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The distribution and character of Venus's impact craters led to the widely accepted idea that Venus underwent global catastrophic resurfacing ca. 500 Ma, and thus Venus records only a short history, encompassing surface evolution since postulated catastrophic resurfacing. Ribbon tessera terrain (RTT), a structurally distinctive unit, represents some of Venus's oldest surfaces, and is widely accepted as forming prior to postulated global catastrophic resurfacing. We constructed a global geologic map of RTT unit exposures and structural trends using National Aeronautics and Space Administration (NASA) Magellan data. Map relations illustrate that RTT displays planet-scale patterns that, together with altimetry, record a rich geologic history that predates proposed global catastrophic resurfacing. [ABSTRACT FROM AUTHOR]

Ungulate and carnivore diversity patterns since 30 Ma in different regions of the western United States suggest abrupt increased diversification at 17-17.5 Ma, followed by decreases ca. 11 Ma, and stasis thereafter. Although global climate change presumably affects evolution, we hypothesize that widespread extensional tectonism in the western U.S. also helped drive diversity increases ca. 17.5 Ma through a topographically induced increase in floral and habitat diversity. The decreases in diversities ca. 11 Ma, as well as the rapid increase in C4 ecosystems (RICE) worldwide at 7-8 Ma may have responded to climate teleconnections and increased seasonality linked to global cooling and growth of orogenic plateaus, particularly the Tibetan Plateau between 13 and 8 Ma. Thus, biodiversity complexly responds both to climate and to tectonics. [ABSTRACT FROM AUTHOR]

In Phanerozoic circum-Pacific orogenic belts, most "postcollisional" S-type granites and associated high-temperature, low-pressure metamorphic complexes formed during early arc extension. The granites are part of a tripartite association consisting of (1) inboard S-type granite, (2) outboard oceanic arc, and (3) intervening, turbidite-filled backarc basin. S-type granites herald the formation of new outboard oceanic arc and extensional backarc systems, but thickening of a preexisting, sediment-dominated backarc basin is a prerequisite for their generation. In these environments, S-type plutonism is triggered by renewal of arc magmatism following thickening, when hot basaltic magmas are intruded into the thickened backarc crust once slab retreat is reestablished. With ongoing extension during retreat, the crust becomes progressively thinned, the sedimentary contribution is diminished, and the granites lose their S-type character. Such tripartite associations involving S-type granite are probably diagnostic of repeated slab-retreat episodes, and the Jurassic U.S. cordillera might be an example. [ABSTRACT FROM AUTHOR]

Recent seismic imaging across the Rio Grande rift, western United States, revealed unexpected structures in the underlying mantle. Low seismic wave velocity anomalies below the Rio Grande rift have been interpreted as being partially of melt origin, and high-velocity structures below the western Great Plains have been proposed to be the result of small-scale convection, i.e., cold downwelling lithospheric material with probably a compositional contribution. We perform a dynamic test of these interpretations using a passive rift model for isochemical convection. The models self-consistently produce a rift localized at approximately the right distance from the border to the nearby thicker Great Plains lithosphere. With realistic upper mantle rheologies, small-scale convection forms, aided by the lithospheric step. The resulting thermal anomalies produce seismic low-velocity anomalies below the rift of amplitudes similar to those imaged seismically, requiring the presence of only small amounts of melt. The lateral extent of the observed low velocities below the Rio Grande rift is as in the models, where it is controlled by the spacing between downwelling limbs of the small-scale convection. The fast velocity structure below the western Great Plains can be produced by cold downwelling lithosphere. The thermal rifting models can predict the amplitudes and size of the main seismic anomalies; compositional heterogeneity may contribute to some of the smaller features observed. [ABSTRACT FROM AUTHOR]

We use three large normal fault arrays in the northeastern Basin and Range Province, western United States, to document catchment development and relief production during fault growth. Fault slip and slip rates increase systematically along strike from zero at the fault tips. Catchment relief and across-strike range width both increase as slip accumulates but reach maximum values at a distance of ∼15 km from the fault tips and remain uniform along strike over much of the footwalls. Catchment outlet spacing also increases away from the fault tips but does not reach a uniform value and may vary by a factor of 5–6 along strike. We infer that catchments first elongate in the across-strike direction as slip accumulates and the range half-width increases. Once the half-width reaches its maximum value, continued catchment growth is possible only by along-strike capture, which increases outlet spacing but not relief. The close correspondence between catchment relief and range half-width suggests that geomorphically limited hillslope and channel gradients are achieved within the 15 km tip zone. Thus, the limiting factor in footwall development is the width of the range, which is controlled by two external agents: the geometry and spacing of the major faults, and the elevations of base level on both flanks. [ABSTRACT FROM AUTHOR]

Summer monsoonal rains (the southwest monsoon) are an important source of moisture for parts of the southwestern United States and northern Mexico. Improved documentation of the variability in the southwest monsoon is needed because changes in the amount and seasonal distribution of precipitation in this semiarid region of North America influence overall water supply and fire severity. Comparison of abundance variations in the planktic foraminifer Globigerinoides sacculifer in marine cores from the western and northern Gulf of Mexico with terrestrial proxy records of precipitation (tree-ring width and packrat-midden occurrences) from the southwestern United States indicate that G. sacculifer abundance is a proxy for the southwest monsoon on millennial and submillennial time scales. The marine record confirms the presence of a severe multicentury drought centered ca. 1600 calendar (cal.) yr B.P. as well as several multidecadal droughts that have been identified in a long tree-ring record spanning the past 2000 cal. yr from west-central New Mexico. The marine record further suggests that monsoon circulation, and thus summer rainfall, was enhanced in the middle Holocene (ca. 6500–4500 14C yr B.P.; ca. 6980–4710 cal. yr B.P.). The marine proxy provides the potential for constructing a highly resolved, well-dated, and continuous history of the southwest monsoon for the entire Holocene. [ABSTRACT FROM AUTHOR]

New compilations of geometry and tectonic transport in the Pennsylvania salient suggest that these features continue uninterrupted from the Piedmont through the Valley and Ridge provinces. These data are the basis for a new two-azimuth transport model for origin of the curvature. Edge geometry of an Eocambrian craton and a thickened stratigraphy acted as templates for Alleghanian regional décollement, first as Reading Prong ∼N35°W motion, followed by ∼N68°W Blue Ridge motion. Both displacements involved some drag rotation and overprinting in a central zone. This model avoids the problem of minimal tangential stretching, explains the anthracite basins as trailing-edge graben and basin structures against the autochthonous Pocono Plateau, and produces the Nittany- Juniata culmination as an overprinted pileup of duplexes in the central zone. The model of detachment followed by changing azimuth of motion of a megadécollement sheet may apply to other salients throughout the world. [ABSTRACT FROM AUTHOR]

The origin of coastal and high-elevation marine gravels on the Hawaiian islands of Lanai and Molokai is controversial, because the vertical tectonics of these islands is poorly constrained. The gravels are either from eustatic highstands or were left by massive tsunamis from offshore giant landslides. In contrast, at Kohala on the island of Hawaii, where continuous subsidence is well estab- lished, lithofacies analysis and dating of a fossiliferous marine con- glomerate 1.5–61 m above present sea level support a tsunami or- igin and indicate a runup of >400 m >6 km inland. The conglomerate age, 110 ± 10 ka, suggests a tsunami caused by the ca. 120 ka giant Alika 2 landslide from nearby Manna Loa volcano. [ABSTRACT FROM AUTHOR]

Newly released reflection seismic and borehole data, combined with sedimentological, provenance, and biostratigraphic data from Upper Cretaceous—Paleocene strata in the proximal part of the Cordilleran foreland-basin system in Utah, establish the nature of tectonic controls on stratigraphic sequences in the proximal to distal foreland basin. During Campanian time, coarse-grained sand and gravel were derived from the internally shortening Charleston-Nebo salient of the Sevier thrust belt. A rapid, regional Campanian progradational event in the distal foreland basin (>200 km from the thrust belt in >8 m.y.) can be tied directly to active thrust-generated growth structures and an influx of quartzose detritus derived from the Charleston-Nebo salient. Eustatic sea-level variation exerted a minimal role in sequence progradation. [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]

Examines the tectonic regulation on greenhouse gas flux to the paleogene atmosphere from the Gulf of Alaska accretionary prism. Implication of methane loading in global warming; Occurrence of deformation, accretion and high heat flow following deposition; Effect of global warming on ocean waters.

Evaluates the volcano edifice collapse hazards at Mount Rainier, Washington. Extent of widespread Osceola Mudflow and electron mudflow deposits; Sources of gravitational stress; Importance of topography in collapse localization; Stability of Mount Rainier's flanks.

Explores the active displacement transfer and differential block motion within the central Walker Lane, Great Basin in Nevada. Velocities of global positioning sites; Influence of block geometries on different displacements concentrated along boundaries as belts of divergent, transcurrent and convergent motion; Localization of block motions.

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.

Examines a natural experiment in river incision into bedrock produced by the 1912 eruption that inundated the Valley of Ten Thousand Smokes in Katmai National Park, Alaska. Influence of the rates and patterns of bedrock channel incision on landscape evolution and long-term interactions among climate, tectonics and erosion; Incision rate data.

Explores the active displacement partitioning and volcanic arc-parallel extension using Global Positioning System geodesy and kinematic analysis in the Aleutian Islands, Alaska. Evidence on a westward forearc migration; Details on partitioning in areas of impeded subduction; Implication for the exhumation of metamorphic rocks in ancient arc systems.

Provides information on the extensional tectonism, ocean-chemistry fluctuations and biological diversification during the late Neoproterozoic Era, preserved by the Chuar Group, exposed only in the Grand Canyon in Arizona. Documentation of shallow-marine deposition; Geochronology of the Chuar Group.

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.

Focuses on some of the tectonics which are located in the northern areas of the United States. Features of tectonics; Findings of studies conducted.

Proposes a genetic relationship between the 43-34 magnesium magmatism, extensional tectonics, and gold mineralization event centered in the Great Basin National Park, Nevada and the development of the Yellowstone hotspot. Features of the hotspot; Description of the Great Basin.

Explains the source of contraction directions along the Cascadia subduction zone in Washington state using a geotectonic model. Inconsistency of the structures with a simple subduction system undergoing northeasternward convergence; Direction of convergence in the south regions of the Cascadian zone.

Describes the sodium-rich (Na-rich) hydrothermal alterations associated with Mesozoic arc igneous rocks of the western United States. Sodium-calcium (NaCa) and sodium altered arc rocks; Mineralogic and compositional changes of NaCa and Na-rich alterations; Evidence of tectonically related geologic environments.

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.

Presents a three-dimensional model of vertical deformation at the southern Cascadia subduction zone in the western United States. Interseismic deformation data; Down-dip width for the locked regions; Prediction of uplift rates; Indication of geologic evidence.

Discusses the possible tectonic significance of observed stratal patterns in the Upper Cretaceous Blackhawk Formation of Utah. Geologic background; Updip termination of marine facies; Effect of changes in sediment supply on stacking patterns of systems tracts.

Studies the flexural subsidence history and basement tectonics recorded in Cenomanian to early Campanian strata in the Cretaceous Western Interior (CWI) basin in the United States. Two-dimensional flexural backstripping; Factors causing observed spatial and temporal trends in the subsidence history of the CWI basin.

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.

Determines exhumation rates for the multiply deformed and metamorphosed Silurian and Devonian metasedimentary rocks that are part of the Central Maine terrace using the relief method. Calculation of exhumation rates; Importance of exhumation rates.

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.

Presents the results of paleoseismologic and neotectonic investigations in the Bluffton area in Beaufort County, South Carolina. Earthquake of magnitude 2.8 in January 1989; Paleoliquefaction features near Bluffton; Paleoseismic events; Possibility of a separate seismic source zone in the South Carolina coastal plain.

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 discussion on the tectonics of the Yellowstone hotspot wake in southwestern Montana. Inference on the southwestern Montana paleovalley markings; Occurrence of unconformities; Effect of the presence of basalts.