Graduate Thesis Or Dissertation
 

Convergence rates across the Ventura Basin, California

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  • Four cross sections are balanced and retrodeformed to 250±50 ka and 975±75 ka to yield crustal shortening amounts and rates for the western Transverse Ranges of California. The cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault (Red Mountain fault) to a blind thrust, to a combination of both a surface reverse fault (San Cayetano fault) and the blind thrust, and to a surface reverse fault (Modelo lobe segment of the San Cayetano fault). Deformation can be separated into three phases: (1) pre- Vaqueros (pre-late Oligocene-early Miocene) tilting in the hanging-wall block of the Oak Ridge fault that is coincident with normal faulting farther south at Big Mountain and to the east in the east Ventura basin, (2) Pliocene reverse faulting and folding, and (3) Quaternary deformation. Crustal shortening rates have increased through time. For the four cross sections, crustal shortening rates were 4±4 mm/y, 5±2 mm/y, 6±5 mm/y, and 14±6 mm/y for the interval between 250 ka and 975 ka. Since 250±50 ka, crustal shortening rates increased to 23±12 mm/y, 29±7 mm/y, 27±11 mm/y, and 25±11 mm/y. But crustal convergence rates determined by Global Positioning System (GPS) surveys, taken over an interval of 2.7 years, indicate a shortening rate of only 7±2 mm/y across the basin. The discrepancy between a rate of strain over a short, recent time period of little seismic activity, and a faster rate determined by the offset of bedrock horizons over several hundred thousand years indicates that much of the present crustal movement is being stored as elastic strain that could result in the release of energy in damaging earthquakes.
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