TECTONICS-THE THREE-DIMENSIONAL PICTURE

The geology and, presumably, the deep structure of southern California illustrated along transect C3 (Figure. 8.6) is grossly two dimensional as far north as the Transverse Ranges. In the Transverse Ranges, the rocks on the southwest side of the San Andreas fault are similar to those in the Chocolate Mountains. These rocks are bounded on the south and west by older, deformed strands of the San Andreas fault system (Figure. 8.7; Powell, 1981). The tectonics also changes in the Transverse Ranges: Crustal-block motion swings to the west to follow the trend of the San Andreas fault, as discussed below.

Using Quaternary geologic and geodetic evidence, Weldon and Humphreys (1986) documented complex motion of crustal blocks in southern California that is not simply predictable from the motion vectors of the Pacific and North American plates. These motion vectors predict a large component of convergence across the San Andreas fault in the Transverse Ranges between the Big Bend and the Salton Trough (Figure. 8.7). For a total offset on the San Andreas fault system of about 300 km (Hill and Dibblee, 1953; Crowell, 1962, 1981; Powell, 1981), a maximum of 45 km of uplift in the Transverse Ranges would be expected (Weldon and Humphreys, 1986). However, the preservation in the Transverse Ranges of upper Cenozoic sedimentary rocks and of offset bedrock features on either side of the San Andreas fault argues against such major uplift and associated consumption of crust, as does the relatively minor crustal root in the Transverse Ranges (Figure. 8.3). Weldon and Humphreys (1986) constructed a kinematic model in which crustal blocks between the San Andreas fault and a system of borderland and other offshore faults rotate counterclock wise, parallel to the San Andreas fault, between the Salton Trough and the Big Bend (Figure. 8.7). Approximately two-thirds of the relative northwestward motion of the Pacific plate past the North American plate is taken up by the San Andreas fault system, including the San Jacinto fault; approximately one-third of it is taken up by the Elsinore fault, a system of borderland faults, and offshore faults in central California, including the San Gregorio-Hosgri fault (Figure. 8.7); and only a minor fraction of it is taken up within the blocks (see Humphreys and Weldon, in press).

A marked advance in the P-wave traveltimes of teleseismic arrivals in southern California is associated with the Transverse Ranges and extends across the San Andreas fault (Hadley and Kanamori, 1977; Raikes, 1980). Tomographic analysis of this anomaly indicates that it results from a vertical slablike region of relatively high velocity in the mantle which extends downward as far as 250 km (Humphreys and others, 1984; Humphreys, 1985; Humphreys and Clayton, in press). The amount of velocity increase, a maximum of 3 percent, is most reasonably explained by a thermal difference in the mantle. This velocity increase, coupled with a velocity decrease in the upper 90 km or so of mantle beneath the Salton Trough, led Humphreys and Hager (1984 and in press) to infer small-scale mantle convection between the Salton Trough and the Transverse Ranges. This convection involves passive rising of asthenosphere beneath the Salton Trough and cooling and sinking of lithosphere beneath the Transverse Ranges. The vertical extent of the inferred lithospheric slab beneath the Transverse Ranges, 250 km, is similar to the 300-km estimate of total offset along the San Andreas fault system. However, because the cooled mantle slab extends across the San Andreas fault, most of the mantle seems to be moving independently of the crust (Figure. 8.8; Hadley and Hanamori, 1977; Humphreys and others, 1984; Humphreys, 1985; Humphreys and Hager, in press). The horizon of decoupling is apparently at or below the Moho because crustal material is not entrained in the slablike feature. Additional decoupling may be occurring in the crust, similar to that postulated for central California (Yeats, 1981; Webb and Kanamori, 1985). Decoupling at the Moho requires that the deformational style and (or) location of the San Andreas fault system change from the crust to the mantle (Figure 8.8). We note that mantle drag on the crust is required to maintain the Big Bend in the San Andreas fault because plate-edge forces alone would tend to "short-circuit" the San Andreas fault south of the Big Bend and cause most plate motion to be taken up on the San Jacinto, Elsinore, or more westerly faults (Kosloff, 1978; Humphreys, 1985).

To summarize, block motions in the region between the Big Bend and the Salton Trough result in only minor interblock convergence in the crust. In contrast, major convergence in the lithospheric mantle is indicated by the presence of an inferred, sinking lithospheric slab.