Abstract

Structural analysis of late Cenozoic folds along the western and southern margins of the San Joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics. The folds have formed by the processes of fault-bend folding and fault-propagation folding, which commonly occur in fold and thrust belts. Our structural interpretation attributes the seismically active Coalinga and Kettleman Hills North Dome anticlines to fault-bend folding above a thrust fault(s) that steps up from a detachment within the Franciscan Assemblage to a detachment at the base of the Great Valley Group. This thrust does not reach the surface (blind thrust), and its slip is consumed in backthrusting and formation of subsurface folds under the San Joaquin Valley. Movement on the postulated thrust(s) would explain the cause of the May 2, 1983, Coalinga earthquake and the August 15, 1985, Avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes. Well-documented examples of fault-bend and fault-propagation folding also occur at Wheeler Ridge, in the San Emigdio Mountains, and at Kettleman Hills South Dome.

Deformation associated with fold and thrust belts can be extremely complicated. Considerable fault movement can occur without accompanying surface rupture or shallow-level faulting. Conversely, surface rupture can occur that has no direct relationship to fault slip at depth. An example would be flexural-slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth. Consequently, traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts.

Our interpretation of the structural style that developed in central California during the late Cenozoic requires that the strain associated with the transpressive motion between the Pacific and North American plates be resolved into normal and tangential components: thrust faulting and folding normal to the plate boundary and strike-slip faulting parallel to the plate boundary (San Andreas fault). The thrust faults root in a déecollement at the brittle- ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction.

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