Geologic map and structural development of the northernmost Sur-Nacimiento fault zone, Central California coast

The Sur-Nacimiento fault exposed along the central California coast (United States) juxtaposes the Salinian block arc against the Nacimiento block accretionary complex, cuts out the majority of the forearc basin and western arc, and requires a minimum of 150 km of orogen-normal 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 Oligocene and younger 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 melange, and have been used to support multiple kinematic models for Late Cretaceous-Eocene 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 melange along several steeply NE-dipping strands of the fault. Between these strands, bedding-parallel gouge zones as much as 2 m wide dip 50°-70°NE and display P-Y fabrics and asymmetric blocks indicating dextral displacement. Kinematic analysis of 401 individual outcrop-scale brittle faults and Y-plane surfaces record dominantly NW-SE extension and NE-SW shortening oblique to the strike of the Sur-Nacimiento fault. At McWay Falls, shear-sense indicators in mylonitic calcite marble found along the McWay fault yield top-South 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 pre-Miocene N-S shortening or dextral transpression adjacent to the Sur-Nacimiento fault, followed by 8-11 km of Neogene dextral slip along the Gamboa fault that reactivated preexisting NW-SE-striking structures along this segment of the Sur-Nacimiento fault. This study highlights the multiple episodes of deformation along the Sur-Nacimiento fault that obscure the fault's early slip evolution with respect to the juxtaposition of the Salinia