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

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

crustal kinematics and deep structure of the northern rhine graben germany surface and subsurface geological data unpublished information gleaned from industrial seismic surveys and results from a deep reflection seismic profile across the tertiary northern rhine graben indicate that crustal extension in excess of 6 km was achieved by asymmetric detachment of the upper crust at a depth of about 15 km comparison of a simple geometric kinematic cross sectional model and its depth extrapolation with the dekorp 9 n deep crustal reflection data suggests that extension below the graben was achieved by slip along a gently westward dipping shear zone which truncated and offset lower crustal planar fabrics and which continues into the uppermost mantle the planar lower crustal reflectors probably originated mainly during late variscan permocarboniferous crustal extension and or magmatic underplating

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

the role of strain in area constant detachment folding abstract the classical constant area relationship for balancing cross sections is reformulated to accommodate layer parallel strain leading to a general equation for the layer parallel strain in a fold formed above a stratigraphically fixed detachment horizon the strain required for area balance can be calculated from a knowledge of the fold shape and the depth to detachment the strains predicted from a simple kinematic model are realistic in the order of 10 30 layer parallel shortening the strain may be penetrative or occur as small scale disharmonic folds and faults the tip top anticline a fold at the leading edge of the wyoming thrust belt is used to illustrate the applicability of this reformulation the computed layer parallel strains in the well controlled upper portion of the structure are less than 5 because most of the shortening is accommodated by second order folds and faults

particle size characterization of historic sediment deposition from a closed estuarine lagoon central california abstract recent studies of estuarine sediment deposits have focused on grain size spectra as a tool to better understand depositional processes in particular those associated with tidal inlet and basin dynamics the key to accurate interpretation of lithostratigraphic sequences is establishing clear connections between morphodynamic changes and the resulting shifts in sediment texture here we report on coupled analysis of shallow sediment profiles from a closed estuarine lagoon in concert with recent changes in lagoon morphology reconstructed from historic sources with a specific emphasis on the ability of suite statistics to provide meaningful insights into changes in sediment transport agency we found that a major reorganization in lagoon morphology dating to the 1940s was associated with a shift in sediment deposition patterns the restricted inlet was associated with deposition of sediments that were finer less negatively skewed and less leptokurtic in distribution than sediments deposited while the lagoon had a more open structure these shifts are associated with a change in transport process from fluvial through flow to closed basin trapping we also found other chemostratigraphic changes accompanying this shift in sediment texture reflecting changes in organic matter source wetland species composition and an increase in sediment organic content as presumably coarse well ventilated floodplain sediments tend to result in mineralization rather than sequestration of organic matter in conclusion we found that grain size analysis in concert with the suite statistics technique reflected changes in coastal configuration supported by historic maps and photos however we also recognize that this analysis was more informative given further context through additional sedimentary analyses these findings provide a basis for the interpretation of particle size distribution in lithostratigraphic sequences associated with bar built estuaries where understanding natural and anthropogenically modified inlet dynamics may help shape conservation management where concerns exist with respect to fish passage water quality and sediment transport

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

structural evolution of the witch ground graben the witch ground graben is a nw se trending extensional basin in the central part of the northern north sea lying between the moray firth and the viking graben ne sw extension occurred from triassic to cretaceous times extension took place on listric normal faults along the sw margin with downthrow to the ne giving rise to the asymmetry of the basin one of the principal phases of fault controlled subsidence occurred in the lower cretaceous from the valanginian to albian compilation of biostratigraphic data from this interval shows the development of an increasing number of stratigraphic breaks across the basin from sw to ne these breaks increasing in time span also in this direction this is a result of rotation of the subsiding fault block on the listric extensional faults the rotation is recorded in dipmeter data and occurred in several distinct episodes the triassic the volgian and the valanginian to aptian in reconstructing a cross section of the basin the interpreted fault curvature is constrained by the observed rotation of bedding surfaces and the amount of extension across the listric faults is constrained by the observed basin subsidence a depth to decollement for the faults of 4 km below sea level and an extension across the basin of 40 are estimated

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

area balance depth to detachment and strain in extension a new lost area cross section construction and validation technique is based on the area balance of a graben system formed above a detachment area balance requires that the area displaced below the original regional level of each bed in a graben the lost area be balanced by the displaced area at the boundary of the system given by the product of the displacement times the depth to detachment this relationship is a straight line on a plot of lost area versus depth the slope of the line is the displacement and the depth intercept is the depth of the detachment in general beds within the graben system undergo both visible and homogeneous layer parallel extension the homogeneous component can be determined from the width of the graben system the lost area and the depth to detachment conversely the detachment depth may be calculated from the total layer parallel extension and the lost area the techniques are illustrated by application to models and to very well constrained natural examples an earthquake induced slope failure in alaska and kilometric sized horsts and grabens in the black warrior foreland basin of alabama a half graben without footwall uplift and a full graben are the same on a lost area diagram a half graben with footwall uplift linked to the hanging wall extension requires that both the hanging wall and footwall areas be included to determine the detachment depth and displacement all the models show layer parallel extension e in the upper levels that are less than the crustal scale strain

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

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

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

excess area and depth to detachment a plot of the excess area in cross section vs the depth to a reference level for several stratigraphic horizons in a compressional fold produces a curve that gives the depth to detachment the curve is a straight line for detachment folds the slope of the line is the displacement on the lower detachment and the depth intercept where excess area zero is the depth to detachment for a fault bend fold the result is a line that abruptly changes slope at the position corresponding to the upper detachment the difference in slope between the two parts of the line is the displacement on the upper detachment added layer parallel shear produces a curve on the graph tested on detachment folds from the jura mountains the method accurately predicts the depth to detachment he tip top field anticline in the wyoming thrust belt is found to be interpretable as either a detachment anticline with a basal detachment close to the top of the basement or as a fault bend fold with a basal detachment at the base of the triassic the excess area diagram is useful for testing a cross section for balance determining the structural style detachment anticline vs fault bend fold and for examining alternatives in the construction of cross sections

structural stratigraphic models for extensional basins of half graben type abstract the equations describing subsidence in isostatically compensated basins formed by lithospheric stretching can be modified to define a notional depth to decollement which relates subsidence to the stretching factor in much the same way as the depth to a physical sole fault relates subsidence to extension in uncompensated thin skinned basins for a constant density basin fill this notional depth to decollement corresponds to the level of no vertical motion within the lithosphere in general the physical sole to a system of detached normal faults will not lie at the notional depth to decollement and must move vertically during extension this vertical movement must be implicitly corrected for before area bed length balancing is used to determine the depth to a physical sole fault the notional depth to decollement has been calculated numerically for an extensional basin loaded by sediments which follow a north sea shaly sand compaction law and increases with from 5 to 9 km a set of model half grabens has been constructed by combining subsidence curves generated by this shaly sand model with geometric relationships inherent in the rotated planar domino mechanism of upper crustal faulting for comparison a similar set of half grabens was constructed for an uncompensated basin with a sole fault at 7 5 km depth in both cases large fault to fault spacings give rise to large basement relief and consequent footwall uplift and erosion small fault spacings give rise to simple listric growth faults intermediate values generate more complex uplift subsidence histories which are capable of giving rise to such commonly observed features as rounded eroded fault block shoulders local regression followed by onlap or exponentially increasing subsidence at the crest of the fault block

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

the mechanics of continental extension and sedimentary basin formation a simple shear pure shear flexural cantilever model abstract deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins major basement faults are generally planar and extend down to 10 15 km depth corresponding to the base of the seismogenic layer beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle a mathematical model is described of the geometric thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle during faulting upper crustal footwall and hanging wall blocks behave as two mutually self supporting flexural cantilevers their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse for extension on multiple faults interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics the flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing horizontal displacement and polarity the effects of fault spacing fault polarity and the amount of extension as well as erosion of footwall uplift on crustal structure and basin geometry during both syn rift and post rift stages of basin evolution are explored models of passive rifting involving lithosphere extension in response to the build up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere appear to conform to the main geological observations the role of active rifting driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

half graben structures balanced models of extensional fault bend folds one common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault this geometry is modeled as being caused by a bend in the master fault the hanging wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend resulting in an extensional fault bend fold the footwall beds remain undeformed and unrotated the major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend important consequences of the model include a relationship between master fault dip above and below a bend and the amount of hanging wall dip the horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend the asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained hanging wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward hanging wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward the model fits measurements taken from a published ramp and flat clay model by e cloos and is used to develop a balanced and restorable cross section of the schell creek fault a basin and range structure in nevada

the geometry of listric normal faults and deformation in their hangingwalls abstract in cross sections containing listric extensional faults area balancing techniques for depth to decollement are usually based on either bed length conservation or displacement conservation listric fault geometry may be constructed from a hangingwall roll over profile using the chevron construction this construction based on conservation of heave necessitates a reduction in fault displacement with decreasing fault dip a modification of this construction utilizing conservation of fault displacement predicts a listric fault that detaches at a shallower depth a new construction based on slip lines uses fault perpendicular displacement segments to generate listric fault shape fault propagation strain may be responsible for anomalous hangingwall geometries and these can be predicted by forward modelling using either a modified chevron construction or a slip line construction

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

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

cross section restoration and balancing as aid to seismic interpretation in extensional terranes structural cross sections in extensional terranes can be improved significantly by restoration and balancing principles originally developed for fold and thrust belts in rift basins and passive continental margins use of restoration techniques in analysis of growth faults elongate salt swells and other extensional structures results in geologically valid seismic interpretations sections must be in depth as opposed to two way time and oriented in the direction of material transport and decompaction must accompany restoration hanging wall deformation is characterized by different processes depending on the mechanical properties of the rocks being extended and can be approximated by various geometric and kinematic models antithetic faulting is analogous to antithetic shear combined antithetic and synthetic faulting is approximated by vertical shear domino style extension corresponds to rigid body rotation and flexural slip flow is modeled by bed length balance methods selection of the appropriate algorithm is crucial for proper restoration and balancing applications of these techniques illustrated here using seismic data from the gulf coast and north sea include 1 evaluating and adjusting seismic interpretations 2 projecting listric fault trajectories to depth 3 delineating more accurately the geometry and extent of hydrocarbon reservoirs 4 identifying new exploration leads 5 determining the deformation history of an area and 6 constraining the timing and direction of hydrocarbon migration and entrapment

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

3 d palinspastic restoration of normal faults in the inner moray firth implications for extensional basin development abstract balanced cross section techniques and the construction of a restored section permit 2 dimensional palinspastic restorations to be made in both compressional and extensional terraines in 3 dimensions an equivalent restoration can be made by assuming conservation of bedding plane area and considering the volume of a stratigraphic interval rather than its cross sectional area extensional basins displaying upper crustal listric normal faulting are particularly amenable to this approach computerised 3 d restorations have been made of the inner moray firth basin offshore scotland this basin is not isostatically compensated and was produced by 7 8 post triassic extension of which 2 5 3 is post jurassic above a detachment surface at 20 25 km depth close to the base of the crust limited lower crustal thinning and lithospheric stretching has affected the eastern part of the basin but this can account for no more than half of the measured upper crustal extension some of this shallow extension is probably coupled by low angle faults or shear zones into major zones of lithospheric stretching such as the north sea grabens where it may help account for discrepancies between estimates of lithospheric thinning and upper crustal extension

rifts and grabens abstract deep basins on continental crust can be divided into two main types basins with normal faults and the basement both tilted in the same direction s type and in opposite directions a type a high intensity of stretching occurs in the basins of the a type under a large angle of block tilting this can also be typical of the s type basins of a very small width l that does not exceed the thickness of the prestretched crust h c 0 l h c 0 only in basins of these two k is extension sufficient to explain the subsidence by stretching they can be called rifts extended basins in the basin and range province are a typical example deep basins of a type with a small angle of block tilting and deep basins of s type with a width l h c 0 cannot be produced by stretching under a width h c 0 l 100 km they can be call grabens the fen wei graben in central china and the viking graben in the north sea belong to this structural type

comment on tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california by john m bartley john m fletcher and allen f glazner

geometry of the hanging wall above a system of listric normal faults a numerical solution the process of hanging wall deformation above a system of listric faults is controlled by the local stress field rock properties and fault geometry although fault geometry provides a boundary condition the strained hanging wall can be adequately described by a process of simple shear during hanging wall collapse a numerical solution of hanging wall deformation above a set of listric normal faults clearly demonstrates the importance of inclined simple shear the true amount of horizontal extension and sense of inclined shear is estimated based on both rollover and fault geometry this forward modeling technique treats listric faults as continuously curved surfaces and may incorporate any number of faults even or unevenly spaced with variable fault geometry and near surface dip basin width is quite sensitive to sense of shear within the hanging wall and depth of detachment as well as the apparent horizontal extension calculated solely upon the basis of bed length balance the model also indicates that antithetic shear is likely to occur near steeply dipping normal faults whereas geologically reasonable basin geometries are generated under synthetic shear for low angle normal faults ramp and flat listric fault geometries can be modeled with inclined simple shear to generate basins care must be taken when assessing the amount of horizontal extension in extended terranes because the percentage of horizontal extension is quite sensitive to fault geometry and angle of internal shear arrays of listric normal faults may develop extensional geometries that resemble domino style extension if an apparent planar fault geometry observed near the surface assumes a listric shape downward and distributed deformation occurs within the hanging wall the resulting estimation of extension based on the angular relationship between faults and beds is likely to be wrong the model successfully describes the overall geometry of half graben type basins and indicates an important link between fault geometry and sense of shear within the hanging wall of listric normal faults near vertical and synthetic shear are determined near low angle normal faults whereas antithetic shear is likely to be observed near steeply dipping normal faults