MULTI-PHASE DEFORMATION IN FRANCISCAN COMPLEX MÉLANGE, SAN SIMEON, CALIFORNIA

Franciscan Complex mélange exposed in seacliff exposures north of San Simeon, California show multiple episodes of deformation and may have implications for the history of nearby, regionally significant faults. Franciscan Complex mélange was formed as part of the California convergent margin subduction complex. Previous researchers working in the area have suggested the observed deformation formed during this accretion-related phase of burial and exhumation. Other researchers have suggested that the mélange in this area was reactivated during subsequent tectonism along the latest Cretaceous–Paleocene Sur–Nacimiento fault, or the Neogene San Gregorio–Hosgri fault. In order to better understand the evolution of strain in the Franciscan Complex mélange in this area, we made a detailed map of a 0.5 km stretch of coastline noting exotic blocks, mélange foliation, high-strain zones, and their relationships with discrete brittle faults.

Along this stretch of coast, foliations in the mélange fabric generally strike NW and dip moderately to the NE. In several locations, high strain zones in the mélange display abundant asymmetric shear fabrics. Measured brittle faults that cut blocks alone do not have any systematic orientation. In contrast, brittle faults that cross-cut the mélange and their enclosed blocks are more systematic with conjugate sets that strike NE–SW with moderate to steep dips. We have also identified multiple generations of calcite veins that are variably concordant with, or cut the mélange foliation and its enclosed sandstone blocks.

Our initial mapping results indicate that the mélange foliation is post-dated by at least one generation of brittle faults, and suggest that multiple episodes of deformation occurred in this area. Future work on this project will utilize calcite vein U–Pb geochronology to attempt to place new timing constraints on the different generations of displacement.