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Satellite synthetic aperture radar interferometry reveals transient strain accumulation along the Blackwater-Little Lake fault system within the Eastern California shear zone. The surface strain map obtained by averaging eight years (1992-2000) of Earth Resource Satellite (ERS) radar data shows a 120-km-long, 20-km-wide zone of concentrated shear between the southern end of the 1872 Owens Valley earthquake surface break and the northern end of the 1992 Landers earthquake surface break. The observed shear zone is continuous through the Garlock fault, which does not show any evidence of left-lateral slip during the same time period. A dislocation model of the observed shear indicates right-lateral slip at 7 [+ or -] 3 mm/yr on a vertical fault below ~5 km depth, a rate that is two to three times greater than the geologic rates estimated on northwest-trending faults in the eastern Mojave area. This transient slip rate and the absence of resolvable slip on the Garlock fault may be the manifestation of an oscillatory strain pattern between interacting, conjugate fault systems. Keywords: strain transient, synthetic aperture radar interferometry, Eastern California shear zone.

Three-dimensional seismic refraction data reveal the structure of the plate boundaries at the Mendocino triple junction. First arrival traveltimes were inverted to obtain a three-dimensional seismic velocity model. The tomography results indicate a strong lateral velocity contrast in the lower crust. This velocity contrast is beneath the surface trace of the San Andreas fault and tracks along the fault to the triple junction and then along the Mendocino fracture zone. Velocity values appropriate for mafic rocks are more than 5 km shallower on the Pacific plate than east of the San Andreas fault, and 1-3 km shallower than under the Cascadia accretionary prism. The model suggests that the San Andreas fault, which is growing as the triple junction migrates northward, cuts steeply through the entire crust. Pacific oceanic crust does not extend beneath North America at this location. The slabless window south of the triple junction and east of the San Andreas fault must be filled with upwelling asthenosphere. The sharp corner of the Pacific plate migrates northward, displacing the Cascadia accretionary prism and forcing the prism material to move north or east. Keywords: seismic tomography, California, plate boundary, San Andreas fault, Mendocino triple junction.

The authors investigate several linear ruptures offshore of the Palos Verdes Peninsula in southern California. They offer the extent of the newly discovered Avalon Knoll fault and estimate likely magnitudes for events along it.

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. Keywords: faulting, kinematics, restoration, Santa Barbara Channel, Ventura basin.

Four of the mountain ranges near Death Valley, California, display exhumed, low-angle normal faults on their flanks, features originally referred to as turtleback structures. These fault surfaces are smooth, planar to curviplanar, and defined by faceted spurs and coincident interfluve crests. The lowest parts of these faults are overlain in tectonic contact by poorly indurated fanglomerate, and locally by Quaternary volcanic rocks. Steep fault scarps in alluvium are present at the foot of each turtleback-type flank. Although these may be examples of active low-angle normal faults, it is difficult to establish the dip histories of these faults. The problem with constraining the dip histories is one of missing and concealed information, because the exposed hanging-wall rocks are some of the youngest material cut and displaced by the (currently) shallow-dipping faults. Another important relationship along these range flanks is the intersection between the low-angle faults and steep, scarp-forming faults that cut alluvium at the range fronts. Although this relationship remains unconstrained in most of the ranges, a shallow-dipping fault is cut by one of the steep faults in Panamint Valley, suggesting that, in one area at least, the low-angle faults are not active. The low-angle faults, and not the steep neotectonic faults, are probably responsible for most of the opening of the present valleys, because each low-angle fault intersects the valley floor at the range front. None of the low-angle faults are perched high in the ranges, as would be the case if the steep faults had uplifted them significantly. Therefore, even if there has been a geologically recent transition from low-angle normal faulting to steep normal (with or without strike slip) faulting, the modern topography still reflects the former regime. Strictly speaking, the Death Valley region may not be an example of active low-angle normal faults and supra-detachment basins. Keywords: Basin and Range province, basins structural, Inyo Mountains, low-angle faults, normal faults, Panamint Range.

Analysis of emergent marine terraces in the San Joaquin Hills, California, and (super 230)Th dating of solitary corals from the lowest terraces reveal that the San Joaquin Hills have risen at a rate of 0.21-0.27 m/k.y, during the past 122 k.y. Movement on a blind thrust fault in the southern Los Angeles basin has uplifted the San Joaquin Hills and has the potential to generate an M(sub w) 7.3 earthquake within this densely populated area. Our structural modeling suggests that the fault dips to the southwest and slips at approximately 0.42-0.79 m/k.y., yielding an estimated minimum average recurrence interval of approximately 1650-3100 yr for moderate-sized earthquakes. Recognition of this blind thrust extends the known area of active blind thrusts and fault-related folding southward from Los Angeles into coastal Orange County.

The Hosgri fault is located immediately offshore of south-central California and is part of the transform boundary between the Pacific and North American plates. This fault terminates to the southeast into east-trending folds and reverse-separation faults of the western Transverse Ranges. Our new structure-contour maps of deformed horizons show a spatial relationship between faulting and folding consistent with right-lateral slip. Restoration of these digital maps quantifies post-Miocene right-lateral slip across the southern Hosgri fault to be 3.5 km. This slip is absorbed by folding, thrust overlap, and clockwise vertical-axis rotation of elongate blocks between strands of the fault. The restored part of a block located to the east has rotated 8 degree clockwise. Extrapolating this restored rotation to the 50 km block length produces an estimate of 7 km of dextral shear for a total, including the 3.5 km fault slip, of 10.5 km of post-Miocene displacement. Our three-dimensional approach precludes interpretations for reverse slip on the Hosgri fault, and is not consistent with models for more than 80 km of late Cenozoic right-lateral fault slip.

This study documents three localities in the Franciscan accretionary complex of northern California, now adjacent to the San Andreas fault, that were overprinted thermally between 13.9 and 12.2 Ma: Point Delgada-Shelter Cove (King Range terrane); Bolinas Ridge (San Bruno Mountain terrane); and Mount San Bruno (San Bruno Mountain terrane). Vein assemblages of quartz, carbonate, sulfide minerals and adularia were precipitated locally in highly fractured wall rock. Vitrinite reflectance (R(sub m)) values and illite crystallinity decrease away from the zones of metalliferous veins, where peak wall-rock temperatures, as determined from R(sub m), were as high as 315 degrees C. The delta 18 O values of quartz and calcite indicate that two separate types of fluid contributed to vein precipitation. Higher delta 18 O fluids produced widespread quartz and calcite veins that are typical of the regional paleothermal regime. The widespread veins are by-products of heat conduction and diffuse fluid flow during zeolite and prehnite-pumpellyite-grade metamorphism, and we interpret their paleofluids to have evolved through dehydration reactions and/or extensive isotopic exchange with accreted Franciscan rocks. Lower delta 18 O fluids, in contrast, evolved from relatively high temperature exchange between seawater (or meteoric water) and basaltic and/or sedimentary host rocks; focused flow of those fluids resulted in local deposition of the metalliferous veins. Heat sources for the three paleothermal anomalies remain uncertain and may have been unrelated to one another. Higher temperature metalliferous fluids in the King Range terrane could have advected either from a site of ridge-trench interaction north of the Mendocino fracture zone or from a 'slabless window' in the wake of the northward migrating Mendocino triple junction. A separate paradox involves the amount of Quaternary offset of Franciscan basement rocks near Shelter Cove by on-land faults that some regard as the main active trace of the San Andreas plate boundary. Contouring of vitrinite reflectance values to the north of an area affected by A.D. 1906 surface rupture indicates that the maximum dextral offset within the interior of the King Range terrane is only 2.5 km. If this fault extends inland, and if it has been accommodating most of the strike-skip component of San Andreas offset at a rate of 3-4 cm/yr, then its activity began only 83-62 ka. This interpretation would also mean that a longer term trace of the San Andreas fault must be nearby, either offshore or along the northeast boundary of the King Range terrane. An offshore fault trace would be consistent with peak heating of King Range strata north of the Mendocino triple junction. Conversely, shifting the fault to the east would be compatible with a slabless window heat source and long-distance northward translation of the King Range terrane after peak heating.

Geodetic observations using the Global Positioning System (GPS) and other techniques record a high rate of north-south shortening in an east-southeast-trending, 5-40-km-wide belt in northern metropolitan Los Angeles, California. Downtown Los Angeles is observed to be converging upon the southern San Gabriel Mountains at 6 mm/yr. Aside from the elastic strain that will be released during earthquakes rupturing the San Andreas and San Jacinto faults, east-west lengthening across northern metropolitan Los Angeles is minor,

A band of seismicity transects the southern Sierra Nevada range between the northeastern end of the site of the 1952 [M.sub.W] (moment magnitude) 7.3 Kern County earthquake and the site of the 1946 [M.sub.W] 6.1 Walker Pass earthquake. Relocated earthquakes in this band, which lacks a surface expression, better delineate the northeast-trending seismic lineament and resolve complex structure near the Walker Pass mainshock. Left-lateral earthquake focal planes are rotated counterclockwise from the strike of the seismic lineament, consistent with slip on shear fractures such as those observed in the early stages of fault development in laboratory experiments. We interpret this seismic lineament as a previously unrecognized, incipient, currently blind, strike-slip fault, a unique example of a newly forming structure.

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 (126 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.

The location of the San Andreas fault in the Shelter Cove area of northern California has been the subject of long-standing debate within the geological community. Although surface ruptures were reported near Shelter Cove in 1906, several subsequent workers questioned whether these ruptures represented true fault slip or shaking-related, gravity-driven deformation. This study, involving geologic and geomorphic mapping, historical research, and excavation across the 1906 rupture zone, concludes that the surface ruptures reported in 1906 were the result of strike-slip faulting, and that a significant Quaternary fault is located onshore near Shelter Cove. Geomorphic arguments suggest that the Holocene slip rate of this fault is greater than about 14 mm/yr, indicating that it plays an important role within the modern plate-boundary system. The onshore trace of the fault zone is well expressed as far north as Telegraph Hill; north of Telegraph Hill, its location is less well-constrained, but we propose that a splay of the fault may continue onshore northward for at least 9 km to the vicinity of Saddle Mountain.

Folds commonly form along transform fault systems. In the San Andreas strike-slip fault, two mechanisms of folding have been described that are regarded as incompatible: (1) wrench tectonics, or distributed shear, and (2) strain partitioning, shortening being perpendicular to the fault. The compatibility and relative importance of these two mechanisms are not well understood, in part because folds are the result of lengthy deformation. This paper synthesizes paleomagnetic data, sedimentary thickness maps, fault patterns, and seismic reflection profiling data along the San Andreas transform fault system that indicate a 20 [degrees] -30 [degrees] clockwise (map view) reorientation of Pliocene en echelon folds to their present position, subparallel to the San Andreas fault. The folds continue to develop today as a result of shortening perpendicular to the fault. Folds apparently rotated with continuing strain in the wrench zone and with increasing shortening across the shear zone. Rotation likely occurred when changing Pacific plate motions increased the component of convergence across the fault system. This field example documents deformation by these two different mechanisms in the same fold belt, which has broad implications for interpreting other fold belts formed in similar settings. The geologic record suggests that the opposing theories of wrench deformation and strain-partitioned shortening perpendicular to the fault are both important and compatible processes that act at different times and places.

The style, timing, and pattern of slip on the Santa Cruz Island fault were investigated by trenching the fault and by analysis of offset late Quaternary landforms. A trench excavated across the fault at Christi Beach, on the western coast of the island, exposed deformation of latest Pleistocene to Holocene sediments and pre-Quaternary rocks, recording repeated large-magnitude rupture events. The most recent earthquake at this site occurred ca. 5 ka. Coastal terraces preserved on western Santa Cruz Island have been dated using the uranium-series technique and by extrapolation using terrace elevations and the eustatic record. Offset of terraces and other landforms indicates that the Santa Cruz Island fault is predominantly left lateral, having a horizontal slip rate of not more than 1.1 mm/yr and probably about 0.8 mm/yr. The fault also has a smaller reverse component, slipping at a rate of between 0.1 and 0.2 mm/yr. Combined with measurements of slip per event, this information suggests a long-term average recurrence interval of at least 2.7 k.y. and probably 4-5 k.y., and average earthquake magnitudes of Mw 7.2-7.5. Sense of slip, recurrence interval, and earthquake magnitudes calculated here for the Santa Cruz Island fault are very similar to recent results for other faults along the southern margin of the western Transverse Range, including the Malibu Coast fault, the Santa Monica fault, the Hollywood fault, and the Raymond fault, supporting the contention that these faults constitute a continuous and linked fault system, which is characterized by large but relatively infrequent earthquakes.

Data from geotechnical boreholes and trenches, in combination with geomorphologic mapping, indicate that the Hollywood fault is an oblique, reverse-left-lateral fault that has undergone at least one surface-rupturing earthquake during latest Pleistocene to middle or late Holocene time. Geomorphologic observations show that the fault extends for 14 km along the southern edge of the eastern Santa Monica Mountains, from the Los Angeles River westward through downtown Hollywood to northwestern Beverly Hills, where the locus of active deformation steps 1.2 km southward along the West Beverly Hills lineament to the Santa Monica fault. Rupture of the entire Hollywood fault, by itself, could produce a [M.sub.w] [approximately]6.6 earthquake, similar in size to the highly destructive, 1994 Northridge earthquake, but even closer to more densely urbanized areas. Assuming a 0.35 mm/yr minimum fault-slip rate consistent with available geologic data, we calculate an average maximum recurrence interval for such moderate events of [less than or equal to][approximately]4000 yr. Although occurrence of such moderate events is consistent with the elapsed time since the poorly constrained age of the most recent surface rupture, the data do not preclude a longer quiescent interval suggestive of larger earthquakes. If earthquakes much larger than [M.sub.w] [approximately]6.6 occurred in the past, we speculate that they may have been generated by the Hollywood fault together with other faults in the Transverse Ranges Southern Boundary fault system.

Measurements of crustal deformation may help determine if mid-crustal reflectors imaged at 15 km depth below the San Francisco Bay area, California, are active detachment horizons that accommodate strike-slip shear and/or convergence across the region. Elastic dislocation models predict distinguishable secular surface displacements for fault geometries involving vertical shear zones or horizontal detachments only where faults step, bend, or branch along their trace. In the southern San Francisco Bay area, the San Andreas, Hayward, and Calaveras fault zones splay from a single trace of the creeping central segment of the San Andreas fault. Whereas measured horizontal surface displacements in the central and northern bay area are equally well predicted by models of strike-slip faults below the major faults and by models involving a detachment, a zone of fault-normal motion observed in the southern bay area is not well explained by models involving only vertical strike-slip faults. A model involving detachment slip parallel to the San Andreas fault appears more consistent with the observations. We find no evidence for significant San Andreas fault-normal regional contraction or detachment faulting. However, additional data and better models are required to more confidently constrain the three-dimensional active fault architecture.

The Laurel-Convict fault is a prominent, northwest-striking, high-angle fault exposed in Paleozoic rocks of the Mount Morrison pendant in the eastern Sierra Nevada of California. The fault zone averages 25-50 m in width and consists of narrow, anastomosing domains of ductile deformation separating lenses of Paleozoic metasedimentary rock. The fault, which cuts structures in middle Permian metasedimentary rocks, is intruded by a relatively undeformed quartz porphyry felsite dike that has yielded a U-Pb zircon upper intercept age of 225 [+ or -] 16 Ma. Displacement on the Laurel-Convict fault is therefore constrained to the interval between middle Permian and Late Triassic. Displacement criteria, including curvature and offset of bedding in the fault zone, rare mesoscopic ductile shear indicators, and restoration of offset lower Paleozoic stratigraphy and structure across splay faults, all indicate a moderate amount of apparent left-lateral strike-slip displacement. Our data do not support previous interpretations of the Laurel-Convict fault as a thrust-faulted terrane boundary or a major right-lateral shear zone. The Laurel-Convict fault is not, therefore, an exposed segment of Intrabatholithic Break 3 (IBB3). Proposed Cretaceous right-lateral displacement of the [Sr.sub.i]= 0.706 isopleth on IBB3 must be accommodated either east of the Mount Morrison pendant in the Owens Valley or west of the pendant in the eastern Sierra Nevada.

Chrysotile-bearing serpentinite is a constituent of the San Andreas fault zone in central and northern California. At room temperature, chrysotile gouge has a very low coefficient of friction ([Mu] [approximately equal to] 0.2), raising the possibility that under hydrothermal conditions [Mu] might be reduced sufficiently (to [less than or equal to]0.1) to explain the apparent weakness of the fault. To test this hypothesis, we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 [degrees] C and axial-shortening velocities as low as 0.001 [[micro]meter]/s. As temperature increases to [approximately equal to]100 [degrees] C, the strength of the chrysotile gouge decreases slightly at low velocities, but at temperatures [greater than or equal to]200 [degrees] C, it is substantially stronger and essentially independent of velocity at the lowest velocities tested. We estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 MPa. Thus, on the sole basis of its strength, chrysotile cannot be the cause of a weak San Andreas fault. However, chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures.

Fault gouge along the Badwater Turtleback fault system displays abundant evidence for mesoscopic ductility, as foliation, isolated and/or rotated inclusions, and isoclinal folds. Fault-gouge thicknesses also vary dramatically. At the outcrop scale, gouge thicknesses can range from 0 to > 1 m, and at the 100 m scale, they can range from 0 to > 5 m. Because these variations are closely associated with other ductility-related features, they probably also formed because of ductile flow. Ductile flow appears to locally strengthen the fault zone by severe attenuation of the gouge and by transport of inclusions. Where gouge pinches out to zero thickness, hanging-wall rock rests directly on footwall rock, a condition that produces an overall stronger fault zone as well as stick-slip behavior in laboratory tests. Transport of inclusions within the gouge zone may create local inclusion 'jams' which may also have a strengthening effect. The presence of these stronger portions of the fault zone may give indirect evidence for ancient seismicity on the fault.

We investigated temporal and spatial patterns of Quaternary fault activity of the San Gabriel fault zone in the Little Tujunga region, southern California, using the electron spin resonance (ESR) plateau dating method. Two major fault zones (San Gabriel and Santa Susana-Sierra Madre) and folds are developed in the Little Tujunga region. The traces of thrust faults and folds of the Santa Susana-Sierra Madre fault zone are roughly parallel to the San Gabriel fault zone, indicating that the maximum horizontal stress was nearly perpendicular to the San Gabriel fault zone. Bends in the main strands of the San Gabriel fault zone yielded local transpressive regimes and changed the direction of maximum horizontal stress to a lower angle to the main strands, resulting in the development of subsidiary faults and folds oblique to the main trend of the fault. These structural features can be explained by the process of low drag-decoupled shear combined with transpression. The temporal pattern of partition between compressional and strike-slip deformation in the Little Tujunga region is not clear yet. ESR dates from both the main strands and subsidiary faults range from 1170 to 40 ka. The dates show temporal clustering into active and inactive periods, analogous to those seen in historic and Holocene earthquake fault activity. Within a given active period, fault activity is widely distributed along a restraining bend in the San Gabriel fault zone. Historic earthquake faults similarly show that fault strain was prohibited in restraining bends and that fault activities spread out up to several kilometers away from the main strand. Some faults were reactivated, producing well-defined bands of reactivated fault gouge within old zones of host fault rock, which exhibit younger ESR dates than those of the host fault rocks. The San Gabriel fault zone was formerly considered to be an exhumed ancient fault of the San Andreas fault system. Results from this study suggest that, although much less active than the San Andreas fault zone (average recurrence interval in Pallett Creek, 132 yr), the long-term (80-120 k.y.) cyclic fault activity of the San Gabriel fault zone continued into the Pleistocene. These results show that ESR dating can be used to evaluate the recurrence intervals of paleo-earthquake fault movements and crustal stability with a temporal resolution of 5%-10%.

We have studied the temporal distribution of paleoseismic records of earthquakes in several regions of California, over time scales ranging from decades to several hundreds of thousands years, by using dates obtained by electron spin resonance plateau dating of fault gouge, 14C dated sediments from fault zones, and records of historic earthquakes. The historical record in the San Andreas fault zone of central California and the paleoseismic record of the San Gabriel fault zone both exhibit self-similarity with a fractal dimension of 0.43-0.46. The fractal dimension of the San Andreas fault zone in southern California both in historic and paleoseismic time scales is 0.67, indicating more evenly distributed fault movements than those of central California. On time scales

Brittle structures in the upper 200 m of the footwall of the Whipple detachment fault are used to characterize the state of stress under which the detachment slipped. They are younger than or coeval with chloritic brecciation, and those that record shear typically are either moderately northeast-dipping normal structures or steep, northwest-striking, southwest-down structures that we interpret as conjugate shears. From this, we infer that the maximum principal stress was oriented 55 [degrees]-80 [degrees] from the detachment while they were active. The angle between the conjugate faults averages about 30 [degrees], indicating failure in the transtensional regime. During conjugate faulting the effective least principal stress was nearly equal to the tensile strength, the Whipple fault probably had a normal coefficient of static friction, and it slipped under hydrostatic to only moderately elevated fluid pressure. Rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength. This causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress.

The coast-parallel elevation pattern of marine terraces along 50 km of coast centered about Santa Cruz, California, provides an opportunity to assess the relative roles of candidate uplift mechanisms. We assemble 130 existing latitude-longitude-elevation inner-edge locations from three of these marine terraces to constrain elastic dislocation models of coseismic uplift. Uplift resulting from repeated oblique slip seismic events similar to the Loma Prieta earthquake (M(sub s) = 6.9) of October 17, 1989, appears to explain well the portion of the terrace uplift pattern south of Santa Cruz. Depending on whether the lowest terrace is assumed to correspond to isotope stage 5c or 5e, the calculated mean repeat time for Loma Prieta events is either 490 or 820 yr, corresponding to either 4.1- or 2.4-mm/yr slip rates on the Loma Prieta fault. Residual uplift not explained by such events increases monotonically to the north toward Ano Nuevo, where the San Gregorio fault zone intersects the coastline. We assess whether this residual uplift may result from repeated events on the San Gregorio fault. Given only the strike of the San Gregorio fault, we use an extensive parameter search to determine the geometry of, and the style of motion on, the implied rupture plane that best explains this residual uplift. The fault parameters that minimize the misfit between data and model imply activity of a moderately steep (70 degrees) northeast-dipping San Gregorio fault that ruptures in oblique thrust slip from approximately 15 km to the surface. The thrust slip component dominates the uplift pattern, resulting in uplift of the coastline from Santa Cruz northward. These parameters match available geological, geodetic, and seismic data on the dip and sense of motion on structures within the San Gregorio fault zone. The San Gregorio fault has therefore been an active late Pleistocene participant in North American-Pacific plate boundary motion. We conclude that the complex pattern of uplift along this 50 km reach of coastline can be explained by coseismic mechanisms tied to San Andreas bend-related fault structures onshore, and to the San Gregorio fault zone offshore. As both of these mechanisms result locally in considerable shore-normal uplift gradients, caution is warranted in applying common correlation techniques based solely on uplift rates in the assignment of terrace ages in this and other such settings. In addition, while it is tempting to use terrace culmination offsets as a means of estimating San Andreas fault dextral slip rates, we provide a formalism to determine under what conditions such calculations are meaningful. We conclude that the terrace offsets along the Santa Cruz coastline result not from dextral slip on the San Andreas, but from differential indentation of the Loma Prieta uplift pattern by the various sea-level highstand coastlines.

Changes in baseline vectors between very long baseline interferometry (VLBI) receiving stations in the Western Transverse Ranges imply that east-west blocks of crust in this region rotate clockwise about vertical axes with respect to the Pacific and North American plates. Minimum apparent rotations, given by the ratios between components of velocity perpendicular to baseline vectors and the lengths of the baselines, imply minimum current rotation rates of a few degrees per million years. The relevant VLBI receivers lie on different crustal blocks that are separated by major active faults. Both geologic and other geodetic observations imply north-south convergence between such blocks at several millimeters per year. Corrections to perpendicular components of velocity for such relative movements between blocks yield likely clockwise rotation rates of 6 degrees/m.y. + or - 2 degrees/m.y., which are indistinguishable from the average rate inferred from paleomagnetic declinations of rocks in the Western Transverse Ranges with ages less than 15 m.y. Thus, rotation seems to have occurred continuously and apparently with only small variations in rate during a period when the tectonics of southern California changed dramatically. This apparent independence of the rotation rate on the changing surface kinematics is consistent (1) with such rotation being a manifestation of continuous deformation at depth in the lower crust and upper mantle, (2) with weak faults separating upper-crustal blocks, and (3) with the important resistance to continental deformation lying in the upper mantle and/or lower crust.

We develop new methods to identify blind-thrust fault systems, determine fault slip rates, and estimate potential earthquake magnitudes and recurrence intervals in active fold-and-thrust belts. These methods are applied to compressive folds along the Offshore Oak Ridge and Blue Bottle trends, which overlie active blind-thrust faults in the eastern Santa Barbara Channel. These folds and their causative faults are interpreted using fault-bend fold theory and are represented in balanced models and cross sections that integrate surface and subsurface data. The structures are mapped using a new technique of axial-surface mapping in seismic reflection grids, which defines three-dimensional structural geometries and shows changes in slip and subsurface fault geometry along strike. Analysis of syntectonic (growth) sediments yields Pliocene and Quaternary fault slip rates of 1.3 mm/yr on a deep thrust (greater than or equal to 16 km) and 1.3 mm/yr on shallower faults (2-5 km). The combined 2.6 mm/yr slip rate represents only part of the 6 mm/yr of shortening measured by geodesy across the channel and estimated from relative Pacific-North American plate motions across the Transverse Ranges. Additional shortening is probably accommodated on other active thrusts in the western Transverse Ranges and in the northern channel along the Santa Barbara coast. Deformed seafloor sediments and a swarm of axial surface seismicity along the fold trends indicate that the underlying thrusts are active and may pose significant earthquake hazards to coastal southern California. Unsegmented fault surfaces are used through empirical relationships between fault surface area and rupture magnitude to estimate the sizes of potential earthquakes. This analysis suggests that a ramp in the Channel Islands fault beneath the Offshore Oak Ridge trend is capable of rupturing in a M(sub s) greater than or equal to 7.2 earthquake. Earthquakes of this magnitude may release approximately 2 m of slip, which, when combined with the estimated slip rate (1.3 mm/yr), yields an earthquake recurrence interval of approximately 1500 yr for this Channel Islands fault ramp.

The Ridge basin, located in southern California between the San Andreas and San Gabriel faults, contains as much as 14 km of Mio-Pliocene marine and nonmarine sedimentary fill. These strata are exposed as a northwest-dipping homoclinal sequence that becomes increasingly younger to the northwest. Pliocene to Holocene deformation, uplift, and dissection of this basin provide an opportunity to study stratal geometry and sedimentary facies in what is commonly cited as a classic 'strike-slip' basin. Newly acquired seismic data from the northern Ridge basin provide new evidence for the subsurface basin geometry. These data lead us to conclude that the principal northern strand of the San Gabriel fault is a listric, east-southeast-dipping, oblique-slip fault rather than a subvertical, strike-slip fault. The principal San Gabriel fault appears to become subhorizontal at the southeastern end of the seismic line at a present-day subsurface depth of approximately 4 km. This interpretation is consistent with preliminary two-dimensional gravity models across the basin and with the homoclinal structure of basin fill. Subsidence within the Ridge basin is explained as the result of northwest-southeast extension locally accommodated along the listric, right-lateral San Gabriel and associated late Miocene strike-slip faults. As a consequence of oblique normal slip on the gently dipping San Gabriel fault, sedimentary strata were continuously tilted and moved southeastward in the hanging wall away from the depocenter. This mechanism of basin formation allowed 14 km of sediment to accumulate without deep burial. The geometry and displacement history of the San Gabriel fault controlled sediment accommodation and resultant stratal geometries within the basin. Slip along the San Gabriel fault and associated subsidence of the Ridge basin correspond temporally with regional tectonic events in southern California, reflecting both changes in the relative motion of Pacific and North American plates and the evolution of strain partitioning across this plate boundary.

The Cenozoic normal fault system exposed in the Chemehuevi Mountains of the southern Cordillera provides constraints on the initiation angle and geometry of an extensional fault system that has accommodated extreme crustal stretching. There, three stacked, brittle, low-angle normal faults that formed at depths as great as 10-12 km cut gently down section northeastward through deformed Proterozoic and Mesozoic crystalline basement. Hanging-wall blocks are displaced relatively northeastward. The upper crust above the Chemehuevi detachment fault was pulled apart along high-angle normal faults that rotated to more gentle dips through time. In contrast, rocks of originally midcrustal affinity in the footwall were only gently rotated and accommodated minor extension (

The Miocene Kingston Range--Halloran Hills detachment fault system of the eastern Mojave Desert, California, delineates part of the eastern breakaway zone for a profoundly extended area between the Sierra Nevada and the Spring Mountains structural blocks. The shallow-dipping, west-rooting detachment fault cuts discordantly across Paleozoic and Precambrian units in the Mesozoic foreland fold-and-thrust belt, exhibits west- to southwest-trending corrugations with structural relief of up to 1.5 km, and underlies the terrestrial Shadow Valley basin. Middle Miocene fault displacement and syntectonic sedimentation in the northern basin were terminated at '12.4 Ma by intrusion of the large ('130 [km.sup.2]), shallow-level ([less than or equal to]4 km depth) Kingston Peak pluton across the detachment fault soon after faulting began. Basin sedimentation and fault displacement southeast of the pluton were not, however, disrupted by its emplacement and continued to evolve. Northern 'pluton-pinned' and southern 'pluton-free' domains were separated by the Blacksmith Hills fault, a northeast-striking, right-slip lateral ramp that accommodated more than 3 km of postpluton differential extension between pinned and nonpinned domains. In more western regions, late phases of detachment-fault--related extension beneath formerly pinned areas (including western and central parts of the pluton itself) are believed to have led to the development of two generations of east-striking left-slip faults. Collectively, these dextral and sinistral transfer faults illustrate the complex patterns of differential extension and accommodation that can characterize detachment-fault systems as adjacent areal domains of extension are generated and die.

Analysis of the unusually diverse right-lateral, left-lateral, reverse, and normal faulting aftershocks of the Loma Prieta earthquake reveals a systematic relation between fault-plane orientation and slip direction; right-lateral aftershock planes strike slightly more northward (10 degrees-15 degrees) than the main shock, left-lateral planes strike slightly more westward, reverse planes dip slightly less steeply, and normal planes dip slightly more steeply than the main shock. We demonstrate that these slip patterns are consistent with an approximately uniaxial stress field acting nearly perpendicular to the main-shock fault plane. These results imply an extremely weak fault zone, possibly resulting from near-lithostatic pore pressure.

Field relations in the footwall of the Badwater Turtleback fault provide evidence that macroscopically ductile deformation was directly responsible for early stages of brittle faulting during extensional denudation. Footwall rocks, which are typically mylonitic, consist of dominantly pegmatite, quartzofeldspathic gneiss, and dolomite and calcite marble. Because of the ductile deformation, these rocks are widely variable in thickness: bodies of rock that exceed 10 m in thickness may pinch out to zero thickness in distances of less than 100 m. The mylonitic rocks also display crosscutting relations that indicate the onset of bulk brittle behavior occurred at different stages of uplift for different rock types. Calcite marble was the last to deform by predominantly crystal-plastic mechanisms and therefore accommodate most of the shear strain during later stages of lower-plate, ductile deformation. Where the calcite marble pinched out into adjacent stronger rocks, the high strains caused brittle faulting in the adjacent rocks. The process described here, of ductile deformation leading to extreme thickness variations and consequent brittle faulting, directly links mid-crustal, ductile deformation with upper-crustal, brittle deformation. It further demonstrates that brittle faulting can initiate at depths where ductile deformation accommodates the bulk strain.

High-angle and reverse faults offset Pleistocene and Holocene deposits across a zone several kilometers wide east of the Calaveras fault near Sunol, California. Alignment of recent epicenters with these and other fault traces suggests that a band of contemporary deformation roughly parallels the Calaveras fault. These exposures provide evidence that active deformation associated with this major transverse fault is distributed over a broad zone. This complex pattern of faulting suggests that other faults throughout central California may be more active than is recognized in seismic hazard assessments.

Timing of the latest movement on the Santa Cruz Island fault, a dramatic physiographic feature of the southern boundary of the California Transverse Ranges, is demonstrated to be latest Pleistocene to Holocene in age. Faulting of dated terrace gravels confirms that the most recent rupture on the fault is not older than 11.78 [+ or -]0.1 ka. This represents an order of magnitude increase over the recency suggested by previous work and requires proportional increases in estimates of the minimum slip rate and seismic hazard posed by the fault. Uplifted lates Pleistocene to Holocene fill terraces are consistent with models of high rates of uplift and high sediment supply. Numerical solution of the interaction of sea-level rise with uplift at the west end of Santa Cruz Island predicts that the youngest strata in the faulted terrace sequence are about 6.1 ka. Reevaluation of high-resolution seismic sections just west of the island supports the latest Pleistocene to Holocene timing of the most recent rupture on the fault. The Santa Cruz Island fault apparently represents an active seismogenic element of southern California, the recency and high rate of activity of which have not been previously recognized.

The late Cenozoic Badwater Turtleback fault separates an upper plate of volcanic and sedimentary rocks from a lower plate of predominantly mylonitic plutonic and metamorphic rocks. The Turtleback fault, however, is not a single continuous surface, but consists of at least three generations of faults. These faults occur as discrete, crosscutting segments that progressively decrease in age and increase in dip to the west. Therefore, they probably began at moderate to steep angles but rotated to lower angles withextensional strain. If so, lower plate mylonitic rocks also restore to steeper dips and suggest that transport of the upper plate occurred on moderate to steeply dipping surfaces in the middle and upper crust. The crosscutting nature of the fault segments and their initial moderate to steep dips, plus a possible offset market on one of the segments, are most consistent with moderate amounts of extension in the Death Valley region.

Mafic meta-igneous supracrustal rocks in the east-central part of the western Triassic and Paleozoic belt are interlayered with and appear to predominantly overlie fine-grained clastic and cherty metasedimentary strata. This complex constitutes a lithostratigraphic terrane exposed in the vicinity of Sawyers Bar, California. Basaltic flows, dikes, and sills are informally referred to as the 'Yellow Dog greenstones.' Dark-colored, amygdaloidal flow breccias rich in titanium, iron, phosphorus, and light rare-earth elements (LREE's) constitute the lowest, mildly alkalic members of the mafic flow series, intimately interlayered with fine-grained terrigenous detritus. Element proportions suggest that the dark, Ti + [Fe.sup.*] + P-rich basaltic lavas may have been extruded in an oceanic intraplate setting. Interstratification with distal turbiditic strata indicates concurrent submarine volcanism and deep-sea sedimentary deposition. The main mass of the stratigraphically higher extrusive sequence is a light-colored, massive tholeiite series, poorer in Ti + [Fe.sup.*] + P + LREE's. Hypabyssal rocks belong exclusively to this second igneous suite. Compositions of the pale-colored, overlying basaltic/diabasic mass of the Yellow Dog greenstone section are consistent with eruption in an immature magmatic arc. Dikes presumably comagmatic with terminal-stage Yellow Dog volcanism cut the overlying, more eastern Stuart Fork terrane. The Yellow Dog greenstones contain relict magmatic clinopyroxene and/or pargasitic hornblende. Both melt series include Mg-rich units ([ave.sub.17]: MgO = 14.4 [+ or -] 1.9 wt. %; Cr = 650 [+ or -] 200 ppm; Ni = 270 [+ or -] 100 ppm). Sea-floor alteration apparently affected these mafic rocks in the marine environment following eruption, as reflected by [[delta].sup.18.O], MgO, and CaO/[Al.sub.2.O.sub.3] values and by minor alkali and silica metasomatism. Yellow Dog igneous activity was extinguished during mid-Jurassic suturing of the oceanward Sawyers Bar terrane against and beneath the pre-existing landward Stuart Fork blueschist complex of Late Triassic metamorphic age. Subsequently, the terrane amalgam was invaded by Middle/Late Jurassic granitoids. Minor chemical alteration, including [sup.18.O] enrichment, also probably accompanied later regional, and especially contact, metamorphism. Nevertheless, many major-, minor-, and rare-earth-element variations in the Yellow Dog greenstones are compatible with igneous fractional crystallization trends but exclude the assimilation of significant amounts of sialic crust.

New geologic mapping, structural studies, geochronology, and diffusion erosion modeling along the Deep Springs fault, California, shed light on its Quaternary faulting history. The Deep Springs fault, a 26-km-long, predominantly north-northeast-striking, west-northwest-dipping normal fault bounding the eastern side of Deep Springs Valley, cuts Jurassic batholithic rocks nonconformably overlain by middle Miocene to Pleistocene stream gravels, coarse-grained sand, tuffaceous sand, unwelded to partially welded tuff, and Bishop ash, as well as Quaternary coarse- to fine-grained alluvial fan deposits. The [sup.40]ArP/[sup.39]Ar geochronology yields ages of 3.09 [+ or -] 0.08 Ma for the unwelded tuff and 753 [+ or -] 4 ka for the Bishop ash. Holocene debris flows, a landslide, and recent alluvium bury the youngest fault scarp. The Deep Springs fault is characterized by multiple fault planes and fault scarps that become progressively younger toward the basin. The dip of the fault plane varies from 20 [degrees] to 87 [degrees] and fault-plane and striation measurements indicate an average orientation of N23E, 47NW, and N77E, 49SW, respectively. The offset of Bishop ash and underlying tuff across the Deep Springs fault indicates horizontal extension and vertical slip rates of ~0.7 and ~0.9 mm/yr, respectively, since the eruption of the Bishop Tuff, and ~0.2 and ~0.2 mm/yr, respectively, since the eruption of the unwelded tuff. If the vertical slip rate since the eruption of the Bishop Tuff has remained constant through time, then slip along the Deep Springs fault began ca. 1.7 Ma. Younger fault scarps to the west of the bedrock fault cut Quaternary deposits; scarp offset ranges from 0.8 to 17.5 m, and scarp slope angle ranges from 8 [degrees] to 37 [degrees]. Topographic profiling of the smallest, least eroded fault scarps, with an average surface offset of 2.7 m, indicates that these scarps developed as the result of a single earthquake and ruptured an ~20-kin-long segment of the fault. Radiocarbon analyses on detrital charcoal, located in the footwall block of one of these scarps, yield an age of 1.960 [+ or -] 0.055 ka. Diffusion erosion modeling of these fault scarps yields an elapsed time of 1.7 [+ or -] 0.5 k.y. since these fault scarps formed. Making reasonable assumptions about the depth of this earthquake and shear modulus, we estimate a moment magnitude, [M.sub.w] [approximately equals] 7.0, for this earthquake. The Deep Springs fault is one of several displacement-transfer normal faults that define a zone of distributed deformation between subparallel right-lateral strike-slip faults east of the Sierra Nevada that make up the northern part of the Eastern California Shear Zone. The young age and recent earthquake activity along the Deep Springs fault are consistent with a model proposed for the kinematic evolution of this part of the Eastern California Shear Zone. Keywords: active faults, active tectonics, argon-argon, fault scarps, mapping, normal faults, structure, tectonics.

Long-term deformation along the Fish Springs fault in Owens Valley, California, is recorded by offset landforms, including a previously dated cinder cone ([sup.39]Ar/[sup.40]Ar, 314 [+ or -] 36 ka, 2[Sigma]), several debris-flow fans and levees deposited by Birch Creek, stream channels, and lava flows of nearby Crater Mountain. The [sup.10]Be and [sup.26]Al model exposure ages (n = 68) delimit fan ages and suggest that deposition stopped after ca. 136, 15, 13, and 8 ka. One fan remains active and has undergone deposition throughout the Holocene. Soil development on three fans, where boulders were sampled for [sup.10]Be and [sup.26]Al analysis, also indicates distinct ages. Together, soil development and cosmogenic isotope data suggest that fan deposits correlate with the Tahoe and late Tioga glaciations. Soil profile development index values from Fish Springs are low compared with those determined elsewhere, suggesting relatively slow rates of soil formation for fan surfaces at Fish Springs and/or modification of soil profiles by surface processes during or following soil formation. Age estimates and measured offsets of the fans are consistent with a long-term vertical slip rate of 0.24 [+ or -] 0.04 m [k.y..sup.-1] for the Fish Springs fault over the past 300 k.y. Keywords: fans, faulting, isotopes, Owens Valley, soils.

Interpretation of seismic reflection data and restoration of depositional geometries of Cretaceous forearc basin strata in the northwest Great Valley of California provide important controls on structural reconstructions of the western margin of the Sacramento Valley and northern Coast Ranges. Monoclinal eastward dips of Great Valley Group strata and fault systems striking northwest-southeast, which are features proposed as evidence for a west-dipping blind Great Valley-Franciscan sole thrust and related backthrusts, instead are expressions of bedding geometry that resulted from folding of the Paskenta and related synsedimentary normal faults, depositional onlap, and a major structural-stratigraphic discontinuity. The discontinuity separates east-dipping Aptian and younger Great Valley Group strata from beds of lower Great Valley Group and Coast Range ophiolite that were deformed and erosionally or structurally truncated by mid-Cretaceous time. Dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient Great Valley forearc, but is also observed in modern forearc basins. Advocates of the Franciscan thrust-wedge model have also proposed that west-dipping, shingled patterns of seismic events imaged beneath the Sacramento Valley are imbricate thrust slices of the Great Valley Group. This hypothesis, however, is incompatible with borehole, potential-field, and seismic-refraction data that characterize the Sacramento Valley basement as ophiolitic. Seaward-dipping reflections in the ophiolitic basement of the Sacramento Valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges. Thus, late-stage tectonic mechanisms are not required to interpret a forearc that owes much of its present-day bedding architecture to processes coeval with deposition. Thickening of the Great Valley Group stratigraphic section (Valanginian-Turonian) in the hanging walls of the Paskenta, Elder Creek, and Cold Fork fault zones, combined with attenuation or complete omission of preextensional units (including the Coast Range ophiolite) and geometric evidence based on seismic reconstructions, suggest that these faults are Jurassic-Cretaceous normal faults that developed in a submarine setting. Down-structure views of the Great Valley outcrop belt simplify otherwise complex map relations and portray the Paskenta and related faults as half-graben bounding faults that accommodated significant northwestward tectonic transport of hanging-wall rocks. It is significant that these faults sole into the Coast Range fault, an enigmatic forearc structure that juxtaposes rocks of the Franciscan Complex (blueschists) with rocks of the Coast Range ophiolite and Great Valley Group that have sustained only zeolite-grade metamorphism. Discovery of Jurassic-Cretaceous crustal-scale extension in the Great Valley forearc suggests that a significant part of Coast Range fault-related attenuation developed early in the history of the subduction complex. Keywords: Coast Range ophiolite, forearc basins, Franciscan complex, Great Valley, Sacramento basin, seismic reflection profiles.

Fluid flow in fractured siliceous mud-stone of the Miocene Monterey Formation of California is inferred to be highly focused toward map-scale faults that locally contain extensive amounts of carbonate and minor silica cement. The distance of cross-stratigraphic flow, as inferred based on the strontium isotopic composition of carbonate fault cement, is close to the thickness of the Monterey Formation of 700 m in one of two study locations, Jalama Beach, and less than the formation thickness at another location, Arroyo Burro Beach. Fluid is thus derived from within the Monterey Formation rather than from underlying older units. Based on mass-balance estimates of the fluid volume required for fault cementation at Jalama Beach, the minimum distance of formation-parallel flow into the fault zone is 4 km and possibly [is greater than] 12 km. The inferred distance of flow parallel to the formation into this fault thus exceeds the distance of cross-formational upward flow along the fault by at least a factor of six. The mass-balance estimate requires that fluid flow along the fault is channeled into a pipe-shaped conduit rather than distributed along fault strike. Fluid flow from the surrounding formation into fault pipes is inferred to follow a radial rather than uni-or bilateral flow symmetry, using bedding-confined sets of extension fractures and stratabound breccia bodies. Radial fluid flow toward fault pipes requires fairly isotropic fracture permeability for flow along bedding and a low permeability across bedding. The inferred flow geometry illustrates the combined effect of fault permeability structure, permeability anisotropy of the surrounding formation, and hydraulic head distribution in controlling basinal fluid flow in faulted sequences. Keywords: faults, fluids, strontium, isotopes, hydrocarbon, Monterey Formation.

The Santa Monica fault is a 40-km-long, oblique left-lateral reverse fault that extends through the densely urbanized northwestern Los Angeles, California, area and offshore parallel to the Malibu coast. The fault exhibits near-surface strain partitioning in paleoseismologic trenches, and has undergone at least six surface ruptures during the past ~50 k.y. Although events may be missing from the older part of the record, and the ages of older events are based on poorly constrained soil age estimates, at least two and probably three events occurred after the well-dated burial of a prominent paleosol ca. 16-17 ka. The resulting ~7-8 k.y. latest Pleistocene-Holocene average recurrence interval for events large enough to cause surface rupture is much longer than the [is less than or equal to] 1.9-3.3 k.y. maximum interval we calculate for hypothetical [M.sub.w] 6.9-7.0 earthquakes generated by rupture of the entire Santa Monica fault. The pronounced disparity between the measured and calculated average recurrence intervals suggests that the Santa Monica fault undergoes infrequent, and therefore probably very large, earthquakes ([M.sub.w] [is greater than or equal to] 7.0). If such large earthquakes have occurred, we speculate that they may have been generated by simultaneous rupture of the Santa Monica fault with other faults in the Transverse Ranges Southern Boundary fault system, such as the Hollywood or Anacapa-Dume faults. The most recent definite Santa Monica fault surface rupture occurred between ca. 10 and 17 ka, although a more recent event probably occurred between ca. 1 and 3 ka. Keywords: earthquakes, faulting, geomorphology, paleoseismology, seismic risk, seismic zoning.

We characterize the seismic hazard of the Elysian Park fault, a blind reverse fault beneath central Los Angeles, by analysis of the Elysian Park anticline, which overlies it. New shallow-subsurface geotechnical data, combined with other surficial stratigraphy and geomorphology, reveal that the Elysian Park anticline is an active 20-km-long structure. From the style and rates of deformation of parasitic folds on the southern limb of the anticline, we estimate a contraction rate of 0.6-1.1 mm/yr. This rate provides a basis for estimating a rate of contraction of the entire Elysian Park anticline, which in turn allows us to estimate a 0.8--2.2 mm/yr time-averaged rate of slip on the underlying fault. At this rate of slip, rupture of the Elysian Park fault could produce a nominal [M.sub.w] 6.2 to 6.7 earthquake every 500 to 1300 yr, on average. Although this Elysian Park earthquake would recur infrequently, its size and recurrence interval may be similar to those estimated for the sources of the destructive 1971 San Fernando and 1994 Northridge earthquakes. Keywords: active faults, fluvial sedimentation, folds, geologic hazards, geomorphology, Los Angeles basin.

Investigation of the tectonic geomorphology of active folding over buried reverse faults at the San Emigdio Mountain front, southern San Joaquin Valley, California, provides insight concerning the tectonic and geomorphic development of mountain fronts produced by active folding and faulting. Monoclinally flexed gravels with dips as great as 50 [degrees] and a minimum age of about 65 ka provide evidence of late Pleistocene deformation at the active range front. Studies of the surface folding of alluvial fans and fluvial terraces indicate a Holocene vertical deformation rate of 1.9-3.0 m/k.y. at the active range front and 0.8-1.3 m/k.y. ~2 km basinward. Geomorphic evidence also indicates that the locus of active folding and vertical deformation along the northern flank of the San Emigdio Mountains has migrated and continues to migrate basinward. This evidence includes a relict mountain front, now within the uplifted block, 5 km from the present active mountain front, and the existence of recently initiated folds in the active alluvial fan 2 km basinward from the mountain front. Northward migration of tectonic activity results in the progressive widening of the uplifted block as the location of active folding moves basinward. This migration of tectonic activity appears to occur through the onset and subsequent increase of vertical deformation along more northerly folds and faults accompanied by reduction and eventual cessation of activity along the older, more southerly structures.

Earliest Miocene (20-24 Ma) faulting and volcanism were widespread in western California, including the San Joaquin-La Honda basin, Coast Ranges, and Transverse Ranges. Evidence for strike slip on faults of this age is lacking. Their arrangement on a pre-San Andreas restoration suggests continuity with early Miocene extended terranes in eastern California and southwestern Arizona. Deformation was probably not an effect of transform or triple-junction-migration tectonics, because it was synchronous over 300 km of latitude and there is no direct evidence for a transform boundary until late in early Miocene time. Magnetic anomalies in the eastern Pacific permit an interpretation that a spreading ridge supplied a still-active subduction took place in an arc-related setting. A transform regime probably formed about 18 Ma, when most basins underwent episodes of deepening or uplift, strike-slip faults became active, and rotation of the western Transverse Ranges block began.

The newly recognized Waterman Hills detachment fault (WHDF) of the central Mojave Desert, California, is significant because it provides the first unambiguous evidence for large-scale core complex-style crustal extension in the central Mojave Desert, and because it has significantly rearranged the pre-Miocene paleogeography of the Mojave Desert. The WHDF places steeply dipping to overturned Miocene volcanic and sedimentary rocks upon mylonitic pre-Teritary basement. The mylonites, which apparently formed during extension, are predominantly L-tectonites which manifest top-to-northeast shear. The WHDF dips to the northeast beneath domino-faulted ranges of the central Mojave Desert and detachment faults of the Colorado River trough, forming an imbricated early Miocene system of detachment faults. Extension continued in the Colorado River trough after extension had ceased in the central Mojave Desert. Tentative correlations of Mesozoic intrusions suggest about 40 km of slip across the WHDF, which carries eugeoclinal Paleozoic rocks in its hanging wall and crational/miogeoclinal Paleozoic rocks in its footwall. Restoration of 40 km of slip (1) removes a prominent kink in the boundary between eugeoclinal and crational/miogeoclinal facies, (2) aligns crational/miogeoclinal strata near Victorville more closely with the late Paleozoic continental margin farther north, (3) places cratonal/miogeoclinal rocks structurally beneath eugeoclinal rocks, implying that the facies were stacked by thrusting, and (4) straightens the western margin of the Late Jurassic Independence dike swarm.

We derived slip rates on the San Andreas fault from displacement of alluvial fans and from approximate dating of the fans by the degree of soil development. Distinctive red-purple sedimentary rocks of the Miocene(?) Potato Sandstone exposed on Yucaipa Ridge in the San Bernardino Mountains were transported down Wilson Creek and shed across the modern trace of the San Andreas fault. Soils developed in alluvial-fan deposits contain red-purple clasts of the Potato Sandstone and are progressively more developed toward the northwest as a result of fault displacement. We used soil development to estimate the ages of these alluvial fans and underlying deposits. Using dated soils from Cajon Pass and from the Central Valley of California, we estimated ages of Wilson Creek deposits, using a statistical method of maximum likelihood. These methods also utilize Monte Carlo simulations of larger data populations and account for soil variability, uncertainty of calibration dates, and the limited number of soils described and sampled. Resulting age estimates have considerable (as much as 80%) uncertainty, resulting largely from quality of calibration dates. Slip rates for three time intervals were obtained by restoring the medial axis of each displaced fan to Wilson Creek. Maximum slip rates were also obtained by restoring the westernmost extent of each fan to Wilson Creek. Best-estimate rates averaged to the present for 0 to 14,000 yr are 14 to 25 mm/yr; for 0 to 30,000 yr, 22 to 34 mm/yr, and for 0 to 65,000 or 90,000 yr, 12 to 16 mm/yr. Maximum rates averaged to present for 0 to 14,000 yr are as high as 65 mm/yr; older rates are as high as 80 mm/yr. When Holocene rates are accounted for, actual or incremental slip rates for the interval 30,000 to 65,000 or 90,000 yr B.P. are probably 6 to 13 mm/yr. These data tentatively suggest that slip rates on the modern trace have increased during the late Quaternary. Uncertainties in slip and age data, however, allow for rates to have been nearly constant or even decreasing toward the present.