Abstract: Coalinga Thrust Wedging Reconsidered

DICKINSON, WILLIAM R.

Interpretations of thrust wedging at depth beneath the Coalinga Anticline rely heavily upon questionable identification of a thick interval of coherent subsurface reflections as part of the Franciscan assemblage (subduction complex) rather than Great Valley Group (forearc basin). Analogous reflection profiles of modern subduction systems consistently display subduction complex either as transparent or as marked by irregular discontinuous reflections. Alternate identification of the key interval of parallel reflections as lower Great Valley instead of Franciscan results in an estimate of Great Valley subsurface thickness comparable to mapped Great Valley successions exposed on nearby Joaquin Ridge and Avenal Ridge. This alternate view is not precluded by interval seismic velocities and permits provisional correlation of selected prominent reflections with major Great Valley sandstone units observed in measured outcrop sections. By contrast, hypothetical insertion of a Franciscan thrust wedge splitting Great Valley apart in the subsurface affords no satisfactory means to account for the full observed Great Valley thickness at the surface. Abandoning the dubious concept of Franciscan thrust wedging at Coalinga challenges the general validity of Franciscan backthrusting coeval with Great Valley forearc deposition elsewhere. Incorporation of steeply dipping Tertiary strata into the limbs of Franciscan-cored folds along the flank of the Great Valley implies that the structures are dominantly of Neogene age. Widespread fork structures within Great Valley strata on reflection profiles are better interpreted as deformed onlap unconformities than as thrust tips. For public geoscience policy, less is evidently known about the 1983 Coalinga earthquake than has been thought.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California