Search

Kinematic assumptions of geodetic inversions for fault slip require that the slip sums to the (plate) boundary velocity. This assumption neglects permanent off-fault deformation, which could account for discrepancies between geologic and geodetic estimates. We use three-dimensional mechanical models to assess if unaccounted permanent strain surrounding faults could contribute to slip rate discrepancies across disconnected faults within the Mojave Desert (California, USA) portion of the eastern California shear zone (ECSZ). We modified fault configurations derived from the Southern California Earthquake Center Community Fault Model to better represent the disconnected nature of active faults in the ECSZ south of the Garlock fault. The models with revised fault geometry produce slip rates that better match geologic strike-slip rates, thus validating the revisions. Within these models, off-fault deformation accounts for 40% ± 23% of the total strain across the ECSZ. This suggests that a significant portion of the discrepancy between the geologic and geodetically modeled slip rates in the ECSZ could be due to the geodetic inversion model assumption of zero permanent off-fault deformation. [ABSTRACT FROM AUTHOR]

Late Cenozoic tectonic reconstructions of southern California (United States) have been hindered for decades by disagreement over the total displacement on the southern San Andreas fault; disparate estimates of dextral offset range from 160 km to 240 km. Prior estimates were based on purely translational models that neglect the effects of transrotational strain on the orientation and distribution of key geologic markers. We present a new reconstruction that integrates published lithologic, structural, and paleocurrent data with the effects of clockwise rotation in the eastern Transverse Ranges (ETR). This model yields 200 ± 14 km of cumulative offset on the southern San Andreas fault system in the Salton Trough. The disparity between our estimate and a lower estimate of 160 km for the Mojave segment can be explained by a combination of transtensional elongation along the southwest boundary of the ETR, transfer of strain to the eastern California shear zone, and diffuse crustal shortening in the Big Bend region. This reconstruction reconciles previously incompatible and long-debated models, and places an important new constraint on the tectonic evolution of the Pacific-North America plate boundary in southern California. [ABSTRACT FROM AUTHOR]

We determine the paleoelevation of the northern Sierra Nevada (California) in the Oligocene based on hydrogen stable isotope compositions of meteoric water preserved within volcanic glass from ignimbrites sampled across the range. A 48‰ decrease in the isotopic composition of hydrated glass from ignimbrites located near paleosea level to ignimbrites 100 km to the east reflects the effect of ancient high topography on precipitation. These data show that 31-28 Ma ago, the northern Sierra Nevada had a steep western gradient and elevations similar to the present. This study, placed in the context of other paleoaltimetry studies, suggests that the range was a high topographic feature throughout the Cenozoic and that the majority of uplift occurred in the Late Cretaceous to early Cenozoic, much earlier than some studies have proposed. [ABSTRACT FROM AUTHOR]

The northern Sierra Nevada and adjacent Basin and Range, western United States, are marked by a widespread regional unconformity at the base of the Eocene-Miocene volcanic and sedimentary section that overlies the Mesozoic batholith and its wall rocks. To help address controversial questions about the origin, uplift, and erosion of the batholith, we compiled a subcrop geologic map of the unconformity prior to Tertiary extension. This simple but underutilized technique reveals the distribution of rock units that could have contributed detritus to the Eocene-Miocene river channels crossing the Sierra Nevada and demonstrates that the Mesozoic Sierra Nevada batholith was continuous to the northeast across the northwestern Basin and Range. More speculatively, the subcrop map implies that Late Cretaceous-Eocene erosional stripping may have been greatest above the axis of the batholith and decreased to the east; thus, drainage in this area may have been eastward and then switched westward in Eocene-Miocene time. [ABSTRACT FROM AUTHOR]

K2O contents have long been recognized as a potential indicator of tectonic processes, and in the Sierra Nevada, California, high-K2O volcanism has been attributed to lithosphere root delamination. However, new data from the central Sierra suggest a very different control: K2O concentrations can be explained by variations in the degree of partial melting in the mantle, where high-K2O volcanics are derived from low-degree partial melts of mantle lithosphere. Field evidence in the central Sierra further suggests that the pulse of high-K2O volcanism there was synchronous with the development of a pull-apart structure along a series of right-stepping dextral transtensional faults at the onset of Walker Lane transtensional faulting. In our alternative interpretation, high-K, low-degree partial melts were tapped by the inception of transtensional stresses, recording the birth of a plate boundary. We speculate that high-K2O lavas in the southern Sierra are similarly related to the onset of transtensional stresses, not delamination. A regional southward increase in incompatible element contents and decrease in erupted volumes are also consistent with a model for crustal thickness controls on magmatism. Depth-integrated density models show that dry mafic magmas beneath thick crust have insufficient buoyancy to erupt, but low-degree partial melts carry sufficient volatiles to allow eruption; as with K2O, degree of partial melting, not source-region heterogeneity, controls water contents and buoyancy. [ABSTRACT FROM AUTHOR]

Paleoseismological data suggest the occurrence of four bursts of seismic moment release in the Los Angeles region during the past 12,000 yr. The historic period appears to be part of an ongoing lull that has persisted for about the past 1000 yr. These periods of rapid seismic displacement in the Los Angeles region have occurred during the lulls between similar bursts of activity observed on the eastern California shear zone in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the greater San Andreas system suppress activity on faults in the eastern California shear zone, and vice versa, can explain the apparent switching of activity between the two fault networks. Combined with the observation that short-term geodetic and longer-term geologic rates co-vary on major southern California fault systems, this suggests that either (1) a temporal cluster of seismic displacements on upper-crustal faults increases ductile deformation on their downward extensions, or (2) rapid ductile slip in the lower crust beneath faults loads the upper crust, driving a seismic cluster. We suggest that alternating periods of rapid seismic displacement may be the expected mode of seismicity when two fault systems accommodate the same plate-boundary motion, and slip on one system suppresses slip on the other. [ABSTRACT FROM AUTHOR]

Evaluation of microstructures in unlithified near-surface sediments provides promising results for differentiating between earthquake rupture-related (coseismic) and creep-related (aseismic) structures formed by the San Andreas fault. Paleoseismic trenches excavated at two sites, Flook Ranch on the creeping section, and Alder Creek on the 1906 rupture trace, show contrasting fault-zone structures and microstructures in near-surface, late Holocene sand. At Alder Creek, the 1–2-m-wide fault zone consists of both faults and 2-10-mm-thick deformation bands. Deformation bands have preferred grain orientations ∼30° counterclockwise from the fault (viewed in the slip-parallel direction), broken and disaggregated grains, smaller than average grain size, and lower porosity than control samples. In contrast, at Flook Ranch, two 4–6-m-wide fault zones consist of multiple faults but lack deformation bands. Silty sand in the fault zone at Flook Ranch has preferred grain orientations ∼10° clockwise from the fault, lacks broken grains, and has comparable grain size but lower porosity than control samples. These microstructures record different deformation mechanisms in near-surface sediment: cataclasis at Alder Creek, and distributed deformation at Flook Ranch. Deformation bands on the 1906 rupture trace of the San Andreas fault at Alder Creek demonstrate that these structures, with their grain rotation, grain breakage, and localized porosity variations, can form coseismically in unlithified sediment. The grain bridge model accounts for fundamental microstructural characteristics of Alder Creek deformation bands, and it provides a connection between these microstructures and laboratory studies of stickslip instability. Deformation bands are easily recognizable in field and trench exposures and may be a useful indicator of coseismic slip. [ABSTRACT FROM AUTHOR]

A high-resolution Compressed High-Intensity Radar Pulse (CHTRP) survey reveals shore-parallel variations in the Holocene sediment thickness offshore La Jolla, California. Sediment thicknesses decrease from >20 m in the south near Scripps Canyon to zero in the north approaching Torrey Pines. In addition to the south-to-north variation in sediment thickness, the transgressive surface observed in seismic lines shoals from Scripps Canyon to the north. Despite these dramatic shore-parallel subsurface changes, the nearshore bathymetry exhibits little to no change along strike. A left jog (i.e., a constraining bend) along the Rose Canyon fault causes local uplift in the region and appears to explain the northward shoaling of the transgressive surface, the decrease in relief on the transgressive surface away from the left jog, and the Holocene sediment thickness variation. This tectonic deformation is shore parallel, and thus the accommodation can be separated into its tectonic and eustatic components. [ABSTRACT FROM AUTHOR]

Late Miocene to early Pliocene deposits at Split Mountain Gorge, California, preserve a record of basinal response to changes in regional tectonics, paleogeography, and evolution of the Colorado River. The base of the Elephant Trees Formation, magnetostratigraphically dated as 8.1 ± 0.4 Ma, provides the earliest well-dated record of extension in the southwestern Salton Trough. The oldest marine sediments are ca. 6.3 Ma. The nearly synchronous timing of marine incursion in the Salton Trough and northern Gulf of California region supports a model for localization of Pacific-North America plate motion in the Gulf ca. 6 Ma. The first appearance of Colorado River sand at the Miocene-Pliocene boundary (5.33 Ma) suggests rapid propagation of the river to the Salton Trough, and supports a lake-spillover hypothesis for initiation of the lower Colorado River. [ABSTRACT FROM AUTHOR]

Using a combination of GPS-measured horizontal velocities of 200 sites and 115,487 range-change rates determined with the permanent scatterer interferometric synthetic aperture radar (InSAR) method in the San Francisco Bay Area, we resolve vertical motions in the region at sub-mm/yr precision. The highest displacement rates are due to nontectonic processes, such as active landslides, subsidence and rebound over aquifers, and rapid settling of unconsolidated sediments along the bay margins. Residual displacement rates are determined by removing the contribution of the GPS-derived horizontal velocity field from the InSAR range-change rates. To isolate vertical tectonic rates, we use only those InSAR measurements made on material that was not Quaternary substrate, which is susceptible to nontectonic and seasonally varying ground motions. The InSAR residuals indicate significant uplift over the southern foothills of the active Mount Diablo anticlinorium, the Mission Hills stepover region of the Hayward and Calaveras faults, and the central and southern Santa Cruz Mountains located along a restraining bend of the San Andreas fault. [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]

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]

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]

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.

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.

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.

The author offers a response to a commentary made on their study on slip rate discrepancies in the eastern California shear zone. Topics discussed include factor affecting the degree to which wholesale clockwise rotation of the region contributes to the geologic-geodetic discrepancy, advantages of their geodetic analysis using block modeling, and factors that may influence estimated slip rates and limit the ability of the geodetic data to resolve left-lateral slip on the Garlock fault.

The article discusses a study by E. L. Evans and colleagues on the geodetic and geologic estimate of slip rate discrepancy on the Garlock fault (GF) in the eastern California shear zone (ECSZ). Topics covered include comparison of the geodetic and geologic estimate of the slip rate on the GF, dextral shear in the ECSZ, and reason that paleomagnetic evidence for rotation is lacking for the GF.

The article discusses the topographic evolution and the uplift of Sierra Nevada in California. According to the interpretation of the geologists, the evolution of topography of the Sierra Nevada and Great Basin represented the relationship between topography and tectonics. It highlights a study that examines the relationship between hydration water in volcanic glass and the uplift of Sierra Madre.

The article focuses on the different geological patterns of earthquakes. Based on the paleoseismological studies at Pallet Creek on the San Andreas fault in California, researchers pointed out the Earth's crust has many unknowns at depths which cannot be observed directly. The establishment of the Southern California Earthquake Center (SCEC) was taken into account. Key information about the eastern California shear zone (ECSZ) in Mojave Desert is further presented including its fault displacement and plate motion.

The article focuses on the study headed by Sue Cashman concerning the rock deformation of unconsolidated and nolithified deposits. This assesses the factors that inhibit the deformation of the deposits, how these materials in the near-surface regime deforms and how the impact of this deformation may be felt in a variety of settings. The study primarily examines the exhumed fault-related rocks along the San Andreas fault and other faults for over 70 years, highlighting on the exhumed fault rock in bedrock. Moreover, it measures the processes at seismogenic depths where seismic energy is radiated. It also addresses two emerging issues such as the deformation in faults at the surface and the deformation process in poorly lithified deposits prior to burial and diagenesis.