Kurt L. Frankel

Cosmogenic 10Be and 36Cl geochronology of offset alluvial fans along the northern Death Valley fault zone: Implications for transient strain in the eastern California shear zone

The northern Death Valley fault zone (NDVFZ) has long been recognized as a major right-lateral strike-slip fault in the eastern California shear zone (ECSZ). However, its geologic slip rate has been difficult to determine. Using high-resolution digital topographic imagery and terrestrial cosmogenic nuclide dating, we present the first geochronologically determined slip rate for the NDVFZ. Our study focuses on the Red Wall Canyon alluvial fan, which exposes clean dextral offsets of seven channels. Analysis of airborne laser swath mapping data indicates ∼297 ± 9 m of right-lateral displacement on the fault system since the late Pleistocene. In situ terrestrial cosmogenic ^(10)Be and ^(36)Cl geochronology was used to date the Red Wall Canyon fan and a second, correlative fan also cut by the fault. Beryllium 10 dates from large cobbles and boulders provide a maximum age of 70 +22/−20 ka for the offset landforms. The minimum age of the alluvial fan deposits based on ^(36)Cl depth profiles is 63 ± 8 ka. Combining the offset measurement with the cosmogenic ^(10)Be date yields a geologic fault slip rate of 4.2 +1.9/−1.1 mm yr^(−1), whereas the ^(36)Cl data indicate 4.7 +0.9/−0.6 mm yr^(−1) of slip. Summing these slip rates with known rates on the Owens Valley, Hunter Mountain, and Stateline faults at similar latitudes suggests a total geologic slip rate across the northern ECSZ of ∼8.5 to 10 mm yr^(−1). This rate is commensurate with the overall geodetic rate and implies that the apparent discrepancy between geologic and geodetic data observed in the Mojave section of the ECSZ does not extend north of the Garlock fault. Although the overall geodetic rates are similar, the best estimates based on geology predict higher strain rates in the eastern part of the ECSZ than to the west, whereas the observed geodetic strain is relatively constant.

Incorporating and reporting uncertainties in fault slip rates

[1] Quantitative slip rate estimates are essential to understanding crustal deformation processes and assessing seismic hazard. Computing slip rates requires two fundamental ingredients: estimates of the age of an offset landforrn or deposit and displacement along the fault of interest. Because both of these measures contain uncertainty, slip rates are inherently uncertain. Methods to compute and report slip rates have not been standardized, and therefore slip rate data are presented inconsistently and are frequently ambiguous; in particular, slip rate uncertainty is often insufficiently characterized. We present a rigorous probabilistic approach to computing and reporting intermediate- to long-term fault slip rates; additionally, we have developed freely available software that provides standard age and displacement uncertainty models. We demonstrate the method using recent observations from the Neodani and Death Valley-Fish Lake Valley fault zones, and we compare slip rates determined using this approach with previously published results.

Spatial and temporal variations in denudation of the Wasatch Mountains, Utah, USA

We evaluate spatial and temporal variations in denudation of the north-central Wasatch Mountains, Utah, by determining catchment-wide denudation rates with 10 Be concentrations in alluvial sediment and comparing these rates with previously published data on rock uplift and exhumation of the range. Catchments draining the range front show relatively little variation in denudation rate (0.07–0.17 mm/yr), while steeper (mean hillslope gradient >30°) catchments in the core of the range show larger variation (0.17–0.79 mm/yr). We attribute the larger spatial variation in catchment-wide denudation in the core of the range to landsliding of hillslopes at threshold gradients; faster denudation in this region may signify landscape adjustment to late Pleistocene glaciations. The mean denudation rate for all catchments (0.2 mm/yr) is generally consistent with longer-term exhumation rates derived from thermochronometers and with shorter-term vertical fault displacement rates, suggesting that denudation of the north-central Wasatch has been roughly steady, or decreasing slightly, over the past 5 m.y. Although 10 Be-based catchment-wide denudation rates are sensitive to localized geomorphic processes and events, overall, they appear to refl ect the larger tectonic forces that have driven denudation of the Wasatch Mountains over longer time scales.

Rates of extension along the Fish Lake Valley fault and transtensional deformation in the Eastern California shear zone–Walker Lane belt

The oblique-normal-dextral Fish Lake Valley fault accommodates the majority of Pacific–North America plate-boundary deformation east of the San Andreas fault in the northern part of the Eastern California shear zone. New rates for the extensional component of fault slip, determined with light imaging and detection (LiDAR) topographic data and 10Be geochronology of four offset alluvial fans, indicate a northward increase in extension rate. The surface exposure ages of these fans range from ca. 71 ka at Perry Aiken Creek and Indian Creek to ca. 94 ka and ca. 121 ka at Furnace Creek and Wildhorse Creek, respectively. These ages, combined with the measured vertical components of slip at each site, an assumed 60° fault dip, and a N65°E extension direction, yield calculated late Pleistocene–Holocene horizontal extension rates of 0.1 ± 0.1, 0.3 ± 0.2, 0.7 +0.3/–0.1, and 0.5 +0.2/–0.1 mm/yr at Furnace Creek, Wildhorse Creek, Perry Aiken Creek, and Indian Creek, from south to north, respectively. Comparison of these rates with geodetic measurements of ∼1 mm/yr of N65°E extension across the northern Eastern California shear zone indicates that the Fish Lake Valley fault accommodates approximately half of the current rate of regional extension. When summed with published rates of extension for faults at the same latitude, the Fish Lake Valley fault data indicate that long-term geologic deformation rates are commensurate with short-term geodetic extension rates. The northward increase in Pleistocene extension rates is opposite the northward decrease in dextral slip rate trend along the Fish Lake Valley fault, likely reflecting a diffuse extensional transfer zone in northern Fish Lake Valley that relays slip to the northeast across the Mina Deflection and northward into the Walker Lane belt.

Timing and rates of Holocene normal faulting along the Black Mountains fault zone, Death Valley, USA

Alluvial fans displaced by normal faults of the Black Mountains fault zone at Badwater and Mormon Point in Death Valley were mapped, surveyed, and dated using optically stimulated luminescence (OSL) and 10 Be terrestrial cosmogenic nuclide (TCN) methods. Applying TCN methods to Holocene geomorphic surfaces in Death Valley is challenging because sediment flux is slow and complex. However, OSL dating produces consistent surface ages, yielding ages for a regionally recognized surface (Qg3a) of 4.5 ± 1.2 ka at Badwater and 7.0 ± 1.0 ka at Mormon Point. Holocene faults offsetting Qg3a yield horizontal slip rates directed toward 323° of 0.8 +0.3/–0.2 mm/yr and 1.0 ± 0.2 mm/yr for Badwater and Mormon Point, respectively. These slip rates are slower than the ∼2 mm/yr dextral slip rate of the southern end of the northern Death Valley fault zone and are half as fast as NNW-oriented horizontal rates documented for the Panamint Valley fault zone. This indicates that additional strain is transferred southwestward from northern Death Valley and Black Mountains fault zones onto the oblique-normal dextral faults of the Panamint Valley fault zone, which is consistent with published geodetic modeling showing that current opening rates of central Death Valley along the Black Mountains fault zone are about three times slower than for Panamint Valley. This suggests that less than half of the geodetically determined ∼9–12 mm/yr of right-lateral shear across the region at the latitude of central Death Valley is accommodated by slip on well-defined faults and that distributed deformational processes take up the remainder of this slip transferred between the major faults north of the Garlock fault.

Temporal variations in extension rate on the Lone Mountain fault and strain distribution in the eastern California shear zone–Walker Lane

The eastern California shear zone (ECSZ) and Walker Lane represent an evolving segment of the Pacific–North America plate boundary in the western United States. Understanding temporal variations in strain accumulation and release along plate boundary structures is critical to assessing how deformation is accommodated throughout the lithosphere. Late Pleistocene displacement along the Lone Mountain fault suggests that the Silver Peak–Lone Mountain (SPLM) extensional complex is an important structure in accommodating and transferring strain within the ECSZ and Walker Lane. Using geologic and geomorphic mapping, differential global positioning system surveys, and terrestrial cosmogenic nuclide (TCN) geochronology, we determined rates of extension across the Lone Mountain fault in western Nevada. The Lone Mountain fault displaces the northwestern Lone Mountain and Weepah Hills piedmonts and is the northeastern component of the SPLM extensional complex, a series of down-to-the-northwest normal faults. We mapped seven distinct alluvial fan deposits and dated three of the surfaces using 10 Be TCN geochronology, yielding ages of 16.5 ± 1.2 ka, 92 ± 9 ka, and 137 ± 25 ka for the Q3b, Q2c, and Q2b deposits, respectively. The ages were combined with scarp profile measurements across the displaced fans to obtain minimum rates of extension; the Q2b and Q2c surfaces yield an extension rate between 0.1 ± 0.1 and 0.2 ± 01 mm/yr and the Q3b surface yields a rate of 0.2 ± 0.1–0.4 ± 0.1 mm/yr, depending on the dip of the fault. Active extension on the Lone Mountain fault suggests that it helps partition strain off of the major strike-slip faults in the northern ECSZ and transfers deformation to the east around the Mina deflection and northward into the Walker Lane. Combining our results with estimates from other faults accommodating dextral shear in the northern ECSZ reveals an apparent discrepancy between short- and long-term rates of strain accumulation and release. If strain rates have remained constant since the late Pleistocene, this could reflect transient strain accumulation, similar to the Mojave segment of the ECSZ. However, our data also suggest a potential increase in strain rates between ca. 92 ka and ca. 17 ka, and possibly to present day, which may also help explain the mismatch between long- and short-term rates of deformation in the region.

Terrestrial Cosmogenic Nuclide Geochronology Data Reporting Standards Needed

Scientists can estimate the time at which rocks at Earths surface became exposed (through glacial scour, faulting, sediment deposition, exhumation, etc.) in a given area using terrestrial cosmogenic nuclide (TCN) geochronology. The idea behind this technique is ingenious in its simplicity. Imagine, for example, a glacier advancing over the landscape and scouring the rocks underneath. As the glacier retreats, fresh rock surfaces become exposed to the atmosphere. Galactic cosmic rays then bombard the fresh minerals exposed at the Earths surface, producing rare nuclides such as beryllium-10 (10Be) and aluminum-26 (26Al) in the process. Thus, measuring the concentration of TCNs in rocks at the Earths surface allows scientists to estimate how long a surface has been exposed and/or the rate of surface denudation.

Latest Pleistocene and Holocene slip rates on the Lone Mountain fault: Evidence for accelerating slip in the Silver Peak‐Lone Mountain extensional complex

Determining the constancy of fault slip rates over time is critical in characterizing strain distribution across plate boundaries such as the Pacific-North American plate boundary in the western U.S. We present results from the Lone Mountain fault, a normal fault within the southern Walker Lane, that suggest slip rates there may have increased approximately twofold since the late Pleistocene. We combine detailed field surficial mapping, topographic surveying, and 10Be cosmogenic nuclide exposure ages to calculate new late Pleistocene and Holocene slip rates on the Lone Mountain fault. Alluvial fans with ages of 14.6 ± 1.4 ka and 8.0 ± 0.9 ka are vertically offset 10.2 ± 0.6 m and 4.7 ± 0.6 m, respectively, yielding vertical slip rates of 0.7 ± 0.1 mm/yr and 0.6 ± 0.1 mm/yr. These slip rates are faster than the rates of 0.1 to 0.4 mm/yr from earlier in the Pleistocene, defining a pattern of accelerating slip on the Lone Mountain fault over a timescale of 104 years. The possibility of accelerating slip rates in parts of the Walker Lane partially reconciles the observed discrepancy between long- and short-term slip rates in this region and elucidates the distribution of strain across an evolving plate boundary.