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Abstract: The plate boundary between the Scotia and Antarctic plates runs along the broadly E–W trending South Scotia Ridge. It is a mainly transcurrent margin that juxtaposes thinned continental and transitional crust elements with restricted oceanic basins and deep troughs. Seismic profiles and regional-scale seismological constraints are used to define the peculiarities of the crustal structures in and around the southern Scotia Sea, and focal solutions from recent earthquakes help to understand the present-day geodynamic setting. The northern edge of the western South Scotia Ridge is marked by a sub-vertical, left-lateral master fault. Locally, a narrow wedge of accreted sediments is present at the base of the slope. This segment represents the boundary between the Scotia plate and the independent South Shetland continental block. Along the northern margin of the South Orkney microcontinent, the largest fragment of the South Scotia Ridge, an accretionary prism is present at the base of the slope, which was possibly created by the eastward drift of the South Orkney microcontinent and the consequent subduction of the transitional crust present to the north. East of the South Orkney microcontinent, the physiography and structure of the plate boundary are less constrained. Here the tectonic regime exhibits mainly strike–slip behavior with some grade of extensional component, and the plate boundary is segmented by a series of NNW–SSE trending release zones which favored the fragmentation and dispersion of the crustal blocks. Seismic data have also identified, along the north-western edge of the South Scotia Ridge, an elevated region – the Ona Platform – which can be considered, along with the Terror Rise, as the conjugate margin of the Tierra del Fuego, before the Drake Passage opening. We propose here an evolutionary sketch for the plate boundary (from the Late Oligocene to the present) encompassing the segment from the Elephant Island platform to the Herdman Bank. [Copyright &y& Elsevier]

The article discusses the late Miocene to early Pleistocene sedimentary rocks in the Fish Creek-Vallecito basin in southern California that records late Cenozoic subsidence and basin filling due to deformation in the San Andreas fault system. Data reportedly show that the section ranges in age from circa 8.0-0.4 million years (Ma). The article states that the onset of sedimentation at circa 8.0-0.4 million years (Ma) preserves the earliest extension or transtension in the area.

Industry seismic reflection data, oil test well data, interpretation of gravity and magnetic data, and seismic refraction deepcrustal profiles provide new perspectives on the subsurface geology of San Fernando Valley , home of two of the most recent damaging earthquakes in southern California. Seismic reflection data provide depths to Miocene-Quaternary horizons; beneath the base of the Late Miocene Modelo Formation are largely nonreflective rocks of the Middle Miocene Topanga and older formations. Gravity and seismic reflection data reveal the North Leadwell fault zone, a set of down-tothe- north faults that does not offset the top of the Modelo Formation; the zone strikes northwest across the valley, and may be part of the Oak Ridge fault system to the west. In the southeast part of the valley, the fault zone bounds a concealed basement high that infl uenced deposition of the Late Miocene Tarzana fan and may have localized damage from the 1994 Northridge earthquake. Gravity and seismic refraction data indicate that the basin underlying San Fernando Valley is asymmetric, the north part of the basin (Sylmar subbasin) reaching depths of 5-8 km. Magnetic data suggest a major boundary at or near the Verdugo fault, which likely started as a Miocene transtensional fault, and show a change in the dip sense of the fault along strike. The northwest projection of the Verdugo fault separates the Sylmar subbasin from the main San Fernando Valley and coincides with the abrupt change in structural style from the Santa Susana fault to the Sierra Madre fault. The Simi Hills bound the basin on the west and, as defined by gravity data, the boundary is linear and strikes ∼N45 E. That northeasttrending gravity gradient follows both the part of the 1971 San Fernando aftershock distribution called the Chatsworth trend and the aftershock trends of the 1994 Northridge earthquake. These data suggest that the 1971 San Fernando and 1994 Northridge earthquakes reactivated portions of Miocene normal faults. [ABSTRACT FROM AUTHOR]

Extension in Death Valley is usually interpreted as a combination of low-angle Basin and Range-style extension and strike slip associated with the developing Pacific-North America plate boundary in western North America, with these two tectonic regimes operating synchronously in Death Valley. Examination of structural, stratigraphic, and timing relationships in the region suggests that this interpretation needs revision. Evolution of Death Valley is best described as a two-stage process. In the first stage, lasting from ca. 18 to 5 Ma, low-angle Basin and Range extension transported allochthons consisting of Late Proterozoic through Early Paleozoic miogeoclinal section along detachment surfaces that, as extension continued, were exhumed from mid-lower crustal levels to the surface. Near the end of this extensional phase and lasting until ca. 3 Ma, deposition of a thick sequence of volcanics, clastics, and some lacustrine carbonates signaled a period of relative tectonic quiescence, with sediments in some areas covering the exhumed detachment surfaces. At ca. 3 Ma, initiation of the East California Shear Zone started development of the present-day topographic depression of Death Valley, formed as a pull-apart basin associated with this strike slip. Faulting broke the older, inactive, Basin and Range detachment surfaces, with high-angle transtensional faulting along the Black Mountains front. The Black Mountains were elevated as a result of footwall uplift, with the well-known turtleback structures being megamullions along these bounding faults. These megamullions are similar to those seen at oceanic spreading centers. The Panamint Range has previously been interpreted as an extensional allochthon, with the entire range transported from on top of or east of the Black Mountains. A new interpretation presented here is that the range is a large core complex similar to the core complex at Tucki Mountain, at the northern end of the range. The Basin and Range extensional detachment tracks over the top of the range, with extensional allochthons perched on the eastern flanks of the range. This modified model for evolution of Death Valley suggests a strong link between timing and style of deformation in the basin with the developing Pacific-North America plate boundary, particularly eastward propagation of this boundary. [ABSTRACT FROM AUTHOR]

Two phases of deformation are needed to describe the Cenozoic tectonic evolution of the Pahrump and Mesquite basins in the southern Great Basin and eastern Mojave Desert, United States. By interpreting seismic refl ection and gravity observations along with bedrock and surfi cial mapping, we infer an extensional phase of basin formation followed by a transtensional phase, in this area straddling the border of southern Nevada and southeastern California.. We reprocessed 220 line km of industry seismic refl ection data from the Pahrump and Mesquite Valleys to emphasize refl ections in the basin fi ll, and combined these results with analysis of gravity data. The seismic lines portray the complex geometry of the Stateline fault system, a major Neogene dextral strike-slip system that passes through these valleys, and provide evidence for multiple ages of faulting along structures that bound the Pahrump basin. Locally thick sequences of preextensional Tertiary sedimentary rocks are cut by large-offset, relatively high- angle normal faults that record a phase of extensional basin formation that preceded transtension. The existence of preextensional basins beneath the Pahrump and Mesquite Valleys bears on tectonic reconstruction of the region and suggests that tilted ranges blocks to the west of these valleys need not restore to positions immediately adjacent to the Spring Mountains to the east. Subsequent dextral offset on the Stateline fault system resulted in the formation of steep-sided basins, local arching and tectonic inversion, and the burial of earlier-formed normal faults with coarse clastic detritus. Gravity models that are constrained to match the basin architecture observed in the seismic lines require lateral variations in basin-fi ll and bedrock density, and they confi rm that the Paleozoic outcrop of Black Butte, a topographic high separating the Pahrump and Mesquite Valleys, is unrooted to underlying bedrock. [ABSTRACT FROM AUTHOR]

Abstract: Stream capture and piracy in tectonically active regions have been described in geomorphic systems worldwide; however, few studies show the influence stream capture has on the rock record. We present an analysis of fluvial fan stratigraphy that developed as a result of multiple stream capture events, building a complex stratigraphic succession beneath the Lawrence Livermore National Laboratory (LLNL), California. The LLNL site is located in the southeast portion of the tectonically active Livermore Basin, a transpressional basin in the California Coast Ranges. Geomorphic evidence for this stream capture include: (1) the Arroyo Seco enters the basin from the south through an uplifted fault block, (2) south of this fault block lies an abandoned Arroyo Seco fluvial fan, (3) north of the fault block, in the Livermore Basin, Arroyo Seco built a 7-km2 fluvial fan, apparently forcing the Arroyo Las Positas, a smaller stream that enters the basin from the east, northward around the Arroyo Seco fan, and (4) a knickpoint exists near the point of capture on Arroyo Seco. Stratigraphic evidence reflecting this shift in the Arroyo Seco position into the Livermore Basin was evaluated through a provenance study of 215 gravel units from 34 boreholes spaced evenly over the 2.6km2 LLNL site. The Arroyo Seco derives its sediment from both the Jurassic–Cretaceous Franciscan Assemblage and the Altamont Hills (which are comprised of Mesozoic Great Valley Group and Tertiary continental sediments). The Arroyo Las Positas drains only the Altamont Hills and thus lacks the Franciscan Assemblage-derived clasts. The origin of the individual gravel units was determined by the percentage of Franciscan Assemblage indicator pebbles (red chert, green chert and blueschist) in the samples. Through this analysis, we determined that high-percentage Franciscan Assemblage-derived clasts were present below a depth of approximately 35m below the surface, low-percentage Franciscan Assemblage-derived clasts were present at depths between 35m and 18m, and high-percentage Franciscan Assemblage-derived clasts were present from depths of approximately 18m to the surface of the fluvial fan. These results indicate that the Arroyo Seco flowed north and deposited sediments at the LLNL site, then was later absent from the basin at which time it formed a fan south of the fault block. During this absence of the Arroyo Seco, the Arroyo Las Positas, a westerly flowing stream, dominated the sediment supply at the LLNL site. The Arroyo Seco was then captured by a gully headward eroding through the uplifted fault block, redirecting the Arroyo Seco into the basin once again. This history of multiple stream captures created three stratigraphic units with alternating overall channel and paleoflow orientations. [Copyright &y& Elsevier]

We present the GIS extension CalHypso to automatically extract multiple hypsometric curves from digital elevation models (DEM). This extension is programmed in Visual Basic 6 and uses the ArcObjects architecture of ArcGIS. It employs a new and easy methodology to extract the hypsometric curve by using the integer data-model properties of ArcGIS and summations algorithms. This provides an optimal integration within the program environment, allowing the representation and comparison of curves directly within the GIS main program. The CalHypso extension can also calculate the main statistics related to the hypsometric curve by applying polynomial fits. We have tested this tool in several basins of the eastern border of the Sierra-Nevada dome in the Betic Cordillera (SE Spain), showing important differences in the north- and south-slope hypsometric curves. These variations are probably related to the tectonically controlled drainage evolution of the southern border of Sierra Nevada. An eastward decrease in tectonic-driven erosion is also suggested by the hypsometric curves analyzed. [Copyright &y& Elsevier]

Structural and stratigraphic analyses along the western margin of the Salton Trough show that the San Andreas fault system was reorganized in early Pleistocene time from a system dominated by two fault zones (the San Andreas fault and the West Salton detachment fault) to a network of dextral faults that include the San Andreas and at least four dextral faults to the southwest. The San Felipe fault zone, one of these dextral faults, has ~5.8 ± 2.8 km of right separation and consists of three principal faults in the Peninsular Ranges. These are the San Felipe fault in the WNW, Sunset fault in the middle, and Fish Creek Mountains fault in the ESE. They form a left-stepping array and bound domains in which the Sunset Conglomerate, the older West Salton detachment fault, and Cretaceous mylonitic rocks below the detachment are folded about WNW-trending folds. A complex flower structure within the left-stepovers probably produced this fault-parallel folding. Because all the rocks within stepovers of the San Felipe fault zone, from Cretaceous to Pleistocene, are deformed about WNW-trending folds and record broadly similar shortening strains, we infer a Quaternary age of deformation. Parts of the San Felipe fault zone cut latest Pleistocene to Holocene surficial deposits, and the fault zone is likely active. Evidence for early Pleistocene initiation of the San Felipe fault zone is preserved in conglomerate NE of the Sunset fault. Poorly sorted angular boulder conglomerate and pebbly sandstone of the Sunset Conglomerate are ~600 m thick and lie in angular unconformity on the Pliocene Palm Spring Group. The conglomerate coarsens upward and toward the fault, and is dominated by plutonic clasts derived from SW of it. Conglomerate beds contain up to 10% sandstone clasts recycled from older basin fill and accumulated in proximal to medial alluvial fans that were shed to the NE from uplifted rocks along the then-active Sunset fault. Based on lithologic, stratigraphic, structural, and compositional similarities, we correlate the Sunset Conglomerate to the Pleistocene Ocotillo Formation. Clasts of recycled sandstone record erosion of detachment-related basin fill that predates the San Felipe fault and once covered the Vallecito and Fish Creek mountains. These crystalline-cored mountain ranges first emerged from beneath basin fill during early slip above the nascent San Felipe fault ca. 1.1-1.3 Ma. Later, the San Felipe fault zone cut upward, folded, cut across, and deactivated the West Salton detachment fault within a ~9-km-wide contractional bend and pair of left-steps. Areas that accumulated sediment within this step-over zone between ca. 1.1 and ca. 0.6 Ma are currently being inverted and folded. Initiation of the San Felipe fault in early Pleistocene time was a significant event in the reorganization of the southern San Andreas fault system. The Quaternary dextral faults broadened the plate boundary zone southwestward from roughly 25 km (during coeval slip on the San Andreas fault and West Salton detachment fault) to 50-70 km, and mark a change in the dominant structural style from transtension to distributed dextral faulting south of the Big Bend. [ABSTRACT FROM AUTHOR]

Deterministic earthquake scenario simulations are playing an increasingly important role in seismic hazard and risk estimation. Our aim is to calculate a substantial number of different finite-source scenarios, embedded in a 3-D structure for a particular fault or fault system, by pre-calculating numerical Green's functions (NGFs). A large seismic fault is divided into subfaults of appropriate size for which synthetic Green's functions at the surface are calculated and stored. Consequently, ground motions from arbitrary kinematic sources can be simulated for the whole fault or parts of it, by superposition. To illustrate this approach and its functionalities we simulate M 7 (up to 0.5 Hz) scenario earthquakes for a simplified model of the Newport-Inglewood (NI) fault in the Los Angeles (LA) Basin. We quantify the variations of surface ground motion (e.g. peak ground velocity PGV and synthetic seismograms) due to source parameters (e.g. hypocentre location and corresponding slip history). The results show a complex behaviour, with dependence of absolute PGV and its variation on asperity location, source directionality and local structure and demonstrate the necessity to combine 3-D structural and finite-source effects to quantify ground motion characteristics and their variations. [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]

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.

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.

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.

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.