Rotation of the southernmost Sierra Nevada, California

Abstract

Structural, paleogeographic, and magmatic trends of the Sierra Nevada batholith swing from a characteristic NNW trend in the northern and central parts of the batholith to a nearly E‐W trend in the Tehachapi and San Emigdio mountains. Paleomagnetic results from eight sites in the upper Cretaceous (80–86 m.y.) Bear Valley Springs pluton in the Tehachapi Mountains yield a mean direction D = 022°, I = 56° (k = 97, α95 = 5.6°). This mean direction is significantly different from the expected late Cretaceous field direction for this locality and suggests that the pluton has undergone a clockwise rotation of 45° ± 14°, in accord with the change in structural trend in that area. Lower to middle Miocene volcanic and sedimentary rocks of the Kinnick Formation overlie the plutonic rocks and yield a mean direction D = 354°, I = 64°. A paleomagnetic pole derived from this mean direction is not significantly different from the expected Miocene pole for North America. This constrains the time of rotation of the pluton to be between about 80 and 20 m.y., probably predating establishment of the present San Andreas transform in this area. We suggest that the observed rotation is part of a large oroclinal bend affecting to different degrees the entire southern Sierra Nevada. The driving mechanism for such a bending may be right‐lateral shear along a proto‐San Andreas transform, for which there is now a significant body of evidence.