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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 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.

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.

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.

A complex structural and metamorphic history is shown by the Orocopia Schist. The overwhelming trend away from the Orocopia fault is nearly NE-SW reflecting the early phase of deformations during subduction. Possible fold formations and times, and a possible detachment faulting during the Tertiary are dsicussed as possible reasons for the regions unique geological status.

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 Ranges from the range front on the east to the continental margin on the west in Central Valley, CA, exhibit a sudden transition along the deformed mountains and less deformed ones. The less deformed structures constitute the California valley. The Coast Ranges of California exhibit major thrusts and faults in the mountain regions.

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 (

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.

The earthquake near Landers, California, on Jun 28, 1992 of magnitude 7.5 shifted static stress to the San Bernardino segment of the San Andreas fault and to segments of the San Jacinto fault. The Mojave and Anza segments of the San Andreas fault also underwent a lessening of static stress. These changes suggest that the timetable of the next earthquake of magnitude 8 may have been advanced about 14 years.

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.

The Del Puerto ophiolite in the northern Diablo Range, California is overlain by fault displacement-sheared Fransiscan metagraywacke. Coast Range ophiolite and Great Valley sequence attentuation was concurrent with the normal faulting and extensional offset representing the displacement. Moving westward across the underlying oceanic plate, the Fransiscan Complex lengthened and thinned due to subduction-caused instability. These movements resulted in blueschist facies alongside only slightly-metamorphic hanging wall units.

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.

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.

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 western Sierra Nevada metamorphic belt is subdivided into six tectonic terranes, which are juxtaposed along mostly cryptic, pre-Late Jurassic faults. The Northern Sierra terrane contains a basement of imbricated and locally chaotic lower Paleozoic quartz sandstone, mafic rocks, chert, argillite, and other rocks, overlain unconformably by Upper Devonian-Middle Jurassic volcanic and siliceous strata. The Feather River terrane is a Paleozoic, possibly polygenetic ophiolite. The Red Ant terrane is a small terrane with an early Mesozoic blueschist-facies overprint. The Calaveras terrane is a chaotic assemblage of upper Paleozoic-Triassic argillite, chert, volcanic rock, and limestone contained in two regionally mapped melange units. The Tuolumne River terrane is a stratigraphic assemblage of disrupted Paleozoic ophiolite overlain by Middle Triassic-Lower Jurassic siliceous and volcanic strata, both intruded by 200 Ma (Lower Jurassic) plutons. The Slate Creek terrane is a 200 Ma pseudostratigraphic sequence of cumulate ultramafic, plutonic, and volcanic rocks. The Northern Sierra and Calaveras terranes contain Permian McCloud fossils, whereas the Tuolumne River terrane contains Permian McCloud and Tethyan fossils in olistoliths within lower Mesozoic deposits. These terranes are bounded mostly by steeply dipping faults of the Late Jurassic Foothills fault system but locally are juxtaposed along earlier, highly deformed, low-angle faults that are remnants of major terrane-bounding faults. A system of early, mostly cryptic, east-dipping faults juxtapose, in descending structural order, the Northern Sierra, Feather River, Red Ant, Calaveras, and Tuolumne River terranes. Each of the four boundaries that separate these five terranes have three or more independent pre-Late Jurassic upper age brackets, including crosscutting plutons, sedimentary provenance, and shared structural fabrics. The Slate Creek terrane tectonically overlies the Tuolumne River, Calaveras, and Red Ant terranes along an isoclinally folded, west-dipping fault that is cut by a 165 Ma pluton. Callovian-Kimmeridgian strata and coeval intrusive rocks, including the 155-165 Ma volcanic rocks, sheeted dikes, and zoned mafic plutons of the Smartville Complex, are deposited on and intruded into the older, previously juxtaposed terranes and constitute an overlap sequence. The well-known faults of the Foothills fault system, including the Melones fault, cut Kimmeridgian strata of the overlap sequence and thus cannot include ancient sutures. These faults reactivated many of the earlier terrane-bounding faults. Pre-Late Jurassic amalgamation of the Sierra Nevada metamorphic belt rules out previous models of Late Jurassic are-continent collision; earlier collisions and/or noncollisional tectonic processes must be invoked to explain strong contractional deformation in the western Sierra Nevada metamorphic belt.

Recently completed sedimentologic and petrologic studies of Oligocene and Miocene strata in the Temblor Range (San Joaquin basin) and Santa Cruz Mountains (La Honda basin) permit detailed reconstructions of paleogeography, as well as new estimates of displacement along the San Andres fault. During the Oligocene and Miocene, the San Juaoquin and La Honda basins were contiguous. The southwestern margin of the San Joaquin-La Honda basin was tectonically unstable and paleogeographically complex, with several small deep-sea fans fed by sediment derived from nearby subaerial uplifts of Franciscan and granitic basement. One of these deep-sea fans, represented by the Temblor Formation of the southern Temblor Range and the Vaqueros Sandstone of the central Santa Cruz Mountains, apparently has been displaced about 315 to 320 km by post-early Miocene (post-Zemorrian, about 23 Ma) right-lateral slip along the San Andreas fault. This new estimate of displacement places an additional constraint on interpretations of San Andres slip history, but ambiguities remain. The time of initiation of San Andres displacement is poorly known, and available data do not permit discrimination between a history of relatively continuous slip versus a history of episodic slip.

Three powerful earthquakes occurring close to where three faults intersect off the northern California coast gave geologists a chance to examine the plate tectonic forces affecting the region. The earthquakes, which occurred on or shortly after Apr 25, 1992 with the respective magnitudes of 6.9, 6.2 and 6.5, resulted from the interaction of the San Andreas fault, the Cascadia subduction zone and the Mendocino transform fault where the Pacific and Gorda plates move past each other.