distribution and mechanism of neogene to present day vertical axis rotations pacific australian plate boundary zone south island new zealand remarkably little knowledge exists about mechanisms of vertical axis rotation in continental crust steeply dipping basement rocks in south island new zealand provide an opportunity to map the distribution of rotations across the pacific australian plate boundary zone and to delineate boundaries of rotated blocks in unusual detail we synthesize new structural data with new and existing paleomagnetic data with geodetic data and with patterns of neogene quaternary faulting in the strike slip marlborough fault system for the past 20 m y vertical axis rotations have been hinged about two crustal scale boundaries near the east coast the ne hinge accommodated 50 of early middle miocene clockwise rotation which caused deformation of the eastern ends of the alpine wairau and clarence strike slip faults the sw hinge has accommodated a further 30 50 of finite clockwise rotation since 4 ma and deflects active fault traces the locus of rotation has shifted southwestward astride a subduction margin that is lengthening in that direction rotating rocks are pinned to the south against a locked collision zone where the continental chatham rise impinges against the margin slip on inland strike slip faults is transformed seaward across a zone of fault termination into rigid body rotation of a large continental block that has been thrust eastward over the downgoing subducted slab of the pacific plate the rotation mechanism is a migrating hinge which resembles a flexed telephone book strike slip faults are translated through a brecciated hinge region that does not coincide with a fixed material line in the rock

late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

the palos verdes anticlinorium along the los angeles california coast implications for underlying thrust faulting 1 the palos verdes anticlinorium pva is a 70 km wide nw trending pliocene quaternary transpressional high that separates the onshore los angeles basin from the offshore san pedro basin the pva southwest limb underlies a 70 km long sw facing seafloor escarpment this escarpment is separated into two segments by a 6 km right step it overlies two levels of ne dipping low angle thrust faults these faults project beneath los angeles toward known sw verging blind thrust faults and may be the same regional faults these conclusions are based on a detailed geometric representation constructed from a regional analysis of 5000 km of multichannel seismic reflection profiles logs and paleontology from 19 wells published seafloor geology and multibeam bathymetric data patterns of sedimentation and erosion indicate that folding of pva initiated during pliocene time and propagated southward with folding of part of the southern pva occurring during the quaternary structural relief continues to grow with regional long term rock uplift of the pva crest and ongoing subsidence in the adjacent basins the low angle ne dipping faults beneath the pva must be late quaternary active in order to maintain hanging wall rock uplift above subsiding footwalls structural geometry requires that the underlying faults have slipped at 1 1 1 6 mm yr over quaternary time

lower crustal deformation beneath the central transverse ranges southern california results from the los angeles region seismic experiment 1 we present a p wave velocity model derived from active source seismic data collected during the 1994 los angeles region seismic experiment our model extends previously published upper crustal velocity models to mantle depths our model was developed by both ray tracing through a layered model and calculating travel times through a gridded model it includes an 8 km thick crustal root centered beneath the surface trace of the san andreas fault north of the highest topography in the san gabriel mountains a simple mass balance calculation suggests that 36 km of north south shortening across the san andreas fault in the central transverse ranges could have formed this root if north south compression began when the big bend in the san andreas fault formed at 5 ma 36 km of shortening implies a north south contraction rate of 7 1 mm yr across the central transverse ranges if instead north south compression began when the transverse ranges formed at 3 4 3 9 ma 36 km of shortening implies a contraction rate of 9 2 10 6 mm yr north of the san andreas fault the mojave desert crust has a low velocity 6 3 km s mid and lower crust and a 28 km deep moho south of the san andreas fault beneath the los angeles and san gabriel valley basins there is a fast 6 6 6 8 km s thick 10 12 km lower crust with a 27 km deep moho farther south still the lower crust of the continental borderland is fast 6 6 6 8 km s and thin 5 km with a shallow 22 km deep moho

late cenozoic tectonics of the east ventura basin transverse ranges california the east ventura basin originated in the middle miocene as a rift system bounded on one side by the oak ridge simi hills structural shelf and on the other side by a granitic ridge parallel to the san gabriel fault this fault began accumulating right slip 10 12 m y ago at a rate of 4 5 9 mm yr depending on whether total slip is 45 or 60 km slowing to about 1 mm yr in the quaternary north of the santa clara river rifting ended prior to deposition of the uppermost miocene lower pliocene towsley formation south of the santa clara river the rift axis shifted southwest toward the oak ridge simi hills shelf as the towsley formation accumulated against a normal fault ancestor of the santa susana fault a change to contractile tectonics occurred in the pliocene with depos tion of the fernando formation when the newhall potrero anticline developed as a monocline above a blind reverse fault the pico anticline to the southeast and the temescal and hopper ranch modelo anticlines to the northwest may have a similar origin tectonic inversion and displacement on the southwest verging santa susana fault began about 0 5 ma based on appearance of locally derived clasts in the upper saugus formation and its equivalents and continues today along with the southwest verging san cayetano fault farther west also active are northeast verging backthrusts occurring in the east ventura basin fold belt and a segment of the san gabriel fault which now acts as a northeast dipping oblique slip reverse fault northeast trending discontinuities and structures divide the present deformation zone into four segments in the hopper canyon segment at the west end of the area in the west ventura basin the san cayetano fault places miocene modelo formation over pliocene pleistocene strata more than 5 km thick largely at maximum burial to the southeast the newhall potrero segment is characterized by north vergent backthrusts within the east ventura fold belt and by southward thrusting of the basin sequence tectonic inversion over the structural shelf on the santa susana fault farther southeast the placerita segment is marked by reverse faulting on both the santa susana fault and the san gabriel fault southeast of the basin in the san fernando valley the sylmar segment contains a thick pli cene pleistocene sequence overridden by basement rocks of the san gabriel mountains as well as the south verging mission hills granada hills and northridge hills fault zones

crustal deformation along the altyn tagh fault system western china from gps we collected gps data from the southern tarim basin the qaidam basin and the western kunlun shan region between 1993 and 1998 to determine crustal deformation along the altyn tagh fault system at the northern margin of the tibetan plateau we conclude from these data that the altyn tagh is a left lateral strike slip fault with a current slip rate of 9 mm yr in sharp contrast with geological estimates of 20 30 mm yr this contrast poses an enigma because the gps data cover a wider region than the geologic data they might be expected to reveal somewhat more slip we also find that the tarim and qaidam basins behave as rigid blocks within the uncertainty of our measurements rotating clockwise at a rate of 11 and 4 5 nrad yr respectively with respect to the eurasia plate the rotation of the tarim basin causes convergence across the tian shan increasing progressively westward from 6 mm yr at 87 e to 18 mm yr at 77 e our data and other gps data suggest that the indo asia collision is mainly accommodated by crustal shortening along the main himalayan thrust system 53 and the tian shan contractional belt 19 eastward extrusion of the tibetan plateau along the altyn tagh and kunlun faults accommodates only 23 of the indo asia convergence

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

crustal velocity field near the big bend of california s san andreas fault we use geodetic data spanning the 1920 1992 interval to estimate the horizontal velocity field near the big bend segment of california s san andreas fault saf more specifically we estimate a horizontal velocity vector for each node of a two dimensional grid that has a 15 min by 15 min mesh and that extends between latitudes 34 0 n and 36 0 n and longitudes 117 5 w and 120 5 w for this estimation process we apply bilinear interpolation to transfer crustal deformation information from geodetic sites to the grid nodes the data include over a half century of triangulation measurements over two decades of repeated electronic distance measurements a decade of repeated very long baseline interferometry measurements and several years of global positioning system measurements magnitudes for our estimated velocity vectors have formal standard errors ranging from 0 7 to 6 8 mm yr our derived velocity field shows that 1 relative motion associated with the saf exceeds 30 mm yr and is distributed on the earth s surface across a band 100 km wide that is roughly centered on this fault 2 when velocities are expressed relative to a fixed north america plate the motion within our primary study region has a mean orientation of n44 w 2 and the surface trace of the saf is congruent in shape to nearby contours of constant speed yet this trace is oriented between 5 and 10 counterclockwise relative to these contours and 3 large strain rates shear rates 150 nrad yr and or areal dilatation rates 150 nstr yr exist near the garlock fault near the white wolf fault and in the ventura basin

continuing evolution of the pacific juan de fuca north america slab window system a trench ridge transform example from the pacific rim abstract many subduction margins that rim the pacific ocean contain complex records of cenozoic slab window volcanism combined with tectonic disruption of the continental margin the series of slab windows that opened beneath california and mexico starting about 28 5 ma resulted from the death of a series of spreading ridge segments and led to piecewise destruction of a subduction regime the timing and areal extent of the resultant slab window volcanism provide constraints on models that depict the subsequent fragmentation and dispersal of the overlying continental margin the initial pioneer slab window thermally weakened the overlying western transverse ranges and california borderlands region starting about 28 5 ma a second thermal pulse occurred in the same region starting about 19 ma during growth of the monterey slab window this additional heating combined with the capture of a partially subducted monterey plate fragment by the cocos plate initiated the pulling apart and rotation of the adjacent continental margin similarly the capture of guadalupe and magdalena plate fragments by the pacific plate and initiation of the guadalupe magdalena slab window about 12 5 ma are coeval with baja california pulling away from the mexico continental margin with the break along the comondu arc in crust already thermally weakened by about 10 my of volcanism in coastal california distributed crustal extension and subsidence accompanied the new transform plate boundary and continued until the slab windows cooled and plate motion coalesced along a through going system of strike slip faults the transform boundary continues to evolve and forward modeling predicts an instability with the current configuration as a result of convergence between the sierra nevada and peninsular ranges batholiths starting about 2 my in the future the instability may be resolved by a shift in the locus of transform motion from the san andreas fault to the eastern california shear zone or by breaking off another fragment of the mojave or southern sierra nevada crustal blocks and translating it northward

formation evolution and inversion of the middle tertiary diligencia basin orocopia mountains southern california the diligencia basin located in the eastern orocopia mountains of southern california contains 1500 2000 m of oligocene miocene continental siliciclastic sedimentary rocks and subordinate limestone and evaporite deposits intercalated with basaltic lavas these rocks are locally intensely folded and faulted defining in present day coordinates an elongate east west trending basin and are unconformably overlain by flat lying late pliocene and pleistocene alluvial deposits the sedimentological stratigraphic and structural history of the basin is compatible with late oligocene early miocene formation as a half graben basin produced by orthogonal basin and range type extension and latest miocene to pliocene basin inversion in either a localized transpressional transverse range regime or a more diffuse compressional regime associated with north south shortening of the entire mojave block facies associations in the lower part of the diligencia basin display a distinctly asymmetric distribution across the basin indicating deposition in a half graben with a steep possibly fault controlled south facing northern escarpment and a more gentle north facing southern slope paleocurrent data particularly from high energy deposits on the northern basin margin indicate stream flow toward the southeast to south southeast oblique to the deformed basin margins and suggest an approximate northeast to east northeast strike for the basin before deformation previously published paleomagnetic data however indicate that the elongate currently east west trending fault block containing the diligencia basin has rotated clockwise by as much as 110 about a vertical axis angle depends on data and model used since deposition ceased if correct this would suggest that the basin may have originally opened to between the northeast and east northeast subparallel to well documented extension directions in miocene age basins in the mojave desert to the east clockwise rotation on east west fault blocks exposed in this region has previously been bracketed as between ca 10 and 4 5 ma structural and paleomagnetic data indicate that inversion of the diligencia basin occurred after block rotation implying a latest miocene to pliocene age for inversion we speculate that basin inversion within a north south and still active compressive stress field resulted from the locking and subsequent internal deformation of this previously passively rotating elongate crustal block

the impact of hydrodynamics authigenesis and basin morphology on sediment accumulation in an upwelling environment the miocene monterey formation at shell beach and mussel rock pismo and santa maria basins central california u s a abstract in the sections at shell beach pismo basin and mussel rock santa maria basin both along the central california coast the miocene monterey formation is composed of laminated biosiliceous organic rich and phosphate rich sediments which are intercalated with carbonates and volcanogenic deposits the presence of small and large scale scours conglomerates gravity flow deposits slumps and condensed intervals reveals the importance of sediment reworking which is related to the presence of bottom currents the general morphology of the basins and recurrent high energy events of likely seismic origin the lithologies reflect strong upwelling enhanced primary productivity and the presence of an oxygen minimum zone which is indicated by ubiquitous lamination a general absence of bioturbation with the rare exception of traces related to the gravity flow displacement of trace makers high pyrite mo and u concentrations and molar ratios of total organic carbon and organic phosphorus toc p org of up to 2000 oxygen depletion was particularly severe during the early langhian and the late serravallian and early tortonian and favored the preservation of organic matter episodes of deposition of organic rich sediment were followed by longer phases of subdued sediment accumulation and early diagenetic phosphogenesis which resulted in the formation of a low porosity less permeable lid in the near surface sediments that assisted in preserving organic matter phosphogenesis occurred throughout the time interval considered approximately 17 5 6 5 ma and phosphate rich sediments formed especially during periods of sedimentary condensation in the langhian and earliest serravallian and the latest tortonian and early messinian a comparison of temporal trends in the type and quantity of deposited sediments between the two sections at shell beach and mussel rock and three sections of the santa barbara basin el capitan state beach naples beach and haskell beach confirms that towards the top of the monterey formation sediments become enriched in biosilica in all three basins the transition into this lithology occurred diachronously during the serravallian in the pismo and santa maria basins and tortonian in the santa barbara basin and was probably related to a diachronous shift to more intense upwelling in all three basins a phase of important condensation and phosphogenesis terminated monterey type sedimentation during the latest tortonian and early messinian in both the santa maria and santa barbara basins the uniform expression of this phase is interpreted as the result of improved connections between the two basins

block kinematics of the pacific north america plate boundary in the southwestern united states from inversion of gps seismological and geologic data 1 the active deformation of the southwestern united states 30 41 n is represented by a finite number of rotating elastic plastic spherical caps gps derived horizontal velocities geologic fault slip rates transform fault azimuths and earthquake derived fault slip vector azimuths are inverted for block angular velocities creep on block bounding faults permanent strain rates within the blocks and the rotations of 11 published gps velocity fields into to a common north american reference frame gps velocities are considered to be a combination of rigid block rotations recoverable elastic strain rates resulting from friction on block bounding faults and nonrecoverable strain rates resulting from slip on faults within the blocks the resulting pacific north america angular velocity is similar to some published estimates and satisfies transform azimuths and one spreading rate in the gulf of california earthquake slip vectors in the gulf of california and alaska and gps velocities along coastal california and within the pacific basin published fault slip rates are satisfied except in the southern mojave desert where the motion of the mohave block relative to north america is faster than can be explained by mapped faults the largest blocks the sierra nevada great valley and the eastern basin and range show permanent strain rates after removing elastic strain of only a few nanostrain per year demonstrating approximately rigid behavior observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short term strains evident in gps velocities are largely elastic in only about 20 of the region is distributed deformation needed to match the data indicating that a plate tectonic style description of the deformation of the western united states is plausible most blocks rotate about vertical axes at approximately the same rate as the pacific relative to north america suggesting that locally spin rates are communicated from block to block arguing against both floating block and ball bearing mechanisms of block rotation the similarities of the blocks spin rates to that of the pacific suggests that the pacific strongly influences their motions through edge tractions however it is shown that the blocks cannot rotate about the pacific north america pole without spinning counter to the sense of pacific north america shear unlike some other broad plate boundaries in the western united states vertical axis rotations take up very little of the slip rate budget across the region

intraplate rotational deformation induced by faults vertical axis rotations provide important constraints on the tectonic history of plate boundaries geodetic measurements can be used to calculate interseismic rotations whereas paleomagnetic remanence directions provide constraints on the long term rotations accumulated over geological timescales here we present a new mechanical modeling approach that links between intraplate deformational patterns of these timescales we construct mechanical models of active faults at their locked state to simulate the presumed to be elastic interseismic deformation rate observed by gps measurements we then apply a slip to the faults above the locking depth to simulate the long term deformation of the crust from which we derive the accumulated rotations we test this approach in northern israel along the dead sea fault and carmel gilboa fault system we use 12 years of interseismic gps measurements to constrain a slip model of the major faults found in this region next we compare the modeled rotations against long term rotations determined based on new primary magnetic remanence directions from 29 sites with known age the distributional pattern of site mean declinations is in general agreement with the vertical axis rotations predicted by the mechanical model both showing anomalously high rotations near fault tips and bending points overall the results from northern israel validate the effectiveness of our approach and indicate that rotations induced by motion along faults may act in parallel or alone to rigid block rotations finally the new suggested method unravels important insights on the evolution timing magnitude and style of deformation along major faults

a detailed image of the continent borderland transition beneath long beach california new crustal images beneath long beach california show the region of the inner borderland to continent transition the cross sections are obtained from stacked autocorrelations of virtual sources generated from oil industry data recorded in the city of long beach ca they show that the moho is dipping at 65 and obliquely truncates an 10 km thick flat lying lower crustal fabric the moho appears to be fault controlled and an integral part of the extrusion of the catalina schist that underlays the inner borderland the basement interface has significant offsets of up to 2 km none of which correspond to the mapped trace of the newport inglewood fault

ultradeep metamorphic rocks the retrospective viewpoint ultradeep or ultra high pressure uhp metamorphic rocks formed from crustal protoliths within the stability field of coesite at pressures 2 5 3 0 gpa corresponding to depths 80 120 km occur locally though regionally distributed in at least five continental areas their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high pressure studies detailed petrographic and microprobe work especially on mineral inclusions in favorable cases allows the derivation of prograde pt paths during subduction and of retrograde ones during exhumation commonly the gneisses adjacent to the uhp rocks do not exhibit signs of ultradeep metamorphism apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower pt conditions during the retrograde path it is also possible however that uhp equilibria were not attained throughout the rock volumes subducted but only along zones of shearing and fluid introduction if it is true that not all uhp metamorphic rocks return to the crustal orogenic belts but some continue to be subducted to greater mantle depths the classical geochemical pattern of a one way mass transfer from mantle to crust throughout the earth s history is at stake the assumed gradual growth of continents may have had a counterpart of continent destruction during collision events most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous mg a1 silicates exist that were not found in nature thus far but may be characteristic minerals in the cold portions of old subduction zones thus extending the water retentivity of subducting slabs to greater and hitherto unexpected depths

oceanside and thirtymile bank blind thrusts implications for earthquake hazards in coastal southern california we define an active blind thrust system in offshore southern california that extends from los angeles south to the united states mexico international border these blind thrusts formed by tectonic inversion of miocene extensional detachments we attribute the 1986 oceanside ml 5 3 earthquake local uplift of marine terraces seafloor fold scarps and observed geodetic convergence to motion on these faults single and multisegment fault rupture scenarios suggest the potential for large m 7 1 7 6 but infrequent earthquakes that would affect the los angeles and san diego metropolitan areas

map restoration of folded and faulted late cenozoic strata across the oak ridge fault onshore and offshore ventura basin california the ventura basin lies within the east west trending active fold and thrust belt of the western transverse ranges california this basin has been the site of significant earthquakes on structures within it and bordering it the purpose of our study is to identify the main structures in the basin and its borders and to quantify their rate of deformation our study includes the onshore and offshore ventura basin the arcuate basin bounding oak ridge reverse fault and the oxnard shelf to the south shortening fault slip and crustal block motions were studied using a three dimensional map restoration technique structure contour maps on the 6 ma surface and other horizons were digitized and restored to the initial horizontal state by unfolding them using the computer program unfold and then fitting the unfolded surfaces across faults comparing the restored and present configuration allows us to estimate total net finite displacements relative to a fixed horizontal reference line average post 5 ma shortening rates estimated from our restoration are slower than both post 1 ma rates and present rates determined by global positioning systems most shortening due to folding in the onshore basin is post 1 ma although slip on the oak ridge fault has occurred both before and after 1 ma displacement due to faulting and folding includes left lateral strike slip motion on the northeast southwest coastal segment of the oak ridge fault and associated clockwise rotation of the adjacent ventura basin the oxnard shelf is bordered to the south by mountains and islands that have been previously interpreted as folds above thrust fault ramps this onshore offshore block moves as one continuous thrust sheet similarly beyond the well studied onshore fault a kinematically continuous offshore oak ridge mid channel left oblique fault system is interpreted to continue at least an additional 100 km westward beneath the santa barbara channel

paleogeographic and tectonic implications of the geology of the tiefort mountains northern mojave desert california geologic mapping petrologic and structural analyses and u pb dating in the tiefort mountains northern mojave desert california provide data on paleozoic paleogeography and mesozoic intra arc tectonics metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late precambrian cordilleran passive margin facies recognition of precambrian augen gneiss and metasedimentary rocks with craton margin protoliths suggests that the craton miogeocline boundary is in or near the tiefort mountains the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area the metasedimentary units are intruded by middle jurassic granite and diorite orthogneisses dated by u pb as ca 164 and ca 160 ma deformed rocks record amphibolite facies metamorphism and mylonitization during middle to late jurassic southeast vergent ductile shear pervasively annealed microstructures indicate that recrystallization outlasted deformation at south tiefort mountain the mylonitic fabric is deformed by northwest trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 14 ma at tiefort mountain synkinematic biotite granite gneiss is dated as ca 105 ma and mylonitic fabrics are folded about north trending axes the presence of the cretaceous orthogneiss suggests that two deformational events jurassic and cretaceous in age may have affected part of the study area dikes dated as ca 82 ma that crosscut all fabrics are interpreted to be part of a regional intrusive event the timing and vergence of shear zones at south tiefort mountain are similar to contractional shear zones in adjacent areas forming a belt of contractional deformation from the central mojave desert to the east sierran thrust system this deformation may be related to a period of late middle to late jurassic sinistral oblique subduction

families of miocene monterey crude oil seep and tarball samples coastal california biomarker and stable carbon isotope ratios were used to infer the age lithology organic matter input and depositional environment of the source rocks for 388 samples of produced crude oil seep oil and tarballs to better assess their origins and distributions in coastal california these samples were used to construct a chemometric multivariate statistical decision tree to classify 288 additional samples the results identify three tribes of 13c rich oil samples inferred to originate from thermally mature equivalents of the clayey siliceous carbonaceous marl and lower calcareous siliceous members of the monterey formation at naples beach near santa barbara an attempt to correlate these families to rock extracts from these members in the nearby cost continental offshore stratigraphic test ocs cal 78 164 well failed at least in part because the rocks are thermally immature geochemical similarities among the oil tribes and their widespread distribution support the prograding margin model or the banktop slope basin model instead of the ridge and basin model for the deposition of the monterey formation tribe 1 contains four oil families having geochemical traits of clay rich marine shale source rock deposited under suboxic conditions with substantial higher plant input tribe 2 contains four oil families with traits intermediate between tribes 1 and 3 except for abundant 28 30 bisnorhopane indicating suboxic to anoxic marine marl source rock with hemipelagic input tribe 3 contains five oil families with traits of distal marine carbonate source rock deposited under anoxic conditions with pelagic but little or no higher plant input tribes 1 and 2 occur mainly south of point conception in paleogeographic settings where deep burial of the monterey source rock favored petroleum generation from all three members or their equivalents in this area oil from the clayey siliceous and carbonaceous marl members tribes 1 and 2 may overwhelm that from the lower calcareous siliceous member tribe 3 because the latter is thinner and less oil prone than the overlying members tribe 3 occurs mainly north of point conception where shallow burial caused preferential generation from the underlying lower calcareous siliceous member or another unit with similar characteristics in a test of the decision tree 10 tarball samples collected from beaches in monterey and san mateo counties in early 2007 were found to originate from natural seeps representing different organofacies of monterey formation source rock instead from one anthropogenic pollution event the seeps apparently became more active because of increased storm activity

rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada 1 we use paleomagnetic data from tertiary volcanic rocks to address the rates and timing of vertical axis block rotations across the central sierra nevada walker lane transition in the bodie hills california nevada samples from the upper miocene 9 ma eureka valley tuff suggest clockwise vertical axis block rotations between ne striking left lateral faults in the bridgeport and mono basins results in the bodie hills suggest clockwise rotations r r 95 confidence limits of 74 8 since early to middle miocene 12 20 ma 42 11 since late miocene 8 9 ma and 14 10 since pliocene 3 ma time with no detectable northward translation the data are compatible with a relatively steady rotation rate of 5 2 ma 1 2 since the middle miocene over the three examined timescales the average rotation rates have probably not varied by more than a factor of two over time spans equal to half of the total time interval our paleomagnetic data suggest that block rotations in the region of the mina deflection began prior to late miocene time 9 ma and perhaps since the middle miocene if rotation rates were relatively constant block rotation in the bodie hills is similar in age and long term average rate to rotations in the transverse ranges of southern california associated with early transtensional dextral shear deformation we speculate that the age of rotations in the bodie hills indicates dextral shear and strain accommodation within the central walker lane belt resulting from coupling of the pacific and north america plates

three stage evolution of the los angeles basin southern california we propose that an episode of transtension dominated development of the los angeles basin area from 12 to 6 ma following middle miocene transrotation and prior to the modern transpressional regime transtension resulted from the releasing bend of the san gabriel chino hills cristianitos fault which acted as the primary transform boundary in southern california during this episode such an interpretation implies that significant transform motion did not occur on the southern san andreas fault prior to 6 ma and that the gulf of california has opened primarily since 6 ma we propose a three stage model for evolution of the los angeles basin and vicinity within the evolving transform fault system transrotation 18 12 ma transtension 12 6 ma and transpression 6 0 ma timing of these stages correlates with microplate capture events which occurred during conversion from a convergent margin to a transform margin

tectonics of the san gabriel basin and surroundings southern california the sangabriel basin is a pliocene pleistocene transverse ranges structure bounded by the san gabriel mountains on the north the san jose and puente hills on the east and the repetto and montebello hills on the west a basement boundary between gneissoid rocks of san gabriel mountains aspect and peninsular ranges rocks strikes east across the basin and connects with the san jose fault this boundary may represent the eastward continuation of the canton fault a middle miocene precursor of the san gabriel fault system with up to 30 km of right slip canton fault displacement is largely canceled by 22 5 km of left slip on a precursor to the raymond fault such that an oligocene dacite in the san jose hills and clasts derived from it show little evidence of large horizontal displacement between the san gabriel mountains and puente hills clockwise rotation in the middle miocene probably influenced the distribution of lateral slip the fill in the northeast trending pliocene pleistocene san gabriel basin is a basal shallow marine sequence overlain by the nonmarine duarte conglomerate in contrast to the deep water fernando formation of the los angeles basin the paleoslope trends south in the direction of the repetto and puente hills evidence that uplift of these hills postdated deposition the triangular san gabriel basin is currently a stable block bounded by the right slip northwest striking east montebello fault on the west and the left slip northeast striking walnut creek fault on the east farther east the san jose and puente hills are underlain by an east trending active fold belt including the san jose reverse fault and anticline the walnut anticline and the puente hills anticline adjacent to the right lateral whittier fault the east montebello fault is bounded on the west by the elysian park anticlinorium a fold belt in downtown los angeles the left slip holocene raymond fault joins the sierra madre reverse fault at its segment boundary marked by the clamshell sawpit fault the triangular stable block between east trending fold belts is bounded on the south by the puente hills blind thrust and on the north by the sierra madre fault documenting north south quaternary convergence east west extension is also indicated although not confirmed by geodetic measurements

crustal thickness variations in southern california from los angeles region seismic experiment passive phase teleseismic travel times the goal of the 1993 los angeles region seismic experiment larse93 passive phase was to collect waveform data from local and distant earthquakes to study lower crust and upper mantle structural features in southern california particularly under the san gabriel mountains and san andreas fault during larse93 approximately 88 stations were deployed in a 175 km long linear array across the los angeles basin san gabriel mountains and mojave desert northeast of los angeles during the four weeks of continuous recording teleseismic events recorded at each site provided a wide range of ray path backazimuths the teleseismic events included a number of intermediate magnitude earthquakes with epicenters in the aleutian island kamchatka kuril island mid atlantic ridge solomon island japan fiji island and chile regions this experiment was followed by larse94 which involved land refraction and deep crustal seismic reflection profiles from offshore and onshore explosion sources we have used the joint data sets in this study to distinguish upper crustal features from adjacent lower crustal and upper mantle structures p wave travel times were determined from 17 teleseisms and upper crustal residual signatures were removed by incorporating larse94 upper crustal velocity model results within each backazimuth range the resulting relative traveltime residuals increase from negative values 0 5 sec average recorded in the northern san gabriel valley southern san gabriel mountain foothills to positive values 0 2 sec average in the central and northern san gabriel mountains the residual patterns display variations for different backazimuths and incidence angles but show almost no lateral spatial shift of maximum or minimum residual along the array indicating that the dominant source of the residual pattern is shallow 50 km the patterns of residuals require a sharp lateral gradient in shallow velocities between the northern san gabriel valley located in the northernmost los angeles basin and the san gabriel mountains over a distance of less than 50 km most of the residual pattern can be explained by laterally varying crustal thickness and a high velocity anomaly in the upper mantle in our model of moho depth variations the northern san gabriel valley to the southwest of the array is underlain by thinned continental crust crustal thickness increases laterally by 12 km over a distance of less than 50 km into the san gabriel mountains this conclusion is supported by widespread surface evidence for rift style volcanism and crustal extension in southern california crustal rocks

the offshore morpho structure and tsunami sources of the viti levu seismic zone southeast viti levu fiji abstract new high resolution seabat 8160 multibeam bathymetry data reveal in detail the complex offshore morphology over part of the viti levu seismic zone vlsz in southeast viti levu the mapped seafloor is scarred by a complex network of linear submarine canyons and numerous submarine slides the submarine canyons are morphologic expressions of graben structures in the underlying basement rock submarine canyon faults are continuous with nearshore shelf faults and onshore fault controlled valleys and structural lineaments together these define continuous onshore offshore fault zones which combined form a structurally complex fault mesh of composite origin we attribute the development of the principal fault sets in the fault mesh to progressive distributed faulting in response to anticlockwise tectonic block rotations within the fiji platform during the neogene low occurrence of large earthquakes and a diffused pattern of seismicity currently observed in the vlsz may reflect complex coseismic faulting through the fault mesh the abundance of submarine slides over the vlsz which typically occur on the outer barrier reef edge as well as the submarine canyon heads and walls and mid slope imply that seismic shaking may be an important factor for triggering slope failure morphometric analysis and empirical modelling of the slides show that slides triggered in shallow water depths within 5 km of the coastline at the outer barrier reef edge and submarine canyon heads produce the largest near field tsunami amplitudes we interpret that such slides represent a significant local tsunami hazard

the san andreas and walker lane fault systems western north america transpression transtension cumulative slip and the structural evolution of a major transform plate boundary abstract relative right lateral pacific north american transform motion across the western us is largely taken up by the san andreas and walker lane fault systems cumulative lateral displacement on active strands of the san andreas system is 3 4 or more times greater than across faults of the walker lane the san andreas system is transpressional while the walker lane system is transtensional the relatively more complex discontinuous and broader system of faults composing the walker lane system is attributed to lower cumulative slip and an attendant extensional component of motion greater slip accompanied by a small component of contraction has yielded the simpler more continuous and generally throughgoing set of faults that comprise the san andreas system with respect to the contrasting amounts of cumulative slip the san andreas is at a more mature stage of structural development than the walker lane despite differences in gross patterns of faulting the systems also share similarities in deformation style slip along each is locally accommodated by clockwise rotation of crustal blocks that has produced structural basins and oblique components of slip are commonly partitioned into subparallel strike slip and dip slip faults

structural geometry and slip of the palos verdes fault southern california implications for earthquake hazards abstract the palos verdes fault pvf is an active structure in southern california comprised of several segments that together form a complex fault system the fault has been active since the miocene and is a major regional seismic source using marine petroleum industry seismic data we define the geometry of the fault and offsets of tertiary stratigraphic units that constrain the along strike segmentation and slip rates on the fault system in san pedro bay patterns of deformed stratigraphic units suggest a deep fault geometry that dips moderately to the southwest and was initially formed as a miocene normal fault plio pleistocene transpression resulted in the reactivation of this normal fault and its subsequent inversion with oblique right lateral reverse displacement to the south the lasuen knoll segment dips steeply to the northeast and is not a reactivated miocene age structure but rather formed in middle pliocene time plio pleistocene transpression has linked these and other segments together to form the currently active pvf system the resulting complex fault geometry and slip patterns suggest that the pvf is a highly segmented fault which may rupture in moderate size earthquakes that involve individual segments or large events that are able to propagate across significant geometric discontinuities estimates of moment magnitudes based on empirical relationships with rupture fault area indicate the possibility of m w 6 6 6 9 earthquakes for single segment ruptures and m w 7 1 7 3 multisegment ruptures with recurrence intervals ranging from 181 to 534 yr given the high inferred slip rates on the fault and its proximity to the urban population these events pose significant earthquake hazards to metropolitan los angeles

mechanical modeling of compressional basins origin and interaction of faults erosion and subsidence in the ventura basin california using geophysical observations and some basic geological ideas on the present structure of the ventura basin we model its possible mechanical evolution during the last 4 million years we use a two dimensional finite element model with frictional and elastoviscoplastic rheologies that allow strain localization this allows us to study the mechanical origin of the san cayetano thrust fault scf and its interaction with the oak ridge fault orf the results of our modeling indicate that the geometry and activity of a shear zone which mimics the san cayetano fault are conditioned both by the preexistence of the oak ridge fault and by the shape of the brittle ductile transition furthermore the presence of a decollement level is shown to favor the development of the san cayetano fault shear zone as a low angle structure the value of the effective friction on the oak ridge fault is evaluated from this model possible values of the viscosity of the sediments are also estimated it is shown that low sediment viscosity may completely hinder the activity of such a fault at the surface while slip at depth continues the stratigraphic profile of the basin is also shown to depend on the relative strengths of the orf and scf which also provide an explanation for the shortening and rapid subsidence of the basin

analysis of teleseismic p waves with a 5200 station array in long beach california evidence for an abrupt boundary to inner borderland rifting we analyze teleseismic p waves from four mw 6 5 earthquakes recorded by a petroleum industry survey in long beach california the survey used a 2 d array with up to 5200 seismometers 120 m mean spacing and 7 10 km aperture at frequencies near 1 hz p wave travel times and amplitudes exhibit coherent lateral variations over scales as short as 400 m including locally delayed travel times and increased amplitudes at the crest of the long beach anticline deeper heterogeneity is indicated by p wave phase velocities that deviate from reference model predictions for events from southwestern azimuths we postulate that a sharp northeastward increase in moho depth from the inner borderland ib to mainland southern california causes the anomalous phase velocities elastic forward modeling finds the travel times are fit well by a moho that dips 65 to the northeast and flattens 10 km southwest of the newport inglewood fault zone constraining the felsic thickness of mainland crust to 28 km requires an 8 km thick layer with a p velocity of 7 km s beneath it which could result from basal accretion of former farallon ocean crust or magmatic underplating during miocene volcanism forward models with a 65 moho dip predict a p to s conversion with a phase velocity of 5 km s deconvolution of the array s mean p wave signal isolates a similar later arriving phase the steep crust thickness transition supports a locally abrupt boundary to ib rifting our results highlight the utility of dense short period arrays for passive imaging at near surface to uppermost mantle depths

lithospheric deformation beneath the san gabriel mountains in the southern california transverse ranges high resolution tomographic images from los angeles region seismic experiment larse array and southern california seismic network scsn teleseismic data suggest that the entire lithosphere below the san gabriel mountains and san andreas fault in the transverse ranges has thickened in a narrow vertical sheet p wave travel time inversions of the combined data support the presence of the well documented upper mantle high velocity anomaly that extends 200 km into the mantle under the northernmost los angeles basin and transverse ranges and is associated with mantle downwelling due to oblique convergence we find that the high velocity high density upper mantle anomaly comprises a 60 80 km wide sheet of mantle material that lies directly below a substantial crustal root in the san gabriel mountains the velocity perturbations are as large as 3 in the anomaly corresponding to a 2 density increase the tomographic images suggest that deformation in the ductile lower crust and mantle lithosphere may be partially coupled mechanically and thermally if the thickening is occurring together in response to convergence and that it may be a local compressional feature

an inverse method for determining three dimensional fault geometry with thread criterion application to strike slip and thrust faults western alps and california abstract in order to draw geological structures such as faults interpolation is generally needed between scattered data the use of an approximation criterion integrating the kinematic properties of the faults helps to document the fault surfaces by adding a compatibility criterion to the data set assuming that two jointed blocks of rocks slipping on each other generate a thread surface an approximation method has been developed which integrates a thread criterion this approximation method is used to solve an inverse problem with least squares criteria including proximity to data points smoothness and thread criteria the aim is to find a smooth surface which is as close as possible to a thread and as close as possible to the observed data set applications to two corrugated fault surfaces with a dense data set located in the western alps france and in the transverse ranges california confirm the validity of the thread assumption despite their difference in mean corrugation wavelength 5 m and 10 km respectively in the type of fault strike slip and thrust fault respectively and in the nature of the faulted rocks limestones and sandstones respectively very similar results are obtained in both cases the observed data fit well with a thread surface and the computed fault displacement fits well with the measured displacement on the fault striae seismic focal mechanism geodetic data restoration the conclusion is that treating a fault as a thread is a valid physical description which gives the slip direction independently of other kinematic indicators the advantage of using a thread criterion in addition to classical proximity and smoothness criteria is that this physical insight allows information from areas where data are relatively dense to help constrain areas where data are relatively sparse these last areas being those that are usually not well constrained by proximity and smoothness criteria

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

sediment yield from the tectonically active semiarid western transverse ranges of california sediment yields from the world s rivers are generally highest from steep drainage basins with weak lithology active tectonics or severe land use impacts here we evaluate sediment yields from the western transverse ranges of california in an attempt to explain why they are two to tenfold greater than the surrounding areas of california we found that suspended sediment yields across the gauged basins of the western transverse range during 1969 1999 varied by approximately an order of magnitude 740 5300 t km2 yr similarly fine sediment concentrations for normalized discharge rates varied by almost two orders of magnitude e g 1 3 110 g l for the mean annual flood for 11 previously unmonitored drainages of the santa ynez mountains areas with high sediment yields consistently have weakly consolidated bedrock quaternary pliocene marine formations and are associated with the highest rates of tectonic uplift of the region 5 mm yr these regions are important to the sediment discharge budgets because 50 of the total suspended sediment discharge from the western transverse range is estimated to be generated within these regions even though they represent only 10 of the total watershed area previous estimates of suspended sediment discharge from the ventura river have likely been underestimated by 50 because the gauging station is located immediately upstream of a high sediment yield region we also found a significant and positive correlation between sediment yield and the percentage of a watershed with grassland and agricultural land use these results suggest that there is adequate variation within the lithology tectonics and land use of the broader western transverse range geologic province to induce large variations in sediment yield at the local scale

upper crustal structure of the inner continental borderland near long beach california a new p wave velocity structural model for the inner continental borderland icb region was developed for the area near long beach california it combines controlled source seismic reflection and refraction data collected during the 1994 los angeles region seismic experiment larse multichannel seismic reflection data collected by the u s geological survey 1998 2000 and nearshore borehole stratigraphy based on lateral velocity contrasts and stratigraphic variation determined from borehole data we are able to locate major faults such as the cabrillo palos verdes thums huntington beach and newport inglewood fault zones along with minor faults such as the slope fault avalon knoll and several other yet unnamed faults catalog seismicity 1975 2002 plotted on our preferred velocity structural model shows recent seismicity is located on 16 out of our 24 faults providing evidence for continuing concern with respect to the existing seismic hazard estimates forward modeling of p wave arrival times on the larse line 1 resulted in a four layer model that better resolves the stratigraphy and geologic structures of the icb and also provides tighter constraints on the upper crustal velocity structure than previous modeling of the larse data there is a correlation between the structural horizons identified in the reflection data with the velocity interfaces determined from forward modeling of refraction data the strongest correlation is between the base of velocity layer 1 of the refraction model and the base of the planar sediment beneath the shelf and slope determined by the reflection model layers 2 and 3 of the velocity model loosely correlate with the diffractive crust layer locally interpreted as catalina schist

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

four dimensional analysis of the inversion of a half graben to form the whittier fold fault system of the los angeles basin abstract analysis of structural and stratigraphic data from hundreds of oil wells surface geologic maps and earthquake focal mechanisms reveals a three phase structural evolution of the whittier fold fault system that evolution progressed from an extensional phase 16 ma accommodated by the formation of a half graben through an episode of graben filling 14 8 ma to lithospheric shortening which began ca 8 ma with contractional reactivation of the graben forming fault as a blind reverse fault and accompanying anticlinal uplift about 1 ma the blind reverse fault broke through onto the sea floor initiating the activity of the present whittier fault system horizontal contraction estimated from reconstructed cross sections ranges from 1 4 to 3 2 km and has been at an average rate of 0 3 mm year in the p direction n s of the 1987 whittier narrows earthquake for the past ca 8 my that rate may have accelerated after breakthrough of the blind reverse fault to as much as 1 0 mm year the ratio of strike slip to dip slip displacement on the whittier fault over the past ca 8 my is about 0 4

quantifying simultaneous discrete and distributed deformation most natural deformation involves simultaneous discrete deformation e g faults and distributed deformation e g penetrative strain in order to properly understand the bulk kinematics of a given deformation the discrete and distributed components must be evaluated simultaneously the displacement diagram method has previously been utilized to quantify strain accommodated by faults or other discrete surfaces the same method is applicable to distributed deformation such as ductilely deformed conglomerates which are typically evaluated by finite strain analysis by mathematically combining the gradients in the displacement fields of both the discrete and distributed components of deformation the bulk deformation can be quantified we apply this method to several examples that contain both distributed and discrete components and characterize the bulk displacement field and finite strain for these deformed systems

lithosphere dynamics and continental deformation the unifying theme in this section is the remarkable weakness of major faults i will consider the diverse new evidence for weakness and the evidence for high pore pressure localized in faults as a fundamental cause with this background one can better understand why faults remain active even after large rotations with respect to stress i will look at large neogene 23 7 million year old rotations about horizontal axes in the basin and range province and about vertical axes along the pacific margin recent developments will be summarized from studies of neogene tectonics neotectonics in california alaska and the mississippi embayment in the context of a weak north american stress field that results mainly from topographic forces to close i will present new geophysical studies relevant to the continuing controversy over whether the basic structure of the north american mantle lithosphere was altered by an early tertiary episode of flat subduction

where is the real transform boundary in california 1 the zone of highest geodetically defined strain rate in california does not everywhere coincide with the surface trace of the san andreas fault saf to determine whether this reflects the pattern of long term permanent deformation we analyzed the velocity field on swaths across the transform located so as to avoid intersections among the major fault strands slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model representing the interseismic strain accumulation our slip rates compare well with current geologic estimates which suggests that the present day velocity field is representative of long term motions we find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the saf and has an overall trend closer to the relative plate motion vector than the saf most sections of the saf take up less than half of the total slip rate and slip is transferred from one part of the system to another in a way that suggests that the saf should not be considered as a unique locator of the plate boundary up to half of the total displacement takes place on faults outside the high strain rate zone distributed over several hundred kilometers on either side our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere around which stress and strain rate decrease on a length scale controlled by the length of the transform

a model for neogene crustal rotations transtension and transpression in southern california two episodes of crustal rotation have occurred in the regions of southern california and the mojave desert since the beginning of miocene time an early miocene episode of clockwise crustal rotation is associated with widespread north south to north northeast south southwest crustal extension throughout southeast california and western arizona this episode is apparently related to changing subduction parameters of the farallon plate a middle miocene and later episode of clockwise rotation in the transverse ranges and bordering areas is related to shear between the pacific and north american plates in both episodes clockwise crustal rotations first occur in a deforming zone which has a component of extension across its trend and shear along its trend the effects of extension are most pronounced in the earlier episode but clear in the later one also the pacific north american deforming zone began contracting in width during pliocene time but clockwise rotations continued and are probably in progress today

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

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

escape tectonics in the los angeles metropolitan region and implications for seismic risk recent damaging earthquakes in california including the 1971 san fernando1 1983 coalinga2 1987 whittier narrows3 and 1994 northridge4 events have drawn attention to thrust faults as both potentially hazardous seismic sources and as a mechanism for accommodating shortening in many regions of southern california consequently many geological studies5 6 have concluded that thrust faults in southern california pose the greatest seismic hazard and also account for most of the estimated 5 7 mm yr 1 of contraction across the greater los angeles metropolitan area7 8 indicated by global positioning system geodetic measurements9 our study demonstrates however that less than 50 of the geodetically observed contraction is accommodated on the principal thrust systems across the los angeles region we integrate the most recent geological geodetic and seismological data to assess the spatial distribution of strain across the los angeles metropolitan region we then demonstrate that a significant component of seismic moment release and shortening in this region is accommodated by east west crustal escape extrusion along known strike slip and oblique slip faults

pliocene transpressional modification of depositional basins by convergent thrusting adjacent to the big bend of the san andreas fault an example from lockwood valley southern california 1 the big bend of the san andreas fault in the western transverse ranges of southern california is a left stepping flexure in the dextral fault system and has long been recognized as a zone of relatively high transpression compared to adjacent regions the lockwood valley region just south of the big bend underwent a profound change in early pliocene time 5 ma from basin deposition to contraction accompanied by widespread folding and thrusting this change followed the recently determined initiation of opening of the northern gulf of california and movement along the southern san andreas fault at about 6 1 ma with the concomitant formation of the big bend lockwood valley occupies a 6 km wide fault bounded structural basin in which converging blocks of paleoproterozoic and cretaceous crystalline basement and upper oligocene and lower miocene sedimentary rocks plush ranch formation were thrust over miocene and pliocene basin fill sedimentary rocks in ascending order caliente formation lockwood clay and quatal formation all the pre quatal sedimentary rocks and most of the pliocene quatal formation were deposited during a mid tertiary period of regional transtension in a crustal block that underwent little clockwise vertical axis rotation as compared to crustal blocks to the south ensuing pliocene and quaternary transpression in the big bend region began during deposition of the poorly dated quatal formation and was marked by four converging thrust systems which decreased the areal extent of the sedimentary basin and formed the present lockwood valley structural basin none of the thrusts appears presently active estimated shortening across the center of the basin was about 30 percent the formerly defined eastern big pine fault now interpreted to be two separate oppositely directed contractional reverse or thrust faults marks the northwestern structural boundary of lockwood valley the complex geometry of the lockwood valley basin is similar to other tertiary structural basins in southern california such those that underlie cuyama valley the ridge basin and the east ventura basin

crustal shortening and vertical strain partitioning in the middle atlas mountains of morocco the ne sw trending middle atlas mountains of morocco are obliquely oriented within the late cenozoic regional stress field resulting in deformation that is partitioned into strike slip faulting and thrust related folding in the central middle atlas thrusting is confined to a 20 km wide fold belt between two relatively rigid crustal blocks that are obliquely converging we suggest that in addition to strain partitioning observed in plan view a partitioning of deformation between the upper and lower crust may be necessary to reconcile estimated crustal thickening and horizontal shortening within the fold belt cross section balancing based on field observations demonstrates a relatively modest amount of cenozoic horizontal shortening 4 7 km normal to the fold belt producing 800 m of structural relief yet geophysical data suggest this contraction has not produced a significant crustal root beneath the fold belt that is the belt does not appear to be isostatically compensated assuming all horizontal shortening was accommodated by crustal thickening beneath the fold belt implies much greater thickening than is suggested by constraints on the preshortened crustal thickness it thus appears that thickening does not accommodate all of the contraction we suggest one possible solution the upper crust shortens by thickening faulting and folding whereas the lower crust deforms laterally

causative source of mw 6 9 sikkim nepal border earthquake of september 2011 gps baseline observations and strain analysis abstract the recent earthquake of mw 6 9 which occurred on september 18 2011 in sikkim nepal border region epicenter 27 72 n 88 06 e depth 20 7 km 68 km nw of the capital city gangtok is the strongest earthquake in the instrumentally recorded history of the region the fault plane solution of this earthquake indicates a strike slip motion however the seismological and geological studies carried out so far after the earthquake could not confirm the causative fault plane in the present study gps observations are used to ascertain causative source in the generation of earthquake and its correlation with the observed seismic data of the region the co seismic displacements recorded by gps show maximum displacement of 11 mm at phodong and 9 mm at taplejung station near the epicenter a simple rigid cross fault model using gps baseline observations was employed to figure out the causative fault plane and seismological characteristic of the region it is inferred that the movement represents the kinematic adjustment of the subsidiary faults as a result of the displacement along the nw se principal plane strain analysis using gps baseline inferred that the region southeast of epicenter has undergone large deformation in addition a significant part of the measured deformation across the surface fault zone for this earthquake can be attributed to post seismic creep

modeling of neogene crustal block rotation case study of southeastern california numerical modeling using a two dimensional distinct element method dem was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the pacific north america transform plate boundary paleomagnetic and structural investigations have shown that large 30 50 horizontal rotations of crustal blocks occurred in the eastern transverse zone and the northeast mojave block southeastern california since the end of early miocene the main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not the simulations were performed using different stress boundary conditions and different parameters of the fault strength applying an assumed ratio of 2 for the horizontal stress components as boundary condition block rotations in the order of 40 only occur if 1 the major horizontal stress sh is oriented n20 e with a maximal deviation of 15 to the east or west and 2 the fault strength described by the friction coefficient is smaller than 0 12 or 14 only of the value 0 85 for intact rocks byerlee s law for a weakening model of the fault strength an initial friction coefficient of 0 85 allows also rotations of about 40 if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0 12 or smaller for the same amount of rotation 40 a decreasing stress ratio requires a lowering of the friction coefficient used whereas an increase of the ratio allows a higher friction coefficient the simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation especially the cumulative slip values discrepancies in the observations and modeling results for the western parts of the mojave desert are the result of a neglect of the three dimensional deformation in our two dimensional modeling

deformation kinematics in the western united states determined from quaternary fault slip rates and recent geodetic data we estimate the horizontal velocity gradient tensor field from quaternary fault slip rates and recent global positioning system gps and very long baseline interferometry vlbi velocity solutions in the western united states transform plate boundary zone the total velocity obtained from the quaternary fault slip rate data across the entire plate boundary is within 1 mm yr of the nuvel 1a predicted pacific pa north american na plate motion velocity but directions are 5 6 anticlockwise of directions given by nuvel 1a the total velocity obtained from inversion of recent geodetic data is 2 3 anticlockwise from the nuvel 1a na pa velocity but the difference between the two is not significant at the 95 confidence level the discrepancy between the total pa na motion obtained from the geological data and nuvel 1a indicates that a marginally significant amount of ne sw shortening possibly as much as 5 mm yr is missing overall in the geologic data shortening may occur in the long term in the offshore and coastal areas of california where such shortening is required in the shorter term geodetic solution the seismic moment released in the last 148 years is 59 of the total moment release rate expected from long term strain rate field including both seismic and aseismic deformation derived from the inversion of geological data with nuvel 1a far field pa na motion constraints the accumulated strain in the areas containing the southern san andreas fault san jacinto fault the san francisco bay area and the area containing the ventura basin in the western transverse ranges in the last 148 years is the equivalent to that which could be released by an mw 7 0 earthquake in each 50 100 15 km3 crustal volume if strain is to be released seismically in these areas

numerical simulation of the microstructure and compression behavior of eckernforde bay sediments abstract transmission electron microscope tem studies conducted on sediments recovered using diver collected cores and gravity cores from eckernforde bay in the baltic sea germany reveal that the sediments consist of silt to clay sized sediment both free and bound into fecal pellets organic matter biogenic materials methane gas and microorganisms the microstructure of the upper 2 m of the sediment is complex with the clay particles arranged in a very open structure with variable inter and intra domain pore space in an effort to understand the relationship between microscopic variables e g microfabric and macroscopic geotechnical properties e g compressibility a numerical study was performed using the discrete element method the objective of this study was to develop an understanding of the micro to macro linkage of mechanical properties specifically one dimensional compression behavior of the sediment was numerically simulated and compared with experimental data compositional parameters of the sediment were obtained from specific tests conducted at the naval research laboratory and from tests conducted by other researchers participating in the project image analysis was used to quantify the microfabric parameters from tem images of the sediment the effect of organic matter was simulated by introducing inter particle contact cementation results indicate that inter particle bonding due to inter particle cementation and or the van der waals attractive force 1 contributes to the apparent overconsolidation observed in the laboratory and 2 holds the highly porous unstable microstructure together until a threshold stress level is reached change in void ratio during compression is due to inter rather than intra domain movement

palaeomagnetism of the pico formation santa paula creek ventura basin california summary in the western ventura basin in southern california marine turbidites in the pico formation are exposed along santa paula creek in a southeast dipping homocline the section is about 3 km thick and contains normal and reverse polarity that was acquired before the development of the homocline although the sediment contains a secondary component of normal polarity that cannot be removed entirely by thermal or alternating field demagnetization the mean declination for 26 sites 12 normal polarity 14 reverse polarity shows a clockwise rotation of 19 8d 9 0d that is similar to the rotation recorded in the saugus formation a coeval non marine deposit in the eastern ventura basin that was studied by levi et al 1986 the partial overprint of the secondary component of normal polarity results in a mean inclination for each polarity that does not match the inclination of an axial dipole field the inclination is too steep in normal polarity and too shallow in reverse polarity still the inclination for the combined sites is close to the expected inclination 52 0d versus 53 9d overall turbidites in the pico formation are suitable for a magnetostratigraphic investigation but caution is required if the palaeomagnetic data are to be used in a tectonic study

paleomagnetic evidence of localized vertical axis rotation during neogene extension sierra san fermin northeastern baja california mexico paleomagnetic data from sierra san fermin in the gulf of california extensional province indicate that localized clockwise rotations about vertical axes occurred during pliocene through recent extension and dextral shear relative declination discordances in upper miocene and pliocene ash flow tuffs indicate a net clockwise rotation of 30 16 clockwise rotation between 12 5 and 6 ma is statistically insignificant 11 17 structural observations and geochronological data suggest that rotations in this area began post 6 ma comprising uniform sense block rotations oblique divergence associated with extension and dextral slip in the northwest striking boundary between the pacific and north american plates northeast striking sinistral slip faults and north striking normal faults accommodate distributed dextral shear in this area allowing fault blocks to rotate in a clockwise sense a model for oblique divergence predicts 21 km of shear in the direction of relative plate motion and 20 7 km ene directed extension perpendicular to the main gulf escarpment a broad region of northeastern baja california may have undergone similar distributed shear two possible dynamic models may explain this shear in one model rotation accumulates above a deep subhorizontal basal shear zone rotating blocks may extend downward to a detachment beneath the extensional province either a low angle eastward continuation of the san pedro martir fault or to a basal shear surface on top of a subducted remnant of the farallon plate alternatively distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults in either case shear may have transferred northward onto faults west of the san andreas fault contributing to late miocene to recent clockwise rotation of the western transverse ranges this shear is not accounted for in the 300 km of dextral slip computed from cross gulf geologic tie points

interpretations of a middle miocene and late quaternary steady dextral transpression in sw japan and the opening tectonics for the japan sea abstract an internal rotation hypothesis for the deformation of sw japan for middle miocene and recent times is studied rotations are known to have occurred from paleomagnetic data for the earlier case two geodetic flow field estimates as well as another tectonic study suggest that the rotations are also taking place at present a further examination of the geodetic flow models suggests that the rotations contained in these flow models are intrinsic to the deformation of sw japan which occurs in an e w dextral transpressive simultaneous pure and simple shear tectonic regime maps of rotation rates one half of vorticity and shear gradients the n s variation of the e w velocity field constructed from the geodetic flow models resemble one another in pattern the estimates of the directions of the principal strain rates from the geodetic models are on the whole n s contractional and or e w extensional which match the geodetic reference coordinate system used in the adjustment of the geodetic data these observations suggest that the vortical flow contained in the geodetic flow models is a frame independent shear induced vorticity as the material is rotating with respect to the principal directions of the strain rates and that the oblique motion of the philippine sea plate is partitioned between shear and arc normal convergence components the rotation rate is 10 12 ma furthermore the kinematical vorticity number estimates from the geodetic data correlate well with the first order tectonic fabric patterns in sw japan super simple shear deformation for large parts of sw japan where the median tectonic line and other large strike slip faults are found is also indicated in the internal kinematical vorticity number estimates a new model for the formation of the japan sea is proposed extrusion of japan due to the okhotsk eurasia collision this model agrees with the fundamental observations of the early middle miocene deformation in the region the application of an indentation extrusion model to the okhotsk eurasia collision results in a better agreement between the predicted and observed structures and leads to simpler more reasonable and coherent interpretations for the origin and development of the structures compared with the achievements of the previous models the nne trending early middle miocene rifts lying along the continental margin of western hokkaido formed as pull apart basins along a sinistral fault related to the collision of the okhotsk plate which segmented in a left handed sense because the collision was oblique the n s trending rifts and the normal faults found in the back arc domain of the ne honshu arc formed by eastward extrusion under the influence of the mariana type subduction in the japan trench these rift basins were subsequently left laterally displaced by movement along nw trending sinistral strike slip faults the shikoku basin opened from the east toward the west and dextral shear was imparted to the sw margin from the nankai trough leading to clockwise rotations in the middle miocene in sw japan the paleomagnetic rotations are accommodated as fully internal structural rotations in the new model for the origin of the japan sea the current episode of deformation in sw japan began 1 5 million years ago after a hiatus during a 15 5 ma interval and is likely a continuation of the middle miocene style of tectonics

structural controls on the continent ocean transition in the northern gulf of california in the gulf of california the pacific north america plate boundary changes character from an oceanic type spreading center and transform fault system to the south to a region of diffuse continental deformation to the north the presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric heat flow gravity or seismic data which indicate significant differences north and south of latitude 30 n we suggest instead that north of 30 n a continent ocean transition begins which we name the wagner transition zone wtz diffuse deformation characterizes the wtz where slip occurs along reactivated north to nnw striking normal faults developed during late miocene or pliocene ene directed extension transtensional deformation varies from ene directed extension along dip slip faults in the west to dextral shear along the coast to dextral oblique slip along inferred north to nnw striking faults submerged in the northern gulf by accounting for rotational and extensional plate motion deformation in northeastern baja california vector constraints require that submerged structures accommodate 30 mm yr of slip in a direction slightly clockwise of the relative plate motion direction the juxtaposition of the discrete spreading center system in the central gulf with the diffuse wtz appears to have been a stable configuration since 4 6 ma perhaps controlling the evolution of spreading center jumps between upper and lower tiburon and delfin basins due to the juxtaposition of kinematically partitioned structural domains different histories of prerift extension and subduction related arc magmatism along the length of the gulf partly related to the migration of the rivera triple junction may explain the location of the continent ocean transition

chasing the garlock a study of tectonic response to vertical axis rotation vertical axis clockwise block rotations in the northeast mojave block are well documented by numerous authors however the effects of these rotations on the crust to the north of the northeast mojave block have remained unexplored in this paper we present a model that results from mapping and geochronology conducted in the north and central owlshead mountains the model suggests that some or all of the transtension and rotation observed in the owlshead mountains results from tectonic response to a combination of clockwise block rotation in the northeast mojave block and basin and range extension the owlshead mountains are effectively an accommodation zone that buffers differential extension between the northeast mojave block and the basin and range in addition our model explores the complex interactions that occur between faults and fault blocks at the junction of the garlock brown mountain and owl lake faults we hypothesize that the bending of the garlock fault by rotation of the northeast mojave block resulted in a misorientation of the garlock that forced the owl lake fault to break in order to accommodate slip on the western garlock fault subsequent sinistral slip on the owl lake fault offset the garlock creating the now possibly inactive mule springs strand of the garlock fault dextral slip on the brown mountain fault then locked the owl lake fault forcing the active leach lake strand of the garlock fault to break

a kinematic model for the evolution of the eastern california shear zone and garlock fault mojave desert california abstract the eastern california shear zone in the mojave desert california accommodates nearly a quarter of pacific north america plate motion in south central mojave the shear zone consists of six active faults with the central calico fault having the fastest slip rate however faults to the east of the calico fault have larger total offsets we explain this pattern of slip rate and total offset with a model involving a crustal block the mojave block that migrates eastward relative to a shear zone at depth whose position and orientation is fixed by the coachella segment of the san andreas fault saf southwest of the transpressive big bend in the saf both the shear zone and the garlock fault are assumed to be a direct result of this restraining bend and consequent strain redistribution the model explains several aspects of local and regional tectonics may apply to other transpressive continental plate boundary zones and may improve seismic hazard estimates in these zones

seismicity of south central coastal california october 1987 through january 1997 earthquakes recorded along south central coastal california from october 1987 through january 1997 by the pacific gas and electric company9s 20 station central coast seismic network and the u s geological survey9s northern and southern california networks are used to study their relation to previous seismicity mapped quaternary faults and regional tectonic features we developed p and s wave velocity models station corrections and duration magnitude calibrations to process 1184 reliable earthquake locations and 212 well constrained focal mechanisms seismicity generally coincides with mapped quaternary faults and areas of tectonic uplift including the southern offshore santa maria basin and areas to the west and the uplifted blocks within the los osos tectonic domain comparison of our data to previous microearthquake locations 1969 1987 and larger m 5 historical earthquakes shows that these patterns have been persistent seismically quiescent areas have few or no mapped quaternary faults and typically are structural lows the base of the seismogenic zone is about 12 km in depth above the relic oceanic crust microearthquake locations and strike slip focal mechanisms document the strike slip nature of the hosgri fault zone and its step over to the san simeon fault along the western boundary of the los osos domain uplifting blocks within the los osos domain are most seismically active toward their western ends along the hosgri fault zone and they exhibit reverse and oblique faulting consistent with block uplift and interaction with the hosgri fault zone the observed patterns and character of microseismicity and macroseismicity clearly show transpressional deformation associated with the hosgri fault zone overprinted by rotation and westward migration of the western transverse ranges within this tectonic setting clockwise rotation of the western transverse ranges causes internal deformation of the los osos domain and westward migration of the ranges appears to deflect the hosgri fault zone westward and causes east west directed crustal shortening in the southern offshore santa maria basin

seismotectonics and fault structure of the california central coast abstract i present and interpret new earthquake relocations and focal mechanisms for the california central coast the relocations improve upon catalog locations by using 3d seismic velocity models to account for lateral variations in structure and by using relative arrival times from waveform cross correlation and double difference methods to image seismicity features more sharply focal mechanisms are computed using ray tracing in the 3d velocity models seismicity alignments on the hosgri fault confirm that it is vertical down to at least 12 km depth and the focal mechanisms are consistent with right lateral strike slip motion on a vertical fault a prominent newly observed feature is an 25 km long linear trend of seismicity running just offshore and parallel to the coastline in the region of point buchon informally named the shoreline fault this seismicity trend is accompanied by a linear magnetic anomaly and both the seismicity and the magnetic anomaly end where they obliquely meet the hosgri fault focal mechanisms indicate that the shoreline fault is a vertical strike slip fault several seismicity lineations with vertical strike slip mechanisms are observed in estero bay events greater than about 10 km depth in estero bay however exhibit reverse faulting mechanisms perhaps reflecting slip at the top of the remnant subducted slab strike slip mechanisms are observed offshore along the hosgri san simeon fault system and onshore along the west huasna and rinconada faults while reverse mechanisms are generally confined to the region between these two systems this suggests a model in which the reverse faulting is primarily due to restraining left transfer of right lateral slip

p wave seismic velocity structure derived from sonic logs and industry reflection data in the los angeles basin california 1 we present a new three dimensional seismic p wave velocity model of the los angeles basin that is based on more than 150 sonic logs and 7000 stacking velocities from industry reflection profiles the model was developed using kriging techniques implemented in gocad a commercial application specializing in the construction of surfaces the analysis of spatially distributed data and the modeling or simulation of properties and is characterized by a heterogeneous spatial varying velocity gradient with maximum velocities of 5000 m s in the sedimentary section variability of the observed p wave velocities increases with depth reflecting complex structural and sedimentological controls the model contains a new representation of the basin shape as defined by the sediment basement interface which was mapped using well control and seismic reflection profiles our model differs substantially in approach and outcome from previous representations of crustal velocity structure in southern california

structures that accommodated differential vertical axis rotation of the western transverse ranges california 1 new paleomagnetic and structural data from the northwestern transverse ranges of california define the location geometry and kinematics of structures that facilitated differential vertical axis rotation paleomagnetic declinations in the southern coast ranges indicate negligible amounts of vertical axis rotation which contrasts sharply with data from rocks of the same age in the western transverse ranges that record approximately 90 of clockwise rotation since early miocene time this change in paleomagnetic declinations occurs across an east west trending zone of reverse faults and folds that includes the western big pine pine mountain fault the santa ynez fault and the structures in between the structures in this zone exhibit increased amounts of shortening to the west such that the zone closed in a fan like fashion resulting in rotation of the southern edge with respect to the northern edge the new data and interpretations refine previous models of vertical axis rotation in southern california by describing the deformation present at rotation boundaries that allowed distinct areas to rotate relative to the surrounding crust

cenozoic block rotation according to a conjugate shear system in central europe indications from palaeomagnetic measurements abstract results of palaeomagnetic measurements in the middle tertiary volcanic provinces of siebengebirge and westerwald northeastern rhenish massif germany indicate that block rotation of the northeastern rhenish massif of the order of 10 to 16 has occurred since the late oligocene contemporaneous subsidence in the upper rhine graben neuwied basin and lower rhine basin and intensive faulting in the hessian depression document a new period of intraplate tectonism in central europe in the tertiary presumably from the eocene to the miocene the axis of maximum horizontal stress rotated from the nne ssw to the nnw sse and the nw se direction respectively the new stress conditions may be responsible for the development of a conjugate shear system in central europe north of the alps a model of a conjugate shear system is presented a sinistral divergent strike slip fault extends ssw nne from the upper rhine graben to the north the fault splits up into two branches in the area of the hessian depression a conjugate dextral wrench fault system develops from the bavarian pfahl zone southeastern germany to the rhenish massif about 200 km to the northeast a parallel dextral strike slip fault is known from the southern lower saxony basin to the north of the osning zone and harz mountains the northeastern block of the rhenish massif was created between dextral strike slip faults the block began clockwise rotation presumably in the late oligocene due to small changes in direction of the maximum horizontal compressive stress from nnw to nw and contemporaneous extension of the western european crust to the southwest it is assumed that the block rotation is responsible for the v shaped opening of the lower rhenish basin and anticlockwise micro block rotation within the hessian depression opening between the rotating and nonrotating crust along the recent rhine river in the central part of the rhenish massif formed triangular transrotational basins basin of neuwied due to movements of crustal blocks in the western rhenish massif bounded by wnw ese dextral faults fault plane solutions from recent earthquakes in the rhine system support the model of block rotation and related crustal movements due to rotation of the northeastern rhenish massif the continuation of the upper rhine graben development to the north is prevented

reconciling disparate estimates of total offset on the southern san andreas fault late cenozoic tectonic reconstructions of southern california united states have been hindered for decades by disagreement over the total displacement on the southern san andreas fault disparate estimates of dextral offset range from 160 km to 240 km prior estimates were based on purely translational models that neglect the effects of transrotational strain on the orientation and distribution of key geologic markers we present a new reconstruction that integrates published lithologic structural and paleocurrent data with the effects of clockwise rotation in the eastern transverse ranges etr this model yields 200 14 km of cumulative offset on the southern san andreas fault system in the salton trough the disparity between our estimate and a lower estimate of 160 km for the mojave segment can be explained by a combination of transtensional elongation along the southwest boundary of the etr transfer of strain to the eastern california shear zone and diffuse crustal shortening in the big bend region this reconstruction reconciles previously incompatible and long debated models and places an important new constraint on the tectonic evolution of the pacific north america plate boundary in southern california

lithospheric structure across the california continental borderland from receiver functions due to its complex history of deformation the california continental borderland provides an interesting geological setting for studying how the oceanic and continental lithosphere responds to deformation we map variations in present day lithospheric structure across the region using ps and sp receiver functions at permanent stations of the southern california seismic network as well as ocean bottom seismometer obs data gathered by the asthenospheric and lithospheric broadband architecture from the california offshore region experiment albacore which enhances coverage of the borderland and provides first direct constraints on the structure of the pacific plate west of the patton escarpment noisiness of obs data makes strict handpicking and bandpass filtering necessary in order to obtain interpretable receiver functions using h and common conversion point stacking we find pronounced lithospheric differences across structural blocks which we interpret as indicating that the outer borderland has been translated with little to no internal deformation while the inner borderland underwent significant lithospheric thinning most likely related to accommodating the 90 clockwise rotation of the western transverse range block west of the patton escarpment we find that the transition to typical oceanic crustal thickness takes place over a lateral distance of 50 km we detect an oceanic seismic lithosphere asthenosphere transition at 58 km depth west of the patton escarpment consistent with only weak age dependence of the depth to the seismic lithosphere asthenosphere transition sp common conversion point stacks confirm wholesale lithospheric thinning of the inner borderland and suggest the presence of a slab fragment beneath the outer borderland

miocene rifting in the los angeles basin evidence from the puente hills half graben volcanic rocks and p wave tomography formation of the puente hills half graben in the northeastern los angeles basin and eruption of the glendora and el modeno volcanics 16 14 ma help to define the timing of extension in the basin normal faulting on the proto whittier fault ca 14 ma established the puente hills half graben in which sedimentary strata accumulated between ca 14 and 10 ma and into which diabase sills intruded north south contraction began to invert the puente hills half graben ca 7 ma leading to formation of the puente hills anticline and the whittier fault our high resolution three dimensional p wave velocity model shows two anomalous higher velocity 6 63 km s bodies at depths between 9 and 18 km which we attribute to dioritic plutons named here for whittier narrows and el modeno the stocklike whittier narrows pluton could have been a source for the glendora volcanics and the diabase sills in the puente hills half graben the sill shaped el modeno pluton was a likely source for the el modeno volcanics the northwesterly alignment of the plutons may mark the location of the northeastern los angeles basin rift boundary which is associated with the clockwise rotation of the western transverse ranges three active faults the elysian park blind thrust the puente hills blind thrust and the whittier fault converge on the whittier narrows pluton which may have played a role in their location and segmentation

seismic refraction evidence for steep faults cutting highly attenuated continental basement in the central transverse ranges california a 2 d upper crustal structural profile with seven steeply dipping faults was constructed from the los angeles region seismic experiment phase ii larse ii seismic refraction data recorded over the north central transverse ranges and adjacent mojave desert in southern california the profile extends to a depth of approximately 4 km the faults were identified from distinctive features in the seismic first arrival data while the velocity structure was determined from travel times and refined by forward modelling the resulting seismic velocity structure is correlative to the geological structure along the profile and is used to help constrain a geological cross section six of the seven faults detected from the seismic data correspond directly to geologically mapped faults in the central transverse ranges and adjacent mojave desert from south to north they are the pelona fault the san francisquito fault the clearwater fault the san andreas fault and two faults that cut the portal ridge the seventh fault is buried beneath quaternary deposits of the western mojave desert and appears to correlate with an additional fault that cuts portal ridge out of the section line the southernmost fault the pelona fault separates the younger vasquez formation in the south from the pelona schist in the north and is determined to be an intermediate 40 60 south dipping normal fault this coupled with field observations indicate that the pelona fault was instrumental in the late oligocene early miocene formation of the soledad basin and the structural ascent and exhumation of the pelona schist along the proto sierra pelona intrabasin ridge analogous neogene extensional deformation is found to have affected the western mojave desert region where faults 6 and 7 appear to have had north side down normal displacement and to have been instrumental in forming the western antelope basin a 6 0 km s 1 basal layer to our shallow seismic structure is correlated with the pelona schist south of the san andreas fault and with the tectonically related rand schist beneath the western mojave desert granitic and gneissic crystalline rocks which form regional upper plate complexes along the vincent thrust above the pelona schist and along the rand thrust above the rand schist yield consistently low seismic velocities as compared with the expected velocities for the constituent rock types these anomalous velocities are reconciled by the degraded structural and textural state of the upper plate crystalline rocks that resulted from extreme shearing brittle fracturing and related retrogressive hydration reactions starting during or shortly after the latest cretaceous early palaeocene underthrusting of the schists extension translation along the san andreas transform system and compressional faulting with the uplift of the transverse ranges further accentuated the basement deformations the integrated result is that the survival of the precambrian through mesozoic granitic crust of the region became a rootless and highly attenuated upper crustal layer

active faulting in the walker lane 1 deformation across the san andreas and walker lane fault systems accounts for most relative pacific north american transform plate motion the walker lane is composed of discontinuous sets of right slip faults that are located to the east and strike approximately parallel to the san andreas fault system mapping of active faults in the central walker lane shows that right lateral shear is locally accommodated by rotation of crustal blocks bounded by steep dipping east striking left slip faults the left slip and clockwise rotation of crustal blocks bounded by the east striking faults has produced major basins in the area including rattlesnake and garfield flats teels columbus and rhodes salt marshes and queen valley the benton springs and petrified springs faults are the major northwest striking structures currently accommodating transform motion in the central walker lane right lateral offsets of late pleistocene surfaces along the two faults point to slip rates of at least 1 mm yr the northern limit of northwest trending strike slip faults in the central walker lane is abrupt and reflects transfer of strike slip to dip slip deformation in the western basin and range and transformation of right slip into rotation of crustal blocks to the north the transfer of strike slip in the central walker lane to dip slip in the western basin and range correlates to a northward broadening of the modern strain field suggested by geodesy and appears to be a long lived feature of the deformation field the complexity of faulting and apparent rotation of crustal blocks within the walker lane is consistent with the concept of a partially detached and elastic brittle crust that is being transported on a continuously deforming layer below the regional pattern of faulting within the walker lane is more complex than observed along the san andreas fault system to the west the difference is attributed to the relatively less cumulative slip that has occurred across the walker lane and that oblique components of displacement are of opposite sense along the walker lane extension and san andreas contraction respectively despite the gross differences in fault pattern the walker lane and san andreas also share similarities in deformation style including clockwise rotations of crustal blocks leading to development of structural basins and the partitioning of oblique components of slip onto subparallel strike slip and dip slip faults

contemporary kinematics of the western united states determined from earthquake moment tensors very long baseline interferometry and gps observations using moment tensors from earthquakes between 1850 and 1995 we determine the horizontal velocity gradient tensor field associated with the seismic deformation in the western u s plate boundary zone the velocity vectors obtained from the integration of the seismic strain rates across the entire plate boundary lie within 5 of the nuvel ia pacific north american plate motion direction the magnitude of the earthquake related velocity is 62 of the nuvel 1a total pacific north american plate motion if earthquake occurrence is a random poisson like process then the large formal errors associated with the model velocity estimates are an indication that the 144 year interval is too short to define the long term seismic moment release rate to within an uncertainty less than the deficit between the observed seismic moment release rate and the total long term moment release rate associated with both the seismic and aseismic deformation to spatially resolve the deficit of the earthquake moment release in the last 144 years we have also estimated the total long term horizontal velocity gradient tensor field using seismic strain rate tensors and recent geodetic velocities both with and without the nuvel 1a pacific north american rigid plate motion constraint we find a systematic difference between the southern california earthquake center scec geodetic velocity map and nuvel 1a north american reference frames with a pole at 38 8 n 124 2 w 0 33 m y 1 this difference is significant at the 99 confidence level and indicates that 4 7 mm yr of ne sw convergence is absorbed in offshore areas of southern california if both the scec velocity map and nuvel 1a correctly describe the relative motion between the pacific and north america plates deficit strain rates calculated from comparison of observed seismic strain rates over the last 144 years with long term model strain rate tensors are largest in areas east of san francisco bay in the transverse ranges and along the southern san andreas fault and san jacinto fault the deficit in each 100 by 50 km grid area within these regions is equivalent to an accumulation of strain in the last 144 years that would be released by an mw 7 earthquake elsewhere the accumulated strain in the last 144 years is small except in northern mojave desert along the garlock fault and in the region southwest of the southern san andreas san jacinto fault zone within each 100 by 50 km grid area in these regions the accumulated strain is equivalent to that released by an mw 6 5 7 0 earthquake

geophysical evidence for miocene extension and mafic magmatic addition in the california continental borderland the california continental borderland is a key region for understanding the effect on crustal structure of tectonic processes associated with the initiation and elongation of the san andreas transform system because the borderland lies near the point where the pacific farallon spreading system first encountered the north american plate margin whereas the central and northern california margins have been extensively investigated by geophysical surveys in recent years the crustal structure of the borderland particularly the outer borderland has received relatively little attention this paper presents three gravity models that stretch from the pacific ocean floor to shore they provide new evidence that the crust of the entire california continental borderland has been extended and infiltrated by mafic melt since early miocene time the gravity models were constructed from interpretations of nearly 750 km of seismic reflection data seismic refraction studies and geologic maps thus minimizing the nonuniqueness of the method the evidence for extension in the models is that the crust of the accretionary complex and forearc basin that make up the borderland is only 22 km thick 35 thinner than comparable crustal sections in central and northern california extension could well have been accommodated by eastward flow of accretionary complex material during middle miocene formation of a metamorphic core complex in the inner borderland the gravity models also show that the accretionary complex is 100 200 kg m 3 more dense than commonly observed to the north this density difference may result from influx of basaltic melts due to mantle upwelling at the rivera triple junction and beneath the highly extended inner borderland the borderland models permit the occurrence of a discrete mafic layer that forms the base of the crust as is seen along the central and northern california coast but do not require such a layer9s presence anywhere except immediately inland of the shelf slope break

paleosecular variation and time averaged field recorded in late pliocene holocene lava flows from mexico 1 this paper presents results from 13 paleomagnetic sites from an area west of mexico city and 7 sites from an area of dispersed monogenetic volcanism in the state of san luis potosi accompanied by seven 40ar 39ar radiometric dates an analysis of secular variation and time averaged paleomagnetic field in the trans mexican volcanic belt tmvb using compiled data both newly obtained and from the literature is presented interpretation can best be constrained after excluding from the data set sites that appear to be tectonically affected the selected data include 187 sites of late pliocene holocene age the mean direction among these sites is dec 358 8 inc 31 6 95 2 0 k 29 this direction does not overlap the expected geocentric axial dipole gad but is consistent with a gad plus a 5 quadrupole the virtual geomagnetic pole scatter of this group of sites 12 7 with lower and upper 95 confidence limits of 11 9 and 14 1 is consistent with the value expected from model g 13 6

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

collapse of southwestern north america and the evolution of early miocene detachment faults metamorphic core complexes the sierra nevada orocline and the san andreas fault system plate reconstructions indicate that early miocene pacific north american relative motion was not parallel to the transform but was instead oblique and transtensional we propose that the western edge of the north american plate east of the transform gravitationally collapsed to the southwest in response to this divergence the northern and eastern edges of the collapsed region underwent extension between 24 and 20 ma whereas extension in southeastern california and southern arizona during this interval was coaxial with the overall s50 60 w collapse direction the extension direction along the northern oundary mojave extensional belt was north south and the result of the combined effects of the southwestward collapse and the independent northwestward translation of the region to the north collapse continued between 20 and 16 ma and extension persisted in southeastern california and southern arizona extension along the northern boundary waned owing to the apparent cessation of motion of the region to the north thus the kinematics of the northern boundary changed to dextral shear and extension along what is here named the trans mojave sierran shear zone this east west zone of mainly right shear oroclinally folded the southern tail of the sierra nevada and the central mojave desert rocks within this 83 110 km wide zone were rotated approx 40 60 clockwise about vertical axes finally as collapse moved the north american plate progressively west across the locus of transform shear its western edge became increasingly subject to the motion of the pacific plate over time the edge of north america was truncated by faults of the san andreas system and transferred to the pacific plate

magnetic stratigraphy and tectonic rotation of the middle eocene matilija sandstone and cozy dell shale ventura county california implications for sequence stratigraphic correlations abstract the matilija sandstone and cozy dell shale in the western transverse ranges of california record a long history of eocene regression from deep marine turbidites to non marine red shales and then transgression to deep marine shales in a rapidly subsiding tectonic basin in its type area along the ventura river north of ojai the matilija sandstone consists of over 800 m of deep marine to non marine sandstones and the cozy dell shale contains over 760 m of deep marine shales and turbidite sandstones paleomagnetic sampling of these rocks yielded a stable magnetic remanence which passed both a fold test and a reversal test some samples were reversed at nrm after dip correction based on calibration from planktonic microfossils the matilija sandstone spans the interval from chron c21r to early chron c20r 46 0 48 5 ma and the cozy dell from late chron c20r to c19r 46 0 42 0 ma the samples show a tectonic rotation of approximately 85 18 clockwise consistent with previous results reported for this tectonic block previous sequence stratigraphic correlations of these rocks are inconsistent with this chronostratigraphy none of the sequence boundaries predicted from these sections matches the global sequence chart

slip partitioning and deformation cycles close to major faults in southern california evidence from small scale faults geometry and kinematics of small scale faults adjacent to large seismically active faults have been analyzed for a large region of southern california the sampled population of small scale faults is scattered in orientation and slip but possesses distinct trends two important characteristics are first the paucity of small scale faults that mimic orientations and mechanisms of the adjacent master faults and second the large variation in slip mechanisms of fault planes oriented e w and n s in order to test the similarity of the sampled fault population to present day deformations data on geometry and slip of the small scale faults have been used for stress inversions these results are compared with those derived from focal mechanisms of low magnitude earthquakes in the same region stress orientations derived from small scale faults are spatially heterogeneous but horizontal axes of stress maintain n s and e w trends over the entire region the similarities between stress inversions from small faults and seismicity support the conclusion that most of the sampled faults are recent and formed during cyclical events of fracturing in a relatively strong crust adjacent to weak master faults

geophysical evidence for the evolution of the california inner continental borderland as a metamorphic core complex we use new seismic and gravity data collected during the 1994 los angeles region seismic experiment larse to discuss the origin of the california inner continental borderland icb as an extended terrain possibly in a metamorphic core complex mode the data provide detailed crustal structure of the borderland and its transition to mainland southern california using tomographic inversion as well as traditional forward ray tracing to model the wide angle seismic data we find little or no sediments low 6 6 km s p wave velocity extending down to the crust mantle boundary and a thin crust 19 to 23 km thick coincident multichannel seismic reflection data show a reflective lower crust under catalina ridge contrary to other parts of coastal california we do not find evidence for an underplated fossil oceanic layer at the base of the crust coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge which represents the transition to the western transverse ranges on the shelf the palos verdes fault merges downward into a landward dipping surface which separates basement from low velocity sediments but interpretation of this surface as a detachment fault is inconclusive the seismic velocity structure is interpreted to represent catalina schist rocks extending from top to bottom of the crust this interpretation is compatible with a model for the origin of the icb as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks the basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until 13 m y ago subduction of the young and hot monterey and arguello microplates under the continental borderland followed by rotation and translation of the western transverse ranges may have provided the necessary thermomechanical conditions for this extension and crustal inflow

recent crustal deformation in southern california deduced from the restoration of folded and faulted strata a rigid element method of restoration unfold is used to restore competent folded and faulted layers of the ventura and los angeles basins to their initial horizontal state comparison of initial undeformed state with present deformed state allows one to estimate both the finite crustal deformation and its associated horizontal displacement field relative to an arbitrary fixed line including data from the santa barbara channel basin the total finite displacement field for the western transverse ranges and vicinity within the pacific plate is inferred from this map restoration and is modeled as a double fan closure this model implies a 10 clockwise rotation of the northeen boundary of the western transverse ranges and a 5 counterclockwise rotation of its northeast boundary lateral variation of the deformation reveals the heterogeneity of the subsurface deformation most of the major thrusts appear to initiate as en echelon structures along the left lateral northern margin and the right lateral northeastern margin of the studied area the resulting deformation and displacement values closely match those derived by other geological methods section balancing techniques or fault slip measurements and by geophysical methods geodetic paleomagnetic and focal mechanism data map restoration thus is a method that can independently quantify both local and regional deformation including folds and faults this method also reveals the zones where problems of compatibility appear with the available geological and geophysical data and thus where the next studies might be focused

clockwise rotation and implications for northward drift of the western transverse ranges from paleomagnetism of the piuma member sespe formation near malibu california 1 new paleomagnetic results from mid tertiary sedimentary beds in the santa monica mountains reinforce the evidence for large scale rotation of the western transverse ranges and anisotropy measurements indicate that compaction induced inclination flattening may resolve a long standing controversy regarding the original paleolatitude of the rotated block previously published paleomagnetic data indicate that post oligocene rotation amounts to 70 110 clockwise affecting the channel islands santa monica mountains and santa ynez mountains the sespe formation near malibu consists of a lower member dominated by nonmarine sandstone and conglomerate and an upper section the piuma member which consists of gray red sandstone and mudstone interbedded with minor tuff and limestone beds the piuma member has a paleomagnetic pole at 36 6 n 326 7 e a95min 5 0 a95max 9 6 obtained by thermal demagnetization of 34 oriented cores from oligocene and early miocene beds after correcting for plunge of the geologic structure the data are consistent with significant clockwise rotation 77 7 of the region relative to stable north america rotation of the western transverse ranges is generally viewed as a consequence of pacific north american plate interactions after 28 ma when east west subduction gave way to northwest transform motion in southern california inclinations from the piuma study indicate a paleolatitude anomaly of 11 7 and are consistent with a mean northward drift that exceeds generally accepted san andreas fault displacement by a factor of 3 however sedimentary inclination error may accentuate the anomaly anisotropy of isothermal remanent magnetization indicates inclination flattening of approximately 8 and correction for the effect reduces the paleolatitude anomaly to 5 3 5 8 compaction may explain the inclination flattening in these sedimentary rocks but the process does not adequately explain lower than expected inclinations found in previous studies of miocene volcanic rocks of the western transverse ranges

a kinematic model for the rinconada fault system in central california based on structural analysis of en echelon folds and paleomagnetism deformation associated with the rinconada fault one of the major strands of the san andreas fault system in central california is accommodated by both discrete fault offset w18 km and distributed off fault deformation en echelon folds adjacent to the rinconada fault were studied in detail at two locations near williams hill and lake san antonio to characterize the magnitude and style of distributed deformation the obliquity between fold hinges and the local strike of the fault was 27 and 14 at these two sites respectively systematic outcrop scale fault displacement measurements along roadcuts indicate that the maximum horizontal elongation occurs parallel to local fold hinges and ranges from 4 to 9 we used the orientation and stretch of fold hinges to construct a transpressional kinematic model for distributed deformation this modeling suggests a 20e50 angle of oblique convergence 5 km of fault parallel wrench deformation and 2e4 km of fault perpendicular shortening between 3 and 16 of clockwise rotation is also predicted by our model this rotation is independently confirmed by a 14 7 vertical