STRUCTURE OF A PORTION OF THE SAN ANDREAS RIFT IN SOUTHERN CALIFORNIA

An area approximately 20 miles long and 5 miles wide along the San Andreas rift in southern California between Palmdale and Elizabeth Lake was mapped. The area includes a portion of the Mohave Desert and two ridge zones separated by a trough along the rift. The ridge zones are principally old crystalline rocks; the trough is underlain by a long, narrow strip of sediments which outcrop in a low “center-trough” ridge. Most of these sediments are part of the Anaverde formation which, on the basis of paleo-botanical evidence, is believed to be transitional between lower and middle Pliocene. Evidence indicating the present relation of this long strip of sediments to the crystalline rocks on either side is conflicting. The older rocks of the ridge zones are in bands striking east or slightly north of east. This trend is cut by the rift which, in this area, has a strike of approximately N. 65° W. Offset of terrace deposits suggests horizontal displacement along the rift of 5–6 miles since the Pleistocene. Streams are offset as much as 1½ miles. The north side of the rift has moved relatively southeastward. The situation of blocks of Pelona schist on opposite sides of the rift suggests displacement of 9 miles in the opposite direction to that indicated by recent faulting. This displacement is believed to be only apparent and possibly the result of differential vertical uplift of two separate Pelona schist blocks. Many and frequent vertical displacements of various blocks are indicated by elevated terrace deposits and old erosion surfaces. There is no definite evidence that strike-slip displacement began earlier than the Pleistocene, although suggestions of greater age of the rift were found. Strike-slip displacements similar to the San Andreas rift are apparently widespread in southern California suggesting that stresses have been exerted throughout a large area, possibly by drag from subcrustal currents.