QUANTITATIVE FOUR-DIMENSIONAL RECONSTRUCTION OF THE CENTRAL SAN ANDREAS FAULT AND ADJACENT CRUST, NORTHERN CALIFORNIA: PROOF OF CONCEPT ALONG THE SANTA CRUZ MOUNTAINS SEGMENT

A preliminary four-dimensional (4D) model depicting the evolution of a folded unconformity along the San Andreas Fault (SAF) provides ‘proof of concept' for building a 4D model of the central SAF (Transverse Ranges-Pt Arena) and adjacent crust through San Andreas time. The unconformity (base of Pliocene Purisima Fm) now is complexly folded into a structural depression (6 x 15 km, 4-5 km deep beneath low hills) adjacent to SAF west of Hollister, CA. These strata were deposited near sea level across the active SAF starting 5.3 Ma and ~75 km NW of their present position (relative to SW side of SAF). The eastside deposits thickened to ~2.5 km and moved relatively SE along the SAF, through the NW ~45 kms of the Santa Cruz Mountains restraining bend.

Present geometry of the unconformity was determined by direct inversion of a gravity low caused by the low-density Purisima strata, constrained by surface geology, wildcat oil wells, seismicity, and tomographic wavespeed models. The inferred structural depression has a complex bottom and is bounded laterally by the SW-dipping SAF, a basement ridge (partly buried) E of the Sargent FZ , and the Calaveras F. East of the ridge, Cenozoic cover is <~2 km thick and the basement surface smooth.

The present unconformity is represented digitally by scattered data points through which a smooth surface is passed. Earlier positions and geometries of the unconformity are defined by specifying trajectories separately for each point (since 5.3 Ma) based on geologic data and reasoning with simplifying assumptions: • Unconformity originally flat and at sea level • Purisima FM overlies homogeneous Mesozoic Franciscan basement • Restraining bend in SAF fixed relative to the SW side for past 5.3 Ma • Folding began when unconformity encountered restraining bend and developed linearly • SAF offset rate constant for past 5.3 Ma. Smooth surfaces are passed through scattered data points extracted from the trajectories appropriate for specific times.

The method can readily accommodate complex time and space ‘histories' for any number of control points. Even this simple model provides intriguing geologic insights. The width of the structural depression, for example, is similar to the amplitude of the restraining bend, as if the edge of the NE block was crumpled just enough to accommodate the ‘space problem'.