Implications of new fault slip rates and paleoseismologic data for constancy of seismic strain release and seismic clustering in the eastern California shear zone.

摘要

The spatial and temporal strain accumulation and release patterns of faults remain an enigma, which has received an enormous amount of attention from geologists. Although the faults of the Eastern California shear zone (ECSZ), including the Garlock fault, are some of the most studied in the world, we still have only limited understanding of their role in the Pacific-North America plate boundary deformation. Geodetic models suggest that the right-lateral northwest-southeast striking ECSZ is the main fault system accumulating strain east of the San Andreas fault, while the left-lateral almost east-west striking Garlock fault has low strain accumulation rates. More geochronologically constrained slip rates are needed from the faults of the ECSZ and Garlock fault in order to determine whether strain storage and release are constant in this region. As part of this dissertation, I focused on several locations along the Garlock fault in southern California, and the Fish Lake Valley Fault (FLVF) in the northern part of the ECSZ, where I used Light Detection and Ranging (LiDAR) digital topographic data to measure normal fault scarps and restore offset alluvial fans to their pre-faulting positions. Combining those restorations with cosmogenic 10Be geochronology of the offset deposits, I was able to determine slip rates along the FLVF and Garlock fault systems.;Besides the major right-lateral component of slip, the Fish Lake Valley fault also exhibits a normal slip component. Offset scarp measurements combined with cosmogenic nuclide geochronology from four sites yield late a Pleistocene-Holocene horizontal extension slip rate that ranges from 0.1 +/- 0.1 mm/yr to 0.7 +0.3/-0.1 mm/yr. Comparison of this slip rate with geodetic measurements of ∼1 mm/yr of extension across the northern ECSZ indicates that the FLVF accommodates approximately half of the current rate of regional extension. When summed with published rates of extension for faults at the same latitude, the FLVF data indicate that long-term geologic deformation rates are commensurate with short-term geodetic extension rates.;Combining an offset measurement of an incised channel across the central Garlock fault with a modeled 10Be surface age, yield a late Pleistocene-Holocene slip rate of 5.3 +1.0/-2.0 mm/yr, and a maximum rate of ∼6.6 +/- 1.2 mm/yr. This rate is similar to previously published geologic slip rates to the west and east of the study site, but it is at least twice as fast compared to reported geodetic strain accumulation rates along the Garlock fault. The similarities, in terms of channel offsets and ages of the incised surfaces between multiple sites along the fault, are indicative of climatically controlled aggradational and degradational events throughout time that are regional in extent. In addition, these data suggest that, as proposed before, a strain transient could be present along the Garlock fault and strain accumulation and release rates have not remained constant through time.;Such a strain transient has also been proposed for the ECSZ south of the Garlock fault. As part of this dissertation, I also studied the paleoseismic record of the Calico fault, the fastest slipping fault in the southern ECSZ. Evidence of surface ruptures between 0.6-2-0 ka, 5.-5.6 ka, 5.6-6.1 (or possibly 7.3) ka, and 6.1 (or 7.3) to 8.4 ka along the Calico fault, coincide with similar clusters elsewhere in southern ECSZ. These data strongly reinforce earlier suggestions that earthquake recurrence in the ECSZ is highly clustered in time and space.