fault slip bed length and area variations in experimental rollover anticlines over listric normal faults influence in extension and depth to detachment estimations abstract to check the behaviour of physical experiments of simple rollover anticlines over listric normal faults several fault slip bed length and area parameters are measured for different beds in two stages of evolution of two physical experiments given a certain amount of extension the heave and dip of the displacement vary for different beds whereas the throw is approximately constant for small amounts of extension and the displacement remains constant for high amounts of extension neither bed lengths nor areas beneath beds remain constant with increasing extension a number of techniques allow amounts of extension and detachment depth to be estimated given one or more marker horizons the portion of the fault between hanging wall and footwall cut off points the depth to detachment and or the shear angle these techniques are applied to the physical experiments of listric normal faults analysed since many of these techniques rely on parameters measured on the physical experiments such as fault slip bed length and area the influence of these parameters on the different magnitudes of extension and detachment depths estimated using different techniques is discussed and the accuracy of the results is compared

laboratory testing of an automatic method for determining normal fault geometry at depth abstract a general kinematic method for determining two dimensional normal fault geometries from a large number of hanging wall horizons has been tested on synthetic data and laboratory models the method assumes that the hanging wall deforms by a combination of arbitrarily inclined bulk simple shear and differential compaction it should be applicable to normal faults which do not change shape as deformation proceeds fault geometry the inclination of simple shear and compaction parameters may all be uniquely determined provided that the geometry of the hanging wall stratigraphy is accurately known prior to and after deformation testing the method on synthetic data has shown that unique solutions can be found solutions obtained using large numbers of hanging wall horizons of laboratory modelled faults are encouraging in each case the method was able to predict a fault geometry close to the fault geometry used in the laboratory model hence bulk simple shear appears to be a good approximation of hanging wall deformation the inclination of simple shear appears to be controlled by the underlying fault geometry rather than the amount of extension or the composition of the hanging wall fill in future it is hoped that a general method similar to this one can be applied routinely to depth converted seismic reflection data

evaluating sand and clay models do rheological differences matter abstract dry sand and wet clay are the most frequently used materials for physical modeling of brittle deformation we present a series of experiments that shows when the two materials can be used interchangeably document the differences in deformation patterns and discuss how best to evaluate and apply results of physical models extension and shortening produce similar large scale deformation patterns in dry sand and wet clay models indicating that the two materials can be used interchangeably for analysis of gross deformation geometries there are subtle deformation features that are significantly different 1 fault propagation and fault linkage 2 fault width spacing and displacement 3 extent of deformation zone and 4 amount of folding vs faulting these differences are primarily due to the lower cohesion of sand and its larger grain size if these features are of interest the best practice would be to repeat the experiments with more than one material to ensure that rheological differences are not biasing results dry sand and wet clay produce very different results in inversion models almost all faults are reactivated in wet clay and few if any are significantly reactivated in sand models fault reactivation is attributed to high fluid pressure along the fault zone in the wet clay a situation that may be analogous to many rocks sand inversion models may be best applied to areas where most faults experience little to no reactivation while clay models best fit areas where most pre existing normal faults are reactivated

tectonic and gravity extensional collapses in overpressured cohesive and frictional wedges two modes of extensional collapse in a cohesive and frictional wedge of arbitrary topography finite extent and resting on an inclined weak decollement are examined by analytical means the first mode consists of the gravitational collapse by the action of a half graben rooting on the decollement and pushing seaward the frontal part of the wedge the second mode results from the tectonics extension at the back wall with a similar half graben kinematics and the landward sliding of the rear part of the wedge the predictions of the maximum strength theorem equivalent to the kinematic approach of limit analysis and based on these two collapse mechanisms not only match exactly the solutions of the critical coulomb wedge theory once properly amended but generalizes them in several aspects wedge of finite size composed of cohesive material and of arbitrary topography this generalization is advantageous to progress in our understanding of many laboratory experiments and field cases for example it is claimed from analytical results validated by experiments that the stability transition for a cohesive triangular wedge occurs with the activation of the maximum length of the decollement it is shown that the details of the topography for the particular example of the mejillones peninsula north chile is however responsible for the selection of a short length scale dynamic instability corresponding to a frontal gravitational instability a reasonable amount of cohesion is sufficient for the pressures proposed in the literature to correspond to a stability transition and not with a dynamically unstable state

controls of listric normal fault styles in the northern gulf of mexico insights from experimental models abstract passive margins such as the gulf of mexico are characterized by two distinct styles of faulting homogenous sand shale packages in offshore texas mostly display basinward dipping listric normal faults with associated rollover structures cut by synthetic and antithetic faults the fault traces are generally long and show a linear trend stratigraphic packages with a ductile substratum salt in offshore louisiana are characterized by basinward and landward dipping short arcuate faults detaching within the salt the structures consist of a series of half grabens with the movement of salt from the front to the back of each fault block clay experimental models are used to study the controls of fault geometries in the two structural styles and their interaction to form complex transfer zones the surface of the clay cake is laser scanned to enable 3d visualization and accurate measurements of structures the results suggest that within homogeneous sand shale packages the dips of the faults and their locations are primarily dependent on the direction of the drop down of the basal detachment along pre existing discontinuities with the slope of the basal discontinuity and the direction of extension providing secondary controls on the other hand the dips of fault systems in packages underlain by a ductile substratum are primarily controlled by the slope of the basal detachment therefore the more common regional roho systems typically form above salt sheets with initial basinward slopes whereas counter regional fault systems form above salt sheets with initial landward slopes the direction of extension and the presence of small pre existing discontinuities impart only secondary controls when ductile basal units are involved the faults initiate at the head of ductile layer and propagate downslope complex transfer zones develop at the boundary of the ductile substratum due to interference between the two fault styles

origin of transverse folds in an extensional growth fault setting evidence from an extensive seismic volume in the western gulf of mexico abstract growth faults in gravity driven extensional provinces are dominated by coast parallel trends but coast perpendicular transverse trends are far less documented the clemente tomas fault in the inner texas shelf has corrugations that are transverse to the fault and that plunge downdip a large 8500 km2 high quality 3d seismic survey allows a uniquely encompassing perspective into hanging wall deformation above this corrugated fault surface synextensional strata in the hanging wall are folded into alternating transverse ridges and synclines typically spaced 10 km apart forward modelling in dip profiles of an extensional fault having three ramps produces ramp basin rollover pairs that compare with the seismically revealed ridges and synclines as they translated down the undulose fault plane ramp basins and rollovers were juxtaposed along strike forming the hanging wall ridges and synclines observed offshore texas fault surface corrugations correlate broadly with footwall structure we infer that corrugations on the clemente tomas fault formed by evacuation of an allochthonous salt canopy emplaced in the late eocene to early oligocene early salt evacuation oligocene created an undulose topography that influenced incipient clemente tomas fault segments as they merged to form an inherently undulose fault late salt evacuation early miocene further deformed this fault surface

growth trishear model and its application to the gilbertown graben system southwest alabama abstract fault propagation folding associated with an upward propagating fault in the gilbertown graben system is revealed by well based 3 d subsurface mapping and dipmeter analysis the fold is developed in the selma chalk which is an oil reservoir along the southern margin of the graben area depth strain analysis suggests that the cretaceous strata were growth units the jurassic strata were pregrowth units and the graben system is detached in the louann salt the growth trishear model has been applied in this paper to study the evolution and kinematics of extensional fault propagation folding models indicate that the propagation to slip p s ratio of the underlying fault plays an important role in governing the geometry of the resulting extensional fault propagation fold with a greater p s ratio the fold is more localized in the vicinity of the propagating fault the extensional fault propagation fold in the gilbertown graben is modeled by both a compactional and a non compactional growth trishear model both models predict a similar geometry of the extensional fault propagation fold the trishear model with compaction best predicts the fold geometry

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

mechanics of extension and inversion in the hanging walls of listric normal faults fault shape material properties and bedding anisotropy determine the style of deformation in the hanging walls of listric normal faults we use numerical models to study this deformation in both extension and inversion during displacement on a variety of master fault shapes elastic plastic material properties in the models allow the development of shear bands which simulate secondary faults within the hanging wall if the master fault is composed of a planar ramp and planar flat separated by a sharp fault bend a series of antithetic normal shear bands develops in the hanging wall propagating up from the fault bend each shear band is progressively abandoned as displacement on the master fault moves it away from the fault bend displacements on the antithetic faults produce the limb of a hanging wall monocline and bound one side of a graben the other side of which is bounded by the master fault the antithetic shear bands are not rotated and layering within the graben remains subhorizontal on the other hand if the fault bend is curved rather than sharp symmetrical nested graben develop in the upper part of the hanging wall above the base of the ramp displacement on the master fault moves these shear bands away from the fault bend after which they are abandoned in favor of new shear bands early formed synthetic shear bands become shallower and concave upward because of the folding and rotation of the hanging wall with increasing radius of curvature of the master fault localization into shear bands decreases and with a large radius of curvature shear bands do not develop if weak bedding layers are included within the hanging wall they become sites of bedding parallel shear bands that accommodate flexural slip folding and replace the synthetic shear bands that develop in homogeneous models if extension is followed by shortening and inversion normal shear bands that developed during extension are reactivated as reverse shear bands but are also crosscut by new reverse shear bands the models produce results that are similar to both natural structures and analogue models and provide explanations for many observations of deformation in seismic profiles through extensional terranes

thermochronology of lower cretaceous source rocks in the idaho wyoming thrust belt lower cretaceous organic rich source rocks that are thermally mature to postmature crop out on the absaroka darby and prospect plates in linear belts that run parallel to the trace of the thrusts in the idaho wyoming portion of the idaho wyoming utah thrust belt although the common assumption is that burial by thrust plates and the synorogenic sediments derived from them have been responsible for thermal maturation of the organic rich strata commercial amounts of hydrocarbons have not been found in structural traps in this portion of the thrust belt in a companion paper burtner and nigrini demonstrated that gravity driven fluid flow in the idaho wyoming portion of the thrust belt was responsible for moving large amounts of heat from the depths of the early cretaceous foreland basin eastward toward the stable platform in this paper we demonstrate through the application of organic maturation indicators and a new refinement of the apatite fission track technique that this process heated lower cretaceous organic rich source rocks to temperatures sufficient to generate hydrocarbons hydrocarbon generation and migration occurred prior to the development of the thrusts that are often assumed to have played a major role in the generation and entrapment of hydrocarbons in this portion of the thrust belt

normal faults and their hanging wall deformation an experimental study we have used clay models to study the effects of fault shape and displacement distribution on deformation patterns in the hanging wall of a master normal fault the experimental results show that fault shape influences the style of secondary faulting and folding mostly antithetic normal faults form above concave upward fault bends whereas mostly synthetic normal faults form above low angle fault segments and convex upward fault bends beds dip toward the master normal fault above concave upward fault bends and away from the master normal fault above low angle fault segments and convex upward fault bends generally secondary faulting and folding are youngest at fault bends and become progressively older past fault bends hanging wall deformation patterns differ significantly when a basal plastic sheet imposes a constant magnitude displacement distribution on the master normal fault in models without a plastic sheet numerous secondary normal faults form in the hanging wall of the master normal fault most secondary normal faults propagate upward and consequently have greater displacement at depth in models with a plastic sheet few visible secondary normal faults develop most of these faults propagate downward and consequently have less displacement at depth hanging wall folding is wider and bedding dips are gentler in models without a plastic sheet than in identical models with a plastic sheet the observed particle paths displacement distributions bedding dips and orientations of the principal strain axes in our physical models with and without a basal plastic sheet are compatible with the assumption that homogeneous inclined simple shear accommodates the hanging wall deformation not all of our modeling observations however are consistent with this assumption specifically the observed variability with depth of the distribution and intensity of deformation is incompatible with homogeneous inclined simple shear as the hanging wall deformation mechanism

the development of seismic reflection sandbox modeling analog sandbox models provide cheap concise data and allow the evolution of geological structures to be observed under controlled laboratory conditions seismic physical modeling is used to study the effects of seismic wave propagation in isotropic and anisotropic media and to improve methods of data acquisition processing and interpretation by combining these two independent modeling techniques the potential exists to expand the benefits of each method for seismic physical modeling the main advantages are that the seismic data collected from these models contain natural variations that cannot be built into conventional solid models which are machined with predetermined structures in addition the cost and construction time to build these models is significantly reduced for sandbox modeling the ability to record three dimensional 3 d seismic images before the model is manually sectioned for conventional two dimensional 2 d structural interpretation allows far more detailed study of subtle 3 d structures than previously possible in the past other workers have attempted to use unconsolidated sands for seismic physical models but these attempts have been unsuccessful because of the lack of control or understanding of the natural variations that occur throughout the models the aim of our research has been to overcome such problems and to develop techniques to alter the acoustic impedance of sand layers thereby allowing the subsequent ultrasonic seismic recording of 3 d fault systems in sand analog geological models

folding thrusting and development of push up structures during the miocene tectonic inversion of the austral basin southern patagonian andes 50 s abstract rift and post rift sections of the austral basin in the southern patagonian fold and thrust belt were examined in outcrop and in 2d seismic reflection sections to evaluate geometric and kinematic aspects of the extension and subsequent inversion the syn rift section is composed of dacitic lava flows of the el quemado complex jurassic the lowermost unit of the basin the coastal sandstones of the springhill formation tithonian berriasian and marine shales of the rio mayer formation berriasian albian are thought to be sag units however field data suggest that these units are also part of the syn rift successions as evidenced by extensional growth strata in the lower levels of the rio mayer formation a dacitic flow that is texturally and compositionally similar to those of the syn rift phase and that is termed here the rio guanaco dacite is interfingered with the basal strata of the rio mayer formation and has a u pb zircon crystallization age of c 141 ma berriasian the outcropping section was inverted by compression in the early miocene according to structural and isotopic data of adjacent areas producing a broad anticline with inverted and fossil extensional faults as well as newly generated thrusts at the seismic scale most of the extensional master faults present negligible reverse slip and shortening was accommodated in hanging wall push up structures this situation is interpreted as a result of the competent metamorphic basement and volcanic syn rift section inducing frictional lock up of the master faults

fault growth and propagation during incipient continental rifting insights from a combined aeromagnetic and shuttle radar topography mission digital elevation model investigation of the okavango rift zone northwest botswana 1 digital elevation models dem extracted from the shuttle radar topography mission srtm data and high resolution aeromagnetic data are used to characterize the growth and propagation of faults associated with the early stages of continental extension in the okavango rift zone orz northwest botswana significant differences in the height of fault scarps and the throws across the faults in the basement indicate extended fault histories accompanied by sediment accumulation within the rift graben faults in the center of the rift either lack topographic expressions or are interpreted to have become inactive or have large throws and small scarp heights indicating waning activity faults on the outer margins of the rift exhibit either 1 large throws or significant scarp heights and are considered older and active or 2 throws and scarp heights that are in closer agreement and are considered young and active fault linkages between major fault systems through a process of fault piracy have combined to establish an immature border fault for the orz thus in addition to growing in length by along axis linkage of segments the rift is also growing in width by transferring motion to younger faults along the outer margins while abandoning older faults in the middle finally utilization of preexisting zones of weakness allowed the development of very long faults 100 km at a very early stage of continental rifting explaining the apparent paradox between the fault length versus throw for this young rift this study clearly demonstrates that the integration of the srtm dem and aeromagnetic data provides a 3 d view of the faults and fault systems providing new insight into fault growth and propagation during the nascent stages of continental rifting

the influence of differential sedimentary loading and compaction on the development of a deltaic rollover abstract three dimensional seismic data and wireline logs from the western niger delta were analyzed to reveal the sedimentary and tectonic history of a major deltaic growth fault depocenter comprising a kilometer scale rollover anticline the seismic units of the rollover show a non uniform thickness distribution with their respective maximum near the main bounding growth fault on the landward side of the system this wedge shaped sediment storage architecture ultimately reflects the non uniform creation of accommodation space in the study area that was controlled by 1 the differential compaction of the hanging wall and footwall strata 2 the lateral variation of fault induced tectonic subsidence above the listric master fault and possibly 3 local subsidence related to the subsurface movement of mobile shale reacting to loading and buoyancy a sequential three dimensional decompaction of the interpreted deltaic rollover units allowed to reconstruct and measure the compaction development of the rollover succession through time documenting that sediment compaction contributed per depositional interval to between 25 and 35 of the generation of depositional space subsequently filled by deltaic sediments the incremental decompaction of sedimentary units was further used to quantify the cumulative amount of accommodation space at and around the studied rollover that was created by fault movement shale withdrawal regional tectonic subsidence isostasy and changes in sea level if data on the regional subsidence and eustasy are available the contribution of these basinwide controls to the generation of depositional space can be subtracted from the cumulative accommodation balance which ultimately quantifies the amount of space for sediments to accumulate created by fault movement or shale withdrawal this observation is important in that it implies that background knowledge on subsidence stratigraphic age and sea level changes allows to reconstruct and quantify fault movement in syn tectonic deltaic growth successions and this solely based on hanging wall isopach trends independent of footwall information

faulting patterns near 19 30 s on the east pacific rise fault formation and growth at a superfast spreading center 1 structural mapping from high resolution sonar and bathymetric data collected near 19 30 s on the southern east pacific rise epr reveals multiple packets of small closely spaced outward dipping faults that form antithetic to larger inward dipping faults the mean spacing and height of inward dipping scarps are 1 6 and 1 9 times greater than those of outward dipping faults and the median spacing and height of inward dipping scarps are 2 0 and 1 7 times greater respectively accordingly inward dipping faults are less abundant 35 45 but accommodate more 50 55 of the roughly 4 0 strain although previous studies have shown that epr fault populations exhibit exponential size frequency distributions one dimensional throw and spacing data from the 19 30 s region are not described well by this model the first major 40 m scarp height fault is observed within 1 4 km of the ridge crest and invariably dips inward further off axis a transition from greater inward dipping to greater outward dipping fault abundance constrains the minimum half width of the active plate boundary zone to be 5 15 km inspection of additional data sets from 16 and 18 30 s indicates a pattern of large inward dipping faults with associated hanging wall deformation similar to that observed in the 19 30 s region this pattern is distinct from the horst and graben morphology observed along the fast spreading northern epr the pervasive hanging wall deformation observed on the flanks of the southern epr suggests master fault geometries that are more sharply curved or kinked at depth relative to their abyssal hill equivalents on the northern epr boundary element modeling indicates the pattern and geometry of faulting observed along the southern epr is consistent with fault nucleation above a shallow sill like magma chamber with internal overpressure since the influence of the sill on fault growth is strongly dependent on the depth of the sill the different patterns of faulting observed along the northern and southern epr may reflect the small difference in axial magma chamber depth between these two regions with a slightly shallower chamber beneath the southern epr

description and kinematics of the se loreto basin fault array baja california sur mexico a positive field test of oblique rift models abstract we present the results of mapping and kinematic analysis on an 18 km2 array of normal and oblique slip faults in the se loreto basin near loreto baja california sur mexico the fault array developed on the western margin of the gulf of california in late pliocene time during the early part of the modern stage of oblique rifting of the gulf the se loreto fault array is an antithetic transfer zone between two large e dipping normal faults the fault array is coherent and formed in one episode the dominant n to nne striking faults primarily dip w and display nearly pure normal slip more ne striking faults show normal sinistral slip and nw striking faults have dextral normal to dextral slip n to nw striking faults have steeper dips and a greater component of dextral slip with more northwesterly strike bulk extension based on strain analysis of faults with striae is oriented nearly e w with little plunge and consistent throughout the fault array the kinematic analysis of the se loreto fault array provides a well constrained test of oblique rift models our results support previous modeling of homogeneous extension in oblique rifted margins when both instantaneous and finite strain are considered positive field tests of analytical and experimental models such as this one indicate that such models may be robust predictors of the obliquity of rifting in ancient mountain belts where brittle fault arrays are preserved but plate motions are unknown

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

late syn rift sequence architecture and sedimentary evolution of a continental rift basin a case study from fulongquan depression of the songliao basin northeast china abstract as an important stage in continental rift basin evolution late syn rift stage is characterized by the combination of broad crustal subsidence and episodic pulses of extension sequence architecture and sedimentary filling process during this period present a certain particularity due to distinctive tectonic subsidence and variable influencing factors this paper analyzes sequence stratigraphy and sedimentary facies development and their evolution in the fulongquan depression a continental rift basin of the songliao basin and discusses the determinants influencing the sequence architecture and depositional systems evolution during late syn rift stage a major transition in the depositional systems from predominantly lacustrine and correlated depositional systems to fluvial floodplain settings was recorded in the evolution of basin fill this gradual and slow peneplanation is the final response to the decrease of differential subsidence the spatial and temporal variability of sedimentary evolution are mainly associated with specific growth history of the major boundary faults sequence stratigraphic units recognized in the fulongquan depression are likely corresponded to different episodic pulses of extension respectively each sequence can be further divided into a lowstand systems tract lst transgressive systems tract tst and highstand systems tract hst maximum differential subsidence and water deepening mainly occur in the lst the expanded tst and hst of each sequence tend to be developed in a broader thermal subsidence regime the depositional systems transition and detrital provenance changes usually occur in the hst throughout the late syn rift stage in addition to major changes of provenance systems tectonic movement is likely to be the main factor in controlling the variations of sediment supply with the decrease of tectonic activity the impact of climate changes and lake level fluctuations on basin filling tends to be increased

geometric models of listric normal faults and rollover folds existing geometric models allow master fault shapes to be constructed given the shape and heave or displacement of a deformed marker horizon in the hanging wall these models assist in projecting normal faults to depth where the fault geometry is poorly constrained by available seismic data currently it is unclear which model best predicts the actual relation between rollover and fault geometries to address this deficiency the relative accuracies of the constant heave constant displacement constant bed length slip line and inclined shear constructions are evaluated using clay model analogs and seismic examples each of the geometric models predicts a different fault shape and depth to detachment for the same rollover shape because different hanging wall deformat on mechanisms are assumed a construction is developed for determining the hanging wall geometry from a known fault geometry this construction is based on the inclined shear model and geometric constraints imposed by material displacement paths viewed from different reference frames fixed to the hanging wall and footwall blocks in clay models the inclined shear antithetic shear greek alpha 20 degrees and slip line constructions produce the best agreement between actual and modeled fault and marker bed shapes in rollovers for subsurface examples the inclined shear model antithetic shear greek alpha 20 40 degrees and the constant displacement model predict fault trajectories that are the most similar to the positions and shapes of the master fault segments interpreted fr m offshore norway and gulf of mexico seismic lines the shear angle required in the inclined shear model can often be estimated from the orientation of minor faults in the hanging wall the shear angle may also be determined through iterative modeling in which the shear angle is systematically varied and the resultant model fault shape is compared to the shape of the shallow segment of the master fault interpreted from the seismic record

geometric and experimental models of extensional fault bend folds the geometric models ranged from simple models of listric normal faults with continuous hanging wall deposition to complex models of normal faults with ramps and flats and footwall deposition and or erosion the inclined shear angle i e the angle between the horizontal and the shear direction was 70 in all models withjack and peterson 1993 describe the modeling technique in the experimental models a 4 cm thick layer of either wet clay or dry sand covered a flat metal base consisting of overlapping plates or precut blocks movement of the plates or blocks produced a listric normal fault in the overlying clay or sand layer we added growth layers after every centimeter of displacement withjack and callaway 2000 describe the properties of the wet clay and dry sand

experimental models of hangingwall deformation in ramp flat listric extensional fault systems abstract the results of three experimental models of ramp flat extensional fault systems are presented the experiments were constructed from sand and mica layers to form an anisotropic sequence that would respond to the imposed deformation by both faulting and folding the geometry of the underlying detachment controls the deformation in the hangingwall sequence ramp flat detachment geometries produce an upper roll over with a crestal collapse graben system a ramp syncline and fold reverse fault zone and a lower roll over with a crestal collapse graben system short ramp geometries produce only minor perturbations in a larger roll over structure as extension progresses the crestal collapse systems migrate and superimpose earlier systems the models of ramp flat extensional structures produce templates for the interpretation of natural ramp flat listric fault systems

listric versus planar normal fault geometry an example from the eisenstadt sopron basin e austria in a gravel pit at the eastern margin of the eisenstadt sopron basin a satellite of vienna basin austria neogene sediments are exposed in the hanging wall of a major normal fault the anticlinal structure and associated conjugated secondary normal faults were previously interpreted as a rollover anticline above a listric normal fault the spatial orientation and distribution of sedimentary horizons and crosscutting faults were mapped in detail on a laser scan of the outcrop wall subsequently in order to assess the 3d distribution and geometry of this fault system a series of parallel ground penetrating radar gpr profiles were recorded behind the outcrop wall both outcrop and gpr data were compiled in a 3d structural model providing the basis for a kinematic reconstruction of the fault plane using balanced cross section techniques however the kinematic reconstruction results in a geologically meaningless normal fault cutting down and up section additionally no evidence for a weak layer serving as ductile detachment horizon i e salt or clay horizon can be identified in stratigraphic profiles instead the observed deflection of stratigraphic horizons may be caused by a displacement gradient along a planar master fault with a maximum displacement in the fault centre decreasing towards the fault tips accordingly the observed deflection of markers in the hanging wall and in a nearby location in the footwall of the normal fault is interpreted as large scale fault drag along a planar fault that records a displacement gradient instead of a rollover anticline related to a listric fault

prediction of normal fault geometries a sensitivity analysis several workers have developed cross section balancing schemes for extensional structures using these geometric models structural interpreters can predict normal fault geometries at depth by specifying the shape of a folded bed in the hanging wall of a normal fault the deformation mechanism that accommodates the folding the footwall counterpart of the folded hanging wall bed and the dip and location of the fault segment between the hanging wall and footwall beds we have systematically determined how uncertainties in these input parameters affect the prediction of normal fault shape and detachment depth our sensitivity analysis shows that reasonable uncertainties in the fold shape near the normal fault the deformation mechanism the footwall correlation and the fa lt dip produce large uncertainties in the predicted fault geometry reasonable uncertainties in the fold shape far from the normal fault and the fault location however produce only small uncertainties in the predicted fault geometry our work suggests that if seismic and well data are lacking or of poor quality then geometric models cannot provide unique answers to questions about the geometry of a normal fault at depth geometric models however can provide a range of possible fault geometries

dolomitized middle proterozoic calcretes bathurst inlet northwest territories canada abstract dolostones within the terrigenous clastic middle proterozoic kanuyak formation are interpreted to be dolomitized calcretes dolocretes which formed on a landscape largely devoid of macroscopic organic life the dolocretes up to 5 m thick form beds or masses which either parallel or cross cut host sedimentary rocks and are composed of five elements 1 curvilinear and rectilinear networks of cracks which are filled with detrital terrigenous clastic and carbonate sediments ooids pisoids and cements sediments and cements are laminated and bedded generally parallel to the margins of cracks 2 micritic fibrous columnar and plumose dolomitic cements that were precipitated as calcareous speleothems ooids and pisoids and masses 3 ooids and pisoids that are concentrically laminat d homogenous or coated grains occurring in reversely graded beds and as isolated particles throughout dolocretes it appears that accretionary particles formed in situ based upon polygonal fitting vadose and grapestone fabrics and gradational contacts between ooids pisoids and host sediments 4 deformation structures that include folded and brecciated detrital sediments within cracks brecciated pisoids and minor teepees and 5 surface karren that locally truncate the dolocretes while missing the features commonly associated with higher plants in post silurian calcretes rhizoconcretions root mouldic porosity etc the presence of an array of other common calcrete fabrics e g cracks cements accretionary grains and deformation structures suggests that physical and chemical processes were probably largely responsible for the carbonate precipitates within kanuyak profiles microbiota most likely also contributed to calcretization however if not significantly altered by dolomitization stable carbon and oxygen isotope values do not indicate any significant influence by organic matter

analog modeling of pull apart basins scaled sandbox models have successfully simulated the geometries and progressive evolution of pull apart basins developed in a weak sedimentary cover above right stepping releasing dextral strike slip fault systems in rigid basement synkinematic basin infill was added progressively as the models were deformed vertical and horizontal sectioning of the completed models has allowed the full three dimensional architecture of the pull apart system to be analyzed we present three representative end member experiments 30 underlapping releasing sidestep 90 sharp nonoverlapping releasing sidestep and 150 overlapping releasing sidestep the pull apart basin geometries are typically sigmoidal to rhombic grabens the geometries of which are dependent upon the architecture of the underlying basement fault systems underlapping releasing sidestepping faults offset angles of 30 75 typically form elongate rhomboidal grabens 90 releasing offset sidesteps produce shorter squat rhomboid pull apart basins and overlapping releasing sidesteps 115 150 produce box like grabens with highly kinked basin sidewalls the pull apart basins are bounded by terraced oblique slip extensional sidewall fault systems that link the laterally offset principal displacement zones pdz of the main dextral strike slip faults the sidewall faults show changes in kinematics from dominantly dip slip extension in copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received june 24 1996 revised manuscript received november 20 1996 final acceptance june 10 1997 2fault dynamics project geology department royal holloway university of london egham surrey tw20 oex united kingdom supported by the fault dynamics project sponsored by arco british limited petrobras u k ltd bp exploration conoco u k limited mobil north sea limited and sun oil britain we also acknowledge additional funding from sun oil britain landsat thematic mapper data courtesy of rtz mining and exploration limited south american division jurrian reijs is thanked for field data relating to the salina del fraile pull apart basin chris elders is thanked for reviewing earlier versions of this manuscript and j crowell c morley and k biddle are thanked for constructive reviews of the manuscript the contents of this paper have been

reverse drag revisited why footwall deformation may be the key to inferring listric fault geometry abstract although reverse drag the down warping of hanging wall strata toward a normal fault is widely accepted as an indicator of listric fault geometry previous studies have shown that similar folding may form in response to slip on faults of finite vertical extent with listric or planar geometry in this study we therefore seek more general criteria for inferring subsurface fault geometry from observations of near surface deformation by directly comparing patterns of displacement stress and strain around planar and listric faults as predicted by elastic boundary element models in agreement with previous work we find that models with finite planar planar detached and listric detached faults all develop hanging wall reverse drag folds all of these model geometries also predict a region of tension and elevated maximum coulomb stress in the hanging wall suggesting that the distribution and orientation of near surface joints and secondary faults may also be of limited utility in predicting subsurface fault geometry the most notable difference between the three models however is the magnitude of footwall uplift footwall uplift decreases slightly with introduction of a detachment and more significantly with the addition of a listric fault shape a parametric investigation of faults with constant slip ranging from nearly planar to strongly listric over depths from 1 to 15 km reveals that footwall fold width is sensitive to fault geometry while hanging wall fold width largely reflects fault depth application of a graphical approach based on these results as well as more complete inverse modeling illustrates how patterns of combined hanging wall and footwall deformation may be used to constrain subsurface fault geometry

preserved extensional structures in an inverted cretaceous rift basin northwestern argentina outcrop examples and implications for fault reactivation 1 during the cretaceous eocene interval a system of intracontinental rift basins the salta group rift evolved in northwestern argentina individual segments of the rift later suffered different degrees of inversion during cenozoic shortening the tres cruces subbasin on the west side of the eastern cordillera was strongly deformed being now part of a thick skinned thrust belt with a predominantly n s structural trend on its eastern border tilting due to folding and thrusting and subsequent erosion have produced exceptional outcrops of preserved east trending extensional structures including half grabens rollover anticlines and extensional fault propagation folds farther west the synrift succession is only intermittently exposed although the interference of north and east trending structures as well as peculiar dome shaped anticlines with spur like extensions suggest that north and east trending cretaceous faults were reactivated particularly near their intersections compilation of published data and analysis of our new data focused on the salta rift indicates three main factors favoring the contractional reactivation of normal faults dip angles lower than approximately 60 especially for faults striking roughly normal to contraction strikes no closer to the contraction direction than approximately 30 and low downdip fault curvatures occasional dip slip reactivation of east trending faults does not match the present and long term andean stress regimes and presents an unresolved problem

chronologic modeling of faulted and fractured reservoirs using geomechanically based restoration technique and industry applications we have developed a geomechanically based restoration method to model reservoir deformation the approach founded on the finite element method simulates the physical behavior of the rock mass and considers heterogeneous material properties bedding slip and the mechanical interaction of faults to demonstrate the method s potential we analyze the deformation and fault growth in the hanging wall of a synsedimentary listric normal fault from a sand box model which provides an analog for evaluating complex faulted reservoirs the numerical model results are then analyzed to investigate the chronology of faulting the numerical model corresponds well to the physical model and provides additional insights about reservoir evolution and deformation the approach is also tested on a natural example of folding using outcrop data to study contractional deformation these examples illustrate how undetected faults and fractures reservoir compartmentalization hydrocarbon migration pathways and hydrocarbon traps can be understood in the context of tectonic processes and how this understanding can be exploited in decision making and reducing risk we conclude that the geomechanically based restoration of faulted and fractured reservoirs has significant potential for industry applications compared to common geometric restoration techniques which lack a mechanical basis

a method for automatically determining normal fault geometry at depth a large number of methods have been described which relate the shape of a normal fault at depth to the shape of a sedimentary horizon in its hanging wall unfortunately the calculated fault geometry is strongly dependent upon the method used here arguments are put forward in favor of a general two dimensional kinematic model which assumes that the hanging wall deforms by a combination of arbitrarily inclined simple shear and differential compaction the method should be applicable to areas where it can be assumed that the fault plane does not change shape as deformation proceeds this assumption may not always be valid especially in the case of large scale basement extending normal faults fault geometry the inclination of simple shear and compaction parameters may all be determined from n beds using a simple inversion scheme based on the general method throughout it is assumed that hanging wall and footwall stratigraphies are accurately known though uncertainty in either could be included as a bounded variable within a formal inversion scheme the most important feature of the model is that it is testable it has first been tested on synthetic data using a range of fault geometries the results of such modeling are encouraging and imply that all unknown parameters including fault geometry can in general be uniquely determined provided that the geometries of two or more beds within the hanging wall are known the greater the number of beds used the greater the degree of confidence in the solution the method has also been applied to published sand box models with surprisingly good results ultimately it is hoped that the method can be applied routinely to depth converted seismic reflection data the preliminary applications presented here suggest that bulk antithetically inclined simple shear combined with differential compaction provide a good approximation for hanging wall deformation in the future the scheme will be generalized to allow for deformation in three dimensions other complexities such as horizontal and vertical variation in the degree of compaction could also be included although at this stage the data do not appear to justify them

predicting reservoir scale faults with area balance application to growth stratigraphy a reservoir that appears to lack faults at one scale of resolution or at one sampling density may nevertheless contain faults that are below the resolution of the observations the area depth relationship from a balanced cross section is shown to contain the necessary information for predicting the sub resolution fault heave existing area depth theory is extended to include growth units allowing structural length and thickness changes to be separated from the depositional changes the technique is validated with a numerical model of a growth full graben and a sand box model of a half graben then field tested in the gilbertown graben a growth structure within the regional peripheral fault trend along the northern margin of the gulf of mexico a cross section developed from wells alone is used to infer the abundance of sub resolution faults by the area balance technique a small but significant amount of sub resolution extension is predicted and then confirmed with a highresolution seismic line q 2003 elsevier science ltd all rights reserved

experimental modeling of extensional fault systems by use of plaster plane strain extension experiments with plaster as the deforming medium produce faults and related structures that closely resemble natural examples the faults with the largest displacement are normally developed at about 25 overall extension 1 25 major faults develop as composite structures and although the main movement may be accommodated by a relatively simple slip surface the fault zones commonly develop internal lense shaped geometries small synthetic faults dipping more steeply than the main zone are typical components of the fault zones particularly on the hanging wall side fault displacements develop over 2 orders of magnitude and the distribution of fault displacements closely follows a power law relationship similar to data from seismic and field mapping the main through cutting faults accommodate about 60 70 of the total deformation while smaller visible faults typically account for ca 10 20 only 20 30 extension is due to faults and ductile deformation that occur below the resolution of the models which roughly corresponds to seismic resolution when scaled up to natural size the internal fault block formation may include zones of normal faults accommodating significant internal shear deformation associated with collapse of fault blocks collapse has been recorded in the upper part of footwalls resulting in a flattening of the upper part of the main faults although internal block deformation by simple shear or other deformations can be shown to play a significant role in the deformation a component of rigid block rotation is demonstrated for example by the rotation of steep normal faults into orientations where they appear as reverse faults

a quantitative simulation study of asymmetrical tectonic subsidence control on non synchronous sequence stacking patterns of eocene lacustrine sediments in bohai bay basin china abstract compared with large scale passive margin marine basins continental basins are susceptive to local or regional tectonic geomorphologic and climatic influences and can form complicated sequence stratigraphic architectures configurations and sedimentary facies we report herein a quantitative simulation study on the influences of asymmetrical tectonic subsidence on sequence stratigraphic stacking patterns in rift basins by varying tectonic subsidence sediment supply and intra rift lake levels it has been demonstrated that isochronous sequences within a rift basin can exhibit both synchronous and non synchronous stacking patterns as a result of differential asymmetric tectonic subsidence in sequence stratigraphy synchronous stacking patterns refer to isochronous sequences at any parts of a basin which are either all retrogradational or all progradational we propose the concept of non synchronous stacking patterns of isochronous sequence stratigraphy for rift basins non synchronous stacking patterns refer to the fact that within the same basin some parts may concomitantly exhibit stacking patterns that are exactly opposite to that of other parts non synchronous stacking patterns can form when the accommodation space increment a is less than that of sedimentary supply s in one area but larger than or equal to that of s in another moreover the duration of non synchronous stacking patterns is controlled by both a and s recognition of non synchronous stacking patterns within isochronous sequence stratigraphy provides a conceptual model for correlation of sequence stratigraphic units in tectonically asymmetric rift basins in eastern china and similar basins around the world

the formation of conjugate normal fault systems in folded sandstone by sequential jointing and shearing waterpocket monocline utah 1 we report conjugate normal fault development by the formation and subsequent shearing of joints during flexure and associated extension of the jurassic wingate and navajo sandstones at the core of the waterpocket monocline utah geometry and structural components across fault zones were quantified from an incipient stage to a relatively well developed stage where the zone is 120 m wide and accommodates more than 3 5 m of slip shearing of joints leads to the formation of splay fractures near the joints tip subsequently these splay fractures are sheared and a new generation of splay fractures forms multiple generations of splay fractures cause the width of the fault damage zone to expand into the surrounding host rock producing unusually wide fault damage zones with respect to fault offset these extensive fault damage zones are stratigraphically confined and develop with low strains at small bed dips during folding the faults formed by this sequential process develop a characteristic fault architecture distinguished by the type of component structures their geometric relationships and their distribution we conclude that cyclic joint formation shearing accompanied by splay fracture formation and repeated shearing produce a geometric pattern similar to a classical conjugate fault system however the mechanism identified here has different implications for fault intersection angles stress inversion and fault architecture

deformation pattern during normal faulting a sequential limit analysis we model in 2 d the formation and development of half graben faults above a low angle normal detachment fault the model based on a sequential limit analysis accounting for mechanical equilibrium and energy dissipation simulates the incremental deformation of a frictional cohesive and fluid saturated rock wedge above the detachment two modes of deformation gravitational collapse and tectonic collapse are revealed which compare well with the results of the critical coulomb wedge theory we additionally show that the fault and the axial surface of the half graben rotate as topographic subsidence increases this progressive rotation makes some of the footwall material being sheared and entering into the hanging wall creating a specific region called foot to hanging wall fhw the model allows introducing additional effects such as weakening of the faults once they have slipped and sedimentation in their hanging wall these processes are shown to control the size of the fhw region and the number of fault bounded blocks it eventually contains fault weakening tends to make fault rotation more discontinuous and this results in the fhw zone containing multiple blocks of intact material separated by faults by compensating the topographic subsidence of the half graben sedimentation tends to slow the fault rotation and this results in the reduction of the size of the fhw zone and of its number of fault bounded blocks we apply the new approach to reproduce the faults observed along a seismic line in the southern jeanne d arc basin grand banks offshore newfoundland there a single block exists in the hanging wall of the principal fault the model explains well this situation provided that a slow sedimentation rate in the lower jurassic is proposed followed by an increasing rate over time as the main detachment fault was growing

geometry and kinematics of extensional structural wedges abstract structural wedges in the compressive environment have been recognized and studied in different locations however extension structural wedges are less well understood based on the normal fault bend folding theory and inclined shear model this paper quantitatively analyses deformations related to extensional structural wedges and builds a series of geometric models for them an extensional structural wedge is a fault block held by two or more normal faults the action of which would fold its overlying strata extensional structural wedges of different shapes will lead to different deformation results for the overlying strata and this paper illustrates both the triangular and quadrangular wedges and their related deformations this paper also discusses differences between the extensional structural wedges and the normal fault bend folding by analysing two seismic sections from langfang gu an sag east china this paper provides two natural examples of the triangular and quadrangular extensional structural wedges where the models can reasonably explain the overlying distinct highs and lows without obvious faults the establishment of a geometric model of extensional structural wedges can provide reference and theoretical bases for future quantitative analysis of deformations in the extensional environment