REGIONAL FEATURES [c2, p15-16]

The San Andreas fault is marked in the landscape by a series of linear valleys and mountain fronts, aligned lakes and bays, elongate ridges, and disrupted or offset stream channels.

On maps, aerial photographs, or satellite images, at almost any scale, the San Andreas fault zone appears as a linear scar across the landscape. At scales small enough to display the entire fault length, valleys, bays, chains of lakes and ponds, linear flanks of mountain ranges, and elongate ridges bounding one side or the other of the fault are the principal features that reveal its location.

Erosion of the softer broken and sheared rocks in the several-hundred-meters- to 1-km-wide fault zone accounts for much of the valley-like expression of the fault, but differential vertical displacements also play a major role. The ratio of local horizontal to vertical displacement may be about 10 or 20 to 1. Differential erosion of the various rock types juxtaposed by faulting also influences the geomorphic expression of the fault.

The San Andreas fault separates some mountain masses from adjacent broad regions of low relief, creating pronounced linear topographic discontinuities. Considering the large lateral displacements that have occurred, juxtaposition of some mountains against flatter, lower areas probably has come about by lateral slip. Some range-size blocks bounded by the fault, however, have risen or dropped hundreds of meters to create the linear topographic features. Elongate blocks of the crust bounded by branches and subparallel strands of the fault have been created and shuffled one block against another by both upward, downward, and lateral differential displacement within the broad shear zone. Such movements are reflected in the topography as elongate ridges and depressions. Whether lateral or vertical block displacement, or warping or folding, has dominated in the development of a specific landform, and what role erosion has played, have yet to be well analyzed for most topographic features within the fault system. Evidently, a complex interaction of tectonic, erosional and depositional processes has influenced the development of each feature, and the result is a linearity of topographic features along and parallel to the San Andreas fault.

From Point Arena southeast ward to the vicinity of San Jose, Calif., the trace of the San Andreas fault is topographically conspicuous on regional maps and images as a series of linear valleys ( see Figure 2.1) and maps at front of book for locations). Aligned linear valleys also mark the fault trace throughout central California between San Jose and the Carrizo Plain, but along that reach another important characteristic is that the fault trace crosses mountain ranges and major ridges at a low angle. These mountain ranges and ridges, many of which are antiformal structures, trend from 50 to 100 more westerly than the strike of the fault. Thus, the altitude of the surface trace of the fault alternately rises and falls along strike.

In the Carrizo Plain-Temblor Range area, the surface trace of the fault does not lie at the base of the range but more within the Carrizo Plain, where the surface expression of the fault is narrowest, clearest, and best defined. Offset streams are especially well preserved here; individual strands of the fault reach a maximum length, from 9 to 18 km, anywhere along the fault.

In the Big Bend area at the south end of the San Joaquin Valley, the fault trace rises to a high altitude as it passes through mountainous terrain. Along the Mojave segment to the northwest and southeast of Palmdale (see maps at front of book for locations), the fault trace is again marked by a distinct narrow, linear valley. In addition, a gross contrast between the high, rugged mountain masses of the Transverse Ranges and the relatively flat Mojave Desert block is apparent ( Figure 2.1). The surface of the Mojave Desert itself stands 700 m or more above sea level and above the San Joaquin Valley. Clearly, the Mojave block has been uplifted, even though the adjacent mountain masses have risen more.

To the southeast of the Mojave segment, the San Andreas fault crosses the Transverse Ranges at a low angle and separates the high San Gabriel Mountains from the San Bernardino Mountains. An extremely complex structural knot, formed by branching of the San Jacinto fault and numerous other faults ( Figure 2.1; see Figure 1.5 is reflected as a complex topographic region surrounding the Cajon Pass area, through which the San Andreas fault passes.

Southwest of the Cajon Pass area, the fault divides into a northern and a southern branch and numerous other smaller faults of different tectonic style. Each fault has its own distinctive geomorphic expression.

Southeast of the zone of major branching, the fault again is less conspicuously marked by contrasts of large topographic features, but it is readily visible on aerial photographs at scales of 1:50,000 and larger (see section below entitled "A Photographic Album of Fault Features").