| A late Pleistocene-Holocene soil chronosequence composed of seven primary members is developed to assess rates of faulting, folding and uplift in the central Ventura Basin. Eight ('14)C dates on charcoal, dendrochronology, and presence of historic debris provide age control on deposits younger than 40,000 ybp. Relative amounts of vertical displacement across active faults, assuming constant slip rates, allows the chronology of older terrace and fan ages to be estimated back to about 200,000 ybp.; Analysis of faulted alluvial fans along the San Cayetano fault indicates that the dip slip rate on the fault increases from 1.05 (+OR-) 0.2 mm/yr at Sisar Canyon eastward to 1.35 (+OR-) 0.4 at Bear Canyon and 2.35 (+OR-) 0.55 at Mud Creek. At Timber Canyon, a minimum slip rate of 3.6 (+OR-) 0.4 is suggested by fanhead segmentation; however, a higher slip rate of 8.7 (+OR-) 1.9 mm/yr is suggested by stratigraphic evidence. Holocene activity is shown by faulted fan alluvium and colluvium at Bear Canyon and Silverthread Oil field respectively, and inferred at Timber Canyon by fanhead segmentation.; Bedding plane faulting in the vicinity of Oak View and Orcutt and Timber Canyons, as well as thrust faulting along the Lion, Big Canyon and Sisar fault systems, is the result of flexural-slip folding of the Santa Clara trough. These faults, with slip rates as high as 1.5 mm/yr, are probably of low seismic hazard because the faults do not extend deeply to rocks of high shear strength.; Two groups of fans in the Ventura Basin, based on relations between drainage basin area (A(,d)) and fan area (A(,f)), are defined by equations: A(,f) = 3.84 A(,d)('0.55); and A(,f) = 0.59 A(,d)('0.8). The first equation with coefficient of 3.84 characterizes fans associated tectonically active mountain fronts coupled with actively folding (tilting) piedmonts.; Mountain front sinuosity (S(,mf)) in the study area varies from 1.01 to 2.73 with the lower sinuosities representing high rates of uplift. Valley width versus valley height ratios (V(,f)) for mountain fronts varies between 0.42 and 1.89. As with S(,mf), V(,f) is lowest for mountain fronts with high rates of uplift.; Decreasing rates of uplift and flank rotation through time on the Ventura Avenue anticline are explained by the mechanics of folding rather than a decrease in the rate of shortening. Assuming constant rates of shortening, relations between uplift, flank dip, rates of flank rotation, rates of uplift and rates of shortening are evaluated mathematically.; The history of folding in Ventura Basin has progressed from north to south with initial uplift in the Santa Ynez and Topatopa Mountains 2.0 to 3.3 mybp followed by folding in the Ojai and Upper Ojai valleys 1.5-2.5 mybp. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI... |
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