trishear kinematic modeling of extensional fault propagation folding we present a new non linear 2 d numerical trishear model that is capable of modeling both symmetrical and asymmetrical trishear zones and a strain prediction approach based on the velocity field within the trishear zone a simple relationship exists between the shape factor r of the velocity field the hanging wall apical angle and footwall apical angle the implication of this relationship is that when dealing with a real deformed state cross section one can measure the hanging wall and footwall apical angles to determine the r value to specify the velocity field within the trishear zone for either the forward modeling or restoration the new trishear model can accurately reproduce the geometry of two experimental clay models and an extensional fault propagation fold from the gulf of suez predicted strain is greatest directly above the fault tip and decreases with distance from the fault tip as in the experimental models greater strain occurs in the hanging wall because of the greater velocity gradient there whereas the footwall is much less intensely deformed consequently secondary faulting and fracturing if present should be better developed along the fault tip line or in the hanging wall the orientation of fractures or minor faults can be predicted from the strain ellipses calculated from the trishear velocity field

defining a 3 dimensional trishear parameter space to understand the temporal evolution of fault propagation folds abstract the application of trishear in which deformation occurs in a triangular zone in front of a propagating fault tip is often used to understand fault related folding a key element of trishear in comparison to kink band methods is that non uniform deformation within the triangle zone allows the layer thickness and length to change during deformation by varying three controlling parameters independently trishear propagation slip ratio trishear apical angle and fault dip we construct a three dimensional parameter space to demonstrate the variability of resultant geometry feasible with trishear we plot published natural examples in this parameter space and identify two clusters and show that the most applicable typical trishear propagation slip ratio is 2 to 3 while the trishear apical angle varies from 30 to 100 we propose that these findings can help estimate the best fit parameters for natural structures we then consider the temporal evolution of specific geometric examples and factors that increase the complexity of trishear including 1 fault dip changes and 2 pre existing faults to illustrate the applicability of the parameter space and complex trishear models to natural examples we apply our results to a sub surface example from the qaidam basin in northern tibetan plateau

geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

cyclical behavior of thrust wedges insights from high basal friction sandbox experiments scaled sandbox experiments with high basal friction simulating the growth of accretionary wedges display cycles alternating between frontal imbricate thrusting and underthrusting of long undeformed sheets by contrast low basal friction experiments with otherwise similar and constant initial conditions produce a classic frontal imbricate fan through repeated failure along frontal thrusts the cyclical behavior observed in high basal friction experiments is expressed by three quantities 1 the average spacing between frontal thrusts 2 the advance and retreat of the deformation front and 3 the frontal slope alpha of the actively deforming wedge as a long sheet is underthrust the front is steepened through slumping until the maximum critical angle is reached then frontal thrusting resumes and the accretion of imbricate slices builds the wedge forward thereby lowering the taper to the minimum critical angle at shallow tapers a long unit is underthrust and subsequently uplifts shortens and steepens the overlying wedge through backthrust deformation thus completing the cycle underthrusting of long units offers a simple mechanism for underplating overlying units it also provides a possible explanation for temporally and spatially varying wedge geometries in nature when basal frictions attain 80 90 of the internal friction

localisation of normal faults in multilayer sequences abstract existing conceptual growth models for faults in layered sequences suggest that faults first localise in strong and brittle layers and are later linked in weak and ductile layers we use the discrete element method for modelling the growth of a normal fault in a brittle ductile multilayer sequence the modelling reveals that faults in brittle ductile sequences at low confining pressure and high strength contrast localise first as mode i fractures in the brittle layers low amplitude monoclinal folding prior to failure is accommodated by ductile flow in the weak layers the initially vertically segmented fault arrays are later linked via shallow dipping faults in the weak layers faults localise therefore as geometrically and kinematically coherent arrays of fault segments in which abandoned fault tips or splays are a product of the strain localisation process and do not necessarily indicate linkage of initially isolated faults the modelling suggests that fault tip lines in layered sequences are more advanced in the strong layers compared with weak layers where the difference in propagation distance is most likely related to strength and or ductility contrast layer dependent variations in fault localisation rates generate fringed rather than smooth fault tip lines in multilayers

mesozoic extension controlling the southern alps thrust front geometry under the po plain italy insights from sandbox models in the southern alps fold and thrust belt east west mesozoic rifting controlled the development of extensional faults the deposition of the sedimentary sequences and the thickness and distribution of the main detachment levels vertical and lateral heterogeneities strongly influenced the structural style of the subsequent north south contractional phase as revealed by surface mapping and by subsurface maps derived from seismic data along the strike of the chain lateral terminations of thrust faults occur at syn rift extensional faults trending normal with respect to the contractional structures from west to east the thrust sheet number decreases and the order of structures and their wavelength changes on a regional scale the effects produced by pre existing heterogeneities on the development of contractional alpine tectonic structures in the external area of the southern alps were evaluated by means of scaled sandbox analogue models modelled lateral heterogeneities produced different wavelength thrust sheets salients and recesses whereas vertical heterogeneities facilitated the development of structures of different orders first order structures detached at the base of the model and second order structures detached at the intermediate relatively weak layers comparison of analogue model results with nature showed good geometrical similarities 2006 elsevier ltd all rights reserved

kinematics of orocline and arc formation in thin skinned orogens the regional traces of folds faults and foliations found in many fold thrust belts are bent in plan view it proves valuable to distinguish between two types of bent orogens based on their kinematic evolution oroclines or rotational arcs are bent orogens in which segments of the orogen change strike during the evolution of the bend and nonrotational arcs are bent orogens in which segments of the orogen do not change strike during development of the bend the kinematic evolution of a bend that forms in a thin skinned orogen can be described in terms of three parameters the displacement path trajectories followed by points along the strike of the orogen the magnitude and distribution of tangential extension along the strike of the orogen and the change in position of the endpoints of the orogen with respect to a reference line only slight differences in the displacement path trajectory pattern determine whether an orogen evolves as a nonrotational arc or as an orocline thus the distinction between these two types of bends is not always of major tectonic significance compressional deformation along irregular continental margins or the impact of indentors on continental margins during collisional orogenies more likely leads to formation of nonrotational arcs as can be simulated with a sand wedge model interaction between a fold thrust wedge with obstacles in the foreland or with a wrench fault more likely leads to orocline formation orocline formation is also associated with noncoaxial reactivation of thrust faults tangential extension is a necessary consequence of certain displacement path trajectory patterns and can accompany development of either nonrotational arcs or oroclines two examples one from the umbrian apennines and one from the makran range demonstrate how movement on cross strike fault arrays can accommodate tangential extension in thin skinned bends

balanced cross sections and their implications for the deep structure of the northwest alps reply abstract the structure of the northwest external french alps around the internal belledonne and mont blanc massifs in the beaufortain area can be explained by foreland wnw propagating thrust tectonics the internal belledonne massif is interpreted as having developed by imbrication of an originally thin skin of basement with a mesozoic cover above this massif all thrusts tend to climb lateral sections to the south to converge with the frontal pennine thrust the southern termination of the mont blanc massif is merely a spectacular example of such a lateral ramp

the cenozoic evolution of the roer valley rift system integrated at a european scale the roer valley rift system rvrs is located between the west european rift and the north sea rift system during the cenozoic the rvrs was characterized by several periods of subsidence and inversion which are linked to the evolution of the adjacent rift systems combination of subsidence analysis and results from the analysis of thickness distributions and fault systems allows the determination of the cenozoic evolution and quantification of the subsidence during the early paleocene the rvrs was inverted laramide phase the backstripping method shows that the rvrs was subsequently mainly affected by two periods of subsidence during the late paleocene and the oligocene quaternary time intervals separated by an inversion phase during the late eocene during the oligocene and miocene periods the thickness of the sediments and the distribution of the active faults reveal a radical rotation of the direction of extension by about 70 80j counter clockwise integration of these results at a european scale indicates that the late paleocene subsidence was related to the evolution of the north sea basins whereas the oligocene quaternary subsidence is connected to the west european rift evolution the distribution of the inverted provinces also shows that the early paleocene inversion laramide phase has affected the whole european crust whereas the late eocene inversion was restricted to the southern north sea basins and the channel area finally comparison of these deformations in the european crust with the evolution of the alpine chain suggests that the formation of the alps has controlled the evolution of the european crust since the beginning of the cenozoic

the oligocene miocene tectonic evolution of the northern outer carpathian fold and thrust belt insights from compression and rotation analogue modelling experiments this paper presents the different analogue scenarios of the tectonic evolution of the northern outer carpathians i e the western and northern eastern outer carpathians which formed as an accretionary wedge in front of the east alpine carpathian pannonian alcapa block during oligocene miocene times currently this fold and thrust belt forms an arc which is asymmetrically convex to the north and wider in its eastern part palaeomagnetic investigations have suggested that the rocks of the arc underwent counter clockwise rotation along almost the whole arc which is difficult to explain as an effect of simple indentation of the triangular indenter in this case two branches of the arc should be rotated in the opposite directions the structural evolution of the western outer carpathians is characterized by superposition of two successive tectonic shortening events directed n s and ne sw the results of the presented analogue modelling suggest that two scenarios of the geodynamic evolution of the studied belt could explain the occurrence of such differently oriented shortening events 1 two phases of differently directed indentation first to the n then to the ne and 2 indenter movement to the ne with simultaneous counter clockwise rotation however the experiment in which the moving indenter is simultaneously rotated produces the most suitable model the counter clockwise rotation of the material is only possible in front of both sides of the convex indenter in this model the results of the analogue modelling also prove that rotation of the alcapa block started after formation of the magura nappe the innermost nappe of the western outer carpathians

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

subsurface structural interpretation by applying trishear algorithm an example from the lenghu5 fold and thrust belt qaidam basin northern tibetan plateau abstract the application of trishear algorithm in which deformation occurs in a triangle zone in front of a propagating fault tip is often used to understand fault related folding in comparison to kink band methods a key characteristic of trishear algorithm is that non uniform deformation within the triangle zone allows the layer thickness and horizon length to change during deformation which is commonly observed in natural structures an example from the lenghu5 fold and thrust belt qaidam basin northern tibetan plateau is interpreted to help understand how to employ trishear forward modelling to improve the accuracy of seismic interpretation high resolution fieldwork data including high angle dips dragging structures thinning hanging wall and thickening footwall are used to determined best fit trishear model to explain the deformation happened to the lenghu5 fold and thrust belt we also consider the factors that increase the complexity of trishear models including a fault dip changes and b pre existing faults we integrate fault dip change and pre existing faults to predict subsurface structures that are apparently under seismic resolution the analogue analysis by trishear models indicates that the lenghu5 fold and thrust belt is controlled by an upward steepening reverse fault above a pre existing opposite thrusting fault in deeper subsurface the validity of the trishear model is confirmed by the high accordance between the model and the high resolution fieldwork the validated trishear forward model provides geometric constraints to the faults and horizons in the seismic section e g fault cutoffs and fault tip position faults intersecting relationship and horizon fault cross cutting relationship the subsurface prediction using trishear algorithm can significantly increase the accuracy of seismic interpretation particularly in seismic sections with low signal noise ratio

kinematics of orocline tightening in the core of an arc paleomagnetic analysis of the ponga unit cantabrian arc northern spain 1 paleomagnetic and structural analyses of the western european variscan belt wevb suggest that the most viable kinematic model for variscan deformation in northern iberia is oroclinal bending of an originally linear belt in a two stage tectonic history this history represents two regional compression phases e w in the late carboniferous and n s in the permian both in present day coordinates which resulted in the refolding about steeply plunging axes of initially north south trending thrusts and folds in the hinge zone and oroclinal tightening due to vertical axis rotation of the belt s limbs however the orocline model has yet to be critically tested in the wevb s core this study reports new paleomagnetic rock magnetic and structural data from the inner core of the wevb in order to test opposing kinematic models for the well documented fault and fold interference structures formed by late stage variscan deformation and to better understand the overall development of the wevb arc map scale structural features within the wevb core have a highly sinuous geometry characterized by transverse and thrust parallel fold systems formed by fault bend folding over footwall ramps the intersections of these two fold systems produce steeply plunging interference folds which are best exposed in the ponga thrust unit a total of 67 paleomagnetic sites were collected in the barcaliente formation of the ponga unit with emphasis placed on detailed spatial coverage of individual structural domains a multicomponent paleomagnetic remanence was measured which is interpreted to be composed of two components a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest stephanian to early permian after initial d1 thrusting and folding bur prior to major secondary rotation rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle domain magnetite these paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction on the basis of the restored ponga unit geometry the steeply plunging interference folds found in the wevb core are best described by a combination of secondary buckling of frontal ramp parallel hanging wall anticlines and modification of d1 lateral oblique ramps as frontal ramps during late stage north south shortening this combination of structural modification of the inner core of the wevb was necessary to accommodate oroclinal bending during the final amalgamation of pangea in the late paleozoic

material transfer in accretionary wedges from analysis of a systematic series of analog experiments the structural evolution of accretionary wedges was modeled in a series of analog experiments with systematically varying initial boundary conditions the two principle parameters investigated were basal friction and thickness of sediment input relative to subducted sediment output material transfer in terms of degree and location of accretion and erosion is quantified as a function of these two parameters low basal friction produces long thin wedges by continuous frontal accretion of imbricate thrusts simultaneously substantial basal erosion may occur with little influence on the frontal dynamics the formation of an outer arc high with an accompanying forearc basin depends upon the relative amount of basal erosion the opening of the subduction window spacing of thrust faults is proportional to the layer thickness confirming the principle of self similar wedge growth from critical wedge theory high basal friction typically generates steep wedges marked by an erosional mass wasting frontal slope in the cases i o this is due to underthrusting and or subduction of the entire sedimentary section the case i o also features the late development of normal faulting and an extensional forearc basin for both low and high basal friction the material balance affects the wedge slope erosive conditions i o for low basal friction the primary modes of material transfer are frontal accretion and basal erosion for high basal friction the primary modes of material transfer are underthrusting underplating and frontal erosion applying the experimental results to active convergent margins parallels may be drawn thus the barbados ridge complex is interpreted to be a good example of a low basal friction wedge with an accretionary material balance while the japan and mariana margins provide examples of an erosive material balance

strain displacement and rotation associated with the formation of curvature in fold belts the example of the jura arc abstract a new simplified genetic classification scheme for arcuate fold thrust belts is proposed based on total strain patterns and displacement vector fields we distinguish three extreme end member models 1 oroclines pure bending of an initially straight belt 2 piedmont glacier with divergent transport directions and 3 primary arcs a simple geometric model set up for the simulation of strain patterns in primary arcs with uniform transport direction demonstrates that divergent strain trajectories and rotations of passive marker lines do not require any divergence in displacement directions these often quoted arguments are insufficient for the identification of oroclinal bending or piedmont glacier type of arc formation only three dimensional restorations of an arc provide the critical information about displacement directions in their absence arc parallel stretches and rotations in comparison with total strains provide the most useful criteria for the distinction of arc formation modes as an example the jura fold thrust belt of the external alps is discussed a large set of strain data includes total shortening estimates based on balanced cross sections local strain axes orientations from the inversion of fault populations homberg c 1996 unpublished phd thesis universite de paris vi france tectonic stylolites and micro strains from twinning in sparry calcite strain trajectories maximum shortening direction computed from these data define a strongly divergent fan with a 90 opening a complete displacement vector field for the entire jura has been determined from balanced cross sections augmented with three dimensional block mosaic restorations philippe y 1995 unpublished phd thesis universite de chambery france displacement vectors diverge by about 40 markedly less than strain trajectories the non parallelism between strain trajectories and transport directions indicates that considerable wrenching deformation did occur in both limbs of the jura arc paleomagnetically determined clockwise rotations of 0 13 from ten sites kempf o et al terra nova 10 6 10 behind the right hand half of the jura arc and two sites with a combined 23 anticlockwise rotation behind the left hand half of the arc are and additional argument in favor of such a wrenching deformation we conclude that the jura arc formed as a primary arc with a minor component of piedmont glacier type divergence in transport directions

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

cross shelf and vertical structure of ichthyoplankton assemblages in continental shelf waters of the southern gulf of mexico abstract the horizontal and vertical variation of ichthyoplankton assemblages in continental shelf waters of the southern gulf of mexico was studied during an annual cycle 1994 1995 at 22 stations distributed across four transects located off the main fluvio lagoon systems of the area samples were collected from five levels of the water column from the surface to a depth of 105 m the bray curtis dissimilarity index identified five assemblages coastal inner neritic outer neritic oceanic and a transition group the main species of each assemblage reflected the habitat season and spawning area of adult organisms the coastal assemblage occupied the uppermost water column levels of the most nearshore stations the oceanic assemblage was restricted to the deepest samplings of the stations of the outer shelf and the neritic assemblages were located on the continental shelf between the coastal and oceanic assemblages embracing all the sampling levels in some periods variation in composition location and abundance of larvae in each assemblage was determined by a series of physical processes that act at different temporal and spatial scales especially the mixing process generated by the wind stress and continental water discharges acting at a mesoscale that influences the distribution of the assemblages along the whole continental shelf as well as a fine scale affecting the composition and vertical distribution of organisms in a more reduced area of just tens of meters in the water column canonical correspondence analysis revealed that the depth of the sample the potential energy and the temperature generated the greatest variability in the system vertical and horizontal salinity gradients play a determinant role in delimiting the coastal and neritic assemblages

fault relays bends and branch lines abstract branch lines between normal faults and their sub parallel splays mapped from 3d seismic reflection data show a range of forms from straight lines to closed loops the different geometries are interpreted as representing stages in the failure of relay zones and in the progressive replacement of fault tip lines with fault branch lines the geometries of these normal fault branch lines are similar to those for thrusts previously inferred from limited two dimensional data the orientation of the axis of a relay and its associated bends relative to a fault slip direction is identified as an important control on the structures developed within the relay neutral restraining and releasing bends can each occur on any fault type normal reverse and strike slip but data bias is a major factor in determining which bend geometry is most often observed with each fault type on normal faults the initial relay zone geometry controls the dominant branch line orientation and the same control is likely on branch lines associated with the other modes of faulting a review of the relay geometries and strains occurring with all three modes of faulting highlights the role of the orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of fault surface and slip directions this relative orientation determines how the relay strain is accommodated and hence the degree of hard linkage and development of branch lines

unusual larval fish distribution pattern in coastal zone of the western mediterranean the coast of catalonia was divided into three zones on the basis of heterogeneity in their physical features and in the structure of the larval fish populations both physical and biological processes are normally more stable overall in the middle zone however in may 1983 an anomalous hydrodynamic event took place over the continental shelf in this zone and resulted in a major alteration in the structure of the larval fish population an intense inverted density front changed the direction of coastal flow the resulting density gradient confined larvae inshore in a zone in which the main larval concentrations are normally located over the shelf break during the rest of the year this anomalous distribution of the larval fish population at that time was a consequence of the unpredictable hydrographic event and contrasted with the structure of the larval population over the rest of the year produced by a more constant hydrodynamic pattern

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

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

a quadshear kinematic model for folding and clay smear development in fault zones abstract a new kinematic model based on the trishear model has been developed to simulate shearing and folding of a ductile interval sandwiched between two brittle layers as faults propagate from the brittle layers into the ductile interval this quadshear model can be used to study fault zone evolution and the development of clay smears which often form when a stratigraphic sequence including ductile clay rich beds is sheared into a developing fault zone the model also gives the critical clay smear factor ccsf the ratio of fault displacement to bed thickness at which a clay smear becomes discontinuous and hence a potential leak point the main control on clay smear development is the position of the clay rich beds relative to brittle fracture prone layers clay rich beds near the centre of a ductile interval can shear to form much longer continuous clay smears than clay rich beds adjacent to brittle layers fault propagation rates are also a strong control on clay smear development slow propagating faults form longer continuous clay smears than fast propagating faults however the width of the shear zone the dip of the fault and the presence of releasing or restraining bends in the fault are minor controls on clay smear development

structural evolution in thrust belts and relative plate motion the upper pennine piemont zone of the internal alps southwest switzerland and northwest italy the earliest ll extension lineation of the polyphase deformation sequence in the combin and zermatt saas fee units of the piemont zone of the upper pennine nappes trends roughly nw se and the geochronological and stratigraphical constraints indicate that the nappe transport and the associated stretching lineation occurred between about 100 and 70 ma this accords in a general way with the relative motion of the african plate to the european plate during the late cretaceous as determined from reconstructions of the opening of the atlantic ocean and suggests that late cretaceous thrusting in the swiss alps was driven fairly directly by the africa europe plate motion

mechanical models of trishear like folds previous workers have formulated velocity descriptions of the trishear kinematic model of fault propagation folds which are inherently non unique we present two complete mechanical models of fault related folding and assess the validity of the assumptions used in the assignment of velocities in the trishear description and to eliminate the untenable situation of an infinite number of possible solutions for velocity fields the mechanical model of forced folding forced fold based on viscous folding theory is used to derive the velocity fields in an anisotropic sedimentary cover overlying faulted and displaced rigid basement blocks the solution of displacements around a stress free fault in an elastic body is used to model fault arrest folds that form around a fault imbedded in a deformed medium the results indicate that the velocity fields assumed in the trishear model more closely resemble the velocities derived in the mechanical forced fold model than the mechanical model of fault arrest folding the forced fold model produces a triangular region of concentrated deformation similar to the trishear region assumed in the kinematic models while the deformation produced by the fault arrest model is not concentrated within a triangular zone

alternative model of thrust fault propagation a widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory we propose an alternative model that applies gretener s beam failure mechanism to a multilayered sequence invoking compatibility conditions which demand that a thrust propagate both upsection and downsection we suggest that ramps form first at shallow levels and are subsequently connected by flat faults this hypothesis also explains the formation of many minor structures associated with thrusts such as backthrusts wedge structures pop ups and duplexes and provides a unified conceptual framework in which to evaluate field observations

the influence of tidal phase on patterns of ichthyoplankton abundance in the vicinity of an estuarine front botany bay australia abstract the influence of oceanography on abundance patterns of ichthyoplankton was compared at different phases of the tide in the vicinity of an estuarine front in botany bay australia fronts were observed at high ebb low and flood tides but the most conspicuous colour discontinuities were found on ebb and flood tides greatest differences in water density were found across the frontal region on ebb tides because rainfall was low density differences were primarily due to differences in temperature there was strong advection of drogues into fronts and along frontal margins on ebb and flood tides thus particles are jetted along fronts total ichthyoplankton was always greatest in fronts on low tides by 1 5 4 times rank abundance of ichthyoplankton in the plume front and ocean at other phases of the tide flood high ebb varied among days great differences in the composition of ichthyoplankton were found between the plume front and ocean at all phases of the tide but differences were most clear on ebb and low tides some types of fish were always most abundant in the front e g exocoetidae or the plume and front e g gobiidae pleuronectidae hemiramphidae blenniidae regardless of the state of the tide some pelagic juveniles were only found in tows that contained drift algae especially in the front the jetting of larvae along topographically stable fronts on flood tides may influence spatial patterns of recruitment in bays and estuaries the intensity of differences in water density across the frontal region between the plume and ocean did not explain the relative abundance patterns of fish the authors argue that a combination of physical advection responses by ichthyoplankton to the biological attributes of fronts e g abundance of food lag effects i e response of plankton to aggregation and differences in larval supply from spawning were responsible for patterns of abundance in different water masses at scales of less than 15 km

structural analysis of the lombard thrust sheet and adjacent areas in the helena salient southwest montana usa abstract the helena salient is a prominent craton convex curve in the cordillera thrust belt of montana usa the lombard thrust sheet is the primary sheet in the salient structural analysis of fold trends cleavage attitudes and movement on minor faults is used to better understand both the geometry of the lombard thrust and the kinematic development of the salient early w e to wnw ene shortening directions in the lombard sheet are indicated by fold trends in the center of the thrust sheet the same narrow range of shortening directions is inferred from kinematic analysis of movement on minor faults and the orientations of unrotated cleavage planes along the southern lateral ramp boundary of the salient as the salient developed the amount and direction of shortening were locally modified as listric detachment faults rotated some tight folds to the nw and as right lateral simple shear caused by lock up and folding of the jefferson canyon fault above the lateral ramp rotated other folds northeastward where the lateral ramp and frontal oblique ramp intersect folds were rotated back to the nw our interpretation of dominant w e to wnw ese shortening in the lombard sheet later altered by local rotations supports a model of salient formation by primary parallel transport modified by interactions with a lateral ramp

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

generation of the benthic foraminiferal assemblage theory and preliminary data abstract the production of benthic foraminiferal communities is filtered through taphonomic mainly destructive processes within the sediments to generate the fossil assemblage both the production and the taphonomy depend on bottom water oxygen content and flux of organic carbon to the seabed an examination of the relationships of processes generating the fossil assemblage to oxygen and organic carbon supply is made using pore water geochemical measurements to estimate carbon flux for locations in the gulf of mexico and the central california margin the locations are plotted in a three dimensional field with bottom water oxygen content organic carbon flux and sediment depth as the axes then the response of foraminiferal standing stock taphonomic processes and the developing fossil assemblage to the field is investigated variation in the vertical stratification of foraminiferal standing stock and test production species stratification taphonomic process intensity and stratification and sediment bioturbation lead to marked differences in the way the fossil assemblage is generated across the oxygen content organic carbon flux field the result is that the oxygen carbon flux field has a significant impact on the fossil assemblage through the interaction of biological and biogeochemical processes in the sediments a model of this interaction is investigated to show how its elements change across the oxygen carbon flux field and how these affect the generation of the fossil assemblage

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

lateral displacement during neogene convergence in the western and central alps although major deformation in the western alps is clearly the result of north northwest directed thrust tectonics there is evidence suggesting that orogen parallel deformation may have been important in the late tectonic history of the western alps the authors propose that this northeast trending deformation includes 1 southwest directed normal fault movement along the simplon line 2 a diffuse zone of northeast striking dextral strike slip deformation along the rhone valley in switzerland between the mont blanc and aiguilles rouges massifs and through the belledonne massif and 3 southwest directed thrusting in the embrunais ubaye and digne nappe systems of southeastern france the correlation of these three broad regions of deformation is based on similar amounts of minimum displacement consistent kinematics and timing of deformation

nutrients plankton and productivity in a warm core ring in the western gulf of mexico argos drift buoy trajectory data showed that a region of anticyclonic circulation about 100 km in diameter was present over the upper continental slope of the nw corner of the gulf of mexico in september october 1988 guided by these data texas am however a maximum of only 36 54 psu at t 25 5 in contrast to as much as 36 88 psu at this density surface in a fresh ring indicated that this feature had spent many months in the western gulf since its separation from the loop current biologically the warm core ring was oligotrophic its surface waters were generally depleted in nitrate to depths of more than 100 m and chlorophyll standing stocks 20 mg m 2 primary productivity 0 4 mg c m 3 h 1 and zooplankton biomass only 4 ml 100 m 3 day and 6 ml 100 m 3 night in the upper 200 m were all extremely low deployments of floating sediment traps within the feature on two consecutive days intercepted fluxes of only 10 20 mg dry weight m 2 d 1 at 50 m and at 100 m by comparison at ring periphery where there was measurable nitrate at 100 m 0 2 1 g at l 1 chlorophyll standing stocks and primary production in the surface mixed layer were 1 5 2 times higher

double edge fault propagation folding geometry and kinematics abstract fault propagation folding is a common folding mechanism in thrust and fold belts and accretionary prisms several geometrical models relating the fold shape to the ramp shape have been proposed in all these models ramps always emanate from a basal fault and propagate upwards we have developed a new kinematic and geometric model of fault propagation folding named double edge fault propagation folding the model simulates folding at thrust ramps as a function of their nucleation site and propagation history within the folded multilayer the fold shape depends on the initial length and location of the ramp its dip and the s p ratio i e incremental ramp slip versus propagation of both the upper and lower ramp tips this solution increases the geometrical flexibility of fault propagation folding reducing for example the direct dependence between the backlimb dip and the ramp dip as double edge fault propagation folding is characterised by a backlimb panel not necessary parallel to the ramp non parallelism between the ramp and the backlimb is commonly observed in thrust related anticlines within fold and thrust belts and accretionary prisms the excess layer parallel shear imposed by the development of double edge fault propagation folding can be easily accommodated by discrete faulting and or penetrative deformation the dependence of the fold shape on the fault behaviour provides a tool for including the role of mechanical stratigraphy and environmental conditions of deformation into kinematic models natural examples of anticlines that could be modelled by double edge fault propagation are presented

change of direction of overthrust shear in the helvetic nappes of western switzerland abstract the lowest of the helvetic nappes in western switzerland the morcles and the doldenhorn nappes are large scale recumbent folds in equivalent tectonic positions in the alpine chain a comparison of the orientations of stretching lineations in both nappes shows that the lineations in the two stratigraphically normal sequences trend n s whereas those in the two stratigraphically inverted sequences trend nw se a similar geometrical relationship has been found from an analysis of the calcite c axis directions of these limestones the two normal sequences show nw se directed c axes whereas the two inverted sequences show w e directed c axes these observations lead to the following conclusions 1 regionally consistent shear movements occurred over a strike distance of more than 60 km and these gave rise to similar deformation patterns in both nappes 2 a progressive rotation of these shear movements from a northerly direction in the earlier formed limbs to northwesterly in the later formed inverted limbs can be deduced the finite strain stretching lineations lag behind the crystallographic fabric axes formed in the later part of the deformation history

crescent like large scale structures in the external zones of the western gibraltar arc betic rif orogenic wedge the external wedge of the gibraltar arc orogenic system western mediterranean is a natural case of a fold and thrust salient although at the scale of the arc a swing of the structural trend can be observed the presence of closed crescent like structures some of them more than 25 km long and scattered over an area of 3500 km 2 characterize the gibraltar arc external wedge this feature makes it very different from a classical arc shaped fold and thrust belt detailed structural maps of representative crescent like structures are presented synorogenic sediments reveal that they probably formed during a short time interval most probably during a single shortening event the crescent like structures are localized in front of a recess formed by the internal zones that acted as backstop this backstop shape and a viscous substrate essentially triassic evaporites below the fold and thrust external wedge constrain the experimental setting of an analogue model in which we were able to reproduce structures that developed originally with a highly non cylindrical shape during a single straight convergence moreover the tectonic transport directions along the thrusts are broadly comparable in the natural case and the model

a modeling study of tidally driven estuarine exchange through a narrow inlet onto a sloping shelf the circulation and particle transport through a narrow inlet kelvin number 0 5 and onto a sloping shelf are numerically examined using a three dimensional hydrodynamic model the model domain consists of an estuarine basin with vertical density stratification separated from a well mixed sloping shelf by a narrow inlet forcing is supplied by semidiurnal tides and buoyancy differential strong ebb flows resulting from this combined forcing transport estuarine water seaward through the inlet to form a radially spreading buoyant surface plume over the shelf a series of radial density fronts corresponding with the ebbing tidal outflow are found on the seaward side of the inlet mouth and are convergence zones for floating particles strong anticyclonic frontal flow advects particles around the boundary of the outflow plume particles released nearest the seaward side of the inlet mouth are able to enter the inlet at depth and remain while those particles released farther offshore rarely enter the inlet this result has clear implications for the shelf to estuary transport of pollutants or the larval phase of marine species asymmetric tidally induced eddies which appear on both sides of the inlet also contribute to the near field circulation calculations of gradient richardson number in the inlet show that regions susceptible to enhanced mixing occur during periods of maximum tidal velocities due to increased shear

arcuate thrust systems in sandbox experiments a comparison to the external arcs of the western alps in the external western alps two regional structural arcs were generated during tertiary northwest directed collision between the apulian indenter and the european passive margin these arcs distinguished by their geographic position and their age are examined by using a new compilation of structural data and comparing these to the results of sandbox analogue experiments the principal western alpine arc comprises two orthogonal synchronous thrust systems in the late eocene early miocene major shortening 105 km was toward the northwest to west northwest and minor shortening 11 km was toward the southwest shortening in each branch decreased toward the core of the arc during the late miocene and pliocene the jura arc accommodated 35 km of northwest directed shortening while 10 5 km of southwest to south southwest directed shortening was accommodated on the digne thrust system sandbox experiments were used to investigate the role of the motion vector of a rigid rectangular indenter orthogonal diagonal curved or rotational paths and the mechanical stratigraphy of the foreland in the evolution of upper crustal arcuate systems e g presence of a basal easy slip silicone horizon comparison of experimental results with the external alpine arc suggests that the indenter followed a slightly diagonal path with respect to the european margin from the eocene to the early miocene and curved counterclockwise by 10 15 in the middle miocene mechanical stratigraphy experiments support the hypothesis that thick triassic evaporites played a primary role in the evolution of the jura arc the influence of mechanical stratigraphy was most prominent during weak deformation at the external boundaries of the alpine orogen jura fold and thrust belt digne thrust

arcuate accretionary wedge formation at convex plate margin corners results of sandbox analogue experiments abstract indentation of a rigid box into a sand layer leads to accretion of sand into a wedge which has an arcuate shape in map view curvature of the wedge is most pronounced at the corners of the indentor where deformation has been investigated in detail in this study several features of the arcuate wedge in the corner region depend on the indentation angle i e the angle between the indentation direction and the normal to the frontal face of the indentor a the ratio between the widths of the lateral and frontal parts of the wedge increases linearly with increasing indentation angle b orogen parallel extension ope in the corner area is initially high and leads thus to large finite ope in the firstly formed nappes but decreases and attains a stable level during progressive convergence finite ope in the corner region is smaller at larger indentation angles c displacement vectors of accreted material exhibit fanning around the arc but in oblique indentation the spread of their orientation is smaller than the change of structural strike nappe traces and fold axes displacement vectors trend mainly in such positions normal to structural strike where displacement is parallel to the indentation direction the results from these experiments can be applied to natural arcuate fold thrust belts if natural conditions do not differ much from experimental boundary conditions such an application may give information about the former plate movement direction across an arcuate plate margin in the example of the eastern carpathian arc a tertiary plate movement direction of approx 125 130 was determined this direction is in accordance with current models of large scale tectonics of the carpathian region which indicates the applicability of the experimental results to natural examples

overthrust shear deformation of a foreland basin structural studies south east of the pelvoux massif se france abstract the late tertiary champsaur prapic fold zone lies to the southeast of the pelvoux basement massif in the footwall of the frontal pennine thrust of the western alps the 0 8 1 3 km thick late eocene oligocene foreland basin succession lies directly on crystalline basement and was deformed by wsw directed shear below the overthrusting internal alpine units on the frontal pennine thrust this deformation decouples from the basement in a top to wsw sw basal shear zone above the turbidite sequence gres du champsaur is folded in chevron like folds a 12 km long continuous profile across the champsaur fold zone documents variations in chevron fold shape axial plane dips and zonal distribution of folding which indicate progressively higher shear strains towards the frontal pennine thrust in the east chevron fold axes trend generally nnwsse except close to the pelvoux massif where fold axes curve to n s possibly due to lateral drag in the western part of the investigated area sw directed thrusting on the champsaur thrust almost doubles the tertiary stratigraphy the champsaur thrust and chevron folding in its footwall deform the overlying internal thrust sheets the champsaur prapic fold zone is a particular example of overthrust shear deformation in the footwall of internal alpine nappes in which deformation is concentrated in diffuse ramp zones to the north late alpine deformation changes to wnw directed partly post dating the wsw sw directed deformation further south therefore in the core of the external western alpine are two sub perpendicular transport directions which barely overlap can be distinguished

a kinematic evolutionary model for the penninic sector of the central ligurian alps the nappe pile presently cropping out in the central sector of the ligurian alps is represented by some principal groups of tectonic units starting from the foreland the outer and lower weakly metamorphic up to 0 3 gpa brianconnais units support the high pressure up to 1 3 gpa ensemble of inner brianconnais nappes in turn overridden by the prepiedmont units sourced from the european continental margin prepiedmont units form two superposed groups the lower is composed only of a pre namurian basement alpine metamorphism up to 0 6 gpa and the upper is mainly composed of a slightly metamorphic greenschist facies post namurian cover at the top lie the high pressure metamorphosed up to 0 8 gpa in the sector here considered ophiolitic units the group of the non metamorphic helminthoid flysch nappes original stratigraphic cover of the ophiolitic units has travelled the greatest distance and is presently mainly set onto the outer part of the chain only events up to the stacking of the nappe pile are discussed disregarding late stage deformation as the examined sector is located at a considerable distance from the collisional zone late processes did not change the overall order of superposition formerly acquired the model proposes the development of two major subhorizontal detachment surfaces the first shallower one confines at the base a very thin skinned set of nappes nearly totally made up of prepiedmont sedimentary covers that are bounded at their top by the helminthoid flysch units both these groups underwent a mainly horizontal outwards transport in contrast the underlying prepiedmont crust and the adjoining brianconnais inner sector separated by the second deeper major detachment surface were progressively dragged into the subduction zone under the ophiolitic units and duplexes were generated exhumation of the metamorphic units occurred along the subduction channel as did stacking of the nappe pile

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

the restoration of thrust systems and displacement continuity around the mont blanc massif nw external alpine thrust belt abstract foreland propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked three dimensional fault geometry this approach has been applied to part of the northwest external french swiss alps around the mont blanc basement massif imbricates of basement and cover sequences on the sw margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km these displacements can be correlated with those in the neighbouring helvetic nappes by transferring movements via lateral branch lines onto the mont blanc thrust by reappraising thrust geometries the helvetic ultrahelvetic nappe complex has been restored to a width of 114 km to the ese of the aiguilles rouges basement massif displacements on the internal se margin of the mont blanc massif estimated by balanced sections and a restoration of the ultrahelvetic klippen in the sub alps exceed 59 km thrust continuity incorporating the restorations of nappes and imbricate geometries around the mont blanc massif is illustrated on a crude restored branch line map which also serves as a preliminary palaeogeographic reconstruction external thrust systems to the east of the external belledonne aiguilles rouges massif restore to a width of at least 140 km in the footwall to the frontal pennine thrust

new constraints for indentation mechanisms in arcuate belts from the jura mountains france the mechanism of indentation in arcuate belts is discussed using the jura mountains as an example using fault inversion analysis from numerous sites the miocene pliocene stress pattern is reconstructed two superimposed compressional stress fields have been recognized considering the theoretical distribution of compression in front of a rigid indenter we interpret the complex stress pattern in the jura mountains as a result of a major change in the mechanism of indentation of the jura by its hinterland initially the indenter was narrower and limited in the southwest by the vuache transfer fault with time it expanded to the southwest and included the entire molasse basin this evolution highlights the importance of decoupling along faults which here controlled the indenter shape

3d cartographic modeling of the alpine arc we built a 3d cartography of the alpine arc a highly non cylindrical mountain belt using the 3d geomodeller of the brgm french geological survey the model allows to handle the large scale 3d structure of seventeen major crustal units of the belt from the lower crust to the sedimentary cover nappes and two main discontinuities the insubric line and the crustal penninic front it provides a unique document to better understand their structural relationships and to produce new sections the study area comprises the western alpine arc from the jura to the northwest up to the bergell granite intrusion and the lepontine dome to the east and is limited to the south by the ligurian basin the model is limited vertically 10 km above sea level at the top and the moho interface at the bottom we discarded the structural relationships between the alps sensus stricto and the surrounding geodynamic systems such as the rhine graben or the connection with the apennines the 3d model is based on the global integration of various data such as the dem of the alps the moho isobaths the simplified geological and tectonic maps of the belt the crustal cross sections ecors crop and nfp 20 and complementary cross sections specifically built to precise local complexities the database has first been integrated in a gis project to prepare their implementation in the geomodeller by homogenizing the different spatial referencing systems the global model is finally interpolated from all these data using the potential field method the final document is a new tri dimensional cartography that would be used as input for further alpine studies

quantifying vertical axis rotation in curved orogens correlating multiple data sets with a refined weighted least squares strike test 1 map scale curvature is a fundamental feature of most contractional orogenic belts and is central to understanding the kinematic and dynamic evolution of mountain systems paleomagnetic analysis combined with detailed structural studies is the most robust means of quantifying vertical axis rotations that produce curvature over a range of temporal and spatial scales this paper explores how vertical axis rotations can best be evaluated for multiple data sets by applying a weighted least squares method to the classic strike test this refined method provides measures of best fit slope confidence interval and goodness of fit between map scale structural trend paleomagnetic rotations and deformation fabric orientations structural trend is estimated by averaging fold axial trace formation contact and bed strike data over kilometer scale areas paleomagnetic and deformation fabric site mean orientations and measurement uncertainties are estimated using vector and bootstrap statistics weighting factors are estimated from combined measurement uncertainty structural noise related to small scale block rotation and stress strain refraction and variations in restoration paths the number of sites needed to obtain a significant confidence interval in strike test slope is a function of combined uncertainty in paleomagnetic or deformation fabric directions and the total range in structural trend around a curved orogen improved estimates of strike test slope and rotation thus require systematic sampling with a wide distribution of sites evaluation of appropriate weighting factors and statistical analysis a case study is presented that highlights application of this refined method to paleomagnetic and deformation fabric data sets from the wyoming salient of the sevier thrust belt paleomagnetic data yield a strike test slope of 0 76 0 11 indicating that the wyoming salient is a progressive arc with 3 4 secondary curvature related to vertical axis rotation synchronous with large scale thrusting and 1 4 initial curvature finite strain anisotropy of magnetic susceptibility and mesoscopic structural orientations which are related to early layer parallel shortening all yield strike test slopes of 0 9 0 1 comparing these slopes with paleomagnetic results indicates that deformation fabrics had initial curvature and thus cannot be used alone to accurately estimate rotations by integrating systematic paleomagnetic and structural data using statistical analysis curvature models for the wyoming salient are closely constrained

examining fault architecture and strain distribution using geospatial and geomechanical modelling an example from the qaidam basin ne tibet the investigation of complex geological setting is still dominated by traditional geo data collection and analytical techniques e g stratigraphic logging dip data measurements structural ground mapping seismic interpretation balance section restoration forward modelling etc despite the advantages of improving our understanding in structural geometry and fault architecture the geospatial modelling applying computer aided three dimensional geometric design visualization and interpretation has rarely been applied to such complex geological setting this study used the lenghu fold and thrust belt in qaidam basin ne tibetan plateau to demonstrate that the application of geospatial and geomechanical modelling could improve our understanding and provide an effective technique for investigating the fault architecture and strain distribution the three dimensional configuration of the lenghu fold and thrust belt was initially derived from traditional analysis techniques such as regional stratigraphic logging cross section construction meso scale ground mapping and landsat image interpretation the high resolution field data and landsat image were integrated to construct the geospatial model which was subsequently used to quantitatively investigate the fault throw changes along the lenghu thrust fault zone and to understand its control on the lateral structural variation the geospatial model was then restored in three dimensions to reveal the kinematic evolution of the lenghu fold and thrust belt geomechanical modelling using a mass spring algorithm provided an effective three dimensional tool for structural strain analysis which was used to predict the strain distribution throughout the overall structure e g normal faults with throws ranging from meters to tens of meters in the hanging wall the strain distribution predicted by geomechanical modelling was then validated by the natural normal faults in the hanging wall the high accordance between the strain prediction and statistics of natural normal faults demonstrates good applicability of geospatial and geomechanical modelling in the complex geological setting of the lenghu fold and thrust belt the geospatial models and geomechanical models therefore can provide a robust technique for analyzing and interpreting multi source data within a three dimensional environment we anticipate that the application of three dimensional geospatial modelling and geomechanical modelling integrating both multi source geologic data and three dimensional analytical techniques can provide an effective workflow for investigating the fault architecture and strain distribution at different scales e g ranging from regional to meso scale

deformation of brittle ductile thrust wedges in experiments and nature 1 even though the rheology of thrust wedges is mostly frictional a basal ductile decollement is often involved by comparison with purely frictional wedges such brittle ductile wedges generally display anomalous structures such as backward vergence widely spaced thrust units and nonfrontward sequences of thrust development laboratory experiments are used here to study the deformation of brittle ductile thrust wedges results are compared with natural systems in the jura mountains and the northern pakistan salt range and potwar plateau two series of three models are used to illustrate the effects of varying the basal wedge angle and shortening rate v these two parameters directly control variations in relative strength between brittle and ductile layers bd coupling wedges with strong bd coupling low and high v give almost regular frontward sequences with closely spaced thrust units and as such are not significantly different from purely frictional wedges weak bd coupling high and low v gives dominantly backward thrusting sequences intermediate bd coupling produces frontward backward oscillating sequences the spacing of thrust units increases as coupling decreases back thrusts develop in parts of a wedge where bd coupling is weak regardless of the thrust sequence wedges with weak bd coupling need large amounts of bulk shortening more than 30 to attain a state of equilibrium at which stable sliding along the base occurs on this basis we argue that a state of equilibrium has not yet been attained in at least some parts of the jura mountains and eastern salt range and potwar plateau thrust systems

characterization of stress perturbations near major fault zones insights from 2 d distinct element numerical modelling and field studies jura mountains abstract we consider and discuss the presence of discontinuities in the crust as a major source of stress perturbations based on 2 d distinct element modelling we reconstruct the local stress field around a vertical discontinuity in various geological contexts the resulting stress distribution reveals that major directional stress changes occur near the tips of the discontinuity so that stress deviations can reach values as large as 50 we establish simple relationships controlling stress changes around a pre existing fault zone as a function of 1 the remote differential stress magnitude 1 3 2 the friction coefficient on the discontinuity and 3 the strike of the discontinuity relative to the far field stress as a geological example we present the morez fault zone in the internal jura paleostress reconstruction in forty two sites indicates that the trends of the mio pliocene compression are n110 on average near the fault whereas they are n130 in the surrounding areas a comparison between the results of the tectonic study and those of theoretical modelling suggests that the 20 counterclockwise deviation is directly related to the reactivation of this large weak zone we thus evaluate the role of mechanical decoupling along pre existing zones of weakness especially with consideration to the accommodation of the alpine deformation in the jura belt

kinematic model for the curvature of the northern subalpine chain france abstract analyses of cross sectional shortening mapped faults and calcite twin strain are used in conjunction with geometric kinematic modeling to interpret the mechanism of curvature formation in the northern subalpine chain of france there is a 45 change in strike around the chain that is convex towards the foreland northwest five models for fold thrust belt curvature are applied to the northern subalpine chain they can reproduce the geometry and the kinematic history of a curved fold thrust belt and can be used to predict the magnitude and orientations of strains and the three dimensional geometry of the curvature the models are pure bending radial thrusting curve parallel simple shear uniform displacement uniform shortening and transport parallel simple shear two principal criteria for distinguishing among the models geometrically are the horizontal tangential strain extension or contraction parallel to strike and cross sectional shortening the transport parallel simple shear model predicts no tangential extension in the central domain and greater radial shortening and significant tangential extension in the northeastern and southern domains this is the pattern observed in the northern subalpine chain therefore the curvature of the northern subalpine chain is attributed to a transport parallel simple shear mechanism

short paper kinematics of the alpine arc is there a continuous sweep in direction of alpine orogenic displacements around the alpine arc or does the overlap of a few distinct directions give this impression the thrust evolution of the sw alps which supports the latter view does not accord with the hypothesis of indenter induced body forces two displacement realms one with directions determined by intraplate configuration and another with directions related simply to the body forces maycoexist within the orogen but this is not yet established one of the longest standing disagreements in alpine geology concerns the nature of the radial pattern of orogenic displacements outwards from the alpine arc in one view there is a continuum of displacement directions sweeping from approximately southward in the maritime alps through west and northwest to approximately northward in austria in the other view there is no continuum but a small number of distinct directions of orogenic displacements each dominating a particular sector which relay one to another around the alpine arc recent papers mark a significant development in both sides of this argument which is given additional importance by the attempt of platt et al 1989 a to use displacement data to constrain plate motions this paper is intended as a contribution to the discussion and suggests one possible resolution of the argument sophisticated syntheses of alpine structure have for a long time accepted the possibility that both of the above views have some validity and goguel 1963 is a good starting point for discussion his displacements of crystalline basement of

multi stage thrusting at the penninic front in the western alps between mont blanc and pelvoux massifs the different segments of the tectonic boundary between external european and internal penninic units in the western alps the so called penninic front pf formed at different times and according to different kinematic scenarios during a first episode eocene the pf corresponds to a transpressive suture zone between penninic and european units north to nnw trending stretching lineations found along internal nappe contacts within the penninic units are related to this episode this subduction zone was sealed by the priabonian flysch of the aiguilles d arves a detrital trench formation that formed during the final stages of subduction during a second episode starting in mid oligocene times the pf imaged along the ecors crop profile acted as a wnw directed thrust this thrust the roselend thrust rt only partially coincides with the pf south of moutiers the rt propagates into the dauphinois units carrying the former eocene pf including the priabonian flysch passively in its hangingwall south of the pelvoux massif the rt finds its continuation along the brianconnais front an out of sequence thrust behind the embrunais ubaye nappes on a larger scale our findings indicate oblique sinistral collision within the future western alps during the eocene followed by westward indentation of the adriatic block

glaciotectonized quaternary sediments at dinas dinlle gwynedd north wales and their bearing on the style of deglaciation in the eastern irish sea abstract the sedimentology and structure of late quaternary sediments exposed in a coastal section at dinas dinlle north wales are described the deposits from two small hills the northern one being more prominent and containing northward dipping units of gravel and diamicton bounded by clay lined thrust faults the southern hill comprises complex folded sands and gravels overlying a basal till surface high resolution seismic surveys along the beach 50 m west of the cliffs show a roughly horizontal rockhead lying some 35 m below beach level and demonstrate that thrust faults penetrate the entire quaternary sequence to a detachment immediately above rockhead thrust units in the exposed cliff section show excellent correlation with the seismic sequence 50 m to the west indicating a strike of 080 260 balanced cross section techniques were applied to an initial simple layered sequence comprising a lower irish sea till overlain by a coarsening upwards sand to gravel unit followed by upper till mixed lithology including welsh and northern provenance and finally an upper gravel unit shortening of approximately 54 is estimated within the thrust sequence it is concluded that the two isolated hills at dinas dinlle are erosional remnants of a formerly more extensive push moraine extending westwards into caernarfon bay which has now been truncated by coastal retreat deglaciation of the area was interrupted by ice sheet readvance towards 170 across a proglacial surface

the curved translation path of the parpaillon nappe french alps the parpaillon nappe is one of the two helminthoid flysch nappes emplaced on the external dauphinois zone of the western alps a structural analysis of the nappe is presented two superposed deformations d1 and d2 are described that are mainly characterized by large scale recumbent folds whose axes are quasi orthogonal ne sw for d1 and nw se for d2 their vergence is northwestward for d1 and southwestward for d2 during the d2 deformation the nappe was separated into two units one of these being thrusted over the other an analysis of incremental strain using quartz and calcite fibre growth indicates that d2 follows d1 without discontinuity therefore the superposition of d1 and d2 structures is interpreted as a progressive deformation instead of two distinct phases of deformation the emplacement of the nappe is discussed under two aspects the relations between displacement and strain and the role of gravity it is concluded that the translation has been twofold first towards the nw and then towards the sw and that the displacement result essentially from gravity forces kinematic implications for the alpine collision are suggested

relationships among high river discharges upwelling events and bowhead whale balaena mysticetus occurrence in the central alaskan beaufort sea abstract aerial surveys for bowhead whales have been conducted with a relatively consistent methodology over the alaskan beaufort sea since 1989 sightings of bowhead whales in the central alaskan beaufort sea in september 1997 and september 2014 were unusual in that hundreds of whales were observed within a few kilometers of local barrier islands we compare bowhead whale sightings river discharge data wind records and satellite imagery to argue that the large numbers of bowhead whales observed nearshore in september 1997 and 2014 resulted from favorable feeding conditions established by high river discharges and prior upwelling events these results are then generalized in a simple binary based mechanistic framework that links the relevant physics to occurrences of observed and potential feeding opportunities for bowhead whales in the central alaskan beaufort sea and in other locales along the beaufort coast

folding by cataclastic flow at shallow crustal levels in the canyon range sevier orogenic belt west central utah abstract folds form by ductile deformation typically involving continuous flow in the elastico frictional regime such deformation may be accomplished by cataclastic flow involving collective movement on a population of fractures and zones the canyon range cr syncline part of the cr thrust sheet in west central utah developed in this regime the cr syncline is composed of thick bedded quartzite units with a small material contrast between layers limiting limb rotation by flexural slip alone thus fracture populations developed to accommodate fold tightening by limb rotation and thinning and the formation of transverse zones across the fold several generations of fracture and deformation zone dz networks are recognized from mesoscopic and microscopic evidence and can be related to stages of folding the net result of the large number of distributed fractures and deformation zones is a continuous deformation that is homogeneous at the scale of the outcrop all these lines of evidence suggest that large scale cataclastic flow accommodated folding by allowing rigid mesoscopic blocks to slide along bounding dzs along its length the cr syncline consists of several segments bounded by transverse zones with different mechanisms accommodating fold tightening in adjacent segments in one segment fold tightening progressed by limb rotation and then out of the core thrusting in contrast fold tightening in the adjoining segments occurred by rotation and thinning of one limb and possible hinge migration with the steeply dipping to overturned limb showing progressive thinning of units on a megascopic scale and progressive increase in the thickness and density of deformation zones at all scales

neogene kinematics of the central and western alps evidence from fission track dating zircon and apatite fission track analysis has established the continuation of a zone of rapid change in mineral ages extending from the simplon fault zone in the simplon alps through the rhone valley and along the southeastern side of the mont blanc and belledonne massifs apatite fission track ages to the northwest of this zone generally range from 1 4 to 6 ma whereas those to the southeast range from 6 to 13 ma corresponding values for zircon are 11 13 and 16 26 ma respectively the zone corresponds to the position of the frontal pennine thrust in the mont blanc belledonne region this structure a prominent feature on recent deep seismic profiles may have been reactivated in the neogene with an important normal fault component this is suggested by apatite and zircon fission track ages in the footwall that are younger than those in the hanging wall and by the observed elimination of the lower pennine or simplon nappes along this fault zone in the rhone valley mont blanc belledonne region resulting in the juxtaposition of north penninic and helvetic ultra helvetic nappes at the current erosion level the distribution of fission track ages reflects this movement and the related neogene exhumation of the mont blanc and belledonne external massifs which has continued until less than 1 4 m y ago i e it is almost certainly still active and has tilted the overlying nappe sequence

mechanics of the salt range potwar plateau pakistan a fold and thrust belt underlain by evaporites the salt range and potwar plateau are part of the active foreland fold and thrust belt of the himalaya in northern pakistan in this region the distance from the main boundary thrust mbt to the front of the fold and thrust belt is very wide 100 150 km because a thick evaporite sequence forms the zone of decollement recent studies have combined seismic reflection profiles petroleum exploration wells bouguer gravity anomalies and surface geology to construct cross sections in the eastern central and western salt range potwar plateau areas in this study the sections are compared with a model that considers the mechanics of a fold and thrust belt to be analogous to that of a wedge of snow or soil pushed in front of a bulldozer chapple 1978 davis et al 1983 dahlen et al 1984 dahlen 1984 models which include the effects of evaporites at the base chapple 1978 davis and engelder 1985 suggest that these thrust belts will have 1 narrow 1 cross sectional tapers 2 larger widths than areas not underlain by evaporites 3 symmetrical structures and 4 changes in deformational style at the edge of the evaporite basin the section across the eastern potwar plateau most closely resembles this latter model having 1 a taper of 0 8 0 1 2 a width of 100 150 km 3 thrust faults that verge both to the north and south and 4 structures rotated 30 counterclockwise with respect to the salt range from the observed taper and pore fluid pressures of the eastern potwar plateau estimates of the values for the yield strength of the evaporites o and the coefficient of internal friction of the overlying wedge are calculated as o 1 33 1 50 mpa and 0 95 1 04 which are then applied to the other cross sections in the central and western sections a basement uplift the sargodha high interferes with the front of the fold and thrust belt this feature causes the ramping of the salt range thrust and produces a relatively steep basement slope 2 4 beneath the potwar plateau this dip in the presence of the weak evaporite decollement is sufficient to provide critical taper no topographic slope is necessary and the thrust wedge of the southern potwar plateau is pushed over the decollement without significant internal deformation the northern potwar plateau is strongly folded and faulted yet the topographic slope remains flat although the deformation suggests that evaporites are not present there the observed taper in the northern potwar plateau is best fitted by the model with evaporites at the decollement combining this with published paleomagnetic and geologic constraints a model for the evolution of the northern potwar plateau suggests that the area deformed as a steeply tapered 3 5 5 5 thrust wedge until approximately 2 million years ago when the southward propagating decollement encountered the evaporites between 2 ma and the present the northern potwar plateau has been pushed along the salt decollement without deformation and erosion has reduced its original steep topographic slope to a nearly level surface

the arc of the western alps in the light of geophysical data on deep crustal structure recently two international deep seismic campaigns in the western and central alps etude continentale et oceanique par reflexion et refraction sismique progetto strategico crosta profonda ecors crop roure et al 1996b and national research program 20 nrp 20 pfiffner et al 1997 have been completed here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the alps collected during the past 40 years including the two above mentioned geophysical projects the tectonic reinterpretation of the ecors crop high resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology combined with previously published interpretations regarding the central alps schmid et al 1996 this study reveals substantial differences in geometry and kinematics between transects across the western and central alps respectively at depth the transition between the central and western alps is marked by the western limits of an adriatic lower crustal wedge shaped structure and the northeastern limits of a similar structure made of european lower crustal material at the surface it coincides with a corridor of dextral strike slip along the tonale and simplon lines in addition the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the penninic realm of the western alps to less than 20 km in the central alps the formation of the western alpine arc was initiated during convergence and collision before 35 myr ago when the adriatic micro plate moving northward with respect to the european foreland caused sinistral transpression in the western alps while the central and eastern alps underwent head on convergence and collision during the post collisional stage i e after 35 myr ago the arcuate shape of the western alps was accentuated by wnw directed movement and anticlockwise rotation of the adriatic microplate decoupled from the central and eastern alps along the tonale simplon dextral shear zone this led to wedging of lower crustal slices both in the western alps and in the central alps the new tectonic interpretation of the ecors crop transect allows a kinematic model to be established for crustal shortening in the western alps during the past 35 myr the ivrea mantle plays the role of a backstop in our tectonic model we distinguish three episodes of post collisional crustal shortening along the ecors crop transect from 35 to 30 ma the brianconnais basement was back thrusted over the gran paradiso units leading to 30 km of crustal shortening first episode in the early miocene movements concentrated mainly along the penninic frontal thrust and resulted in about 60 km shortening second episode post 12 ma shortening within the external massifs is associated with folding in the jura mountains when the crust was shortened by an additional 30 km third episode

fault related folding in sandbox analogue models of thrust wedges abstract the nucleation and growth of thrust fault related folds has been simulated using high resolution sandbox coulomb wedge type analogue models thrust related anticlines grew progressively from initial layerparallel shortening to decollement folding thrust tip folding and eventually thrust ramp folding thrust ramps nucleated in the central sector of the overturned forelimb of decollement folds as a consequence of limb thinning thrust tip folding occurred during outward migration of the ramp tips the fault propagation rate was greater at the lower ramp tip which quickly joined the basal thrust flat while folding continued beyond the upper tip the development of the analogue model thrust fault related folds by progressive changes in kinematics from decollement folding to thrust tip folding and finally thrust ramp folding compares well with analyses of naturally occurring thrust fault related folds

extensional fault related folding northwestern red sea egypt the duwi and hamadat areas on the northwestern margin of the red sea rift system exhibit superb outcrop examples of kilometric scale extensional fault related folds in this area the nakheil and hamadat faults of the rift border fault system consist of a series of wnw and nw trending segments that link through breached relay ramps the fault hanging walls are characterised by a series of offset synclines with non linear axial traces sub parallel to the faults the synclines are doubly plunging and in the immediate hanging wall to the normal faults the strata dip sub parallel to the fault these folds result from along strike displacement variations on the individual fault segments together with extension related fault propagation folding as the faults propagated upwards through a highly anisotropic pre rift sedimentary section in the duwi area the axial traces of the synclines are offset and bend in the regions of relay ramps in the hamadat area the synclines associated with individual fault segments are separated by transverse hanging wall anticlines that formed where segment linkage occurred numerical simulations of extension related fault propagation folding using a trishear model produce geometries that closely match those of the duwi and hamadat folds the dimensions of these hanging wall synclines and their relationships to fault displacement variations indicate that they are formed by extensional fault propagation folding rather than by frictional drag along the fault surface

effects of layering and anisotropy on fault geometry the geometry and orientation of faults are examined using several field examples of small scale 1 with dihedral angles usually of about 50 as predicted by the coulomb theory of failure in layered rocks the geometry of faults varies with the orientation of layering with respect to the stress field where 1 is approximately normal to layering or anisotropy conjugate faults also develop symmetrically about 1 where rocks have interbedded layers with different mechanical behaviours however faults tend to initiate orthogonal to the more brittle layers i e originate as extension fractures sub parallel to 1 but oblique to the less brittle layers as the fault steps through the layering pull aparts are developed which may reduce the dihedral angle where 1 is oblique c 25 75 to anisotropy one set of faults is developed at a high angle to layering with another at a low angle usually showing a ramp flat geometry large dihedral angles up to 90 may result and 1 does not bisect this angle where 1 is approximately parallel to layering two cases can be recognized where 3 is approximately normal to layering faults with layer parallel flats and contractional ramps develop where 2 is approximately normal to layering conjugate faults develop which are symmetrical about 1 the geometry resembling that in isotropic rocks these observations are in agreement with rock deformation experiments which show the strong effect of anisotropy on fault orientation but the observations incorporate the effects of layering of materials with different deformation characteristics

fault displacement gradient folds and the structure at lost hills california u s a abstract geometric models of fault propagation folds require that the underlying fault tip propagate upward across layering however related folds can be generated if the tip remains stationary or even moves back along the ramp as a pre existing fault is reactivated fault propagation folds detachment folds as well as those produced when there is no propagation are generated by a displacement gradient along the underlying fault and as a general class can be called fault displacement gradient folds new geometric equations are derived which relate fold shape to displacement gradient and fault propagation folds are a special case of these more general equations growth sequences associated with these structures have been used to distinguish fault propagation folds from fault bend folds when the underlying structure is obscured a displacement gradient fold in which the fault tip retreats back down the ramp generates a growth sequence that in some cases could be confused with that of a fault bend fold the structure at lost hills in the san joaquin valley of california u s a has been interpreted as a fault bend fold but it may be a displacement gradient fold which formed as a pre existing fault was reactivated during a pliocene change in plate motion

mechanical and fracture stratigraphy using examples from core studies this article shows that separate identification of mechanical stratigraphy and fracture stratigraphy leads to a clearer understanding of fracture patterns and more accurate prediction of fracture attributes away from the wellbore mechanical stratigraphy subdivides stratified rock into discrete mechanical units defined by properties such as tensile strength elastic stiffness brittleness and fracture mechanics properties fracture stratigraphy subdivides rock into fracture units according to extent intensity or some other observed fracture attribute mechanical stratigraphy is the by product of depositional composition and structure and chemical and mechanical changes superimposed on rock composition texture and interfaces after deposition fracture stratigraphy reflects a specific loading history and mechanical stratigraphy during failure because mechanical property changes reflect diagenesis and fractures evolve with loading history mechanical stratigraphy and fracture stratigraphy need not coincide in subsurface studies current mechanical stratigraphy is generally measurable but because of inherent limitations of sampling fracture stratigraphy is commonly incompletely known to accurately predict fractures in diagenetically and structurally complex settings we need to use evidence of loading and mechanical property history as well as current mechanical states

first paleomagnetic data from the sedimentary cover of the french penninic alps evidence for tertiary counterclockwise rotations in the western alps abstract we present a paleomagnetic study performed in the brianconnais sedimentary cover of the western alpine arc france sampling was focused on upper jurassic rocks of the brianconnais cover in the briancon guillestre area more than 100 samples from 11 sites were collected essentially from ammonitico rosso limestones and sedimentary dykes of the upper jurassic thermal and alternating field demagnetizations revealed three components of magnetization carried dominantly by magnetite 1 a low temperature component with present day magnetic field direction of viscous origin 2 a well defined reverse polarity component with a maximum unblocking temperature of 440 c a component and 3 a weak high temperature component b component that experienced a viscous overprint during heating and was difficult to isolate the a component exhibits a negative fold test and shows a better clustering when corrected for tilting associated with late alpine extension this component is interpreted as a secondary remagnetization acquired during the cooling path related to late eocene early oligocene metamorphism the mean a component direction corrected for late alpine extensional tilting is d 142 i 57 k 44 95 8 this direction significantly differs by 47 13 from the expected direction for eurasia we interpret this difference to be related to a post eocene counterclockwise rotation about a vertical axis of the penninic zone relative to stable europe

thrust zone kinematics in a basement cover imbricate stack eastern pelvoux massif french alps abstract thrust imbrication is now widely accepted as a major process in controlling basement cover relationships in the external parts of the western alps complications in the simple geometries expected from thrusting alone arise from alpine deformation affecting pre existing extensional faults inherited from the old mesozoic basins this contribution describes the structural geology of part of the eastern pelvoux massif centred on the rocher de l yret where alpine deformation has masked any pre existing structures a series of major thrusts carry basement onto cover these lie west of and in the footwall to the pennine front a major zone of thrusting which carries the internal alps onto the external zones stretching lineations in calc mylonites derived from eocene limestones deposited on the basement indicate a tectonic transport axis of wnw ese plunging ca n100 e this direction is supported by the orientation of striae and corrugations on faults which cut basement rocks shear criteria in the calc mylonites imply top to the wnw shearing despite this simple kinematic picture basement cover relationships cannot be explained by thrusting alone a series of faults cut down through the thrust stack towards the wnw and show net extensional offsets with respect to the tectonic pile these faults and associated shears in the calc mylonites cannot be traced to depth rather they appear to branch onto earlier thrust surfaces this geometry is interpreted as due to distributed shearing in the hanging wall to the local floor thrust perhaps reflecting late reactivation of the pennine front

relay zones between mesoscopic thrust faults in layered sedimentary sequences abstract we examine the geometry formation and destruction of relay zones in well bedded sedimentary sequences containing thrust fault segments with maximum observed displacements of 0 64 8 m contractional extensional and neutral relay zones are distinguished based on the stepping sense of the interacting fault segments and the orientation of the associated fault slip vectors large displacement gradients 0 17 1 0 in relay zones are mainly accommodated by folding with aggregate displacements maintained across segment boundaries and a significant component of the fault slip converted to fold related shortening folds in contractional relay zones verge in the direction of hanging wall motion with the steeper limb defining a parallelogram shaped zone between thrusts fault segment lengths are established after minimal displacement and folds in relay zones grow between segments as displacement accrues on the thrusts with increasing displacement upper and lower thrust tip lines in contractional and extensional relay zones are fixed at stratigraphic boundaries while the lateral thrust tip lines may propagate in a strike parallel direction upper and lower tip lines may eventually propagate to bypass relay zones that formed early in the fault growth history contractional relay zones remain intact at higher displacements than extensional relay zones

present day and future tectonic underplating in the western swiss alps reconciliation of basement wrench faulting and decollement folding of the jura and molasse basin in the alpine foreland abstract the western alps form a geodynamically active mountain belt showing the typical features of an evolving orogenic wedge with its pro wedge geometry to the nnw and its retro wedge structures to the sse renewed tectonic underplating of european continental crust occurred after the orogenic wedge underwent major dynamic disequilibrium following the break off of the southward subducting slab of the european passive margin the most important of these basement imbricates are the mont blanc aiguilles rouges and gastern aar crystalline massifs also forming the alps highest mountains the upper plate present day orogenic wedge of the western alps includes the molasse basin and the jura fold and thrust belt both decoupled from the basement over a basal decollement surface the overall geometry of this wedge appears to be strongly unstable according to simple wedge models in its attempt to regain stability out of sequence thrusts form in the existing basement nappes but also new basement nappes should develop beneath the southern portion of the molasse basin new out of sequence thrusts in the cover trigger higher than average uplift rates concentrated around the newly forming structures and are accompanied by a concentration of earthquakes tectonic underplating is further corroborated by neotectonics and the tectonic structures observed in the prealpes molasse basin and jura similarly uplift rates and earthquakes along the southern edge of the jura mountains seem to witness the development of a new incipient basement nappe at depth partial inversion of former permo carboniferous grabens in the basement a possible spatial coincidence of areas with strong earthquake activity and zones with uplift rates above surrounding values suggest a common mechanism for their origin in the western swiss alpine foreland combined with information from basement geometry and wedge dynamics it is proposed that the common mechanism is the development of basement imbricates by tectonic underplating the proposed model for ongoing and possible future tectonic underplating beneath an active alpine orogenic wedge also allows to reconcile the models of basement wrench faulting in the molasse basin and jura with the distant push theory where the molasse basin and jura develop over a basal decollement horizon

localization patterns in sandbox scale numerical experiments above a normal fault in basement abstract the finite element program elfen is used to study the effect of basement fault dip on the evolution of shear band patterns in unconsolidated sand the material properties and boundary conditions of the model were chosen to correspond to generic sandbox experiments model results reproduce the range of structural styles found in corresponding sandbox experiments with a basement fault dip of 60 and lower a graben structure is formed composed of a synthetic shear band followed by one or more antithetic shear bands with a basement fault dip of 70 and steeper a reverse precursor shear band forms first followed by a synthetic normal shear band that accommodates all further displacement the dip of the synthetic shear band is close to the basement fault dip for basement fault dips between 60 and 70 we observe a transition in localization patterns an analysis of the stress fields and velocity vectors in the model explains the first order aspects of the relationships observed we consider the observed precursor dominated and graben dominated structural domains to be important components of normal fault systems in which the first order structural style and deformation patterns are only weakly dependent on the details of the rheology of the model materials and explore the interesting problem of the change in structural style from precursor dominated to graben dominated structural domains above a normal fault in basement we find similar structural domains in sandbox experiments for the same set of boundary conditions but with slightly different material properties suggesting that the modeled patterns are robust within these two structural domains i e will occur over a range of similar material properties and boundary conditions the results of this study contribute to our ability to validate numerical models against experiments in order to finally better simulate natural systems

kinematics of the alpine arc and the motion history of adria the alps have been interpreted traditionally in terms of north or northwest directed convergence between africa and europe1 with adria the continental underpinnings of the adriatic sea and northern italy acting as a promontory of the african continent2 this has been supported by recent analyses of africa europe relative plate motions3 5 which indicate dominantly north south directed convergence during the tertiary phase of intracontinental shortening we show here however that kinematic indicators from alpine tectonites are difficult to reconcile with this motion history figs 1 and 2 and are explained best by roughly west northwest directed adria europe motion since about 70 million years ago combined with an arc normal pattern of motion driven by local body forces this implies that adria has moved independently of both africa and europe since at least the late cretaceous

controls on the geometry of fold thrust belt salients salients convex to the foreland curves in the map traces of orogens typically involve fold thrust belts formed along the foreland margin of orogens an examination of the map view shape degree of asymmetry dimensions and structural trend line patterns i e the variation in fold trend and fault strike along the length of the belt of 20 fold thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed in order to understand these relationships we used simple sandbox models to simulate the formation of three of the several types of salients specifically we examined indenter controlled salients the geometry of which reflects interaction of a fold thrust belt with a hinterland indenter basin controlled salients the geometry of which reflects lateral variations in the thickness of predeformational basin fill and detachment controlled salients the geometry of which reflects lateral variations in the strength of a detachment surface our models demonstrate that thrusts in these three types of salients initiate with a curved shape and that rocks within thrust sheets on the limbs of salients can undergo a vertical axis rotation in response to simple shear without requiring major rotation of the fault trace furthermore trend line patterns distinguish basin controlled from indenter controlled salients specifically structural trend lines in basin controlled salients converge at the end points of the curve while in indenter controlled salients they diverge at the end points of the curve the asymmetry of a salient correlates with the asymmetry of the predeformational basin and or of the indenter and with the convergence direction thus our results explain geometric characteristics of individual salients and address controversies concerning paleomagnetic data in salients they also may help prioritize petroleum exploration targets in salients foreland basin oil fields concentrate at the apex of basin controlled salients because the geologic factors that generate the salient also contribute to localizing oil fields

kinematic modelling of extensional fault propagation folding abstract many studies have shown that in extensional basins discrete faulting at depth is commonly linked to more distributed deformation in particular folding at higher levels such extensional fault propagation folds are particularly common where there is a distinct mechanical contrast between faulted basement and sedimentary cover outcrop 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 strain the folds are cut by faults as they propagate upwards into the cover to date however there has been little investigation into the kinematics of linked basement faulting and extensional fault propagation folding here we present a two dimensional kinematic model of linked basement faulting and fault propagation folding which is based upon trishear the model allows investigation of the influence of shear zone geometry and the rate of fault propagation upon the style of folds and the strains associated with them the evolution of linked basement faulting and folding predicted by the model is compared in detail to that observed in an analogue model the kinematic model reproduces well many of the features seen both in the analogue model and reported from outcrop and seismic studies

formation of segmented normal faults a 3 d perspective the interpretation of fault kinematics from geometric data is an essential step in developing an understanding of the growth of fault systems constraints on fault geometry are however often restricted to 2 d maps or cross sections in this article we consider the extent to which kinematic interpretations of faulting benefit from a 3 d rather than 2 d geometrical perspective concentrating on relay zones and segmented normal fault arrays we suggest that very different interpretations of their evolution arise from the recognition that the propagation directions of faults and fault segments will rarely be contained within the inspection plane of 2 d data a 3 d perspective favours an interpretation in which the segments of a fault array are kinematically interrelated from their initiation individual segments in such systems may link into a single fault surface out of the plane of inspection or may be unconnected in 3 d we argue that this interpretation which conflicts with the often suggested model of incidental overlap of originally isolated faults should be the preferred model for the generation and growth of segmented normal fault arrays

the spatial distribution of larvalfishes about the mississippi river plume the distribution of larval fishes is shaped by the mississippi river plume at both coarse kilometers and fine tens to hundreds of meters spatial scales density estimates of larval fishes no m 3 based upon ichthyoplankton samples collected about the mississippi river plume in february 1982 december 1982 and november 1983 were often greater by a factor of 10 and sometimes by several orders of magnitude at the plume front than they were inside within or outside of the plume greater densities at the plume front were most apparent for larval brevoortia patronus a clupeid and unidentified clupeid larvae frontal convergence apparently affects the accumulation of larval fishes at the frontal interface but high densities were not evident everywhere along the front an index of potential convergent velocity indicates maximal velocities of the order of 0 lo0 30 m s l but this index does not explain variation in the accumulation of larval fishes frontal convergence may be localized and affected by secondary circulations