GEOLOGIC MAP AND STRUCTURAL GEOLOGY OF THE SUR–NACIMIENTO FAULT ZONE, MCWAY FALLS TO GAMBOA POINT, CENTRAL CALIFORNIA COAST

The Sur–Nacimiento fault juxtaposes the Salinian block arc terrane against Nacimiento block accretionary mélange and requires a minimum of 150 km of forearc crustal excision within the Mesozoic California convergent margin. Despite this significant strain, the kinematic evolution of the Sur–Nacimiento fault remains poorly understood with diverse hypotheses suggesting sinistral, dextral, thrust, or normal displacement along the fault. This Late Cretaceous–Paleogene strain history is complicated by the location of the fault within a belt of subparallel faults that have accommodated significant Neogene dextral displacement between the Pacific and North American plates. In the vicinity of Big Creek along the Big Sur coast, steeply-bounded bedrock enclaves of Salinian block affinity are enclosed within Nacimiento block mélange, and have been used to support multiple kinematic models for Sur–Nacimiento slip.

The work presented here targets coastal outcrops from McWay Falls to Gamboa Point where our new mapping documents Salinian enclaves within Franciscan mélange along several steeply NE-dipping strands of the fault. Between these strands, bedding-parallel gouge zones up to 2 m wide dip 50−70º NE and display S–C fabrics and asymmetric blocks indicating dextral displacement. Kinematic analysis of over 200 individual outcrop-scale brittle fault surfaces record dominantly NW−SE extension and NE−SW shortening oblique to the strike of the Sur−Nacimiento fault. At McWay Falls, mylonitic calcite marble found along the McWay fault yields top−S thrust displacement of Salinian basement over Salinian sedimentary rocks. South of the McWay fault, Salinian sedimentary rocks are overturned adjacent to, and within strands of the Sur−Nacimiento fault, and display a subvertical E−W striking disjunctive cleavage. These results are consistent with Neogene dextral displacement along this segment of the Sur–Nacimiento fault through either progressive transpressional wrenching or reactivation of pre-existing NE–SW-striking structures, and which obscures fault’s Late Cretaceous slip history. Recent dextral displacement along the Sur–Nacimiento fault and other faults with the Nacimiento block may partially accommodate differential displacement along San Gregorio–Hosgri fault.