MODELS FOR SLIP ON BLIND THRUST FAULTS: THE SANTA MONICA MOUNTAINS-CHANNEL ISLANDS THRUST AND ANTICLINE, CALIFORNIA

The northern Channel Islands and the Santa Monica Mountains are the crest of the same post-Miocene anticline, but are separated by a structural saddle exhibiting little evidence for late Quaternary folding. There is agreement that this anticline is the result of slip on blind thrust faults, but no agreement on the appropriate way to model fault slip and geometry. Ramp-flat models with planar fault segments predict that backlimb width increases with fault slip, and that there is no progressive tilting of fold limbs. In contrast, the listric thrust model predicts a progressively-tilting fold backlimb above the concave-up part of the fault. Backlimb dip, not width, increases with continuing slip. Neither of these models account for the response of the footwall to the load applied by the growing hanging-wall fold.

We surveyed uplifted late Quaternary coastal terraces on the northern Channel Islands, and interpreted USGS high-resolution seismic reflection data that image subsided shelf-edge deltas north of these islands. Uplift of the islands and subsidence of the shelf represents a down-to-the north tilt, consistent with a listric thrust model. This tilt causes low-stand deltas that prograde farther north onto the fold limb to subside more rapidly, and older deltas to be deeper and more tilted than younger ones. Planar erosion surfaces all around the islands suggest regional subsidence and fault footwall deformation. Such footwall deformation means that slip should be inferred using growth of structural relief or tilt rates of fold limbs, not uplift with respect to sea level. The maximum tilt rate the backlimb of the Channel Islands anticline is ~0.005 deg./ka. Such a tilt rate would not be recognized in the typical trench or coreholes on comparable structures in San Fernando or Los Angeles basins. Active axial surfaces (kinks) are not required by the listric model and their lack does not preclude active blind thrust slip.