Fault slip rates, constancy of seismic strain release, and landscape evolution in the eastern California shear zone.

摘要

The constancy of strain accumulation and release in time and space is one of the most fundamental issues in tectonics. Models of geodetic data suggest the Death Valley-Fish Lake Valley fault zone (DV-FLVFZ) is storing most of the Pacific-North American plate boundary strain in the northern eastern California shear zone (ECSZ). However, the scarcity of geochronologically constrained slip rates on the DV-FLVFZ has made it difficult to determine whether strain storage and release are constant in this region. I used airborne laser swath mapping (ALSM) digital topographic data to restore offset alluvial fans to their pre-faulting positions, combined with cosmogenic 10Be and 36Cl geochronology to determine slip rates along the DV-FLVFZ.; Offset measurements combined with cosmogenic nuclide geochronology yields a slip rate of ∼4.5 mm/yr for the DV-FLVFZ in northern Death Valley. Summing this rate with known rates on the major faults at similar latitudes suggests a late-Pleistocene geologic slip budget across the northern ECSZ of 8.5 to 10 mm/yr. This rate agrees with the geodetic rate and implies the strain transient in the southern ECSZ does not extend away from the structurally complex zone near the Big Bend of the San Andreas fault.; Combining offset measurements for two alluvial fans in Fish Lake Valley with 10Be ages yields late Pleistocene rates of ∼2.5 mm/yr and ∼3 mm/yr for the northern DV-FLVFZ. These rates are slower than those determined for the system in Death Valley, indicating rates decrease northward. These data suggest that at ∼37.5°N latitude, significant deformation must be accommodated on structures east of Fish Lake Valley, or strain accumulation and release rates have not remained constant through time.; Furthermore, comparison of surface roughness values derived from the ALSM data shows that eight mapped alluvial fans are statistically unique at the 99% confidence level. The roughness metric indicates that fans become smoother from the active channel to a surface dated at 70 ka. Beyond 70 ky, alluvial landforms become rougher with age, suggesting that fans in arid regions smooth out with time until a threshold is crossed where roughness increases with age due to headward tributary incision.