Kinematic analysis of outcrop-scale structures, southern Big Sur segment of Highway 1, Monterey and San Luis Obispo Counties, California




Trasko, Keith Patrick

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The Nacimiento Block is located in the Southern Coast Ranges of California, and consists mainly of Franciscan Complex accretionary prism rocks. It is cross-cut by the San Gregorio-Hosgri Fault Zone, a major right-lateral strand of the San Andreas Fault System. The Nacimiento Block is bounded on the east by the Nacimiento Fault, of debated timing and kinematics, which separates it from the Salinian Block. The Salinian Block is a piece of the Sierra Nevada Batholith, and both the Salinian and Nacimiento Blocks have been displaced from southern California by right-lateral slip on the San Andreas Fault System. To address the question of fault kinematics, a 48 kilometer long section of the Nacimiento Block was examined along California Highway 1 between Lopez Point and Ragged Point. Exposure occurs along approximately 20 kilometers of the transect, and landsliding obscures approximately half of the exposure. The remaining 10 kilometers of outcrop were mapped. Kinematic data were taken on 29 outcrops, totaling 542 minor faults, 406 with slickenlines and 258 with sense of slip indicators, along with 314 veins. Of the faults, 202 are dip-slip (60-90° rake), 113 are oblique-slip (31-59° rake), and only 91 are strike-slip (0-30° rake). The dominant mode of minor faulting is normal, with 111 faults observed, compared to 25 reverse, 24 left-lateral, and 28 right-lateral strike-slip. Two sets of vein and one set of dike orientations were measured. Stereographic analysis reveals the normal and reverse faults dip steeply to the southwest and strike northwest-southeast, subparallel to the coast and San Gregorio-Hosgri and Nacimiento Faults. There is no dominant orientation to the strike-slip faults. Faults of all types cut 17 slab-window related andesitic dikes, which are likely Early Miocene in age according to apatite and zircon fission track ages. The character of all fault planes is similar, indicating they are coeval. Three stages of deformation are recognized. Subduction generated mélange, the dominant lithology in this area, and "broken formations". A second stage of deformation is recorded in the emplacement of dikes and one set of veins. A third stage of deformation is recorded in the minor planar faults that were measured in this study. It is proposed that this latest phase of deformation is caused by the gravitational collapse of the western edge of the Santa Lucia Range. The normal faults parallel the coastline and local slope angles are up to 40°. Coeval strike-slip associated with the San Gregorio-Hosgri Fault Zone is superimposed on this deformation. Apatite fission track ages (n=3) indicate that the dikes mapped along Highway 1 cooled to 110°C at approximately 11 Ma. This indicates an unroofing rate on the order of 300 m/my. This anomalously fast unroofing is accomplished by side-inwards gravitational collapse and erosion


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