rise and fall of the southern santa cruz mountains california from fission tracks geomorphology and geodesy the santa cruz mountains are closely associated with a left bend along the right lateral san andreas fault the loma prieta area on the northeast side of the san andreas consists of fault bounded blocks that rise along active deeply rooted reverse and oblique slip faults six samples from a transect across this area yield concordant apatite fission track ages averaging 4 6 0 5 ma these ages date the time of cooling below 110 c and suggest that about 3 km of unroofing has occurred over the last 4 6 m y allowing for current elevations of about 1 km this suggests an average uplift rate of the order of 0 8 mm yr over the last 4 6 m y to further define the extent and distribution of this young uplift we used morphometric analyses of the youthful topography of the area steep drainage slopes and high local relief indicate that the area northeast of the san andreas forms a well defined zone of high uplift in contrast the region on the southwest side of the san andreas is characterized by broad upwarping and folding more subdued topography old fission track ages and mean quaternary uplift rates of 0 1 0 4 mm yr geodetic data 1906 san francisco earthquake and subsequent strain transients 20 years of interseismic deformation 1989 loma prieta earthquake and 2 years of post loma prieta earthquake strain show that the southern santa cruz mountains repeatedly rise and subside through a complex sequence of bay area deformation events an additional deformation element that involves reverse slip averaging 2 3 mm yr along the foothills thrust system must occur to explain the longer term uplift pattern in the loma prieta area since the late pliocene the asymmetry in deformation on opposite sides of the san andreas probably reflects the contrasting rock types on opposite sides of the fault the influence of preexisting structures and the interaction with neighboring faults of the san andreas system

pacific north america plate boundary reorganization in response to a change in relative plate motion offshore canada 1 the transition from subduction in cascadia to the transform queen charlotte fault along western canada is often drawn as a subduction zone yet recent studies of gps and earthquake data from northern vancouver island are not consistent with that model in this paper we synthesize seismic reflection and gravity interpretations with microseismicity data in order to test models of 1 microplate subduction and 2 reorganization of the preexisting strike slip plate boundary we focus on the critical region of outer queen charlotte sound and the adjacent offshore on much of the continental shelf several million years of subsidence above thin crust are a counterindicator for subduction an undated episode of compression uplifted the southernmost shelf but subsidence patterns offshore show that recent subduction is unlikely to be responsible previously unremarked near vertical faults and a mix of extensional and compressional faults offshore indicate that strike slip faulting has been a significant mode of deformation seismicity in the last 18 years is dominantly strike slip and shows large amounts of moment release on the revere dellwood fault and its overlap with the queen charlotte fault the relative plate motion between the pacific and north american plates rotated clockwise 6 ma and appears to have triggered formation of an evolving array of structures we suggest that the paleo queen charlotte fault which had defined this continental margin retreated northward as offshore distributed shear and the newly formed revere dellwood fault propagated to the northwest

general circulation model sensitivity to 1982 83 equatorial pacific sea surface temperature anomalies abstract three control and anomaly simulation pairs run with the goddard laboratory for atmospheric sciences glas climate model have been analyzed in order to investigate the atmospheric response to the 1982 83 tropical sea surface temperature anomalies the observed 1982 83 sst anomalies obtained from the climate analysis center were applied to two separate 75 day control simulations starting on 16 december 1982 and 16 december 1979 respectively and a third 60 day control simulation starting on 1 january 1975 in each experiment the equatorial pacific precipitation increased significantly in a wide band stretching from just east of the dateline to the south american coast in agreement with observed outgoing longwave radiation olr anomalies west of this region the precipitation was reduced in the anomaly simulations as in previous gcm experiments the major contributor to the tropical precipitation changes was the low level moisture convergence the largest evaporation differences were wound 4 mm

nappes tectonics of oblique plate convergence and metamorphic evolution related to 140 million years of continuous subduction franciscan complex california this paper presents a new synthesis of franciscan complex tectonics with the emphasis on the pre san andreas fault history of these rocks field relations suggest that the franciscan is characterized by nappe structures that formed during sequential accretion at the trench the presence of these structures along with other field relations including the lack of evidence for large offset of conglomerate suites indicates that strike slip fault systems of large displacement 500 km did not cut the franciscan complex during subduction regional geology and comparisons to modern arc trench systems suggest that strike slip faulting associated with oblique subduction took place inboard east of the franciscan in the vicinity of the magmatic arc the franciscan varies along strike because individual accreted elements packets of trench sediment seamounts etc did not extend the full length of the trench different depths of underplating distribution of post metamorphic faulting and level of erosion pro

mars south polar spring and summer behavior observed by tes seasonal cap evolution controlled by frost grain size thermal emission spectrometer tes observations of the recession phase of mars south polar cap are used to quantitatively map this recession in both thermal and visual appearance geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior for most of the cap while temperatures remain near the co2 frost point albedos slowly increase with the seasonal rise of the sun then drop rapidly as frost patches disappear over a period of 20 days a cryptic region remains dark and mottled throughout its cold period tes observations are compared with first order theoretical spectra of solid co2 frost with admixtures of dust and h2o the tes spectra indicate that the cryptic region has much larger grained solid co2 than the rest of the cap and that the solid co2 here may be in the form of a slab the mountains of mitchel remain cold and bright well after other areas at comparable latitude apparently as a result of unusually small size of the co2 frost grains we found little evidence for a significant presence of h2o although co2 grain size may be the major difference between these regions incorporated dust is also required to match the observations a self cleaning process carries away the smaller dust grains comparisons with viking observations indicate little difference in the seasonal cycle 12 martian years later the observed radiation balance indicates co2 sublimation budgets of up to 1250 kg m 2 regional atmospheric dust is common localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm

deformation around stepovers in strike slip fault zones abstract this stuck addresses the kinematics of stepovers in strike slip fault zones where left lateral faults step rightward or right lateral faults step leftward a stepover is regarded as containing an internal strike slip fault whose ends link to transform faults surrounded by uniformly distributed deformation the kinematics are governed by k the ratio of slip rates on the internal fault and transform faults and gq the angle between these taults the convergence direction across the deformation at angle to the internal fault is an azimuth along which vectors do not rotate around vertical axes the lebanon stopover on the dead sea fault zone and the big bend stepover on the san andreas fault zone provide case studies in lebanon the time averaged deformation since 15 ma involves k 0 06 32 and 34 it accounts for 30 of anticlockwise rotation of cretaceous magnetization vectors in the lebanon mountains and 50 rotation of jurassic vectors the present phase of deformation around the big bend which began at 5 7 ma and imolves k 0 89 20 and 98 has instead only caused small rotations both clockwise and anticlockwise much of this region s structure including the dramatic up to 80 clockwise rotation of magnetization vectors in the transverse ranges is shown to relate instead to an earlier deformation phase with much lower k

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

dextral translation and progressive emergence of the pleistocene merced basin and implications for timing of initiation of the san francisco peninsula segment of the san andreas fault

formation of sequential basins along a strike slip fault geophysical observations from the dead sea basin abstract the dead sea basin is often cited as one of the classic examples for the evolution of pull apart basins along strike slip faults despite its significance the internal structure of the northern dead sea basin has never been addressed conclusively in order to produce the first comprehensive high resolution analysis of this area all available seismic data from the northern dead sea lake lower jordan valley land were combined results show that the northern dead sea basin is comprised of a system of tectonically controlled sub basins delimited by the converging western and eastern boundary faults of the dead sea fault valley these sub basins grow shallower and smaller to the north and are separated by structural saddles marking the location of active transverse faults the sedimentary fill within the sub basins was found to be relatively thicker than previously interpreted as a result of the findings of this study the classic model for the development of pull aparts based on the dead sea is revised the new comprehensive compilation of data produced here for the first time was used to improve upon existing conceptual models and may advance the understanding of similar basinal systems elsewhere

improved general circulation models of the martian atmosphere from the surface to above 80 km we describe a set of two new generation general circulation models of the martian atmosphere derived from the models we originally developed in the early 1990s the two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations the vertical resolution near the surface has been refined and the vertical domain has been extended to above 80 km these changes are accompanied by the inclusion of state of the art parameterizations to better simulate the dynamical and physical processes near the surface boundary layer scheme subgrid scale topography parameterization etc and at high altitude gravity wave drag in addition radiative transfer calculations and the representation of polar processes have been significantly improved we present some examples of zonal mean fields from simulations using the model at several seasons one relatively novel aspect previously introduced by wilson 1997 is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi global angular momentum conserving hadley cell which has no terrestrial equivalent within such a cell the coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km this winter polar warming had been observed but not properly modeled until recently in fact thermal inversions are generally predicted above one and often both poles around 60 70 km however the mars middle atmosphere above 40 km is found to be very model sensitive and thus difficult to simulate accurately in the absence of observations

mechanical evolution of transpression zones affected by fault interactions insights from 3d elasto plastic finite element models abstract the mechanical evolution of transpression zones affected by fault interactions is investigated by a 3d elasto plastic mechanical model solved with the finite element method ductile transpression between non rigid walls implies an upward and lateral extrusion the model results demonstrate that a transpression zone evolves in a 3d strain field along non coaxial strain paths distributed plastic strain slip transfer and maximum plastic strain occur within the transpression zone outside the transpression zone fault slip is reduced because deformation is accommodated by distributed plastic shear with progressive deformation the 3 axis the minimum compressive stress rotates within the transpression zone to form an oblique angle to the regional transport direction 9 10 the magnitude of displacement increases faster within the transpression zone than outside it rotation of the displacement vectors of oblique convergence with time suggests that transpression zone evolves toward an overall non plane strain deformation the slip decreases along fault segments and with increasing depth this can be attributed to the accommodation of bulk shortening over adjacent fault segments the model result shows an almost symmetrical domal uplift due to off fault deformation generating a doubly plunging fold and a positive flower structure outside the overlap zone expanding asymmetric basins subside to negative flower structures on both sides of the transpression zone and are called transpressional basins deflection at fault segments causes the fault dip fall to less than 90 86 89 near the surface 1 5 km this results in a pure shear dominated triclinic and discontinuous heterogeneous flow of the transpression zone

along fault migration of the mount mckinley restraining bend of the denali fault defined by late quaternary fault patterns and seismicity denali national park preserve alaska abstract the tallest mountain in north america denali formerly mount mckinley 6 190 m is situated inside an abrupt bend in the right lateral strike slip denali fault this anomalous topography is clearly associated with the complex geometry of the denali fault but how this restraining bend has evolved in conjunction with the regional topography is unknown to constrain how this bend in the denali fault is deforming we document the quaternary fault related deformation north of the denali fault through combined geologic mapping active fault characterization and analysis of background seismicity our mapping illustrates an east west change in faulting style where normal faults occur east of the fault bend and thrust faults predominate to the west the complex and elevated regional seismicity corroborates the style of faulting adjacent to the fault bend and provides additional insight into the change in local stress field in the crust adjacent to the bend the style of active faulting and seismicity patterns define a deforming zone that accommodates the southwestward migration of this restraining bend fault slip rates for the active faults north of the denali fault derived from offset glacial outwash surfaces indicate that the mount mckinley restraining bend is migrating along the denali fault at a late pleistocene holocene rate of 2 6 mm yr ongoing thermochronologic and structural studies of the mount mckinley restraining bend will extend these constraints on the migration and evolution of the restraining bend deeper in time and to the south of the denali fault

hirise observations of gas sublimation driven activity in mars southern polar regions iii models of processes involving translucent ice abstract enigmatic surface features known as spiders found at high southern martian latitudes are probably caused by sublimation driven erosion under the seasonal carbon dioxide ice cap the mars reconnaissance orbiter mro high resolution imaging science experiment hirise has imaged this terrain in unprecedented details throughout southern spring it has been postulated kieffer h h titus t n mullins k f christensen p r 2000 j geophys res 105 9653 9700 that translucent co 2 slab ice traps gas sublimating at the ice surface boundary wherever the pressure is released the escaping gas jet entrains loose surface material and carries it to the top of the ice where it is carried downslope and or downwind and deposited in a fan shape here we model two stages of this scenario first the cleaning of co 2 slab ice from dust and then the breaking of the slab ice plate under the pressure built below it by subliming ice our modeling results and analysis of hirise images support the gas jet hypothesis and show that outbursts happen very early in spring

perennial water ice identified in the south polar cap of mars the inventory of water and carbon dioxide reservoirs on mars are important clues for understanding the geological climatic and potentially exobiological evolution of the planet1 from the early mapping observation of the permanent ice caps on the martian poles2 3 the northern cap was believed to be mainly composed of water ice whereas the southern cap was thought to be constituted of carbon dioxide ice however recent missions nasa missions mars global surveyor and odyssey have revealed surface structures4 altimetry profiles5 underlying buried hydrogen6 and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide7 here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes at a resolution of 2 km during the local summer when the extent of the polar ice is at its minimum we observe that this south polar cap contains perennial water ice in extended areas as a small admixture to carbon dioxide in the bright regions associated with dust without carbon dioxide at the edges of this bright cap and unexpectedly in large areas tens of kilometres away from the bright cap

late holocene slip rate and recurrence of great earthquakes on the san andreas fault in northern california the slip rate of the san andreas fault 45 km north of san francisco at olema california is determined by matching offset segments of a buried late holocene stream channel stream deposits from 1 800 plus minus 78 yr b p are offset 42 5 plus minus 3 5 m across the active 1906 fault trace for a minimum late holocene slip rate of 24 plus minus 3 mm yr when local maximum coseismic displacements of 4 9 to 5 5 m from the 1906 earthquake are considered with this slip rate the recurrence of 1906 type earthquakes on the north coast segment of the san andreas fault falls within the interval of 221 plus minus 40 yr both comparable coseismic slip in 1906 and similar late holocene geologic slip rates at the olema site and a site 145 km northwest at point arena prentice 1989 suggest that the north coast segment behaves as a coherent rupture unit

sublimation of mars s southern seasonal co2 ice cap and the formation of spiders 1 in this paper we define and describe morphological features that have colloquially been termed spiders and map their distribution in the south polar region of mars we show that these features go through a distinct seasonal evolution exhibiting dark plumes and associated fan shaped deposits during the local defrosting of the seasonal cap we have documented the seasonal evolution of the cryptic region and have found that spiders only occur within this terrain these observations are consistent with a geyser like model for spider formation association with the transparent cryptic portion of the seasonal cap is consistent with basal sublimation and the resulting venting of co2 gas also consistent with such venting is the observation of dark fan shaped deposits apparently emanating from spider centers spiders are additionally confined to the polar layered deposits presumably due to the poorly consolidated and easily eroded nature of their upper surface

the effect of the hadley circulation on the meridional propagation of stationary waves the effect on the meridional propagation of stationary planetary waves of the inclusion of the zonal mean hadley circulation in linearized models of the atmosphere is examined in a forced barotropic vorticity equation model a meridional wind in the basic state enables wave propagation through tropical easterlies in the direction of the wind this is shown to be consistent with the wkb solution in a multi level linearized steady primitive equation model with an idealized meridional circulation tropical forcing can still produce response through easterlies although propagation from one mid latitude to the other is much less than in the barotropic result this is explained through wkb and wave tracing methods for a simplified three dimensional vorticity equation the near non divergence of the unforced tropical motion permits a wave to travel largely within a layer of favourable winds in a twelve level model using observed january and july zonal mean conditions the hadley circulation changes the response to earth topographic forcing mainly in the subtropics however tropical heat forcing can now produce a large mid latitude response in the winter hemisphere especially the resulting subtropical jets suggest a mechanism for the maintenance of those observed over australia during the northern monsoon season and adjacent to the anomalous forcing of the 1983 el nino

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

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

analog models of restraining stepovers in strike slip fault systems scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike slip fault systems in rigid basement models were run both with and without synkinematic sedimentation which was added incrementally to cover the growing antiformal structures vertical and horizontal sections of the completed models permit the full three dimensional 3 d structure of the pop ups to be analyzed in detail three representative end member experiments are described 30 underlapping restraining stepovers 90 neutral restraining stepovers and 150 overlapping restraining stepovers the experimental pop ups are typically sigmoidal to lozenge shaped antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults underlapping restraining stepovers typically form elongate lozenge shaped pop ups 90 neutral restraining stepovers produce shorter squat rhomboidal pop ups and overlapping restraining stepovers produce sigmoidal antiformal pop ups trans pop up cross fault systems are characteristic at large displacements on the basement fault system above the offset principal displacement zones the pop ups are commonly small narrow positive flower structures whereas in the stepover region they widen out and become markedly asymmetric this pop up asymmetry switches across the center of the stepover where the pop ups are largely symmetical maximum rotations measured within the central highly uplifted region of the pop ups increase from 7 counterclockwise for the underlapping 30 stepovers to 14 counterclockwise for the neutral 90 stepovers to 16 counterclockwise for the overlapping 150 stepovers in models where no synkinematic sediments were added during deformation the pop up structures are bound by convex flattening upward oblique slip reverse fault systems that link downward to the offsets in the basement fault system in contrast in the begin page 234 experiments where synkinematic sediments were added incrementally during deformation the pop ups are formed by oblique slip reverse faults that steepen upward into the synkinematic strata with the formation of fault propagation growth folds the analog models are compared with natural examples of pop up structures and show strong similarities in structural geometries and stratal architectures these models may provide structural templates for seismic interpretation of complex contractional structures in offset strike slip fault systems

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

abrupt along strike change in tectonic style san andreas fault zone san francisco peninsula seismicity and high resolution aeromagnetic data are used to define an abrupt change from compressional to extensional tectonism within a 10 to 15 km wide zone along the san andreas fault on the san francisco peninsula and offshore from the golden gate this 100 km long section of the san andreas fault includes the hypocenter of the mw 7 8 1906 san francisco earthquake as well as the highest level of persistent microseismicity along that 470 km long rupture we define two distinct zones of deformation along this stretch of the fault using well constrained relocations of all post 1969 earthquakes based a joint one dimensional velocity hypocenter inversion and a redetermination of focal mechanisms the southern zone is characterized by thrust and reverse faulting focal mechanisms with ne trending p axes that indicate fault normal compression in 7 to 10 km wide zones of deformation on both sides of the san andreas fault a 1 to 2 km wide vertical zone beneath the surface trace of the san andreas is characterized by its almost complete lack of seismicity the compressional deformation is consistent with the young high topography of the santa cruz mountains coast ranges as the san andreas fault makes a broad restraining left bend 10 o through the southernmost peninsula a zone of seismic quiescence 15 km long separates this compressional zone to the south from a zone of combined normal faulting and strike slip faulting focal mechanisms including a m l 5 3 earthquake in 1957 on the northernmost peninsula and offshore on the golden gate platform both linear pseudo gravity gradients calculated from the aeromagnetic data and seismic reflection data indicate that the san andreas fault makes an abrupt 3 km right step less than 5 km offshore in this northern zone a similar right stepping dilatational geometry is also observed for the subparallel san gregorio fault offshore persistent seismicity and extensional tectonism occur within the san andreas right stepover region and at least 15 km along strike both to the se and nw the 1906 san francisco earthquake may have nucleated within the san andreas right stepover which may help explain the bilateral nature of rupture of this event our analysis suggests two seismic hazards for the san francisco peninsula in addition to the hazard associated with a m 7 to 8 strike slip earthquake along the san andreas fault the potential for a m 6 normal faulting earthquake just 5 8 km west of san francisco and a m 6 thrust faulting event in the southern peninsula

structure deformation and strength of the loma prieta fault northern california usa as inferred from the 1989 1990 loma prieta aftershock sequence analysis of aftershock focal mechanisms of the october 1989 m 7 1 loma prieta earthquake reveals details of structure and deformation aftershocks below 4 6 km define an alignment plane oriented 131 65 this alignment includes three planar segments defining a restraining bend in a blind strike slip fault dextral strike slip dominates in the south and reverse dextral slip dominates in the north a distribution similar to slip during the main shock above this fault dominantly reverse faults striking roughly 110 lie en echelon along the main fault trend in the 17 sets of spatially clustered aftershocks a more complex substructure is defined by local planar hypocenter alignments and by preferred orientations of aftershock shear planes the maximum shortening axes make angles with the normal to these planes that average 46 and 53 respectively these angles are incompatible with a weak fault for which 0 20 orientations of preferred aftershock shear planes resemble riedel shears relative to local hypocenter alignment planes the similarity of slip distributions inferred from the aftershock inversions and from the main shock implies that the stress drop during the main shock was incomplete and thus that the loma prieta fault is not unusually weak triaxial brittle deformation is partitioned into a pair of plane strains rotated relative to each other about a common principal axis the shape of the triaxial strain ellipsoid determines the angle of rotation evidence supports our micropolar model of brittle deformation which predicts that the antisymmetric part of the seismic moment tensor records effects of block rotations

reactivation of a collisional suture by miocene transpressional domes associated with the red river and song chay detachment faults northern vietnam abstract elongate miocene gneissose and granitic domes in northern vietnam formed in a dextral transpressional ductile shear regime possibly associated with large scale restraining step overs along dextral faults initial anticlinal d1 doming involved folding of both basement and hanging wall rocks with d1 secondary folds that verge toward the anticlinal axes such folds reflect dome scale flexural slip folding with continued shortening d2 detachment faults developed on the flanks of the anticlines along the hanging wall basement interface so that the basement was extruded vertically into the overlying hanging wall rocks the detachment faults were associated with d2 drag folds that verge away from the anticlinal axes the hanging wall assemblage lacks a well ordered stratigraphy displaying primarily block in matrix fabric we identified bedded cherts associated with umbers and alkalic basaltic intrusions within these hanging wall rocks a first report of such rocks from vietnam the association of cherts umbers and basaltic intrusions and extrusions with block in matrix units with clastic rocks strongly suggest that the hanging wall rocks comprise part of a subduction complex because the base of a subduction complex is a former subduction megathrust horizon the hanging wall basement interface represents a reactivated collisional suture such a suture was probably associated with the indosinian orogeny and the basement should be the indochina continental block this structure may have influenced the position of miocene dextral faulting in addition to controlling the position of the dome detachments the well known red river fault marks the boundary of one of the domes but in this region it appears to be a detachment normal fault rather than a dextral strike slip fault however the association with the dome evolution with large scale restraining step overs suggests that dextral faulting associated with dome development may lie further away from the dome axes than the detachment

no signature of clear co2 ice from the cryptic regions in mars south seasonal polar cap the seasonal polar ice caps of mars are composed mainly of co2 ice a region of low 30 albedo has been observed within the south seasonal cap during early to mid spring the low temperature of this cryptic region has been attributed to a clear slab of nearly pure co2 ice with the low albedo resulting from absorption by the underlying surface here we report near infrared imaging spectroscopy of the south seasonal cap the deep and broad co2 absorption bands that are expected in the near infrared with a thick transparent slab of co2 ice are not observed models of the observed spectra indicate that the low albedo results from extensive dust contamination close to the surface of a co2 ice layer which could be linked to atmospheric circulation patterns the strength of the co2 absorption increases after mid spring so part of the dust is either carried away or buried more deeply in the ice layer during the co2 ice sublimation process

interpreting quaternary uplift rates at the mendocino triple junction northern california from uplifted marine terraces analysis of the altitudinal spacing of 14 flights of marine terraces indicates a spatial pattern of varying uplift rates that agrees with that determined from previously dated terraces for the past 3 81 ka and temporal changes in uplift rates from 1 m ka to 3 5 m ka that may reflect response to changes in tectonic regime during passage of the mendocino triple junction mtj a possible mechanism for regional uplift is growth of a slab window south of the mtj the region of most rapid uplift is 25 43 km south of the mtj immediately south of the northern boundary of the slab window the coastline is tilted upward to the south in the region directly above the southern edge of the subducted gorda plate at point delgada 55 km south of the present mtj where the northern edge of the slab window passes approximately 300 ka uplift rates have been 1 2 m ka for at least 330 ka more than 1 4 m y after passage of the southern edge of the subducted slab at the mendocino coast uplift rates have been less than 0 4 m ka for at least 330 ka

variations of midlatitude transient dynamics associated with enso abstract the winter response of the atmosphere to el nino events in the pacific is studied both from a 14 year integration of the center for ocean land atmosphere gcm using observed ssts from january 1979 to february 1993 and from the corresponding analyses of ecmwf emphasis is put on the shift in the high frequency transients that define the pacific storm track during warm events warm and normal ensembles are defined on the basis of the gcm s diabatic heating field in the tropical pacific which falls in one of two states during the winters of 1982 83 1986 87 and 1991 92 the heating averaged between 6 s and 6 n lies in the range of 100 200 w m 2 all the way across the basin the remaining 10 normal years all show no large tropical diabatic heating anomalies in the mid or eastern pacific the difference between warm and normal ensembles for the mean fields of zonal wind u and height z indicates an eastward and equatorward extension of the midlatitude pacific jet associated with a similar extensio

late neogene and quaternary tectonics associated with northward growth of the san andreas transform fault northern california strain patterns within the forearc at a convergent margin adjacent to a passing fault fault trench triple junction show a systematic evolution we describe late neogene and quaternary deformation in northern coastal california in the last 3 m y associated with the migration of the mendocino triple junction deformation in the north american forearc near humbolt bay consists of a 30 km wide zone of on land contraction with a narrower zone of translation to the east the zone of subaerial contraction is part of the relatively wide southernmost extent of forearc contraction within the cascadia subduction zone net quaternary northeast southwest contraction across forearc thrust faults is at least 7 9 km and minimum fault slip rates on the six major thrust faults are 0 8 2 3 mm yr net right slip within the zone of translation is a minimum of 3 km south of the forearc translation is predominant in the north american plate and is accommodated along two major right lateral fault zones of the san andreas transform boundary the lake mountain and garberville fault zones discontinuous exposures of late neogene sediments along these faults near covelo and garberville respectively indicate that crustal contraction expressed by thrust faults and associated folding predated translational deformation at each site inception of contraction near covelo was related to inception of internal deformation of the gorda plate at 3 ma this contraction due to coupling of the convergent plates migrated northward in the forearc in advance of and at the rate of migration of the triple junction present day deformation near humboldt bay thus reflects contraction associated with subduction of the gorda plate and translation farther east associated with oblique convergence at the plate boundary as the triple junction migrates the strike slip faults of the san andreas transform boundary will extend to the north northwest into the present forearc and forearc strike slip faults will become faults of the san andreas system contractional structures near humboldt bay will then cease to be active and will be preserved in the geologic record in the same way as contractional structures are preserved farther south

distribution of displacement on and evolution of a young transform fault system the northern san andreas fault system california this paper presents a working model for the spatial and temporal distribution of dextral slip on the northern san andreas fault system of coastal california based primarily on field relations in the san francisco bay area and offers insight into the evolution of a young transform fault system a fundamental difference between this and previous models of slip distribution is that this model assigns negligible slip to the pilarcitos fault which has been suggested to have 120 to 250 km of post early miocene dextral slip in previous models because separation on the san francisco peninsula reach of the san andreas fault is about 25 km and displacement on the san andreas fault in central california is 310 to 320 km more than 250 km of late cenozoic dextral slip must be accommodated east of the san francisco peninsula the distribution of this dextral slip is constrained by offset late cenozoic and basement units slip distribution combined with ages of offset features and plate boundary kinematic constraints show that the distribution of slip rates on groups of faults along the transform boundary has changed in an irregular fashion through time in contrast to existing models that propose progressive eastward migration of active faulting in the san andreas system in addition to the shifting patterns of displacement a migrating transition zone connecting the eastern faults of the strike slip system to the mendocino triple junction may have resulted in distributed dextral faulting and shortening in the northernmost and youngest part of the transform fault system

interpretation of winter warming on northern hemisphere continents in 1977 94 abstract northern hemisphere december march near surface temperature and pressure anomalies during 1977 94 relative to those during 1946 76 are considered to a first approximation these anomalies can be decomposed into two components the first one reaches its maximum during 1977 88 the main features of the anomalies of 1977 88 can be obtained as the atmospheric general circulation model agcm response to sea surface temperature anomalies observed during that period the second component reaches its maximum in 1989 94 the main features of anomalies of 1989 94 relative to 1977 88 can be obtained as the agcm response to lower stratosphere ozone depletion observed in 1989 94

diverse pliocene quaternary tectonics in a transform environment san francisco bay region california the san francisco bay region occupies part of the diffuse transform boundary zone between the pacific and north american plates although dextral strike slip faulting is dominant the plate motion is expressed in a variety of ways some strike slip faults are parallel with the plate boundary but some are slightly oblique the major strike slip faults are zones in which an interplay occurs between strands some of which are en echelon this interplay may be responsible for some apparent pull apart basins and presumed normal faults along the calaveras fault zone in extensive areas between strike slip fault zones there are many compressional structures folds and reverse faults that are pliocene quaternary in age hence largely coeval with the strike slip faults some of the compressional structures are oblique to and others are parallel with the major strike slip faults we have divided the san francisco bay region into domains on the basis of the types orientations and relationships of structures or in one case lack of strong deformation each domain has responded in its own way to the regional northwest southeast dextral shear imposed by the relative plate motion in an attempt to understand the origin of various structures and the interrelationships among them we compared established models with our observations some observed geometric relationships agree quite well with classical coulomb anderson and simple shear models but others require further explanation models based on fault interaction seem to apply to certain cases for example the east bay hills domain which is between the left stepping calaveras and hayward rodgers creek fault zones is under compression resulting from interaction between the two strike slip zones

late quaternary deformation and slip rates in the northern san andreas fault zone at olema valley marin county california abstract quaternary sedimentary deposits along the structural depression of the san andreas fault saf zone north of san francisco in marin county provide an excellent record of rates and styles of neotectonic deformation in a location near where the greatest amount of horizontal offset was measured after the great 1906 san francisco earthquake a high resolution gravity survey in the olema valley was used to determine the depth to bedrock and the thickness of sediment fill along and across the saf valley in the gravity profile across the saf zone quaternary deposits are offset across the 1906 fault trace and truncated by the western and eastern boundary faults whose youthful activity was previously unknown the gravity profile parallel to the fault valley shows a basement surface that slopes northward toward an area of present day subsidence near the head of tomales bay surface and subsurface investigations of the late pleistocene olema creek formation qoc indicate that this area of subsidence was located further south during deposition of the qoc and that it has migrated northward since then localized subsidence has been replaced by localized contraction that has produced folding and uplift of the qoc this apparent alternation between transtension and transpression may be the result of a northward diverging fault geometry of fault strands that includes the valley bounding faults as well as the 1906 saf trace the vedanta marsh is a smaller example of localized subsidence in the fault zone between the 1906 saf trace and the western boundary fault analyses of holocene marsh sediments in cores and a paleoseismic trench indicate thickening and probably tilting toward the 1906 trace consistent with coseismic deformation observed at the site following the 1906 earthquake new age data and offset sedimentary and geomorphic features were used to calculate four late quaternary slip rate estimates for the saf at this latitude luminescence dates of 112 186 ka for the middle part of the olema creek formation qoc the oldest quaternary deposit in this part of the valley suggest a late pleistocene slip rate of 17 35 mm year which replaces the unit to a position adjacent to its sediment source area a younger alluvial fan deposit qqf basal age 30 ka is exposed in a quarry along the medial ridge of the fault valley this fan deposit has been truncated on its western side by dextral saf movement and west side down vertical movement that has created the vedanta marsh paleocurrent measurements clast compositions sediment facies distributions and soil characteristics show that the bear valley creek drainage now located northwest of the site supplied sediment to the fan which is now being eroded restoration of the drainage to its previous location provides an estimated slip rate of 25 mm year furthermore the bear valley creek drainage probably created a water gap located north of the qqf deposit during the last glacial maximum 18 ka the amount of offset between the drainage and the water gap yields an average slip rate of 21 30 mm year finally displacement of a 1000 year old debris lobe approximately 20 m from its hillside hollow along the medial ridge indicates a minimum late holocene slip rate of 21 25 mm year similarity of the late pleistocene rates to the holocene slip rate and to previous rates obtained in paleoseismic trenches in the area indicates that the rates may not have changed over the past 30 ka and perhaps the past 200 400 ka stratigraphic and structural observations also indicate that valley bounding faults were active in the late pleistocene and suggest the need for further study to evaluate their continued seismic potential

dark spots and cold jets in the polar regions of mars new clues from a thermal model of surface co2 ice observations of the martian co2 ice cap in late winter and spring have revealed exotic phenomena unusual dark spots fans and blotches form as the south polar seasonal co2 ice cap retreats the formation mechanisms of these features are not clearly understood theoretical models suggest that photons could penetrate deep into the co2 ice down to the regolith leading to basal sublimation and gas and dust ejection we have developed a detailed thermal model able to simulate the temporal evolution of the regolith co2 ice layer atmosphere column it takes into account heat conduction radiative transfer within the ice and the atmosphere and latent heat exchange when there is a phase transition we found that a specific algorithm fully coupling these three components was needed to properly predict ice sublimation below the surface our model allows us to determine under what conditions basal sublimation is possible and thus when and where it can occur on mars our results show that basal sublimation is possible if we consider large pathlengths and very little dust content within the ice moreover the model can explain how dark spots can appear very early after the end of the polar night at high latitudes we also evaluate the importance of the different parameters in our simulations contrary to what was suggested by theoretical models the role of seasonal thermal waves is found to be limited solar radiation alone can initiate basal sublimation which therefore only depends on the co2 ice properties three main modes were identified one where condensation sublimation only occurs at the surface in the case of small grains and or high dust content one where basal sublimation is possible large pathlengths and very little dust content and an intermediate mode where sublimation within the ice may occur we suggest that these different modes could be keys to understanding many processes that occur at the surface of mars like the anticryptic area behavior or the recent reported activity in gullies

formation of intracontinental basins in the opposite corners of the tabas block as coeval structures controlled by transpressional faulting iran the interaction of strike slip faults in their restraining junctions allowed for the coeval formation of the tabas and abdoughi basins and led to their inversion during the late cenozoic the intracontinental basins filled with neogene and quaternary deposits were controlled by large scale dextral transpression along major faults that bounded the tabas block which is a part of the central iranian block the anastomosing strike slip fault pattern facilitated the development of both basins in opposite corners of the tabas block the subsided areas were formed as a result of interaction between the restraining junctions of strike slip faults and thrusts flexural loading caused by the uplifted series of thrust sheets resulted in the depression of the opposite fault slabs which permitted deposition of neogene sediments deformation according to the bookshelf mechanism can be considered as a consequence of accommodation of the shortening of the area north of the main zagros thrust and externally imposed shearing along the great kavir doruneh fault during the collision of the arabian and eurasian plates related processes of transpression and counterclockwise rotation of the tectonic blocks included in the central iranian block favored the interaction of strike slip faults the change of far field stress and continuous transpression caused inversion of the basins and formation of neogene folds in the northern and southern corners of the tabas block the geomorphic features observed along these strike slip faults and on the thrust surfaces bounding the folds display their recent activity consistent with present day seismicity and geodetic measurements within the central iranian block

exhumation of triassic hp lt rocks by upright extrusional domes and overlying detachment faults ishigaki jima ryukyu islands abstract the tomuru formation of ishigaki jima in the southernmost part of the ryukyu arc comprises blueschist facies subduction complex rocks metamorphosed in triassic time d1 structures related to subduction blueschist facies mineral growth and possibly early stages of exhumation are deformed by d2 structures that appear to reflect the last stage of exhumation d2 structures define several anticlines with parasitic overturned folds verging away from anticlinal axes the shortening recorded by this deformation appears to reflect upward extrusion relative to flanking material the anticlines are flanked by detachment faults with normal sense of shear parallel to d2 vergence hanging wall rocks that include the fusaki formation an accretionary prism with early cretaceous metamorphic ages and late eocene limestone conglomerate and andesitic volcanics the eocene strata contain metamorphic detritus derived from the tomuru and fusaki formations indicating pre late eocene surface exposure of these units ultramafic rocks and gabbro blocks of the tomuru formation were incorporated by sedimentary sliding into the trench prior to subduction and high pressure metamorphism rather than being emplaced as diapirs along a post metamorphic fault as previously proposed geochronologic metamorphic and thermal considerations suggest exhumation of the tomuru formation to relatively shallow crustal depths prior to or concurrent with early cretaceous metamorphism of the fusaki formation arcward vergent thrusting may have placed the younger and formerly structurally lower subduction complex fusaki formation over the older one tomuru formation d2 extrusional doming began after the emplacement of the fusaki formation at high structural levels the d2 transport directions are subparallel to the strike of the orogen suggesting that the upright extrusion may have occurred along a forearc strike slip fault system this final stage of exhumation concluded in the late eocene with extensional collapse and the development of detachment faults the progression from initial exposure of the tomuru and fusaki formations deposition of late eocene strata extrusional doming and late detachment faulting may have been associated with migrating step overs rather than changes in regional tectonics

present tectonic motion across the coast ranges and san andreas fault system in central california geodetic results from very long baseline interferometry vlbi satellite laser ranging slr and the global positioning system gps are used to estimate angular velocities between the sierran microplate pacific plate and north american plate the sierra pacific pole of rotation lies nearer to the san andreas fault than does the pacific north america pole of rotation and leads to different tectonic implications than if the latter is used the angular velocities show that the san andreas fault system and central california coast ranges accommodate motion of 39 2 mm yr mainly by strike slip faulting all confidence limits following signs in this paper are 95 confidence limits fault normal motion is small is mainly convergent at rates up to 3 3 1 0 mm yr and varies along the coast but is divergent at 2 6 1 2 mm yr across san pablo bay and associated topographic lows across which the sierran and central valley watershed drains to the pacific ocean the mountain ranges tend to be larger where the fault normal convergence rates are larger the low convergence rate 0 5 1 8 mm yr normal to the san andreas fault in the carrizo plain differs sharply from that previously inferred 8 2 1 2 mm yr and 4 9 1 6 mm yr by feigl et al 1993 the difference is due to differences between their and our elastic strain accumulation models and between how their and our pacific plate reference frames are defined the ranges in most places require a minimum of 4 2 1 m y of fault normal convergence at the present rate to attain their present cross sectional area if erosion is neglected more if it is not the amount of convergence previously estimated from a balanced cross section across the diablo range in central california requires 10 8 3 m y of convergence at the present rate the former is consistent with widely held views about the onset of the coast range orogeny but the latter is not both are consistent however with the recent plate reconstructions by s cande j stock and colleagues which indicate that pacific plate motion relative to north america changed to a more convergent direction 20 25 clockwise of its prior direction at ca 8 to 6 ma and not at 3 5 ma as had been previously inferred the inferred change in direction of plate motion is large compared with the present angle of convergence across the straight and narrow segment of the san andreas fault of 0 7 4 7 from which we infer that the sierran microplate changed motion relative to north america at the same time ca 8 to 6 ma as did the pacific plate we further infer that the motion accommodated across the great basin must also have changed at the same time we also examine the hypothesis that stable sliding occurs along the san andreas fault and other northwest striking strike slip faults in central california where the fault normal convergence rate is low or negative and that these faults are unstable where the fault normal convergence rate is high such a relationship appears to hold in general but fails in detail in particular there are substantial sections of fault with small inferred rates of fault normal convergence across which the san andreas fault is locked moreover the creeping section of the san andreas fault i e the section between parkfield and the calaveras junction is the locus of greater fault normal convergence 3 2 1 4 mm yr than is the locked part of the fault 0 5 1 8 mm yr south of parkfield thus this hypothesis is at best a partial explanation for the observed distribution of locked and nonlocked sections of the fault

a comparison between mid paleozoic new england usa and the modern western usa subduction of an oceanic ridge transform fault system abstract a detailed comparison is made between the mid paleozoic norumbega fault system nfs in maine usa and the san andreas fault system safs of coastal california usa and their tectonic settings the safs formed following subduction of an oceanic ridge transform system and the nfs is interpreted as having formed the same way the parallel evaluation of mid crustal processes associated with the nfs and surface processes associated with the safs gives a unique perspective on modern and inactive crustal scale strike slip fault systems the safs is separated from the cascadia subduction zone by the mendocino triple junction similarly the mid paleozoic nfs is interpreted as having been separated from a convergent zone to the southwest by the interpreted norumbega triple junction the safs cuts a subduction complex the franciscan complex that formed when the margin was still convergent no equivalent subduction complex associated with the nfs has been recognized perhaps because it was eroded away or displaced or because it never formed the safs experienced periods of transpression and transtension whereas the nfs shows transpressive structures only slab window magmatic rocks exist along the safs devonian to earliest carboniferous plutonic rocks along the nfs may in part be slab window magmatic rocks although the effects of the acadian and prior orogenies in the appalachians complicate their identification magmatic rocks along the safs are offset due to the high rate of displacement 20 40 mm yr magmatic rocks along the nfs show little offset consistent with the low rate of displacement 3 mm yr in the model transverse structures are associated with both the safs and nfs other mendocino type triple junctions may have existed in the past but may be difficult to recognize where a past crustal scale intracontinental strike slip fault system terminates on an along strike contemporaneous convergent system such a fault fault trench triple junction may be considered

superposed deformation and inherited structures in an ancient dilational step over zone post mortem of the rengali province india abstract in the eastern indian shield a dextral strike slip system juxtaposed the archaean singhbhum province against the proterozoic eastern ghats belt at 490 470 ma two wnw ese trending strands of the strike slip system enclose a multiply deformed d1 to d3 intervening domain called the rengali province with d3 representing dextral shearing in a granulite lens within the province an early fabric sgr was deformed by an amphibolite facies d1 d2 deformation continuum in the late archaean time forming cylindrical folds in the surrounding quartzofeldspathic gneisses quartzites and mica schists of the province superimposition of syn d3 shortening on d1 d2 folds generated complex non cylindrical geometries the granulites escaped d3 strain microstructures in the province bounding shear zones confirm that d3 deformation was associated with mylonitization dynamic recrystallization and greenschist facies metamorphism in the quartzites syn d3 folds can be correlated with rotation of d1 d2 structures through the shortening zone of bounding dextral shears since the province bounding shears form a step over zone the structural complexity within the rengali province arises from superposition of syn d3 shortening structures on initially asympathetically oriented inherited cylindrical d1 d2 folds hydrous fluid channeling causing greenschist facies metamorphism and quartz vein emplacement accompanied d3 as the step over zone was dilational in nature

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