Abstract: Late Cenozoic Growth of the Mt. Diablo Fold-and Thrust Belt, Central Contra Costa County, California, and Implications for Transpressional Deformation of the Northern Diablo Range

J. R. Unruh, T. L. Sawyer

Late Cenozoic tectonics of central Contra Costa County are dominated by growth of the west-northwest-trending, south-southwest-vergent Mt. Diablo fold-and-thrust belt. The Mt. Diablo anticline, which is the largest structure in the fold belt, is approximately 25 km wide and attains a maximum structural relief of approximately 7-10 km. The Mt. Diablo fold-and-thrust belt is bounded on the east by the dextral Greenville-Marsh Creek fault system, and on the west by the Calaveras and Concord faults. We propose that northwest dextral slip on the Greenville-Marsh Creek system is transferred primarily to the Concord fault across a 20-km-wide restraining left-stepover. Crustal shortening in the stepover region is accommodated by growth of the Mt. Diablo fold-and-thrust belt. Prel minary structural analysis indicates that the rate of late Cenozoic shortening necessary to generate uplift of the Mt. Diablo anticline (3-5 mm/yr) is comparable to the creep rate on the Concord fault (3±1 mm/yr) and our calculated slip rate on the Greenville fault (3.2-3.8 mm/yr), indicating that northwest dextral slip is conserved by contractional deformation in the stepover region. Additional structural analyses are planned to test this interpretation and develop models for blind thrust faults beneath the Mt. Diablo anticline and related structures.

The west-northwest trend of the Mt. Diablo fold-and-thrust belt and conservation of dextral slip in central Contra Costa County strongly indicate that crustal shortening is driven by distributed northwest dextral shear, consistent with geodetic analyses that show no significant northeast-directed contraction normal to faults of the San Andreas system in the northern Coast Ranges. The <=3 mm/yr northeast-directed shortening normal to the Pacific/North American plate boundary probably is accommodated by growth of northwest-trending folds along the western Great Valley margin. We thus interpret that the northwest-trending Altamont anticline, located east of and parallel to the Greenville fault in eastern Contra Costa County, probably accommodates northeast-directed shortening normal t the plate boundary.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California