Publication Date:
2014-04-05
Description:
The dextral-slip Mohawk Valley fault zone (MVFZ) strikes northwestward along the eastern margin of the Sierra Nevada and is the westernmost member of a network of active right-lateral strike-slip faults in the northern Walker Lane. Recent geodetic block modeling indicates that the MVFZ may accommodate as much as ~3 mm/yr of regional dextral strain, implying that it is the highest slip-rate strike-slip fault in the region; however, only limited geologic data are available to constrain the system's slip rate and earthquake history. We mapped a complex, anastomosing network of MVFZ strands using high-resolution, airborne lidar data and field observations, and identified a site near Sulphur Creek for paleoseismic investigation. At this site, oblique dextral-normal faulting on the steep valley margin has created a closed depression that floods annually during spring snowmelt to form an ephemeral, shallow pond. We excavated three fault-perpendicular trenches at the site and exposed fine-grained, pond sediment that interfingers with multiple colluvial packages eroded from the scarp that bounds the eastern side of the pond. We interpret the colluvial packages, as well as other stratigraphic and structural relationships as evidence of four surface-rupturing earthquakes (E1-E4) on this strand of the MVFZ. OxCal modeling of radiocarbon and luminescence ages indicate these earthquakes occurred at: E4 = 14.0 ± 1.0 ka, E3 = 12.8 ± 1.4 ka, E2 = 5.7 ± 3.0 ka, and E1 = 1.9 ± 0.1 ka (2 sigma). These times yield a closed mean recurrence time of 4.0 ± 3.0 kyr. The mean 4.0 kyr recurrence interval is inconsistent with slip rates of ~3 mm/yr derived from geodetic block models; these relatively high rates imply surface ruptures of more than 10 m per event, which is geologically implausible for the subdued geomorphic expression and 60 km length of the MVFZ. We propose that unidentified structures that are not yet incorporated into geodetic models may accommodate a significant amount of regional dextral shear across the northern Walker Lane, highlighting the role of distributed deformation in this region.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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