fault block tilting the gebel zeit example gulf of suez abstract the gebel zeit consists of the eroded crest of a tilted block in the southern part of the suez rift the zeit block displays a typical asymmetrical geometry it is bordered to the east by a 35 45 e dipping normal fault with kilometric throw and has a 30 sw dipping homoclinal flank part of the pre rift sedimentary sequence has been preserved on this flank and is unconformably covered by upper burdigalian nn4 globigerina marls in the southern part of the block crest the complete pre rift series has been eroded and evaporites of langhian age nn5 rest directly on the precambrian basement field evidence indicates alternations from erosion to sedimentation at the crest of the zeit block in an attempt to characterize its tilting subsidence curves were computed along a cross section with the backstripping method results indicate three stages in the evolution of the block 1 a rapid subsidence between 22 and 16 my 2 a pause between 16 and 10 my and 3 a slow subsidence until present time during the tectonic quiescence the sedimentary loading effect alone produced an increase of 8 in the tilt angle a simple kinematic model of tilting along a circular fault is proposed to quantify the zeit rotation depth of the brittle ductile transition is estimated at 10 km to explain the tilting strong driving of the tilting by the listric fault induces conjugate movements between the crest and trough of the block and explains the discrepancy between the zeit and regional tectonic subsidence

a simple kinematic model for crustal deformation along two and three dimensional listric normal faults derived from scaled laboratory experiments abstract we have derived a simple kinematic model of the deformation that results from extension accommodated by movement of a crustal block along two and three dimensional listric fault surfaces the model accurately reproduces deformation observed in a series of scaled analogue models the kinematic model is based on the simple assumption that lines within the hangingwall that are normal to the fault surface before deformation remain so following deformation an additional constraint built into the model is that of incompressibility deformation in the hangingwall block as observed in the laboratory experiments and predicted by the kinematic model is characterized by 1 a key stone structure or crestal collapse graben at some finite distance from the fault tip and 2 pure solid body rotation of the hangingwall head area near the tip of the fault in three dimensions the central region of the model undergoes extension in a direction normal to the direction of imposed displacement in such a way that the direction of dip of the upper surface of the hangingwall is aligned with the direction of extension this result provides quantitative support for the use of dip analysis to infer tectonic transport direction we also show how the distribution of extension within the hangingwall is affected when the constraint of constant displacement along the fault is relaxed

probabilistic modeling of faults below the limit of seismic resolution in pelican field north sea offshore united kingdom faults which can compartmentalize a reservoir and which are too small to be seen on seismic can be predicted using a self similar or fractal model of faulting at various scales a computerized method has been developed for statistical analysis of a fault pattern interpreted from three dimensional 3 d seismic in order to fractally derive the distributions of fault size frequency and spatial variations and to verify their invariance within the seismic resolution range this analysis can be used for quality control of seismic interpretation and as input for extrapolation toward smaller scales using probabilistic techniques the application of this program to pelican field united kingdom north sea validated the interpretation of the complex fault pattern of that field but suggested that only limited extrapolations could be confidently made probabilistic maps showing distribution of faults down to 3 m 10 ft of throw and not resolved by 3 d seismic were made these maps subsequently were used as a qualitative aid in full field reservoir simulations to model differently zones of potential dense faulting from zones of reduced faulting this was done because nearby field behavior had shown that faulting is a significant factor in reservoir behavior and ultimately field deliverability field development planning therefore could be optimized

the importance of small scale faulting in regional extension a recurring observation in many studies of extensional basins has been that the amount of extension visible on normal faults for example on seismic reflection profiles is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence1 5 one mechanism suggested to account for this discrepancy is small scale faulting with offsets too small to be resolved seismically6 7 but earthquake studies8 10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area scholz and cowie11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements here we present fault displacement data that directly conflict with scholz and cowie s conclusions and imply that up to 40 of the extension may be missed by summing fault offsets on basin profiles the fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time

drill hole design for dilational ore shoot targets in fault fill veins ore along fault fill veins typically has an uneven distribution with semilinear high grade ore shoots separated by lower grade zones or barren fault segments therefore a successful drill campaign must be optimally designed for intersection of ore shoots and not the intervening zones however the typical drill hole design for fault fill veins which targets the vein directly downdip from the highest grade outcrops will miss most ore shoots that rake less than 80 to 90 in the structure dilational ore shoots form along pre or synmineralization faults in localized elongated structural openings perpendicular to the slip vector the rake of such ore shoots in fault fill veins can be predicted using models of stress controlled fault fracture kinematics equations are presented that use fault kinematic data to design drill holes for optimal success in targeting such ore shoots the equations calculate the universal transverse mercator utm coordinates of the drill target given fault fill vein orientation slip vector rake usually determined from slickenlines vertical depth to intersect the ore shoot and the utm coordinates of geochemically anomalous outcrops along the fault vein although the method assumes that fault kinematics control ore shoot orientation and is not applicable to all fault vein ore shoots such as those formed at intersections of faults or shear zones with permeable beds other faults or dikes the method increases the probability of success for drill testing ore shoots through optimal drill hole design

populations of faults and fault displacements and their effects on estimates of fault related regional extension abstract measured populations of fault displacements derived from regional seismic oilfield seismic coalmine plans and outcrop data show a power law distribution with exponents s of 0 45 to 0 95 for singleline samples across an array of faults the more negative values indicate relatively larger numbers of smaller faults an expression for a population of active faults derived from the gutenberg richter magnitude frequency relation for earthquakes is log n a b d log d where d maximum displacement of a fault n number of faults of maximum displacement d or greater and b d 1 0 and has the same value as b for the corresponding earthquake population populations of dead faults existing at the end of a tectonic episode have been numerically derived using a fault growth model and satisfy the relation log n a e log d where e has a value between 1 6 and 2 0 numerically derived populations of fault displacements in a dead fault population have slopes of s where s e 1 the contribution of an individual fault to the regional strain varies with the lifetime seismic moment of the fault and is proportional to d 2 estimation of fault related extension by summing heaves on faults of a limited size range is valid only if the measured size range of faults accommodates most of the extension correction can be made if the s value of the fault displacement population is known

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

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

fractal dimension of fault systems in japan fractal structure in rock fracture geometry at various scales based on fault maps whether or not the fracture geometry of rocks is self similar was examined by using a box counting algorithm the statistical self similarity fractal structure of the fault fracture systems holds well at the scale of about 2 to 20 km the fractal dimension in japan varied from 1 05 to 1 60 the fractal dimension is about 1 5 1 6 at the central part of the japan arc and decreases with distance from the center at a smaller scale the fractal structure also holds well in the rock fracture geometry the fractal dimension of the north izu peninsula fault system branching faults is 1 49 at the scale of 0 625 to 10 km the fractal dimension of rock fracture geometry at the scale order of 10 1 to 10 2 meters is about 1 49 1 61 the upper limit of the fractal dimension of rock fracture geometry is about 1 6 judging from the estimation of fractal dimension on actual fracture geometry of rocks this value may impose a restraint on modeling of faulting and the fracture process of rocks

evidence for flexural shear folding associated with extensional faults the maintainance of bed length and bed thickness within the hanging wall of extensional fault systems requires the development of bed parallel shear four field examples of bed parallel shear related structures are described from a number of positions within the hanging wall and footwall to extensional faults flexural shear folding is a characteristic of both fault bend and fault propagation folding the dominant direction of the over riding shear is directed toward the fault surface in the case of fault bend folds the fault surface may be considered to represent the finite neutral surface of the fold in extensional fault propagation folds the direction of the overriding shear is toward the synclinalfold axial surface the observations suggest that the direction of flexural shear for extensional and contractional folds is opposite for folds of geometrically similar form

geochemical characteristics of biogenic gases in china abstract a number of contributions have demonstrated that the stable carbon and hydrogen isotopic compositions together with molecular compositions have a great significance on determination for gas origin and gas generation pathways of biogenic gases together with some data from published work a total of 143 gas samples were compiled from different biogenic gas producing locations in china to investigate the gas origin and gas generation pathways by the molecular composition stable carbon and hydrogen isotopes the results demonstrate that biogenic gases in china are dominated by methane with low level of c2 3 generally less than 0 5 and low 13cch4 values generally 55 the relatively high level of c2 3 up to 11 79 in the biogenic gases from the baise and songliao basins indicates the possible mixture of low mature thermogenic gases biogenic gases from the jiangzhe coastal area including shanghai zhejiang and jiangsu provinces and yingqiong basin have dch4 values greater than 190 indicating marine salt water depositional environments however all other gases have dch4 values less than 190 possibly indicating a terrestrial freshwater or freshwater to slightly saline water depositional environment the methanogenic pathway of biogenic gases in china is dominated by biogenic carbonate reduction though a number of these biogenic gases were generated in a terrestrial freshwater or freshwater to slightly saline water depositional environments compared to the bacterial carbonate reduction methanognic pathway in the luliang basin gas formation mechanism in the baoshan basin was more toward bacterial methyl type fermentation likely due to the relatively higher geothermal gradient in the baoshan basin

dips of normal faults in british coal measures and other sedimentary sequences fault dip measurements n 3291 from 122 faults in british coalfields are presented two classes of normal fault are recognized a gently dipping 55 post coalification tectonic faults formed at depths of about 2 5 km which comprise 90 of the faults analysed dip data from the steeply dipping faults are variable standard deviation 9 both on single faults and between faults and 70 is a better approximation of fault dip than the commonly accepted value of 60 data on tectonic normal faults with comparable depths of formation using the modified griffith fracture criterion the dips of faults at varying depths up to 4 km are calculated for both unsaturated dry and saturated rocks the dips of normal faults are expected to vary from 90 at the surface to 60 at 3 4 km and this variation is consistent with the limited field data available the coalfield data are consistent with fault formation at standard hydrostatic pore fluid pressures at depths of about 2 5 km below depths of a few kilometres normal faults are expected to flatten out gradually to a dip of 45 but such flattening is not necessarily an indication of an underlying detachment

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

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

fault and bed rotation during continental extension block rotation or vertical shear abstract for almost a century the view has existed that the tilting of blocks between closely spaced planar normal faults is rigid body rotation this interpretation requires only simple geometry and has consequently found widespread application however it is not consistent with the deformation expected around normal faults given the present knowledge of stress fields and rheology in basement in the brittle upper crust which is better regarded instead as distributed vertical simple shear rigid body rotation and vertical shear involve different relations between fault and bed tilting and thus predict different initial fault dips for particular present day dips of faults and beds these two schemes also predict different amounts of extension and it is consequently important to establish which is correct with this aim in mind we examine normal faults associated with neogene extension in western turkey and the western united states and with mesozoic extension in the north sea except where extension and the associated tilting are minimal rigid body rotation predicts unrealistically steep initial fault dips some extensional basins also exhibit reversals of normal fault polarity and tilt polarity of beds which are incompatible with rigid body rotation we therefore conclude that the general cause of the tilting is vertical shear not rigid body rotation this has three main observational consequences first the heave on any normal fault equals the amount of extension across it second no feature near a normal fault can rotate through the vertical a normal fault thus cannot rotate through the vertical and appear as a reverse fault third any initially vertical feature near a normal fault will remain vertical a vertical dyke in the tilted surroundings of a normal fault is thus not necessarily younger than the extension that caused the tilting

strain and scaling of faults in the chalk at flamborough head u k analysis has been made of the orientations displacements and spacings of 1340 extensional faults with displacements of up to 6 m along an almost completely exposed 6 km length of cliff this data set has been used to study how fault populations account for strain in a region and to study relationships between different scales of fault strains have been estimated the maximum and intermediate extensions are sub horizontal with approximately equal extension e 0 01 in all horizontal directions it can be inferred that the minimum extension maximum compression was sub vertical but that the wide variety of fault orientations and cross cutting relationships resulted from variable horizontal extensions some faults have oblique slip slickenside lineations which imply a period of later dominantly nnw sse extension which possibly developed as the exposure scale faults linked up e w striking larger scale normal faults effectively forming a single wide fault zone graphs of displacement per unit distance are used to illustrate variations in displacement the scaling of fault displacement appears to follow a power law relationship the differences in orientation between the small scale and large scale faults precludes a simple estimation of the total strain over all scales

relations between normal fault geometry tilting and vertical motions in extensional terrains an example from the southern gulf of suez abstract earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern gulf of suez are approximately planar with dips of 30 40 from the surface to around 10 km depth these faults and the blocks they bound appear to rotate about a horizontal axis as they move causing tilting this tilting is seen both in young vertical movements of the coastline such as raised beaches and marshlands and in the distribution of middle miocene marine rocks which are uplifted to elevations of 400 500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens the absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw tooth topography on a regional subsidence caused by crustal and lithospheric thinning the observations required for this simple model are the present day fault dip the amount of tilting and the width of the rotating blocks the virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents

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

quantitative analysis of populations of small faults abstract tectonic strain includes contributions from small faults with length less than the thickness of the brittle upper crust this study derives algebra to determine the strain associated with small faults that follow a power law i e fractal distribution independent of assumptions concerning how they relate to large faults in the same region and discusses how to apply this algebra for a range of small fault sample types the method is illustrated using as case studies the jurassic extension in the north sea the miocene and younger extension in the gulf of suez and the hercynian strike slip faulting the badajoz cordoba fault zone in southern spain my preferred estimates for the strain accommodated on small normal faults with length

extensional fault systems in sedimentary basins a review of analogue model studies abstract scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems this paper reviews recent results of an on going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems four fundamental model styles scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust are reviewed extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults these experimental results are directly comparable to structures found in intracontinental rifts extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino style fault arrays in the pre rift sequence and listric growth faults in the syn rift sequence these results compare with highly extended rift basins and terranes such as the basin and range province usa extension above a simple listric fault produces a characteristic hangingwall roll over and crestal collapse graben system at high values of extension superposed crestal collapse grabens occur and a fan of listric growth faults is developed these structures bear striking similarities to growth faults in progradational delta systems ramp flat listric extensional fault systems using sand mica models produce a characteristic family of structures with an upper roll over crestal collapse system a ramp syncline and reverse fault fold zone and a lower roll over crestal collapse system an example of a ramp flat fault system from the gulf coast usa is presented as a comparison with the analogue model results preliminary results of three dimensional extensional model experiments are presented to illustrate the complexities in such fault systems in all of the experimental models it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall the analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins