COUPLING GEOPHYSICAL AND GEOMORPHIC ANALYSES TO IDENTIFY BLIND THRUST STRUCTURES IN THE SAN FRANCISCO BAY REGION

Blind thrust faults pose a significant earthquake hazard, both for the damage they can cause, but also because of the difficulty in locating and characterizing them. In the San Francisco Bay region, the potential impact of blind thrusts has been recognized in the East Bay and Santa Cruz mountains. However, there is still significant uncertainty about even the existence of such structures elsewhere in the region. Here we propose that the Marin County – Mt Tamalpais region is underlain by one or more blind thrust structures. Regional determinations of fault kinematics, allow the possibility that 2-4 mm/yr of ~N-S shortening could be occurring across the Marin county region. Geologic slip rates along the San Andreas abruptly increase from the San Francisco Peninsula segment (~ 17 mm/yr) to the San Andreas north of the Golden Gate (~ 24 mm/yr). This increase on the San Andreas has been attributed to the transfer of slip from the San Gregorio fault in the vicinity of Point Reyes. However slip rate determinations further south along the San Gregorio indicate only3-4 mm/yr of slip. We propose that some of the observed slip increase may reflect motion obliquely transferred to the San Andreas from the East Bay fault system along blind structures. We have combined tectonic interpretation of local geomorphology with crustal kinematics and deformation modeling. One of the primary manifestations of the proposed blind thrusts would be active differential uplift across the region. The topographic edifice of Mt Tamalpais is somewhat enigmatic - relief is locally 4-5 times greater than relief in lithologically similar portions of the peninsula to the south – potentially implying a significant difference in rock uplift rates. Analysis of fluvial and hillslope gradients along the Bolinas ridge, north and west of Mt. Tamalpais reveals spatial variations in landscape morphology. Although hillslope gradients remain roughly similar within the uniform lithology of the ridge, channel gradients (normalized for upstream drainage area) exhibit a four-to-six-fold increase from north to south. We interpret this increase to reflect spatial variations in erosion rate, driven by differential uplift across the region. Coupled geomorphic analyses and deformational model thus provide a means to place constraints on the spatial extent, geometries and kinematics of this potential earthquake source.