Abstract: Folding and Faulting Along the Mojave Segment of the San Andreas Fault Near Palmdale, CA: Implications for Simple Shear Mechanics
KESSEL, LOWELL, University of California, Santa Barbara, Santa Barbara, CA; ARTHUR SYLVESTER, University of California, Santa Barbara, Santa Barbara, CA
The purpose of this study was to determine the mechanics related to folding and faulting exposed in the Hwy 14 road cut near Palmdale. The roadcut is about 27 m high and about 600 m long and oriented nearly perpendicular to the San Andreas fault (exposed at the southern end of the cut) and the Little Rock fault (located about 76 m north of the north end of the cut) and can be considered to be a large "trench". The Little Rock fault is a right slip fault with about 20 km estimated offset. The road cut exposes complexly folded and faulted, lower to middle Pliocene, gypsiferous, lacustrine rocks of the Anaverde Formation. They are unconformably overlain by undeformed Pleistocene gravels, suggesting that folding and faulting occurred prior to their deposition.
Recent re-excavation of the road cut provided exposure from which folds and faults exposed on one side of the cut were correlated with folds and faults on the opposite side. Faults parallel to the San Andreas fault are subsidiary segments or fractures; the faults oriented approximately 20? to the San Andreas fault have the proper orientation to be Riedel shears; and the single fault oriented 30? to the San Andreas fault is a thrust fault. The orientation of these faults suggests that they formed as a result of simple shear. Two models may accommodate the structural constraints; (1) coupling of the San Andreas fault with the Little Rock fault or (2) simple shear related exclusively to the San Andreas fault. Establishing the temporal relations between folding, movement along the faults, and formation of fractures may resolve which model is correct.
AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California