Abstract

Examination of declination anomalies within a well-controlled chronologic framework can permit precise delineation of the progression of a tectonic rotation. In the past, declination anomalies from numerous spatially separated and individually dated localities have been synthesized in order to define regional, progressive rotations. Rarely, however, have magneto-stratigraphic data from a single locality been used successfully to chronicle a sequential rotation that resulted from movement along a nearby fault. Magnetostratigraphic studies from the El Paso basin in southern California reveal progressively smaller amounts of counterclockwise rotation for strata dated between 11 and 7 Ma. Two tests indicate that this is a temporally controlled, rather than a spatially controlled, rotation. This rotation is interpreted to have resulted from sinistral shear along the Garlock fault. As such, it appears to delimit the initiation of Miocene movement along the western segment of this major strike-slip fault.

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