Unraveling the History of the Peninsula Segment of the San Andreas Fault Using a 4-D Geologic Map

We use a three-dimensional geologic model to explore the tectonic evolution of the Peninsula segment of the San Andreas fault in the San Francisco Bay area. We effectively create a four-dimensional model by quantitatively retrodeforming specific surfaces (e.g. unfolding paleohorizontal horizons, removing fault slip) in the model, allowing us to place constraints on the geometric evolution of both rock bodies and faults over several million years

We divide the three-dimensional model into fault-bounded blocks, subdivided into lithologic units, by modeling the inter- and intra-block boundaries. Geologic mapping provides the foundation for the model. Structural analysis and borehole data allow extrapolation to a few kilometers depth. Geometries of active faults are inferred from double-difference relocated earthquake hypocenters. Gravity and aeromagnetic data provide constraints, respectively, on the geometry of low density Cenozoic deposits on denser basement and the geometry of highly magnetic marker units. Considering these several datasets together allows us to construct a model of the first-order geologic features in the upper ~15 km of the crust. Major features in the model include the active San Andreas fault surface, an abandoned strand of the San Andreas fault (the Pilarcitos fault), an active fold and thrust belt, regional topography of the basement surface, and several Cenozoic syntectonic basins. Retrodeformation of these features requires constraints from all available tools (structure, geochronology, paleontology, etc.).

Both construction of the three-dimensional model and retrodeformation scenarios are non-unique, but significant insights follow from restricting the range of possible geologic histories. For example, we can investigate how the crust responded to migration of the principal slip surface from the Pilarcitos fault to the modern San Andreas fault trace between ~5 and ~3 Ma. In short, our approach allows us to begin holistic evaluation of deformation in complex tectonic settings.