dynamical lagrangian remeshing dlr a new algorithm for solving large strain deformation problems and its application to fault propagation folding abstract we present a new algorithm to solve the problem of rock deformation the method is based on a classical lagrangian finite element solver in which the connectivity between the nodes of the numerical mesh is updated at the end of each time or deformation step the method which we term dynamic lagrangian remeshing dlr relies on the theory of delaunay triangulation and produces optimal finite element meshes in the sense that nodes are always connected to their set of nearest neighbour nodes because it is based on a lagrangian method in which the numerical grid is attached to material particles and advected with the deformation the dlr algorithm is ideally suited to track material boundaries and properly represent free surfaces in addition in comparison to classical lagrangian solvers the dlr method is not limited to small strain problems and may be used to model discontinuities such as faults and narrow shear zones we have used the dlr method to study the evolution of a crustal layer undergoing finite shortening driven by a velocity discontinuity along its base the numerical model predicts the formation of a large fold that is progressively cut by a fault originating at the velocity discontinuity and propagating through the crustal layer the model predictions provide support for the fault propagation folding model of suppe 1

chemistry of uranium thorium and radium isotopes in the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal abstract the most comprehensive data set on uranium thorium and radium isotopes in the ganga brahmaputra one of the major river systems of the world is reported here the dissolved 238u concentration in these river waters ranges between 0 44 and 8 32 1 and it exhibits a positive correlation with major cations na k mg ca the 238 u cations ratio in waters is very similar to that measured in the suspended sediments indicating congruent weathering of uranium and major cations the regional variations observed in the 234 u 238 u activity ratio are consistent with the lithology of the drainage basins the lowland tributaries chambal betwa ken and son draining through the igneous and metamorphic rocks of the deccan traps and the vindhyan bundelkhand plateau have 234 u 238 u ratio in the range 1 16 to 1 84 this range is significantly higher than the near equilibrium ratio 1 05 observed in the highland rivers which drain through sedimentary terrains the dissolved 226ra concentration ranges between 0 03 and 0 22 dpm 1 the striking feature of the radium isotopes data is the distinct difference in the 228ra and 226ra abundances between the highland and lowland rivers the lowland waters are enriched in 228ra while the highland waters contain more 226ra this difference mainly results from the differences in their weathering regimes the discharge weighted mean concentration of dissolved 238u in the ganga at patna and in the brahmaputra at goalpara are 1 81 and 0 63 1 respectively the ganga brahmaputra river system constitutes the major source of dissolved uranium to the bay of bengal these rivers transport annually about 1000 tons of uranium to their estuaries about 10 of the estimated global supply of dissolved uranium to the oceans via rivers the transport of uranium by these rivers far exceeds that of the amazon although their water discharge is only about 20 of that of the amazon the high intensity of weathering of uranium in the ganga brahmapura river system can also be deduced from the 232 th 238 u and 230 th 238 u activity ratios measured in the suspended sediments 230th is enriched by about 19 in the suspended sediments relative to its parent 238u the flux of excess 230th supplied to the bay of bengal via these river sediments is 980 1012 dpm a about six times more than its in situ production from seawater in the entire bay of bengal

transport of u and th series nuclides in a baltic shield watershed and the baltic sea the transport of th ra and 210 pb from a continental source region and through an estuarine environment was investigated unlike previous studies here both short and long lived nuclide data were obtained for river inputs river water and estuarine waters the mire rich kalix river drainage basin was chosen as a typical northern shield area because this river may represent typical waters flowing into the arctic and northern seas groundwaters from bedrock and glacial tills have comparable th isotope concentrations and do not exhibit significant th isotopic shifts relative to host rocks the extensive peat deposits of the basin receive groundwater discharges and concentrate th and u but not ba and ra which cause high 230 th 232 th ratios in mire waters however mire outflows do not have a significant impact on th and ra isotopic compositions of the river overall weathering characteristics for the basin are obtained from the river data the 230 th 232 th 228 ra 226 ra and 226 ra ba river ratios are comparable to those of source rocks consistent with similar release rates of these nuclides from u th and ba bearing minerals river ratios of 230 th 238 u ar and 226 ra 238 u ar are 1 so that th and possibly ra are accumulating in the weathering regions and the weathering profile is still evolving low 228 ra 232 th ar and 226 ra 230 th ar ratios indicate that th is preferentially retained over ra river 234 th 238 u ar ratios are greater than 230 th 238 u ar ratios and suggest that in systems where river inputs are well characterized these ratios can be used to calculate th transit times through the watershed filtration data indicate that although a dominant fraction of the th is transported in the river on particles the rest is almost entirely carried by colloids the kalix river discharges into the baltic sea model calculations for the transport of th and ra isotopes in the baltic sea show that the high ratios of 234 th 238 u ar found here reflect long th residence times relative to particle scavenging of 50 d the water column 232 th budget is dominated by eolian inputs the 226 ra concentrations may be higher than those of water inflows with 60 derived from underlying sediments a greater fraction of 228 ra is derived from sediments to balance the decay of 228 ra within the water column during the 35 yr residence time of water in the baltic the baltic 228 ra 226 ra ar ratios which are relatively constant over a range of salinities are fortuitously similar to those of the river inflows

time scale and conditions of weathering under tropical climate study of the amazon basin with u series the rio solimoes amazonas amazon river and its major tributaries have been analyzed for u series nuclides 238 u 234 u 230 th 226 ra disequilibria have been measured in the dissolved 0 2 lm as well as bed sands u series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering moreover while the dissolved load records present day weathering suspended particles integrate the erosion his tory over much longer time scales 100 ka lowland rivers are characterized by long time scales of chemical erosion p100 ka result ing in a high weathering intensity moreover exchange between suspended particles and the dissolved load may explain the u series signature for these rivers by combining u series and pb isotopes in suspended particles we show that erosion in the rio madeira basin occurred as a multi step process whereby the pristine continental crust was eroded several hundreds of ma ago to produce sediments that have then been integrated in the cordillera by crustal shortening and are currently eroded in contrast recent erosion of a pristine crust is more likely for the rio solimoes amazonas 10 ka the suspended particles of the rivers draining the andes solimoes ama zonas madeira suggest time scales of weathering ranging between 4 and 20 ka this indicates that suspended particles transported by those rivers are not stored for long periods in the andean foreland basin and the tropical plain the sediments delivered to the ocean have resided only a few ka in the amazon basin 6 3 1 ka for the rio amazonas at obidos nevertheless a large fraction of the sediments coming out from the andes are trapped in the foreland basin and may never reach the ocean erosion in the andes is not operating in steady state u series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady state erosion and that is a consequence of soil destruction greater than production by relating this observation to the short time scales of weathering inferred for the andes a few ka it appears that the erosion regime has been recently perturbed resulting in high denudation rates a possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies our results indicate that erosion responds rapidly to high frequency climatic fluctuations

particle transport of 234u 238u in the kalix river and in the baltic sea the role of particles for u isotope transport was investigated in the kalix river watershed a particle poor fe mn rich river in northern sweden and in the baltic sea estuary particles 0 45 m are strongly enriched in u and contain 20 50 of the total riverine uranium budget and 1 of the total u in brackish waters 3 7 psu the particles have high 234 u which is close to that of dissolved u in the associated water indicating that u on particles is dominantly nondetrital and isotopically exchanges rapidly with the ambient dissolved u particles at the river mouth are dominated by nondetrital fe mn oxyhydroxides uranium and fe are strongly correlated clearly demonstrating that secondary fe oxyhydroxide is the major carrier of u in river water there is no evidence for significant association of u with mn oxyhydroxide apparent u distribution coefficients k d fe were calculated for u between the authigenic fe on particles and the solution these values appear to be relatively constant throughout the year this suggests an equilibrium between fe in solution and authigenic fe oxyhydroxides on detrital particles high values of k d fe calculated for one summer as well as high u concentrations in brackish waters can be explained by u scavenging by biogenic phases with low authigenic fe content

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

structures and mechanisms associated with development of a fold in the cantabrian zone thrust belt nw spain a detailed history of progressive fold development based on mesostructural analysis is provided for a metre scale fold in the cantabrian fold and thrust belt of northwestern spain the well exposed asymmetric syncline displays a wide variety of structures including extensional faults thrusts veins tectonic stylolites bedding plane faults drag folds and a main limb thrust that transects the entire fold important constraints on fold development are provided by the spatial distribution and style of deformation angular relations and cross cutting relations among the structures fold mechanisms varied as a function of structural position while the northwest limb was subjected to pure shear shortening in the form of contractional faults the southeast limb was bent under tangential longitudinal strain resulting in a combination of outer arc extensional faulting in

mechanical constraints on the chronology of fracture activation in folded devonian sandstone of the western moroccan anti atlas the three dimensional meter scale fracture networks observed on exposed folds between the towns of tata and akka western moroccan anti atlas consist mostly of planar discontinuities which are sub perpendicular to the bedding and partitioned in three main sets the chronology of their activation is proposed in five stages since the hercynian orogeny stage 1 predates folding and involves the horizontal compression of the emsian sandstone it involves fracture set i composed of systematic joints parallel to the direction of compression stages 2 4 correspond to the folding and are marked in the outer arc by the activation of fracture set ii composed mainly of joints parallel to the fold axial plane stage 5 is a regional shear event during which sets i and iii separated by an angle close to 608 are activated in a conjugate manner to throw light on the recurrent difficulty in discriminating between activation of inherited and new fractures an elasto plastic model is used to construct a stress path in the pervasively fractured medium idealized as a continuum each fracture set obeys the mohr coulomb criterion truncated in tension to describe both sliding and opening activations finite element simulations of a simple buckling event accounting for the field fracture sets are presented it is shown that set i cannot be generated by folding and thus does belong to stage 1 set ii is activated at a later stage of folding than expected from the field interpretation set iii cannot be activated during stage 2 confirming its role in stage 5 the advantages and limitations of the proposed modeling are finally discussed q 2002 elsevier science ltd all rights reserved

relationship between structural style overpressures and modern stress baram delta province northwest borneo christopher k morley mark tingay and richard hillis australian school of petroleum university of adelaide adelaide australia rosalind king

the strontium isotopic budget of himalayan rivers in nepal and bangladesh abstract himalayan rivers have very unusual sr characteristics and their budget cannot be achieved by simple mixing between silicate and carbonate even if carbonates are radiogenic we present sr o and c isotopic data from river and rain water bedload and bedrock samples for the western and central nepal himalaya and bangladesh including the monsoon season central himalayan rivers receive sr from several sources carbonate and clastic tethyan sediments high himalayan crystalline hhc gneisses and granitoids with minor marbles carbonates and metasediments of the lesser himalaya lh and miocene recent foreland basin sediment from the siwaliks group and the modern flood plain in the tethyan himalaya rivers have dissolved sr 6 mol l and 87sr 86sr 0 717 with a large contribution from moderately radiogenic carbonate rivers draining hhc gneisses are very dilute with sr 0 2 mol l and 87sr 86sr 0 74 lesser himalayan streams also have low sr 0 4 mol l and are highly radiogenic 87sr 86sr 0 78 highly radiogenic carbonates of the lh do not contribute significantly to the sr budget because they are sparse and have very low sr in large rivers exiting the himalaya sr systematics can be modeled as a mixture between tethyan rivers where slightly radiogenic carbonates mean 87sr 86sr 0 715 are the main source of sr and lesser himalaya waters where extremely radiogenic silicates 0 8 are the main source of sr hhc waters are less important because of their low sr rivers draining the siwaliks foreland basin sediments have sr 4 mol l and 87sr 86sr 0 725 weathering of silicates in the siwaliks and the flood plain results in a probably significant radiogenic 0 72 0 74 input to the ganges and brahmaputra g b but quantification of this flux is limited by uncertainties in the hydrologic budget the g b in bangladesh show strong seasonal variability with low sr and high 87sr 86sr during the monsoon sr in the brahmaputra ranges from 0 9 mol l and 0 722 in march to 0 3 mol l and 0 741 in august we estimate the seasonally weighted flux from the g b to be 6 5 108 mol yr with 87sr 86sr 0 7295

a diagenesis model for geomechanical simulations formulation and implications for pore pressure and development of geological structures forward basin modelling is routinely used in many geological applications with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented here a new temperature dependent kinetic diagenesis model is formulated and implemented within a hydromechanical framework the model simulates the macroscopic effects of diagenesis on 1 porosity loss 2 sediment strength 3 sediment stiffness and compressibility 4 change in elastic properties 5 increase in tensile strength due to cementation and 6 overpressure generation a brief overview of the main diagenetic reactions relevant to basin modelling is presented and the model calibration procedure is demonstrated using published data for the kimmeridge clay formation the calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation the incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 mpa due to both stress independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability finally a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behaviour to be captured due to the increase in strength that results in an over consolidated stress state this is illustrated by comparison of the present day structures predicted by a geomechanical only model where a ductile fold forms and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted

how do horizontal frictional discontinuities affect reverse fault propagation folding abstract the development of new reverse faults and related folds is strongly controlled by the mechanical characteristics of the host rocks in this study we analyze the impact of a specific kind of anisotropy i e thin mechanical and frictional discontinuities in affecting the development of reverse faults and of the associated folds using physical scaled models we perform analog modeling introducing one or two initially horizontal thin discontinuities above an initially blind fault dipping at 30 in one case and 45 in another and then compare the results with those obtained from a fully isotropic model the experimental results show that the occurrence of thin discontinuities affects both the development and the propagation of new faults and the shape of the associated folds new faults 1 accelerate or decelerate their propagation depending on the location of the tips with respect to the discontinuities 2 cross the discontinuities at a characteristic angle 90 and 3 produce folds with different shapes resulting not only from the dip of the new faults but also from their non linear propagation history our results may have direct impact on future kinematic models especially those aimed to reconstruct the tectonic history of faults that developed in layered rocks or in regions affected by pre existing faults

from spatial variation of fracture patterns to fold kinematics a geomechanical approach 1 a vertical compression parallel joint set apparently formed at sheep mountain anticline sma during the early laramide orogeny prior to significant folding field data indicate that these joints occur in fewer locations in the forelimb than in the hinge and backlimb using three dimensional elastic dislocation models we show that early slip along an underlying thrust fault can locally perturb the surrounding stress field inhibiting joint formation above the fault thus we are able to constrain the forelimb kinematics finding that the forelimb was originally located in the hangingwall above the thrust fault furthermore the mode of folding at sma is constrained the anticline developed with a fixed hinge and rotating limbs while the underlying basement rocks deformed internally

bedding plane slip in initial stages of fault related folding abstract frictional slip along bedding planes contributes to fault related folding of layered rocks we use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under layerparallel contraction and extension within the numerical experiments contraction boundary conditions produce asymmetric anticlines and extension produces asymmetric synclines the fold shape may be used to infer dip of the underlying fault in situations where the fault may not be observable additionally sense of slip along bedding planes may indicate proximity to the fault tip under uniform remote tectonic strain fault slip induces deformation in both the hangingwall and the footwall at depths as shallow as 1 km there is no significant difference between fold amplitudes in the hangingwall and the footwall this result is contrary to many kinematic models currently in use kinematic models of fault related folds commonly include the development of flat ramp and flat ramp flat fault geometries which may be attributed to initial ramp thrusting and later flat development our mechanical models show that fault flats may be produced from fault ramps due to slip along frictional bedding planes near the thrust fault tips our numerical experiments also evaluate joint initiation joints perpendicular to bedding are promoted in extensional environments we compare the results of the mechanical model to kinematic models of fault propagation folds and conclude that mechanical models offer important insights to better understand the folding process

influence of outcrop scale fractures on the effective stiffness of fault damage zone rocks we combine detailed mapping and microstructural analyses of small fault zones in granodiorite with numerical mechanical models to estimate the effect of mesoscopic outcrop scale damage zone fractures on the effective stiffness of the fault zone rocks the bear creek fault zones were active at depths between 4 and 15 km and localize mesoscopic off fault damage into tabular zones between two subparallel boundary faults producing a fracture induced material contrast across the boundary faults with softer rocks between the boundary faults and intact granodiorite outside the boundary faults using digitized fault zone fracture maps as the modeled fault geometries we conduct nonlinear uniaxial compression tests using a novel finite element method code as the experimental laboratory apparatus map measurements show that the fault zones have high nondimensional facture densities 1 and damage zone fractures anastamose and intersect making existing analytical effective medium models inadequate for estimation of the effective elastic properties numerical experiments show that the damage zone is strongly anisotropic and the bulk response of the fault zone is strain weakening normal strains as small as 2 can induce a reduction of the overall stiffness of up to 75 fracture induced effective stiffness changes are large enough to locally be greater than intact modulus changes across the fault due to juxtaposition of rocks of different lithologies therefore mesoscopic fracturing is as important as rock type when considering material or bimaterial effects on earthquake mechanics these results have important implications for earthquake rupture mechanics models because mesoscopic damage zone fractures can cause a material contrast across the faults as large as any lithology based material contrast at seismogenic depths and the effective moduli can be highly variable during a single rupture event

mechanical restoration of gravity instabilities in the brunei margin n w borneo in offshore brunei the baram delta displays pliocene to recent gravitational deformations on top of the overpressured setap shales we use the limit analysis implemented in slamtec to constrain their kinematics and propose first order values of compatible pore pressures using the critical coulomb wedge theory we confirm that extreme overpressure are needed to account for the gravitational deformations currently observed in the delta slamtec simulations quantify the role of the migration of the deltaic system on the deformations and suggest that this overpressure has existed since the initiation of the gravity driven system an inverse analysis is performed to identify the fluid overpressure and sedimentation pattern that best reproduces the forward structural model this inverse approach validates the proposed kinematics since we can reproduce quantities describing the structural style with overpressure coherent with the present day observations it points out the existence of key events explaining the structural interpretations which leads to propose new kinematic models enriched with compatible pore pressure and sedimentation rates this mechanical study highlights how mechanical modelling helps design a cross section with an appropriate kinematics based on mechanical solutions and discuss on the basis of objective mechanical criteria the ranges of pore pressure consistent with a prescribed structural evolution

northern latitude chemical weathering rates clues from the mackenzie river basin canada abstract the main scope of this study is to investigate parameters controlling chemical weathering rates for a large river system submitted to subarctic climate more than 110 river water samples from the mackenzie river system northern canada have been sampled and analyzed for major and trace elements and sr isotopic ratios in the dissolved phase the three main morphological units are reflected in water chemistry rivers from the canadian shield are very dilute dominated by silicate weathering millot et al 2002 whereas the rivers of the rocky and mackenzie mountains as well as the rivers of the sedimentary interior platform are dominated by carbonate weathering and are so 4 rich compared to the rivers of the mackenzie and rocky mountains the rivers of the interior plains are organic silica and na rich and constitute the dominant input term to the mackenzie river mainstream rivers of the canadian shield area do not significantly contribute to the mackenzie river system using elemental ratios and sr isotopic ratios a mathematical inversion procedure is presented that distinguishes between solutes derived from silicate weathering and solutes derived from carbonate weathering carbonate weathering rates are mostly controlled by runoff which is higher in the mountainous part of the mackenzie basin these rates are comparable to the carbonate weathering rates of warmer areas of the world it is possible that part of the carbonate weathering is controlled by sulfide oxidative weathering but its extent remains difficult to assess conversely to what was stated by edmond and huh 1997 overall silicate weathering rates in the mackenzie basin are low ranging from 0 13 to 4 3 tons km 2 yr na k ca mg and confirm the negative action of temperature on silicate weathering rates for river basins in cold climates in contrast to what has been observed in other large river systems such as the amazon and ganges rivers silicate weathering rates appear 3 to 4 times more elevated in the plains than in the mountainous headwaters this contradicts the raymo hypothesis raymo and ruddiman 1992 isotopic characterization of suspended material clearly shows that the higher weathering rates reported for the plains are not due to the weathering of fine sediments produced in the mountains e g by glaciers and deposited in the plains rather the relatively high chemical denudation rates in the plains are attributed to lithology uncompacted shales high mechanical denudation and the abundance of soil organic matter derived from incomplete degradation and promoting crystal lattice degradation by element complexation the three to fourfold factor of chemical weathering enhancement between the plains and mountains is similar to the fourfold factor of enhancement found by moulton et al 2000 between unvegetated and vegetated watershed this study confirms the negative action of temperature on silicate weathering for cold climate but shows that additional factors such as organic matter associated with northern watersheds are able to counteract the effect of temperature this acceleration by a factor of 4 in the plains is equivalent to a 6 c increase in temperature

3 d deformation pattern analysis and evolution of the anisclo anticline southern pyrenees understanding the genetic relationships of folding related deformation structures is of importance for both academic and industrial purposes we analysed the three dimensional deformation pattern of the anisclo anticline in the southern pyrenees the specifically designed statistical method includes sequential steps of statistical and spatial data analysis and classification our results indicate that pressure solution cleavage frequency exhibits a spatial distribution that depends on the position within the fold conversely such a dependence does not occur in joints and veins consequently our data suggest that pressure solution cleavage is the most appropriate deformation structure for unravelling fault fold kinematics in carbonate multilayer in particular we found that the spacing of solution cleavages s is related to the corresponding bed thickness h this supports the use of h s value for deformation intensity quantification instead of spacing in the anisclo anticline analysis of h s indicates the occurrence of three main deformation panels whose restoration suggests that fold evolution progressed from layer parallel shortening to decollement folding up to thrust breakthrough and fault propagation folding comparison of h s values with finite strain data indicates that line length was not preserved during fold evolution this evidence has important implications for the kinematic interpretation of layer parallel shortening and cross section balancing

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

mechanical models of fracture reactivation and slip on bedding surfaces during folding of the asymmetric anticline at sheep mountain wyoming abstract we use finite element methods to investigate the reactivation of fractures opening and shearing and the development of bedding surface slip during the deformation of the asymmetric anticline at sheep mountain wyoming a series of numerical simulations were run to show the effect of mechanical stratigraphy layer thickness slip on bedding surfaces and tectonic shortening of the fold on the response of a representative three layer ductile brittle ductile two dimensional system the model uses large deformation frictional contact mechanics to capture the response of existing fractures and slip along bedding surfaces and considers both elastic and elastoplastic layer properties the computational results demonstrate the relationships among overall configuration of the multilayer slip on bedding surfaces and the sequence and mode of deformation opening versus shearing of bed perpendicular fractures we show that fractures located in the hinge are mainly reactivated as joints and that those in the forelimb are predominantly reactivated as thrust faults a flexural slip mechanism develops during folding when the layers bounded by frictional bedding surfaces have similar stiffnesses in contrast when the difference in the layer stiffnesses is significant softer outer layers the deformation is accommodated within the softer units without exceeding the frictional strength of the bedding surfaces a reduction of the middle layer thickness from 100 m to 10 m has a minor quantitative effect on the slip along the bedding surfaces we compare the numerical results with fracture data collected at sheep mountain anticline and discuss the similarities and differences between the field observations and the model results

numerical modeling of fold initiation at thrust ramps abstract the movement of the hangingwall over the footwall at thrust ramps produces a variety of structures found commonly at different scales in fold and thrust belts the nature of the structure depends on the relative rigidity of the hangingwall and footwall friction along the fault fault dip fault displacement and the boundary conditions of the deformation structures include fault bend style folds fault propagation style folds and wedge folds we investigate the initial stages of development of such structures using the finite difference code flac the rock layers are represented as continua with elastic plastic mohr coulomb constitutive relations and the fault and bounding bedding planes are assigned normal and shear stiffnesses and coefficients of friction under conditions with all layers compressed antisymmetric fault propagation style folds develop in both the hangingwall and footwall with rigid footwall and restricted far field slip in the hangingwall a single fault propagation style fold develops with far field displacement of the hangingwall allowed broader antisymmetric wedge folds develop if hangingwall and footwall are deformable and a single fault bend style fold develops if the footwall is rigid all structures become accentuated with increasing slip on the fault where both hangingwall and footwall are deformable the deformation reduces the ramp angle and tends to minimize distortion of the rock adjacent to the fault fault propagation style folds paired wedge folds and fault bend style folds are common in nature and small scale examples can be found in various stages of development continued slip on thrust faults may lead to the mature structures commonly seen in fold and thrust belts

paleomagnetic dating of alleghanian orogenesis and mineralisation in the mascot jefferson city zinc district of east tennessee usa abstract the mascot jefferson city m jc mississippi valley type mvt deposits are in the valley and ridge province of the appalachian orogen in east tennessee they have been a major source of zinc for the usa but their age is uncertain and thus their genesis controversial about 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods the sites sample limestones dolostones breccia clasts and sphalerite dolomite mvt mineralisation from mines in the lower ordovician kingsport and mascot formations of the knox group the characteristic remanent magnetisation chrm is carried by magnetite in the limestones by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation mineralized sites have a more intense chrm than non mineralised indicating that the mineralising and magnetisation event are coeval paleomagnetic breccia tests on clasts at the three sites are negative indicating that their chrm is post depositional remagnetisation and a paleomagnetic fold test is negative indicating that the chrm is a remagnetisation and a post dates peak alleghanian deformation the unit mean chrm direction for the a limestones gives a paleopole at 129 e 12 n d p 18 d m 26 n 3 indicating diagenesis formed a secondary chemical remanent magnetisation during the late ordovician early silurian b dolomitic limestones and dolostone host rocks gives a paleopole at 125 3 e 31 9 n d p 5 3 d m 9 4 n 7 recording regional dolomitisation at 334 14 ma 1 and c mvt mineralisation gives a paleopole at 128 7 e 34 0 n d p 2 4 d m 4 4 n 25 showing that it acquired its primary chemical remanence at 316 8 ma 1 the mineralisation is interpreted to have formed from hydrothermal fluid flow either gravity or tectonically driven after peak alleghanian deformation in eastern tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 ma prior to and during peak deformation

discrete element modelling of contractional fault propagation folding above rigid basement fault blocks many studies have shown that discrete blind faults at depth are commonly linked to more distributed deformation in particular folding at higher levels one category of fault related folds forced folds is common where there is a distinct mechanical contrast between faulted basement and sedimentary cover outcrop numerical and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation monoclines above discrete faults at depth with increasing displacement the folds are often cut by faults as they propagate upwards into the cover while the trishear kinematic model of fault propagation folding appears to approximately represent the geometric development of such structures comparatively little is known of the mechanical controls on their development here we present a 2d discrete element model of sedimentary cover deformation above a contractional fault in rigid basement the elements consist of a series of soft spheres that obey newton s equations of motion and initially interact with elastic forces under the influence of gravity particles are bonded until the separation between them exceeds a defined breaking strain at which time the bond breaks simulated by the transition from repulsive attractive forces to solely repulsive forces the model is used to investigate the influence of basement fault dip and sedimentary cover strength on the geometry of the folds developed and the rate of fault propagation in all cases an upward widening monocline occurs above the basement fault we find that shallow basement fault dips produce homogenous thickening of the monocline limb while steeper dips produce contemporaneous thinning and thickening within the monocline thinning and thickening within the monocline are accommodated by a combination of small scale faulting and folding with decreasing cover strength the zone of deformation becomes wider localization does not occur on a single fault and fold geometries resemble trishear fold profiles with low propagation to slip ratios p s 1 in contrast a stronger cover produces a narrower zone of deformation localization on a single fault and more rapid fault propagation similar to trishear fold profiles where p s 2 3 the fault propagates into the cover at approximately the same angle as the basement fault the model reproduces well many of the features observed in analogue modelling and reported from outcrop and seismic studies

age of the sherman type zn pb ag deposits mosquito range colorado the sherman type zn pb ag dolomite deposits in central colorado are hosted in dolostones of the early mississippian leadville formation paleomagnetic analysis using progressive alternating field and thermal demagnetization and isothermal remanent magnetization acquisition methods was performed on specimens from samples at 37 sites in the sherman type continental chief peerless ruby sacramento and sherman deposits in their host rocks in the 72 ma pando porphyry sill s and in the 40 ma leadville type black cloud massive sulfide deposit paleomagnetic fold contact and breccia tests were performed to test for the antiquity of the magnetizations the results are interpreted to indicate that the leadville carbonates were regionally dolomitized at 308 6 1 ma in the early pennsylvanian and that the sherman type deposits were emplaced at 272 18 1 ma during the early permian after northeast trending block faulting karstification and 4 1 km of sedimentary burial possibly as the result of subsurface gravity driven fluid flow related to the ouachita marathon orogen following late ouachita marathon or earliest laramide late cretaceous folding the remanence in the sherman type deposits and the leadville dolostone rocks within the contact alteration zone of the 72 ma pando porphyry sill s was reset to acquire a late cretaceous normal characteristic remanent magnetization thereafter the black cloud leadville type massive sulfide deposit was magnetized in the eocene to acquire a reversed polarity characteristic remanent magnetization that was not found in the sherman type deposits

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

hydraulic fracturing of soil during laboratory experiments part 1 methods and observations hydraulic fracturing appears to have useful environmental and geotechnical applications but the details of fracture morphology and propagation in soil are poorly known to identify those details

geometry and kinematics of mykonos detachment cyclades greece evidence for slip at shallow dip 1 the eastern part of mykonos island cyclades greece shows the detailed internal structure of a two branch shallow dipping extensional detachment system of miocene age this paper shows that the last stage of slip along the detachment during the deposition of syn rift sediments occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension extensional deformation during and after a 13 ma old granite intrusion is taken up by two main shallow dipping shear zones 1 the lower livada detachment which extends laterally across tinos island is a ductile structure located at the interface between the granite and the upper cycladic nappe metabasites a pervasive top to the ne shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact later brittle faults shallowly and steeply dipping rework the ductile deformation with the same overall shear sense 2 the upper mykonos detachment is brittle and separates the metabasites from late miocene sandstones and conglomerates the detachment fault dips 12 15 toward the nne and its sense of shear is consistent with that of the livada detachment soft sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone indicating that the overall direction of extension is nne ssw with a sense of shear toward the nne and the dip of the fault is throughout very low the two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the aegean sea which emphasizes the importance of low angle extensional faults and shear zones in extensional tectonics

rb sr dating of sphalerites from tennessee and the genesis of mississippi valley type ore deposits mississippi valley type mvt ore deposits commonly consist of some combination of lead zinc and iron sulphides accompanied by barite fluorite dolomite and calcite they are thought to form by fluid expulsion from sedimentary successions1 3 but their exact origin has remained controversial because of the scarcity of reliable geochronological data in a recent and widely quoted model the mvt deposits of the central and eastern united states formed as a result of large scale lateral fluid flow driven by the tectonic squeezing of sedimentary successions during the late palaeozoic alleghenian orogeny 330 250 myr 4 here we report the first direct rb sr dating of sphalerites from an mvt deposit and propose this as a useful geochronological technique for sulphide mineralization with poor age control rb sr data for sphalerites from coy mine east tennessee define an isochron age of 377 29 myr ruling out any genetic connection with the alleghenian orogeny instead mvt mineralization in the eastern united states was apparently generated from fluids expelled during thrusting associated with the older acadian orogeny 380 350 myr

development of crestal normal faults associated with deepwater fold growth deepwater anticlines during their early stages of development approximately between interlimb angles of 180 and 140 grow and accumulate weak fine grained and poorly lithified sediment on their crests growth of the anticlines induced gravitational instability within the syn kinematic sediments and development of a range of shallow gravity driven failure mechanisms in addition to shallow tens of metres depth to detachment rotational landslides deeper seated normal fault swarms also affect the fold crests and upper fold limbs these faults tend to penetrate to depths between 200 and 800 m are 100s of metres to a few kilometres long planar to weakly curved in cross section and display throws in the order of tens of metres the faults do not sole out into a detachment but die out downwards they can occur as single sets or tiers of faults that have developed at different times in map view the fault patterns range from straight to anastamosing to highly curved with complex cross cutting relationships the number and orientation of faults corresponds to the maximum slope dip and direction and consequently the faults are inferred to be mainly the product of gravity induced deformation although tangential stresses arising from outer arc bending of a fold or crestal collapse in response to a mobile sedimentary unit may contribute to some extent

deep water niger delta fold and thrust belt modeled as a critical taper wedge the influence of elevated basal fluid pressure on structural styles we use critical taper wedge mechanics theory to show that the niger delta toe thrust system deforms above a very weak basal detachment induced by high pore fluid pressure the niger delta exhibits similar rock properties but an anomalously low taper sum of the bathymetric slope and dip of the basal detachment compared with most orogenic fold belts this low taper implies that the niger delta has a very weak basal detachment which we interpret to reflect elevated pore fluid pressure 0 90 within the akata formation a prodelta marine shale that contains the basal detachment horizon the weak basal detachment zone has a significant influence on the structural styles in the deep water niger delta fold belts the overpressured and thereby weak akata shales ductilely deform within the cores of anticlines and in the hanging walls of toe thrust structures leading to the development of shear fault bend folds and detachment anticlines that form the main structural trap types in the deep water fold belts moreover the low taper shape leads to the widespread development of backthrust zones as well as the presence of large relatively undeformed regions that separate the deep water fold and thrust belts this study expands the use of critical taper wedge mechanics concepts to passive margin settings while documenting the influence of elevated basal fluid pressures on the structure and tectonics of the deep water niger delta

subandean thrust and fold belt of northwestern argentina geometry and timing of the andean evolution the subandean ranges of northwestern argentina are an active thin skinned fold and thrust belt the main detachment level in silurian shales dips 2 3w and all the major east verging faults detach from it important intermediate detachment levels in the devonian shales generate lift off structures and the decoupling of the lower and upper structural levels the subandean thrust belt has a minimum shortening of about 60 km 36 at about 2240 latitude the deformation started at about 8 5 9 ma with the uplift of the el pescado range and the formation of an important back thrust at the cinco picachos range fault generation gets young to the east the pintascayo range uplift started at 7 6 ma while the baja orn range uplift began at about 6 9 ma these two ranges continued growing simultaneously until at least 4 7 ma at san antonio range fault movement began at approximately 4 4 ma and the aguarage uplift started at about 2 7 ma an important stage dominated by out of sequence movements spans from about 4 5 ma to present for both proposed models of shortening the quaternary rates of shortening between 8 and 11 mm yr coincide well with global positioning system results from the area the hydrocarbon generation and migration is contemporaneous with the deformation enhancing the possibilities of hydrocarbon entrapment in the area

faulting fracturing and in situ stress prediction in the ahnet basin algeria a finite element approach abstract many low efficiency hydrocarbon reservoirs are productive largely because effective reservoir permeability is controlled by faults and natural fractures accurate and low cost information on basic fault and fracture properties orientation in particular is critical in reducing well costs and increasing well recoveries this paper describes how we used an advanced numerical modelling technique the finite element method fem to compute site specific in situ stresses and rock deformation and to predict fracture attributes as a function of material properties structural position and tectonic stress presented are the numerical results of two dimensional plane strain end member fem models of a hydrocarbon bearing fault propagation fold structure interpretation of the modelling results remains qualitative because of the intrinsic limitations of numerical modelling however it still allows comparisons with the little available geological and geophysical data in all models the weak mechanical strength and flow properties of a thick shale layer the main seal leads to a decoupling of the structural deformation of the shallower sediments from the underlying sediments and basement and results in flexural slip across the shale layer all models predict rock fracturing to initiate at the surface and to expand with depth under increasing horizontal tectonic compression the stress regime for the formation of new fractures changes from compressional to shear with depth if pre existing fractures exist only sub horizontal fractures are predicted to open thus defining the principal orientation of effective reservoir permeability in models that do not include a blind thrust fault in the basement flexural amplification of the initial fold structure generates additional fracturing in the crest of the anticline controlled by the material properties of the rocks the folding induced fracturing expands laterally along the stratigraphic boundaries under enhanced tectonic loading models incorporating a blind thrust fault correctly predict the formation of secondary syn and anti thetic mesoscale faults in the basement and sediments of the hanging wall some of these faults cut reservoir and or seal layers and thus may influence effective reservoir permeability and affect seal integrity the predicted faults divide the sediments across the anticline in several compartments with different stress levels and different rock failure and proximity to failure these numerical model outcomes can assist classic interpretation of seismic and well bore data in search of fractured and overpressured hydrocarbon reservoirs

mobility of u series nuclides during basalt weathering an example from the deccan traps india abstract u series disequilibria have been measured by tims and mc icp ms in dissolved phases and suspended sediments of the main basaltic rivers from the deccan trap region india for dissolved phases 234u 238u 230th 238u and 226ra 238u range between 1 11 and 1 28 between 0 03 10 2 and 1 35 10 2 and between 0 02 and 0 16 respectively for suspended particles 234u 238u 230th 238u and 226ra 238u range from 0 95 to 1 1 from 1 23 to 1 59 and from 0 1 to 0 47 respectively 230th 238u and 234u 238u systems show direct links with indices of weathering the data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady state because present day physical erosion rates are higher than expected 100 400 mm ka versus 40 mm ka for a steady state process for the narmada and the tapti basins models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40 90 ka in contrast with basins located in northern latitudes erosion in the deccan basins does not appear to have been significantly disturbed during the last glaciation 234u 238u disequilibria in the dissolved phase of large rivers world wide are also explored and two key factors are highlighted in this study the dissolution of carbonates releasing dissolved u at secular equilibrium and the silicate weathering rate estimated at the scale of the watershed in particular a positive correlation is found between dissolved 234u 238u and basaltic weathering rates for the deccan that could reflect a key role of physical erosion since for silicate monolithological basins chemical erosion rates are primarily controlled by physical erosion rates for basins with mixed lithology dissolved organic matter also plays a key role in the preferential release of 234u from silicate minerals 234u 238u ratios in rivers therefore reflect a mixture between u released by carbonate dissolution and 234u preferentially released from silicate minerals located in high physical erosion and peat areas

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

testing different tectonic models for the source of the mw 6 5 30 october 2016 norcia earthquake central italy a youthful normal fault or negative inversion of an old thrust

critical state finite element models of contractional fault related folding part 1 structural analysis abstract fourteen lagrangian finite element models with a critical state mechanics constitutive model illustrate some of the primary controls on the formation of fault propagation fault bend and diffuse folds the models demonstrate how variable mechanical stratigraphy initial fault dip and inter layer detachments affect the way faults propagate and thus exert a significant control on resultant fold layer geometry for example models of uniform sandstone properties exhibit efficient strain localization and clear patterns of fault tip propagation uniform shale properties tend to inhibit fault propagation due to distributed plastic deformation models with mixed inter layered sandstone and shale deform in a disharmonic manner resembling lobate cuspate arrangements that are common to many folds observed in outcrop detachments accommodate shortening by bed parallel slip resulting in fault bend fold kinematics and poorly expressed fault propagation across layers structural analysis of the numerical model results reveals that contractional deformation is a composite of lateral compaction pure shear shortening fault propagation along narrowly localized zones of reverse shear and flexure of layers the relative proportions of these shortening components vary in time and with mechanical properties shale vs sandstone depth to detachment calculations performed on selected numerical models suggest that reasonably accurate predictions can be made for detachment fold thrust belts and toe of slope contractional systems however our study suggests that applications to mid crustal level detachments within the basement beneath a sedimentary cover may be inaccurate

the role of bed parallel slip in the formation of blind thrust faults abstract a finite element code is used to model the mechanical behaviour of elastoplastic sedimentary layers that are deformed by the movement of a faulted deformable basement the propagation of the blind thrust fault is analysed in terms of the evolution of strain localization in the elastoplastic layers of strain softening and strain hardening materials the role of layer thickness bedding parallel slip and fault dip are also studied we found that if no interlayer slip is allowed blind thrusts tend to propagate with constant dip regardless of the rheological properties of the sediment layers however if bedding parallel slip is present a mechanical decoupling appears between layers allowing stress transfers along large distances and favouring the formation of backthrusts fault dip also has significant influence on the qualitative behaviour when interlayer slip is possible a steep fault will tend to split

assessing the implications of tectonic compaction on pore pressure using a coupled geomechanical approach abstract overpressure prediction in tectonic environments is a challenging topic the available pore pressure prediction methods are designed to work in environments where compaction is mostly one dimensional and driven by the vertical effective stress applied by the overburden furthermore the impact of tectonic deformation on stresses porosity and overpressure is still poorly understood we use a novel methodology to capture the true compaction phenomena occurring in an evolving 3d stress regime by integrating a fully coupled geomechanical approach with a critical state constitutive model to this end numerical models consisting of 2d plane strain clay columns are developed to account for compaction and overpressure generation during sedimentation and tectonic activity we demonstrate that a high deviatoric stress is generated in compressional tectonic basins resulting in a substantial decrease in porosity with continuing overpressure increase the overpressure predictions from our numerical models are then compared to those estimated by the equivalent depth method edm in order to quantify the error induced when using classical approaches based on vertical effective stress in tectonic environments the stress paths presented here reveal that a deviation from the uniaxial burial trend can substantially reduce the accuracy of the edm overpressure predictions

critical state finite element models of contractional fault related folding part 2 mechanical analysis abstract finite element modeling of contractional fault related folding with critical state mechanics concepts reveals fundamentally different behaviors at two levels depending on 1 mechanical stratigraphy and 2 initial fault geometry deviatoric plastic strain rate and strain patterns show that strong layers like normally consolidated sandstone or overconsolidated shale develop strain localization in form of narrow plastic shear bands soft layers like normally consolidated shale fail to develop localizations and respond by diffuse distributed plastic strain inter layered overburdens display both deformation modes models with listric thrust fault seeds develop basement cored anticlines in the hangingwall that are characterized by locally tensile stress regimes within overall compressive regional stress fields the anticline crests of these models show evidence for strain softening by contrast anticlines emerging in detached fault related fold models are subject to locally compressive stress regimes consistent with the applied far field shortening these anticline crests strain harden during deformation stresses resulting from the finite element models deviate from stress estimates based on integrated densities the discrepancy is small for basement involved models but significant for detached style models suggesting that stress predictions based on integrated densities may be inaccurate for contractional systems first order predictions of plastic deformation suggest that footwall synclines in basement involved fold styles may host reservoirs with relatively poor quality owing to horizontal and vertical compaction anticlines found in detached deformation styles are bounded by compactive domains whereas the crestal regions are less affected by volumetric plastic deformation thus producing relatively better reservoir quality

reply to discussion on mississippi valley type lead zinc deposits through geological time implications from recent age dating research by s e kesler and c w carrigan 2001 mineralium deposita 36 711 740

paleostress magnitudes associated with development of mountain belts insights from tectonic analyses of calcite twins in the taiwan foothills determinations of stress magnitudes based on inversion of calcite twin data from fault related folds in the taiwan foothills show a decrease of differential stresses during synfolding erosion and exhumation suggesting that paleodepth of burial largely controlled the levels of differential stresses sustained by rocks regional frictional conditions linked to shallow and or deep decollement tectonics probably also influenced stress magnitudes combination of the twin inversion technique with fracture analysis and rock mechanics data provides first order estimates of principal stress values related to the major pleistocene shortening event in taiwan these stress estimates are compared to previous stress estimates in fold thrust belts and to available data on differential stress magnitudes in foreland and hinterland domains of orogens at the scale of an entire orogenic system the most striking point is a decrease of differential stresses from the hinterland toward the foreland this decrease not only reflects tectonic stress attenuation away from the plate boundary but also suggests a major control by the depth of deformation differential stresses estimated from natural deformation consequently provide an independent support to the depth dependent strength and the frictional behavior of the upper continental crust deduced from laboratory experiments and stress measurements in deep boreholes

influence of mechanical stratigraphy and initial stress state on the formation of two fault propagation folds abstract kinematic and mechanical modeling of the rip van winkle se new york usa and la zeta sw mendoza argentina anticlines illustrate the influence of mechanical stratigraphy and initial stress state on the kinematics of fault propagation folding in both anticlines faults nucleating at distinct stratigraphic levels open upward into triangular zones of folding folding intensity and finite strain attenuates with distance from the fault tip trishear reproduces the bulk geometry and finite strain of the relatively homogeneous limestone sequence of the rip van winkle anticline and predicts an initial location of the fault tip consistent with the field observations folding of the heterogeneous sedimentary section of la zeta anticline however cannot be simulated by the trishear model alone an additional mode of deformation involving transport of material from the backlimb into the hinge area and extension parallel to the direction of fault propagation is necessary to reproduce the geometry and finite strain of the anticline mechanical distinct element modeling dem of the anticlines indicates that their contrasting kinematics could have resulted from differences in mechanical stratigraphy and initial stress state folding of a homogenous normally consolidated assemblage initial horizontal to vertical stress ratio k o 1 is trishear like and resembles the rip van winkle anticline folding of a heterogenous layered such as la zeta over consolidated assemblage k o 1 departs from the trishear model and resembles la zeta anticline based on the dem simulations we postulate that the rip van winkle anticline formed at high depths high overburden loads and lithostatic stress conditions and that la zeta anticline formed at shallow depths after substantial uplift and erosion of the andean mountain front which induced over consolidation and high k o

recent and future trends in paleopiezometry in the diagenetic domain insights into the tectonic paleostress and burial depth history of fold and thrust belts and sedimentary basins paleopiezometry provides an access to the past stress magnitude key to better understand the behaviour of the earth s crust over long period of time this contribution presents a review of some paleopiezometric techniques that can be used in the diagenetic domain in fold and thrust belts and sedimentary basins calcite twinning and stylolite roughness techniques have been selected and are presented through a critical description of their methodologies along with approaches to further reconstruct the complete effective stress tensor major geological lessons learned over the past decades from published studies are summarized and discussed along with a way forward to potential breakthroughs

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

erosion timescales derived from u decay series measurements in rivers the relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades the principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates in this study 234u 238u 230th 238u and 226ra 238u were measured by tims and by mc icp ms in the dissolved and suspended loads of rivers from the mackenzie basin northwest territories canada the data show a complementary nature between 234u 238u 230th 238u in the dissolved and suspended loads while 226ra is characterized by a more complex behavior modeling of fractionation of u series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed 9 28 10 ka as well as the rates of release of u series nuclides the results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the mackenzie river

multiphase structural evolution of a continental margin during obduction orogeny insights from the jebel akhdar dome oman mountains the structural evolution of the carbonate platform in the footwall of the semail ophiolite emplaced onto the passive continental margin of arabia helps to better understand the early stages of obduction related orogens these early stages are rarely observable in other orogens as they are mostly overprinted by later mountain building phases we present an extensive structural analysis of the jebel akhdar anticline the largest tectonic window of the oman mountains and integrate it on different scales outcrop observations can be linked to plate motion data providing an absolute timeframe for structural generations consistent with radiometric dating of veins top to s overthrusting of the semail ophiolite and hawasina nappes onto the carbonate platform during high plate convergence rates between arabia and eurasia caused rapid burial and overpressure generation and migration of hydrocarbons and bedding confined veins but no major deformation in the carbonate platform at reduced convergence rates subsequent tectonic thinning of the ophiolite took place above a top to nne crustal scale ductile shear zone deforming existing veins and forming a cleavage in clay rich layers in early campanian times ongoing extension occurred along normal to oblique slip faults forming horst graben structures and a precursor of the jebel akhdar dome campanian to maastrichtian this was followed by ne sw oriented ductile shortening and the formation of the jebel akhdar dome deforming the earlier structures thereafter exhumation was associated with low angle normal faults on the northern flank of the anticline we correlate the top to nne crustal scale shear zone with a similar structure in the saih hatat window to develop a unified model of the tectonic evolution of the oman mountains

discrete element modelling of the influence of cover strength on basement involved fault propagation folding abstract a discrete element model is used to investigate the influence of sedimentary cover strength on the development of basement involved fault propagation folds we find that uniformly weak cover best promotes the development of classical trishear like fault related folds showing marked anticlinal thinning and synclinal thickening with cover dips increasing downwards towards the fault tip uniformly strong cover results in more rounded fold forms with only minor hinge thickening thinning and significant basement fault propagation into the sedimentary cover heterogeneous layered cover sequences with marked differences in strength promote the development of more complex and variable fold forms with a close juxtaposition of brittle and macroscopically ductile features which diverge from the predictions of simple kinematic models in these structures the upper layers are often poor indicators of deeper structure in addition we find that in layered cover sequences fault propagation into the cover is a complex process and is strongly buffered by the weaker cover units

multiple geometries and modes of fault propagation folding in the canadian thrust belt abstract a multitude of fold models have been proposed to explain the variety of fold geometries which develop in front of thrust faults detailed field fabric and photogrammetric studies of 4 fault cored asymmetrical folds in the thin skinned canadian thrust belt were used to test models of fault propagation folding fold geometries include combinations of angular and rounded fold surfaces highly contorted anticlinal hinge areas and minimal penetrative deformation or changes in bedding thickness interlimb angles generally decrease with increasing shortening indicating progressive fold tightening about fixed anticlinal hinges extensive flexural slip thrusting toward the anticlinal axes of angular folds suggests that kink folding in thin skinned thrust belts is aided by material transfer from both fold limbs into hinge areas fold geometries change dramatically along the strike of individual structures demonstrating the non uniqueness of fault propagation fold geometries no single mode of fault propagation folding can explain the diverse fold geometries seen in the canadian thrust belt this geometric variability can be ascribed to the complex interplay of multiple modes of folding in strata near the causal thrusts oblique shear and flexural slip in triangular shear zones distribute thrust displacements into both rounded and angular folds simultaneous angular folding of overlying strata commonly occurs by progressive kink folding where folds tighten by flexural slip on all fold limbs until the thrust breaks through the fold regional and local differences in the amount of pervasive top to the craton shear needed for progressive kink folding may be partially responsible for the variability of fault propagation fold geometries in thin and thick skinned orogens world wide

chemical mobilizations in laterites evidence from trace elements and 238u 234u 230th disequilibria geochemical and mineralogical investigations including measurements of major and trace elements sr isotope ratios and 238u 234u 230th activity ratios were made on an old african laterite to reconstruct its formation steps and assess recent chemical mobilization the present data support a scenario of discontinuous formation for the laterite with different bedrock weathering conditions during the formation of each unit rather than a scenario of continuous formation absolute accumulation of fe u and lanthanides in the uppermost ferruginous unit suggests an autochthonous origin of this iron cap by leaching of an older overlying profile present chemical distributions of lanthanides as well as of rb k ba and sr within the profile cannot be linked to the mineralogical distribution of both relictual primary and authigenic secondary phases complementary lanthanide patterns indicate that these elements were primarily accumulated in the uppermost ferruginous unit before further remobilization and accumulation in the underlying horizons these redistribution processes may be related to the chemical instability of the ferruginous cap the 238u 234u 230th disequilibria indicate that recent u mobilization occurs in the whole profile and that as for lanthanides there is a vertical redistribution of u from the uppermost ferruginous unit to the underlying horizons moreover these data show that both u losses and gains exist at each level of the profile a simple modeling of this double u mobilization process is proposed to interpret the 238u 234u 230th data differences in the mobilization and fractionation intensities of the u input and removal processes can account for the two evolution trends which distinguish the ferruginous unit from the underlying ones furthermore on the basis of this modeling the profile appears to be in a transient state because of recent changes in the u mobilization conditions which could correspond to major pleistocene climatic variations

numerical models for the control of inherited basin geometries on structures and emplacement of the klippen nappe swiss prealps cover nappes commonly deform above a shallow basal detachment surface located above rigid crystalline basement rocks inherited basin geometries are presumed to control the kinematic evolution and detachment of cover nappes in an accretionary wedge we compare results from two dimensional finite element modelling with the structural style of a natural cover nappe the klippen nappe which was detached from its basement along an evaporitic detachment horizon and thrust over a distance of roughly 100 km first order characteristics of the klippen nappe paleo basin include a complex distribution of weak detachment rocks and various changes in the sediment composition and layer thicknesses two types of structural style can be distinguished an imbricate fan at the rear end of the nappe prealpes medianes rigides and a fold dominated region at the front prealpes medianes plastiques the model results resemble the first order characteristics of the klippen nappe the model experiments suggest that the formation of imbricates and fault related folds are controlled by the discontinuity of detachment horizons discontinuities are locations where thrust ramps are triggered and layer heterogeneities control the locations for the initiation of detachment folds basement horsts may lead to the formation of recumbent folds which develop a melange zone on the highly sheared inverted limb the experiments also suggest that the thickness of the basal detachment horizon of the medianes plastiques decreases gradually from the foreland to the hinterland

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

mechanical models of fault propagation folds and comparison to the trishear kinematic model abstract faults propagating through the earth generate a wave of deformation ahead of their tip lines we have modeled this process to understand the relationship between fold geometry and fault propagation using finite element modeling fem we investigate the response of incompressible frictionless and frictional materials and a compressible frictional material with associated flow to vertical and dipping faults whose tip lines propagate at rates 3 3 5 times their slip rates the fold geometries finite strain and velocity fields in models with incompressible materials are very similar to those produced by the trishear kinematic model even though the latter uses a purely ad hoc linear velocity field furthermore when the trishear grid search is applied to the final geometry of the mechanical folds the best fit kinematic models have approximately the same propagation to slip ratio as was used in the fem experiments however when the compressible frictional material is used the mechanical models exhibit a main triangular shear band in front of the tip line and a conjugate shear band in the fold backlimb both migrating with the propagating tip line the conjugate shear band antithetic to the fault produces a gentle anticlinal back limb even though there is not a bend in the fault

deepwater fold and thrust belt classification tectonics structure and hydrocarbon prospectivity a review abstract deepwater fold and thrust belts dwftbs are classified into near field stress driven type 1 systems confined to the sedimentary section and type 2 systems deformed by either far field stresses alone or mixed near and far field stresses dwftbs can occur at all stages of the wilson cycle up to early stage continent continent collision type 1 systems have either weak shale or salt detachments they occur predominantly on passive margins but can also be found in convergent related areas such as the mediterranean and n borneo examples include the niger and nile deltas the west coast of africa and the gulf of mexico type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin dwftbs continent convergence zones cover dwftbs developed during continent arc or continent continent collision and those in a deepwater intracontinental setting e g w sulawesi makassar straits active margins include accretionary prisms and transform margins the greatest variability in dwftb structural style occurs between salt and shale detachments and not between tectonic settings changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to dwftb type in comparison with shale salt detachment dwftbs display a lower critical wedge taper more detachment folds long and episodic duration of deformation and more variation in vergence structures unique to salt include canopies and nappes accretionary prisms also standout from other dwftbs due to their relatively long continuous duration rapid offshore propagation of the thrust front and large amount of shortening in terms of petroleum systems many similar issues affect all dwftbs these include the oceanward decrease in heat flow offshore increase in age of mature source rock and causes of trap failure e g leaky oblique and frontal thrust faults breach of top seal by fluid pipes one major difference between type 1 and type 2 systems is reservoir rock high quality continent derived quartz rich sandstones are generally prevalent in type 1 systems more diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in type 2 systems or many are simply poor in well developed turbidite sandstone units however some type 2 systems particularly those adjacent to active orogenic belts are partially sourced by high quality continent derived sandstones e g nw borneo s caspian sea columbus basin in some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts coupled with uplift due to collision results in accretionary prism related fold belts that pass laterally from sub aerial to deepwater conditions e g s caspian sea indo burma ranges the six major hydrocarbon producing regions of dwftbs worldwide gulf of mexico niger delta nw borneo brazil west africa s caspian sea stand out as differing from most other dwftbs in certain fundamental ways particularly the very large volume of sediment deposited in the basins and or the great thickness and extent of salt or overpressured shale sdetachments

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

hydromechanical modeling of stress pore pressure and porosity evolution in fold and thrust belt systems we present coupled critical state geomechanical fluid flow simulations of the evolution of a fold and thrust belt in nw borneo our modelling is the first to include the effects of both syntectonic sedimentation and transient pore pressure on the development of a fold and thrust belt the present day structure predicted by the model contains the key first order structural features observed in the field in terms of thrust fault and anticline architectures stress predictions in the sediments show two compressive zones aligned with the shortening direction located at the thrust front and back limb between the compressive zones near to the axial plane of the anticline the modelled stress field represents an extensional regime the predicted overpressure distribution is strongly influenced by tectonic compaction with the maximum values located in the two laterally compressive regions we compared the results at three notional well locations with their corresponding uniaxial strain models the 2d thrust model predicted porosities which are as much as 7 5 lower at 2 5 km depth and overpressures which are up to 6 4 mpa higher at 3 km depth these results show that one dimensional methods are not sufficient to model the evolution of pore pressure and porosity in contractional settings finally we performed a drained simulation during which pore pressures were kept hydrostatic the predicted geological structures differ substantially from those of the coupled simulation with no thrust faulting these results demonstrate that pore pressure is a key control on structural development

geomechanical analysis of a welding salt layer and its effects on adjacent sediments abstract we simulate welding of the source layer of a salt diapir with a forward finite element model and study stresses and deformation in the salt layer and the diapir as well as in their adjacent sediments welded salt layers are abundant in mature salt basins where most or all of the salt has withdrawn into diapirs however there is little understanding of the stress field in these layers and their adjacent sediments we show that salt flow along the source layer leads to significant stress anomalies inside the layer and in adjacent sediments in the source layer salt pressure becomes higher than overburden stress in nearly welded areas and becomes lower than overburden stress in adjacent thicker areas when the source layer welds stresses increase significantly in sediments near the weld tip which helps compaction of these sediments and possibly their fracturing and faulting our model illustrates that all sediments overlying the weld experience this stress increase and the associated material changes as the weld tip propagates along the weld we present natural examples fitting our predictions and discuss the importance of our results for the exploration characterization and production of reservoirs near welded salt layers

palaeomagnetic dating of widespread remagnetization on the southeastern border of the french massif central and implications for fluid flow and mississippi valley type mineralization summary palaeomagnetic dating techniques have been applied to determine the age of fluid migration that produced the mississippi valley type mvt pb zn ba f deposits in the cevennes region of southern france 15 sampling sites in two gently deformed areas around the largentiere and croix de pallieres mines on the cevennes border were selected for palaeomagnetic study they yielded a very well defined direction of remagnetization corresponding to an early middle eocene age this remagnetization cannot be related to the formation of magnetite as a result of the transformation of smectite to illite because the latter has been well dated as a mesozoic event the magnetic overprint in this area is related to a chemical phenomenon during fluid migration the age of remagnetization corresponds to a major uplift in the pyrenees mountains located to the south of the cevennes this implies that fluid migration occurred from the south to the north as a result of hydraulic head established in the pyrenees orogenic belt during orogenesis and suggests that the mvt deposits in the cevennes region formed from a gravity driven fluid system as described by garven freeze 1984a b

initiation and reactivation of faults during movement over a thrust fault ramp numerical mechanical models abstract with numerical continuum models we investigate the effects of fault geometry fault friction material properties and anisotropy on the initiation and reactivation of faults in the hanging wall of a thrust fault ramp the models use an elastic plastic frictional dilatant cohesion softening material in which deformation may localize as shear bands with a sharp lower fault bend backthrust shear bands propagate up from the concentration of differential stress at the fault bend with a rounded lower fault bend bedding parallel shear bands develop above the fault bend in the center of the layer where differential stress is highest friction on the ramp enhances the development of backthrusts and causes them to initiate at shallower dips increased overburden decreases the amount of localization within shear bands and the spacing between them shear bands that develop at the lower fault bend are weaker than the surrounding material and are thus potential sites of reactivation during subsequent deformation because of this potential reactivation the style of deformation at the upper fault bend depends on the deformation that accumulates at the lower fault bend at the upper fault bend backthrusts are reactivated as extension faults and are crosscut by more steeply dipping extension faults bedding parallel shear bands are reactivated at the upper fault bend with top to the hinterland sense of shear low angle extension faults are listric into the reactivated bedding parallel shear bands producing hinterland verging extensional duplexes above the upper flat

stress and strain patterns kinematics and deformation mechanisms in a basement cored anticline sheep mountain anticline wyoming 1 in order to characterize and compare the stress strain record prior to during and just after folding at the macroscopic and the microscopic scales and to provide insights into stress levels sustained by folded rocks we investigate the relationship between the stress strain distribution in folded strata derived from fractures striated microfaults and calcite twins and the development of the laramide basement cored sheep mountain anticline wyoming tectonic data were mainly collected in lower carboniferous to permian carbonates and sandstones in both rock matrix and veins calcite twins recorded three different tectonic stages the first stage is a pre laramide sevier layer parallel shortening lps parallel to fold axis the second one is a laramide lps perpendicular to the fold axis and the third stage corresponds to laramide late fold tightening with compression also perpendicular to the fold axis stress and strain orientations and regimes at the microscale agree with the polyphase stress evolution revealed by populations of fractures and striated microfaults testifying for the homogeneity of stress record at different scales through time calcite twin analysis additionally reveals significant variations of differential stress magnitudes between fold limbs our results especially point to an increase of differential stress magnitudes related to laramide lps from the backlimb to the forelimb of the fold possibly in relation with motion of an underlying basement thrust fault that likely induced stress concentrations at its upper tip this result is confirmed by a simple numerical model beyond regional implications this study highlights the potential of calcite twin analyses to yield a representative quantitative picture of stress and strain patterns related to folding

mechanics of non critical fold thrust belts based on finite element models abstract the mechanics of fold thrust belts and accretionary wedges is investigated using a two dimensional plane strain elastic plastic cohesive mohr coulomb mechanical model solved with the finite element method results show that when a layer with an initially non critical geometry is compressed from the rear it does not form a wedge that is at failure throughout as assumed in critical wedge theory rather the wedge consists of narrow plastic shear zones that propagate sequentially outward with time loading rocks ahead while unloading rocks behind not only are stress states within the wedge not everywhere at failure but principal stress orientations vary strongly in time and space particularly across shear zones near the basal detachment and in the hanging wall of active structures where local surface extension may be observed the reason the investigated wedges are not stressed to compressive failure throughout is related to strength reduction associated with strain localisation that enables material outside shear zones to unload and return to an elastic stress state this mechanism is intrinsic to elastic plastic materials and occurs regardless of any material degradation such as loss of cohesion even though the stress state of the investigated wedges is generally non critical the overall geometry may still be consistent with cohesionless critical wedge theory since the local surface slope is created when a particular part of the wedge is at a limit state prowedge tapers display non self similar growth through time but eventually evolve to the minimum critical taper retrowedges on the other hand may get caught within this initial transient state and thus may have tapers anywhere between the minimum and maximum critical taper however if the basal detachment is such that lateral propagation is not kinematically inhibited retrowedges are shown to also eventually evolve towards minimum critical tapers resulting in a symmetrical doubly vergent orogen because all of these wedges have non critical stress states both pro and retrowedges are predicted to display stable non equilibrium taper angles in the presence of external perturbations for example related to erosion or sediment deposition

determination of principal stress magnitudes using calcite twins and rock mechanics data abstract this paper proposes a new method for determining principal stress magnitudes based on the use of the deviatoric stress tensor determined from computer inversion of calcite twin data additional evaluation of the vertical stress weight of overburden or of the mean stress is enough to calculate the complete stress tensor we emphasize that the best reasoning consists of combining calcite twin analysis with rock mechanics data failure friction criteria because the determination of the six parameters of the absolute stress tensor is overconstrained