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Normal faulting at convergent plate boundaries: Mylonitic extensional fabrics in the Franciscan subduction complex in Del Puerto Canyon, California, revisited

Using a strain and rotation analysis we tested the hypotheses that top‐east mylonitic extensional structures in the uppermost Franciscan subduction complex in Del Puerto Canyon, California, accomplished exhumation of the Franciscan blueschists. We found no evidence of strongly noncoaxial deformation, instead our data indicate overall coaxial deformation in the proposed zone of mylonitic extensional deformation. There are no extensional strains, moderate vertical shortening occurred without horizontal extension and was compensated by modest deformation‐related volume loss. There is also no strain gradient toward and within the proposed shear zone. Therefore the results of our work indicate that mylonitic extensional structures in Del Puerto Canyon do not exist. Overall, we conclude that there is no currently identifiable structure adjacent to or within the Franciscan in California that could have accommodated large‐scale horizontal extension. Exhumation of the Franciscan blueschists was mainly achieved by erosion of an emergent forearc high. Our strain results and published data are also inconsistent with the development of a supercritically tapered Franciscan wedge that ultimately should have triggered normal faulting. Strain magnitudes are low despite the relatively long residence time of the rocks in the accretionary wedge. From this we infer that strain rates were low and that the wedge material flowed with a relatively high viscosity. Therefore ductile flow probably was not fast enough to form a supercritically tapered Franciscan wedge.