COMPLEXITIES OF THE FAULT SYSTEM
OTHER RELATED FAULTS [c1, p7-8]

In addition to the right-lateral strike-slip faults that characterize the San Andreas fault system, faults displaying left-lateral strike slip, as well as thrust faults and reverse faults of many sizes, are present (see maps at front of book). Normal faults are less common but are present in some places, for example, in zones of extension at the crest of folds associated with the major faults, in the bordering ranges, and at jogs in the fault where local extension is to be found.

Most conspicuous of the faults displaying left-lateral slip is the Garlock fault which intersects the San Andreas fault at about lat 35° N. and extends northeastward and eastward from there for 240 km ( Figure 1.3; see maps at front of book). On the west side of the San Andreas fault, the southwest- and east-west-trending Big Pine fault joins the San Andreas fault a few kilometers northwest of the point where the Garlock fault joins the San Andreas fault.

The left-lateral Pinto Mountain fault zone joins the San Andreas fault on its east side at about lat 34° N. and extends northeastward, in a pattern not unlike that of the Garlock fault. The Blue Cut fault is another left-lateral fault in the same general area.

On a broad regional scale, thrust faults and detachments that accommodate subduction of the lower crust are significant, and various interpretations and speculations have been offered ( Figure 1.4; Weldon and Humphreys, 1986; Namson and Davis, 1988). Intermediate-scale thrust faults that border the Transverse Ranges on the south side are characterized by such faults as the San Fernando fault zone (Grantz, 1971) and the Cucamonga fault zone ( Figure 1.5). At the north end of the San Andreas fault system, where it joins the Mendocino Fracture Zone, such thrust faults as the Kings Range fault similarly accommodate crustal shortening. Smaller thrust faults that flank the San Andreas fault and dip toward it are common along many parts of the fault zone.

A range of fault types, and the complexities that typify much of the fault zone, are well illustrated near Cajon Pass and southeast of there ( Figure 1.5), where the San Andreas fault zone splits into a northern and a southern branch. The strike of the San Gorgonio Pass fault zone in the east-central part of Figure 1.5 changes in several places, and depending on the trend of a given segment, strike or dip slip may predominate. In this same area, the San Andreas fault, which to the northwest is relatively continuous and linear, ends as a surface feature, and the style of deformation changes from strike slip to primarily dip slip at the surface. Left-lateral slip characterizes the Pinto Mountain fault in the eastern part of Figure 1.5, and small normal faults can be found throughout this area.