Folding and faulting associated with the San Andreas fault, along highway 14 near Palmdale, California.

 

Abstract by Lowell Kessell     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 27 m high and 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 ìtrenchî. 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 gravel, 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. Preliminary results suggest that model two was initially controlling the fault zone. Model one may have become more important later in the evolution of the area. This may suggest that the LRF became active after the SAF had been active and is younger fault compensating relative motion between the Pacific plate and the North American plate.

View northwestward along the San Andreas fault with the Mojave Desert to the right and the San Gabriel Mountains to the left. Use topographic map for reference and location.

1952 Aerial photograph of the area prior to construction of highway 14 and excavation of the road cut. Fold and fault map of the area centered on the road cut. The map has traces of fault segments, folds, and the location of the San Andreas fault and the Little Rock fault. A photo-mosaic of the road cut may be posted here someday and will illustrate all the structures within the road cut.