DID THE GABILAN MESA FORM BECAUSE OF THE CHANGE IN SLIP BEHAVIOR ON THE SAN ANDREAS FAULT?

The Gabilan Mesa covers more than 2000 square kilometers in central California between the Salinas valley in the west and the San Andreas fault in the east. While the mesa is dissected by numerous drainages, the ridge tops are roughly concordant and define a surface that slopes 3˚ or less away from the San Andreas fault. Unlike other near-fault regions, the rocks in the Gabilan Mesa are largely undeformed, which may be due to relatively rigid Salinian basement beneath the mesa. The present-day position of this landform coincides with the portion of the San Andreas fault where slip behavior changes from locked slip to creeping slip, near the town of Parkfield. Previous studies of the mesa have attributed tilting to the slight convergent component of plate motion in central California.

We propose an alternative model for the formation of the mesa based on the results of physical experiments. Our silicone model contains a dextral strike-slip fault that creeps along part of its extent and is locked along the remaining extent. We track vertical displacements and hence the topographic development of the silicone using photogrammetry. In our experiments, an elevation gradient develops near the locked-to-creeping transition in approximately the same position where the Gabilan Mesa is located in central California. This experimental gradient is the result of contraction, accommodated via uplift, on one side of the fault and extension, accommodated via subsidence, on the opposite side of the fault.

This physical modeling approach, which allows us to examine the evolution of the system, forms a useful conceptual basis for understanding why a topographic tilt might develop because of a change in slip behavior. With this context in mind, we examine the position, shape, drainage patterns, and uplift history of the Gabilan Mesa in relation to slip behavior along the fault. By understanding how this landform may have formed in tandem with a locked-to-creeping transition, we can interpret topography near faults around the world that also have slip rate changes including the Philippine, Chaman, and North Anatolian faults.