Development of offset channels across the San Andreas fault

Abstract

The San Andreas fault offsets some small gullies incising the relatively flat surfaces in the Carrizo Plain, California. Among these gullies, those with larger drainage basins have larger offsets, probably because gullies that have larger floods can keep the channels entrenched and subjected to the fault displacement longer. On the other hand, older offset channels in the area, which have much larger offsets, indicate that the gradual shift to straighter courses seems probable only for offset channels able to keep and accumulate the offset for a long time. The strike-slip fault displacement offsets a channel as much as the channel can follow with some modification by fluvial processes until overbank floods at the upper bend induce channel avulsion to a straighter course, or until the channel is captured by the beheaded and abandoned downstream reach of the adjacent channel moved with the fault displacement. The offset channel promotes aggradation upstream from the upper bend, and this makes the channel increasingly susceptible to overbank floods. The older offset channels are confined in valleys, and this condition is apparently preventive of channel avulsion or capture. The strike-slip fault displacement also deforms longitudinal profiles of channels. Assuming that the elimination of discontinuities in the longitudinal profile is the main response of fluvial processes to the deformation by fault displacement, nearly continuous longitudinal profiles of some old offset channels seem to indicate the full adjustment, the condition in which no more changes will occur as a response to fault displacement until the next fault slip deforms them. The irregularities in longitudinal profiles of other offset channels indicate that the channel adjustment by fluvial processes may be occurring but not enough to eliminate the irregularities. An offset river can attain a continuous longitudinal profile if it has enough time to adjust its longitudinal profile after the fault displacement. Otherwise, the discontinuity reflecting a various degree of adjustment would remain in the longitudinal profile until channel avulsion or capture occurs. The degree of adjustment and the probability of channel avulsion or capture depend highly on the fluvial ability and condition of each river as well as the time after the deformation.