MORHOLOGIC FEATURES OF OFFSET CHANNELS CROSSING THE SAN ANDREAS FAULT, CALIFORNIA

Channel course geometry of some offset rivers crossing the San Andreas fault measured in the field indicates that the offset length has a good correlation with the drainage area above the fault for gullies cutting the surfaces of alluvial fans or pediments in the Carrizo plain, while older offset channels with longer offset reaches show a good correlation between offset length and the angle of deflection. For relatively new channels with expanding drainage area, the drainage area indicates the period after the channel initiation, during which the fault slip accumulates. The old offset channels, on the other hand, are in valleys and have existed for a longer time without taking new courses; and therefore, they received larger fault displacement. How long they can preserve their offset courses determines the offset length and the angle of deflection in this case. The angle of deflection seems to be significant only for rivers preserving the offset courses for an extended period of time.

The deformation of channels by strike-slip faults can be classified into three types by the relationship between fault displacement and channel gradient. Type A is a case in which the ratio of vertical component to horizontal component of fault displacement (offset slope) is smaller than channel gradient. The offset slope is steeper than channel gradient in Type C, and equal in Type B. Considering the possible response of a stream, such as shifting of an offset reach towards the steepest possible course, damming effects of an offset course, and the tendency to smooth the profile, offset rivers of Type B are considered to preserve the deformation the best. The deformation in the longitudinal profile would be preserved well in Type C offset rivers, and Type A offset rivers probably easily modify the deformation. This estimation is roughly compatible with the results of field measurements.