displacement transfer at thrust terminations in southern appalachians saltville thrust as example transition from faulting to noncylindrical folding involving rapid plunge change at the fold closure is the primary displacement transfer mechanism at the northern end of the south appalachian foreland fold and thrust belt regional thrust faults including the saltville narrows and st clair terminate in regional overturned asymmetric anticlines this mechanism of displacement transfer is distinct from thrust terminations where fold complexes of smaller noncylindrical folds converge toward the end of the fault trace the saltville thrust transition to the sinking creek anticline is representative of the southern appalachian structures deformation intensity patterns reflected by cleavage development mesoscopic folding and faulting suggest that the sinking cree anticline formed due to upwarping from shear thrusting at depth laboratory deformation analogs of folds formed above ramps support this contention experimentally produced folds are asymmetric overturned structures with high strains on overturned limbs and very low strains on upright limbs a deformation sequence involving shear thrusting with subsequent break thrusting may characterize fault propagation at thrust termini this will be important for layered sedimentary sequences where thick rigid members low in the sedimentary pile are overlain by thinner interbedded mechanically anisotropic strata

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

selective reverse reactivation of normal faults and deformation around reverse reactivated faults in the mesozoic of the somerset coast abstract normal faults exposed in the triassic jurassic limestones and shales of the somerset coast were formed during the mesozoic development of the bristol channel basin reverse reactivation of some of these normal faults occurred during late cretaceous to early tertiary north south contraction the contraction is also evident from thrusts and conjugate strike slip faults preferential reactivation of the normal faults is attributed to 1 decreased fault plane friction 2 domino block rotation 3 displacement magnitude and 4 fault connectivity the geometries of overlapping and underlapping zones in reactivated fault zones are dependent on the existing structural geometry two distinctive styles of displacement accommodation occur between reverse reactivated normal faults 1 formation of a network of strike slip faults conjugate about nne ssw and 2 oblique steeply dipping reverse faults interaction between strike slip and an existing fault is dependent on whether the normal fault was reactivated the range of structures related to the north south contraction has been incorporated into a single deformation model controlled by the northwards movement of the hanging wall of the quantock s head fault pure dip slip movement occurred in the centre of its curved fault trace with a sinistral component at the western tip and a dextral component of displacement and strike slip block rotations occurred at the eastern tip shortening of these blocks was achieved through development of a strike slip fault network and nw striking thrusts in an underlap zone loading of the footwall by the hanging wall block modified the local stress system to allow formation of oblique steeply dipping reverse faults

propagation of an aseismic dislocation through asperities with smooth borders abstract a 2 d model is presented for the propagation of a somigliana dislocation along a fault with nonuniform friction fault slip is driven by a uniform ambient shear stress slowly increasing with time the dislocation is nucleated in the lowest friction region of the fault plane and is confined by the surrounding higher friction regions asperities the case studied involves asperities which have smooth borders characterized by a constant friction gradient for values of ambient shear stress near to the weak zone friction the propagation of dislocation is slowed down by the presence of asperities only when it goes beyond the border does the dislocation front move at increasing velocity the model shows that at a given value of ambient shear stress the slip amplitude is larger in the case of finite and constant friction gradient than in the case of asperities with sharp borders unlike the propagation velocity slip rate is ever increasing during the dislocation process the model shows to what extent a dislocation is influenced by the distribution of friction on the fault a detailed knowledge of slip rate and slip history is needed to understand the mechanism of frictional instability on faults

numerical modeling of propagation and overlap of thrust faults with application to the thrust fold belt of central alberta abstract we use two computer programs to analyze the propagation of multiple thrust faults and their influence on the geometry of a thrust belt they use algorithms based on current equations of fault propagation to generate graphical simulations the simulations are used to demonstrate a model of thrust propagation and thrust belt development that fits current knowledge of fault propagation an alternative to the thrust transfer zone model is proposed called thrust overlap zones the computer simulations provide useful information about the generation and behavior of multiple faults the number of faults the positions of their nucleation points and hence their density the rock properties and rate of fault propagation govern the uniformity of the shortening along the thrust belt thus shortening of a thrust belt can be distributed more evenly by means of propagation of a large number of small faults than by a few large faults the final geometry of a large thrust fault on a geological map provides few clues about its origin i e whether it was produced by the coalescence of a series of en echelon thrust faults or from only a single fault a simple kinematic model based on map length and position of known thrust faults imitates the curved fault map patterns of a segment of the thrust fold belt of central alberta

mineralogy and origin of the state line talc deposit pennsylvania the state line talc deposit in lancaster county pennsylvania near the maryland border occurs at the boundary between a serpentinite body and surrounding metamorphosed pelitic rocks blackwall type zones are found in the talc body the zones are composed of variable amounts of tremolite chlorite biotite and dolomite however trace element chemistry indicates that the zones are not a product of simple blackwall metasomatic processes the ratios of al 2 o 3 cr zr cr ti cr and al 2 o 3 ni in the blackwall type zones are intermediate between the ratios found in the serpentinite and the pelite supporting the hypothesis that these zones result from the sedimentary mixing of pelitic and serpentinite bearing material on the sea floor with later modifying metasomatism the distribution of tremolite a contaminant that affects significantly the market value of a talc deposit is the result of small scale metasomatic reactions that modified the original layers of serpentinite bearing and pelitic detritus and larger scale metasomatism involving silica water and carbon dioxide that fixed tremolite as the saturating calcium phase in the proximity of the pelitic country rock and dolomite in the proximity of the serpentinite

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

chaos fractals nonlinear phenomena in earth sciences many problems in geophysics can be solved using linear equations examples include seismic waves the earth s gravity and magnetic field and problems involving heat flow and elasticity however other problems are essentially non linear the classic example is fluid turbulence in the past twenty years major advances have been made in the understanding of nonlinear problems in the past four years many of these advances have been applied to geophysical problems

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

relay ramp forms and normal fault linkages canyonlands national park utah fieldwork within a series of mesoscale grabens in southeast utah has revealed a particularly well exposed system of interlinked extensional faults a series of down faulted grabens are developed within a 460 m thick brittle layer of upper paleozoic sandstone and shale which overlies a ductile layer with a high gypsum content all the major grabens consist of two or more overlapping elements which are composed of fault segments these segments may be hard linked fault surfaces are joined or soft linked fault surfaces are isolated but linked by ductile strain of the rock volume between them in map view relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments in the canyonlands grabens the transfer of displacement between soft linked fault segments is characterized by well de fined dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments relay ramps develop as ephemeral structures eventually becoming breached by hard linkage of the fault segments breakdown of ramps by breaching is part of the process of fault growth by segment linkage within the canyonlands graben system four orders of segmentation are present this may be a consequence of different scales of mechanical heterogeneity but evidence from the canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage

modes of reverse reactivation of domino type normal faults experimental and theoretical approach experiments with sand cement models show that reverse reactivation of a set of parallel faults bounding domino blocks takes place in two principal modes in mode i reactivation the domino blocks undergo counter rotation during contraction while normal faults are reactivated as reverse faults mode ii reactivation occurs by fault block translation we present a theoretical analysis of the two modes which considers forces and their rotational moments in a domino block and show that low angle normal faults are commonly reactivated in mode ii by contrast steep faults are reactivated in mode i furthermore fault reactivation depends on the initial fault spacing a large initial fault spacing opposes fault reactivation especially in mode i instead the system deforms along new thrusts dipping opposite to the normal faults we generate reactivation parameters for the conditions of the two modes as functions of the ratio r between fault spacing and effective thickness of fault horizon the fault dip the coefficient of friction and the ratio of principal stresses sr during late contraction

strike slip relay ramps areas of reorientated bedding at contractional oversteps between strike slip faults are here called strike slip relay ramps metre scale examples are described from the jurassic sediments at east quantoxhead somerset u k larger strike slip relay ramps occur in the rio de peixe basin ne brazil along the newport inglewood trend california and in the bovey basin sw england although the geometry and development of strike slip relay ramps are similar to those of relay ramps in normal fault systems there are differences in the structures which accommodate the transfer of displacement between the overstepping faults whereas strike slip relay ramps are typically transpressional with pressure solution often occurring relay ramps in normal fault systems are dominated by extension or transtension care needs to be taken when interpreting areas of reorientated bedding between overstepping faults particularly when displacement directions are unknown for example when using seismic data this is because relay ramps can occur in both strike slip and normal fault zones

relationship between seismicity and subsurface fluids central coast ranges california seismicity is irregularly distributed in the central coast ranges of california with several large areas having virtually no seismic activity since 1968 aseismic areas correlate with zones of abnormally high pore fluid pressures ahps 130 of hydrostatic level line surveys suggest that aseismic areas have experienced significant historical uplift moreover these aseismic areas show a remarkable correlation with perennial saline springs that represent technically expelled formation fluid as indicated by their perennial flow 18o and d enrichment to 6 3 and 13 respectively elevated b to 331 ppm and chemistry dominated by na and cl aseismic areas are dominantly located at the center of structural blocks that are under compression and are bounded by major faults and seismicity belts volumetric strain within these blocks creates pockets of overpressured formation fluid that greatly reduce seismic activity while promoting aseismic uplift seismic cross sections reveal three dimensional pockets of aseismicity that are floored and surrounded by scattered hypocenters we suggest that overpressured formation fluids originate at depths of 1 to 7 km within these pockets on the basis of na k ca geothermometry estimates that are consistent with stratigraphic information and available drill hole data these overpressured fluid pockets may also influence seismicity on major faults such as the upper 3 km of the active greenville fault which is relatively aseismic where it separates two overpressured pockets decreasing flow rates observed at some saline springs may represent a local transition to a period of increased seismic activity while many studies have linked generation of earthquakes to man made fluid overpressures our data show that the dominant long term effect of ahps is to reduce seismicity and promote uplift