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Some of the evidence of tectonic and chemical processes that occur in subduction zones is preserved in blueschists and eclogites, as validated by the geochemical data presented from high-grade-metamorphic tectonic blocks from the Franciscan Complex in central California. Combining major elements, compatible and incompatible trace element concentrations and ratios, and Sr-Nd-Pb isotopic data, we conclude that the protoliths of these high-grade Franciscan blocks were arc lavas with no continental crust-derived components. Although the protoliths underwent high-grade metamorphism at the inception of subduction, the tectonic affinity of the protoliths can be distinguished by our geochemical study. Our geochemical data are significant in that (1) the protoliths of these high-grade blocks have a different mode of origin than the protoliths of the lower-grade Franciscan basalts that were incorporated later in the subduction complex, as the latter generally are considered to be oceanic-island basalts (OIB) and derived from mid-oceanic-ridge basalt (MORB); and (2) the protoliths of the high-grade blocks were formed over a pre-Franciscan subduction zone. Based on the geochemical evidence for an arc origin and the available geochronological data, we suggest that protoliths of the high-grade tectonic blocks of the Franciscan Complex and the Coast Range Ophiolite formed in the same infant-arc setting, initiation of Franciscan subduction in this arc crust led to the formation of the high-grade blocks beneath the young Coast Range Ophiolite. Continued subduction resulted in subduction of older oceanic crust of MORB and OIB origin, some of which was scraped off to form lower-grade metabasites within the Franciscan Complex. [ABSTRACT FROM AUTHOR]

New continuous and differential global positioning system (GPS) measurements of recent slip rates and 30 yr alignment-array offsets from the central creeping segment of the San Andreas fault yield a maximum right-lateral slip rate of 25 ± 1 mm/yr. This slip rate is 20% slower than the 30 mm/yr slip rate accepted for this segment of the fault and 35% slower than the 39 mm/yr slip rate predicted between the Sierra Nevada-Great Valley block and the Pacific plate. New continuous GPS measurements between pairs of sites that flank the creeping segment at intersite distances of 1.0 km and 70 km give relative fault-parallel slip rates of 28 ± 2 and 30 ± 2 mm/yr, respectively. These observations indicate that right-lateral deformation rates increase with distance from the fault. Possible explanations for the gradient observed in the geodetic data are elastic strain accumulation along the creeping segment or significant distributed deformation on off-fault structures. [ABSTRACT FROM AUTHOR]

Salinia is an out-of-place granitic terrane in central coastal California whose debated origin is critical to understanding the tectonic history of southwestern North America. Salinian metasedimentary and sedimentary rocks that respectively host and cover its predominant arc rocks should contribute important data about its origin and kinematic history, but pervasive intrusion, high-grade metamorphism and Cenozoic erosion of the Salinian block have inhibited their widespread characterization and correlation. To further address these problems, we report 605 U-Pb detrital-zircon geochronologic ages collected by laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-Ni C-ICP-MS) from seven Salinian metasedimentary framework (Sur Series) and sedimentary cover samples. Samples collected from the Sur Series contain Late Archean (23–2.9 Ga), late Paleoproterozoic (1.6–1.9 Ga), Mesoproterozoic (0.9–1.5 Ga), Neoproterozoic (0.65–0.8 Ga), Paleozoic (250–450 Ma), and possibly Mesozoic U-Pb detrital-zircon ages. Samples collected from Upper Cretaceous cover units have various age-peak distributions, which collectively include late Paleoproterozoic (1.6–1.8 Ga), early Mesoproterozoic (1.35–1.55 Ga), Permo-iriassic (220–290 Ma), and Jurassic-Cretaceous (80–190 Ma) peaks. From these data, several interpretations are made. (1) Maximum depositional ages of the Sur Series and cover intervals are 280–360 Ma and 78–90 Ma, respectively. (2) The presence of Late Archean, early Paleoproterozoic, and Neoproterozoic zircons in Salinian metasedimentary rocks suggest that uplift and erosion of adjacent basins recycled sediment onto Salinia. (3) The abundant pre-Mesoproterozoic detrital-zircon ages in Sur Series and cover units preclude the possibility that Salinia originated in southern Mexico, as has been previously suggested. (4) Five of six key detrital-zircon age peaks identified in Salinian basement and cover units are nowhere more closely arranged than in the Mojave Desert-Peninsular Ranges region of Baja and southern Alta California. (5) Paleozoic and early Mesozoic detrital zircons in Sur Series and cover units match the ages of several plutonic events that occurred along the western margin of North America-however, Permian ages favor a Mojave Desert origin over other candidates. Collectively, these and other data suggest that Salinia resided in the Mojave Desert-Peninsular Ranges region from the late Paleozoic until the Late Cretaceous, after which it was rapidly exhumed, deposited upon, and then translated outboard and northward to its current position. [ABSTRACT FROM AUTHOR]

A 2-D upper crustal structural profile with seven steeply dipping faults was constructed from the Los Angeles Region Seismic Experiment phase II (LARSE II) seismic refraction data recorded over the north central Transverse ranges and adjacent Mojave desert in southern California. The profile extends to a depth of approximately 4 km. The faults were identified from distinctive features in the seismic first arrival data, while the velocity structure was determined from travel times and refined by forward modelling. The resulting seismic velocity structure is correlative to the geological structure along the profile and is used to help constrain a geological cross-section. Six of the seven faults detected from the seismic data correspond directly to geologically mapped faults in the central Transverse ranges and adjacent Mojave desert. From south to north, they are the Pelona fault, the San Francisquito fault, the Clearwater fault, the San Andreas fault and two faults that cut the Portal ridge. The seventh fault is buried beneath Quaternary deposits of the western Mojave desert and appears to correlate with an additional fault that cuts Portal ridge out of the section line. The southernmost fault (the Pelona fault) separates the younger Vasquez formation in the south from the Pelona schist in the north and is determined to be an intermediate (40°–60°) south-dipping normal fault. This coupled with field observations indicate that the Pelona fault was instrumental in the late Oligocene—early Miocene formation of the Soledad basin, and the structural ascent and exhumation of the Pelona schist along the proto-Sierra Pelona intrabasin ridge. Analogous Neogene extensional deformation is found to have affected the western Mojave desert region, where faults 6 and 7 appear to have had north side down normal displacement and to have been instrumental in forming the western Antelope basin. A 6.0 km s−1 basal layer to our shallow seismic structure is correlated with the Pelona schist south of the San Andreas fault and with the tectonically related Rand schist beneath the western Mojave desert. Granitic and gneissic crystalline rocks, which form regional upper plate complexes along the Vincent thrust above the Pelona schist and along the Rand thrust above the Rand schist, yield consistently low seismic velocities as compared with the expected velocities for the constituent rock types. These anomalous velocities are reconciled by the degraded structural and textural state of the upper plate crystalline rocks that resulted from extreme shearing, brittle fracturing and related retrogressive hydration reactions starting during or shortly after the latest Cretaceous—early Palaeocene underthrusting of the schists. Extension, translation along the San Andreas transform system and compressional faulting with the uplift of the Transverse ranges further accentuated the basement deformations. The integrated result is that the survival of the Precambrian through Mesozoic granitic crust of the region became a rootless and highly attenuated upper crustal layer. [ABSTRACT FROM AUTHOR]

Jurassic ophiolitic complexes in California and Oregon are widely distributed outboard of a Jurassic and older continental margin. Whether these outboard complexes were exotic and collided with the Jurassic continental margin, or whether they were simply part of a wide forearc to a Jurassic continental magmatic arc, has long been at issue. We have studied two such complexes and crosscutting intrusions in the fault-bounded Central and Western belts of the northern Sierra Nevada. We report new and revised U-Pb (zircon) dates for the Early Jurassic and younger Slate Creek complex (ca. 203–171 Ma) in the Central belt; the Late Jurassic Smartville complex (159 ± 3 Ma) in the Western belt; and crosscutting intrusions ranging from Middle Jurassic to Early Cretaceous age. The new ages imply that the Central and Western belts have been part of the same terrane since Middle Jurassic time and that the fault separating them has accommodated only minor displacements since ca. 160 Ma. Isotopic compositions of common Pb indicate derivation from mantle sources, but evidence for inheritance of Precambrian zircon is common in plutons within the Early and Late Jurassic complexes and in Middle Jurassic, Late Jurassic, and Early Cretaceous crosscutting intrusions. Although there are several potential sources for such contamination, the ages of the inherited zircon are consistent with a North American origin. This contamination would have been accomplished most plausibly near the North American margin. [ABSTRACT FROM AUTHOR]

Pliocene (ca. 3.5 Ma) removal of dense eclogitic material under the Sierra Nevada has been proposed from variations in the petrology and geochemistry of Neogene volcanic rocks and their entrained xenoliths from the southern Sierra. The replacement of eclogite by buoyant, warm asthenosphere is consistent with present-day seismologic and magnetotelluric observations made in the southern Sierra. A necessary consequence of replacing eclogite with peridotite is that mean surface elevations and gravitational potential energy both increase. An increase in potential energy should increase extensional strain rates in the area. If these forces are insufficient to significantly alter Pacific—North American plate motion, then increased extensional strain rates in the vicinity of the Sierra must be accompanied by changes in the rate and style of deformation elsewhere. Changes in deformation in California and westernmost Nevada agree well with these predictions. Existing geologic evidence indicates that a period of rapid uplift along the Sierran crest of more than ∼1 km occurred between 8 and 3 Ma, most likely as a consequence of removal of lower lithosphere. About this same time, extensional deformation was initiated within ∼50 km of the eastern side of the Sierra (5–3 Ma), and regional shortening began to produce the California Coast Ranges (5–3 Ma). We suggest that these events were induced by the >1.2 × 1012 N/m increase of gravitational potential energy generated by the Sierran uplift. Evidence for Pliocene uplift, adjoining crustal extension, and shortening in directly opposing parts of the Coast Ranges is found along nearly the entire length of the Sierra Nevada and implies that lithosphere was removed beneath all of the present-day mountain range. The uplifted area lies between two large, upper-mantle, high-P-wave-velocity bodies under the south end of the San Joaquin Valley and the north end of the Sacramento Valley. These high-velocity bodies plausibly represent the present position of material removed from the base of the crust. Lithospheric removal may also be responsible for shifting of the distribution of transform slip from the San Andreas Fault system to the Eastern California shear zone, a prediction that awaits better-defined slip histories on both faults. Overall, the late Cenozoic deformational history of the Sierra Nevada and vicinity illustrates that locally derived forces can influence deformation kinematics within plate-boundary zones. [ABSTRACT FROM AUTHOR]

The response of ephemeral alluvial streams to active tectonics is not as well established as those documented for perennial alluvial streams. This study documents the response of ephemeral streams transverse to growing folds along the north flank of the San Bernardino Mountains in southern California. The growing folds emerge amid a broad, sloping piedmont mantled by north-dipping alluvial fans and underlain by coarse, angular gravel and sand of mixed provenance. The study area contains fans composed of two distinct lithologies that control the expression of the folds. Sediment transport processes differ among alluvial perennial and ephemeral channels and play a primary role in defining the range of responses that ephemeral streams have to an actively rising fold. We find that ephemeral streams respond by (1) changing pattern from a single, slightly incised channel with well-defined banks to a braided channel upstream of the fold axis, (2) incising across the fold axis, preserving a terrace and braid bars, and (3) returning to a single-thread, less incised channel downstream of the fold axis. This channel response is documented for both a topographically obvious anticlinal fold, the Cougar Buttes anticline, as well as a suspected, but not topographically obvious anticline, the Pitzer Buttes anticline. Variations from this general model appear to be correlated with locations of slow fold growth l and/or channel alluvium that is fine grained and of low cohesion. A growing fold, such as the Cougar Buttes anticline, provides a laboratory for the investigation of the development of transverse streams with respect to position along strike with the fold axis. Some streams crossing the Cougar Buttes anticline are antecedent, whereas others are consequent to the growth of the fold. These observations lay the foundation for a conceptual model for ephemeral stream response to active tectonics, particularly useful in identifying previously unrecognized actively rising folds. In this study, the fold axis of the Cougar Buttes anticline is revealed to extend at least 1 km beyond its current obvious topographic expression. Because ephemeral streams are sensitive to tectonic deformation, they can be used locally in paleo-seismological investigations, regionally to understand the strain partitioning between the Big Bear and Mojave blocks, and conceptually to constrain geodynamic models investigating the interaction between surface processes and the geometry of and slip rates on faults responsible for fold growth. [ABSTRACT FROM AUTHOR]

The San Gabriel Basin is a Pliocene—Pieistocene Transverse Ranges structure bounded by the San Gabriel Mountains on the north, the San Jose and Puente Hills on the east, and the Repetto and Montebello Hills on the west. A basement boundary between gneissoid rocks of San Gabriel Mountains aspect and Peninsular Ranges rocks strikes east across the basin and connects with the San Jose fault. This boundary may represent the eastward continuation of the Canton fault, a middle Miocene precursor of the San Gabriel fault system with up to 30 km of right slip. Canton fault displacement is largely canceled by ∼22.5 km of left slip on a precursor to the Raymond fault such that an Oligocene dacite in the San Jose Hills and clasts derived from it show little evidence of large horizontal displacement between the San Gabriel Mountains and Puente Hills. Clockwise rotation in the middle Miocene probably influenced the distribution of lateral slip. The fill in the northeast-trending Pliocene—Pleistocene San Gabriel Basin is a basal shallow-marine sequence overlain by the nonmarine Duarte Conglomerate, in contrast to the deep-water Fernando Formation of the Los Angeles Basin. The paleoslope trends south in the direction of the Repetto and Puente Hills, evidence that uplift of these hills postdated deposition. The triangular San Gabriel Basin is currently a stable block bounded by the right-slip, northwest-striking East Montebello fault on the west and the left-slip(?), northeast-striking Walnut Creek fault on the east. Farther east, the San Jose and Puente Hills are underlain by an east-trending active fold belt, including the San Jose reverse fault and anticline, the Walnut anticline, and the Puente Hills anticline adjacent to the right-lateral Whittier fault. The East Montebello fault is bounded on the west by the Elysian Park anticlinorium, a fold belt in downtown Los Angeles. The left-slip Holocene Raymond fault joins the Sierra Madre reverse fault at its segment boundary marked by the Clamshell-Sawpit fault. The triangular stable block between east-trending fold belts is bounded on the south by the Puente Hills blind thrust and on the north by the Sierra Madre fault, documenting north-south Quaternary convergence. East-west extension is also indicated, although not confirmed by geodetic measurements. [ABSTRACT FROM AUTHOR]

Clay mineral assemblages have been investigated in a sequence of Late Quaternary age sediments (160 ka) from the Santa Barbara Basin off California. These predominantly terrigenous sediments contain an assemblage of clay minerals dominated by smectite mostly derived from discharges of the Santa Clara River, together with illite and kaolinite from adjacent relief. Besides its characteristic clay mineral signature, the Santa Clara River has the largest drainage basin in this region and contributes most of the terrigenous load to the Santa Barbara Basin. Smectite contents of the sediments in the basin may therefore be used as a proxy for precipitation and run-off. Distinct changes in the relative abundance of smectite allow reconstruction of the pattern of precipitation, and its relations with regional environments and global climate. This showed that major intervals of precipitation coincide with local development of pine during glacials, and oak during terminations and interglacials; some expansion of mountain glaciers in the Sierra Nevada, especially when they occur during local minima of insolation; and lacustrine developments in the now arid continental interior of California. Major precipitation intervals started during glacials, and persisted during terminations and early interglacials, associated with a weak California Current and subsequent expansion of warm Pacific surface waters. In detail, major precipitation intervals consist of alternating episodes of higher and lower rainfall modulated by orbital precession, with a significant correlation between episodes of high rainfall and minima in precession, which were times of increased meridional gradients, enhanced poleward heat transfer and northward expansion of tropical waters. [Copyright &y& Elsevier]

We investigate the Quaternary slip rate for the Blackwater fault, Eastern California shear zone, through mapping and geochronology of offset volcanic rocks. Basalt flows of the Black Mountains support the presence of faulting at 3.77 ± 0.11 Ma, 1.8 ± 0.1 km of subsequent slip, and a well-constrained long-term slip rate of 0.49 ± 0.04 mm/yr. Total slip diminishes northward, evidenced by a 0.3-1.8 km offset of a 7.23 ± 1.07 Ma dacite flow in the Black Hills and fault termination in the Lava Mountains, 5 km short of the Garlock fault. Slow long-term slip rate together with sparse evidence for Holocene rupture contradict predictions of rapid slip rate from tectonic geodesy. These results support the conclusion that as much as 95% of geodetic strain accumulation across the Blackwater fault, and thus from 1 to 6 mm/yr of geodetic strain measured across the Eastern California shear zone, is a transitory phenomenon. Discrepant geologic and geodetic results may indicate an increased near-term seismic hazard, but merit caution for interpretation of fault slip rates from geodesy alone. [ABSTRACT FROM AUTHOR]

All offshore zone of faulting approximately 10 km from the southern California coast connects the seismically active strike-slip Newport-Inglewood fault zone in the Los Angeles metropolitan region with the active Rose Canyon fault zone in the San Diego area. Relatively little seismicity has been recorded along the offshore Newport-Inglewood Rose Canyon fault zone, although it has long been suspected of being seismogenic. Active low-angle thrust faults and Quaternary folds have been imaged by seismic reflection profiling along the offshore fault zone, raising the question of whether a through-going, active strike-slip fault zone exists. We applied a waveform cross-correlation algorithm to identify clusters of microseismicity consisting of similar events. Analysis of two clusters along the offshore fault zone shows that they are associated with nearly vertical, north-northwest-striking faults, consistent with an offshore extension of the Newport-Inglewood and Rose Canyon strike-slip fault zones. P-wave polarities from a 1981 event cluster are consistent with a right-lateral strike-slip focal mechanism solution. [ABSTRACT FROM AUTHOR]

A case study from southern California illustrates the value of detailed geologic data for understanding the kinematics of complex fault systems. The neotectonic behavior of faults at the junction of the Eastern California shear zone (ECSZ) and Transverse Ranges has implications for the interaction of intersecting, segmented fault systems and regional plate boundary evolution. Paleoseismic observations indicate that the North Frontal thrust system (NFTS) has ruptured once in the Holocene with 1.7-m displacement, despite previous speculations of inactivity based on its dissection by younger strike-slip faults. Simple polyphase deformation, in which dextral shear has replaced and overprinted thrusting, is thus not a valid explanation for this system of intersecting faults. This illustrates the limitations of inferring rupture behavior from mapped fault patterns alone. Neotectonic and geomorphic observations along the thrust system also suggest that the thrust segment west of the intersection with the dextral Helendale fault is significantly more active than the segment to the east. This is consistent with a simple block velocity model, in which dextral slip on the Helendale fault is balanced by convergence on the western thrust segment, dextral motion on the poorly studied Pipes Creek fault to the southeast, and inactivity on the eastern thrust segment. This divides the San Bernardino Mountains into domains dominated by thrusting (west) and strike-slip (east), the union of which is a quasi-stable triple junction. We speculate that this union has migrated to the west as the Mojave Desert has been translated southwards along the San Andreas fault. [Copyright &y& Elsevier]

Faults in complex tectonic environments interact in various ways, including triggered rupture of one fault by another, that may increase seismic hazard in the surrounding region. We model static and dynamic fault interactions between the strike-slip and thrust fault systems in southern California. We find that rupture of the Sierra Madre-Cucamonga thrust fault system is unlikely to trigger rupture of the San Andreas or San Jacinto strike-slip faults. However, a large northern San Jacinto fault earthquake could trigger a cascading rupture of the Sierra Madre-Cucamonga system, potentially causing a moment magnitude 7.5 to 7.8 earthquake on the edge of the Los Angeles metropolitan region. [ABSTRACT FROM AUTHOR]

Studies field, microstructural, and geochronologic evidence indicating that the Late Cretaceous McDoogle pluton of the eastern Sierra Nevada batholith was emplaced as a subvertically sheeted complex into a steep reverse-sense shear zone. Age, location, and kinematics of other shear zones; Isotope dates from various minerals.

Analyzes the structural geology and kinematic history of rocks formed along low-angle normal faults in Death Valley, California. Finite shear strains increasing progressively upward, toward the principal slip plane; Strain gradients and corresponding changes in grain size in the shear zones except in their symmetry.

The White Mountains represent the westernmost range of the central northern Basin and Range province. They are situated to the east of the unextended Sierra Nevada and represent a crustal block that is bounded along its western flank by the high-angle White Mountains fault zone. The fault zone accommodates up to ∼8 km of total dip-slip displacement. Investigation of the structural and thermal history of the White Mountains indicates a two-stage Cenozoic tectonic evolution. Preextensional Miocene volcanic rocks preserved along the eastern side of the range unconformably overlie Mesozoic granitic basement and currently dip up to 25° to the east, recording the total Cenozoic tilt of the crustal block. Apatite fission-track and (U-Th/He) thermochronological data indicate that the White Mountains underwent rapid exhumation and eastward tilting in the middle Miocene, starting at ca. 12 Ma. Geologic mapping (1: 10,000), fault kinematic analysis, and dating of younger volcanic sequences show that following middle Miocene east-west extension, the White Mountains have been dominated by right-lateral transtensional deformation related to the Walker Lane belt. The eruption of late Miocene and Pliocene volcanic sequences in the eastern White Mountains postdates the majority of the uplift of the range, as evidenced by infilling of paleodrainages and the presence of east-directed flow fabrics. Fault kinematic indicators from the White Mountains fault zone are characterized by apparent overprinting of dip-slip fault-motion indicators by right-lateral slickenfibers and fault striations, demonstrating that the range-bounding fault system along the western side of the White Mountains was reactivated as a dextral strike-slip fault system. At the northern and southern ends of the range, Pliocene right-lateral transtension along this northwest-southeast-trending fault systems resulted in the formation of northeast-trending pull-apart basins that truncate the mountain range... [ABSTRACT FROM AUTHOR]

We identify two surfaces in the shallow subsurface on the Eel River margin offshore northern California, a lowstand erosion surface, likely formed during the last glacial maximum, and an overlying surface likely formed during the most recent transgression of the shoreline. The lowstand erosion surface, which extends from the inner shelf to near the shelfbreak and from the Eel River to Trinidad Head (∼80 km), truncates underlying strata on the shelf. Above the surface, inferred transgressive coastal and estuarine sedimentary units separate it from the transgressive surface on the shelf. Early in the transgression, Eel River sediment was likely both transported down the Eel Canyon and dispersed on the slope, allowing transgressive coastal sediment from the smaller Mad River to accumulate in a recognizable deposit on the shelf. The location of coastal Mad River sediment accumulation was controlled by the location of the paleo-Mad River. Throughout the remainder of the transgression, dispersed sediment from the Eel River accumulated an average of 20 m of onlapping shelf deposits. The distribution and thickness of these transgressive marine units was strongly modified by northwest–southeast trending folds. Thick sediment packages accumulated over structural lows in the lowstand surface. The thinnest sediment accumulations (0–10 m) were deposited over structural highs along faults and uplifting anticlines. The Eel margin, an active margin with steep, high sediment-load streams, has developed a thick transgressive systems tract. On this margin sediment accumulates as rapidly as the processes of uplift and downwarp locally create and destroy accommodation space. Sequence stratigraphic models of tectonically active margins should account for variations in accommodation space along margins as well as across them. [Copyright &y& Elsevier]

Vertical-axis, clockwise block rotations in the Northeast Mojave block are well documented by numerous authors. However, the effects of these rotations on the crust to the north of the Northeast Mojave block have remained unexplored. In this paper we present a model that results from mapping and geochronology conducted in the north and central Owlshead Mountains. The model suggests that some or all of the transtension and rotation observed in the Owlshead Mountains results from tectonic response to a combination of clockwise block rotation in the Northeast Mojave block and Basin and Range extension. The Owlshead Mountains are effectively an accommodation zone that buffers differential extension between the Northeast Mojave block and the Basin and Range. In addition, our model explores the complex interactions that occur between faults and fault blocks at the junction of the Garlock, Brown Mountain, and Owl Lake faults. We hypothesize that the bending of the Garlock fault by rotation of the Northeast Mojave block resulted in a misorientation of the Garlock that forced the Owl Lake fault to break in order to accommodate slip on the western Garlock fault. Subsequent sinistral slip on the Owl Lake fault offset the Garlock, creating the now possibly inactive Mule Springs strand of the Garlock fault. Dextral slip on the Brown Mountain fault then locked the Owl Lake fault, forcing the active Leach Lake strand of the Garlock fault to break. [ABSTRACT FROM AUTHOR]

The Salinian block in the Salinas Valley region of central California consists of arc granitic and metasedimentary rocks (schist of Sierra de Salinas) sandwiched between coeval high-pressure, low-temperature mé1ange belts. U-Pb zircon ages of three granitic plutons from this region range from 88 to 82 Ma, and coexisting biotite yielded 4°Ar/[sup 39]Ar cooling ages of 76-75 Ma. The U-Pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117-81 Ma igneous provenance. Muscovite and biotite [sup 40]Ar/ [sup 39]Ar cooling ages of 72-68 Ma from the nearby schist are distinctly younger than those from the granitic plutons. These data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks, contradicting the prevailing view that the schist comprises the local framework for the Salinian arc. We propose that the schist of Sierra de Salinas was thrust beneath the Salinian magmatic arc along a Campanian thrust fault that has not been recognized. This hypothesis implies that the Salinian arc originated as a klippe of basement rocks derived from the vicinity of the western Mojave Desert. Thrusting initiated southeastward-migrating Laramide tectonism of a style similar to that which formed the Vincent thrust and the latest Cretaceous and Paleocene Pelona and Orocopia Schists of southern California and southwestern Arizona. [ABSTRACT FROM AUTHOR]

Examines the effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin in California. Location of the resulting complicated tectonic setting forms; Causes of the structural deformation of the neogene strata; Observation of distinct deformation regimes.

Understanding the way fluids flow in fault zones is of prime importance to develop correct models of earthquake mechanics, especially in the case of the abnormally weak San Andreas Fault (SAF) system. Because fluid flow can leave detectable signatures in rocks, geochemistry is essential to bring light on this topic. The present detailed study combines, for the first time, C–O isotope analyses with a comprehensive trace element data set to examine the geometry of fluid flow within a significant fault system hosted by a carbonate sequence, from a single locality across the Little Pine Fault–SAF system. Such a fault zone contains veins, deformation zones, and their host rocks. Stable isotope geochemistry is used to establish a relative scale of integrated fluid–rock ratios. Carbonate δ18O varies between 28‰ and 15‰ and δ13C between 5‰ and −7‰. From highest to lowest delta values, thus from least to most infiltrated, are the host rocks, the vein fillings, and the deformation zone fillings, respectively. Infiltration increases toward fault core. The fluids are H2O–CO2 mixtures. Two fluid sources, one internal and the other external, are found. The external fluid is inferred to come essentially from metamorphism of the Franciscan formation underneath. The internal (local) fluid is provided by a 30% volume reduction of the host limestones resulting from pressure solution and pore size reduction. Most trace elements, including the lanthanides, show enrichment at the 100-m scale in host carbonate rocks as fluid–rock ratios increase toward the fault core. In contrast, the same trace element concentrations are low, relative to host rocks, in veins and deformation zone carbonate fillings, and this difference in concentration increases as fluid–rock ratio increases toward the fault core. We suggest that the fluid trace elements are scavenged by complexation with organic matter in the host rocks. Elemental complexation is especially illustrated by large fractionation of Y–Ho and Nb–Ta geochemical pairs. Complexation associated with external fluid flow has a significant effect on trace element enrichment (up to 700% relative enrichment) while concentration by pressure solution associated with volume decrease of host rocks has a more limited effect (up to 40% relative enrichment). Our observations from the millimeter to the kilometer scale call for the partitioning of fluid sources and pathways, and for a mixed focused–pervasive fluid flow mechanism. The fluid mainly flows within veins and deformation zones and, simultaneously, within at least 10 cm from these channels, part of the fluid flows pervasively in the host rock, which controls the fluid composition. Scavenging of the fluid rare earth elements (REE) by host rocks is responsible for the formation of REE-depleted vein and deformation zone carbonate fillings. Fluid flow is not only restricted to veins or deformation zones as commonly believed. An important part of fluid flow takes place in host rocks near fault zones. Hence, the nature of the lithologies hosting fault zones must be considered in order to take into account the role of fluids in the seismic cycle. [Copyright &y& Elsevier]

Examines the association of backarc and arc-related tectonic units in California. Influence of alkalic and arc-related magnetic processes to backarc basin; Initiation of the Franciscan subduction; Formation of the Round Mountain serpentine melange.

Examines the tectonic motion across the Coast Ranges and San Andreas fault system in central California. Use of the assumption of the rigidity of plate interiors, a velocity model of plates and sites based on very long baseline interferometry; Observed strike of the faults; Testing of the hypothesis that stable sliding occurs along the San Andreas fault and other northwest-strikign strike-slip faults.

Abstract A major problem with the current use of porphyroblast–matrix microstructural relationships to infer orogenic histories, such as multiple orthogonal orogenic events, is that other evidence for these events is typically lacking. For example, a comparison of regional relationships and local structures formed in and adjacent to porphyroblasts present in contact aureoles in the Foothills Terrane, Sierra Nevada, California, shows that: (1) except in shear zones, contact aureoles and local zones along lithological contacts, the Foothills Terrane has a single regional cleavage, although locally formed by multiple processes; (2) the regional cleavage and locally developed porphyroblast inclusion trails have variable orientations, and neither dataset supports the formation of dominantly subhorizontal and subvertical cleavages in this orogen; (3) structural and metamorphic heterogeneities occur at all scales and can markedly affect inclusion trail patterns in porphyroblasts; (4) complex porphyroblast growth features and internal inclusion trail patterns can form in porphyroblasts that grow during short time intervals in contact aureoles, indicating that local complexity in porphyroblasts does not imply regional complexity. Because of these conclusions, multiple datasets, rather than data acquired only from porphyroblasts, should be considered when attempting to understand the evolution of orogens. Furthermore, using microstructural information preserved only in porphyroblasts to infer orogenic processes and plate motions is generally unjustified. [ABSTRACT FROM AUTHOR]

Investigates the paleogeographic and tectonic implications of the geology of Tiefort Mountains in California. Use of geologic mapping, petrologic analysis and U-Pb dating; Data on the Paleozoic paleogeography and Mexozoic intra-arc tectonics; Account on the contractional deformation from central Mojave Desert to East Sierran.

Discusses the stratigraphic and sedimentologic analysis of the lower Diligencia Formation and the postdepositional tectonic structures in the basin. Asymmetric distributions of facies associations; Geologic setting; Thickness of the Diligencia Formation; Depositional systems; Alluvial fan deposits; Alternative models for Miocene basin evolution.

Studies the crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California. Maximum depth of the Los Angeles basin; Formation of north dipping buttress by the Duarte branch of the Sierra Madre fault zone; Dip and depth extension of the San Andreas fault line.

Analyzes the stream profiles in the Mendocino triple junction region of northern California. Use of shear stress bedrock-incision model; Description of the tectonic history, lithology, climate and channel morphology of the Mendocino triple junction area; Components of bedrock lithology.

Examines the cretaceous dikes within the Jurassic Independence dike swarm in eastern California. Use of uranium-lead geochronology readings; Confirmation that some mafic dikes in the swarm are coeval with previously dated Late Jurassic felsic dikes; Comparison with cretaceous dikes in the eastern Sierra Nevada batholith.

Examines the changes in the tectonic regime from subduction to strike-slip in the Mendocino triple junction. Structure of the San Andreas Fault system; Changes in the asthenospheric mantle; Penetration of strike-slip faults through the entire crust to the upper mantle.

Studies how time-dependent postseismic stress transfer between the three subparallel strike-slip faults in California depends on the presence and geometry of lower crustal shear zones. Mechanics of time-dependent stress transfer; Comparison of models of Maxwell viscoelastic relaxation of the mantle, lower crustal detachment surface and vertical shear zones.

Studies the regional abundance of fluid expulsion in Monterey Bay, California. Bay tectonics and seeps; Sources of fluid flow; Potential fluid conduits; Site by site description of fluid expulsion features in the bay; Interpretation of the bay's fluid venting and subsurface hydrogeology.

Presents a mechanical model of the evolution of the geological structure of the Ventura basin in southern California. Kinematic constraint of the basin; Principles and parameters used in the model; Findings.

Presents research which demonstrated that a significant component of seismic moment release and shortening in the Los Angeles, California, region is due to extrusion along known strike-slip and oblique-slip faults. Prevalence of thrust faults in the area; Use of Global Positioning System geodetic measurements.

Investigates the mechanism of the distinct deformation styles in fault-bounded crustal blocks in the San Francisco Bay region in California with the use of a two-dimensional map-view model. Description of the model used; Rheology of crustal blocks; Description of modeling procedure.

Presents strontium and neodymium isotope data for a group of mafic and ultramafic xenoliths from the Cima volcanic field in southeastern California. Similarity of the magma source to a mid-ocean ridge basalt (MORB); Late Cretaceous intrusion age; Invasion of the lower crust by asthenospheric magmas; Crustal heating by ascent of asthenosphere.

Observes the formation of southern California and northern Baja California within two distinct tectonic settings rather than the previously proposed single phase of continental rifting. New rare earth element indicative of components of a different magmatic system; Data refuting presence of crystalline basement beneath the Salton trough.

Explains the tectonic rotation of the western Transverse Ranges block in California. Subduction of the Monterey microplate; Explanation of the timing of initial rotation and basin formation; Pacific-North America strike-slip motion; Evolution of a transform system.

States that contraction and extension were significant at upper- and mid-crustal depths in the Chuckwalla Mountains region during the Late Jurassic in California. Factors for the Jurassic extensional tectonics; Speculation on the impact of the combined influence of oblique convergence and the opening of the Gulf of Mexico.

Characterizes the Newberry Mountains detachment fault of the early Miocene Mojave extensional belt. Determinations of the original dip of the fault taken from fission-track thermochronology and geologic observations; Dip modification caused by footwall unloading.

Presents a study which investigated the active tectonic geomorphology of Wheeler Ridge, southern San Joaquin Valley, California. Information on the hypothesis which indicated that climatic perturbations are responsible for producing geomorphic surfaces such as alluvial fan segments and river terraces; Methodology used to conduct study; Results of study; Conclusion reached.

Examines the changes in the tectonothermal history of the four phases of the Sierra Sand Pedro Martir pluton and Baja California. Speculation into the cause and occurrence of tilting; Suggestions from data collected on the phases of the pluton; Results received from samples of the pluton.

Describes the Laurel-Convict fault, a prominent, northwest-striking, high-angle fault exposed in Paleozoic rocks of the Mount Morrison pendant in the eastern Sierra Nevada of California. Geology of the Mount Morrison pendant; Average width of the fault zone; Laurel-Convict fault as an exposed segment of Intrabatholithic Break 3.

Analyzes regional tectonic rotation in the Mojave Desert of California. Paleomagnetic data suggesting oldest to youngest rotation; Occurrence of two episodes of clockwise vertical axis rotation; Rotation after tilting as evidenced by the original north-south direction of the Miocene extension; Correlation of rocks and structures in the southwest Cady Mountains.

Discusses neodymium (Nd) and strontium (Sr) isotopic geochemistry and tectonics of ridge subduction and middle Cenozoic volcanism in western California. Geologic background of the study area; Major and trace element geochemistry; Petrogenesis of the Coast Range and Santa Maria volcanic rocks; Tectonic models for volcanism.

Studies variations in deformation fields during the development of a large-volume magmatic arc in central Sierra Nevada (CSN), California during the Late Cretaceous Period. Structures in the granitic rocks; Cooling rate of plutons in the CSN; Solid-state shear zones; Extension during pluton emplacement; Models for batholith emplacement.

Proposes a model for progressive arcward contraction of the fore-arc basin in the Rumsey Hills region, southwestern Sacramento Valley, beginning in Cretaceous and continuing episodically through Cenozoic time. Seismic reflection data used to develop the forward model; Major tectonic and depositional events; Stratigraphic and structural relationships.

Studies the structural geology of the Rattlesnake Creek terrain in Klamath Mountains, California. Regional geologic setting; Geologic framework; Basement assemblage; Geochemistry of igneous rocks; Deformation and metamorphism.

Studies the tectonic and depositional history of the California continental margin as it change from a convergent tectonic regime to a translational regime during the evolution of the San Andreas transform fault systems. New evidence from the Gualala and Point Arenas basins.

Attempts to develop finite element codes that would permit computational experiments with different postulated fault rheologies. Technique of modeling fault networks with thin-plate finite elements; Application of techniques to California region; Grid construction; Rheologic model; Estimation of seismic hazard.