P-residual studies suggest that the Pacific-American plate boundary in the upper mantle lies in the central Mojave Desert, northeast of its crustal position at the San Andreas fault. Focal mechanisms of some relatively deep microearthquakes suggest a flattening with depth of north-dipping thrust faults, resulting in a mid-crustal detachment in which the central Transverse Ranges move south relative to the mantle. This makes sense geologically if the central Transverse Ranges and western Mojave Desert are considered as tectonic flakes of brittle crystalline rocks underlain by ductile Pelona Schist or Catalina Schist, which makes up the zone of decollement. Such ductile schists would occur today at depths comparable to those at which they were metamorphosed. The detachment breaks through to the surface at the Point Dume–Malibu Coast fault zone and the Red Mountain–San Cayetano–Santa Susana–Sierra Madre–Cucamonga fault zone, the latter coinciding with the southern edge of the Palmdale bulge. The Transverse Ranges flake now moves west-northwest relative to the western Mojave flake, south-southwest relative to the Pacific plate, and northwest relative to the Great Valley–Sierra Nevada. The mantle boundary may rejoin the San Andreas fault north of a southwest-trending zone of high seismicity in the Tehachapi Mountains which may be produced by convergence. The age of the present tectonic system may be the same as the age of the break-through thrust faults; these are probably no older than 2 m.y. and may be even younger.

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