SEISMOGENIC FLATTENING OF MOUNTAINS - A POSSIBLE EXAMPLE NEAR THE BIG BEND OF THE SAN ANDREAS FAULT, SOUTHERN CALIFORNIA

Mount Pinos and Frazier Mountain form two prominent mountain massifs, underlain mostly by granitic and gneissic rocks, just south of the Big Bend segment of the San Andreas Fault. Each massif is broad, rounded, and has a large (several square kilometers) flat summit, previously interpreted as a late Tertiary erosional surface. However, a lack of lag or stream gravels on the surfaces puts the erosion-surface interpretation in doubt. At most places, the mountain flanks are relatively gentle, sloping typically at angles less than 20 degrees. Extensive landslide deposits, mostly old and deeply incised although some are still active, mantle the flanks of both massifs. Recent studies demonstrate that a pervasive fracture network underlies both massifs. Mesoscale faults are clearly evident in roadcuts and form a randomly oriented mosaic of gouge zones. Ridges are generally sinuous in plan view, and in profile form a "hill-and-saddle" topography. The hills generally contain outcrops, whereas the saddles lack outcrops and probably are zones of relatively dense fracturing. Sackungen structures (trenches and scarps suggesting deep-seated rock creep) are common. In some places, scarps bound flat surfaces on the downhill side, suggesting deep-seated rotational slumps. The Big Bend is a region of concentrated transpression along the San Andreas Fault system. At most places, crystalline rocks of the Mt. Pinos and Frazier Mountain massifs have been thrust generally southward along low-angle faults (e.g., Mt. Abel and Frazier Mountain thrusts) over sedimentary rocks as young as Pliocene. Latest movement on these thrusts probably is Quaternary, although the thrusts apparently are not currently active. During thrusting, ground accelerations in the hanging wall, particularly near thrust tips, have been shown to be particularly high. I propose that seismic shaking, from movement along both thrusts and the San Andreas Fault, pervasively fractured the hanging-wall rocks, thereby weakening them, producing essentially a pile of loose blocks. The lowered shear stresses necessary for failure, coupled with ongoing seismic activity, reduced gravitational potential by spreading the mountain massifs, which triggered flanking landslides and produced broad, flat-topped mountains.