Nathan A. Niemi

Contemporary strain rates in the northern Basin and Range province from GPS data

We investigate the distribution of active deformation in the northern Basin and Range province using data from continuous GPS (CGPS) networks, supplemented by additional campaign data from the Death Valley, northern Basin and Range, and Sierra Nevada–Great Valley regions. To understand the contemporary strain rate field in the context of the greater Pacific (P)–North America (NA) plate boundary zone, we use GPS velocities to estimate the average relative motions of the Colorado Plateau (CP), the Sierra Nevada–Great Valley (SNGV) microplate, and a narrow north-south elongate region in the central Great Basin (CGB) occupying the longitude band 114–117°W. We find that the SNGV microplate translates with respect to the CP at a rate of 11.4 ± 0.3 mm yr^(−1) oriented N47 ± 1°W and with respect to NA at a rate of ∼12.4 mm yr^(−1) also oriented N47°W, slower than most previous geodetic estimates of SNGV-NA relative motion, and nearly 7° counterclockwise from the direction of P-NA relative plate motion. We estimate CGB-CP relative motion of 2.8 ± 0.2 mm yr^(−1) oriented N84 ± 5°W, consistent with roughly east-west extension within the eastern Great Basin (EGB). Velocity estimates from the EGB reveal diffuse extension across this region, with more rapid extension of 20 ± 1 nstr yr^(−1) concentrated in the eastern half of the region, which includes the Wasatch fault zone. We estimate SNGV-CGB relative motion of 9.3 ± 0.2 mm yr^(−1) oriented N37 ± 2°W, essentially parallel to P-NA relative plate motion. This rate is significantly slower than most previous geodetic estimates of deformation across the western Great Basin (WGB) but is generally consistent with paleoseismological inferences. The WGB region accommodates N37°W directed right lateral shear at rates of (1) 57 ± 9 nstr yr^(−1) across a zone of width ∼125 km in the south (latitude ∼36°N), (2) 25 ± 5 nstr yr^(−1) in the central region (latitude ∼38°N), and (3) 36 ± 1 nstr yr^(−1) across a zone of width ∼300 km in the north (latitude ∼40°N). By construction there is no net extension or shortening perpendicular to SNGV-CGB relative motion. However, we observe about 8.6 ± 0.5 nstr yr^(−1) extension on average in the direction of shear from southeast to northwest within the Walker Lane belt, comparable to the average east-west extension rate of 10 ± 1 nstr yr^(−1) across the northern Basin and Range but implying a distinctly different mechanism of deformation from extension on north trending, range-bounding normal faults. An alternative model for this shear parallel deformation, in which extension is accommodated across a narrow, more rapidly extending zone that coincides with the central Nevada seismic belt, fits the WGB data slightly better. Local anomalies with respect to this simple kinematic model may reveal second-order deformation signals related to more local crustal dynamic phenomena, but significant improvements in velocity field resolution will be necessary to reveal this second-order pattern.

Comparison of geodetic and geologic data from the Wasatch region, Utah, and implications for the spectral character of Earth deformation at periods of 10 to 10 million years

The Wasatch fault and adjacent fault zones provide an opportunity to compare present-day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows, ranging from the last millennium up to 10 Myr. The three easternmost GPS sites of the Basin and Range Geodetic Network (BARGEN) at this latitude define a ∼130-km-wide region spanning three major normal faults extending east-west at a total rate of 2.7 ± 0.4 mm/yr, with an average regional strain rate estimated to be 21 ± 4 nstrain/yr, about twice the Basin and Range average. On the Wasatch fault, the vertical component of the geologic displacement rate is 1.7 ± 0.5 mm/yr since 6 ka, <0.6 mm/yr since 130 ka, and 0.5–0.7 mm/yr since 10 Ma. However, it appears likely that at the longest timescale, rates slowed over time, from 1.0 to 1.4 mm/yr between 10 and 6 Ma to 0.2 to 0.3 mm/yr since 6 Ma. The cumulative vertical displacement record across all three faults also shows time-variable strain release ranging from 2 to 4 mm/yr since 10 ka to <1 mm/yr averaged over the past 130 kyr. Conventional earthquake recurrence models (“Reid-type” behavior) would require an accordingly large variation in strain accumulation or loading rate on a 10-kyr timescale, for which there appears to be no obvious geophysical explanation. Alternatively, seismic strain release, given a wide range of plausible constitutive behaviors for frictional sliding, may be clustered on the 10-kyr timescale, resulting in the high Holocene rates, with comparatively low, uniform strain accumulation rates on the 100-kyr timescale (“Wallace-type” behavior). The latter alternative, combined with observations at the million-year timescale and the likelihood of a significant contribution of postseismic transients, implies maxima of spectral amplitude in the velocity field at periods of ∼10 Myr (variations in tectonic loading), ∼10 kyr (clustered strain release), and of 100 years (postseismic transients). If so, measurements of strain accumulation and strain release may be strongly timescale-dependent for any given fault system.

Community Fault Model (CFM) for Southern California

We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strike- slip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehen- sive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment.

Tectonic implications of a dense continuous GPS velocity field at Yucca Mountain, Nevada

[1] A dense, continuous GPS network was established in the Yucca Mountain area in 1999 to provide the most reliable measurements possible of geodetic strain patterns across the nation’s only proposed permanent repository for high-level radioactive waste. The network lies astride a boundary between the geodetically stable central Great Basin and the active western Great Basin, which at the latitude of Yucca Mountain is undergoing distributed right-lateral shear at a rate of � 60 nstrain/yr. Monitoring from 1999 to 2003 (3.75 years) yields a velocity field characterized by nearly homogenous N20� W rightlateral shear of 20 ± 2 nstrain/yr (net velocity contrast of � 1.2 mm/yr across a 60 km aperture) in the vicinity of the proposed repository site. Comparison of time series of continuous results with earlier campaign surveys indicating � 50 nstrain/yr of westnorthwest extension from 1991 to 1997 suggests that the more rapid rates were in part transient motions associated with the 1992 Ms 5.4 Little Skull Mountain earthquake. Postseismic motions do not appear to affect the 1999–2003 velocity field in either campaign or continuous data. The magnitude of the velocity contrast across the area, the overall linearity of the gradient, and the large area of undeforming crust to the east of Yucca Mountain are difficult to explain by elastic bending of the crust associated with the Death Valley fault zone, a major right-lateral strike-slip fault about 50 km west of the repository site. These observations, along with apparent local variations in the velocity gradient, suggest that significant right-lateral strain accumulation, with displacement rate in the 1 mm/yr range, may be associated with structures in the Yucca Mountain area. The absence of structures in the area with equivalent late Quaternary displacement rates underscores the problem of reconciling discrepancies between geologic and geodetic estimates of deformation rates. INDEX TERMS: 1208 Geodesy and Gravity: Crustal movements— intraplate (8110); 1243 Geodesy and Gravity: Space geodetic surveys; 8107 Tectonophysics: Continental neotectonics; 8109 Tectonophysics: Continental tectonics—extensional (0905); KEYWORDS: geodesy, tectonics, Yucca Mountain

Long-term continental deformation associated with transpressive plate motion: The San Andreas fault

A synthesis of transpressive mountain building, as evidenced by rock uplift and topography along the entire San Andreas fault, reveals a complex crustal response to oblique plate motion. Convergent deformation increases toward the fault but does not correlate with the angle of plate-motion obliquity. The shortening estimated from rock uplift is also insufficient to account for the fault-normal motion based on relative plate velocity. This suggests that near-field convergence is influenced by local structural complexity and is not purely driven by regional transpression, and that the fault-normal component of plate motion is partly accommodated elsewhere. Heterogeneity in deformation and degree of slip partitioning highlight the importance of other factors in shaping transpressive continental deformation, including surface processes, material anisotropy, and strain weakening.

Active megadetachment beneath the western United States

Geodetic data, interpreted in light of seismic imaging, seismicity, xenolith studies, and the late Quaternary geologic history of the northern Great Basin, suggest that a subcontinental-scale extensional detachment is localized near the Moho. To first order, seismic yielding in the upper crust at any given latitude in this region occurs via an M7 earthquake every 100 years. Here we develop the hypothesis that since 1996, the region has undergone a cycle of strain accumulation and release similar to “slow slip events” observed on subduction megathrusts, but yielding occurred on a subhorizontal surface 5–10 times larger in the slip direction, and at temperatures >800°C. Net slip was variable, ranging from 5 to 10 mm over most of the region. Strain energy with moment magnitude equivalent to an M7 earthquake was released along this “megadetachment,” primarily between 2000.0 and 2005.5. Slip initiated in late 1998 to mid-1999 in northeastern Nevada and is best expressed in late 2003 during a magma injection event at Moho depth beneath the Sierra Nevada, accompanied by more rapid eastward relative displacement across the entire region. The event ended in the east at 2004.0 and in the remainder of the network at about 2005.5. Strain energy thus appears to have been transmitted from the Cordilleran interior toward the plate boundary, from high gravitational potential to low, via yielding on the megadetachment. The size and kinematic function of the proposed structure, in light of various proxies for lithospheric thickness, imply that the subcrustal lithosphere beneath Nevada is a strong, thin plate, even though it resides in a high heat flow tectonic regime. A strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern Great Basin, deformation microstructures and low water content in dunite xenoliths in young lavas in central Nevada, and high-temperature microstructures in analog surface exposures of deformed lower crust. Large-scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere, as inferred by such proxies as low P wave velocity and mafic magmatism.

Characterization of site‐specific GPS errors using a short‐baseline network of braced monuments at Yucca Mountain, southern Nevada

[1] We use a short-baseline network of braced monuments to investigate site-specific GPS effects. The network has baseline lengths of 10, 100, and 1000 m. Baseline time series have root mean square (RMS) residuals, about a model for the seasonal cycle, of 0.05–0.24 mm for the horizontal components and 0.20–0.72 mm for the radial. Seasonal cycles occur, with amplitudes of 0.04–0.60 mm, even for the horizontal components and even for the shortest baselines. For many time series these lag seasonal cycles in local temperature measurements by 23–43 days. This could suggest that they are related to bedrock thermal expansion. Both shorter-period signals and seasonal cycles for shorter baselines to REP2, the one short-braced monument in our network, are correlated with temperature, with no lag time. Differences between REP2 and the other stations, which are deep-braced, should reflect processes occurring in the upper few meters of the ground. These correlations may be related to thermal expansion of these upper ground layers, and/or thermal expansion of the monuments themselves. Even over these short distances we see a systematic increase in RMS values with increasing baseline length. This, and the low RMS levels, suggests that site-specific effects are unlikely to be the limiting factor in the use of similar GPS sites for geophysical investigations.

Stateline fault system: A new component of the Miocene-Quaternary Eastern California shear zone

The Eastern California shear zone is an active, north-northwest–trending zone of intraplate right-lateral shear that absorbs ∼25% of Pacific-North America relative plate motion. The Stateline fault system (SFS), which includes several previously recognized, discontinuously exposed Quaternary structures along the California-Nevada border, is in this paper defined as a continuous, 200-km–long zone of active dextral shear that includes (from south to north) the Mesquite, Pahrump, and Amargosa Valley segments. Recognition of this system expands the known extent of the Eastern California shear zone ∼50 km to the east-northeast from its traditionally recognized boundary along the Death Valley fault system. Proximal volcanic and rock avalanche deposits offset across the Mesquite segment of the SFS indicate 30 ± 4 km of slip on this structure since 13.1 ± 0.2 Ma. This offset is an order of magnitude larger than previous estimates across this section of the SFS, but it is consistent with larger offsets previously proposed for the central and northern sections. The total offset and averaged slip rate since mid-Miocene time (2.3 ± 0.35 mm/yr) are similar to those of other major faults across this portion of the Basin and Range, which, from east to west, include the Death Valley, Panamint Valley-Hunter Mountain, and Owens Valley fault systems. However, in contrast to these faults, the average post–mid-Miocene slip rate on the SFS is approximately twice that estimated from present-day geodetic observations and an order of magnitude greater than estimates of average post–mid-Pleistocene slip rates. This discrepancy between long-term, short-term, and geodetically derived slip rates differs from other geologic-geodetic, slip-rate discrepancies in the Eastern California shear zone, where geodetic slip rates are significantly faster than both long-term and short-term geologic slip rates. This suggests that either the slip rate on the SFS has diminished over time, such that the system is an abandoned strand of the relatively young Eastern California shear zone, or that the present-day slip rate represents a transient period of slow slip, such that strands of the shear zone must accommodate a complex spatial and temporal distribution of slip.

Distribution and provenance of the middle Miocene Eagle Mountain Formation, and implications for regional kinematic analysis of the Basin and Range province

Conglomeratic strata from middle Miocene sections in the central Resting Spring Range and nearby Eagle Mountain, California, contain a clast assemblage including marble, orthoquartzite, fusulinid grainstone, and coarse (∼1 m) monzogabbro, interstratified with tephras yielding laser-fusion ^(40)Ar/^(39)Ar ages of 11.6, 13.4, and 15.0 Ma. Petrographic and geochronologic evidence ties the clast assemblage to a source area in the southern Cottonwood Mountains, California, >100 km west-northwest of their present location. In the upper 100 m of the Resting Spring Range section, conglomerates are derived almost exclusively from the southern Cottonwoods source, and sandstone modes are as much as 50% angular plagioclase derived from the monzogabbro. The lack of dilution of this detritus by other sources and sedimentary features in both sections indicate (1) that deposition occurred on an alluvial fan with a north- northeast paleoslope and (2) that transport of the gravels by sedimentary processes was probably <20 km north-northeast, in a direction normal to the present azimuth to their source. Therefore, we interpret most or all of the net east-southeast transport as a result of extensional and strike-slip faulting between the Cottonwood Mountains and the Resting Spring Range since 11–12 Ma. Restoration of these deposits to a position 10–20 km north-northeast of the eastern margin of the monzogabbro source (east margin of the Hunter Mountain batholith) yields a net tectonic displacement of the Cottonwood Mountains relative to the Resting Spring Range of 104 km N67°W. This result confirms previous reconstructions based on the restoration of isopachs in the Cordilleran miogeocline, pre-Cenozoic structural features, and other proximal Tertiary deposits in the region.

Controls on the Spatial Variability of Modern Meteoric δ18O: Empirical Constraints from the Western U.S. and East Asia and Implications for Stable Isotope Studies

We analyze a dataset of multiannual modern precipitation and meteoric water records from the western U.S. (n = 206) and east Asia (n = 478) to (1) determine which environmental parameters correlate best with and account for spatial variability of meteoric water isotopic (δ18O and δD) compositions and (2) assess the degree to which this variability is a function of physiography and climatology. Multivariate linear regression analysis including five environmental parameters [latitude, longitude, elevation, mean annual temperature (MAT), mean annual precipitation (MAP)] reveals that latitude and elevation are consistently strongly correlated with meteoric δ18O distributions throughout much of the western U.S. and east Asia, but correlations with site longitude, MAT, and MAP can also be significant, depending on region. Our analysis also indicates that isotope-environment relationships are region-dependent. Isotope-elevation gradients are reduced by a factor of two or more in continental interior rainshadows (for example, Basin and Range) and high elevation continental plateaus (for example, Tibetan Plateau) in comparison to isotope-elevation gradients observed in orographic settings with a single dominant moisture source and relatively simple storm track trajectories (for example, Sierra Nevada and Himalaya). Published global and continental predictive equations for the calculation of modern meteoric water isotopic compositions that do not account for this regional variability are shown to be characterized by significant predictive uncertainties for modern δ18O values (∼ ±3-4‰), particularly in regions of complex moisture source interactions, continental moisture recycling and increased convective storm activity where reduced isotope-elevation gradients are also observed. We present new empirical relationships between environmental parameters and isotopic compositions that inform stable isotope-based reconstructions of climate change, timing and location of groundwater recharge, and paleoaltimetry by providing quantitative constraints on the statistical relationship between precipitation δ18O and individual environmental parameters (for example, latitude, elevation, temperature) and identifying how isotopic relationships are influenced by physiography and climatology. In particular, paleoelevation estimates in regions of low-magnitude isotope-elevation gradients (Basin and Range, Tibet) are shown to be prone to significant uncertainties (in some cases > ±2 km). These relationships improve the interpretation of stable isotope proxy data, particularly in cases where the paleogeographic setting of proxy formation can be constrained.