The eastern boundary of the Eastern California shear zone (ECSZ) is defined by the Stateline-Amar... more The eastern boundary of the Eastern California shear zone (ECSZ) is defined by the Stateline-Amargosa Valley fault zone (SAVFZ), a NW striking zone of dextral faulting that extends 150 km along the CA-NV border in Clark and Nye counties. This fault system has accumulated ~ 30 km of slip since 12.5 Ma, yielding a long-term slip rate of ~ 2
A short-baseline network that we have established at Yucca Mountain, southern Nevada, is enabling... more A short-baseline network that we have established at Yucca Mountain, southern Nevada, is enabling us to put new constraints on the level at which individual site motion can be detected by continuous GPS stations. We use this network to investigate and quantify aspects of the GPS error budget at a level of precision that would have been hard to imagine
Understanding the paleoelevation history of orogens is fundamental to answering a number of signi... more Understanding the paleoelevation history of orogens is fundamental to answering a number of significant questions about continental deformation. Most paleoaltimeters rely on proxies for measuring paleoelevation that may be affected by factors other than elevation, such as climate. The basalt vesicle paleoaltimeter avoids problems associated with such proxies by directly measuring the paleo-atmospheric pressure recorded by vesicle size-distributions in basalt
A fundamental issue in continental dynamics is whether deformation within the broad Pacific-North... more A fundamental issue in continental dynamics is whether deformation within the broad Pacific-North America plate boundary in the southwestern United States is better described by the relative motions of rigid microplates or by diffuse crustal flow. Whereas the Colorado Plateau and Sierra Nevada-Great Valley regions appear to be fairly undeformed over geological time scales, crustal extension between these regions in the Basin and Range province based on prevalent Quaternary faulting appears to have been relatively homogeneous. However, historic seismicity, which is largely confined within the relatively narrow Central Nevada Seismic Belt (CNSB) and the Intermountain Seismic Belt (ISB), may suggest that modern deformation of the Basin and Range province is also confined to these province bounding belts and that the central part of the Basin and Range province, the Central Great Basin region, is presently relatively rigid. We use site velocities from several campaign and continuous GPS...
The ongoing Arabian-Eurasian collision provides an opportunity to understand strain localization ... more The ongoing Arabian-Eurasian collision provides an opportunity to understand strain localization within a nascent orogen. The central third of this collision lies between the Black and Caspian Seas, with late Cenozic deformation extending over 700 km across strike, from the Zagros in the south to the Greater Caucasus (GC) in the north. The GC range is flanked to the south
New field observations from the Gonga Shan (7756m) region in eastern Tibet show a spatial associa... more New field observations from the Gonga Shan (7756m) region in eastern Tibet show a spatial association between high topography, intense erosion and exhumation and extensional faulting. The Gonga Shan massif is a region anomalously high topography, approximately 1 - 3 km in excess of the surrounding plateau elevations, and incised by rapid fluvial and glacial erosion. The massif is bounded
The successful quantification of long-term erosion rates underpins out understanding of landscape... more The successful quantification of long-term erosion rates underpins out understanding of landscape formation, the topographic evolution of mountain ranges, and the mass balance within active orogens. The measurement of in situ-produced cosmogenic radionuclides (CRNs) in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide long-term erosion rates over a wide variety of landscapes and erosional
The successful quantification of long-term erosion rates underpins our understanding of landscape... more The successful quantification of long-term erosion rates underpins our understanding of landscape formation, the topographic evolution of mountain ranges, and the mass balance within active orogens. The measurement of in situ produced cosmogenic radionuclides (CRNs) in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide such long-term erosion rates. Although deep-seated bedrock landsliding is an important erosional process in active orogens, its effect on CRN-derived erosion rates is largely unquantified. We present a numerical simulation of cosmogenic nuclide production and distribution in landslide-dominated catchments to address the effect of bedrock landsliding on erosion rates. Results of the simulation indicate that the temporal stability of erosion rates determined from CRN concentrations in sediment decreases with increased ratios of landsliding to bedrock weathering rates within a given catchment area, and that as the frequency of landsliding increases, larger catchment areas must be sampled in order to accurately evaluate long-term erosion rates. In addition, results of this simulation suggest that sediment sampling for CRNs is the appropriate method for determining long-term erosion rates in regions dominated by mass-wasting processes, whereas bedrock surface sampling for CRNs may generally underestimate long-term erosion rates. Response times of CRN concentrations to changes in erosion rate indicate that climatically driven cycles of erosion may be detected, but that complete equilibration of CRN concentrations to new erosional conditions may take tens of thousands of years. Comparison of simulated CRN-derived erosion 0012-821X/$ -see front matter D Earth and Planetary Science Letters 237 www.elsevier.com/locate/epsl rates with a new data set of such rates from the Nepalese Himalaya underscore the conclusions drawn from the model results. D
The Stateline fault zone is the easternmost active dextral structure in the Eastern California Sh... more The Stateline fault zone is the easternmost active dextral structure in the Eastern California Shear Zone and its northern segment lies only ~20 km southwest of the proposed Yucca Mountain nuclear waste repository. This ~150 km long fault zone follows the California-Nevada state line from Amargosa Valley to Ivanpah Valley. The geometry, kinematics, and displacement history of this structure are
ABSTRACT Erosion in the Himalaya is driven largely by a strongly coupled system of extreme climat... more ABSTRACT Erosion in the Himalaya is driven largely by a strongly coupled system of extreme climatic conditions and active tectonic processes. Spatial and temporal variations in erosion rates along strike are presumably controlled by differences in local climate, seismicity, deformation rates, and lithology. Quantifying the contribution of each of these parameters to the erosional budget of the Himalaya, however, is a nontrivial problem. The easternmost portion of the Himalayan arc offers a natural laboratory to explore the role of climatic influence on erosion rates. Deformation and uplift of the Shillong Plateau since ~8 Ma has created an orographic barrier ~400 km long that shields the eastern Himalaya, in Arunachal Pradesh, India, from a significant proportion of the precipitation carried by the South Asian Monsoon. Long-term exhumation rates derived from the Himalaya west and east of this orographic barrier have been shown to differ by a factor of ~2, a difference ascribed to reduced climatic forcing of erosion in the lee of the Shillong Plateau. Here we present apatite (U-Th)/He thermochronology data from modern detrital samples collected from northeast India. Between 18-20 single grain ages from each catchment were analyzed in order to calculate erosion rates on a 106 yr timescale. Recently developed Bayesian techniques for the inverse modeling of detrital data were used to derive time-temperature histories for each sample. Recent erosion rates modeled for a single south-facing catchment on the Shillong Plateau are modest, ~0.25 km Myr-1, and show a clear increase in exhumation rates at ~8 Ma from rates of
X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been ... more X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of low-grade metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in low-grade metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Canyon to epizone-low anchizone conditions near Death Valley. Most of the regional variation in illite parameters occurs between Frenchman Mountain and the southern Nopah Range, corresponding to the transition from cratonic to miogeoclinal facies and also to the leading edge of the Sevier thrust belt. In contrast to this "horizontal" transect that highlights metamorphic reactions, a vertical transect through the Paleozoic stratigraphy of the Grand Canyon targets temporal variations in the flux of detrital muscovite. In the Grand Canyon, illite crystallinity reaches a maximum at the base of the Pennsylvanian Supai Group before decreasing upsection. Deposition of the Supai Group was coeval with formation of basementcored uplifts during the Ancestral Rocky Mountains orogeny, and we suggest that the upsection change in illite composition at the Grand Canyon reflects input of detrital muscovite eroded from these uplifts.
1] The Wasatch fault and adjacent fault zones provide an opportunity to compare present-day defor... more 1] 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 timescaledependent for any given fault system.
1] A dense, continuous GPS network was established in the Yucca Mountain area in 1999 to provide ... more 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 M s 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.
The eastern boundary of the Eastern California shear zone (ECSZ) is defined by the Stateline-Amar... more The eastern boundary of the Eastern California shear zone (ECSZ) is defined by the Stateline-Amargosa Valley fault zone (SAVFZ), a NW striking zone of dextral faulting that extends 150 km along the CA-NV border in Clark and Nye counties. This fault system has accumulated ~ 30 km of slip since 12.5 Ma, yielding a long-term slip rate of ~ 2
A short-baseline network that we have established at Yucca Mountain, southern Nevada, is enabling... more A short-baseline network that we have established at Yucca Mountain, southern Nevada, is enabling us to put new constraints on the level at which individual site motion can be detected by continuous GPS stations. We use this network to investigate and quantify aspects of the GPS error budget at a level of precision that would have been hard to imagine
Understanding the paleoelevation history of orogens is fundamental to answering a number of signi... more Understanding the paleoelevation history of orogens is fundamental to answering a number of significant questions about continental deformation. Most paleoaltimeters rely on proxies for measuring paleoelevation that may be affected by factors other than elevation, such as climate. The basalt vesicle paleoaltimeter avoids problems associated with such proxies by directly measuring the paleo-atmospheric pressure recorded by vesicle size-distributions in basalt
A fundamental issue in continental dynamics is whether deformation within the broad Pacific-North... more A fundamental issue in continental dynamics is whether deformation within the broad Pacific-North America plate boundary in the southwestern United States is better described by the relative motions of rigid microplates or by diffuse crustal flow. Whereas the Colorado Plateau and Sierra Nevada-Great Valley regions appear to be fairly undeformed over geological time scales, crustal extension between these regions in the Basin and Range province based on prevalent Quaternary faulting appears to have been relatively homogeneous. However, historic seismicity, which is largely confined within the relatively narrow Central Nevada Seismic Belt (CNSB) and the Intermountain Seismic Belt (ISB), may suggest that modern deformation of the Basin and Range province is also confined to these province bounding belts and that the central part of the Basin and Range province, the Central Great Basin region, is presently relatively rigid. We use site velocities from several campaign and continuous GPS...
The ongoing Arabian-Eurasian collision provides an opportunity to understand strain localization ... more The ongoing Arabian-Eurasian collision provides an opportunity to understand strain localization within a nascent orogen. The central third of this collision lies between the Black and Caspian Seas, with late Cenozic deformation extending over 700 km across strike, from the Zagros in the south to the Greater Caucasus (GC) in the north. The GC range is flanked to the south
New field observations from the Gonga Shan (7756m) region in eastern Tibet show a spatial associa... more New field observations from the Gonga Shan (7756m) region in eastern Tibet show a spatial association between high topography, intense erosion and exhumation and extensional faulting. The Gonga Shan massif is a region anomalously high topography, approximately 1 - 3 km in excess of the surrounding plateau elevations, and incised by rapid fluvial and glacial erosion. The massif is bounded
The successful quantification of long-term erosion rates underpins out understanding of landscape... more The successful quantification of long-term erosion rates underpins out understanding of landscape formation, the topographic evolution of mountain ranges, and the mass balance within active orogens. The measurement of in situ-produced cosmogenic radionuclides (CRNs) in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide long-term erosion rates over a wide variety of landscapes and erosional
The successful quantification of long-term erosion rates underpins our understanding of landscape... more The successful quantification of long-term erosion rates underpins our understanding of landscape formation, the topographic evolution of mountain ranges, and the mass balance within active orogens. The measurement of in situ produced cosmogenic radionuclides (CRNs) in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide such long-term erosion rates. Although deep-seated bedrock landsliding is an important erosional process in active orogens, its effect on CRN-derived erosion rates is largely unquantified. We present a numerical simulation of cosmogenic nuclide production and distribution in landslide-dominated catchments to address the effect of bedrock landsliding on erosion rates. Results of the simulation indicate that the temporal stability of erosion rates determined from CRN concentrations in sediment decreases with increased ratios of landsliding to bedrock weathering rates within a given catchment area, and that as the frequency of landsliding increases, larger catchment areas must be sampled in order to accurately evaluate long-term erosion rates. In addition, results of this simulation suggest that sediment sampling for CRNs is the appropriate method for determining long-term erosion rates in regions dominated by mass-wasting processes, whereas bedrock surface sampling for CRNs may generally underestimate long-term erosion rates. Response times of CRN concentrations to changes in erosion rate indicate that climatically driven cycles of erosion may be detected, but that complete equilibration of CRN concentrations to new erosional conditions may take tens of thousands of years. Comparison of simulated CRN-derived erosion 0012-821X/$ -see front matter D Earth and Planetary Science Letters 237 www.elsevier.com/locate/epsl rates with a new data set of such rates from the Nepalese Himalaya underscore the conclusions drawn from the model results. D
The Stateline fault zone is the easternmost active dextral structure in the Eastern California Sh... more The Stateline fault zone is the easternmost active dextral structure in the Eastern California Shear Zone and its northern segment lies only ~20 km southwest of the proposed Yucca Mountain nuclear waste repository. This ~150 km long fault zone follows the California-Nevada state line from Amargosa Valley to Ivanpah Valley. The geometry, kinematics, and displacement history of this structure are
ABSTRACT Erosion in the Himalaya is driven largely by a strongly coupled system of extreme climat... more ABSTRACT Erosion in the Himalaya is driven largely by a strongly coupled system of extreme climatic conditions and active tectonic processes. Spatial and temporal variations in erosion rates along strike are presumably controlled by differences in local climate, seismicity, deformation rates, and lithology. Quantifying the contribution of each of these parameters to the erosional budget of the Himalaya, however, is a nontrivial problem. The easternmost portion of the Himalayan arc offers a natural laboratory to explore the role of climatic influence on erosion rates. Deformation and uplift of the Shillong Plateau since ~8 Ma has created an orographic barrier ~400 km long that shields the eastern Himalaya, in Arunachal Pradesh, India, from a significant proportion of the precipitation carried by the South Asian Monsoon. Long-term exhumation rates derived from the Himalaya west and east of this orographic barrier have been shown to differ by a factor of ~2, a difference ascribed to reduced climatic forcing of erosion in the lee of the Shillong Plateau. Here we present apatite (U-Th)/He thermochronology data from modern detrital samples collected from northeast India. Between 18-20 single grain ages from each catchment were analyzed in order to calculate erosion rates on a 106 yr timescale. Recently developed Bayesian techniques for the inverse modeling of detrital data were used to derive time-temperature histories for each sample. Recent erosion rates modeled for a single south-facing catchment on the Shillong Plateau are modest, ~0.25 km Myr-1, and show a clear increase in exhumation rates at ~8 Ma from rates of
X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been ... more X-ray diffraction (XRD) analysis of the illite to muscovite transition in pelitic rocks has been widely used in regional studies of low-grade metamorphism. Variations in detrital muscovite content of sediments are also measurable with XRD and may, in fact, be difficult to distinguish from metamorphic gradients. We utilize field transects to isolate detrital muscovite versus metamorphic reaction components of illite-muscovite variations in Paleozoic rocks of the western United States. One transect focuses on a single stratigraphic interval (Middle Cambrian pelites) and extends from Death Valley in the west to the Grand Canyon in the east. Detrital muscovite content is roughly constant along this 500-km-long transect, so illite-muscovite variations are ascribed to differences in low-grade metamorphism. In terms of both illite crystallinity and polytype composition, there is an overall increase in metamorphic grade to the west along the transect, from the diagenetic metapelitic zone at the Grand Canyon to epizone-low anchizone conditions near Death Valley. Most of the regional variation in illite parameters occurs between Frenchman Mountain and the southern Nopah Range, corresponding to the transition from cratonic to miogeoclinal facies and also to the leading edge of the Sevier thrust belt. In contrast to this "horizontal" transect that highlights metamorphic reactions, a vertical transect through the Paleozoic stratigraphy of the Grand Canyon targets temporal variations in the flux of detrital muscovite. In the Grand Canyon, illite crystallinity reaches a maximum at the base of the Pennsylvanian Supai Group before decreasing upsection. Deposition of the Supai Group was coeval with formation of basementcored uplifts during the Ancestral Rocky Mountains orogeny, and we suggest that the upsection change in illite composition at the Grand Canyon reflects input of detrital muscovite eroded from these uplifts.
1] The Wasatch fault and adjacent fault zones provide an opportunity to compare present-day defor... more 1] 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 timescaledependent for any given fault system.
1] A dense, continuous GPS network was established in the Yucca Mountain area in 1999 to provide ... more 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 M s 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.
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