FAULTING IN THE LOPEZ POINT-RAGGED POINT SEGMENT OF HIGHWAY 1, SOUTHERN BIG SUR AREA, CALIFORNIA: IMPLICATIONS FOR THE NACIMIENTO FAULT

The Nacimiento Block is located in the Southern Coast Ranges of California, and consists mainly of Franciscan Complex accretionary prism rocks. The block is bounded on the west by the San Gregorio-Hosgri fault zone, and to the east by the Nacimiento Fault. The Nacimiento Fault separates the Nacimiento Block from the Salinian Block, a displaced piece of the Sierra Nevada batholith. The San Gregorio-Hosgri fault zone is an active strand of the San Andreas fault system. The Nacimiento Fault is inactive, and the timing, magnitude, and direction of movement are debated. The Nacimiento fault is of particular interest because it juxtaposes the Franciscan Complex and the Salinian Block, apparently omitting the Great Valley Group and Coast Range Ophiolite of the forearc basin. To address the question of fault kinematics, ten kilometers of outcrop along a 48 kilometer long section of California Highway 1 between Lopez Point and Ragged Point were mapped. Kinematic data was taken on 29 outcrops, totaling 541 minor faults ,407 with slickenlines, and 313 veins traceable for more than one meter. Of the faults, 217 are dip-slip (60-90° rake), 116 are oblique (31-59° rake), and only 84 are strike-slip (0-30° rake). The dominant mode of minor faulting is normal, with 122 faults observed, compared to 26 reverse, 22 left-lateral, and 23 right-lateral strike-slip. Stereonet analysis reveals the normal and reverse faults dip steeply to the southwest and strike northwest-southeast, subparallel to the coast and bounding San Gregorio-Hosgri and Nacimiento faults. There is no dominant orientation to the strike-slip faulting. Faults of all types cut 16 slab-window related felsic dikes of probable Late Oligocene-Early Miocene age. The character of all three types of fault planes is similar, indicating they are coeval. Faults along this section of Highway 1 do not mimick the overall regional right-lateral transform system of the San Andreas. Two possibilities are proposed for minor fault formation: 1) Uplift of the Franciscan Complex was accommodated by normal faulting subparallel to and along the Nacimiento Fault during Late Cenozoic time, with local reverse and strike-slip faulting. 2) All fault types are part of a Reidel shear network associated with the nearby major strike-slip faults.