PAST AND PRESENT TECTONIC REGIMES

The Mendocino triple junction has moved northward through offshore central California during approximately the past 20 Ma, and subduction of the Farallon plate (or its derivative) was replaced by transform motion of the Pacific plate past North America (see chap. 3; Atwater, 1970, 1989). If tectonic wedging occurred during the late Mesozoic and Cenozoic, in association with all of the episodes of tear faulting or compression outlined above, then clearly it was driven during both subduction and transform regimes. At present, it is being driven by a transform regime. At least two additional arguments can be made that wedge motion - indeed, probably a major fraction of wedge motion - occurred during the subduction regime. The first argument is simply based on geometry: The east boundary of the Coast Ranges, inferred to coincide approximately with the buried tip of the wedge, largely parallels Mesozoic structures in the Sierran foothills and the Great Valley rather than the late Cenozoic San Andreas fault (Wentworth and Zoback, 1989; C.M. Wentworth, oral commun., 1990). The second argument, developed below, is based on the total apparent displacement of the wedge.

If the inferred tectonic wedge of Franciscan assemblage extends to the San Andreas fault, as we have shown (Figure. 8.4B), then a minimum shortening of about 70 km has occurred along faults at the top and bottom of the wedge in the Diablo Range. Likewise, in the northern Coast Ranges, the inferred tear faults in the plate above the wedge have a total displacement - and, thus, shortening - of many tens of kilometers (Wentworth and others, 1984), possibly as much as 100 km (Jones and Irwin, 1971).

Although a transform regime has replaced a subduction regime in central California over approximately the past 20 Ma, plate-margin compression, necessary to drive the wedge, has persisted for only approximately the past 5 Ma (Page and Engebretson, 1984). At about 5.5-4.5 Ma, transform motion was also transferred from offshore faults to the modern San Andreas fault system (see chap. 3; Atwater, 1989; Humphreys and Weldon, in press). Present plate-margin compression is understandable from (1) the slight misalignment of the direction of relative plate motion (N. 35° W.; Minster and Jordan, 1978) and the strike of the San Andreas fault (N. 40° W.), and (2) the opening of the Basin and Range province. Crouch and others (1984) calculated from these two effects a rate of shortening across the Coast Ranges that, integrated over the past 5.5 Ma, predicted a total shortening of 28 to 72 km. Most of this shortening could be accounted for in small fault displacements and folds distributed throughout the Coast Ranges (Crouch and others, 1984). Thus, the minimum shortening of 70 to 100 km represented by the tectonic wedge, as discussed above, would appear to equal or exceed the maximum shortening calculated for the transform regime, a result suggesting that some, if not most, of the wedge motion occurred during the subduction regime.

Shear coupling between the subducting plate and overlying accretionary prism (Franciscan assemblage) could conceivably drive the wedge during the subduction regime. Such a mechanism has been postulated for southern Alaska by Fuis and Plafker (in press). To drive the wedge during a transform regime appears to require a less obvious mechanism, such as plate-margin compression combined with differing deformation in the upper and lower crust. Such a mechanism is developed below.

Sibson (1982) pointed out, on the basis of strength considerations, that ductile flow could be expected in the middle crust, below the maximum depth of earthquake hypocenters. Several workers (Crouch and others, 1984; Namson and Davis, 1988; Eaton and Rymer, in press) have postulated a decollement near the base of the seismicity in the Coast Ranges (avg 15-km depth; see chap. 5; Wesson and others, 1973) into which thrust and oblique-slip faults on both sides of the Coast Ranges sole. They envision differential movement between upper and lower crust caused by differing alignment of the transform faults in these two layers, or by shortening of the lower crust by ductile thickening.

We have incorporated the idea of a Coast Range-wide detachment in our cross section (Figure. 8.4B). In the Diablo Range, we show a young thrust fault at the base of the inferred tectonic wedge soling into the brittle-ductile transition zone, which in this area is, coincidentally, near the interface between Franciscan rocks and mafic crust. Although we also indicate soling of the San Gregorio-Hosgri fault into such a zone and underthrusting of the Salinian block by the early Tertiary accretionary prism, focal mechanisms in this region indicate pure strike slip on the San Gregorio-Hosgri fault (see chap. 5) and argue against this interpretation. Such an interpretation of a Coast Ranges-wide midcrustal detachment requires that the deformational style and (or) location of the San Andreas fault system change from the upper to the lower crust.

If we have correctly inferred the geologic history of wedge movement, it is remarkable that such movement has apparently occurred in two quite different tectonic regimes, a subduction regime and a transform regime.