Abstract

In the southern California region, published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time, chiefly since the opening of the Gulf of California. These rates, together with the known geometries of active faults, are used to constrain a hand-fit block model for present horizontal velocity of the crust. The assumptions employed are that these rates apply to the present, that the blocks are internally rigid down to the greatest depth of earthquakes, and that the over-all Pacific–North American plate motion is correctly given by global models. The resulting model fits 67 data within their uncertainties, 10 more with a small discrepancy (≤ 1 mm/yr), and fails to accommodate only 4 data (2 of which are inconsistent with other data). Nominal precision is good (∼3 mm/yr), but accuracy and uniqueness are both suspect, because many block boundaries are questionable.

Despite its many local defects, this model suggests four reliable regional conclusions. First, the rate of shortening on the south side of the Transverse Ranges varies from 6 to 14 mm/yr, increasing eastward, which implies an unrecognized seismic hazard, unless the motion is taken up by folding or fault creep. Second, the pattern of block rotation rates is quite different from the pattern of rotations shown by Tertiary paleomagnetic declinations, which implies some major Pliocene(?) reorganization of blocks. Third, the “great bend” (left step) of the plate boundary fault system in the Transverse Ranges appears to be straightening gradually, by a 3°/m.y. clockwise rotation relative to both plates. Finally, if it is assumed that plate shortening has been occurring at this bend since the early Pliocene and that crust and upper-mantle velocities are similar, then this model predicts mantle-lithosphere downwelling to at least 250-km depth beneath the Transverse Ranges. This prediction is consistent with upper-mantle seismic-velocity structure.

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