Abstract

The displacement history of the San Andreas fault is characterized by three phases: a post–mid-Cretaceous to early Eocene period of active right slip, an Eocene to Miocene period of inactivity, and a post-Miocene period of accelerating right slip. It has been suggested that the proto–San Andreas offsets resulted from oblique convergence in the early Tertiary. This concept requires a mid-Tertiary period of normal convergence correlative with the period of inactivity and thus provides a basis for testing reconstructed plate motions.

Assuming that hot spots constitute a fixed frame of reference and that the motion of North America has been constant, Farallon–North America convergence vectors have been computed for three Cenozoic phases of plate motion ending at 55, 42, and 30 m.y. B.P. Although there is some overlap between the early Tertiary period of oblique convergence and the mid-Tertiary phase of inactivity on the San Andreas, the calculated convergence vectors are generally consistent with the displacement history of the fault: moderate oblique convergence is indicated for the early Cenozoic, with some reduction in the oblique component between 55 and 42 m.y. B.P., and a period of nearly normal convergence is indicated for the period between 42 and 30 m.y. B.P. Thus, the two hypotheses, that the proto–San Andreas was driven by oblique convergence and that hot spots approximate a fixed frame of reference, are not mutually exclusive. Although this type of study cannot prove either hypothesis, the results lend some support to each.

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