LONG-TERM OFFSET HISTORY OF THE CENTRAL SAN ANDREAS FAULT SYSTEM: NEW INSIGHTS INTO OLD QUESTIONS USING SEDIMENTARY SYSTEMS ANALYSIS (Invited Presentation)

The long-term offset history of strike-slip faults is critical for developing tectonic models and understanding geohazards. The identification of multiple pre- and syn-kinematic piercing points, offset geologic features that can be correlated across the trace of an evolving fault, provides the basis for constructing age-displacement and slip rate curves that inform tectonic and hazards models. However, the resolving power of piercing points is subject to uncertainties related to the uniqueness of the offset feature and to the temporal and spatial uncertainty of the correlation.

Since recognition of large-magnitude offset across strike-slip faults of the San Andreas system in the 1950’s, multiple generations of workers identified piercing points and constrained offset histories of the numerous strands of this complex fault system. We focus on two outstanding problems that bear on the history of strike-slip motion along the Pacific-North American plate boundary. First, restoration of 315 km of Neogene right-slip on the central San Andreas fault yields a residual latest Cretaceous-Paleogene offset of 50-75km; the timing and mechanism for this early phase of motion remain poorly resolved. Second, existing tectonic models propose 80-100 km offset across the southern segment of the San Gregorio-Hosgri fault, a conclusion that is not supported by the structural evolution of the adjacent Santa Maria basin.

This talk will detail recent analysis of Eocene and Miocene sedimentary systems in the California Coast Ranges and suggest that two critical, previously accepted piercing points are not reliable, reconciling these two key problems. First, detrital zircon geochronology and subsurface mapping suggest that the Eocene Point of Rocks Sandstone is not correlative with the Butano Sandstone. Second, stratigraphic observations and detrital zircon geochronology of Miocene sediments near Point Sal and Point Piedras Blancas suggest these basins contain few characteristics that would allow them to be confidently correlated. Together, these findings help to clarify the tectonic history of the central San Andreas fault system, including the timing and mechanism of its initiation and the way in which it accommodated vertical axis rotation of the western Transverse Ranges.