mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

comment on magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes by g j borradaile and d gauthier anisotropy of magnetic susceptibility is used as a proxy for the determination of magmatic flow direction in mafic dykes here we take advantage of the dataset from g j borradaile and d gauthier to comment three points 1 the sampling strategy 2 the geometric relationship between magnetic axis dyke and 3 an alternative interpretation to obtain a flow direction the magnetic lineations published by borradaile and gauthier correspond to the zone axis of the dyke and magnetic foliation poles questioning the reliability of the magnetic lineation as a flow estimate an alternative interpretation is based on the use of the tiling of the magnetic foliation plane against the dyke wall

ridge regression algorithm for gravity inversion of fault structures with variable density we derive an analytical expression for gravity anomalies of an inclined fault with density contrast decreasing parabolically with depth the effect of the regional background particularly the interference from neighboring sources of a fault structure is ascribed by a polynomial equation we have developed an inversion technique employing the ridge regression iterative algorithm to infer the shape parameters of the fault structure in addition to the effect of regional background we demonstrate the validity of the proposed technique by inverting a gravity anomaly of a theoretical model both with and without adding a regional background the technique is insensitive to the effect of regional background two density depth models of the godavari subbasin in india are used in our interpretation of the gravity anomalies of the ahiri cherla master fault the interpreted results of a parabolic density model are found to be more geologically reasonable in comparison with the constant density model the variations of the misfit function of the theoretical and observed gravity anomalies the damping factor and the shape parameters of the fault against the iteration number indicate the reliability of the interpretation

the relationship between depth age and gravity in the oceans we reassess the applicability of the thermal plate cooling model to the subsidence of the north pacific atlantic and north indian ocean basins we use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments and the ridge temperature structure is consistent with the thickness of the oceanic crust we first attempt to exclude thickened crust from our data set and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate wavelength gravity and topography we find that the average variation of depth with age is consistent with conventional half space models until about 90 myr thereafter the departure from the half space cooling curve is more rapid than predicted using simple conductive plate cooling models the depth age curves in the pacific and atlantic show 250 m of temporary shallowing between the ages of 90 130 myr a result consistent with the outcome of experiments on the initiation of small scale boundary layer convection the results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography a 90 km thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the pacific ocean lithosphere in the pacific the broad topographic undulations associated with the line island swell the hawaiian swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mgal km 1 and are likely to result from convective circulation in the upper mantle in the northeast atlantic the intermediate wavelength admittance over the cape verde swell is similar in the northwest atlantic over the bermuda swell it is slightly larger but not as well constrained

gravity inversion of blocky basement relief using l 0 norm constraint with exponential density contrast variation depicting the basement relief of a sedimentary basin with gravity data is vital to predicting the hydrocarbon potential of a sedimentary basin and guiding exploration work we have developed a gravity inversion method to estimate the depth to a blocky basement of a sedimentary basin the basement rocks are assumed to be homogeneous and have uniform density while the density of the sediment over the basement increases exponentially with depth the density contrast between the sediment and the basement at the surface varies horizontally the decay factor of density contrast is also nonuniform the sediment above the basement is divided into vertically juxtaposed prisms and the depth of the bottom of each prism represents the depth to the basement and is the parameter to be estimated the l0 norm is introduced to limit the gradient of the parameter vector to obtain the model constraint function we then establish the objective function for inversion by combining the gravity data misfit function the known depth constraint function and the model constraint function the inversion is performed by minimizing the objective function using the nonlinear conjugate gradient algorithm the inversion method is evaluated using a 2d and a 3d sedimentary basin model the results show that our proposed method is capable of delineating the blocky basement relief of a sedimentary basin and the result is sharper than that obtained using the l1 norm constraint the method is applied to real data from the western part of the zhu 1 depression in the pearl river mouth basin northern south china sea the solution reveals a strongly faulted basement which is in accordance with the known tectonic information indicating the basin is a fully developed graben

automatic gravity optimization of 2 5d strike listric fault sources with analytically defined fault planes and depth dependent density an automatic gravity inversion technique in the space domain simultaneously estimates the parameters of strike limited listric fault sources and regional gravity background from a set of observed bouguer gravity anomalies the fault profile and regional effect are described by unknown polynomial functions of arbitrary but prescribed degree furthermore the density contrast within the fault structure is presumed to be known according to a prescribed parameterized nonlinear function of depth in geologic settings where the detached downthrown block consists of a series of sedimentary beds whose density increases with depth the inversion is automatic in that it initializes and determines polynomial coefficients for the fault boundary and regional gravity background from a set of observed bouguer gravity anomalies and improves them iteratively until the modeled gravity anomalies mimic the observed anomalies an analysis of a set of gravity anomalies attributable to a synthetic model of a listric fault structure in the presence of pseudorandom noise with and without regional background has disclosed that the algorithm yields reliable interpretations with modest error in model geometry even in the presence of pseudorandom noise in the presence of regional gravity background and pseudorandom noise the estimated parameters of the structure deviate marginally from the true ones the derived density depth model of the jharia coal basin in india a pull apart basin has been used to analyze the observed bouguer gravity anomalies of a boundary fault the interpretation has yielded information consistent with drilling results and geologic setting of the basin

three dimensional interpretation of gravity data from sedimentary basins using an exponential density depth function in mapping the topography of the basement of deep sedimentary basins by gravity modelling the accuracy can be improved by incorporating an exponential increase in density with depth for calculating the gravity effect of a three dimensional 3d structure with such an exponential density depth relation a frequency domain forward algorithm based on series expansion is presented the numerical evaluation of which can be performed efficiently by fast fourier transform the algorithm can be applied in a recursive procedure to give the inverse solution in terms of basement relief the inversion procedure is satisfactorily tested on a 2d synthetic example and a 3d field example of gravity data from the western margin of the pannonian basin in eastern austria where up to 2 2 km of tertiary sediments overlie an igneous or metamorphic basement the results are confirmed by basement intersections in several wells

strain and fabric analyses based on porphyroclast interaction abstract two dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history the models provide templates for a known clast density and clast ellipticity that correlate clast imbrication angle of collision and clast shape fabric to strain and kinematics of deformed rocks modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric upon collision between clasts the trains formed can remain fixed or rotate if they rotate imbricated clasts can become independent when they are no longer forced together by the flow based upon the mode of clast and train rotation using the march rotation of passive lines and the jeffery rotation of rigid particles as end members three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions the march fixed train march rotating train and jeffery rotating train models simulate solid state intermediate and magmatic conditions respectively modeling shows that the two march models produce similar results but the jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric application of the modeling to the study of a syntectonic granite in the sierra nevada suggests that deformation was dominated by simple shear some k feldspar imbrication and fabric development took place initially in the magmatic stage and culminated in the solid state to produce a large proportion of imbricated k feldspar porphyroclasts which define a strong shape fabric this type of analysis provides a rationale to determine syn vs post emplacement deformation in the granite

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

the dilemma of asymmetric porphyroclast systems and sense of shear abstract asymmetric porphyroclast systems have long been used as independent shear sense indicators based on their clast tail asymmetry and or stair stepping tail geometry over the last two decades advances in exploring the mechanical behavior of rigid clasts in a viscous flow has shed new light on understanding the formation of porphyroclast systems embedded in noncoaxial viscous flow and subsequently quantifying many aspects of flow kinematics using examples from naturally deformed rocks combined with results from recent advances our study indicates that 1 in general only porphyroclast systems composed of equant shaped clasts can be treated as independent shear sense indicators 2 special attention should be given to type porphyroclast systems composed of high angle intermediate aspect ratio clasts which may contain meaningful information on bulk flow kinematics 3 bookshelf structures cannot serve as independent shear sense indicators multi criteria and thorough three dimensional shape analyses are suggested for a proper interpretation of flow kinematics in ductile shear zones

rolling structures at large shear strain abstract at large shear strain shear criteria are often obliterated or become ambiguous from examination of both natural examples and experimental models we describe a new criterion called rolling structure widely represented in sheared rocks a typical rolling structure is composed of a rigid or competent object e g porphyroclast boudin fossil etc with two tails asymmetrically disposed around it in most cases tails are of the same material as that of the rotating object and result from strain softening and grain size reduction at the object periphery forming a mantle z and s asymmetries of rolling structures represent dextral and sinistral senses of shearing respectively tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense besides the determination of the sense of shear the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks since rolling structure length is proportional to shear strain

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow

shape fabrics of particles in low concentration suspensions 2d analogue experiments and application to tiling in magma abstract jeffery s equations ascribe a theoretical cyclic nature to the shape fabric of non interacting rigid particles immersed in a viscous fluid undergoing simple shear flow this theoretical behaviour is confirmed at low shear strains 5 first experiment 134 identical particles or 1 second experiment 178 particles with two size classes then become stable at 17 first experiment and at more than 50 second experiment of the population of particles due to the contribution of the tiled particles the shape fabric becomes asymmetrical in its orientation distribution with a maximum lying above the shear plane the latter result provides a new shear sense indicator in addition to the statistical determination of the tiled features the study also suggests that crystalline fabrics in magmas could be acquired at high melt fractions i e early in the crystallization history of the magma

recent changes in antarctic sea ice in contrast to the arctic total sea ice extent sie across the southern ocean has increased since the late 1970s with the annual mean increasing at a rate of 186103km2 per decade 1 5 per decade

syn tectonic emplacement of the late mesozoic laojunshan granite pluton in the eastern qinling central china an integrated fabric and geochronologic study abstract the porphyritic monzogranite of laojunshan eastern qinling in central china forms a w e elongated pluton characterized by a varying abundance of cm scale potassium feldspar kfs megacrysts with or without plagioclase mantles mafic microgranular enclaves mmes and associated mafic dykes the measurements of the shape preferred orientation of kfs megacrysts and the tiling of megacrystic pairs throughout the pluton suggest that these megacrysts constitute magmatic foliations subparallel to the pluton s margin possibly resulting from eastward non coaxial magmatic flow felsic mafic magma mixing and mingling were important during magma emplacement as indicated by abundant mmes coeval mafic dykes and disequilibrium textures within kfs megacrysts quantitative analyses of the crystal size distribution and the in situ chemistry of kfs megacrysts revealed that the temperature fluctuation in the deep magma chamber and the resultant textural coarsening are the most likely important mechanisms for the growth of kfs megacrysts the syn tectonic and two main stage emplacement of the pluton in a sinistral transtensional regime is inferred by the mechanical coupling between magmatic submagmatic solid state fabrics in the pluton and wall rock deformations as well as zircon u pb dating results which produce eastward younging emplacement ages spanning from 122 ma to 119 ma combined with other studies our work supports the idea that the qinling dabie orogen was in a sinistral transtensional regime during the early cretaceous which may correspond to the eastward extrusion tectonics in eastern china

sampling strategies and the anisotropy of magnetic susceptibility of dykes abstract most studies of the anisotropy of magnetic susceptibility ams of dykes have assumed that susceptibility axes should define a unique orientation relative to the dyke walls and magma flow direction theoretical considerations however predict systematic variations of ams as a function of the amount of shear experienced by the flowing magma although this feature of ams might seem undesirable at first sight based in a theoretical model it has been suggested that actually we can take advantage of such variation in the orientation of the ams axes to infer magma flow directions more confidently than until now if proper attention is given to the scheme followed during sample collection in this work we report the results of a pilot study made to test those theoretical predictions we resampled two dykes from the ko olau dyke complex in o ahu and for which magma flow direction was previously inferred albeit with some uncertainty the resampling was made following the guidelines suggested by the theoretical work as closely as possible i e collecting more than one profile parallel to the dyke walls and across the dyke the results of our measurements provide the first empirical evidence supporting the occurrence of a cyclic fabric acquisition during the emplacement of at least some dykes the procedure described in this paper therefore constitutes a self contained approach that can be followed to test in a case by case basis the occurrence of such fabric behavior this in turn should provide enough elements to increase the reliability of the interpretation in the obtained results

strain insensitive preferred orientation of porphyroclasts in mont mary mylonites abstract the shape preferred orientation spo of porphyroclasts was determined in high temperature mylonites the porphyroclasts approach rhomboidal sillimanite or elliptical garnet plagioclase sillimanite shapes and exhibit aspect ratios r as high as 11 particles with r 3 are dominantly rhomboidal the long axis of the best fit ellipse defines a very strong spo inclined at 5 10 to the mylonitic foliation in an antithetic sense with respect to the shear direction this angle is independent of r the inclination of the long sides of rhomboidal sillimanite increases from 10 to 20 with decreasing r in contrast the short sides have a constant orientation of 15 to 17 irrespective of r and are parallel to extensional crenulation cleavage low aspect ratio mainly elliptical objects show low intensity spo close to the shear plane the two spos appear strain insensitive in the case of r r 3 a stable position is acquired this is not explained by any of the current theoretical and experimental models of spo

natural rotated inclusions in non ideal shear abstract kinematic analysis of rotated winged inclusions is often couched in terms of ideal simple shear with reference to a specific model based upon porphyroclasts of circular section however many natural winged inclusions are not porphyroclasts commonly their geometries as well as the orientation distribution of the inclusions themselves reflect the non ideal nature of the shearing deformation during which they formed stair stepped sigma shaped geometries may reflect the immaturity of a structure developed in non ideal shear or a mature structure developed in ideal simple shear stair stepped delta shaped geometries indicate that deformation approximated to ideal simple shear in plane delta shaped geometries and skewed inclusion orientation distributions are indicators of non ideal shear

a three dimensional gravity inversion applied to sao miguel island azores gravimetric studies are becoming more and more widely acknowledged as a useful tool for studying and modeling the distributions of subsurface masses that are associated with volcanic activity in this paper new gravimetric data for the volcanic island of sao miguel azores were analyzed and interpreted by a stabilized linear inversion methodology an inversion model of higher resolution was calculated for the caldera of furnas which has a larger density of data in order to filter out the noncorrelatable anomalies least squares prediction was used resulting in a correlated gravimetric signal model with an accuracy of the order of 0 9 mgal the gravimetric inversion technique is based on the adjustment of a three dimensional 3 d model of cubes of unknown density that represents the island s subsurface the problem of non uniqueness is solved by minimization with appropriate covariance matrices of the data resulting from the least squares prediction and of the unknowns we also propose a criterion for choosing a balance between the data fit which in this case corresponds to residues with rms of the order of 0 6 mgal and the smoothness of the solution the global model of the island includes a low density zone in a wnw ese direction and a depth of the order of 20 km associated with the terceira rift spreading center the minimums located at a depth of 4 km may be associated with shallow magmatic chambers beneath the main volcanoes of the island the main high density area is related to the nordeste basaltic shield with regard to the caldera furnas in addition to the minimum that can be associated with a magmatic chamber there are other shallow minimums that correspond to eruptive processes

gravity effect of sediment compaction examples from the north sea and the rhine graben abstract a fourier domain expression for calculating the gravity effect of a continuously varying density structure is used to investigate the way in which sediment compaction modifies the shape of the gravity anomaly across a sedimentary basin in general sediment density increases with depth in a basin as the overburden thickness increases the effect of the increase in sediment density is to reduce the gravity contribution from the density contrasts in the deeper parts of the basin relative to near surface contributions for a theoretical uncompensated basin the gravity effect of the sediments is calculated for a density depth variation described by 1 a simple exponential increase in sediment density with depth and 2 an exponential modified to include a local density inversion representative of sediment overpressuring it is shown that for both cases the calculated gravity does not necessarily reflect the morphology of the sediment basement interface the gravity effect is most sensitive to the distribution of the youngest stratigraphic units within the basin results of modeling observed gravity anomalies across the viking and rhine graben show that the small peak to trough amplitude of the gravity anomalies across these basins can be attributed to the increase in sediment density with depth rather than the compensation of the basin for the rhine graben it is further shown that the wavelength of the gravity anomaly is strongly controlled by the flexural strength of the lithosphere together these results suggest that while the amplitude of gravity anomalies across extensional basins may be primarily reflecting compaction of the sediment infill the anomaly wavelength is more sensitive to the compensation mechanism

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

determination of crustal interface topography from potential fields we present a method for determination of the topography of an interface separating two uniformly but differently magnetized media the technique is developed within the framework of discrete linear inverse theory and exploits the quasi linearity of the problem to derive a simple efficient algorithm suitable for use on large gridded data sets variation of auxiliary parameters allows a suite of acceptable models to be produced rapidly these parameters can be appraised in light of available geologic and geophysical evidence because very large matrices are involved when large amounts of data are interpreted approximate resolution measures requiring no matrix inversion provide useful information concerning averages of the constructed topography

paleocurrent direction measurements in a volcanic setting by means of anisotropy of magnetic susceptibility a case study from the lower miocene tepoztlan formation transmexican volcanic belt central mexico sources of ancient volcanic rocks are often unknown if they are either eroded and or covered by younger deposits this problem as well as the provenance of reworked volcaniclastic fluvial and mass flow deposits can be partially solved by the application of anisotropy of the magnetic susceptibility ams for massive and poorly sorted volcaniclastic rocks in particular this may be the only way of finding reliable transport directions and therefore allowing for paleogeographic reconstructions here we present a data set of 428 ams measurements and 249 measurements of sedimentary paleocurrent indicators from the miocene tepoztlan formation at the southern edge of the transmexican volcanic belt central mexico the highest degree of reliability of ams measurements is gained for data from lava samples and the lowest from mass flows sedimentary structures in sandstones and conglomerates such as trough cross stratification asymmetric ripple marks and the shape of scours and channels could be used to calibrate the results from ams data and to prove their reliability ams data on fluvial deposits point to a drainage system with a w e flow direction indicating an outflow of the river system into the ancient gulf of mexico

investigation of sediment compaction in the pannonian basin using 3d gravity modelling abstract the great number of geophysical and geological data available in hungary has made possible the direct modelling of the density distribution of the upper crust without considering lateral inhomogeneities a simple 1d theoretical model was introduced for the relationship between bouguer gravity anomalies and basement depths to derive a general formula describing the vertical change of density in the sediments the validity of the correlation model was investigated by direct 3d gravity modelling the 1d model proved a satisfactory approximation so it was applied to the observed gravity field over a selected area of the pannonian basin and an extensive analysis of the sediment compaction was carried out based on a high resolution digital depth model ddm and volume model of the sediments covering the pannonian basin with an average thickness of 2 km the derived density depth function of the selected area differs from the generally accepted vertical change of density estimated by geological geophysical exploration although the effect of the sediment compaction is clearly visible the obtained density contrast function resulted in apparent density values for the sediments higher than given by the reference model used in 3d gravity field model computations this systematic deviation may be a consequence of the crustal thinning which is supposed to be a consequence of basin evolution a crust mantle model of the investigated area extending below the moho discontinuity was used to support this assumption

magnetic and shape fabrics of magnetite in simple shear flows abstract the magnetite fabrics measured by anisotropy of magnetic susceptibility ams and by shape preferred orientation spo optical methods are classically used as flow kinematics indicators in lava flows the development of magnetite fabrics during simple shear strains 20 was performed using a suspension of 1 volume fraction of multidomain magnetite randomly contained in a mixture of silicone and wax we measured ams fabric and spo ellipsoids by calculating a quadratic shape tensor from oriented thin sections for jeffery 1922 fabrics are usable to determine the flow kinematics including the amount of applied finite strain for 8 fabric elements foliation and lineation are stabilised closely parallel to the flow plane and the shear direction respectively two and three dimensional numerical simulations using measured aspect ratios of magnetite point out that the large scattering of aspect ratios and the initial orientation distribution of particles are together responsible for a wide ranging loss of periodicity the stable ams and spo fabrics observed at large strains in experiments are the result of these primary fabric properties combined to collisions between particles and possibly their complex three dimensional shapes in addition the constant angular relationship observed at large strains between fabrics and flow components is related to the transient collisions consequently the determination of the lava flow kinematics by using fabric properties measured either by ams or by spo analyses should be indubitably associated to a detailed study of the three dimensional shape of the solid carriers regularly shaped populations of low elongated particles will be capable to produce cyclic to oscillating fabrics while the fabric of elongated particles will be more sensitive to the shape parameters and collisions ultimately favouring stable fabrics at large strains

development of symmetrical and asymmetrical fabrics in sheet like igneous bodies the role of magma flow and wall rock displacements in theoretical and natural cases abstract two orientations of fabrics are recognised in vertical dykes a symmetric orientation associated with magma injected into non deforming wall rocks and an asymmetric orientation usually associated with external non coaxial shearing or magma injected into an active fault the notion of two fabric arrangements stems from two planes of symmetry the dyke symmetry plane dsp and the fabric symmetry plane fsp this paper uses conceptual models to simulate all possible orientations of magma flow indicators that might be produced within a vertical dyke emplaced under coeval internal and external stresses the effect of magmatic buoyancy related stresses are portrayed in terms of magma flow velocities mfv and the external non coaxial tectonic related stresses in terms of wall displacement velocities divided by two wmv 2 the tectonic setting is that of a sinistral transcurrent fault using these assumptions five cases are proposed mfv wdv 2 mfv wdv 2 mfv wdv 2 mfv

3d and 2 d modelling of gravity anomalies with variable density contrast1 the decrease of density contrast in sedimentary basins may be approximated by a quadratic function a sedimentary basin may be viewed as a number of prisms placed in juxtaposition equations in closed form for the gravity anomalies of 3d and 2 d prismatic models are derived approximate equations for these models are also derived for rapid calculations efficient methods are developed for anomaly calculation by an appropriate use of the exact and approximate equations and hence for 3d and 2 d modelling the depths to the basement are adjusted iteratively by comparing the calculated anomalies with the observed anomalies these methods are applied for analysis of the residual anomaly map of the los angeles basin california

two stage deformation of the mont louis andorra granite pluton variscan pyrenees inferred from magnetic susceptibility anisotropy the mont louis andorra granodiorite pluton eastern pyrenees france andorra and spain was the subject of a thorough study of its internal structures principally the measurement of the anisotropy of its magnetic susceptibility and the classification of its microstructures regularly spaced sampling 281 sites allowed the precise mapping of the susceptibility magnitudes consistent with mapped rock types the magnetic structures fall into two distinct families which occur in separate parts of the pluton and have different microstructures family i is observed almost everywhere and has close to horizontal lineations remarkably constant in ne sw azimuth magmatic to submagmatic microstructures characterize these rocks in contrast family ii is confined within nw se trending corridors where moderate to steep nw plunges of the lineations are recorded and subsolidus microstructures are observed magnetic structures of family i are argued to equate with the kinematic framework of the deforming magma during its emplacement family ii structures represent a late to post consolidation reworking and therefore record a second stage of deformation in the pluton it is concluded that magma emplacement was related to a ne sw trending strike slip and or thrust fault system coeval with the sw verging thrust episode documented in the pyrenees the second stage corresponds to dextral shear movements with a thrust component on nw se striking planes corridors containing family ii structures are shear zones responsible for dissection of the massif into three blocks the eastern block representing the former base and the western block the top of the pluton restoration of the pluton9s shape along these corridors demonstrates the importance of the early variscan ne sw kinematic direction because the family ii structures that form shear zones affecting the pluton have similar orientation and kinematics as the compressive steep main phase of deformation ubiquitous in the pyrenees the pluton of mont louis andorra was probably emplaced before this main phase of deformation

3d gravity modeling of buyuk menderes basin in western anatolia using parabolic density function abstract a method to model 3d sedimentary basins with parabolic density contrast is applied to buyuk menderes basin in western anatolia the measured gravity fields reduced to a horizontal plane are assumed to be available at grid nodes of a rectangular square mesh juxtaposed 3d vertical prisms with their geometrical epicenters on top coincide with grid nodes of a mesh to approximate a sedimentary basin the algorithm based on newton s forward difference formula automatically calculates the initial depth estimates of a sedimentary basin assuming that 2d infinite horizontal slabs can generate the measured gravity fields and among these slabs the density contrast varies with depth the lower boundary of a sedimentary basin is formulated by estimating the depth values of the 3d prisms with in predetermined limits measured gravity fields pertaining to the buyuk menderes basin turkey where the density contrast varies with depth are interpreted to show the applicability of the method

schlieren formation in diatexite migmatite examples from the st malo migmatite terrane france schlieren are trains of platy or blocky minerals typically the ferromagnesian minerals and accessory phases that occur in granites and melt rich migmatites such as diatexites they have been considered as 1 unmelted residue from xenoliths or the source region 2 mineral accumulations formed during magma flow 3 compositional layering and 4 sites of melt loss in order to help identify schlieren forming processes in the diatexites at st malo differences in the size shape orientation distribution and composition of the biotite from schlieren and from their hosts have been investigated small biotite grains are much less abundant in the schlieren than in their hosts schlieren biotite grains are generally larger have greater aspect ratios and have except in hosts with low 10 biotite contents a much stronger shape preferred orientation than host biotite the compositional ranges of host and schlieren biotite are similar but schlieren biotite defines tighter sharper peaks on composition frequency plots hosts show magmatic textures such as imbricated tiled unstrained plagioclase some schlieren show only magmatic textures tiled biotite no crystal plastic strain features but many have textures indicating submagmatic and subsolidus deformation e g kinked grains and these schlieren show the most extensive evidence for recrystallization magmas at st malo initially contained a significant fraction of residual biotite and plagioclase crystals smaller biotite grains were separated from the larger plagioclase crystals during magma flow since plagioclase was also the major early crystallizing phase the plagioclase rich domains developed rapidly and reached the rigid percolation threshold first forcing further magma flow to be concentrated into narrowing melt rich zones where the biotite had accumulated hence increasing shear strain and the degree of shape preferred orientation in these domains schlieren formed in these domains as a result of grain contacts and tiling in the grain inertia regime final amalgamation of the biotite aggregates into schlieren involved volume loss as melt trapped between grains was expelled after the rigid percolation threshold was reached in the biotite rich layers

shape fabric development in rigid clast populations under pure shear the influence of no slip versus slip boundary conditions abstract shape fabrics of elliptical objects in rocks are usually assumed to develop by passive behavior of inclusions with respect to the surrounding material leading to shape based strain analysis methods belonging to the rf family a probability density function is derived for the orientational characteristics of populations of rigid ellipses deforming in a pure shear 2d deformation with both no slip and slip boundary conditions using maximum likelihood a numerical method is developed for estimating finite strain in natural populations deforming for both mechanisms application to a natural example indicates the importance of the slip mechanism in explaining clast shape fabrics in deformed sediments

modified jeffery model influence of particle concentration on mineral fabric in moderately concentrated suspensions 1 a modified jeffery model involving particle interactions allows the simulation of preferred orientation of crystals and the anisotropy of magnetic susceptibility in magmatic flow with up to 40 of rigid particle fraction numerical simulations are compared to analogue and natural fabrics a study of the role of particle concentration shows that the increasing percentage of crystal fraction in dilute suspensions leads to progressive attenuation of the fabric intensity and cyclicity at high strains in simple shear flow the lineation converges to a constant value close to the shear plane with increasing particle concentration the fabric diverges from the shear direction reflecting the effect of increased crystal tiling the model can be applied to quantify mineral preferred orientations of crystal mushes in magmatic fabric studies

polymineralic orientation analysis of magmatic rocks using electron back scatter diffraction implications for igneous fabric origin and evolution abstract electron back scatter diffraction ebsd which provides an easy way of acquiring large numbers of individual crystallographic orientation data from different phases has been applied to the study of magmatic fabrics using this technique the crystallographic preferred orientation cpo of plagioclase biotite orthopyroxene hornblende and quartz in natural tonalites and quartzdiorites from the santa olalla igneous complex sw iberia deformed during the magmatic stage have been determined plagioclase is the coarser phase defining the main fabric in each sample whereas biotite can display either the same fabric as plagioclase or a completely different one the differences between these two phases occur because 1 smaller phases interact with the larger ones to produce more random orientations 2 under simple shear finer phases can completely rotate giving a girdle included in the xz plane 3 finer phases can more easily preserve relict fabrics while the coarser phases are completely reoriented by the last stress tensor the last phases to crystallize show weak to completely random cpos hornblende or completely random distributions quartz the study was completed with a shape preferred orientation analysis using the intercept method in order to detect weak magmatic lineations and numerical modelling simulations of theoretically equivalent simple shear situations for each sample

optimum expression for computation of the gravity field of a homogeneous polyhedral body1 the formula for the computation of the gravity field of a homogeneous polyhedral body is derived and transformed into the form which is most suitable when considering speed and simplicity of numerical calculation

a review of criteria for the identification of magmatic and tectonic foliations in granitoids abstract foliations in granitoids can form by magmatic flow submagmatic flow high temperature solid state deformation and moderate to low temperature solid state deformation a review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent diapiric emplacement and expansion emplacement during regional deformation or regional deformation post dating emplacement however a magmatic origin is favoured for foliations defined by the alignment of igneous commonly euhedral minerals particularly where the foliation is parallel to internal or external pluton contacts foliations formed during expansion or ballooning of diapirs may be strictly magmatic in origin although some studies suggest that solid state deformation also may occur if so we would hope to find evidence of deformation of crystal melt systems and that the solid state deformation occurred at high temperatures the inference of syntectonic foliations is most convincing where magmatic and high temperature solid state foliations are subparallel these foliations are continuous with regionally developed foliations in the wall rocks synkinematic porphyroblasts are present in the wallrocks and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage a strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals no alignment of igneous minerals occurs the foliation is locally at high angles to pluton wallrock contacts and the foliation is continuous with a regionally developed cleavage

experimental modelling of mantled porphyroclasts abstract mantled porphyroclasts are commonly used as sense of shear indicators in mylonites analogue experiments have been carried out to model the development of such objects rigid spheres mantled by a viscous plasticene putty mixture with power law behaviour were suspended in an optically transparent newtonian polymer matrix the samples were deformed between two cylinders one placed within the other and with coinciding axes this induces a non coaxial flow between the cylinders and allows the achievement of high finite strain the viscous mantle around the rigid sphere remained undeformed deformed into an ellipsoid or developed wings the final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix which is a function of the flow induced stress on the surface of the mantle winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small if this critical induced viscosity contrast is exceeded the mantled sphere remains undeformed or obtains a stable ellipsoidal shape even at high finite strain shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin

magnetic interactions at the origin of abnormal magnetic fabrics in lava flows a case study from kerguelen flood basalts summary anisotropy of low field magnetic susceptibility ams of basaltic lava flows can give some clues about post emplacement tilting occurring in volcanic sequences such a study has been carried out on a sequence of 19 successive lava flows from kerguelen archipelago surprisingly two different patterns were observed the first one herein called normal fabric is consistent with the lava flow direction inferred from geological observations whereas the second one observed for about 70 per cent of the analysed samples herein called abnormal fabric appears to be unrelated to the shear history of the lava flows during their emplacements these abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin the unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high resolution electron backscattered diffraction measurements the aim of this study is to propose a physical interpretation of these fabrics a first step in our understanding was to clearly identify the nature and the magnetic properties of thefe tioxidesassemblage thus wepresentacomprehensive rockmagneticanalysisrelying on low and high temperature thermomagnetic curves k t crossover diagrams first order

3d gravity analysis in the spatial domain model simulation by multiple polygonal cross sections coupled with exponential density contrast an automatic 3d modeling technique is developed in the spatial domain to analyze the gravity anomalies produced by a concealed density interface with mass density contrast differing exponentially with depth the sedimentary column above the interface is described with a stack of multiple vertical polygonal sections of unit thickness each for such a case the depth ordinates of the vertices of the cross sections become the unknown parameters to be estimated from gravity data forward solution of the model space is realized in the spatial domain by a technique that combines both analytic and numeric approaches initial depths to the interface are calculated based on the bouguer slab approximation and subsequently improved iteratively based on the ratio of the product of the observed gravity anomaly and existing depth parameter to the corresponding model gravity response the iterative process continues until one of the predefined termination criteria is accomplished unlike the existing methods the advantage of the proposed method is that the observed gravity anomalies need not necessarily be sampled available at regular spatial grid intervals the applicability of the proposed model is exemplified with a set of noisy gravity anomalies attributable to a synthetic structure before being applied to a real world gravity data in the case of the synthetic example the method has yielded a structure that was compatible with the assumed structure even in the presence of random noise application of the proposed method to the gravity data set from the los angeles basin california using a prescribed exponential density function has yielded a model that concurs reasonably well with the published models

flowage differentiation in an andesitic dyke of the motru dyke swarm southern carpathians romania inferred from ams csd and geochemistry abstract two dykes of different thickness 5 5 m for tj31 and 23 m for tj34 from the late pan african calc alkaline motru dyke swarm s carpathians romania have been studied by electron microprobe mineral chemistry crystal size distribution csd anisotropy of magnetic susceptibility ams and whole rock geochemistry all the physical and chemical variations observed across the dyke s width point to concordant results and show that the variations of both modal abundance and size of the amphibole and biotite microphenocrysts inside the dykes deduced from the classical csd measurements are the result of a mechanical segregation of suspended crystals during magmatic transport despite a pene contemporaneous regional tectonic the flow induced differentiation in the thicker dyke is characterized by the concentration of pre existing ti rich pargasite tschermakite clinopyroxene and plagioclase crystals in the core of the dyke and of the extracted differentiated liquid near the walls this mechanical differentiation induces a chemical differentiation with a basaltic andesite composition for the core of the dyke whereas the margins are andesitic thus the chilled margins appear as a slightly more evolved liquid with a newtonian behaviour when compared to the average composition of the dyke the localization of the liquid on both sides of the dyke has certainly facilitated the ascent of the central part of the dyke that behaved as a binghamian mush

experimental modeling of viscous inclusions in a circular high strain shear rig implications for the interpretation of shape fabrics and deformed enclaves 1 deformation experiments with initially spherical and prolate viscous inclusions suspended in a viscous newtonian matrix in a circular high strain annular shear rig provide insights on the shape development of inclusions in high strain shear zones during progressive deformation inclusions with a specific viscosity ratio with respect to the matrix material show distinct types of three dimensional shape development for instance at a high viscosity ratio between matrix and inclusion a pulsating ellipsoid develops which both continuously rotates and changes its shape from a sphere to an ellipsoid and back to a sphere the experiments show that the shape of an inclusion that has a viscosity different to the matrix material cannot be taken as a reliable indicator for the magnitude or type of finite strain in naturally formed shear zones in which strain rates are heterogeneous from boundary to center viscosity contrast and power law flow will result in a large number of different shape developments trends for inclusions such as mafic and microgranitic enclaves pebbles or mineral aggregates

an integrated geophysical approach for imaging subbasalt sedimentary basins case study of jam river basin india an integrated geophysical strategy comprising deep electrical resistivity and gravity data was devised to image subbasalt sedimentary basins a 3d gravity inversion was used to determine the basement structure of the permian sediments underlying the cretaceous formation of the jam river basin in india the thickness of the cretaceous formation above the permian sediments estimated from modeling 60 deep electric sounding data points agrees well with drilling information the gravity effect of mass deficit between the cretaceous and permian formations was found using 3d forward modeling and subsequently removed from the bouguer gravity anomaly along with the regional gravity field the modified residual gravity field was then subjected to3d inversion to map the variations in depth of the basement beneath the permian sediments inversion of gravity data resulted in two basement ridges running almost east to west dividing the basin into three independent depressions it was found that the katol and kondhali

shear criteria and structural symmetry abstract during the last decade it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures e g pressure shadows shear bands c s structures fabrics tension gashes folds veins the correspondence between coaxial or non coaxial deformation and symmetric or asymmetric particle velocity fields respectively accounts for the use of structural symmetry as an indicator of strain history the application of this symmetry concept to various field examples emphasizes that i the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system and ii the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern even where the progressive deformation results in a complex pattern which cannot be directly interpreted

evidence of sheared sills related to flank destabilization in a basaltic volcano piton des neiges basaltic volcano la reunion has been deeply dissected by erosion exposing large volumes of debris avalanche deposits to shed light on the factors that led to volcano flank destabilizations we studied the structure the crystallographic and magnetic fabrics of the substratum of a debris avalanche unit this substratum is a complex of n50 seaward dipping sills that has been exposed by the avalanche structural observations show that the sill plane in contact with the avalanche is one of the latest intrusions in the sill complex in this uppermost sill the anisotropy of magnetic susceptibility ams is correlated to the crystallographic preferred orientation of magmatic silicate minerals allowing us to use ams as a proxy to infer the magmatic flow the ams fabric across the intrusion is strongly asymmetric which reveals that the contact sill was emplaced with a normal shear displacement of its hanging wall the shear displacement and the magma flow in the intrusion are both directed toward the nne i e toward the sea which is also the direction of the slope and of the debris avalanche runout because all the sills in the intrusion complex have a similar dip and dip direction it is likely that several of them also underwent a cointrusive slip toward the nne we conclude that this cointrusive normal slip repeated over many intrusions of the sill complex increased the flank instability of the volcano this incre mental instability may have ended up into the observed debris avalanche deposit at piton de la fournaise the active volcano of la reunion sill intrusion and cointrusive flank displacement have been inferred from geophys ical studies for the april 2007 eruption by providing direct evidence of sheared sills our study substantiates the idea that repeated sill intrusions may eventually trigger flank destabilizations in basaltic volcanoes