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In this paper we report U Pb zircon ages from a syn-tectonic magmatism related to a contractional thick-skinned deformation pulse in the Mexican Fold and Thrust Belt. The study area corresponds to the Concepcion del Oro Block (COB), which is located in northern Zacatecas, Mexico. The COB is a NW-SE trending elongated tectonic structure located south of the Parras Transversal Sector in the Mexican Fold Thrust Belt (MFTB) in north-central Mexico. In this area, regional thin-skinned folds were formed between the late Turonian and Campanian and they define the core of a major northeastward-convex orocline around Concepcion del Oro mining district, Zacatecas, Mexico. The regional folds were cut by four Eocene-early Oligocene plutons (43-32 Ma; U Pb zircon), which in places modified the older thin-skinned structures. Magma was emplaced along thrust faults and modified the attitude of inverted limbs of overturned folds, and the orientation or position of the fold axis at the core of anticlines, causing the formation of complex emplacement-related deformation. Two NW-SE trending lineaments bound the COB, and both cut the periclinal ends of the thin-skinned older folds, at different time. The eastern lineament is here interpreted as a high angle reverse fault, which tilted polymictic conglomeratic continental succession that includes cobbles derived from one of the intrusive bodies and an andesitic syn-tectonic lava flow (40.7 ± 0.6 Ma; U Pb zircon). The western lineament is a normal fault generated by later regional tectonic extension related to the Basin and Range province. Altogether suggests that a late Eocene-early Oligocene contractional thick-skinned deformation pulse affected the COB. This pulse was contemporaneous with the Eocene-Oligocene magmatism in the Concepcion del Oro, Zacatecas region, and is similar in style and age to some of the structures produced by the Laramide orogeny in the southern Rocky Mountains and in some isolated areas in northern Mexico.

Extension orientation is a key parameter in rift tectonics. While a clockwise rotation of the extension orientation is widely accepted for the southern Rio Grande rift and the Basin and Range province in the western U.S., in which direction(s) the northern Rio Grande rift opened has been controversial. Moreover, slip re-orientation, a phenomenon caused by local stress rotation on weak faults under a uni-directional regional extension, imposes a serious impact on kinematic interpretations. We investigate the presently oblique Tusas-Abiquiu segment of the Rio Grande rift in north-central New Mexico to assess the rifting kinematics with the presence of pre-existing crustal weaknesses. Fault-slip data show an overall NE-SW extension on the NW-trending Tusas border faults, and WNW-ESE extension on the NNE-trending Abiquiu border and internal faults. The slip vectors on the border faults show a wide variety of oblique-slip component, whereas those on the internal faults are predominantly pure dip-slip. The majority of our field measurements can be reproduced by the slip re-orientation model, although this model's setting does not satisfy regional geological records. Clockwise rotation of the extension axes from NE-SW to WNW-ESE can also explain our fault-slip data, as well as other structures in the rift. Tectonic reactivation during the extension also influenced the landscape evolution of the early rift. The Rio Grande rift shared similar extension orientation with the southern Basin and Range during the late Oligocene. The extensional strain field of the southwestern North American plate shows a progressive, time-transgressive rotational pattern, implying that plate boundary processes, instead of asthenospheric upwelling, governed the initiation and early evolution of this intracontinental rift.

The accretion of the Avalon and Meguma terranes to the Laurentian margin was a major event in the development of the Appalachian orogen. The Minas Fault Zone delineates the boundary between these terranes. A strongly lineated and foliated granite intrusion (Kelly Brook pluton) exposed along the West River St. Mary's Fault intruded into Meguma Supergroup metasedimentary rocks. Well-developed C-S fabrics characterize the pluton with muscovite aligned in the S fabric, and a shallow (~10°) westerly plunging quartz stretching lineation. New U-Pb (zircon, LA-ICP-MS) geochronological data yield a crystallization age of 375.0 ± 4.6 Ma for the Kelly Brook pluton that is indistinguishable from other Late-Devonian intrusive rocks in the Meguma terrane. Petrographic data for muscovite indicate that it grew prior to deformation. A 100% plateau 40Ar/39Ar age of 369.0 ± 1.2 Ma on a single muscovite grain constrains the time of cooling of the granite to approximately 450 to 420 °C. Apatite grains also demonstrate pre- to syn-deformational fabrics and yield a U-Pb (LA-ICP-MS) age of 361.2 ± 5.6 Ma indicating the temperature of the granite was still above 350 °C at this time. Earliest Tournaisian fossils in the unconformably overlying Horton Group strata indicate that the granite was exposed by ca. 359 Ma. Exhumation rates of the Kelly Brook pluton and the Meguma terrane from approximately 9.5–13.3 km depth from ca. 375 Ma to surface exposure at ca. 359 Ma are 0.06 to 0.08 cm yr−1. Estimates for the Upper Devonian regional geothermal gradient in the northern Meguma terrane are high between 39.5 °C km−1 and 67.8 °C km−1, gradients that are common in extensional tectonic environments such as the Basin and Range province. An apatite fission-track age of 215 ± 14 Ma indicates Mesozoic reburial and reheating of the granite prior to opening of the Atlantic Ocean.

International audience; Breaking the lithosphere in extension without exceeding the driving far-field forces available on Earth is a tough quantitative modeling problem. One can tear it apart by propagation of an existing oceanic basin or weakness zone or one must assist rifting with magmatic processes, which drop the effective stress and weakens locally the lithosphere. While previous 3D models have demonstrated that non-cylindrical plumes produce almost cylindrical rift structures in a lithosphere under slight far-field loading, our contribution goes one step further by producing models of complete continental break-up. We investigate in details how the rheological stratification of the continental lithosphere interacting with active mantle plume influences the geometry and dynamics of rifting to continental break-up in 3D. We find that, irrespective of the rheological stratification, a plume-induced rifting process always occurs in two stages: an early crustal rifting stage and a late lithospheric necking (breakup) stage. In case of a rheologically decoupled lithosphere, initial brittle deformation is concentrated in the upper crust and strongly localized due to compensating ductile flow of lower-crustal material (core complex extension mode). On the contrary, rheological coupling between upper crust and lithospheric mantle results in highly distributed brittle deformation in the crust above mantle plume head (wide rift mode). Both core complex-like and wide rifting are followed by an abrupt transition to narrow rift stage when a localized ascent of mantle plume material focuses high strain along faults zones breaking through the entire lithosphere. The Main Ethiopian Rift, the Basin and Range province, and the East Shetland Basin may be natural examples of regions that have passed through these two stages of extension. Across-strike and along-strike asymmetry of break-up patterns arising spontaneously within initially symmetrical and laterally homogenous environment seems to be an intrinsic characteristic of plume-induced rifting.

The spread of grasslands in the Miocene and of C4 grasses in the late Miocene-Pliocene represents a major development in terrestrial plant evolution that affected the climate system and faunal evolution. The macrofossil record of grasses is sparse, likely due to the limited preservation potential of grasses. Diagnosis of the C3 or C4 photosynthetic pathway depends on preservation of both cellular structures and organic carbon for isotope analysis. Here we analyze the anatomical and isotopic composition of newly-collected grass fossils from the Dove Spring Formation, Red Rock Canyon State Park, California, USA, located in the El Paso Basin on the western side of the Basin and Range Province, a site previously identified as one of the earliest known C4 grass fossil bearing localities. We analyzed the anatomical and geochemical characteristics of these new grass fossils dated to 12.01–12.15 Ma. The fossils analyzed in this study include grass shoots and in cross-section display anatomy indicative of the C3 photosynthetic pathway. We isolated organic carbon from the stem fossils and determined the carbon isotopic composition to be − 24.8 ± 0.5‰. Together, the anatomical and geochemical analyses confirm that these plants used the C3 photosynthetic pathway. Our findings are consistent with dietary evidence based on tooth enamel from grazing mammals of available C3 resources in the same sections. These newly reported Miocene-age C3 grass fossils contribute to a sparse macrofossil record of grass evolution. Overall, paleoecological reconstructions at this site indicate more humid conditions during the Miocene compared to the modern Mojave Desert with C3 grasses and diverse grazing mammals.

Pluvial lake deposits in the Basin and Range province have long been exploited for records of hydrologic and climatic change. To obtain the most accurate reconstructions, a thorough understanding of the geomorphic evolution of such lake basins must be grounded in field stratigraphy and mapping, which are often lacking or are not well integrated with core studies. We conducted extensive stratigraphic investigation, mapping, and dating of exposed strata in the Lake Manix basin, former terminus of the Mojave River, California, during the Pleistocene to provide such a background and to supplement interpretations from a 45-m-long core. In this paper, we emphasize the geomorphic and tectonic processes that shaped the evolution of the Manix basin after the arrival of the Mojave River in the early-middle Pleistocene. The main processes include (1) sedimentary infilling of the lake basin, (2) interaction among the subbasins as controlled by internal sills and sill failures, (3) tectonics, and (4) climate change. From about 500 to 190 ka, Lake Manix was confined to its western subbasins, and fluctuated in response to climate change and perhaps to brief diversions of the river into the upstream Harper Lake basin. During this time, about 24 m of sediment accumulated near the confluence of the Mojave River and Manix Wash, and a 15-km-long clastic wedge built into the western basins, gradually filling the accommodation space. Two basin-integration events subsequently occurred, both caused by failure of sills that lay on sheared deposits along the Manix fault zone. When Lake Manix rose at the onset of glacial marine isotope stage 6, the sill on the south flank of Buwalda Ridge failed and triggered a catastrophic flood that entered the new, lower elevation, previously endorheic Afton subbasin. Subsequent lake-level changes affected both subbasins, but the thickest deposits after the flood are preserved in the Afton subbasin. The area upstream of Buwalda Ridge was only submerged during relatively high lake levels. Thus, sediment records interpreted from the core site and nearby outcrops in part reflect a geomorphic event (basin integration) rather than climatic conditions. The sill of Lake Manix after the integration event lay at the northeast end of the Afton subbasin, athwart strands of the Manix fault zone. At about 25 ka, this eastern sill was rapidly incised, and the river advanced to terminate in Lake Mojave in the Soda and Silver Lake basins. Recognition and dating of integration events, common for lake-river interactions in active tectonic settings, provide important context for interpreting paleoclimate records from sediments and provide temporal and spatial constraints on the biogeography of aquatic species.

Rivers are key elements of the landscape sensitive to changes in the rate of rock uplift. For the case where rock uplift is rapid, rivers experience a drop of their base-level leading to the formation of a knickpoint that propagates through the fluvial network. Such response is expected in extensional settings where rivers incise across the fault scarp direction. The NW portion of the state of Sonora (NW Mexico), belongs to the Basin and Range province where the extensional activity produced the arrangement of mountain ranges with NNW to SSE orientation. The extension started in the Late Oligocene and continued in the Miocene; however, when the landscape responded to the tectonic activity across the river basin remains unknown. The basin morphometry and knickpoint propagation ages are used to explore evidence of the first extensional activity traced back from the modern landscape of the Sonora river basin (SRB). The results of basins morphometric analysis indicate that the landscape steepness is high at the northeastern, central and southern portion of the SRB. Such trend is in agreement with the estimated ages of knickpoint retreat that range from ~19 to ~10 Ma. The results obtained in this study indicate that the landscape of the SRB has been affected by many phases of tectonic events, probably occurred in the Pliocene and Quaternary, and the landscape has not reached the equilibrium.

The Gulf of California has a long tectonic history. Before the subduction of the Guadalupe and Magdalena plates ceased, extension of the Gulf began to the east, at the Basin and Range province. Later, it was focused west of the Sierra Madre Occidental and the opening of the Gulf started. Currently, the Gulf rifting has different characteristics to the north than to the south. In this study, we analyze the lithosphere thickness in the Gulf of California region by means of P -wave and S -wave receiver functions. We grouped our lithosphere-thickness estimates into five froups: 1) North of the Gulf, with a thin lithosphere (~ 50 km) related to the extension observed in the Salton Through region; 2) the northwestern part of Baja California, with a thicker lithosphere (~ 80 km), thinning towards the Gulf due to the extension and opening processes (~ 65 km); 3) central Baja California, with no converted phase corresponding to the lithosphere-asthenosphere boundary but evidence of the presence of a slab remnant; 4) the southern Baja California peninsula, showing a shallow lithosphere-astenosphere boundary (LAB) (

Degassing of carbon monoxide (CO), which plays a significant role in the contribution of deep carbon to the atmosphere, commonly occurs within active fault zones. CO degassing from soil to the atmosphere in the Basin and Range province, west of Beijing (BRPB), China, was investigated by in-situ field measurements in the active fault zones. The measured concentrations of CO in soil gas in the BRPB ranged from 0.29 × 10 −6 to 1.1 × 10 −6 with a mean value of 0.6 × 10 −6 , which is approximately twice as large as that in the atmosphere. Net fluxes of CO degassing ranged from −48.6 mg m −2 d −1 to 12.03 mg m −2 d −1 . The diffusion of CO from soil to the atmosphere in the BRPB was estimated to be at least 7.6 × 10 3 ton/a, which is comparable to the corresponding result of about 1.2 × 10 4 ton/a for CO 2 . CO concentrations were spatially heterogeneous with clearly higher concentrations along the NE-SW trending in the BRPB. These elevated values of CO concentrations were also coincident with the region with low-velocity and high conductivity in deep mantle, and high Poisson’s ratio in the crust, thereby suggesting that CO degassing from the soil might be linked to upwelling of the asthenospheric mantle. Other sources of CO in the soil gas are suggested to be dominated by chemical reactions between deep fluids and carbonate minerals (e.g., dolomite, limestone, and siderite) in country rocks. Biogenic processes may also contribute to the CO in soil gas. The spatial distribution patterns of CO concentrations are coincident with the stress field, suggesting that the concentrations of CO could be a potential indicator for crustal stress field and, hence is potential useful for earthquake monitoring in the BRPB.

One highly disputed topic about the Jurassic igneous rocks in the eastern South China Block (SCB) is whether they were products of the Pacific plate subduction or the intra-continental extension of the SCB. The early Jurassic mafic rocks, which are regarded as important tracers for the mantle nature, geodynamic processes and tectonic setting, are of great importance to solve this controversy. This paper presents the detailed zircon U-Pb geochronological, elemental geochemical and Sr-Nd-Hf isotopic data for the Xialan gabbroic intrusion from the Eastern Nanling Range. SIMS zircon U-Pb dating results for olivine gabbro and hornblende gabbro yield weighted mean 206 Pb/ 238 U ages of 190.6 ± 1.3 Ma and 196.0 ± 2.7 Ma, respectively. Olivine gabbros are high in MgO, Mg-numbers and CaO and low in SiO 2 and depleted in LILEs and LREEs. They have significant Ba, Sr and Eu positive anomalies with e Nd (t) of +0.8 - +2.9, suggestive of their origination from an ancient lithospheric source. Hornblende gabbros have a wide compositional range of SiO 2 , MgO, Cr and Ni and are characterized by enrichment in LILEs and LREEs and insignificantly negative Nb-Ta anomalies, along with positive e Nd (t) values ranging from +1.7 to +6.2 and zircon e Hf (t) values from +9.7 to +12.7, indicative of their origination from the asthenosphere-lithosphere interaction. The estimated mantle potential temperature (1341–1407 °C) of hornblende gabbros is similar to that of the western Basin and Range Province (1350–1450 °C). In combination with the E-W-trending early Jurassic rock-association in the Nanling Range, it is herein proposed that the early Jurassic igneous rocks most likely formed in an intra-continental extension setting in response to the asthenospheric upwelling.

High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

Queen Creek exemplifies a moderately-sized tributary drainage of the Salt River, central Arizona, USA. In contrast to the exoreic Salt River and its immediate neighbor to the south, the exoreic Gila River, Queen Creek has remained an ephemeral stream, losing its discharge as it debouched from its mountain drainage basin throughout the late Pliocene and Quaternary. As a result, only the largest discharge events extended out into the center of the endorheic Higley Basin and then, later, to the Salt River. Data presented here demonstrate that the Queen Creek drainage existed long before the arrival of the Salt River in the Higley Basin, and continued to exist after aggradational piracy diverted the early Salt River around the north side of South Mountain. Thus, Queen Creek not only provides information about the history of internal drainage within the Higley Basin, but it also provides valuable insight about how a local endoreic stream reacts to drainage integration.

The Salt River and Verde River watersheds provide downstream metropolitan Phoenix, Arizona, USA with much of its water supply, and this paper explains how these rivers integrated in an extensional tectonic setting. Near the end of the Pliocene, segments of the proto-Salt and proto-Verde watersheds of central Arizona consisted of local drainage networks supplying water and sediment into internally drained basins, including depressions occupied by late Pliocene natural lakes occupying the Verde Valley and Tonto basins. A key location, the lower Verde River valley (LVRV), is where the modern-day drainages of the Salt and Verde now meet downstream of these Pliocene lakes. At the time of the Nomlaki tuff deposition ~3.3 Ma, a condition of sediment overfill existed in the LVRV, although there was no exoreic drainage and a playa was still maintained. A fanglomerate unit, named here the Rolls formation, spilled over a bedrock sill and an alluvial-fan ramp transported sediment into the Higley Basin that underlies the eastern part of metropolitan Phoenix. Lithologies of preserved remnants of this Pliocene alluvial-fan system match well cuttings of buried sediment in the Higley Basin with a cosmogenic burial isochron age of 3.90 ± 0.70 Ma. Based on cosmogenic burial isochron ages, ancestral Salt River gravels started depositing on top of this fan ramp between 2.8 and 2.2 Ma. Deposition of Salt and Verde river gravels in the Higley Basin continued for ~2 million years and eventually led to an aggradational piracy event that overtopped a bedrock ridge immediately east of Phoenix's Sky Harbor Airport. A cosmogenic burial age of 460 ± 23 ka is a rough maximum age for this river avulsion that relocated the Salt River into the Luke Basin that underlies western metropolitan Phoenix. Available chronometric data are not precise enough to determine whether the Salt River or Gila River integrated first. All of the exoreic rivers of western North America's Basin and Range Province — the lower Colorado, Gila, Rio Grande, Salt, and Verde rivers — employed lake overflow to integrate across half-graben, graben, rift and supradetachment tectonic settings.

The Arizona-Sonora porphyry copper cluster is one of the most productive provinces of this type on Earth. There, upper-crustal deposits emplaced between Late Cretaceous and Early Eocene, subsequently exhumed and developed supergene enrichment profiles. Despite the Basin and Range regional extension, these shallow intrusions and associated mineralization were preserved in eastern Sonora in the exhumed footwalls of normal faults. In order to better constrain their exhumation history, we performed a thermobarometrical (Al-in-hornblende) and thermochronological (apatite U–Pb and fission-track) study on the La Caridad and Suaqui Verde porphyry copper deposits. On the one hand, thermobarometry allows us to determine that the Suaqui Verde porphyry copper emplaced at 4.8 ± 0.9 km-depth, which implies that Cu–Mo mineralization may occur deeper than the ~2–4 km depth classically proposed for porphyry copper. On the other hand, using thermobarometry and inverse modeling of thermochronologic data obtained for individual and drill core samples, we reconstruct the thermal history of these porphyry copper. After a very fast post-emplacement cooling, numerical models suggest that cooling of rocks was slow and mainly monotonous during exhumation. Indeed, our data do not record an acceleration of the exhumation in eastern Sonora during the Basin and Range regional extension (between ~25 and 12 Ma). However, inverse modeling of fission-track data obtained along a drill core in La Caridad allows us to reconstruct the evolution of the geothermal gradient through time, which is in agreement with the regional geodynamics. Overall, it cannot be excluded that Oligocene volcanic rocks protected the ore deposits from erosion, even if they remained relatively thin (

The dynamics of river systems are most easily studied by observing their present geomorphology exposed at Earth's surface. However, the bulk of these often long-lived transportation and depositional systems is mostly buried at depth and hidden from view. Accessing these subsurface archives is often not possible. The Phoenix ‘basin’ of Arizona, USA, with its rich collection of water well drilling information, offers a unique opportunity. Clast assemblages of sand and gravel deposits contained in subsurface drill cuttings enable estimates of sediment provenance and, importantly, reveal changes of provenance over time. This study represents more than a decade of drill cuttings analysis and presents a method for evaluating the depositional history of how the Phoenix basin developed a large external river system that crossed a long-lived internally drained basin. This configuration developed after Basin and Range faulting and extension had ceased, allowing the basin to fill to capacity with sediment. When placed in the context of a series of new isopach maps of the subsurface ancestral Salt River deposits, our clast assemblage analysis supports the conclusion that the Salt and Verde rivers appeared suddenly in the geologic record. The ancestral Salt River deposits and other nearby streams crossing the basin each possess a unique clast assemblage that can be used to ‘fingerprint’ sediments with similar sources. We use these observations to draw conclusions about the age and origin of these streams. Used in conjunction with other types of analyses such as cosmogenic burial dating and detrital zircon analyses, determining clast assemblages of drill cuttings in the context of mapping these assemblages is a powerful new method for estimating the source of sediment and for understanding how the Phoenix basin, and basins in general, fill with sediment. This information is of value in modeling both basin development and groundwater resources in similar contexts in Arizona and elsewhere.

Studies of the evolution of drainage systems in extensional settings like the Basin and Range Province of western North America benefit from well drilling as a means of acquiring valuable insight. Cuttings from two wells drilled into sediments of the Phoenix metropolitan region, Arizona, USA, offer new insights into the drainage history of the Salt and Verde river drainage basins. Analyses of detrital zircons with U Pb dating reveal a different signature for Ancestral Salt River Deposits (ASRD) as compared to the underlying basin fill. Trace element, 87Sr/86Sr ratios, and electron microprobe analyses of basalt fragments in the basal deposits of the ASRD in two different wells from Mesa, Arizona, show matches for outcrop sources near Bartlett Dam in the Verde drainage and near Stewart Mountain Dam in the Salt drainage. This indicates that the Salt and Verde rivers were transporting these basalts when the Salt River first flowed into the metropolitan Phoenix area. We employed tephrochronology to determine that the 3.3-Ma Nomlaki tuff accumulated in closed-basin playa deposits located near the present-day junction of the Salt and Verde rivers, providing a maximum-limiting age for the integration of both rivers. Because the age of mountains crossed by the Salt and Verde rivers pre-date 12 cm/yr across multiple bedrock uplands; and (iv) Mescal Limestone clasts on the highest Salt River strath terrace cannot be explained by headward erosion. However, all analyzed evidence are consistent with the process of lake overflow. Based on our findings and the use of geomorphic criteria ( Douglass et al., 2009 ), we conclude that lake overflow is the most likely cause of drainage integration of both the Salt River and the Verde River.

The Gila River and its tributaries in southern Arizona and adjoining states incorporate several dozen individual extensional basins of the central Basin and Range province into a single integrated drainage network. Forty basins in the Gila domain contain more than 1000 m (maximum ~ 3500 m) of post-12 Ma basin fill. Subsurface evaporites in many basins document internal drainages terminating in isolated playa lakes during early phases of basin history. The nature of intrabasinal and interbasinal divides and of eroded or sedimented stream passages through mountain ranges intervening between the basins reveal the geomorphic mechanisms that achieved drainage integration over late Miocene to early Pleistocene time. Drainage integration accompanied by headward erosion eastward toward Gila headwaters was a response to Miocene opening of the Gulf of California, into which the Gila River debouched directly before the Pliocene (

Tectonic activity generates topography, and the variability of tectonic forcing is responsible for topographic patterns and variability of relief in fluvial landscapes. Despite this basic relation, the inverse problem, by which features of the topography are used for inferring tectonic uplift rates, has proven challenging. Here we develop formal linear inversion schemes to infer a record of the rate of relative uplift as a function of space and time from the long profiles of rivers. The relative uplift rate is the difference between the rates of rock uplift and of the base level change. The inversion schemes are based on a closed-form analytic solution to the transient linear stream power model, and to increase model resolution they make use of the multiplicity of information made available by multiple rivers and their tributaries. The distribution of the fluvial response time to tectonic perturbations is a key component of the inversion scheme, as this determines which tectonic events are preserved in the topography. We develop two inversion parameterizations that differ in their assumptions about the tectonic forcing: space-invariant and time-space variability with an assumed spatial distribution. The inversion schemes are applied to the Inyo Mountains, an uplifted block along the western boundary of the Basin and Range Province in California. Inversion results indicate that the range has been experiencing an acceleration of the relative uplift in the past ∼2-3 Ma. We use the inversion results to constrain the paleotopography and paleo-erosion rate along the range and to recover the throw rate history along the fault that bounds the Inyo range.

This study identifies anthropogenically disturbed areas and barren playa surfaces as the two primary dust source types that repeatedly contribute to dust storm events in the eastern Great Basin of western Utah, U.S.A. This semi-arid desert region is an important contributor to dust production in North America, with this study being the first to specifically identify and characterize regional dust sources. From 2004 to 2010, a total of 51 dust event days (DEDs) affected the air quality in Salt Lake City, UT. MODIS satellite imagery during 16 of these DEDs was analyzed to identify dust plumes, and assess the characteristics of dust source areas. A total of 168 plumes were identified, and showed mobilization of dust from Quaternary deposits located within the Bonneville Basin. This analysis identifies 4 major and 5 secondary source areas for dust in this region, which produce dust primarily during the spring and fall months and during moderate or greater drought conditions, with a Palmer Drought Index (PDI) of − 2 or less. The largest number of observed dust plumes (~ 60% of all plumes) originated from playas (ephemeral lakes) and are classified as barren land cover with a silty clay soil sediment surface. Playa surfaces in this region undergo numerous recurrent anthropogenic disturbances, including military operations and anthropogenic water withdrawal. Anthropogenic disturbance is necessary to produce dust from the vegetated landscape in the eastern Great Basin, as evidenced by the new dust source active from 2008 to 2010 in the area burned by the 2007 Milford Flat Fire; this fire was the largest in Utah's history due to extensive cover of invasive cheatgrass ( Bromus tectorum ) along with drought conditions. However, dust mobilization from the Milford Flat Burned Area was limited to regions that had been significantly disturbed by post-fire land management techniques that consisted of seeding, followed by chaining or tilling of the soil. Dust storms in the eastern Great Basin negatively impact air quality and transportation in the populated regions of Utah; this study details an improved forecasting protocol for dust storm events that will benefit transportation planning and improve public health.

Recent national programs in Canada (Lithoprobe) and the U.S. (EarthScope) are providing vast quantities of data and many new scientific insights concerning crustal and lithospheric mantle structure and evolution. More modest but significant results are also being produced in and about Mexico. This paper primarily focuses on results published since 2005 and presents an updated Moho map of North America. This overview of crustal structure does reveal a number of interesting observations. For example, two buried distinct crustal blocks have been discovered in the past 20 years. One was caught in a Precambrian suture zone in Canada, and the other is buried under the Gulf Coast region and is still of unknown origin. The Precambrian assembly of the continent has been documented by the mapping of numerous suture zones, and shallow portions of subducted slabs have been preserved in several cases. Not surprisingly, the internal structure of these suture zones is complex and highly variable. Isostatic adjustment has been shown to be a complex process involving the lithospheric mantle not just crustal structure variations. Specifically, the correlation of crustal thickness with elevation is only approximate at best. Several crustal blocks that have been stable over very long periods of time possess unusually thick crust whose lower portion is usually fast (Vp > 7.0 km/s). Rifting has been shown to affect the crust in a variety of ways ranging from the massive magmatic modification that produced the Mid-continent rift system to the broad region of extension in the Basin and Range province. Crustal structure along the active modern western margin of North America is highly complex as would be expected as a result of transitions between subduction and transform faulting and variations in volcanism and intraplate magmatism. We clearly still have a lot to learn about the structure and evolution of this continent.

This paper presents results of the study of attenuation of seismic waves in the lithosphere and upper mantle of the northern part of the Basin and Range Province (BRP) (Fig. 1). In this study, the coda-wave method [Aki, Chouet, 1975] is applied to process data collected in the seismic experiment conducted in 1988–1989, PASSCAL Basin and Range Passive Seismic Experiment [Owens, Randall, 1989], including records of 66 earthquakes and explosions (Mb=1.1–5.0) which occurred in BRP (Fig. 2). The effective seismic quality factor by the coda is calculated using the single-backscattering model [Aki, Chouet, 1975]. The QC values are calculated for 18 values of the lapse time window W from 10 to 95 sec with the step of 5 sec at six (6) central frequencies (0.3, 0.75, 1.5, 3.0, 6.0, and 12.0 Hz). In total, 7776 individual measurements of QC were done. It is observed that the quality factor QC is strongly dependent on the frequency and the lapse time window W: QC increases from 12±6 to 359±17 for the central frequencies of 0.3 and 12.0 Hz when the lapse time window is W=10 sec and from 87±6 to 1177±87 for the same frequencies when W=95 sec (Fig. 6). On the basis of the QС values obtained for all the lapse time windows W empirical relationships of quality factors and frequencies are calculated according to [Mitchell, 1981], and values of quality factor Q0 at reference frequency f0 (f0=1 Hz) and frequency parameter n (which is close to 1 and varies depending on the heterogeneity of the medium [Aki, 1981]) are obtained. In this study, Q0 varies from 60±8 to 222±17, the frequency parameter ranges from 0.57±0.04 to 0.84±0.05, and the attenuation coefficient δ varies from 0.015 to 0.004 km–1, depending on W (Fig. 8); similar values of attenuation parameters are typical of regions with high tectonic activity [Mak et al., 2004]. In the single-backscattering model, the dependence of the attenuation parameters from the lapse time window can be explained in terms of the depth of formation of the coda [Pulli, 1984]: a larger value of W corresponds to a greater depth through which the coda-waves go. As shown by the analysis of variations of attenuation coefficient δ and frequency parameter n for the Basin and Range Province, both parameters decrease irregularly with depth – the slope of the curve showing variations of δ is considerably changed at the depth of 150 km. At the top of the graph (to the depth of 150 km), an abrupt change of δ with depth is observed; it is clearly seen in the graph of gradient δ (Fig. 9 and Fig. 10); such behaviour is also characteristic of n. At the depth of 140 km, parameter n is increased. In the middle section (at depths of 150–200 km), the slope of the δ curve increases, and gradients of δ and the frequency parameter are significantly reduced. At the bottom of the profile (> 200 km), the value of δ is almost constant, and an abrupt increase of n is observed (Fig. 9 and Fig. 10). Figure 10 shows the high-speed profile of the area under study, which is published in [Wagner et al., 2012]. The profile shows the low velocity mantle under the Basin and Range Province, actually starting underneath the Moho (at the depth of 50–60 km). The lower boundary of the low-velocity mantle is located at the depth of 130–160 km. Thus, there are grounds to conclude that the change in the slope of the curve showing dependence of δ from the depth is related to the deep structure of the medium. The abrupt changes of δ and n are associated with the velocity discontinuities of the medium. The high values of δ and n, which are characteristic of the upper part of the profile, indicate the high degree of heterogeneity of the medium, which is also confirmed by the low velocities of seismic waves in the area under study [Wagner et al., 2012]. The reduction of parameters δ and n in the middle and lower parts of the profile suggests a more homogeneous structure of the medium at larger depths. As a result of the study of the characteristics of seismic wave’s attenuation in the lithosphere and the upper mantle of the northern part of the Basin and Range Province, it is established that the effective seismic quality factor QC is highly dependent on the frequency in the range of 0.5–16.0 Hz. The empirical relationships of Q(f) for various lapse time windows are obtained; it is shown that increasing the lapse time window causes the values of the effective seismic quality factor to increase, which may be interpreted as reduction of attenuation with depth. By comparing the depth variations of the attenuation coefficient and the frequency parameter against the velocity structure, it is shown that there is a distinct change in attenuation of seismic waves at the velocity discontinuities in the northern part of the Basin and Range Province.

The modern Great Basin of the interior western United States is characterized by surface winds with considerable spatial and temporal variabilities. Wind records from the second half of the 20th century for 12 Great Basin localities, analyzed with standard aeolian-sediment transport methods developed elsewhere in the world, reflect this complexity. The drift potential (DP) for aeolian deposits is generally moderate (DP 200–400) in the western Great Basin and weak (DP 300) at the eastern edge of the Great Basin where the dominant prevailing wind direction is south-southwesterly. Both DP and resultant drift direction (RDD) are consistent with synoptic meteorological observations of the evolution of cold fronts in the Great Basin. Meteorological observations show that effective winds to produce dunes are most commonly the result of late winter–early spring cyclogenesis. There has been considerable temporal variability of DP in the latter half of the 20th century. Most of the Great Basin has experienced decreasing wind strength since 1973, consistent with recent studies of wind strength in North America and elsewhere. Dune morphology matches both localized RDD and temporal variations in DP reasonably well in the Great Basin. The results demonstrate that local topography can have an important influence on wind directionality, thus providing a cautionary note on the interpretation of dune morphology in the paleoclimatic and stratigraphic record.

Relations among geothermal system dynamics, evolution of an 8-m thick biomorphic mound, and requirements to protect critical habitat of Gila boraxobius , a fish protected under the Endangered Species Act of 1973, were explored at Borax Lake, a shallow thermal lake in the tectonically active northwestern Basin-and-Range Province. Neutral pH, thermal water at ~ 105 °C discharges within a carrot-shaped vent at a depth of 29 m and rapidly mixes within the vent and lake which is mostly − 1 and proximity of seismic sources indicates the mound may be susceptible to liquefaction and slope failure during dynamic earthquake loading. Borax Lake is the unique product of the geothermal system and the biological communities it supports. Portions of the mound may have failed in the past during seismic loading, eroded, and been rebuilt by processes similar to those active today.

We provide the first synthesis of seismic reflection data and active present-day crustal deformation for the greater Wasatch fault zone. We analyzed a number of previously unpublished seismic reflection lines, horizontal and vertical crustal velocities from continuous GPS, and surface geology to investigate the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults across the eastern third of the northern Basin and Range Province. The seismic reflection data show recent activity along high-angle normal faults that become listric with depth and appear to sole into preexisting decollements, possibly reactivating them. We interpret these listric normal faults as reactivated Sevier-age structures that are connected at depth with a regionally extensive detachment horizon. These observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension. We modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space. The estimated model results include a low-angle dislocation (~ 8–20°) at a locking depth of ~ 7–10 km and slipping at 3.2 ± 0.2 mm/yr. Despite the model's relative simplicity, we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data, and suggests an active regionally extensive sub-horizontal surface in the eastern Basin and Range. This result may imply that this surface represents aseismic creep across a reactivated low-angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle–ductile transition zone under the present-day Basin and Range extensional regime. This result may also have implications for crustal rheology, and suggests that geodesy might, under some circumstances, serve as an appropriate tool for inferring deeper crustal structure.

Seismic reflection profiling demonstrates that the active, significant-offset Surprise Valley Fault that marks the western boundary of the Basin-and-Range Province in northernmost California dips at a moderate angle, only ~ 30° to ~ 2 km depth. A nearby seismic refraction/wide-angle reflection profile, albeit of lower-resolution, shows the fault-plane remains approximately planar to at least 7 km depth. Variably-tilted volcanic strata of known ages within the Surprise Valley basin demonstrate that ≥ 7 km of normal slip occurred along the Surprise Valley Fault within the past 8–4 Myr at a time-averaged slip rate of ~ 1–2 mm/yr, and that the fault rotated from an initial dip of as much as 60° to its present dip of 30°. The rotation of this isolated fault was not accomplished by domino-style rotation as seen elsewhere in the Basin-and-Range province, but rather by flexural rift-shoulder uplift aided by mid-crustal flow.

Surprise Valley in northeastern California offers an ideal opportunity to examine the structural setting of a developing extensional basin due to its late Miocene to recent activity in isolation from other major normal fault-bound basins. Seismic velocity and potential field modeling help determine the nature of basin fill and identify intra-basin faults. Based on a detailed gravity and magnetic profile, we identify shallow subsurface basalt flows and several faults within the valley that may accommodate hundreds of meters of vertical offset, possibly cutting and offsetting the ~ 30° east-dipping Surprise Valley fault that rotated during footwall tilting of the adjacent Warner Mountains. Some of these intra-basin faults correspond with mapped Quaternary fault scarps, but others have no surface expression. These faults may represent the currently active fault system within the basin. If so, they would indicate that basin development is transitioning away from the main range-front normal fault to a new set of steep intra-basin faults that are more favorable for accommodating regional transtensional strain.

Approximately 25% of the differential motion between the Pacific and North American plates occurs in the Walker Lane, a zone of dextral motion within the western margin of the Basin and Range province. At the latitude of Lake Tahoe, the central Walker Lane has been considered a zone of transtension, with strain accommodated by dip-slip, strike-slip, and oblique-slip faults. Geologic data indicate that extension and strike-slip motion are partitioned across the central Walker Lane, with dip-slip motion resulting in E–W to ESE–WNW extension along the present-day western margin of the central Walker Lane since approximately 15 Ma, and dextral strike-slip motion across a zone further east since as early as 24 Ma. GPS velocity data suggest that present-day strain continues to be strongly partitioned and localized across the same regions established by geologic data. Velocity data across the central Walker Lane suggest a minimum of 2 mm/yr extensional strain focused along the western margin of the belt, with very little extension across either the central or eastern portions of the Walker Lane. These data indicate very little dextral motion across the central and western portions of the domain, with dextral motion of 3–5 mm/yr presently focused along a discrete zone of the eastern part of the central Walker Lane, coincident with existing, mapped strike-slip faults. Historic seismic data reveal little seismic activity in areas of Late Holocene dip-slip motion in the west or dextral motion in the east, suggesting a period of quiescence in the earthquake cycle and the likelihood of future activity in both areas. Based on this and previous studies, it is likely that a combination of pre-Cenozoic crustal structure, a relatively weak lithosphere beneath the Walker Lane, and long-term low stress ratios in the crust have permitted the long-term partitioning of dextral and extensional strain exhibited across the central Walker Lane. The present-day location of dextral strain in the central Walker Lane is subparallel with dextral deformation documented in the northern Walker Lane, suggesting that as strain continues to accumulate, these two discrete zones could become a continuous strike-slip system which will play a more important role in the future accommodation of relative Pacific–North American plate motion.

This work constitutes the first attempt to understand the seismic hazard in northeast Mexico. We present a compilation of regional seismicity in northeast Mexico (24–31°N, 97–106°W), finding 148 earthquakes for the 1787–2006 period. The study area lies within three morphotectonic provinces: Basin, Range, and Rio Grande rift; Sierra Madre Oriental; and Gulf coastal plain. Peak ground acceleration (PGA) maps were computed for three different scenarios: 1928 Parral, Chihuahua ( M W = 6.5); 1931 Valentine, Texas ( M W = 6.4); and a hypothetical earthquake located in central Coahuila. Ground motion values were computed using attenuation relations developed for central and eastern North America and the Basin and Range provinces. The earthquake in central Coahuila is considered a critical scenario for the main cities of northeast Mexico. The damage associated with this hypothetical earthquake could be severe because most buildings were constructed without seismic criteria. The expected PGA values in Monterrey, Saltillo, and Monclova are between 30 and 70 cm/s 2 . This earthquake might also produce or trigger significant landslides and rock falls in the Sierra Madre Oriental, where several cities are located on the mountain range.

Records of lithospheric extension and mountain-range uplift are most continuously contained within syntectonic sedimentary rocks in basins adjacent to large structural culminations. In southeastern Arizona, metamorphic core complexes form mountain ranges with the highest elevations in the region, and supposedly much less extended terranes lie at lower elevations. Adjacent to the Santa Catalina-Rincon metamorphic core complex, within the Tucson Basin, stratigraphic-sequence geometries evident in a large suite of 2-D seismic reflection data suggest a two-phase basin-evolution model controlled by the emplacement and subsequent uplift of the core complex. In its earliest stage, Phase I of basin formation was characterized by extensive faults forming relatively small-scale proto-basins, which coalesced with the larger basin-bounding detachment fault system. Synextensional sedimentation within the enlarging basin is evidenced by sediment-growth packages, derived from adjacent footwall material, fanning into brittle hanging-wall faults. During this phase, volcanism was widespread, and growth packages contain interbedded sediments and volcanic products but, paradoxically, no mylonitic clasts from the adjacent metamorphic core complex. Phase II of basin evolution begins after a significant tectonic hiatus and consists of a symmetric deepening of the central basin with the introduction of mylonitic clasts in the basin fill. This is coupled with the activation of a series of high-angle normal faults ringing the core complex. These observations suggest a two-phase model for metamorphic core complex evolution, with an initial stage of isostatic core complex emplacement during detachment faulting that resulted in little topographic expression. This was followed, after a significant tectonic hiatus, by late-stage exhumation and flexural uplift of the Santa Catalina-Rincon metamorphic core complex through younger high-angle faulting. Moreover, the geometry of upper basin fill units suggests an extremely low effective elastic thickness in the region and that flexural uplift of the core complex induced asymmetric transfer of ductile mid-crustal rocks from beneath the subsiding Tucson Basin to the uplifting mountain range.

Floodplains in arid lands can be agriculturally productive, but they can also be geomorphically unstable environments. Many of the floodplains of higher order basins in southern portion of the Basin and Range Province of the American Southwest have been farmed intermittently for hundreds of years, and continuously for the past century. In order to be productive, however, these floodplains were modified considerably by human agency. This paper reviews the different types of human manipulations to floodplains that may result in land degradation and geomorphic change, including vegetation, topography, soils, hydrology, and channel banks. Emphasis is on the direct and indirect consequences of geomorphic modification, and the long-term viability of such changes in the contexts of landscape sensitivity and sustainability. Evidence documents transformation of natural floodplains to agricultural surfaces, with examples from southern Arizona, USA, and Sonora, Mexico.

Following a protracted phase ( � 155–60 Ma) of crustal shortening and mountain building, widespread extension and magmatism have dominated the tectonic history of the North American Cordillera since early Tertiary. Although decades of intensive investigations have made North American Cordillera one of the best studied continental regions in the world, many fundamental issues, including the cause of metamorphic core complexes and basin-and-range extension and their relationship with the overlapping magmatism, remain controversial. Recent studies have emphasized the role of gravitational collapse in causing both extension and magmatism in the Cordillera, but the geodynamics of gravitational collapse are not well understood. Using simple thermal–rheological and thermomechanical modeling, we address the following questions. (1) Could gravitational collapse of the thickened Cordilleran crust have formed the metamorphic core complexes? (2) Did gravitational collapse induce the intensive mid-Tertiary volcanism in the Cordillera? (3) What caused basin-and-range extension and the associated basaltic volcanism? Our results show that, although a thickened crust at isostatic equilibrium is dynamically unstable and tends to collapse, major postorogenic extension happens only when the lithosphere is sufficiently weakened by thermal processes associated with orogenesis, including thermal relaxation, radioactive heating, shear heating, and mantle thermal perturbations. Ductile spreading within the lower crust plays a major role in postorogenic gravitational collapse. This mechanism can explain most metamorphic core complexes and the associated plutonism. However, it is unlikely to have induced major mantle upwelling required by the voluminous silicic eruption during the mid-Tertiary. The cause of the mid-Tertiary mantle upwelling remains speculative, but strong mantle thermal perturbations under the northern Basin and Range province may have persisted since mid-Miocene. Thermomechanical modeling shows that this mantle upwelling may have caused significant ductile deformation within the surrounding lithosphere, with lithospheric material being pushed away and downward from the upwelling asthenosphere. The loci of maximum lithospheric thinning are near the margins of the upwelling mantle and have migrated outward during basin-and-range extension. This process can explain some of the spatial–temporal evolution of extension and volcanism in the Cordillera since mid-Miocene. The resultant lithospheric structures are consistent with geophysical observations in the Basin and Range province. D 2001 Elsevier Science B.V. All rights reserved.

A detailed compilation of the historical seismicity of northeastern Sonora and northwestern Chihuahua (28–32°N, 106–111°W) for the period 1887–1999 from catalogs, archives, and newspaper reports yielded 64 events (excluding aftershocks). The most significant ones are the 3 May 1887 Bavispe, Sonora (MW=7.4±0.3), 26 May 1907 Colonia Morelos, Sonora (Imax=VIII, MI=5.2±0.4), 17 May 1913 Huasabas, Sonora (Imax=VIII, MI=5.0±0.4), 18 December 1923 Granados–Huasabas, Sonora (Imax=IX, MI=5.7±0.4) and 28 October 1965 Nicolas Bravo, Chihuahua (mb=5.0) earthquakes. Most of the compiled seismicity is concentrated in the epicentral region of the 1887 Bavispe earthquake, whose surface rupture is >100 km long. Other seismicity clusters have been located in the Valle de Guaymas graben and in the regions of Fronteras–Nacozari, Granados–Huasabas, and Ciudad Juarez–El Paso. These are most likely tectonic earthquakes related to normal faults of the southern Basin and Range province and the Rio Grande rift.

Extensional deformation in the western United States, particularly in the Basin and Range, has frequently been ascribed to buoyancy forces arising from horizontal density contrasts. The Rio Grande rift is subject to buoyancy forces 2-3 times larger than those in the tectonically active northern Basin and Range, suggesting that it should be extending considerably faster than the northern Basin and Range, yet paradoxically is extending more slowly. Using a thin viscous sheet model, we test the hypothesis that regions surrounding the Rio Grande rift, which have rheologically stronger lithosphere, resist deformation and reduce the rate of extensional deformation in the Rio Grande rift to values consistent with observations. We find that these regions must have lithospheric strength at least twice that of the Rio Grande rift. This modest contrast in lithospheric strength is consistent with independent estimates derived from observations of heat flow, seismicity, and crustal thickness.

Models of the evolution of fault scarp morphology provide time elapsed since slip initiated on a faulted surface and may therefore provide more accurate estimates of slip rate than the rate calculated by dividing scarp offset by the age of the ruptured surface. To accomplish this task, linear and nonlinear models of sediment transport are calibrated from the morphology of Lake Bonneville shoreline scarps and fault scarps formed by multiple, surface-rupturing earthquakes along the Wasatch Fault Zone (WFZ). Profile modeling of scarps formed by several events distributed through time is done using a constant slip rate (CSR) solution and yields a value of A/κ (1/2 slip rate/diffusivity). Time elapsed since slip initiated on a fault is determined by establishing a value for κ and measuring total scarp offset. CSR nonlinear modeling (κ0 = 2.8 ± 1.1 m2/kyr, WFZ) of faults along the west slope of the Oquirrh Mountains indicates a slip rate of ∼0.1 mm/yr since 50 to 65 ka, which is corroborated by cosmogenic dating (10Be/26Al age = 75 ka). The slip rate along the west flank of the Stansbury Mountains varies from 0.04 to 0.2 mm/yr for time frames of 10 to >100 ka, with the most recent rupture on the northern portion of the fault zone ∼10 ka. Scarp analysis of the southern end of the Nephi segment, WFZ, suggests either temporal clustering or variable slip rate as indicated by differences in the short-term (1.3 mm/yr for 4.3 ka) versus long-term (0.4 mm/yr for 70 ka) slip rates.

We calculate a density model of the lithosphere of the southwestern United States through an integrated analysis of gravity, seismic refraction, drill hole, and geological data. Deviations from the average upper mantle density are as much as ±3%. A comparison with tomographic images of seismic velocities indicates that a substantial part (>50%) of these density variations is due to changes in composition rather than temperature. Pronounced mass deficits are found in the upper mantle under the Basin and Range Province and the northern part of the California Coast Ranges and adjacent ocean. The density structure of the northern and central/southern Sierra Nevada is remarkably different. The central/southern part is anomalous and is characterized by a relatively light crust underlain by a higher-density upper mantle that may be associated with a cold, stalled subducted plate. High densities are also determined within the uppermost mantle beneath the central Transverse Ranges and adjoining continental slope. The average density of the crystalline crust under the Great Valley and western Sierra Nevada is estimated to be up to 200 kg m−3 higher than the regional average, consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region.

Shear wave splitting and P, SKS, and S travel time residuals are calculated for teleseismic arrivals recorded on the Colorado Plateau-Great Basin Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) portable broadband seismic deployment and for permanent stations in the western United States. Little shear wave splitting is observed for broadband recordings in the northern Colorado Plateau, the Rocky Mountains, or the central Great Basin. The transition between the Colorado Plateau and the Great Basin is marked by moderate shear wave splitting (1.0 s) and unusually late teleseismic phase arrivals. This suggests material with a higher content of mantle melt or volatiles than regions to either side. Splitting in the transition between the Colorado Plateau and Great Basin is part of a pattern of fast polarizations that align in a semicircle, surrounding a central Great Basin region of null (no splitting) measurements. Away from the California plate boundary, splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the North American plate. No simple spatial relation of splitting with geological and geophysical features such as mountain ranges, velocity anomalies, gravity, magnetics, or heat flow is evident in most of the western United States. However, splitting in the Great Basin is compatible with asthenospheric flow. The smallest shear wave splitting delay times coincide with the Eureka Low in heat flow, also having low S velocity at 300 km depth. We suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the Great Basin is caused by mantle flow, by the interaction of mantle up welling and the absolute motion of the North American plate.

A new compilation of reliable isotope age data indicates that Cretaceous magmatism in SE China occurred in four major episodes during 136–146 Ma, 122–129 Ma, 101–109 Ma and 87–97 Ma. A-type granitic and within-plate basaltic magmatism from 140–90 Ma suggests a dominant extensional environment in the region. Voluminous coeval high-K calc-alkaline rocks, which have geochemical features similar to those formed in continental back-arc and post-collision extension settings, are interpreted to have been generated in response to lithospheric extension. Cretaceous magmatism, NNE-trending wrench faulting and formation of extensional basin systems favour an extensional tectonic regime in SE China at that time, which was probably similar to present-day Basin and Range Province in the western US.

A three-dimensional P wave velocity model of south-central California from the Coast Ranges to the Sierra Nevada shows that the crust under most of the southern Sierra Nevada batholith has seismic velocities (5.9–6.3 km/s) below the continental average. The crust is not much thicker (on average about 35 km) than in the adjacent Great Valley and Basin and Range province apart from a small, northward thickening, crustal root under the western Sierra Nevada that reaches a depth of 42 km. Crustal velocities above the continental average are observed beneath much of the Great Valley due to a high-velocity body underlying the sedimentary basin and the Foothills metamorphic belt (6.4–7.0 km/s). Upper mantle velocities are generally low (7.8 km/s) but span a wide range (7.4–8.2 km/s). We display the velocity model in several cross sections and maps of Moho depth and average crustal velocity. The measured velocities in the upper and mid crust of the Sierra Nevada batholith are in good agreement with laboratory measurements on Sierra Nevada tonalites after corrections for density and temperature. Peridotite xenoliths from the eastern Sierra Nevada suggest strong upper mantle anisotropy, which could explain some of the velocity heterogeneity in the Sierra Nevada mantle. By the time Cretaceous subduction-related magmatism ceased, the Sierra Nevada arc must have had a thick mafic lower crust; yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout. A subcrustal layer with velocities below normal Pn velocities (

Middle‐late Miocene (proto-Gulf; ~12‐6 Ma) extension around the Gulf of California (Gulf Extensional Province) is commonly interpreted as resulting from partitioning of oblique Pacific‐North American plate motion into strike‐ slip displacement along the margin and east‐northeast extension perpendicular to the margin within the North American plate. We propose that this mechanism also applies to kinematically similar, predominantly east‐northeast extension that occurred at the same time throughout the southern Basin and Range province, from southern Arizona and New Mexico to the Trans-Mexican Volcanic Belt. New field and 40Ar/39Ar data in Sinaloa and Durango confirm that this episode of extension occurred on the mainland side of the Gulf and in the Basin and Range east of the Sierra Madre Occidental, which is generally considered the eastern margin of the Gulf Extensional Province. Published data indicate the middle‐late Miocene episode also occurred across the northern and southern ends of the Sierra Madre where the Gulf Extensional Province connects with the Basin and Range: (1) from central Sonora into southern Arizona and New Mexico, and (2) from Nayarit into central Mexico north of the Trans-Mexican Volcanic Belt. This episode appears to have aVected an area that continues to the eastern edge of the Basin and Range province in Texas and San Luis Potosi. Recognition that this episode of extension aVected the entire southern Basin and Range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the Gulf Extensional Province alone. Published plate reconstructions require 160 to 110 km of east‐northeast extension between ~12 and 6 Ma. If taken up solely within the Gulf Extensional Province, this would have generated 66 to 78% extension, which is much greater than observed. Spread across the entire southern Basin and Range it requires only ~20% total extension, which is more consistent with observations of cumulative extension between 12 and 6 Ma. Extension was partitioned into the Gulf Extensional Province because (1) it lies between two stable batholith belts (Mesozoic Peninsular Ranges on the west and mid-Tertiary Sierra Madre Occidental on the east) that resisted extension and (2) the Gulf was thermally weakened by immediately preceding arc magmatism. Extension in the main Basin and Range province in part probably avoided the relatively strong, batholithic crust of the Sierra Madre Occidental. © 2000 Elsevier Science B.V. All rights reserved.

Low-angle normal faulting is precluded by elementary rock mechanics arguments. However, the existence of many low-angle normal faults in regions such as western North America has long suggested that this familiar theory is incomplete. Initiation of a low-angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes, which necessitates a substantial shear stress within the vertical plane. Nonetheless, no-one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low-angle dip of ∼30° in preference to the steeper dip of ∼50–60° predicted by standard theory. Despite previous claims to the contrary, this study shows for the first time that appropriately oriented stress fields can exist. However, they require the shear stress to locally reach ∼100 MPa near the base of a ∼10-km-thick brittle layer. This is only possible under extreme conditions, as it requires dramatic lateral variations in the state of stress across the extending region. It is suggested that a shear stress of this order can develop due to the combined effects of lower-crustal flow (which imparts a horizontal shear traction at the base of the brittle layer) and loading (which imparts shear traction in the vertical plane) associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region. Regional patterns of low-angle normal faulting in western North America are thus interpreted in terms of changes to the geometry of subduction of the Farallon plate.

The fault-segmentation method is commonly used to estimate the potential earthquake size. Segment boundaries play an important role in arresting earthquake ruptures from event to event. In the Basin and Range Province, earthquake rupture terminations are commonly associated with structural discontinuities, but not all-structural discontinuities have the capability to terminate an earthquake rupture. The size of structural discontinuities with respect to the rupture length and displacement may play an important role in controlling rupture termination. Studies of the geometric pattern of seven well-documented historical earthquake ruptures in the Basin and Range Province reveal three important relationships between rupture termination and the size of structural discontinuities. Firstly, terminations of normal faulting earthquakes are often associated with structural discontinuities, at least for the five historical ruptures studied in this paper. Secondly, the sizes of structural discontinuities at the end of earthquake rupture zones are generally the largest among the structural discontinuities within the rupture zone. Thirdly, there appears to be a trend that larger earthquake ruptures are stopped by larger structural discontinuities. The size of structural discontinuities that stopped the earthquake seems to scale with the length and displacement of earthquake surface rupture.

A multi-disciplinary (geomorphology, ecology, sedimentology, geology, gravimetry, ground penetrating radar, and direct current resistivity) study established a relation among the subsurface structure of Laguna El Fresnal, the geomorphic units, and the vegetation. According to gravity data, Laguna El Fresnal is an asymmetric fault bounded basin typical of the Basin and Range province and the Rio Grande rift. The geomorphic units along the eastern and western sides of El Fresnal basin show marked differences. The western slope is characterized by a variety of sedimentary deposits and geomorphic features, such as alluvial fans, debris flows, braided stream deposits, sheet flood deposits, erosional surfaces, paleosols and fluvial paleosystems of dendritic and parallel types. In contrast, the eastern slope has only aeolian deposits covering structures of alluvial origin. The vegetation also shows this asymmetry. Along the western slope vegetation is composed, from the highest to the lowest elevations, of Juniperus ssp., yucca, Echinocereus ssp., and Prosopis ssp.; whereas, on the eastern slope, the vegetation is mostly composed of small Prosopis ssp. associated with Larrea tridentata communities. The Juniperus ssp. vegetation represents remnants of an environment (Upper Pleistocene age?) more humid than at present. The stratigraphy of the morphological units was tentatively established. Three depositional lobes (QI, QII, and QIII), and three generations of alluvial fans (QIV, QV, and QVI) were defined. Silty-clayey sediments of aeolian origin (loess) from the basin constitute unit QVII. The structure of the first 3–4 m of units QVII and QI were established using a ground penetrating radar. Vertical electric soundings indicate at least 40 m of clay sediments beneath the plain. The contrasting differentiation between geomorphic units and vegetation at both slopes of the basin indicates that the western border of the basin is or was uplifting.

The partitioned waveform inversion method [Nolet, 1990] was used to obtain a new crustal thickness map of the western United States. We fitted 200 seismograms containing higher- and fundamental mode Rayleigh waves for events and stations in the western United States, which provided average crustal thickness constraints between each source-receiver pair. All these constraints were combined in a linear inversion to obtain a crustal thickness map. Local crustal thickness estimates from published seismic reflection/refraction work were used as additional constraints to overcome deficiencies of path coverage in some areas. The resulting crustal thickness map shows thicker (∼45 km) crust beneath the Cascades, thick (∼40 km) crust beneath the Sierra Nevada, thin (∼30 km) crust beneath the Basin and Range, and thick (greater than 45 km) crust beneath the Colorado Plateau. The boundary between the Colorado Plateau and the southern leg of the Basin and Range is a prominent feature of the model. Thick crust is seen at the bend of the San Andreas fault near Los Angeles. Our model is broadly consistent with the crustal thickness map of Mooney and Weaver [1989]. A quantitative comparison with the crustal thickness model 3SMAC [Nataf and Ricard, 1996] was performed by inverting for a model that deviates least from it. Results show that we are able to retain most of the features of that model while maintaining a good fit to the surface wave and point constraint data.

Deformation in the southern Walker Lane Belt region of the southwestern Great Basin accommodates significant portions of both Pacific-North America transform motion and Basin and Range extension. Apparent kinematic inconsistencies between geodetic and fault slip data in this region have made it difficult to understand the nature of the interaction between these deformation processes and to infer the regional kinematics. We model the kinematics of this region in a manner that enforces kinematic consistency and includes fault geometry and slip rate data, Global Positioning System (GPS) survey data, and very long baseline interferometry/very long baseline array (VLBI/VLBA) site velocity data. A model is found that is consistent with the set of available data, and this model differs significantly from prior models for the region. Our model has the Sierra Nevada block, which bounds the western margin of the Great Basin, moving N50°W±5° at 12.7±1.5 mm/yr, with only minor amounts of counterclockwise rotation. Left-lateral slip on the western Garlock Fault is a consequence of the relatively great westerly component of Sierra Nevada velocity. We also find that the Basin and Range extension east of our study area occurs at a rate of about 2.5 mm/yr, which is slower than the rate in the central Great Basin.

Problems about the nature of extended crust in the Basin and Range include the role of plastic flow during extension, the possibility of fluids in the crust, and the amount of mantle-derived material added to the crust. To address these problems, the University of Wyoming conducted a multicomponent wide-angle seismic experiment in the Ruby Mountains of the Basin and Range. The northern Ruby Mountains expose upper and middle crustal rocks of a metamorphic core complex, whereas the southern Ruby Mountains consist of low-grade miogeoclinal rocks. The wide-angle experiment consisted of a 95 km long N-S profile that extended from the southern to the northern Ruby Mountains. Travel time inversion of the wide-angle reflection data reveals relatively high seismic velocities in the midcrust to lower crust which suggest that fluid-filled pores, if they exist, do not reduce the seismic velocities significantly. Therefore the midcrustal to lower crustal porosity is probably much smaller than 1–2 vol %. The average P wave velocity for the crust is 6.4–6.5 km/s in the north and 6.1–6.2 km/s in the south. This difference suggests that larger amounts of mantle-derived material were added during the Tertiary to the core complex crust of the northern Ruby Mountains than to the “normal” Basin and Range crust of the southern Ruby Mountains. The velocity profile is consistent with a maximum of 6–7 km of mantle-derived material in the south and a maximum of 12–15 km of mantle-derived material in the north. Thus volumetric mafic intrusions probably accommodated a maximum of 7 km of core complex exhumation. Shear wave splitting indicates that the upper to middle crust is seismically anisotropic probably due to superposed Tertiary plastic flow patterns. Doming and inflation of a midcrustal layer in the seismic model are compatible with Tertiary flow of material from the south to the north that compensated about 2–5 km of core complex exhumation. Precritical Moho reflections (0–70 km offset, 10–11 s normal two-way travel time) are weaker and less continuous than midcrustal reflections (7–9 s normal two-way travel time). Strong and continuous Moho reflections occur at postcritical (>80 km) offsets only. Modeling shows that the Moho depth increases from about 32.5 km in the south to about 34.5 km in the north.

Reflection seismic data from the Great Salt Lake Basin, Utah, show that the major basin-bounding normal faults decrease in dip from ∼60° at the surface to ∼10°–20° at depths as shallow as 4–6 km. This rapid decrease in fault dip at depths shallower than the brittle-ductile transition zone in the Basin and Range Province suggests an explanation other than a gradual change of rheology and stress orientations with depth. Using a dense grid of seismic data, gravity data, borehole data, and published geologic information from islands in the lake, we constrain the position of the Sevier age Willard thrust and a footwall imbricate and show their reactivation as normal faults during Tertiary extension. In the absence of surface geologic information, we use available subsurface information from the lake to draw an analogy with the Ogden duplex in the Wasatch Front, where Cenozoic normal faulting was superimposed on an earlier Sevier age thrust regime to give rise to listric normal faults. Our interpretations are consistent with finite element modeling results, which demonstrate that extensional slip on preexisting thrust ramps leads to the formation of energetically favored synthetic normal faults, some of which may merge with the thrust ramp and obtain listric geometries. Further slip on these listric faults gives rise to secondary synthetic and antithetic faults resulting in hanging wall grabens.

21 well-distributed teleseisms (30° to 100° distance) were recorded by mixed broad-band (BB) and short-period (SP) seismic arrays at Mineral King (MK) and Horseshoe Meadow (HM) in the southern Sierra Nevada and at Darwin Plateau (DP) between the Inyo and Argus ranges. These arrays permit identification and separation of direct P and S arrivals, reflections from topography, scattered energy, and arrivals from different back azimuths (multipath arrivals). P-to-S conversions can be identified from beamed BB or SP seismograms. Least squares time domain processing recovered single-event receiver functions from these beamed seismograms. Converted phases attributable to the Moho are clear and uncomplicated at DP; Ps-P times of 3.9–4.2 s correspond to crustal thicknesses of 31–33 km, assuming a mean P crustal velocity of 6.2 km/s and a Poisson's ratio (ν) of 0.255. Ps-P times at HM (eastern Sierra) are 3.9–4.6 s (32–37 km) excepting some unusual seismograms from events at back azimuths of 225°–239°. MK (western Sierra) times for Ps-P show a strong E-W asymmetry: 3.9–4.1 s (31–33 km) from the east, and 4.8–5.3 s (39–42 km) from the west. The MK arrivals from the east are multiple and substantially weaker by comparison with HM and DP. These results confirm the absence of a thick crust under the southern High Sierra inferred from both a refraction experiment coincident with this experiment and some earlier studies. At DP the Moho event follows an intracrustal negative polarity event defining the top of an S wave low-velocity zone. This feature dips west under the Sierran crest at HM but is absent farther west at MK. This feature appears to be a manifestation of extensional strain and thus indicates that the surficially undeformed Sierra overlies a tectonized lower crust. Sub-Moho energy is absent under the Basin and Range (DP) but is conspicuous under the High Sierra at positive arrivals ∼7.3 s (MK) and ∼9 s (HM) after the P. These arrivals might be from the base of a low-velocity, low-density upper mantle body supporting the Sierra.

More than 36,000 km of Deep Seismic Sounding (DSS) profiles have been collected in China since 1958. However, the results of these profiles are not well known in the West due to the language barrier. In this paper, we summarize the crustal structure of China with a new contour map of crustal thickness, nine representative crustal columns, and maps showing profile locations, average crustal velocity, and Pn velocity. The most remarkable aspect of the crustal structure of China is the well known 70+ km thickness of the crust of the Tibetan Plateau. The thick (45–70 km) crust of western China is separated from the thinner (30–45 km) crust of eastern China by the north-south trending seismic belt (105°E). The average crustal velocity of China ranges from 6.15 to 6.45 km/s, indicating a felsic-to-intermediate bulk crustal composition. Upper mantle (Pn) velocities are 8.0 ± 0.2 km/s, equal to the global continental average. We interpret these results in terms of the most recent thermo-tectonic events that have modified the crust. In much of eastern China, Cenozoic crustal extension has produced a thin crust with a low average crustal velocity, similar to western Europe and the Basin and Range Province, western USA. In western China, Mesozoic and Cenozoic arc-continent and continent-continent collisions have led to crustal growth and thickening. Inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by DSS profiles and other seismological studies. A high velocity (7.0–7.4 km/s) lower-crustal layer has been reported in western China only beneath the southernmost Tibetan Plateau. We identify this high-velocity layer as the cold lower crust of the subducting Indian plate. As the Indian crust is injected northward into the Tibetan lower crust, it heats and assimilates by partial melting, a process that results in a reduction in the seismic velocity of the lower crust in the central and northern Tibetan Plateau.

Landsat Thematic Mapper data is used to estimate instantaneous regional-scale surface water and energy fluxes in a semi-arid Great Basin desert of the western United States. Results suggest that it is possible to scale from point measurements of environmental state variables to regional estimates of water and energy exchange. This research characterizes the unifying thread in the classical climate-topography-soil-vegetation relation-the surface water and energy balance-through maps of the partitioning of energy throughout the landscape. The study was conducted in Goshute Valley of northeastern Nevada, which is characteristic of most faulted graben valleys of the Basin and Range Province of the western United States. The valley comprises a central playa and lake plain bordered by alluvial fans emanating from the surrounding mountains. The distribution of evapotranspiration (ET) is lowest in the middle reaches of the fans where the water table is deep and plants are small, resulting in low evaporation and transpiration. Highest ET occurs in the center of the valley, particularly in the playa, where limited to no vegetation occurs, but evaporation is relatively high because of a shallow water table and silty clay soil capable of large capillary movement. Intermediate values of ET are associated with large shrubs and is dominated by transpiration.

Hafnium isotope Hafnium analyses of 52 Cenozoic basalts from the southwestern United States document the differences and similarities of the mantle beneath the continents, as compared to the suboceanic mantle. One of the major conclusions of this work is documentation of a widespread Nd-Hf isotope array that is oblique to that of the oceanic island basalt (OIB) array, indicating that the subcontinental mantle forms an important component to the Nd-Hf isotope balance of the Earth. Basalts from the Basin and Range province have Sr, Nd, and Pb isotope compositions that overlap those of OIB, and have been interpreted by many workers to have been derived from an OIB-like mantle plume [e.g., Fitton et al, 1991; Kempton et al., 1991]. However, the Hf isotope compositions of Basin and Range basalts do not overlap those of OIB, and are instead more similar to mid-ocean ridge basalt (MORB) mantle. Recognition of a MORB source for high-eNd Basin and Range lavas is possible only with addition of Hf isotope data. The Pb, Sr, Nd, and Hf isotope compositions and trace element contents of Basin and Range basalts can be matched by mixing melts that were derived from small degrees of partial melting of a depleted MORB mantle, with partial melts of incompatible element enriched pyroxenite veins. In order to match the low Lu/Hf ratios measured in these basalts, one of these components (pyroxenite veins or depleted peridotite) must have been garnet bearing. Moreover, to match the positive eNd and eHf values of Basin and Range basalts, these lithophile element-enriched pyroxenite veins must be relatively young; we suggest they reflect mantle veining during earlier widespread Mesozoic or Cenozoic magmatism. Basalts from the Rocky Mountains and western Great Basin have similar trace element contents, high lithophile element contents and high large-ion lithophile element (LILE) to high-field-strength element (HFSE) ratios. The Sr, Nd, and Pb isotope compositions of most western Great Basin samples plot along the enriched mantle (EM) II array of Zindler and Hart [1986], but basalts from the Rocky Mountains plot along the EM I array. The Hf isotope compositions of western Great Basin and Rocky Mountain samples overlap. However, these Hf isotope compositions are anomalous relative to the OIB array; most samples have higher 8nf values at a given eNd value as compared to OIB. The lead isotope compositions of both the Rocky Mountain and western Great Basin samples plot significantly above the northern hemisphere reference line in terms of 207Pb/204Pb ratios, supporting models for input of crustal material into the mantle. Crustal recycling into the mantle can produce a mantle source region that has high LILE/HFSE ratios, which are a characteristic of the source of basalts from the western Great Basin and Rocky Mountains. To produce the negative eNd and eHf values, but high eHf values at a given eNd value (as compared to the OIB mantle), the crustal material input into the mantle must be a blend of pelagic and turbidite sediments. Ancient subduction and storage of pelagic sediments in the mantle will produce a source region with negative eNd values but positive eHf values. In contrast, subduction of turbidite sediments will produce a mantle that has negative eHf and eNd values, but low eHf values at a given eNd value as compared to the OIB mantle. A mixture of pelagic to turbidite sediments that has a ratio greater than 1:1.2 can produce the negative eHf and eNd values, and high eHf values at a given eNd value (relative to OIB).