Miocene Paleogeography of Southwestern California and Its Implications Regarding Basin Terminology

Abstract

A new palinspastic reconstruction for the latest Oligocene of southwestern California is created by: (1) backsliding the eastern part of the western Transverse Ranges west of the curved San Gabriel-Sierra Madre fault in a counterclockwise, rotational, left-slip direction in order to close the gap created by medial Miocene transrotational extension of the easternmost Los Angeles basin; and (2) bending the rocks in the westernmost Transverse Ranges, west of the Santa Monica Mountains, through an additional counterclockwise backrotation until they are in contact with the present southern California coast. This reconstruction restores Eocene forearc basin deposits of the southern California coast and the western Transverse Ranges regions into straight alignment with Eocene forearc-basin deposits to the north in the San Rafael Mountains. During the Oligocene, this forearc basin was divided into two depositional basins that received mostly nonmarine deposits. The northern of the two basins is named the DiSoCuMa depositional basin and the southern one the AnaVent depositional basin. Subsidence occurred during the early Miocene and flooded the two basins with marine deposits. During the medial Miocene, subsidence continued as the western side of the AnaVent basin (today's western Transverse Ranges) began to pull away from the southern California coast and rotate clockwise around its northern end. This transrotational process served to enlarge both basins by crustal extension and produced extensive areas of volcanism. Throughout the late Miocene, transrotation and subsidence continued, the basins grew in size and depth to their present configuration, and internal isostatic adjustments produced several new marine depositional basins within the previous two-basin framework.