LOCAL GEOMORPHIC FEATURES
WITHIN THE FAULT ZONE [c2, p16-18]

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 ( Figure 2.2).

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.

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 ( Figure 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 ( Figure 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..