We conducted five traverses across strike of more than 14 km of Cretaceous through Quaternary section in the Ventura basin to assess the contribution of aseismic deformation to regional shortening in southern California during the last 5 Ma. Recent studies (e.g., Dolan et al., 1995) show that deformation rates determined by geologic and geodetic studies are far in excess of that accounted for by historical seismicity and conclude that a deficit of moderate and/or large earthquakes exists in southern California. These authors discount the possibility that aseismic deformation may contribute to regional strain because of the absence of microseismicity on thrust ramps and the lack of surficial disruption. Many studies, however, show that earthquakes account for only 10%-70% of the measured geologic deformation in orogenic belts (e.g., Ekstrom and England, 1989). Therefore, aseismic deformation mechanisms, such as pressure solution, must be considered when forecasting earthquakes from deficits in the seismic budget. Deformation by pressure solution at depth would not produce microseismicity, nor would it necessarily cause surface disruption.
The seismogenic zone is believed to be dominated by brittle deformation mechanisms; however, hypocenters for moderate to large earthquakes occur at depths where ductile deformation mechanisms operate. Dolan et al. (1995) showed that potential hypocentral depths for large-magnitude earthquakes in southern California are between 15 and 20 km. For a modest geothermal gradient of 20 °C/km, hypocentral depths of 10-20 km correspond to ambient temperatures between 200 and 400 °C. These temperatures are well within the thermal range of several aseismic deformation mechanisms including pressure solution (e.g., Bailey et al., 1994), reaction softening (e.g., Evans, 1988), and crystal-plastic deformation (e.g., Hirth and Tullis, 1992). Pressure solution has been documented in rocks deformed at temperatures below 100 °C (e.g., Alvarez et al., 1978) and is clearly the dominant process in the production of rock cleavage in orogenic belts (T = 250-350 °C).
Based on experimental work, theoretical considerations, and fabrics preserved in exhumed parts of ancient contractional orogenic belts, we suggest that nonbrittle deformation mechanisms may contribute to the observed shortening. We suggest that aseismic strain may occur either interseismically or postseismically and may contribute substantially to regional deformation. Postseismic deformation has been documented following the 1992 Landers and the 1994 Northridge earthquakes (Massonnett et al., 1996; Peltzer et al., 1996; Bock, et al., 1997) and could reflect accelerated rates of pressure solution due to microfracturing and pore fluid flow. In addition, the 11+3 mm yr-1 north-south shortening rate determined geodetically by Donnellan et al. (1993) occurred in the absence of moderate to large earthquakes. This shortening may be a manifestation of aseismic deformation.
In five cross-strike transects , we examined intervals over 14 km of stratigraphic section and studied more than 80 oriented rock specimens for microstructural analysis. The entire 14-km-thick section from the Cretaceous Jalama Formation through the Pliocene Pico Formation shows unequivocal evidence for pressure solution at both the mesoscopic and microscopic scales. Mesoscopic structures indicative of pressure solution include bedding-normal spaced cleavage and interpenetration features. Microscopic structures indicative of pressure solution include selvages of insoluble residues that define cleavage domains, grain-shape fabric produced by preferential dissolution of grains normal to the shortening direction, and grain impingements that indicate dissolution along grain boundaries. Although a well-constrained estimate for the total pressure solution strain across the basin will require more detailed studies that specifically address the spatial variability in pressure solution activity, a rough estimate can be made based on our preliminary grain-shape analysis. The observed elongation of pressure solved grains is consistent with 10-20 % tectonic shortening based on comparison with Elliot's (1970) shape factor plot.
To determine the prefolding orientation of cleavage, bedding was restored to horizontal for five data sets of Eocene through Pliocene rocks (Fig. 4). Four of the data sets, the Oligocene through Pliocene Sespe, Monterey, Towsley, and Pico Formations, indicate a restored cleavage orientation of nearly east-west and subvertical, consistent with regional north-south shortening. Data from Eocene rocks suggest a northeast-southwest shortening direction, which is broadly compatible with north-south shortening. Deviation in the Eocene rocks from ideal north-south shortening fabrics may reflect modest pre-Miocene deformation in the area or block rotation about vertical axes (e.g., Nicholson et al., 1994).
Our field and microstructural observations show that pressure solution may have contributed significantly to permanent strain in the Ventura basin. This mechanism may occur either interseismically, postseismically, or both. Failure to consider the contribution of aseismic deformation mechanisms to bulk strain may lead to an overestimate of seismic risk.
Figure Captions
Figure 1. Map of western Transverse Ranges, California, showing major faults and physiographic features. Locations of sample traverses are shown by heavy black lines.
Figure 2. Equal-area, lower-hemisphere projection with Kamb contour of poles to pressure solution cleavage planes (bedding restored to horizontal). Contour interval is 2 s. Large circle represents best-fit pole to all data and great circle represents best-fit cleavage plane to all data. Small circles represent best-fit poles to cleavage planes for individual formations. Orientations are consistent with regional north-south shortening.
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