estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

strain partitioning and stress perturbation around stepovers and bends of strike slip faults numerical results abstract stepovers and bends along strike slip faults are where push up ranges and pull apart basins are formed they are also commonly where fault ruptures terminate field study and analogue models suggest that the configuration of faults plays a key role in crustal deformation around bends and stepovers but the related mechanics of stress perturbation strain partitioning and fault evolution remains poorly understood here we present results of systematical mechanical models of stress changes and strain partitioning around simple stepovers and bends using three dimensional viscoelasto plastic finite element code our model predicts elevated deviatoric stress around all stepovers and bends with higher stresses around the restraining ones narrow stepovers localize strain between the fault gaps to form connecting faults whereas wide stepovers localize strain on the tips of fault segments so the stepovers may evolve into subparallel faults we explored how various configurations of stepovers and bends change the stress field and strain distribution and show that these results can help explain some key differences between the pull apart basins in the dead sea trough and death valley and the push up ranges along the san andreas fault

anatomy and evolution of a pull apart basin stellarton nova scotia the stellarton basin is a late paleozoic pull apart basin located close to the meguma avalon terrane boundary in the canadian appalachians at the stepover between the cobequid and hollow strike slip faults the basin contains 3 km of rapidly deposited pennsylvanian clastic sedimentary rocks representing lacustrine and deltaic environments extensively documented through coal related mining and drilling coal seams and oil shales allow stratigraphic correlation within the basin permitting reconstruction of basin subsidence and structural evolution coal seams represent approximately paleo horizontal surfaces thickness variations corrected for tilt and compaction in the most coal rich part of the basin fill show that the south basin margin subsided rapidly during deposition acting as a trap for coarse sediment and allowing coal forming mires to develop in the north abundant soft sediment deformation structures reflect synsedimentary tectonic activity north striking normal faults dissected the basin fill during and soon after deposition early in the diagenetic history contouring of mine plans allows fault heaves to be identified and also shows that both coal seams and faults were folded by east to northeast trending folds consistent with 1 an environment of deformation involving dextral strike slip motion and 2 clockwise rotation of fault blocks during progressive strain later north northeast striking contractional structures within the basin and a positive flower structure at the northwest margin probably represent a transition from transtension to transpression the basin illustrates the role of rotational progressive strain in the reorientation of structures generated during transtension it displays evidence for changes in tectonic style through time from transtension to transpression that may typify basins developed on major strike slip faults

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

new aspects of rhomb structures abstract the acute angles of 48 rhombic trapezoidal and triangular grabens horsts and upthrusts throughout the world were determined the results indicate an angular spread of 7 73 in most structures the two acute angles differ considerably most rhomb structures are grabens which show affinities to seismic faults and only a few are horsts and upthrusts in this connection modes of static crustal fracture propagation are compared to dynamic propagation rhomb structures occur along strike slip faults and in rifts these results question the universality of the pull apart model as a mechanism for rhomb structure development the two mechanisms that can lead to the formation of rhomb structures are 1 interaction of en echelon or non coplanar fractures by shear and extensional modes and 2 fracture bifurcation in an extensional mode the two mechanisms may result in faults of different shapes characterized by curved and straight boundaries respectively the grabens and horsts in extensional regions are the consequence of early fracture and later vertical displacements

rapid creation and destruction of sedimentary basins on mature strike slip faults an example from the offshore alpine fault new zealand abstract seismic reflection profiles and multibeam bathymetric data are integrated to analyse the structure of the 25 km long strike slip dagg basin associated with the marine section of the alpine fault fiordland new zealand the basin is developing in almost 3000 m water depth at a releasing bend whilst contemporaneous transpression results in inversion of its southern end fiord derived glacio marine sediments flooded the basin during the last glaciation and provide a stratigraphic framework for structural analysis geometrical analysis enables an estimation of 450 1650 m of dextral displacement on the alpine fault at the releasing bend since the development of an unconformity estimated to have formed at between 30 and 110 ka this implies a dextral slip rate ranging from a possible minimum of 4 mm yr to the maximum of 35 mm yr constrained by the pacific australian plate motion rate despite total dextral displacement of 480 km on the alpine fault zone and a growth history spanning 15 20 myr this geomorphically well expressed and structurally complex strike slip basin developed and evolved rapidly during the late pleistocene and thus represents only the latest phase in the evolution of the alpine fault upward splaying structures within the fault zone exhibit a rapid spatial evolution in pleistocene strata which may reflect the interactions between high fault slip rate voluminous sedimentation supply inherited structural complexities in the basement rocks and deeper cover sequence and interactions between adjacent faults the present through going releasing bend at the northern end of the basin may have evolved from a more complex pull apart basin that developed between separate segments of the alpine fault the results from dagg basin illustrate the rates at which structural complexities and sedimentary basins can develop within highly active very mature through going continental wrench faults strike slip basins on the scale of 40 80 km 2 on such faults may be ephemeral features that can be developed and destroyed on a time scale of 10 5 10 6 years

modelling the influence of pre existing brittle fabrics on the development and architecture pull apart basins abstract we use new analogue modelling experiments to analyse the development of pull apart basins in an upper crust characterised by the presence of pre existing discrete fabrics as in previous models lateral movement of rigid basal plates induced strike slip deformation of a sand pack local extension allowing the formation of a pull apart basin was produced by the step over geometry of the master faults in this area a basal silicone layer was introduced to distribute deformation and reproduced a weaker crust in the basin conditions of neutral overlapping and underlapping interacting master faults were reproduced the model upper crust modelled by a sand mixture was characterised by the presence of pre existing structures the orientation of these inherited heterogeneities was systematically varied in different experiments model results indicate that depending on their orientation with respect to the strike slip displacement pre existing structures can be reactivated both within and at the margins of the pull apart basins inside the basin reactivation occurs when the pre existing structures are orthogonal or sub orthogonal to the strike slip displacement in this case the pre existing fabrics delay the development and linkage of cross basin faults and increase the complexity of the deformation pattern giving rise to a new set of faults characterised by an atypical trend pre existing fabrics oblique to the local extension direction may be partly reactivated in the central part of the basin as segments of cross basin faults at the margins of the pull apart reactivation occurs if the fabrics spatially coincide with the lateral boundaries of the silicone layer in these conditions reactivation allows a faster development of the border faults which are less segmented than in the homogenous models this also results in a more regular final geometry of the pull apart

experiments on simultaneous faulting and folding above a basement wrench fault abstract wrench zones commonly show en echelon patterns of folds in the stratified cover above a basement fault examples are the darien basin in panama and the inglewood fault trend in california folds are often associated with synthetic and antithetic wrench faults but the time relationships between folds and faults may not be obvious we have done experiments where simultaneous faulting and folding occur above a basement fault four experiments were performed using a simple apparatus capable of imposing localised strike slip motions this apparatus consists of two plastic plates one can be moved laterally past the other by a piston thus modelling lateral motion on a single basement fault models were properly scaled to account for gravitational forces brittle behaviour of sandstone and ductile behaviour of clay or shales in the upper crust each model is a vertical succession of silicone layers and sand layers each layer is 2 mm thick except for the basal silicone layer which is 2 4 6 or 8 mm thick in experiments 1 2 3 or 4 respectively all models were deformed at the same slip rate on the basal fault 1 cm h 1 experiments 1 2 and 3 with 8 cm total horizontal sinistral offset were designed to investigate the influence of basal silicone layer thickness on the deformation of a multilayered model experiment 4 with 20 cm of total offset was designed to investigate the influence of an increasing left lateral offset on folding and faulting with a 2 mm thick basal silicone layer experiment 1 synthetic strike slip faults appeared en echelon above the basement fault no folds were visible at the surface of the experiment but the interfaces between sand and silicone layers showed millimetric folds en echelon above the basement fault with a 4 mm thick basal silicone layer experiment 2 en echelon synthetic strike slip faults and folds appeared above the basement fault faults and folds rotated progressively anticlockwise and synclines were activated as dextral antithetic shear zones with a 6 mm thick basal silicone layer experiment 3 synthetic faults and en echelon folds appeared as before but the fold amplitude was larger than in experiment 2 an increasing left lateral offset produced anticlockwise rotation of faults and folds as before and synclines were activated as dextral antithetic shear zones in experiment 4 after 16 cm of left lateral offset the anticlockwise rotation of the folds was such that antithetic dextral shear zones became almost parallel to the basement fault and were then reactivated as synthetic sinistral shear zones

large scale erosional response of se asia to monsoon evolution reconstructed from sedimentary records of the song hong yinggehai and qiongdongnan basins south china sea

computerized x ray tomography analysis of sandbox models examples of thin skinned thrust systems computerized x ray tomography applied to analog sandbox experiments performed in a normal gravity field makes possible the analysis of the kinematic evolution as well as the three dimensional geometry of models that simulate tectonic deformations most of the plastic or viscous analog materials generally used in a normal gravity field for such models have x ray attenuations compatible with medical scanner images models using soft cohesionless granular materials were studied faults in granular materials corresponding to dilatant zones with lower by 30 50 x ray attenuation are thus clearly visible in the tomography images kinematic analysis was done on imbricate thrust propagation in materials with an internal friction angle of 30 thrust propagation is controlled mainly by a triangular pop up structure resulting from the nucleation of two conjugate thrusts at the basal decollement the pop up structure is created immediately in front of the previous thrust three dimensional geometry analysis was tested on lateral ramps parallel to the transport direction lateral ramps were induced by changing the frictional boundary conditions at the bottom of the model though the lateral ramps strike almost parallel to the transport direction their dip angle is comparable to the dip angle of frontal ramps

complex geometry and segmentation of the surface rupture associated with the 14 november 2001 great kunlun earthquake northern tibet china abstract the 14 november 2001 kunlun china earthquake with a moment magnitude m w 7 8 occurred along the kusai lake kunlun pass fault of the kunlun fault system we document the spatial distribution and geometry of surface rupture zone produced by this earthquake based on high resolution satellite landsat etm aster spot and ikonos images combined with field measurements our results show that the surface rupture zone can be divided into five segments according to the geometry of surface rupture including the sun lake buka daban hongshui river kusai lake hubei peak and kunlun pass segments from west to east these segments each 55 to 130 km long are separated by step overs the sun lake segment extends about 65 km with a strike of n45 75 w between 90 05 e 90 50 e along the previously unrecognized west sun lake fault a gap of about 30 km long exists between the sun lake and buka daban peak where no obvious surface ruptures can be observed either from the satellite images or field observations the buka daban hongshui river kusai lake hubei peak and kunlun pass segments run about 365 km striking n75 85 w along the southern slope of the kunlun mountains between 91 07 e 94 58 e this segmentation of the surface rupture is well correlated with the pattern of slip distribution measured in the field detailed mapping suggest that these five first order segments can be further separated into over 20 second order segments with a length of 10 30 km linked by smaller scale step overs or bends our result also shows that the total coseismic surface rupture length produced by the 2001 kunlun earthquake is about 430 km excluding the 30 km long gap which is the longest coseismic surface rupture for an intracontinental earthquake ever recorded finally we suggest a multiple bilateral rupture propagation model that shows the rupture process of the 2001 m w 7 8 earthquake is complex it consists of westward and eastward rupture propagations and interaction of these bilateral rupture processes

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

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

sequence stratigraphy basin dynamics and petroleum geology of the miocene from eastern tunisia on the eastern margin of tunisia miocene limestones marl and siliciclastic deposits crop out poorly and are lacking in age diagnostic faunal content the biostratigraphic and lithostratigraphic subdivisions of these series are not clearly defined a regional study of subsurface sequences of this margin cap bon gulf of hammamet and sahel by means of sequence stratigraphy and subsurface structural analyses permits the identification of seven third order sedimentary sequences of inferred langhian to messinian age the boundaries of which are characterized by downlap and onlap toplap relationships these sequences include turbidites sands and sandstones deposited in connection with eustatic sea level changes and tectonic movements of east west and south north deep se ted faults due to the alpine and atlassic paroxysm stratal sequences are organized around miocene syndepositional grabens half grabens platforms and folds occurring inside and outside of regional tectonic corridors the geodynamic evolution of miocene basins has led to the deposition of turbiditic black argillaceous source rocks and sandstone and carbonate reservoirs that present new miocene petroleum targets the basin subsidence in response to the alpine atlassic orogeny has permitted the maturation of miocene source rocks oil generation and the formation of oil traps stratigraphic pinch outs and structural enclosures on the flanks of folds and on the borders of grabens

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

the anatomy of a pull apart basin seismic reflection observations of the dead sea basin the dead sea basin dsb located along the african arabian plate boundary constitutes a good example of a pull apart basin because of its large dimensions its structural simplicity and its active subsidence a coherent three dimensional picture of the dsb has been constructed on the basis of analysis of seismic stratigraphy together with the interpretation of previously published geological and geophysical data despite the large known vertical offsets across the basin deformation takes place mainly along the transverse and longitudinal faults and the intervening sediments are relatively undeformed and are hardly tilted comparison between e w seismic lines indicate that the basin has widened by the collapse and tilting of arcuate blocks from the western margin but that its original shape is a full graben the southern and central parts of the basin are divided into equidistant segments 20 30 km long by transverse faults activity along these faults commenced only during the pleistocene long after the dead sea strike slip fault system was formed and migrated northward with time a likely scenario for the development of the dsb is one in which the basin grows northward with time by a simultaneous propagation of the southern strand of the dead sea fault and a retardation of the northern strand

structural localization and origin of compartmentalized fluid flow comstock lode virginia city nevada bonanza grade orebodies in epithermal style mineral deposits characteristically occur as discrete zones within spatially more extensive fault and or fracture systems empirically the segregation of such systems into compartments of higher and lower permeability appears to be a key process necessary for high grade ore formation and most commonly it is such concentrations of metals that make an epithermal vein district world class in the world class silver and gold producing comstock mining district nevada several lines of evidence lead to the conclusion that the comstock lode is localized in an extensional stepover between right lateral fault zones this evidence includes fault geometries kinematic indicators of slip the hydraulic connectivity of faults as demonstrated by veins and dikes along faults and the opening of a normal fault bounded asymmetric basin between two parallel and overlapping northwest striking lateral to lateral oblique slip fault zones during basin opening thick generally subeconomic banded quartz adularia veins were deposited in the normal fault zone the comstock fault and along one of the bounding lateral fault zones the silver city fault as deformation continued the intrusion of dikes and small plugs into the hanging wall of the comstock fault zone may have impeded the ability of the stepover to accommodate displacement on the bounding strike slip faults through extension within the stepover a transient period of transpressional deformation of the comstock fault zone ensued and the early stage veins were deformed through boudinaging and hydraulic fragmentation fault motion inversion and high and low angle axial rotations of segments of the fault planes and some fault bounded wedges this deformation led to the formation of spatially restricted compartments of high vertical permeability and hydraulic connectivity and low lateral hydraulic connectivity bonanza orebodies were formed in the compartmentalized zones of high permeability and hydraulic connectivity as heat flow and related hydrothermal activity waned along the comstock fault zone extension was reactivated in the stepover along the occidental zone of normal faults east of the comstock fault zone volcanic and related intrusive activity in this part of the stepover led to a new episode of hydrothermal activity and formation of the occidental lodes

three dimensional geometry and kinematics of experimental piggyback thrusting the three dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box strike parallel geometric elements simulated include lateral ramps eyed sheath folds splays and thrust thrust interference fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum eroding technique a kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport despite the steady forward advance of a rear wall the piggyback wedge accreted episodically recording different strain domains in longitudinal cross sections strain partitioning in single layers by bed length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth

three dimensional modeling of pull apart basins implications for the tectonics of the dead sea basin we model the three dimensional 3 d crustal deformation in a deep pull apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the dead sea basin the brittle upper crust is modeled by a boundary element technique as an elastic block broken by two en echelon semi infinite vertical faults the deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress free shear zone in the vicinity of the fault zone the bottom displacement represents the regional relative plate motion results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin bounding faults as the width of the shear zone increases the depth of the basin decreases the rotation around a vertical axis near the fault tips decreases and the basin shape the distribution of subsidence normalized by the maximum subsidence becomes broader in contrast two dimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone our models also predict full graben profiles within the overlapped region between bounding faults and half graben shapes elsewhere increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin we suggest that the observed structure of the dead sea basin can be described by a 3 d model having a large overlap more than 30 km that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km

the dinevar transtensional pull apart basin nw zagros mountains iran a geological study and comparison to 2d finite element elastic models the dinevar transtensional pull apart basin is a nw se trending basin located in the northwest part of the zagros mountains sanandaj sirjan tectonic zone iran this basin was formed by oblique extension in a strike slip fault system which was controlled by the pre existing sahneh and the mianrahan faults in addition to a field study we constructed a 2d finite element elastic model based on the major fault segments according to the field study various geological structures are formed around the pre existing fault segments including pencil structures with 13 aspect ratio 19 tight to open asymmetric folds oblique normal faults synthetic and antithetic faults normal and reverse fault drags and negative and hybrid type flower structure fe model results show that around the fault step is a zone of displacement gradient that indicates extension transtension within the overlapping zone consequently subsidence is predicted over the releasing fault step the modelled stress distributions within the fault step and around the fault segment tips are asymmetric elastic modelling shows that the mean normal stress within the step is tensional which would impede plastic yield while the region outside the step is characterised by compressive mean normal stress the mean normal stress pattern is characterised by a regional minimum localised inside the extensional step transtension zone and two local maximum and minimum couples formed at the advancing and retreating sides respectively in the fault segment tips the results show that two main subsidence regions depocentres or sub basins form along and in the proximity of the fault segment tips the imposed relative motion is entirely accommodated by creep on the faults which causes plastic strain to accumulate outside the faults mostly localised within the fault step this localised strain between fault segments could initiate connecting faults or reactivate earlier faults generally at high angles relative to the master fault segments as suggested by the stress and strain patterns the rotation and magnitude variation in the stress and strain trajectories reflects heterogeneous deformation in general the dinevar transtensional pull apart basin is characterised by an elongate rhomboidal to trapezoidal shaped geometry the modelled stress distribution and strain localisations within the basin are in general agreement with the locations of fault segments within the dinevar transtensional pull apart basin

step over and bend structures along the 1999 duzce earthquake surface rupture north anatolian fault turkey the 12 november 1999 duzce earthquake m w 7 1 which occurred on the duzce fault of the north anatolian fault zone formed a 40 km long surface rupture which extended between golyaka in the west and kaynasli in the east the earthquake nucleated close to the beykoy step over a 0 8 km wide and 1 8 km long pressure ridge with bilateral rupture propagation both to the west and the east the beykoy kaynasli and cevizlik step overs through which the 1999 duzce rupture propagated are less than 2 5 km wide the eften and bakacak step overs where the rupture was arrested are more than 4 km wide these exceptional dimensions of step overs associated with the 1999 duzce rupture were investigated for their influ ence on rupture propagation the earthquake broke three distinct geometric segments from west to east the eften dagdibi and kaynasli fault segments separated by left restraining step overs at beykoy and kaynasli to the west of the main shock 3 5 m of average slip on the eften fault segment propagated into the 2 km wide cevizlik releasing double bend where it was largely arrested with smaller 0 5 m slip continuing westward on the golyaka intervening segment west of lake eften this is similar to the ter mination of slip observed on the karadere fault segment in the 1999 izmit earthquake where 1 5 m of slip was largely terminated by a releasing double bend at the golyaka intervening segment with minor slip continuing eastward near lake eften to the east of the main shock 5 m of slip decreased along the dagdibi fault segment to about 3 5 m at the 1 km wide kaynasli step over from here to the east the 2 5 m of average slip on the kaynasli fault segment continued to decrease reaching zero at the releasing step overs to the elmalik fault the terminations of the 1999 duzce surface rupture are defined by distinct struc tural complexities the rupture was arrested in the west at the 4 km wide eften releasing double bend where the eastern termination of the 1999 izmit rupture also occurred to the east the rupture terminated at the 4 to 5 km wide bakacak releasing double step overs both eastern and western ends of the duzce rupture have short fault segments that experienced minor rupture during the 1999 duzce earthquake the golyaka intervening segment experienced minor rupture during both the 1999 izmit and duzce earthquakes the duzce rupture propagated across 0 8 to 2 km wide step overs but it was arrested at step overs 4 km wide or wider these results agree with the observations on ratios between step over width in kilometers and strike slip displacement in meters collected along strike slip faults worldwide let tis et al 2002

how normal faulting and sedimentation interact to produce listric fault profiles and stratigraphic wedges abstract in a series of scale experiments we generated listric normal faults by extending models composed of colored sand layers deposited at regular time intervals throughout deformation during syntectonic sedimentation progressive bulk extension along a single set of faults caused domino type tilting of fault blocks underlying tilted faults propagated upwards with constant dips into newly sedimented layers hence fault profiles become progressively more listric tilting of the free surface resulted in asymmetrical depressions and in fan shaped deposits stepwise or continuous geometric models of domino tilting and synchronous sedimentation yield the same structural and stratigraphic features as observed in the sand models the continuous geometric model shows that a critical parameter governing fault curvature is the ratio r between rates of bulk strain and sedimentation both types of models reveal that synchronous sedimentation and bulk extension are capable by themselves of generating listric normal faults

modelling the geometry of coulomb thrust wedges internal geometry and detachment of coulomb thrust wedges is examined in a combined gravity and pressure force field under various boundary conditions and coulomb parameters frontally buttressed sand wedges accreted with narrow tapers above soles with negligible friction in these models serial imbricates stepped up in conjugate sets which operated synchronously or serially with increases in sole friction model coulomb wedges accreted in piggyback style with enhanced taper but narrower spacing of imbricates in this mode flat topped box fold anticlines nucleated at the frontal tip of a sole thrust prior to the fore limb failing as a ramp the initial ramp thrusts subsequently rotated and steepened to acquire concave upwards listric geometry when cohesion was a significant component of strength and the sand grains interlocked thoroughly ramp climb during piggyback thrusting occurred through the generation of a stack of backthrusts the taper of cohesionless sand wedges which lack any length scale was consistent with the critical cohesionless coulomb wedge taper in contrast the taper of experimental cohesive sand wedges which are scale dependent was consistent with the theoretical cohesive coulomb narrow taper only when the cohesive length scale was lr sr rar 10 6 this was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass

four dimensional tectono stratigraphic evolution of the southeastern peri tyrrhenian basins margin of calabria italy abstract we use seismic reflection profiles of diverse resolutions integrated with high resolution bathymetry and calibrated by deep wells piston cores and field data to derive a structural and stratigraphic interpretation of the southeastern peri tyrrhenian basins the study area is located in the southeastern tyrrhenian sea offshore calabria we document a tectono stratigraphic evolution of the sedimentary basins from the serravallian to the present the basins formation started during the serravallian and the area was successively affected by e w normal faulting and approximately n s transfer faults in the messinian lower pleistocene time range during the lower pleistocene the tectonic depressions completely filled with sediments were affected by folding associated with transcurrent tectonics along a nw trending left lateral fault finally during the middle pleistocene a change in the slip direction produced minor pull apart basins that overprinted the folds the basin analysis revealed the stratigraphic signature of the tectonic events and the uplifting and subsiding areas and the direction from which sediments arrive into the tectonic depression the four dimensional geologic reconstruction of the margin of calabria furnished new constrains for the kinematic evolution of the southeastern peri tyrrhenian basins we suggest that the superposition of different structural styles reflects a variation of the relative velocity between the southern apennine arc and the calabria arc during the formation of the apennine thrust belt

extinction of pull apart basins numerous pull apart basins are present along the haiyuan fault zone in northwestern china detailed geological mapping of these basins reveals an evolution of faulting and subsidence that leads toward extinction of some pull apart basins the most common cause of pull apart basin extinction appears to be the development of new strike slip faults along one of the basin margin normal faults or diagonally across the basin the strike slip faults may develop together with normal faults to form a complex zone within the basin the migration of boundary strike slip faults toward the basin center can also cause the extinction of pull apart basins the extinction of these pull apart basins suggests that there is probably a tendency for a strike slip fault zone to straighten itself and the examples show how this takes place

2 45 ga break up of the archaean continent in northern fennoscandia rifting dynamics and the role of inherited structures within the archaean basement abstract constraints from restoration of minor rift related faults preserved from subsequent palaeoproterozoic overprint along the southern margin of the perapohja belt northern fennoscandia indicate local ne sw extension directions prevailed at the time of 2 44 ga rifting of the archaean continent this palaeostress field is attributed to pull apart setting of the perapohja belt at a left overstepping zone of major sinistral n s trending deformation zones consequently the variably overprinted nw se trending structures of the perapohja belt including the nw se central graben were originally generated as normal faults the proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step over zone between strike slip faults by contrast the faults bounding e w to ene wsw trending basement horsts are the result of reactivation of older basement structures underlying the pull apart basin based on the results we infer that the final break up of the archaean continent in northern fennoscandia utilized the 2 44 ga rift related nw se trending crustal anisotropy parallel with dyke swarms with pre existing archaean n s trending structures reactivated as second order shear zones in between the voluminous syn rift 2 5 2 4 ga and later pulses of mafic magmatism 2 32 1 98 ga are considered indicative of active type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the superior hearne and karelian kola cratons the presented model supports the continuity of the archaean craton beyond the caledonian orogen towards north west and explains its segmentation suggesting a model for the localization of the palaeoproterozoic supracrustal belts of northern fennoscandia moreover this paper provides an approach to understand the basement cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits

lithofacies and depositional environments of miocene deposits from tectonically controlled basins red river fault zone northern vietnam abstract the lao cai yen bai and bao yen basins connected with the red river fault zone rrfz are filled with over 1000 m thick neogene miocene clastic deposits of different type and origin coarse grained alluvial deposits predominate along the south western margin of the lao cai and yen bai basins while different types of conglomerates and sandstones even siltstones occur in their central and along their north eastern parts such asymmetry in the cross section basin fill points to the south western active margin of these basins three main facies associations composed of 12 lithofacies have been recognized in the miocene succession filling the studied basins 1 debris flow dominated alluvial fan association 2 sheetflood dominated alluvial fan association and 3 lacustrine association both alluvial fan facies associations are broadly spread in the studied basins especially along their south western margins the debris flow dominated deposits are represented by decimetre to several meters thick sand and mud supported disorganized pebble to cobble size occasionally boulder breccia and conglomerate beds the sheetflood dominated deposits are represented by alternating planar bedded pebble and cobble gravel couplets the occurrence of conglomerate bodies within and overlapping fine deposits of the lacustrine facies association may suggest deposition in a debris flow and sheetflood dominated alluvial fan setting associated with narrow elongated basins that could be occupied by lakes and bordered by active fault scarps the facies pattern of the particular basins is connected with the evolution of the rrfz the origin of these basins may be correlated with a sinistral transtensional regime which occurred in the early to middle miocene moreover the high grade metamorphic core of the day nui con voi massif presently the main structural element of the rrfz was probably not exposed during sedimentation of most of the strata in the studied basins

northward migration of the southern diagonal fault of the hula pull apart basin dead sea transform northern israel abstract the hula valley located in northeastern israel is a 4 million year old pull apart basin bounded by two left stepped segments of the dead sea transform basic models of pull apart basins suggest four faults at all sides of the basin a situation which is not always the case in the hula pull apart basin the longitudinal n s faults are clearly evident while the transversal e w faults are less explicit an nw oriented lineament expressed as a north facing continuous scarp 1 3 m high was recognized in the southern part of the valley morphological seismic and sedimentary considerations suggest that this scarp is the surface expression of a deep seated fault this fault termed the yesod hama ala fault forms the present southern boundary of the hula pull apart basin and appears to be the southern diagonal fault which links the jordan fault in the east with the yammunneh fault in the west it is suggested that this fault scarp formed a shallow morphological barrier that partly blocked the southern outlet of the valley enabling a permanent lake in the southern part of the hula valley thus the uplifted southern block determined the sw shoreline of the lake it is proposed that the hula basin is a version of the classic pull apart model it is also suggested that in the absence of upwelling mantle material and extension along the length of the basin a crustal thinning and subsidence might have occurred through normal faulting of the upper brittle crust the thick depositional sequence within the southern part of the basin shows that the present margins have been a major depositional site for a long time and that the hula valley has thus developed as an asymmetric pull apart rhomb shaped basin bounded by steep north striking boundary normal as well as strike slip faults w nw striking and northward dipping normal faults and a south facing bending flexure as activity within the basin continued and northward migration of the faulting occurred a new fault developed according to this perception the present active fault is the yesod hama ala fault while the future fault will be located northward as can be presumed from the present micro seismic activity in the valley

identification of wrench faults using subsurface structural data criteria and pitfalls 1 many structures imaged with reflection seismic can resemble a wrench fault system because of the great structural diversity of strike slip faults and the wide range of contractional and extensional features associated with these faults because of these resemblances numerous faults are misidentified as wrench faults by workers using subsurface structural data credible identification requires combinations of profile and map criteria 1 narrow long straight throughgoing solitary master fault or zone of deformation 2 steep to moderate dip of the master fault at depth 3 offset of the top of basement 4 changes in relative upthrown side separation sense and or fault dip direction at depth or along the strike of the master fault 5 narrow fault slices within the master fault zone that steepen and join at depth negative and positive flower structures 6 different separation sense and orientation of relative upthrown side at these fault slices and 7 coeval en echelon flanking structures the identification should be confirmed with other criteria that refute alternative interpretations the main pitfalls to a reliable identification are not considering alternative structural styles and the misinterpretation of features or patterns that superficially resemble wrench systems the profile geometries of basement involved reverse or normal faults particularly those with structural inversion can resemble the single strand of convergent or divergent wrench faults profile geometries of flower structures can be mimicked by narrow contractional horsts or narrow extensional grabens complexly faulted anticlines or synclines splay faults fault zones intruded by salt or shale and overprinted structures pitfalls in interpreting map relationships are possible when structural patterns combine several tectonic influences are controlled by rock anisotropy or include elements hat are not coeval and if the patterns characteristic of other styles are misinterpreted many erroneous identifications of wrench faults can be eliminated by demonstrating that the investigated fault or feature lacks a straight trace lateral length or throughgoing continuity

tectonic evolution of the penglai 7 6 structure and its implications for petroleum exploration in a releasing bend of a strike slip fault system tan lu fault zone east china abstract the penglai 7 6 pl7 6 structure is located on the tan lu fault zone a huge fault zone in east asia tectonism of the tan lu fault zone produced complex structural deformation and was an important factor in petroleum accumulation three dimensional seismic data were employed to document the structural geometry and evolutionary process of the pl7 6 structure and implications for petroleum exploration in a releasing bend of a strike slip fault system were discussed systematic analysis of the seismic data indicates that the main fault the bd2 fault of the tan lu fault system slipped right laterally during the cenozoic strike slip movement of the bd2 fault peaked during deposition of e 1 2 k to e 3 d 3 and decreased largely in intensity during e 3 d 1 2 deposition in the neogene and quaternary the bd2 fault was reactivated formation of the bodongnan low uplift is related to the connection between the bodongnan low uplift n and bodongnan low uplift s the bodongnan low uplift n is represented by an anticline produced by strike slip movement of the bd2 fault and the bodongnan low uplift s is represented by the relatively uplifted strata in the vicinity of an extensional fault the bd3 fault the pl7 6 structure developed above a releasing bend formed by the subsequent strike slip movement of the curved bd3 fault orientations of the shallow faults in the pl7 6 structure are not only associated with the lower bd3 fault but also controlled by the basement high unlike traditional releasing bends that are generally disadvantage to petroleum accumulation due to the lower structural elevation in the center the releasing bend in the pl7 6 structure is favorable for petroleum accumulation because of the three reasons 1 adjacent to generative kitchens 2 located on a regional structural high and 3 served by the releasing bend related faults acting as conduits for vertical petroleum migration

effects of oceanic ridge subduction on accretionary wedges experimental modeling and marine observations sandbox modeling is used to study the deformation of accretionary wedges caused by the subduction of oceanic ridges the first experiment incorporates a massive ridge within a sand wedge the wedge thickens and shortens when the forward propagation of the basal decollement ceases the wedge thickening results in taper change reactivation of preexisting thrusts and retreat of the frontal part of the sand wedge similar mechanisms may have affected some margins that have undergone ridge subduction such as the tonga margin after the subduction of the oblique louisville oceanic ridge the second experiment shows the effects of an active basement thrust slice as it enters a subduction zone this process may have happened in the eastern nankai accretionary wedge initially the wedge in this experiment behaved similarly to that of the first experiment rapidly the topographic slope changed the wedge thickened above the basement slice generating a slope break in the topography a deeper propagating accretionary wedge again characterized by a small taper developed these results when compared with observations made in the eastern nankai trench are in agreement with the past subduction of a basement thrust slice in this area

fine scale partitioning of contemporary strain in the southern walker lane implications for accommodating divergent strike slip motion treating the crust as a micropolar continuum and inverse modeling a high quality catalog of earthquake focal mechanisms reveals that km scale heterogeneous strain is partitioned at km scales into pairs of plane strains with sub perpendicular principal axes these earthquakes occurred in a shallow crustal volume centered on the western indian wells valley of eastern california the site of divergent right lateral strike slip motion along the boundary between the sierran microplate and the southern basin and range province the seismic events defining the plane strains occur in a diffuse seismic source zone and cannot be additionally subdivided on the basis of the type of strain they accommodate the results indicate that seismic moment release for background earthquakes is partitioned into events associated with either fault zone perpendicular extension by normal faults or horizontal shearing by conjugate strike slip faults the spatial scales over which this partitioning occurs suggest that outcrop scale brittle faults may in some cases record nearly perpendicular plane strains related to a single continuous deformation

fault structure and related basins of the north aegean sea and its surroundings 1 fault structure and basin evolution in the northern aegean sea and its surroundings have been investigated using bathymetry available onshore and offshore seismic profiles onshore fault patterns from ne greece nw turkey three islands in the aegean sea surface ruptures associated with the 1978 thessaloniki earthquakes gps measurements from north aegean islands and a number of earthquake focal plane solutions and historic seismicity bathymetric and isopach data show that the north aegean trough can be divided into the sporades basin and the saros trough post miocene sediment thickness differences in these basins offset of distinct horizons and frequent seismicity along basin bounding faults indicate that sediment accumulation in the area is fault controlled offshore seismic profiles show that the basin bounding faults are dominantly characterized by apparent normal components as well as strike slip components inferred from typical fault zone profiles additional evidence for existence of strike slip faulting comes from earthquake focal plane solutions we refined the pattern and kinematics of the faults controlling the basins and classified them into two major groups the faults in the first group trend ne sw and ene wsw and are here interpreted to be dominantly right lateral strike slip with normal or reverse components these faults accommodate a similar amount of deformation like the ganos fault and thus represent the westernmost extension of the north anatolian fault zone the faults in the second group trend nw se and are dominantly normal faults with an occasional left lateral component these faults are believed to manifest the termination of the north anatolian fault zone because of the segmentation of the right lateral strike slip faults and the preexisting ne trending tectonic grain the basins are compartmentalized into smaller depocenters basins are orientated with their long axes either parallel to the ne sw to ene wsw trending echelon strike slip faults or in the zone north of the strike slip faults parallel to the nw se trending normal faults the interpretations regarding the kinematics of the faults are consistent with the data compiled from the islands near the faults and from the land in ne greece and nw turkey the northern aegean basins and their spatial relationships to the fault architecture representing the diffused termination of the north anatolian fault system are consistent with the mechanical principles of the strike slip tectonics

experiments on faulting in a two layer cover sequence overlying a reactivated basement fault with oblique slip abstract two series of experiments were undertaken to investigate the development of faults in a cover sequence overlying a reactivated basement fault with oblique slip the experiments represent physical models properly scaled to account for gravitational forces brittle behaviour of the upper cover and ductile behaviour of evaporites in the lower cover models were made with a coulomb layer sand overlying a newtonian ductile layer silicone putty in each series the thickness of the silicone layer was varied from each experiment to the next in all normal wrench experiments a domain of faults appeared in the brittle layer vertical offsets were mostly of normal sense but some reverse faults occurred in experiments with thick silicone layers the thicker the silicone the wider the fault domain the deformation zone was in all cases limited to the faulted domain in the reverse wrench experiments discrete reverse riedel faults appeared without a silicone layer as the silicone layer became thicker the importance of the reverse faults diminished with the thickest layer of silicone 3 cm no faults appeared in the overburden but a monoclinal flexure accommodated the dip slip component in the presence of silicone the deformation zone was not limited to a faulted domain and the strike slip component was accommodated by a bulk shearing of the overburden in the uplifted compartment these experiments demonstrate that in basins deformed in oblique slip mode any deformation that is localized along discrete faults at the basement level becomes distributed over a much wider zone of overburden when this overlies an interval of evaporites

analogue models of pull apart basins sandbox analogue models of pull apart basins that developed in sedimentary strata above releasing steps in underlying basement faults are characterized by rhombic basins that are flat bottomed box grabens with a subhorizontal synkinematic basin infill steep to nearly vertical sigmoidal oblique slip and segmented oblique extensional faults are the dominant bounding structures of the pull apart basins cross basin short cut faults link the offset principal displacement zones that are characterized by flower structure development the structural architectures of the physical models compare directly in form and dimensions to natural examples of strike slip pull apart basins

experiments on basin inversion above reactivated domino faults abstract results are presented from scaled sand box experiments which investigate the structural inversion of half graben basins generated by the contractional reactivation of extensional domino fault systems the overburden was deformed above a series of rigid basement fault blocks that were extended in a domino manner the models were constructed from either 1 sand with mica interlayers anisotropic behaviour 2 pure sand isotropic behaviour or 3 a sand clay mixture behaviour with increased competence during the extensional phase of deformation sand or alternating sand mica layers were deposited in teh half grabens associated with individual domino faults to simulate a syn extensional stratigraphic sequence together with a component of overall subsidence after termination of the extensional phase sand layers were added to represent a post rift passive subsidence stratigraphy subsequent contraction of the experimental configuration resulted in progressive reactivation of the domino fault system which was accompanied by back rotation of both the footwall and hanging wall in each of the individual fault blocks this produced null points along the profile section of the fault planes which separated net contraction above from net extension below continued shortening of the system led to further contraction being accommodated along footwall shortcut faults it is suggested that these shortcuts developed in response to 1 the accommodation of space problems generated by increased bed length in the graben acquired during extension and 2 by more efficient low angle fault trajectories in keeping with coulomb failure during horizontal to subhorizontal compression during inversion in the anisotropic model folding in the footwall partially suppressed the initiation of footwall shortcut faults footwall shortcut faulting did not occur in experiments where the dips of the original extensional faults were decreased before the inversion in all inversion experiments the contractional deformation associated with the shortening of the half graben was concentrated in the footwall to each domino basin bounding fault a control experiment in which an undeformed sand pack was deposited above an extended series of basement fault blocks revealed that the early extensional architecture formed in the other experiments had a profound influence on the resultant inversion geometries the results show clear similarities to interpreted examples of natural basin inversion structures associated with reactivated domino fault systems as well as to inferred strike slip induced flower structures

structure of the dead sea pull apart basin from gravity analyses analyses and modeling of gravity data in the dead sea pull apart basin reveal the geometry of the basin and constrain models for its evolution the basin is located within a valley which defines the dead sea transform plate boundary between africa and arabia three hundred kilometers of continuous marine gravity data collected in a lake occupying the northern part of the basin were integrated with land gravity data from israel and jordan to provide coverage to 30 km either side of the basin free air and variable density bouguer anomaly maps a horizontal first derivative map of the bouguer anomaly and gravity models of profiles across and along the basin were used with existing geological and geophysical information to infer the structure of the basin the basin is a long 132 km narrow 7 10 km and deep 10 km full graben which is bounded by subvertical faults along its long sides the bouguer anomaly along the axis of the basin decreases gradually from both the northern and southern ends suggesting that the basin sags toward the center and is not bounded by faults at its narrow ends the surface expression of the basin is wider at its center 16 km and covers the entire width of the transform valley due to the presence of shallower blocks that dip toward the basin these blocks are interpreted to represent the widening of the basin by a passive collapse of the valley floor as the full graben deepened the collapse was probably facilitated by movement along the normal faults that bound the transform valley we present a model in which the geometry of the dead sea basin i e full graben with relative along axis symmetry may be controlled by stretching of the entire brittle and ductile crust along its long axis there is no evidence for the participation of the upper mantle in the deformation of the basin and the moho is not significantly elevated the basin is probably close to being isostatically uncompensated and thermal effects related to stretching are expected to be minimal the amount of crustal stretching calculated from this model is 21 km and the stretching factor is 1 19 if the rate of crustal stretching is similar to the rate of relative plate motion 6 mm yr the basin should be 3 5 m y old in accord with geological evidence

three dimensional structure and evolution of an asymmetric pull apart basin the structure of a 250 m long asymmetrical pull apart basin developed in carbonate rocks at the galilee heights israel is herein analyzed the reconstruction of the basin geometry is based on detailed mapping and lidar measurements of fault scarps the architecture of faults is then used as a boundary condition for calculating the stress pattern in the vicinity of the basin using a dislocation model the basin is found to be an asymmetrical v shaped structure bordered by two longitudinal oblique right lateral strike slip faults the strike of one of the faults is bent at the eastern edge of the basin generating a transverse boundary fault which joins the second boundary fault orthogonally the overall lateral displacement is smaller than the basin length and no transverse or diagonal fault is observed in the western end of the basin the deformation around the basin is mostly displayed by fractures yet folds and fault branches are observed near the tips of the boundary faults and near kink points of fault segments stress analysis obtained by the three dimensional model is in general agreement with the orientation of fractures and location of high deformation regions in and around the basin based on our observations we present a conceptual model which demonstrates the development of asymmetric basins in releasing bends and sheds light on the structures of similar large pull apart basins

four dimensional transform fault processes progressive evolution of step overs and bends many bends or step overs along strike slip faults may evolve by propagation of the strike slip fault on one side of the structure and progressive shut off of the strike slip fault on the other side in such a process new transverse structures form and the bend or step over region migrates with respect to materials that were once affected by it this process is the progressive asymmetric development of a strike slip duplex consequences of this type of step over evolution include 1 the amount of structural relief in the restraining step over or bend region is less than expected 2 pull apart basin deposits are left outside of the active basin and 3 local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes this type of evolution of step overs and bends may be common along the dextral san andreas fault system of california we present evidence at different scales for the evolution of bends and step overs along this fault system examples of pull apart basin deposits related to migrating releasing right bends or step overs are the pliopleistocene merced formation tens of km along strike the pleistocene olema creek formation several km along strike along the san andreas fault in the san francisco bay area and an inverted colluvial graben exposed in a paleoseismic trench across the miller creek fault meters to tens of meters along strike in the eastern san francisco bay area examples of migrating restraining bends or step overs include the transfer of slip from the calaveras to hayward fault and the greenville to the concord fault ten km or more along strike the offshore san gregorio fold and thrust belt 40 km along strike and the progressive transfer of slip from the eastern faults of the san andreas system to the migrating mendocino triple junction over 150 km along strike similar 4d evolution may characterize the evolution of other regions in the world including the dead sea pull apart the gulf of paria pull apart basin of northern venezuela and the hanmer and dagg basins of new zealand d 2004 elsevier b v all rights reserved

structure of the hanmer strike slip basin hope fault new zealand hanmer basin 10 x 20 km located in northern south island new zealand is evolving where two major segments of the dextral strike slip hope fault are projected to converge across a 6 to 7 km wide releasing step over the structural geometry and development of hanmer basin does not conform to traditional pull apart basin models the respective fault segments do not overlap but are indirectly linked along the southwest margin of the basin by an oblique normal fault the hope river segment terminates in an array of oblique normal faults along the northwestern basin range front and east west striking normal faults on the west hanmer plain faulted holocene alluvial fan surfaces indicate west hanmer basin is actively subsiding evolving under north south extension the conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces small pop up ridges and graben depressions early basin fill sediments of pleistocene age are being folded elevated and dissected as the eastern part of hanmer basin is progressively inverted and destroyed by north south contraction the north margin of the basin is defined by a series of topographic steps caused by normal faulting outside of the area of the releasing step over these normal faults we interpret to reflect large scale upper crustal collapse of the hanging wall side of the hope fault new seismic reflection data and geologic mapping reveal a persistent longitudinal and lateral asymmetry to basin development four seismic stratigraphic sequences identified in the eastern sector of the basin thicken and are tilted southward with insequence lateral onlaps occurring to the north and east and also onto basement near the fault controlled basin margins the basin depocenter currently contains 1000 m of sediment adjacent to the south margin and is disrupted by faulting only at depth in the western part of the basin the sediment fill is thinner today the rate of basin deepening under transtension at the western end is matched by its progressive inversion and destruction under transpression in the eastern sector with the oldest basin fill now being recycled we propose a hybrid model for hanmer strike slip basin one in which geometric elements of a fault wedge basin downward and upward tipped spindle shaped ends are combined with those of a pull apart basin step over region between the major fault segments we also conclude that changes in fault geometry releasing and restraining bends and step overs at a variety of scales and over short distances control the development of the extensile and contractile parts of the basin and three dimensional basin asymmetry strain partitioning is complex and cannot be related simply to local reorientation of the regional stress field

experimental fracture strain and subsidence patterns over en echelon strike slip faults implications for the structural evolution of pull apart basins abstract claybox experiments show that with progressive left lateral displacement on basal left stepping en echelon strike slip faults an extremely complicated zone of evolving fracture strain and subsidence patterns develops in the overlying clay the fracture pattern is more complex than predicted by theoretical models and consists of a progressively widening array of riedel shears conjugate riedel shears and tension gashes above basal master faults conjugate riedel shears rotate anticlockwise and lengthen forming s shaped fractures riedel shears rotate both anticlockwise and clockwise those that rotate anticlockwise eventually open as tension gashes other tension gashes open where the ends of riedel and conjugate riedel shears link strain contours in the deformed clay show an asymmetrical z shaped pattern over the stepover area between basal en echelon faults in the early stages of displacement strain is distributed penetratively over a progressively broadening zone in later stages of displacement strain is partitioned into a narrow middle zone maximum strain is sustained always in the middle of the sheared zone and increases exponentially with progressive displacement subsidence is accommodated in the stepover area by oblique slip on many riedel and conjugate riedel shears distributed throughout the stepover area subsidence y exhibits a linear relationship with master fault displacement x described by y 0 36 x 1 4 the experimental fracture strain and subsidence patterns in clay over en echelon faults can be used as interpretive and predictive tools with which to constrain interpretations of static field examples especially in pull apart basins on the crustal scale where many similarities with the models exist

structural model of the san bernardino basin california from analysis of gravity aeromagnetic and seismicity data 1 the san bernardino basin is an area of quaternary extension between the san jacinto and san andreas fault zones in southern california new gravity data are combined with aeromagnetic data to produce two and three dimensional models of the basin floor these models are used to identify specific faults that have normal displacements in addition aeromagnetic maps of the basin constrain strike slip offset on many faults relocated seismicity focal mechanisms and a seismic reflection profile for the basin area support interpretations of the gravity and magnetic anomalies the shape of the basin revealed by our interpretations is different from past interpretations broadening its areal extent while confining the deepest parts to an area along the modern san jacinto fault west of the city of san bernardino through these geophysical observations and related geologic information we propose a model for the development of the basin the san jacinto fault related strike slip displacements started on fault strands in the basin having a stepping geometry thus forming a pull apart graben and finally cut through the graben in a simpler bending geometry in this model the san bernardino strand of the san andreas fault has little influence on the formation of the basin the deep central part of the basin resembles classic pull apart structures and our model describes a high level of detail for this structure that can be compared to other pull apart structures as well as analog and numerical models in order to better understand timing and kinematics of pull apart basin formation

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

quantifying the origin and geometry of circular sag structures in northern fort worth basin texas paleocave collapse pull apart fault systems or hydrothermal alteration three dimensional seismic data reveal numerous subcircular sag structures in the northern fort worth basin the structures are defined by concentric faults that extend vertically upward 760 1060 m 2500 3500 ft from the lower ordovician ellenburger group the largest structures remained active into the lower desmoinesian strawn group criteria are outlined for defining seismically resolvable sag structures and a detailed quantitative analysis of the geometries of these circular features was undertaken results are compared and contrasted with reviews of subsurface collapse mechanisms and strike slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view in this manner we develop several constraints for differentiating collapse related sag structures from strike slip related sag structures qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void into which the overburden subsequently sags and collapses quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced collapsed paleocaves within the ellenburger group observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins

investigating role of the hormuz salt bodies in initiation and evolution of the strike slip faults in the fars zone of the zagros fold and thrust belt insights from seismic data and sandbox modeling the zagros belt has examples of strike slip faults affected by precursor hormuz salt bodies we attempted to examine the role of salt walls in localizing the strike slip faults using field observations seismic profiles and understandings from the sandbox modeling setting and performance of the pre hormuz features in connection to the salt tectonics are major questions the inherited topography below the salt is an important parameter which controlled the structuration of the basin the antecedent troughs provided accommodation for the thick hormuz salt differential loading above the salt helped generation of salt related structures the salt walls caused formation of elongated highs these highs were re shaped in form of strike slip faults during the zagros orogeny the intersection of the salt walls and the zagros induced deformation fronts are location of potential diapirs needless of bending points along the tear faults in contrast the underlying deep seated basement faults are hardly believed to be strike slip

sandbox model studies of inversion tectonics abstract scaled plane strain sandbox models have been used to simulate basement controlled structural inversion in a sedimentary overburden two basement configurations were tested in the first one reversal of movement along vertical and normal faults that controlled a graben relief induced only a partial recovery of displacement along the pre existing overburden faults the inversion produced new reverse faults in the footwall of the pre existing overburden faults the reactivated normal faults and the new reverse faults defined wedge shaped fault blocks at the edges of the inverted structure in the second configuration the relationship between inversion and tilted fault blocks was tested when a tilting basement block does not move parallel to its bounding fault a compression is induced in the overburden which generates an inverted structure bounded by new reverse faults both experiments show that structural inversion not only reactivates pre existing faults but also generates new faults in the overburden faults related to inversion are the result of horizontal compression either associated with rotation of one of the fault blocks or with vertical uplift of basement blocks

stress tensor and focal mechanisms along the dead sea fault and related structural elements based on seismological data the dead sea fault is among the largest active strike slip fault of the world this study is focused on the southern part of this fault from the sea of galilee to the gulf of aqaba as monitored mainly by the jordanian and israeli seismic networks the data of arrival times and polarities allowed relocation of earthquakes with a better azimuthal coverage and computation of focal mechanisms this last step has been realized by inverting the polarities to determine a unique stress tensor for the region and the compatible focal mechanisms inversion with different subsets of the data set based on tectonic regionalization has also been performed to evaluate the impact of each cluster of earthquakes on the global solution the population of focal mechanisms is clearly dominated by strike slip events with the notable exception of a cluster of earthquakes south of the dead sea which displays several normal focal mechanisms this last cluster forces 1 to be vertical and 2 to be horizontal a large number of fault planes however are close to the vertical inhibiting the action of the vertical component of the stress tensor and acting like under strike slip stress regime we observed a good agreement between the location of the earthquakes and the active faults based on geological data in addition there is a good agreement between the fault plane solutions and the orientation of the active faults

deep crustal structure and tectonics in the offshore southern santa maria basin california reprocessed deep penetration seismic reflection profiles across the offshore southern santa maria basin suggest a complex history of tectonic deformation the top of subducted oceanic crust imaged at the north end of edge line ru 10 is disrupted and eventually disappears to the south farther south a strong lower crustal reflection ramps up and may reflect a possible north dipping low angle detachment or a change in crustal geometry associated with the subducted morro fracture zone above a time transgressive unconformity miocene and younger sediments onlap to the south and water bottom deepens suggesting a major regional tilt reversal in post miocene time below the unconformity basement is displaced by largely northwest striking normal faults that we believe form tilted crustal blocks it is possible that cretaceous and paleogene sections are preserved in resulting half grabens some of these normal faults including splays off the outer santa lucia bank fault system were reactivated as minor reverse faults in post miocene time there is no evidence that requires any post 17 5 ma tectonostratigraphic terrane boundary through this area rather much of the structure of the southern santa maria basin appears to be continuous with the rotated western transverse ranges and northern channel islands

structural and sedimentary characteristics of the liangjia wangchang strike slip region yitong basin china abstract the yitong basin a typical strike slip basin is part of the northern segment of the world famous tanlu strike slip fault area in china research is needed to understand how the strike slip tectonic environment affects the development of structures and depositional systems within the basin the aim of this paper is to analyze how the right lateral strike slip movement of the northwestern boundary faults in this region controls extensional faulting and to analyze differences in two main sedimentary sequences termed sysq1 and sysq2 that result from strike slip movement the study is based on geological and geophysical data the following conclusions are drawn 1 the two periods of strike slip movement of the boundary fault generated two episodes of extensional fault inside the basin the extensional faults have a characteristically episodic activity related to the episodic strike slip movement the extensional faults have small fault throw and their activity lags behind the strike slip movement of the northwestern boundary faults 2 the strike slip movement of the boundary faults generated the extensional faults inside the basin and changed the locus of depocenters with complimentary changes in the paleogeomorphology and in the distribution of depositional facies it is this strike slip movement that caused the differences in sedimentary and structural characteristics between sequences sysq1 and sysq2

the structural evolution of dilational stepovers in regional transtensional zones we propose a theoretical model supported by a field study to describe the patterns of fault fracture meshes formed within dilational stepovers developed along faults accommodating regional scale wrench dominated transtension the geometry and kinematics of the faulting in the dilational stepovers is related to the angle of divergence and differs from the patterns traditionally predicted in dilation zones associated with boundary faults accommodating strike slip displacements where 0 for low values of oblique divergence 30 and low strain the fault fracture mesh comprises interlinked tensile fractures and shear extensional planes consistent with wrench dominated transtension at higher values of strain a switch occurs from wrench to extension dominated transtension leading to the reactivation and or disruption of the early formed structures these structural processes lead to the development of a geometrically complex and kinematically heterogeneous fault pattern which may affect and or perturb the development of a through going fault linking and facilitating the slip transfer between the two overlapping fault segments as a result dilational stepover zones will tend to form long lived sites of localized extension and subsidence in regional transtensional tectonic settings cyclic increases decreases of structural permeability will be related to slip on the major boundary faults that control the distribution of fluid flow paths and consequently the long and short term structural evolution of these sites our model also predicts complex and more realistic subsurface fluid migration pathways relevant to our current understanding of hydrothermal ore deposits and hydrocarbon migration and storage

fault scarp features and cascading rupture model for the mw 7 9 wenchuan earthquake eastern tibetan plateau china on 12 may 2008 the m w 7 9 wenchuan earthquake ruptured two northeast striking imbricated reverse faults and one northwest striking reverse fault along the middle longmen shan thrust belt at the eastern margin of the tibetan plateau the morphology of the coseismic scarp varies drastically along strike we distinguish eight different categories 1 simple thrust scarp 2 hanging wall collapse scarp 3 simple pressure ridge 4 dextral pressure ridge 5 fault related fold scarp 6 backthrust pressure ridge 7 local normal fault scarp and 8 pavement suprathrust scarp the coseismic surface ruptures confirm that the wenchuan earthquake is dominated by reverse faulting with some right lateral component that varies from site to site the surface rupture can be divided into two parts the yingxiu segment and the beichuan segment when the earthquake is split into two subevents accordingly they correspond to an m w 7 8 event and an m w 7 6 event respectively these two segments can in turn be divided into four second order subsegments which are equivalent to four subevents of m w 7 5 m w 7 7 m w 7 0 and m w 7 5 respectively the segmentation of the rupture is consistent with a cascading rupture pattern responsible for the total 110 s of the earthquake rupture in addition to surface ruptures the focal mechanisms determined for the aftershocks recorded by the local seismic network are used to constrain the fault geometry of the subsegments they show that the dip of the fault responsible for the earthquake varies along strike and the fault tends to flatten at depth in addition the fault plane gets steeper northward enabling the fault to accommodate a larger strike slip component along a lateral ramp this major earthquake confirms that crustal shortening could be the main driving force for the growth of high topography along the eastern margin of the tibetan plateau and that lower crustal flow is not required

shallow structure and deformation along the san andreas fault in cholame valley california based on high resolution reflection profiling the mapped active traces of the san andreas fault are separated by a 1 km wide right stepping offset in cholame valley the geometry of this offset defined in other strike slip systems as a releasing bend or a dilational jog has resulted in the formation of a pullapart basin various researchers have inferred that this offset served as a rupture terminus for earthquakes on both strands of the san andreas fault 1966 parkfield and 1857 fort tejon thus this en echelon offset may represent a barrier to the propagation of rupture between two segments of the san andreas fault we collected 18 km of high resolution seismic reflection data specifically designed to image the san andreas fault zone in the shallow crust surrounding this offset four short profiles 3 3 km long of mini sosie reflection data 1 s of two way travel time 1 5 km deep were collected perpendicular to the san andreas fault three of these profiles were tied by a 7 km long profile that trended northwest through cholame valley subparallel to the san andreas fault a zone of incoherent energy narrow at the surface but widening with depth underlies the mapped active traces of the san andreas fault and abruptly terminates shallow reflections on both sides the reflection profiles and available well data indicate that west of the mapped active traces of the san andreas fault the shallow subsurface structure of the crust consists of thin 400 m thick offset packages of reflections laterally coherent on the scale of tens of meters overlying deformed clastic sedimentary rocks east of the san andreas fault the structure of the shallow crust in southern cholame valley is characterized by thick packages of reflections laterally coherent on the scale of kilometers overlying the franciscan complex all of the strata east of the fault within cholame valley dip toward the san andreas fault and the offset into an approximately 1 km deep sedimentary basin abutting the south strand of san andreas fault the offset in cholame valley is characterized by a gentle downwarping of sediments into the offset the presence of many small faults and discontinuous reflections consistent with areally distributed shear between the traces of the main fault localized subsidence abutting the main strike slip fault the formation of a basin near the offset that is about as deep as the jog is wide and the southward propagation of the deformation associated with the offset strain field modeling based on simple geometries of the san andreas and associated faults successfully predicts the general features of the observed topography and subsurface structure of southern cholame valley including subsidence and basin formation near the offset a discontinuous san andreas fault plane and at least one fault in southeastern cholame valley

rb sr and k ar ages on minerals at temperatures of 300 400 c from deep wells in the larderello geothermal field italy rb sr and k ar ages have been obtained on six biotites two muscovites and one hornblende from samples of micaschist gneiss and amphibolite of lower paleozoic to precambrian age at a depth exceeding 2 000 m in basement rocks of the larderello travale geothermal region most of the data cluster in the range 2 5 3 7 ma revealing the existence of a pliocene thermal event to which the origin of the field may be attributed the resulting duration of the larderello geothermal field is unexpectedly long in the basement levels of the two wells examined unstabilized minimum temperatures of 290 and 380 c were measured all the biotites show almost complete 40ar and 87sr retention at the measured well temperatures petrologic evidence stilpnomelane stability and experimental data activation energies and diffusion coefficients also favour a closure temperature above 400 c for rb sr and k ar in biotites in agreement with recent direct experimental determinations

lithospheric scale centrifuge models of pull apart basins abstract we present here the results of the first lithospheric scale centrifuge models of pull apart basins the experiments simulate relative displacement of two lithospheric blocks along two offset master faults with the presence of a weak zone in the offset area localising deformation during strike slip displacement reproducing the entire lithosphere asthenosphere system provides boundary conditions that are more realistic than the horizontal detachment in traditional 1 g experiments and thus provide a better approximation of the dynamic evolution of natural pull apart basins model results show that local extension in the pull apart basins is accommodated through development of oblique slip faulting at the basin margins and cross basin faults obliquely cutting the rift depression as observed in previous modelling studies our centrifuge experiments suggest that the angle of offset between the master fault segments is one of the most important parameters controlling the architecture of pull apart basins the basins are lozenge shaped in the case of underlapping master faults lazy z shaped in case of neutral offset and rhomboidal shaped for overlapping master faults model cross sections show significant along strike variations in basin morphology with transition from narrow v and u shaped grabens to a more symmetric boxlike geometry passing from the basin terminations to the basin centre a flip in the dominance of the sidewall faults from one end of the basin to the other is observed in all models these geometries are also typical of 1 g models and characterise several pull apart basins worldwide our models show that the complex faulting in the upper brittle layer corresponds at depth to strong thinning of the ductile layer in the weak zone a rise of the base of the lithosphere occurs beneath the basin and maximum lithospheric thinning roughly corresponds to the areas of maximum surface subsidence i e the basin depocentre

experiments on strike slip faulting and block rotation analogue model experiments investigating strike slip faulting in zones of distributed shear deformation show that fault orientations and fault evolution indicate modifications of the stress field with increasing bulk shear strain dextral bulk shear deformation at low strain is accommodated dominantly by synthetic strike slip faults riedel shears at higher strain secondary antithetic and synthetic faults develop mostly between earlier formed major riedel shears closely spaced parallel antithetic faults cross faults delimit domains that rotate about vertical axes with continuing shear deformation rotation of fault bounded domains results in sigmoidal antithetic faults that have a dip slip component and a dip direction that changes along strike there is good agreement between models and natural examples of distributed shear zones where block rotations about vertical axes have been documented

clay modeling of rift asymmetry and associated structures abstract a clay model is used to simulate the development of half graben of alternating asymmetry in a rift zone the sense of asymmetry is determined by the spatial arrangement and movement pattern of plates below the clay the plates can be thought of as analogous to low angle detachment horizons below rifts the change in half graben asymmetry occurs across a transfer zone marked by a highly faulted anticlinal saddle whose axis is oblique to the rift trend

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

influence of releasing step overs on surface fault rupture and fault segmentation examples from the 17 august 1999 izmit earthquake on the north anatolian fault turkey the 17 august 1999 izmit earthquake broke four distinct structural segments of the north anatolian fault from east to west the karadere sakarya sapanca and golcuk fault segments these segments are separated from one another and the adjoining yalova and duzce fault segments to the west and east respectively by distinct right releasing step over and or gaps in the fault trace these include from east to west the eften lake step over 2 4 km wide akyazi gap 6 13 km long sapanca step over 1 2 km wide golcuk step over 1 2 km wide and karamursel step over 4 5 km wide the earthquake nucleated within or near the golcuk step over with bilateral rupture propagation to the west and east to the west about 4 5 5 m of surface rupture up to 7 m of subsurface rupture on the golcuk segment was mostly if not entirely arrested by the 4 to 5 km wide karamursel step over to the east about 3 4 m of surface rupture about 4 m of subsurface rupture propagated through the 1 to 2 km wide sapanca step over triggering up to 4 5 5 5 m of surface rupture up to 5 6 m of subsurface rupture on the sakarya segment rupture on the sakarya segment stepped across the akyazi gap triggering 1 1 5 m of rupture on the karadere segment rupture on the karadere segment was arrested by the 2 to 4 km wide eften lake step over modeling of the transient stress field associated with the rupture propagation agrees well with the field observations the modeled fault rupture of 5 m on the golcuk segment appears to decrease the transient stress on the eastern tip of the adjoining yalova segment across the 4 to 5 km wide karamursel step over even if we allow rupture on the golcuk segment to extend along the entire southern margin of the step over our model shows that regardless of the amount of slip in a generic sense it is difficult to increase stress on the tip of an adjoining fault segment across a 5 km wide step over on the other hand fault rupture of 3 m on the western tip of the sapanca fault segment significantly increases the transient stress on the adjoining sakarya segment across the 1 to 2 km wide sapanca step over allowing rupture propagation across the basin an evaluation of 30 historical strike slip earthquakes involving 59 step over basins supports our field observations and stress modeling of the izmit fault rupture historical strike slip events with small to large displacements usually propagate through step overs less than 1 2 km wide with increasing displacement larger and larger step overs can be ruptured through the empirical data generally show a ratio ranging from 2 1 to 1 1 between step over width in kilometers and strike slip displacement in meters for a through going rupture however step overs of 4 to 5 km width appear to always arrest fault rupture regardless of the amount of displacement these results therefore provide important constraints for evaluating the probability of multisegment ruptures on segmented strike slip faults manuscript received 20 august 2000

tan lu fault system and its significance in oil accumulation in the central liaodong bay subbasin bohai bay basin china the tan lu fault system in the central liaodong bay subbasin bohai bay basin has complex structural characteristics and its tectonism during the cenozoic is an important factor in oil accumulation three dimensional seismic data were used to document the structural features and evolution of the system variations in source rock occurrence oil catchment area and faulting intensity were comprehensively evaluated to discuss the heterogeneity of oil occurrence along the system detailed analysis of the seismic data indicates a right lateral slip for the tan lu fault system transpression and transtension occur on two branches of the tan lu fault with different orientations indicating a likely slip azimuth that is between the two orientations i e 30 35 the strike slip began at the middle stage of deposition of the shahejie formation and reached its climax during deposition of the first and second members of the dongying formation to date the slip is still continuing in the transpressional system the lower strata on opposite sides of the strike slip fault were shortened and folded whereby the upper strata were stretched and normal faulted as a result a negative flower structure developed immediately above a positive flower structure strain distribution in this system is similar to that in a classic fold moderate tectonic deformation could enlarge the oil catchment area benefiting oil accumulation weak or intense deformation is unfavorable for commercial oil accumulation because of a small oil catchment area and poor oil preservation respectively

transform normal extension and asymmetric basins an alternative to pull apart models continental transforms are commonly associated with relatively small scale pull apart basins however much larger scale basins which are not consistent with the pull apart model commonly exist adjacent to transforms structures associated with several basins along the dead sea rift and el pilar fault illustrate this these basins are remarkably asymmetric bounded by essentially linear transform segments on one side and subparallel normal faults on the other suggesting simultaneous strike slip motion and transform normal extension this observation which is incompatible with classical faulting theory can be explained along divergent plate boundaries if the transform fault is much weaker than the adjacent crust analysis of the orientation of the horizontal principal stress direction near a weak transform fault embedded in an otherwise strong crust shows that convergent and divergent plate motion results in a stress field characterized by nearly fault normal compression and extension respectively structural styles along the san andreas dead sea and el pilar transform systems show that predictable changes between these two states have occurred through time along strike of the transforms