magnetic susceptibility fabrics in slates structural mineralogical and lithological influences abstract a single penetrative tectonic deformation of archean greywackes accompanied by low grade metamorphism generated low field magnetic susceptibility fabrics that are coaxial with the rock s schistosity although there are multiple sources of magnetic susceptibility pre metamorphic magnetite and the metamorphic minerals pyrrhotite biotite and chlorite variety thuringite daphnite aphrosiderite are responsible for the anisotropy of susceptibility these minerals grew with preferred crystallographic orientations influenced by the prevailing stress regime on its own rotation cannot be invoked to explain the alignment of the metamorphic minerals there are varying proportions of at least two of these minerals in most specimens for 153 specimens the degree of anisotropy increases as the mean susceptibility increases over the range from 100 to 700 in units of 10 6 s i theory shows that this is to be expected either due to increasing the traces of strongly oriented ferrimagnetics or due to increasing the proportion of chlorite in the latter case this simultaneously increased its preferred orientation by closer packing stress influenced crystallization of chlorite rapidly achieves saturation alignment and a limiting magnetic anisotropy although strain may continue to increase samples taken through individual graded beds show that the susceptibility ellipsoid varies from oblate in slaty rocks to a neutral shape in sandier rocks however its orientation is always structurally controlled with the minimum susceptibility axis perpendicular to schistosity

magnetic susceptibility anisotropy strain and progressive deformation in permian sediments from the maritime alps france abstract the relationships among magnetic susceptibility anisotropy finite strain and progressive deformation have been studied in permian red shales and slates of the maritime alps southeastern france these rocks contain deformed reduction spots which serve as finite strain indicators the magnetic fabric of undeformed regions is modified during deformation to yield characteristic magnetic susceptibility anisotropy patterns and a magnetic equivalent of the deformation path derived from strain measurements the magnetic fabric changes progressively from oblate to prolate and back to oblate as deformation increases the quantitative relationships between natural strain and magnetic anisotropy in these rocks have been determined they differ between the less and more deformed areas perhaps due to a change in deformation mechanism accompanying an increase in metamorphism the relationships provide a rapid means of strain determination using magnetic measurements but their variation emphasizes the need for local structural control

tectonic evolution of a continental collision zone a thermomechanical numerical model we model evolution of a continent continent collision and draw some parallels with the tectonic evolution of the himalaya we use a large scale visco plasto elastic thermomechanical model that has a free upper surface accounts for erosion and deposition and allows for all modes of lithospheric deformation for quartz olivine rheology and 60 mm yr convergence rate the continental subduction is stable and the model predicts three distinct phases during the phase 1 120 km or 6 of shortening deformation is characterized by back thrusting around the suture zone some amount of delaminated lower crust accumulates at depth during phase 2 120 km 420 km or 6 22 of shortening this crustal root is exhumed medium to high grade rocks along a newly formed major thrust fault this stage bears similarities with the period of coeval activity of the main central thrust and of the south tibetan detachment between 20 16 myr ago during phase 3 420 km or 22 of shortening the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge deformation is localized around frontal thrust faults this kinematics should produce only low to medium grade exhumation this stage might be compared with the tectonics that has prevailed in the himalaya over the last 15 myr allowing for the formation of the lesser himalaya the experiment is conducted at constant convergence rate which implies increasing compressive force considering that this force is constant in nature this result may be equivalent to a slowing down of the convergence rate as was observed during the india asia collision

sample preparation effects on illite crystallinity measurement grain size gradation and particle orientation abstract specimen thickness is the main experimental factor controlling the results of illite crystallinity ic or crystallite size measurements on sedimentation slides different values obtained from thick and thin preparations are due to grain size gradation effects which may exclude larger and higher ordered grains from contributing to the diffraction orientation effects control the measured peak intensity the change from poor particle orientation in thick slides to high orientation in very thin slides is marked by an increase in specimen density diminishing non basal reflections and by a strong increase in peak intensity a plateau with constant peak breadths is observed if thin slides of well ordered platy illites are used a similar plateau can be recognized for thick preparations of specimens from less ordered materials but not from well ordered ones therefore it is suggested that ic is determined on very thin sedimentation slides with a thickness of 0 25 mg cm2 or less ultrasonic and h2o2 treatments enhance the degree of particle orientation by destruction of grain aggregates and organic compounds leading to smaller peak breadths and higher intensities

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

tectonic versus mineralogical contribution to the magnetic fabrics of epimetamorphic slaty rocks an example from the ardennes massif france belgium abstract the discrimination of the respective contribution of strain and mineralogy of the bulk anisotropy of low field magnetic susceptibility ams of rocks is addressed by a study of the ardennes variscan thrust belt unmetamorphosed to low medium grade rocks in the study area define a succession of magnetic parageneses hematite maghemite hematite maghemite magnetite magnetite pyrrhotite a direct control of the magnetic mineralogy on the magnitude of ams axes is shown in addition in all cases the interpretation of the directions of ams axes as kinematic indicators also applies to these rocks we show a progressive change in shearing direction from the south of the rocroi massif where the nappe transport is n directed in the north the vergence changes to northwestward

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

the influence of deformation mechanisms on magnetic fabrics in weakly deformed rocks abstract magnetic fabrics of weakly deformed rocks are not simply correlated with the principal finite strain directions depositional magnetic fabrics persist at 30 shortening where the rocks are deformed by particulate flow grain boundary sliding rocks with a spaced stylolitic cleavage believed to be formed by pressure solution and shortened by 20 50 have maximum susceptibility parallel to the intersection of cleavage and bedding this type of magnetic fabric is attributed to the interference of a depositional magnetic fabric between cleavage surfaces with a tectonic magnetic fabric in the cleavage surfaces

slaty cleavage development and magnetic anisotropy fabrics the shale to slate transition preserved in the ordovician martinsburg formation at the lehigh water gap pennsylvania provides an opportunity to study the relationship between magnetic anisotropy fabrics and the development of slaty cleavage our previous work has indicated that anisotropy of magnetic susceptibility ams does not record changes in finite strain associated with cleavage development in these rocks but instead measures the degree of dissolution and new growth of ehlorite additional ams data presented in this paper lend further support to this conclusion conversely anhysteretie remanent magnetization anisotropy arma which is not affected by paramagnetie ehlorite accurately reflects the strain induced rock fabrics associated with cleavage formation arma results show that magnetite dimensional orientations vary from bedding parallel in shale samples to cleavage parallel in samples with well developed slaty cleavage samples with weak and pencil cleavage display scattered arma orientations which lie in between bedding and cleavage these intermediate orientations may be due to either passive rotation of magnetite from bedding parallel to cleavage parallel or re crystallization of magnetite if rotation occurred grain rotation was highly heterogeneous in the samples with incipient cleavage the intermediate arma orientations may also reflect the varying contribution of two magnetite preferred orientations a depositional orientation parallel to bedding and a new growth orientation parallel to cleavage

recognition and implication of tectonic loading induced reheating in the northern variscan front belgium and northern france based on an illite kubler index and oxygen isotope study illite kubler index ki and oxygen isotope brachiopods and micrites investigations have been performed on more than 300 frasnian limestones sampled in one borehole and numerous outcrops in the dinant synclinorium belgium northern france of the northern variscan front the illite kubler index and 18o data of a 3 km thick tectonically repeated frasnian series from the focant borehole are compared with their surrounding surface correspondents and document in situ reheating induced by variscan tectonic loading which post dated sedimentary burial alteration the boundary between these two thermal processes sedimentary burial and tectonic loading on the focant profile corresponds to an important location where the heat induced by the tectonic loading was equivalent to that frasnian strata suffered during maximum sedimentary burial mainly based on this knowledge and on a former conodont colour alteration index study the thickness of the eroded thrust sheet in the focant area is estimated to be around 3 000 m oxygen isotopic exchange in these frasnian closed carbonate systems occurring under highest grade diagenesis and anchimetamorphism records two events brachiopods present a quite different and more homogeneous pattern due to their higher resistance to heat alteration these thermal events caused both 18o records to become increasingly lighter than the presumed original seawater signature the comparison between ki and 18o profiles indicates that illite ki analysis is more appropriate than 18o in highlighting the temperature variations in the burial metamorphism at the periphery of orogenic belts

magnetic fabric study of the se rhenohercynian zone bohemian massif implications for dynamics of the paleozoic accretionary wedge the progressive deformation recorded in the magnetic fabric of sedimentary rocks was studied in the se rhenohercynian zone rhz eastern margin of the bohemian massif czech republic almost 800 oriented samples of the lower carboniferous mudstones and graywackes were collected from the sse part of the czech rhz so called the drahany upland the anisotropy of magnetic susceptibility ams is predominantly controlled by the preferred orientation of paramagnetic phyllosilicates mainly iron bearing chlorites a regional distribution of the magnetic fabric within the drahany upland revealed an increasing deformation from the sse to the nnw in the se the magnetic fabric is bedding parallel with magnetic lineation scattered in the bedding plane or trending n s to nne ssw further to the nw the magnetic foliation rotates from the bedding parallel orientation to the orientation parallel to the evolving cleavage

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

pre and synorogenic burial metamorphism in the ardenne and neighbouring areas rhenohercynian zone central european variscides abstract the rhenohercynian zone of the central european variscides contains in the central ardenne eifel brabant campine region of belgium france germany and luxemburg an omnipresent thermal overprint which reaches in places uppermost epizonal conditions because of its distribution and structural relationships this metamorphism cannot be explained by a singular tectonometamorphic event field and microstructural observations new illite crystallinity and other data relating to this metamorphism are presented together with a review of existing petrological illite crystallinity vitrinite reflectance conodont alteration index and fluid inclusion data and their relationship to small and regional scale geological structures three distinct geotectonic situations can be described which overlap and alternate in space and time 1 the oldest metamorphism which reached epizonal conditions can be localized in the pre devonian basement rocks of the brabant massif and with minor evidence of the stavelot massif further south it is clearly related to a pre variscan caledonian deformation event and could be of burial origin 2 the main regional metamorphism affected all devonian and carboniferous rocks and the lower palaeozoic basement highs of the ardenne eifel area it does not exceed diagenetic conditions in some areas but reaches maximal values of 500 c and 400 mpa in the serpont and rocroi massif areas theye and fransolet 1993 robion et al 1995 it is pre to synkinematic to the main variscan contractional penetrative deformation and correlates well with the age and thickness of the sedimentary cover rocks the metamorphism is associated with the pre carboniferous preorogenic rifting stage of the rhenohercynian basin and documents the peak of subsidence and sediment accumulation we interpret it as diastathermal metamorphism it reaches its maximum in the low grade metamorphic zones of the ardenne area which are inverted synsedimentary structures 3 the diastathermal metamorphism is overprinted in the central part of the ardenne eifel area middle devonian to carboniferous rocks of the dinant synclinorium by an anchizonal beginning epizonal metamorphism in the foreland basins north of the midi fault zone it reaches diagenetic to lowermost anchizonal conditions it is the expression of the burial by thick synorogenic mid to upper carboniferous cover rocks which were deposited in piggyback and foreland basins this clastic wedge relates to the carboniferous contraction of the rhenohercynian basin whose metamorphosed rocks were themselves affected by the deformation of the advancing orogenic front

diagenetic versus detrital origin of the magnetic susceptibility variations in some carbonate frasnian famennian boundary sections from northern africa and western europe implications for paleoenvironmental reconstructions to provide a new insight into the diagenetic versus detrital origin of the magnetic susceptibility variations in ancient carbonate sequences a study was conducted within four frasnian famennian platform carbonate sections from germany france and morocco the study includes along section magnetic susceptibility and carbonate content measurements complemented by analyses of magnetic hysteresis parameters inorganic geochemistry and clay mineralogy our results show that the magnetic susceptibility evolution is dominantly controlled by the variations in the concentration of low coercivity ferromagnetic magnetite grains and to a lesser extent of paramagnetic clays in more detail hysteresis ratios suggest the coexistence of two magnetite populations with significantly different grain size 1 a dominantly coarse grained detrital fraction including a mixture of multi domain and single domain particles 2 an authigenic fine grained fraction composed of a mixture of single domain and superparamagnetic particles despite a diagenetic imprint on the clay assemblages no relationship is established between magnetic susceptibility and illite crystallinity therefore discarding a noticeable distortion of primary within section magnetic susceptibility evolution the overall inherited character of the magnetic susceptibility fluctuations is corroborated by a significant correlation of magnetic susceptibility with terrigenous proxies zr th the poorer correlation of magnetic susceptibility with the fe content is consistent with the existence of a very fine grained authigenic magnetite component that possibly induces a global magnetic susceptibility increase at the section scale but no distortion of the within section evolution the magnetic susceptibility curves presented here provide a general record of climate driven detrital influx and carbonate productivity through frasnian lower famennian times

salt redistribution during extension and inversion inferred from 3d backstripping abstract a 3d backstripping approach considering salt flow as a consequence of spatially changing overburden load distribution isostatic rebound and sedimentary compaction for each backstripping step is used to reconstruct the subsidence history in the northeast german basin the method allows to determine basin subsidence and the salt related deformation during late cretaceous early cenozoic inversion and during late triassic jurassic extension in the northeast german basin the deformation is thin skinned in the basinal part but thick skinned at the basin margins the salt cover is deformed due to late triassic jurassic extension and late cretaceous early cenozoic inversion whereas the salt basement remained largely stable in the basin area in contrast the basin margins suffered strong deformation especially during late cretaceous early cenozoic inversion as a main question we address the role of salt during the thin skinned extension and inversion of the basin in our modelling approach we assume that the salt behaves like a viscous fluid on the geological time scale that salt and overburden are in hydrostatical near equilibrium at all times and that the volume of salt is constant because the basement of the salt is not deformed due to decoupling in the basin area we consider the base of the salt as a reference surface where the load pressure must be equilibrated our results indicate that major salt movements took place during late triassic to jurassic e w directed extension and during late cretaceous early cenozoic nne ssw directed compression moreover the study outcome suggests that horizontal strain propagation in the salt cover could have triggered passive salt movements which balanced the cover deformation by viscous flow in the late triassic strain transfer from the large graben systems in west central europe to the east could have caused the subsidence of the rheinsberg trough above the salt layer in this context the effective regional stress did not exceed the yield strength of the basement below the rheinsberg trough but was high enough to provoke deformation of the viscous salt layer and its cover during the late cretaceous early cenozoic phase of inversion horizontal strain propagation from the southern basin margin into the basin can explain the intensive thin skinned compressive deformation of the salt cover in the basin the thick skinned compressive deformation along the southern basin margin may have propagated into the salt cover of the basin where the resulting folding again was balanced by viscous salt flow into the anticlines of folds the huge vertical offset of the pre zechstein basement along the southern basin margin and the amount of shortening in the folded salt cover of the basin indicate that the tectonic forces responsible for this inversion event have been of a considerable magnitude

rheological and geodynamic controls on the mechanisms of subduction and hp uhp exhumation of crustal rocks during continental collision insights from numerical models while subduction of crustal rocks is increasingly accepted as a common scenario inherent to convergent processes involving continental plates and micro continents its occurrence in each particular context as well as its specific mechanisms and conditions is still debated the presence of ultra high pressure uhp terranes is often interpreted as a strong evidence for continental subduction subduction of continental crust since the latter is seen as the most viable mechanism of their burial to uhp depths yet if one admits nearly lithostatic pressure conditions in the subduction interface or channel the presumed links of continental subduction to exhumation of high and ultra high pressure hp uhp units also remain a subject of controversy despite the fact that recent physically consistent thermo mechanical numerical models of convergent processes suggest that subduction can create specific mechanisms for uhp exhumation we hence review and explore possible scenarios of subduction of continental crust and their relation to exhumation of hp and uhp rocks as inferred from last generation of thermo mechanical numerical models accounting for thermo rheological complexity and structural diversity of the continental lithosphere the inferences from these models are matched with the petrology data in particular with p t t paths allowing for better understanding of subtleties of both subduction and burial exhumation mechanisms numerical models suggest that exhumation and continental subduction are widespread but usually transient processes that last for less than 5 10 myr while long lasting 10 15 myr subduction can take place only in rare cases of fast convergence of cold strong lithospheres e g india the models also show that tectonic heritage can play a special role in subduction exhumation processes in particular when thicker continental terrains are embedded in subducting oceanic plate exhumation of uhp terranes results in the formation of versatile metamorphic belts and domes and in series of slab roll back and exhumation events with remarkably different p t t records

rheological and tectonic modeling of salt provinces abstract technological improvements and a more rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden the fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth converted seismic sections scale models are therefore useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps although even unsealed models may help generating ideas the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes previously published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden this paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other intrinsic material properties scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics including those with synkinematic sedimentation

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

a comparative study of magnetic anisotropy measurement techniques in relation to rock magnetic properties abstract magnetic anisotropy measurements are becoming increasingly common to many studies within the different disciplines of geology involving sedimentary igneous and metamorphic rocks a plethora of techniques exists for measuring magnetic anisotropy of rocks some are rapid and non destructive while others are more labor intensive or may result in alteration of the magnetic minerals all however have the potential of revealing a wealth of information when measured and interpreted correctly in broad terms anisotropy techniques subdivide into measurements of susceptibility remanence and torque here we consider the first two of these anisotropy of magnetic susceptibility ams is by far the most utilized and measures composite fabrics magnetic susceptibilities in high fields and low temperatures however are being increasingly used to isolate the paramagnetic contribution to the fabrics when distinguishing between fabrics carried by different ferromagnetic phases or to separate these from the diamagnetic and paramagnetic contributions to the fabric then remanence anisotropy techniques become necessary anisotropies of thermal remanence atrms of anhysteretic remanence aarm and of isothermal remanence airm are the most common examples remanence anisotropy may be measured over the full spectrum of magnetic coercivities or over a targeted range e g partial or aparm moreover anisotropies may be calculated using only the resolved field parallel component of the vector in which case a minimum of six different orientations is necessary to obtain a complete symmetric tensor or using the three components full vector of the measured magnetic vectors e g avarm in which case three orthogonal applied magnetizations are the minimum requirement in this study we utilize a variety of magnetic remanence room temperature techniques to measure remanence anisotropy of selected coarse and finer grained gneiss granulitic specimens with well pronounced fabrics results are compared to room temperature ams and are interpreted in terms of the applicability of instrumentation technique to specific rock magnetic properties

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

a numerical approach for investigation of stress states induced by salt structures abstract the main oil and gas reserves around the world are found in regions containing evaporitic rocks owing to their low porosity and low permeability these rocks provide favorable conditions for hydrocarbon trapping increasing the probability of success in oil and gas exploration salt rocks exhibit time dependent strain behavior when submitted to deviatoric stresses this behavior is described by creep constitutive laws based on deformation mechanics one of the major challenges in modeling stress states in sedimentary basins in the presence of salt structures is to predict the magnitude of stress perturbations around salt bodies this work presents a geo mechanical approach to take into account the specific weight variation throughout the lithology and the effects of buoyancy in the resulting forces on the interface salt body formation modeling is driven by the fact that salt bodies cannot sustain deviatoric stresses numerical analyses using the finite element method under plane strain conditions provide the results for the comparison of the methodologies found in the literature with the one proposed by the authors two synthetic models with salt dome structures are investigated the proposed approach provided more realistic stress fields in the vicinity of salt structures

estimation of elastic moduli of mixed porous clay composites we have developed a procedure for estimating the effective elastic properties of various mixtures of smectite and kaolinite over a range of confining pressures based on the individual effective elastic properties of pure porous smectite and kaolinite experimental data for the pure samples are used as input to various rock physics models and the predictions are compared with experimental data for the mixed samples we have evaluated three strategies for choosing the initial properties in various rock physics models 1 input values have the same porosity 2 input values have the same pressure and 3 an average of 1 and 2 the best results are obtained when the elastic moduli of the two porous constituents are defined at the same pressure and when their volumetric fractions are adjusted based on different compaction rates with pressure furthermore our strategy makes the modeling results less sensitive to the actual rock physics model the method can help obtain the elastic properties of mixed unconsolidated clays as a function of mechanical compaction the more common procedure for estimating effective elastic properties requires knowledge about volume fractions elastic properties of individual constituents and geometric details of the composition however these data are often uncertain e g large variations in the mineral elastic properties of clays have been reported in the literature which makes our procedure a viable alternative

the anisotropy of magnetic susceptibility ams in low grade cleaved pelitic rocks influence of cleavage bedding angle and type and relative orientation of magnetic carriers

rifting through a stack of inhomogeneous thrusts the dipping pie concept 1 orogenic building leaves a complex heritage consisting of a stack of nappes that may have contrasting lithologic structures resulting in heterogeneous mechanical behavior of the system during the postorogenic stages while the thermal state of the region is reequilibrating strong lateral variations of the depth to the brittle ductile transition develop as a consequence of these preexisting heterogeneities we use a thermomechanical model to quantify how an inherited weak nappe influences the development of fault patterns resulting from postorogenic extension the competence contrast between the nappe and the rest of the upper crust as well as the strength of the crust itself are the principal variable parameters of our experiments the results suggest that a dipping weak nappe introduces a lateral velocity discontinuity and serves as a localization factor for deformation the presence of a preexisting nappe with a low competence contrast is sufficient to localize strain along the nappe leading to the formation of a flexural rolling hinge in this case the migration of the basin is slow continuous and limited by gravity driven processes that lead to the rise of hot weak material under the subsiding basin in case of a high competence contrast overall crustal strength is reduced by a dipping pie effect assuming overall high crustal strength the presence of a contrasting nappe leads to a bimodal fault pattern governed by two types of faults crustal scale planar faults rooting on the brittle ductile transition of the crust and thin skinned listric faults rooting on the nappe itself this bimodality results in a jump like migration of the basin downward along the dipping weak nappe applying this model to the case of the gulf of corinth greece allows us to explain in the case of assumed high competence contrast the observed microseismicity patterns the asymmetry of the gulf and the kinematics of fault migration within the basin

transpressional deformations at lateral boundaries of propagating thrust sheets the example of the meuse valley recess within the ardennes variscan fold and thrust belt n france s belgium abstract the 3d kinematics of the fold thrust structures within the meuse valley recess mvr ardennes variscan thrust belt n france s belgium have been investigated using an integrated approach combining geological mapping cross section analyses and studies of magnetic fabrics and paleomagnetic data at a total of 52 sites distributed within the study area fold thrust geometries strain distribution and thrust sheet rotational patterns lead to a conceptual structural model illustrating natural deformation processes at lateral boundaries of propagating thrust sheets the mvr is shown to result from distributed transpressional deformation of the ardennes basal thrust hanging wall above a buried lateral oblique ramp inherited from the lower middle devonian extensional tectonics of the sedimentary wedge this deep lateral oblique discontinuity basically controls the thickness and basal friction of the ardennes thrust wedge thus resulting in contrasting thrust propagation west and east of the mvr out of plane strain within the transfer zone includes some varying components of normal to bedding shear and lateral shortening depending on the proximity with the buried lateral discontinuity a gradient of lateral shortening is evidenced towards this discontinuity thus acting as a buttress zone we emphasize the high friction conditions prevailing along the lateral borders of the thrust sheets which potentially result in drag effects during foreland directed thrust propagation such additional shear strain is suggested to be accommodated by late pervasive thrust sheet rotation and lateral broadening of the deformed zone subsequent to oblique folding close to the lateral oblique boundary

applicability of magnetic fabrics in rocks associated with the emplacement of salt structures the bicorb quesa and navarres salt walls prebetics se spain abstract this work assesses the applicability of the analysis of the anisotropy of magnetic susceptibility ams to rocks associated with salt structures magnetic fabrics from 16 sites sampled on upper triassic clays keuper facies interbedded within the salt layers of the bicorb quesa and navarres salt walls prebetic zone se spain are mostly characterized by oblate magnetic ellipsoids with their magnetic foliation parallel to the bedding and magnetic lineation contained in the bedding plane but without a constant relationship with dip direction to evaluate the quality and significance of the low field ams several experiments were carried out to characterize the magnetic mineralogy of representative samples and enable separation of the contributions from paramagnetic and ferromagnetic s l minerals the comparison between the orientation of the observed magnetic lineation and that of the structural elements characterizing the internal geometry of the studied salt structures antiforms minor folds faults and shear zones and flanks indicates that the magnetic fabric is controlled by deformation processes affecting the salt walls during their growth and or evolution samples were also analyzed from sixteen sites from miocene rocks outcropping in several syn diapiric miocene half grabens related to the salt walls studied their magnetic fabric is interpreted to reflect the dominant stretching direction at the syn diapiric half grabens during and or shortly after deposition our results indicate that caution is required in interpreting magnetic lineations related to salt flow mechanics from rocks associated with salt structures

late palaeozoic palaeomagnetic and tectonic constraints for amalgamation of pangea supercontinent in the european variscan belt abstract this paper provides a review of the evolution of palaeomagnetic directions and sequence of tectonic events in the european variscan belt during late palaeozoic times these data are correlated with palaeomagnetic records from gondwana and baltica in order to provide a large scale geodynamic framework during the carboniferous to permian the early carboniferous palaeomagnetic records reported mainly from the rhenohercynian and saxothuringian magmatic arcs indicate a 70 anticlockwise rotation while late carboniferous to permian magnetic directions from various rocks of western and central europe are consistent with 120 clockwise rotation of the eastern and central parts of the variscan belt our review of the chronology of tectonic events is based on a robust database of geochronologically constrained deformation metamorphic and magmatic events which allow discretizing the 80 ma long orogenic evolution into five principal events in this scenario the variscan belt can be regarded as a linear sub plate isolated from gondwana and laurussia by the rhenohercynian and palaeotethysian oceans during devonian times this linear composite belt was segmented by transform faults and boundaries in the late devonian to early carboniferous times 360 335 ma during progressive e w closure of rhenohercynian ocean synchronously with collision between saxothuringian and moldanubian blocks subsequent relocation of subduction to the northern boundary palaeotethysian ocean was responsible for n s shortening almost orthogonal to the ancient sub plate n s elongation at around 335 325 ma this deformation resulted in dextral reactivation of transform boundaries associated with anticlockwise rotation of intermittent blocks at the end of this rotation the faults were parallelized to the teysseire tornquist zone the southern margin of baltica while the lozenge shaped blocks of the former variscan sub plate were further shortened during continuous contraction this evolution can fully explain the 70 anticlockwise rotation from cn3 to cn2 palaeomagnetic directions which are regarded as successive carboniferous magnetizations subsequently the variscan belt suffered a giant transtensional event from 325 to 310 ma that was related to the development of extensional syn magmatic core complexes over the whole belt and significant dextral reactivation of earlier nw se trending transform faults this extensional event was associated with important tilts recorded by cp magnetic overprint resulting from a major thermally induced remagnetization during this event new sets of sinistral transfer nne ssw trending faults originated that partly reactivated boundaries of the principal tectonic zones blocks delimited by second order sinistral and first order dextral faults then rotated in a clockwise manner by 80 the whole system subsequently suffered a period of nne ssw shortening that affected the variscan belt namely along the former laurussian and former variscan sub plate contact in the north and in the south where the giant cantabrian orocline developed at around 310 300 ma due to hard collision with gondwana this deformation is associated with the clockwise rotation of laurussia together with the accreted northern sector of the variscan belt and the anticlockwise of gondwana this clockwise 30 rotation is achieved by a1 remagnetizations while the southern part of iberia suffered anticlockwise rotation the final stage of rapid clockwise rotation affecting the northern limb of the iberian arc including corsica and sardinia is attributed to giant n s extension affecting the whole variscan belt at the onset of permo triassic opening of the tethys ocean this complex evolution is regarded as a result of the readjustment of inhomogeneous and thermally and mechanically instable mobile space in between reorganizing the gondwana and laurussia megaplates and the opening of the palaeotethys ocean during the final stages of formation of the pangea supercontinent finally the palaeomagnetically constrained rotations and tectonic evolution of the variscan belt are explained using a pinned model of internal and external rotations of blocks driven by activation of dextral shear zones by n s compression and e w transtension in particular

geomechanical modeling of stresses adjacent to salt bodies part 2 poroelastoplasticity and coupled overpressures we use a fully coupled poroelastoplastic geomechanical model to study how stresses and pore pressures evolve in sediments bounding a spherical salt body drained analyses pore pressures remain hydrostatic demonstrate that sediments yield in response to loading by the salt which leads to a redistribution of stresses and to deformations larger than predicted by poroelastic or solid coulomb plastic models undrained analyses overpressures develop while no dissipation occurs illustrate that salt loading induces pore pressures that extend kilometers away from the salt body we also model the flow and consequent dissipation that occur in the sediments because of this undrained salt loading we show that with time the pressure field dissipates and expands the dissipation process takes millions of years which suggests that pore pressure perturbations caused by salt loading should still be present in mudstones near many salt bodies under drained conditions stress perturbations generate low minimum principal stresses above and below the salt resulting in convergence of pore pressure and minimum principal stress at these locations such conditions are challenging to drill through in undrained systems sharp drops in pore pressure may occur above and below the salt whereas both the pore pressure and the minimum principal stress rise next to the salt in contrast to previous models that do not couple changes in stress to changes in pore pressure the coupled approach presented here has the potential to predict in situ stresses and pore pressures more accurately in a wide variety of geologic settings

to conserve or not to conserve mass in numerical models mass conservation is a fundamental physical law yet computer models aiming to simulate earth evolution commonly fail to respect it indeed most geodynamic models implement phase transitions of rocks such as metamorphism solid solid phase change or melting solid liquid phase change following a simplifying assumption dating back to 1897 which is conserving volume rather than mass the underlying problem is present at different scales illustrated here by three examples metamorphism in the continental crust phase changes in the mantle transition zone and melt crystallization during columnar jointing these illustrate that phase changes may become a driving force of a system s deformation and point to important differences with respect to simplified models developing and applying mass conserving approaches in future modelling tools are therefore not an option but a necessity

influence of uniform basement extension on faulting in cover sediments abstract numerical predictions are compared with the results obtained from sandbox experiments intended to reproduce the effects of uniform basement extension for mechanically homogeneous or uniformly layered sediments that are not strain softened faults are not expected to develop during uniform basement extension even in heterogeneous or strain softening sediments subject to uniform basement extension the only faults to develop reveal an irregular and uncharacteristic distribution of displacements along the fault plane it is inferred that minor and inevitable gradients in strain rate are the most important reason why faulting was observed in the sandbox models irregularities such as topographic variation can also lead to faulting numerical experiments show that with increasing thickness of a basal elastic layer rubber membranes in sandbox models faulting of the cover sediments can cause the strain in the underlying elastic basement layer to become non uniform faulting during uniform basement extension is controlled by irregularities particular to each example and thus does not give rise to a characteristic structural style in the overlying sediments thus sandbox experiments of this type cannot be used as templates for structural interpretation except for the rare case when the initial distribution of sediment and basement material properties are known and their influence is explored from extensive numerical experimentation

interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data clay mineral crystallinity and crystallite domain size data determined by x ray diffraction xrd are methods extensively used in the characterization of very low grade metamorphic conditions however the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained a consequence of the different machine conditions measurement methods and sample preparations used during analysis a calibration approach to the standardization of data using rock chip standards is presented which allows data sets produced by different research groups to be directly and quantitatively compared a standardized scale the crystallinity index standard cis is proposed with illite crystallinity anchizonal boundary limits of 0 25d 2 and 0 42d 2 and equivalent illite crystallite sizes of 52 and 23 nm respectively determined by the warren averbach method calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments and should improve the accuracy and reliability of correlations made between xrd data and other indicators of very low grade metamorphism hence increasing the value of such clay mineral studies

disentangling magnetic subfabrics and their link to deformation processes in cleaved sedimentary rocks from the internal sierras west central pyrenees spain abstract here we present a detailed study of the magnetic fabrics and subfabrics of remagnetized upper cretaceous limolites that crop out in the internal sierras west central pyrenees affected by a penetrative pressure solution cleavage the bulk magnetic fabrics of these rocks rt ams show variable orientations that do not conform to what is typically reported for cleaved sedimentary rocks in contrast the paramagnetic subfabrics lt ams show remarkably constant directional properties so that their k max and k min axes cluster parallel to the intersection lineation and to the poles to bedding respectively these lt ams subfabrics indicate a preferred orientation of phyllosillicates that is consistent with a syn sedimentary late cretaceous period of nne oriented layer parallel shortening noticeably these phyllosillicate subfabrics are not further altered by the subsequent formation of cleavage in the late eocene early oligocene the ferrimagnetic subfabrics aarm also show remarkably constant orientation so that their k max axes are strikingly parallel to the shortening direction in the area we interpret this preferred orientation of ferrimagnetic grains as being caused by subhorizontal shear associated to cleavage formation which is consistent with the age and mechanisms authigenic growth and rotation of pre existing magnetite grains proposed for the pervasive remagnetization that affects the studied rocks

composite magnetic fabric in weakly deformed black shales abstract the anisotropy of magnetic susceptibility ams of weakly deformed sediments of the subalpine terres noires french alps is due to phyllosilicates matrix contribution and detrital titanomagnetites ferromagnetic contribution of variable grain size various rock magnetic and anisotropy techniques have been carried out to understand the origin of the scattering of maximum and minimum susceptibility axes we demonstrate the major contribution of the matrix both to susceptibility and ams anisotropy even when mean matrix susceptibility is weak this suggests that phyllosilicates are better oriented within the foliation plane and that intrinsic anisotropy of the detrital titanomagnetite grains is probably weaker than expected also the separation of the two magnetic subfabrics of three selected sites by coupling together low field ams hfa high field ams and aas anisotropy of anhysteretic remanence demonstrates the existence of a competition between two magnetic lineations and an interchange of the intermediate and minimum ams axes as a result of fine titanomagnetite this is the first evidence of the so called intermediate fabric as predicted by theory

paleoremanence dispersal across a transpressed archean terrain deflection by anisotropy or by late compression the quetico belt is an archean metasedimentary terrain sandwiched between greenstone subprovinces a single regional schistosity is subparallel to magnetic foliation defined by anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanence aarm aligned micas are the primary cause of ams aligned pyrrhotite is the sole cause of aarm each magnetic subfabric is due to minerals that grew in overlapping intervals in the strain history their slight obliquity may be due to crystallization during noncoaxial strain magnetic fabric ellipsoids suggest varying severity of flattening very strong flattening on the northern boundary less severe on the southern boundary and least severe in the interior the interior and the southern boundary of the belt stretched horizontally and the shortening direction varies gradually from ns in the north to nnw in the south the paleomagnetic record carried by pyrrhotite shows characteristic remanences dispersed around a great circle subparallel to the regional schistosity and magnetic foliations late stress effects rotated paleoremanences into this pattern we discount deflection of the paleofield by anisotropy the pattern of dispersal indicates uniaxial oblate flattening of the region with nnw sse shortening late in the metamorphic cooling history 300 c this postdates schistosity ams and aarm fabrics but it is controlled by the same kinematic pattern

mechanical basis for slip along low angle normal faults 1 the existence of active low angle normal faults is much debated because 1 the classical theory of fault mechanics implies that normal faults are locked when the dip is less than 30 and 2 shallow dipping extensional fault planes do not produce large earthquakes m 5 5 however a number of field observations suggest that brittle deformation occurs on low angle normal faults at very shallow dip to reconcile observations and theory we use an alternative model of fault reactivation including a thick elasto plastic frictional fault gouge and test it at large strain by the mean of 2d mechanical modeling we show that plastic compaction allows reducing the effective friction of faults sufficiently for low angle normal faults to be active at dip of 20 as the model predicts that these faults must be active in a slip hardening regime it prevents the occurrence of large earthquakes however we also evidence the neoformation of riedel type shear bands within thick fault zone which we believe may be responsible for repeated small earthquakes and we apply the model to the gulf of corinth greece

channel flow and the development of parallel dipping normal faults 1 in a series of numerical experiments arrays of parallel dipping normal faults formed only if the model contained a thin viscous layer sandwiched between the brittle faulted layer on top and a lower boundary that resisted vertical displacement with this particular vertical stratification parallel dipping faults developed even if the viscous layer had a shear stress free lower boundary condition this observation contradicts previous studies relating parallel dipping normal faults to consistent horizontal shear stress in the brittle layer we explain the formation of parallel dipping normal faults in our experiments through the properties of flow in a thin viscous channel and how this flow accommodates and delocalizes faulting in the brittle layer on top first for closely spaced normal faults an array of parallel dipping faults minimizes the viscous work rate dissipation in the viscous layer as it minimizes the distance between sources and sinks in the flow pattern second a thin layer of moderate viscosity leads to the required formation of closely spaced single faults a highly viscous substratum tends to reflect faults at the base of the brittle layer and hence promotes the formation of a local graben at each necking site in the brittle layer with lower viscosities a thick layer or a floating lower boundary condition usually leads to the formation of widely spaced core complexes in the brittle layer our scheme can explain varying dip polarity along strike which is observed in many rift systems and difficult to explain through consistent shear stress

environmental changes at the frasnian famennian boundary in central morocco northern gondwana integrated rock magnetic and geochemical studies rock magnetic magnetic susceptibility and hysteresis parameters and geochemical analyses major and trace elements were carried out on whole rock samples of two frasnian famennian boundary sections anajdam and bou ounebdou in the central morocco western meseta during the frasnian the decreasing trend of the magnetic susceptibility signal mainly carried by low coercivity magnetite grains indicates a gradual reduction of detrital influx this decrease in detrital input parallels a frasnian long term sea level rise in the late frasnian kellwasser horizons that are classically considered to represent highstand deposits the magnetic signal exhibits the lowest intensities in connection with maximum diamagnetic contribution of the carbonate fraction with respect to geochemical data the two black carbonate rich kellwasser horizons are characterized by noticeable positive anomalies of bottom water dysoxic proxies and of marine primary productivity markers our data thus suggest that in central morocco the late frasnian marine environments were marked by a relatively important biogenic productivity favouring the onset of oxygen depleted conditions during periods of maximum transgression on the continental platforms

composite magnetic anisotropy fabrics experiments numerical models and implications for the quantification of rock fabrics housen b a richter c and van der pluijm b a 1993 composite magnetic anisotropy fabrics experiments numerical models and implications for the quantification of rock fabrics tectonophysics 220 1 12 magnetic fabrics from rocks with multiple mineral preferred orientations may have anisotropy ellipsoids whose shape and orientation arise from the addition of two or more component fabrics our numerical models and experiments demonstrate that such composite magnetic fabrics do not directly reflect the shapes and or orientations of the individual mineral fabrics and we provide criteria for the recognition and interpretation of composite fabrics in natural rocks these criteria include 1 the orientation of the maximum susceptibility axis is located at the intersection of two planar fabrics and 2 the shape of the susceptibility ellipsoid changes from oblate to prolate and the degree of anisotropy decreases as the relative intensity of two planar component fabrics becomes equal and as the angle between the planar fabrics increases composite magnetic fabrics are observed in the shales and slates of the martinsburg formation lehigh gap pennsylvania modeling of the ams anisotropy of magnetic susceptibility and arma anhysteretic remanent magnetization anisotropy behavior constrains the relative degree of anisotropy of the bedding parallel and cleavage parallel fabrics in particular arma model results allow a good estimate of magnetite fabric strength we conclude that in the presence of composite magnetic fabrics quantitative measures of finite strain in deformed rocks are limited by the ability to accurately determine the degree of anisotropy and relative susceptibility of each component fabric such determinations require knowledge of the mineral s that are responsible for the measured magnetic fabric and their behavior during deformation

variscan very low grade metamorphism in southwest england a diastathermal and thrust related origin abstract regional metamorphism in the external variscides of southwest england varied from diagenetic level to greenschist facies there is a fundamental difference in the metamorphic character between the northern and southern regions of the area in the north m1 metamorphism is of a sedimentary burial character associated with high heat flow whilst to the south it is related to tectonic burial during thrust thickening processes with lower geothermal gradients this pattern appears to be related to the character of basin development and its subsequent tectonic evolution the northern region has features that accord with a diastathermal extensional origin for the very low grade metamorphism whilst in the southern region the very low grade metamorphism is linked to thrusting as a consequence of variscan compression the tintagel high strain zone presents an anomaly in this regional pattern where an m2 metamorphic phase is attributed to localized d2 thrust stacking along the southern margin of the culm basin there is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region however there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith excluding the north devon area this link is attributed largely to the late stage structural up doming of the higher grade areas over the roof of the batholith

behaviour of dry sand tested in a large triaxial apparatus in this paper the results from an extensive experimental study of the stress strain behaviour of dry sand in triaxial compression tests are presented the experiments were performed in a new triaxial apparatus for large specimens emphasis is given to sources of error such as end restraint bifurcation and imperfection sensitivity the behaviour at small and large strains is presented and the rigid granular model of sand its practical significance and limitations are discussed dans cet article on presente les resultats d une recherche experimentale sur le comportement rheologique du sable sec en compression triaxiale l appareil triaxial est d un genre nouveau qui contient de tres grands echantillons une etude detaillee des erreurs de mesures dues au frottement des plaques a la bifurcation et a l influence des imperfections est faite le comportement rheologique est presente pour de petites et grandes deformations le modele du sable avec grains rigides sa signification pratique et ses limites seront

magnetic mineralogy and metamorphic zonation in the ardennes massif france belgium abstract in the ardennes area pelitic schists of cambrian to early devonian age have experienced a low grade polyphase metamorphism which can be characterised by changes in magnetic mineralogy using mainly rock magnetic tools we have defined various iron oxides and sulphide assemblages allowing a magnetic mineralogy zoneography of the ardennes metamorphism to be constructed low temperature and three axial irm measurements identify pyrrhotite in the southern part of the massif this mineral had not been previously recognised in the ardennes area in the devonian cover toward the metamorphic apex the appearance of coarse grained haematite is characterised by low coercivity values the trends of low field and anhysteretic susceptibilities allow a discrimination between the lithologic and metamorphic effects to be made additionally the relative contributions of an early metamorphic event and the classical metamorphism linked to the variscan shortening are discussed

numerical models for continental break up implications for the south atlantic we propose a mechanism that explains in one unified framework the presence of continental break up features such as failed rift arms and high velocity and high density bodies that occur along the south atlantic rifted continental margins we used 2d and 3d numerical models to investigate the impact of thermo rheological structure of the continental lithosphere and initial plume position on continental rifting and break up processes 2d experiments show that break up can be 1 central mantle plume induced and directly located above the centre of the mantle anomaly 2 shifted mantle plume induced and 50 to 200 km shifted from the initial plume location or 3 distant self induced due to convection and or slab subduction delamination and 300 to 800 km off set from the original plume location with a 3d perfectly symmetrical and laterally homogeneous setup the location of continental break up can be shifted hundreds of kilometres from the initial position of the mantle anomaly we demonstrate that in case of shifted or distant continental break up with respect to the original plume location multiple features can be explained its deep seated source can remain below the continent at one or both sides of the newly formed ocean this mantle material glued underneath the margins at lower crustal levels resembles the geometry and location of high velocity high density bodies observed along the south atlantic conjugate margins impingement of vertically up welled plume material on the base of the lithosphere results in pre break up topography variations that are located just above this initial anomaly impingement this can be interpreted as aborted rift features that are also observed along the rifted margins when extension continues after continental break up high strain rates can relocalize this relocation has been so far attributed to rift jumps most importantly this study shows that there is not one single rift mode for plume induced crustal break up

composite ferromagnetic fabrics magnetite greigite measured by ams and partial aarm in weakly strained sandstones from western makran iran summary we present examples of composite magnetic fabrics ams and partial aarm in five sites from the western makran accretionnary prism these rocks are weakly strained sandstones of mio pliocene age rock magnetic investigations show a large occurrence of magnetite and also some greigite at two sites partial anhysteretic remanent magnetization coercivity spectra reveal that grain size is not homogeneous suggesting several grain fractions ams foliations are close to bedding except at one greigite bearing site where ams foliation is almost perpendicular to bedding soft aarm 0 50 mt foliations are slightly oblique both to bedding and ams foliation surprisingly hard aarm 50 100 mt foliations are perpendicular to bedding comparison of two limbs of one fold suggests that ams foliations are pre tilting while hardaarm foliations are post tilting corrected degree of anisotropy parameters display similar trends for all sites p 1 04 p 1 11 p hardaarm 1 19 we suggest that ams soft aarm and hard aarm reflect distinct magnetic grain populations however a possible relationship between hard aarm and a component of natural remanent magnetization is possible our study shows that two orthogonal fabrics exist in these weakly strained rocks coarse grains are parallel to bedding while fine grains are aligned perpendicular to bedding

palaeozoic clay mineral sedimentation and diagenesis in the dinant and avesnes basins belgium france relationships with variscan tectonism abstract clay mineral investigations have been performed on more than 500 limestones and shales sampled in lower devonian emsian to lower carboniferous namurian outcrops in the dinant and avesnes basins ardenne massif nw europe clay mineral data have been placed in the palaeoenvironmental and structural histories documented by previous lithological stratigraphical palaeontological diagenetic and tectonic contexts the clay associations are dominated by illite and chlorite derived partly from the erosion of land masses surrounding the marine domain the geothermal gradient estimated from correlation with conodont colour alteration index ranges between 40 and 70 c km a diachronous northwards migration of the diagenesis metamorphism interface links to uplift caused by late carboniferous compressional folding and overthrusting associated clay minerals include smectite locally preserved from diagenetic changes mainly by early pore closure that reflect lagoonal or quiet offshore marine conditions smectite and subordinate kaolinite abundances decrease upwards during the devonian in three successive intervals suggesting alternations of sub arid to drier climates the local occurrence of corrensite ordered chlorite smectite mixed layer is attributed to the moderate diagenetic transformation of pre existing smectite

a comparison of the effectiveness of clast based finite strain analysis techniques to ams in sandstones from the sevier thrust belt wyoming abstract geological strain analysis of sedimentary rocks is commonly carried out using clast based techniques which rely on assumptions regarding clast shapes and orientations as well as distances between clast centres to evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility ams in sandstone samples from the wyoming salient the cordilleran mountain belt of north america is one of the world s classic foreland fold and thrust belts the sevier belt represents the thin skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of devonian to cretaceous strata eastward deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility ams to clast based strain analyses studies attempting to define the relationship between ams and finite strain have been in vogue since the link between layer parallel shortening and ams was first established this relationship despite proven strong correlations between the ams tensors and tectonic directions is generally complicated by competing sub fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the ams fabric even with these complications we conclude that ams is much more sensitive to incipient tectonic fabrics than clast based methods

finite thickness of shear bands in frictional viscoplasticity and implications for lithosphere dynamics permanent deformations in the lithosphere can occur in the brittle as well as in the ductile domain for this reason the inclusion of viscous creep and frictional plastic deformation is essential for geodynamic models however most currently available models of frictional plasticity are rate independent and therefore do not incorporate an internal length scale which is an indispensible element for imposing a finite width of localized shear zones therefore in computations of localization either analytical or numerical a width of the shear zone is obtained which tends to zero in numerical computations this manifests itself in a severe mesh sensitivity moreover convergence of the global iterative procedure to solve the non linear processes is adversely affected which negatively affects the reliability and the quality of predictions the viscosity which is inherent in deformation processes in the lithosphere can in principle remedy this mesh sensitivity however elasto visco plastic models which are commonly used in geodynamics assume a series arrangement of rheological elements maxwell type approach which does not introduce an internal length scale here we confirm that a different rheological arrangement which put a damper in parallel to the plastic slider kelvin type approach introduces an internal length scale as a result pressure and strain and strain rate profiles across the shear bands converge to finite values upon decreasing the grid spacing we demonstrate that this holds for non associated plasticity with constant frictional properties and with material softening with respect to cohesion finally the introduction of kelvin type viscoplasticity also significantly improves the global convergence of non linear solvers