Neogene Tectonic History of Los Angeles Basin Area: Evidence from Conglomerates of Santa Ana and Santa Monica Mountains Areas

Charles L. Lane

Conglomerate clast petrology is used as an aid in determining the Neogene history of basin development and paleogeography of the Los Angeles basin area. In particular, clast petrology of upper Oligocene through middle Miocene arkoses from the Santa Ana and Santa Monica Mountains is analyzed with respect to transport direction and source terrane. Clast-petrology data are compared with basement-terrane data in an effort to determine the most likely provenance and transport system(s).

Approximately 1,500 clasts were collected from 26 different localities within the Santa Ana and Santa Monica Mountains. A comparison of clast-petrology and basement-terrane data indicates that the eastern Transverse Ranges-western Mojave Desert area was the source for plutonic rocks transported into the Los Angeles basin during the Neogene. Exotic rhyolite clasts are also present and are considered to be far traveled. Paleocurrent data from both mountain ranges are consistent with a model for rotational opening of the Los Angeles basin beginning in the early Miocene.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.