MODE OF DEFORMATION ASSOCIATED WITH RISE OF THE SANTA LUCIA RANGE, AS CONSTRAINED BY NEW THERMOCHRONOLOGIC DATA AND THERMOKINEMATIC MODELIN

The youthful topography of the Santa Lucia Range, California is attributed to oblique convergence across the Pacific-North American transform boundary, starting at about 6 Ma. This conclusion is well supported by geologic and geodetic studies, but the mode of deformation remains poorly understood. We use new (U-Th)/He ages for zircon (n=34) and apatite (n=39), along with thermo-kinematic modeling to test models for the mode of deformation of the Santa Lucia Range. The Point Sur coast southwest of the San Gregorio - Hosgri fault is distinguished by a relatively old HeA cooling age (~15 Ma) and a wide continental shelf to the west, indicating slow uplift and erosion at the coast. Uplift and erosion increase dramatically just inland of the coast, as indicated by young HeA ages (3-5 Ma) at the western foot of the range. The HeA and HeZ ages increase in age with elevation within relief transects on the west side of the range. Equal elevation samples tend to young towards the SW. A best-fit solution for HeA cooling ages south of Point Sur indicates a well-defined set of isochrone surfaces dipping at ~8 degrees to the northeast. Paleogene and Neogene sediments exposed across the southerly portion of the range exhibit similar NE dips.

We consider three structural models to account for these observation: 1) an oblique vise model (McKenzie and England, 1983), 2) a convergent flower structure (Harding, 1985), and 3) an oblique ramp-flat thrust fault. Thermokinematic models that produce cooling age patterns based on prescribed velocity and thermal fields will be generated for each of these three test cases. Preliminary results show that the first two models predict SW- dipping isochrones, which is inconsistent with our isochrone fitting results and the geology. Our tentative conclusion is that the pattern of cooling ages is best fit by a moderately NE dipping ramp-flat structure surfacing at the age break along the Point Sur coast. We draw parallels to Alpine fault of New Zealand, which is a oblique strike-slip fault with a ramp-fault geometry (Little et al., 2002). Notably, the ramp for the the Alpine fault has a dip of ~40 degrees, which is contrary to the expectation that faults within a strike-slip system are generally steep.