Figure 2.1

      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 l-km-wide fault zone accounts for much of the valley-like expression of the fault, but differential vertical displacements also playa 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 with in 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 southeastward 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 fig. 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 5° to 10° 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 (fig. 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 (fig. 2.1; see fig. 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").

      Within the fault zone, various geomorphic features are found that have their origin in both the lateral and vertical shuffling of fault-bounded slices, as well as in the persistent, large strike slip. These features include sag depressions and sag ponds, shutter ridges and medial ridges, offset and deflected stream channels, linear benches along valley walls, aligned notches and saddles on spurs, offset marine and river terraces, scarps, fault-controlled drainage, and folds and pressure ridges (fig. 2.2).

Figure 2.2 - Common landforms along the San Andreas fault system (from Vedder and Wallace, 1970).

      Along its entire length, the fault zone exhibits peculiar, anomalous drainage patterns. In regions where tectonic activity is less pronounced, streams generally flow more or less perpendicular to mountain blocks and highlands, and grade more or less regularly to the lowlands; not so along an active fault like the San Andreas. When drainage flowing from highlands meets the San Andreas fault, it is diverted subparallel to the trends of the highlands or is interrupted or blocked completely. In less active areas, erosion generally is the dominant factor in carving geomorphic forms, but displacements are so rapid within the fault zone that tectonic effects overwhelm erosion, and so the geomorphic features directly express fault movement.
      Movement within the network of branching and anastomosing fault strands jostles the intervening blocks, compressing some, rotating some, or causing extension across others. Because the principal slip is horizontal and lateral, the blocks tend to be elongate parallel to the trend of the fault. Blocks under compression tend to be squeezed upward to form elongate ridges, whereas blocks under extension may drop downward to form sags, and laterally displaced slices or ridge spurs create shutters across drainage channels.
      The dominantly lateral slip across the fault zone and the rate of slip, from 1 cm to a few centimeters per year, make stream channels that are offset right laterally, a common and characteristic geomorphic feature. Stream channels can be completely beheaded or merely offset while maintaining continuity of flow.
      In addition to the effects of lateral slip, streams are extremely sensitive to vertical slip on faults and warping of the land surface. For example, only a small upward movement of a block on the downstream side of a fault crossing a stream may divert the stream either to the left or right, thus mimicking lateral slip on the fault. Similarly, warping of the land surface over folds adjacent to the fault or on pressure ridges within the fault zone can distort the patterns of streams. Combinations of these different tectonic processes can produce many unusual features. Both the tectonic and the erosional changes at times may occur almost instantaneously, and so the dominance of one or the other process suddenly may change. Between such periods of sudden change, very little may happen for decades or even centuries. The relative rates of erosional and tectonic processes, and the timing of sudden events, are critical to the landforms created. Some of the patterns of streams found in the Carrizo Plain area are illustrated in figure 2.3, and an example is shown in figure 2.5.

Figure 2.3 - Diagrammatic representation of patterns of fault-related stream channels found in the Carrizo Plain area (from Wallace, 1975): A, misalignment of single channels directly related to amount of fault displacement and age of channel- no ridge on downslope side of fault, beheading common; B, paired stream channels misaligned; C, compound offsets of ridge spurs, and offset and deflection of channels, both right and left deflection; D, trellis drainage produced by multiple fault strands, sliver ridges, and shutter ridges; E, exaggerated or reversed apparent offset, caused by offset plus deflection by shutter ridge; F, "Z" pattern, caused by capture by adjacent channel followed by right-lateral slip; G, false offset caused by differential uplift or warping; H, false offset caused by echelon fractures over fault zone, followed by subsequent streamflow.

Figure 2.4 - Topographic map of a segment of the San Andreas fault in the Carrizo Plain area, showing some characteristic small-scale geomorphic features. Markers (SAF) at the left and right margins indicate main fault trace. Between 300 and 500 m (see scale at bottom) is one of the best examples of a stream offset by right-lateral slip on the San Andreas fault (see fig. 2.20). To the northwest, between 100 and 300 m, is an abandoned channel of the same stream. To the southeast, between 600 and 700 m, small streams are offset about 10 m; a few of these streams record multiple offsets of about 8 to 10 m. The last offset presumably was during the great Fort Tejon earthquake of 1857. Between 2,150 and 2,250 m is a pair of streams that has been offset a few tens of meters. At 2,000 m, the downstream segments of those two streams have been beheaded completely, and at 2,100 m is another possible beheaded segment of the streams (see fig. 2.21). Between 2,300 and 2,700 m is a sag depression about 7 m deep, resulting from downdrop of a narrow block into the San Andreas fault zone. From Sieh and Wallace (1987); map prepared by U.S. Geological Survey from aerial photographs taken January 13, 1966.

      The geomorphic forms created represent the results of a continuing contest between erosional changes and changes related to fault slip, folding, and warping. Where streams are large and rainfall is greater, only displacements of hundreds of meters or more are preserved for longer than a few centuries. In desert climates, however, as in the Carrizo Plain, the rate of fault slip outruns erosion, and the effects of only a few meters of fault slip may be preserved for hundreds of years, if not millennia, where small channels cross the fault (fig. 2.4).
      As an example of how erosion and sedimentation interact with the faulting process, a straight channel that formerly crossed the fault at right angles is shown after having been offset by right-lateral strike slip (fig. 2.5). The strike slip partly or temporarily dams the stream, causing upstream alluviation at C. A fresh fault scarp is formed in the vicinity of A, and successive offsets expose new scarp areas to the left of A. The dam at B is eroded, and the alluvium deposited earlier at C is dissected. As offset progresses further, the channel segment along the fault trace, between B and A, continually elongates, thus lowering the channel gradient more and more. Because of this decreasing gradient, alluvium is deposited upstream from A to and beyond C, and eventually the stream, having difficulty maintaining a channel along that elongate course, spills across the fault trace and creates a new channel more nearly in alignment with the segment upstream from the fault.
      After fault movement has progressed sufficiently, the downstream segments of other channels are brought into alignment, or nearly so, with the original channel. For example, in the vicinity of D in figure 2.5, drainage flowing to the right in an adjacent channel would tend to erode headward toward C, and capture of the original stream would take place. The gradient of this capturing segment, in which flow is to the right, progressively increases because right-lateral slip shortens the channel, thus accelerating erosion; at the same time, the channel flowing to the left elongates, gradient decreases, and erosion decelerates. An example of some geomorphic features that result is shown in figure 2.5.

Figure 2.5 - General features and conditions produced where a stream channel is offset by strike slip on a fault. See text for discussion of A-D.

DETAILED MAPS OF THE FAULT SYSTEM

      The U.S. Geological Survey and the California Division of Mines and Geology have prepared numerous detailed maps of the faults within the San Andreas fault system. Maps prepared by the California Division of Mines and Geology address the problem of "active faults" as defined under the Alquist-Priolo Special Studies Zones Act of 1972. These maps, too numerous to list here, were indexed and described by Hart (1985); they constitute a rich data set about the San Andreas and other faults in California. Those readers interested in examining the features of the fault system in more detail or in the field are referred to the published "strip" maps and special fault maps, an index map of which is in figure 2.6.

Figure 2.6 - Index map of California, showing locations of selected maps of surface traces of the San Andreas fault system.

A PHOTOGRAPHIC ALBUM OF FAULT FEATURES

REFERENCES CITED