RISE AND FALL OF THE SOUTHERN SANTA-CRUZ MOUNTAINS, CALIFORNIA, FROM FISSION TRACKS, GEOMORPHOLOGY, AND GEODESY

The Santa Cruz Mountains are closely associated with a left bend along the right-lateral San Andreas fault. The Loma Prieta area on the northeast side of the San Andreas consists of fault-bounded blocks that rise along active, deeply rooted, reverse and oblique-slip faults. Six samples from a transect across this area yield concordant apatite fission track ages averaging 4.6 +/- 0.5 Ma. These ages date the time of cooling below similar to 110 degrees C and suggest that about 3 km of unroofing has occurred over the last 4.6 m.y. Allowing for current elevations of about 1 km, this suggests an average uplift rate of the order of 0.8 mm/yr over the last 4.6 m.y. To further define the extent and distribution of this young uplift, we used morphometric analyses of the youthful topography of the area. Steep drainage slopes and high local relief indicate that the area northeast of the San Andreas forms a well-defined zone of high uplift. In contrast, the region on the southwest side of the San Andreas is characterized by broad upwarping and folding, more subdued topography, old fission track ages, and mean Quaternary uplift rates of 0.1-0.4 mm/yr. Geodetic data (1906 San Francisco earthquake and subsequent strain transients, 20 years of interseismic deformation, 1989 Loma Prieta earthquake, and 2 years of post-Loma Prieta earthquake strain) show that the Southern Santa Cruz Mountains repeatedly rise and subside through a complex sequence of bay area deformation events. An additional deformation element that involves reverse slip averaging 2-3 mm/yr along the Foothills thrust system must occur to explain the longer-term uplift pattern in the Loma Prieta area since the late Pliocene. The asymmetry in deformation on opposite sides of the San Andreas probably reflects the contrasting rock types on opposite sides of the fault, the influence of preexisting structures, and the interaction with neighboring faults of the San Andreas system.