olds out of sequence thrusting by the theux tunnel fault formed the esdre nappe the latter is the northern part of the ardenne allochthon he main discrepancies with the ramp flat thrust model are emphasized articularly the vesdre nappe which is the highest thrust sheet is ot the youngest event of deformation common elements of the variscan ront zone between northern france and western germany are discussed he theux tunnel fault has the same kinematics significance as the idi aachen fault the footwall of the vesdre nappe is the eastward rolongation of the brabant foreland the complex structure of the theux indow reflects imbrications under the allochthon and can be compared ith other wedges and thrust sheets c 1999 elsevier science b v all ights reserved

the variscan front and the midi fault between the channel and the meuse river abstract the ecors seismic profile allows a new insight into the deep geology of northern france various geological data such as ancient coal mines surface mapping and boreholes are used to interpret the seismic profile a new cross section directly superimposed on the ecors seismic profile cambrai dreux is compared with a previous one drawn along the meuse river through the ardenne massif the cylindrism and length balancing hypotheses are discussed by comparing the two cross sections the main geological results are a the emplacement of the dinant nappe is later than most of its internal deformation and b major folding and thrusting give an approximate 30 35 shortening without taking into account the internal deformation minor folding and cleavage and the unknown net translation vector of the dinant nappe 50 km at least possibly 150 km

laminated veins and hydrothermal breccia as markers of low angle faulting rhenish massif germany abstract the emplacement of bedding veins and vein breccia in a fold and thrust belt setting is discussed we relate vein types to thrusting regimes using the rhenish massif in germany as case example the rhenish massif is a slate belt in a hercynian fold and thrust system the slate belt cleavage generally dips to the southeast and locally has been rotated into low angle shear zones parts of these are parallel to bedding and marked by quartz carbonate veins both shear veins and extensional veins are distinguished on the basis of fibrous crack seal microstructures most shear veins grew by incremental pull apart between en echelon slip surfaces s0 the overlap of which produced a vein stack the stacks have a lineation l0 resulting from a combination of slip fibre growth and dissolution pressure solution slip the attitude of l0 relative to fold axes and the asymmetry of vein microstructures are consistent with vein emplacement during the hercynian thrusting early shear zone veins were thrusted mylonitized and finally boudinaged or folded depending on their setting the shear veins include coeval sheets of vein hosted breccia the breccia contains angular fragments of mylonitized veins and slate the orientation of s0 and s1 in the breccia varies randomly per fragment indicating random fragment rotation unencumbered spin of country rock debris implies rock expansion possibly by rapid pull apart events during the thrusting

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

mechanical analysis of a natural example of onland gravity gliding the role of river incision and deposition gravity gliding implies rigid translation of a body down a slope where displacements are parallel to a tilted detachment plane although large scale gravity gliding is commonly observed offshore under conditions of high fluid overpressure and abundant upslope sedimentary supply its occurrence on land is debated we investigate the mechanical feasibility of such a process as well as the role of fluvial incision and sedimentation down the slope in the initiation of the gliding we use a two dimensional 2 d finite element model combined with a 2 d failure analysis approach the numerical models simulate the deformation and provide quantitative estimates of the failure criteria at the head and toe of the overburden analytical solutions approximate the numerical results by taking into account the fluvial incision and sedimentation the internal friction angle and the thickness and length of the overburden our models are based on a field example in the andean foothills of argentina where gravity gliding of a 1000 m thick section is suspected above a crustal scale anticline the incision and sedimentation reduce and strengthen respectively the downslope resistance to contractional failure the critical slope at which the gliding is initiated is reduced by fluvial incision and increased by sedimentation we show that tectonic uplift may lead to large scale gravity gliding on land where the overburden thickness is less than 2000 m incision facilitates and localizes the frontal shortening incision greater than 1000 m may trigger gliding for overburden up to 4000 m thick while sedimentation thicker than 1000 m inhibits gliding these results show that thin skinned onland gravity gliding could be common in tectonically active regions where incision is important

evidence for continent continent collision zone in the south indian shield region abstract with a view towards understanding the evolutionary history of the complex south indian shield several geological and geophysical studies have been carried out recent geophysical studies include magnetotelluric mt deep seismic sounding dss gravity magnetic and deep resistivity soundings drs in the present study mt results along 140 km andiyur turaiyur east west profile is presented the data are subjected to groom bailey decomposition and static shift correction before deriving a 2 d model the 2 d modeling results have shown that the upper crust up to about 15 km towards western part of the profile have exhibited high resistive character of about 40 000 ohm m as compared to the eastern part less than 5 000 ohm m the mid lower crust has shown a decrease in resistivity in western part of the profile the order of resistivity being 2 000 ohm m an anomalous steep conductive feature less than 100 ohm m is observed near sankari at mid lower crustal depths 20 km towards middle part of the profile this feature is spatially correlatable with the well known moyar bhavani shear zone mbsz the features obtained in the present study are consistent with earlier mt studies in this region and correlatable with other geophysical studies dss studies near the study region gave an evidence for differing crustal structure on either side of mbsz variation in geoelectric character along the profile both in the upper crust and mid lower crust indicate a block structure in the sgt with shear zones acting as boundaries the new evidence in the form of distinct geoelectric structure and also variation in seismic structure indicate a continent continent collision zone in this region and plays an important role for the gondwana reconstruction models of south indian shield

variscan transpressive inversion in the northwestern central rhenohercynian belt of western germany abstract important orogen parallel strike slip components during the convergence of the variscan orogen of central europe have been increasingly recognized these oblique slip faults often develop from pre existing tectonic boundaries and are influenced by pre existing structural trends in the basement in this paper evidence is presented for a 3 6 km wide distributed shear zone here named the monschau shear zone in epizone metamorphic cambro ordovician and lower devonian clastic rocks of the northwestern rhenohercynian belt kinematic indicators including field observations of anomalous mullion and fold axes patterns a set of two cleavages stretching lineations and shear and kink bands microscopical observations of stretching parallel asymmetric pressure shadows of quartz on prekinematic epizone metamorphic pyrite porphyroblasts and microfabrics of phyllosilicates by x ray texture goniometry point to a sinistral tranpressive shear zone with emplacement of the southeast hangingwall under retrograde conditions coincidence of the anchizone epizone metamorphic boundary with the change along strike from penetrative shearing to distributed thrusting suggests a ductile brittle transition the monschau shear zone is situated on the backlimb of an allochthonous anticlinorium located above a basement ramp that corresponds roughly to a pre orogenic synsedimentary normal fault its continuation shows up clearly in a dekorp seismic reflection profile to a depth of 12 15 km

new evidence of a meridional current system in the equatorial ionosphere an interesting variation in the geomagnetic d component has been found by analysis of the magsat data characteristics of this variation are that it appears everyday on the low latitude duskside and is antisymmetric about the dip equator negative in the northern hemisphere that the magnitude from 5 to 25 nt depends upon the geographic longitude and altitude and that no corresponding change can be seen on the ground magnetic data the change suggests a meridional current system in the equatorial ionosphere produced in association with the electrojet current along the dip equator this effect should be taken into account in the study of geomagnetic anomaly of internal origin with the magsat data

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

the origin of the basse normandie duplex boulonnais france abstract two hercynian duplexes are developed in visean limestones in the basse normandie quarry the lower duplex is completely exposed in a subvertical quarry wall the partially exposed upper duplex lies immediately above the lower duplex the duplexes are both located in the footwall of the hydrequent thrust which emplaced devonian clastic sediments above the visean limestones the lower duplex exposes all the internal thrusts a reference bed of chalky limestone the roof and floor thrusts and the duplex tip the duplex has been graphically restored to its pre deformation geometry by line length and area balancing and its resultant geometry is close to the model of boyer elliott the lower duplex shortened by two different mechanisms an initial phase of layer parallel shortening which produced no cleavage followed by thrust imbrication the average contraction of the front portion of the duplex was 49 natural strain of which 27 is layer parallel shortening and 22 is thrust imbrication however locally the bulk shortening increases from zero at the duplex tip to over 120 in a down dip direction the area balancing provides the most accurate estimates of bulk shortening line length balance calculations give minimum estimates only an area balance on the whole of the lower duplex gives a bulk shortening of 84 an area balance of the upper duplex yields an average contraction of 75 and the total contraction produced by both duplexes is 92

strain partitioning across a fold and thrust belt the rhenish massif mid european variscides abstract strain data across the entire western rhenish massif illustrate the pattern of deformation partitioning within a fold and thrust belt from the foreland on the northern front of the european variscides to the suture with the internal zones of higher metamorphic grade shortening perpendicular to cleavage increases from 16 to 27 constant volume assumed in the northern rhenish massif to 51 at the southern border these values correspond to a weak general layer parallel shortening of the basin filling in cross section on the order of about 5 15 and more in the case of volume loss the geometry of apparent finite strain is nearly plane strain except in the venn anticlinorium where strain is apparent prolate and near the southern margin of the massif where it is apparent oblate net orogenic shortening in the upper crust of the fold and thrust belt is approximately 42 it is mainly achieved by folding and tectonic stacking of the deformed basin filling which was apparently detached from the lower crust distribution of strain in the orogenic wedge is controlled by different factors at different scales heterogeneity is caused by changes in lithology and average grain size by the structural position of samples in folds and their distance from thrusts in the hangingwall at deeper crustal levels and by the regional distribution of the metamorphic grade i e by the dominant deformation mechanism this last factor causes an exponential increase of strain with approach to the ductile lower crust the superposition of strain partitioning mechanisms on different scales creates a complex regional strain pattern the southern part of the thrust belt apparently suffered late westward motion in gently dipping imbricates or strike parallel motion in steeply dipping imbricates which is probably due to oblique convergence between the rhenohercynian and saxothuringian microplates

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

the bolivian altiplano conductivity anomaly 1 a long period magnetotelluric study was carried out in the central andes between latitudes 19 5 s and 21 s along two almost parallel profiles of 220 and 380 km length respectively the investigation area extends from the pacific coast to the southern altiplano plateau in the back arc of the south american subduction zone the main geoelectrical structure resolved is a broad and probably deep reaching highly conductive zone in the middle and deeper crust beneath the high plateau although the data show deviations from two dimensionality a two dimensional approach is justified for large parts of the profiles sensitivity studies were carried out in order to constrain the depth extent another electrically conductive structure was resolved in the middle crust of the chilean forearc thought to be connected with the precordillera fault system the andean continental research program ancorp seismic reflection profile carried out along the same line at 21 s revealed highly reflective zones below the altiplano in good correlation with the upper boundary of the altiplano conductor this highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp vs ratio and although not well resolved with low qp seismic quality factors taking into account the enhanced heat flow and a derived temperature model the most probable explanation lies in the assumption of granitic partial melts the good conductor below the volcanic arc which was found in regions farther south at 22 s gradually vanishes toward the north this is consistent with the results of seismic tomography concerning qp values and a gap of recent volcanism

storm time changes of geomagnetic field at magsat altitudes 325 550 km and their comparison with changes at ground locations the values of h x y and z at magsat altitudes were first expressed as residuals h x y and z after subtracting model values hmd xmd ymd and zmd the storm time variations of h showed that h dusk was larger negative than h dawn and occurred earlier indicating a sort of hysteresis effect effects at magsat altitudes were roughly the same 10 accuracy as at ground indicating that these effects were mostly of magnetospheric origin the y component also showed large storm time changes the latitudinal distribution of storm time h showed north south asymmetries varying in nature as the storm progressed it seems that the central plane of the storm time magnetospheric ring current undergoes latitudinal meanderings during the course of the storm

sensitivity of shear zones in orogenic wedges to surface processes and strain softening abstract we investigate the internal deformation of orogenic wedges growing by frontal accretion with a two dimensional numerical model our models are limited to crustal deformation and assume a horizontal detachment as observed for various natural orogens e g alaska and costa rica the model wedges develop as a result of convergence of a brittle sediment layer in front of a strong backstop we find that our reference model develops in sequence forward thrusts which propagate upward from the basal detachment for this reference model we investigate the sensitivity of shear zone activity to surface processes and strain softening model results show that diffusive or slope dependent erosion enhances material transport across the wedge and slows down forward propagation of the deformation front frictional strain softening focuses deformation into narrow shear zones and enhances displacement along them this has also been postulated for natural thrusts such as the glarus thrust in the swiss alps and the moine thrust in the scottish caledonides a second series of models investigates the effects of regularly spaced weak inclusions within the sediment layer which simulate remnants of previous deformation phases these inclusions facilitate and focus internal deformation influence the thrust dip and thrust vergence and enable thrust reactivation in the internal part of the wedge our results show that inactive thrusts in the internal part of the wedges may be reactivated in models with diffusive surface processes strain softening or weak inclusions thrust reactivation occurs as models seek to maintain their critical taper angle first order characteristics of our numerical models agree well with natural orogenic wedges and results from other numerical and analogue models

orogenic mass transfer and reflection seismic patterns evidence from dekorp sections across the european variscides central germany abstract orogenic wedges evolve by different mass transfer modes which control different styles of deformation metamorphic pattern and bulk reflectivity structural patterns and reflectivity strongly vary depending mainly on the length thickness ratio of thrust sheets and on their displacement length ratio reevaluation of dekorp seismic sections across the central european variscides lines dekorp 1a c 9n 2n 2s 3b underlines the usefulness of the integrated interpretation strategy in establishing orogenic growth mode and evolution the results are in contrast with reflectivity patterns predicted from critical taper theory the northern flank of the mid european variscides is formed by a doubly vergent orogenic wedge system which evolved from collision of a passive margin rhenohercynian zone with a continental arc mid german crystalline rise mgcr of the saxothuringian zone the two fold and thrust wedges show inward dipping shear zone related reflections above subhorizontal mid crustal reflection bands evidence of dominantly frontal accretion of upper crustal material in both wedges in contrast the sections crossing the axial zone mgcr either show arched reflection stacks related to underplated amphibolite facies rocks in exposed antiformal nappe stacks or a transparent wedge shaped unit related to low grade calcalkaline magmatics and steeply dipping devono carboniferous sediments of the former upper plate magmatic arc analysis suggests that part of this arc was tectonically eroded prior to collision in combination with isograd and structural patterns the observed features indicate that heterogeneous basal accretion was the dominant accretion mode in the axial zone during collision the discussed patterns are suggested to reflect the material transfer site into the orogenic wedge and therefore also growth mode and material provenance in collisional belts

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

influence of basin architecture on the style of inversion and fold thrust belt tectonics the southern adelaide fold thrust belt south australia abstract the southern adelaide fold thrust belt involves neoproterozoic adelaidean strata deposited during protracted rift and sag phases and an overlying cambrian kanmantoo sequence represented by a thick wedge of turbidites rapidly deposited in an eastwards deepening basin deeply incised into the underlying rocks the belt was deformed during the delamerian orogeny developed at the southeastern margin of the australian craton during early palaeozoic nw directed displacement during this shortening event the margin of the kanmantoo basin was reactivated and formed a significant strain guide that controlled key features of the fold thrust belt strain integrated and restored sections reveal contrasting geometric and kinematic styles along the strike of the belt a transpressional zone is developed at the steep basin margin in the south oriented oblique to the structural grain the central parts of the belt are dominated by strong buttressing growth fault reactivation and basement involved footwall shortcut thrusting in the northern part of the belt shortening is accommodated by homogeneous folding above a master de collement these variations in regional structural style and internal strain partitioning reflect significant variations in the original kanmantoo basin margin geometry rheological parameters control the partitioning of strain associated with pervasive fabrics for example cleavage and transposed bedding folding is accompanied by negative line length changes shortening in the northern part of the belt in its thrusted central part pelites commonly show positive line length changes psammites were least affected by line length changes and thus provide key beds for section balancing pervasive strain makes a full restoration of cross sections impossible however incorporating regional strain data assists establishment of balanced and restorable sections which provide a powerful tool in understanding both pre deformational basin geometry and its shortening

electrical conductivity beneath the bolivian orocline and its relation to subduction processes at the south american continental margin 1 a long period magnetotelluric data set was obtained during 2002 and 2004 in the central andes to study the deep electrical conductivity structure in the region of the bolivian orocline between latitudes 17 s and 19 s the profile extends from the coastal cordillera in northernmost chile crosses the volcanic arc and the altiplano high plateau in central bolivia and ends in the eastern cordillera two dimensional inversion revealed several well defined conductivity anomalies in upper crustal levels the conductive sedimentary basins of the central altiplano and the resistive arequipa block beneath the western altiplano are imaged earlier seismological and magnetotelluric investigations on the southern altiplano inferred a large highly conductive partially molten body in the mid to deep crust it was assumed that this structure would be underlying the entire plateau but this is not the case according to the new models instead the most prominent feature in the new investigation area is a high conductivity zone at upper mantle depths below the high plateau which may be interpreted as an image of partial melts and fluids triggered by water supply from the subducting nazca slab this conductor would be in accordance with the standard subduction scenario it is however laterally offset by almost 100 km from the volcanic arc in contrast the deep crust and upper mantle beneath the arc is moderately resistive both observations may hint at an emerging shift of the magmatic fluid system in the central andes

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

structural inheritance and cenozoic stress fields in the jura fold and thrust belt france abstract based on an analysis of 8000 minor fault slip data in the jura mountains france we discuss the influence of pre existing discontinuities on the development of fold and thrust belts we present palinspastic maps showing the stress fields and active faults during the cenozoic pre orogenic events in the jura belt prior to the main late miocene fold and thrust tectonics during the eocene a n s strike slip regime produced a few nne ssw sub vertical strike slip faults in the central external jura and a few e w reverse faults in the eastern jura near the future frontal thrust during the oligocene an average wnw ese extension with irregular stress trajectories resulted in normal faulting along n s to ne sw trends in the external part of the belt wnw ese trends along the future northern and northeastern frontal thrust and nw se trends in the internal jura the late miocene tectonics began with a strike slip regime with a fan shaped compressional trajectory it was followed by a stress field with similar stress direction but local 2 3 stress permutation resulted in strike slip regime domains contrasting with reverse regime domains stress deflections and permutations occurred near inherited cover and basement discontinuities major deformation zones like the jura frontal thrust onto the foreland the thrust of the internal central jura onto the external jura and the narrow deformation bands within the flat lying plateaus formed close to the inherited faults the structural style of the jura belt thus partly mimics the pre orogenic fault pattern stress deflections point to the pre orogenic faults express the indentation process of the jura by its hinterland and highlight successive slip events along major faults during the fold and thrust tectonics this case study illustrates the relevance of minor fault slip studies for characterizing both the pre orogenic tectonics and the kinematics of the deformation

a two stage exhumation in western french massif central new geochronological evidences of syn collisional extension abstract systematic 40ar 39ar dating of granitoids and surrounding metamorphic rocks has been performed in the limousin area w of french massif central in order to provide new constrains concerning its late orogenic thermal and tectonic evolution the same 40ar 39ar age of ca 335 338 ma is found on the present day outcrop surface for most of the limousin metamorphic rocks hence indicating that the entire massif was uniformly cooled down at this period these ages are very close to the 40ar 39ar ages proposed in the sioule and brevenne areas in the literature but also to the coupled tectonic and magmatic activity in the south limousin these results help to document the existence of two exhumation stages each one associated with an extensional stage a first exhumation stage has been active at ca 335 340 ma available data indicate that this extension and associated exhumation and cooling occurred over a distance of more than 200 km in the internal zones of the belt whereas compression was still active in the northern and southern parts of the belt a second exhumation stage may be related to the late orogenic extension and collapse of the belt at ca 310 305 ma and is characterized by the uplift of the st sylvestre complex by unroofing of the overlying metamorphic and granitoidic formations

moho geometry beneath the eastern margin of the andes northwest argentina and its implications to the effective elastic thickness of the andean foreland the seismically constrained shape of the moho beneath the eastern margin of the northwest argentine andes 24 s is interpreted in the context of regional isostasy to constrain the flexural rigidity of the lithosphere in this actively deforming orogen seismic travel time residuals from regional intermediate depth earthquakes are inverted for three dimensional variations in the moho depth beneath a portable seismic network spanning the eastern margin of the puna plateau and the santa barbara foreland system beneath the easternmost santa barbara ranges the moho is relatively flat and lies at 40 45 km depth farther west beneath the eastern margin of the puna the moho dips 10 15 to the west before flattening out at a depth of 60 65 km beneath the plateau the moho inversion model and coincident gravity data constrain the flexural rigidity of the foreland lithosphere to be between 1021 and 1022 m the corresponding effective elastic thickness of 6 12 km is a factor of 2 to 4 less than that estimated in previous published studies for the bolivian altiplano and subandean ranges farther to the north these results indicate that in contrast to the bolivian andes the foreland lithosphere in northwest argentina does not provide significant flexural support to the plateau margin and the eastern margin of the andes is nearly in a state of local isostatic compensation the north to south decrease in the effective elastic thickness of the andean lithosphere supports published seismic wave attenuation studies which indicate that south of 23 s the lithospheric thickness decreases beneath the plateau and foreland

electrical conductivity beneath the volcanoes of the nw argentinian puna the detection of a high conductivity anomaly below the northern bolivian part of the altiplano puna plateau motivated a new analysis of magnetotelluric mt data which were formerly collected further south in nw argentina after appropriate dimensionality analysis from a profile located in the magmatic arc and backarc of the south central andean subduction zone two dimensional 2 d electrical conductivity modelling of the crust and upper mantle in the eastern sector could be performed i e in the eastern puna and backarc zone the 2 d conductivity models show a conductive zone beneath the eastern puna shoshonitic volcanoes and nearby tuzgle volcano which reaches from the upper crust to the upper mantle we suggest that this conductor is related to the puna volcanism of enriched mantle composition whether the western puna is also characterized by anomalous conductivities at crust mantle depths is still an open question

lithospheric flexure and bending of the central andes gravity anomaly and topography data are used to define the effective elastic thickness of the lithosphere t in the bend region of the central andes values of z increase from nearly zero north and south of the bend to values greater than 50 km at the bend in bolivia there is a close correlation between t and the style and magnitude of the shortening in the central andes since the late miocene in the bend region where the lithosphere is flexurally strong with large t foreland deformation is concentrated into a thin skinned fold and thrust belt above a basal decollement which has absorbed more than 100 km of shortening further north and south where the lithosphere is flexurally weak with low t foreland deformation is more complex involves basement and has absorbed less shortening the along strike gradients in foreland shortening have accommodated both clockwise and anticlockwise rotations about a vertical axis of almost the entire width of the bolivian andes we speculate that the observed variation in t is related to the proximity of the brazilian shield and has been an important factor in controlling the nature and amount of foreland deformation and hence bending of the central andes the flexural properties of the lithosphere may play an important role in determining the large scale evolution of mountain belts

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

the basse normandie duplex boulonnais n france evidence for an out of sequence thrusting overprint abstract the thrust sheets outcropping in the basse normandie quarry near the hydrequent village boulonnais n france represents an exceptionally well exposed section of the nw european variscan thrust front these structures developed in the footwall of the main hydrequent thrust have been often described as a classic example of a duplex structure only the lower part of the structure satisfies however the geometric and kinematic criteria defining a duplex and more precisely an intraformational hinterland dipping duplex the upper thrust sheets of the imbricate stack exhibit a much more complex pattern of deformation than a simple piggy back duplication of the same rock sequence restoration of these thrust sheets based upon the definition of two marker beds as well as the analyses of fold thrust relationships and strain markers argues for a late ne verging thrust event that progressed within the thrust system from the tip of the upper thrust sheets towards the hydrequent thrust in a local break back style of thrust propagation this out of sequence thrusting event induced refolding and cross cutting of the forelimb of a hangingwall anticline developed previously above the footwall ramp of the nne verging basal thrust of the structure within the whole thrust system the lower duplex represents only a minor structure developed during the initial phase of thrusting in the foreland of the major anticline as a frontal second order duplex the structural data presented in this paper illustrate the tectonic processes acting within the deformed zones lying in the footwall of major thrusts and emphasize the out of sequence style of thrust migration that arises from the sequential blocking of thrust propagation towards the foreland