Reappraisal of hypothetical Franciscan thrust wedging at Coalinga: Implications for tectonic relations along the Great Valley flank of the California Coast Ranges

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Abstract
Correlation of the character and orientation of seismic reflectors imaged on profiles across the Coalinga anticline with known surface geology indicates that local Franciscan thrust wedging, favored for past geodynamic interpretations of the 1983 Coalinga earthquake, is an untenable hypothesis. The inference that a subhorizontal detachment surface of simple shear is present beneath the anticline is consequently unnecessary, and unlikely in view of the coherent areal pattern of wrench folding within the southern Diablo Range. Elsewhere along the structural front of the California Coast Ranges at the western flank of the Great Valley, interpretations of similar thrust wedging based in part on analogy with supposed structural relations at Coalinga are equally questionable. In a wider context, the seismogenic behavior of blind thrust and reverse faults in California cannot be interpreted with the invalid concept of thrust wedging at Coalinga as a guiding template for geotectonic interpretations.