Holocene Slip Rate Constraints for the Northern Greenville Fault, Eastern San Francisco Bay Area, California: Implications for the Mt. Diablo Restraining Stepover Model
Abstract
We conducted a preliminary paleoseismic investigation of the dextral Greenville fault at a site near Laughlin Road in northeastern Livermore Valley to evaluate the Holocene slip rate on the fault, and to test a kinematic model for restraining transfer of slip from the Greenville fault to the Concord fault across the late Cenozoic Mt. Diablo fold-and-thrust belt (MDFTB). The Greenville fault is expressed by a west-facing scarplet that extends across a laterally offset alluvial fan at the site. Two trenches excavated parallel to and on opposite sides of the fault exposed a correlative sequence of well-defined and laterally restricted channel deposits offset by right-lateral displacement. These trenches were connected with two fault-normal trenches that documented the location of the Greenville fault and provided evidence of surface faulting from the 1980 Livermore earthquake sequence (M<5.9), which reportedly produced minor but continuous surface rupture through the site. A secondary fault trace, 30 m to the east of the `main' trace, evidently did not rupture during the 1980 earthquake sequence, and was not investigated in this study. Alluvial-fan deposits in the fault-normal trenches were extensively disrupted by a zone of near-vertical to steeply west-dipping faults, some exhibiting sub-horizontal slickensides on clay gouge. Several fractures extend upwards from the fault zone to near the modern ground surface that we interpret to be related to the 1980 earthquake sequence. Two channel-fill units and the margin of a large paleo-channel exposed in the fault-parallel trenches are right-laterally offset 17 to 25 m by the Greenville fault, based on detailed trench logging, total-station surveying, and comparisons of channel morphology, stratigraphy and soil relationships. The geomorphic offset of the alluvial fan apex is comparable, about 25 m, based on analysis of a 2-foot contour map. Available age constraints for these offset features currently are limited to 6 AMS radiocarbon dates on pedogenic calcium carbonate, that range from 4.1 to 8.5 ka. We assume that these are minimum-limiting ages for the offset deposits because we know of no detrital or aeolian sources of calcium carbonate in the region. These data provide a preliminary estimate of the Holocene right-lateral slip rate of 4.1 + 1.8 mm/yr, or less, for the main trace of the northern Greenville fault. Coupled with data on the Holocene slip rate and secular creep rate of the Concord fault (about 3 mm/yr; Borchardt et al., 1999; Galehouse, 1992), our preliminary results support the hypothesis that Mt. Diablo is an active fault-propagation fold formed in a restraining stepover between the Greenville and Concord strike-slip faults (Unruh and Sawyer, 1995). Tectonic-geomorphic studies indicate that late Quaternary uplift and deformation rates of the Mt. Diablo anticline and associated surface folds are on the order of about 0.6 mm/yr to several mm/yr (Sawyer, 1999), consistent with accommodation of about 4 mm/yr slip from the Greenville fault by growth of contractional structures in the MDFTB.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.T62F..03S
- Keywords:
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- 1734 Seismology;
- 1744 Tectonophysics;
- 7221 Paleoseismology;
- 7230 Seismicity and seismotectonics;
- 8010 Fractures and faults