UCLA

The Vincent Gap region of the eastern San Gabriel Mountains in southern California is a small but important piece of an originally continuous terrane separated into the Tejon, Soledad and Orocopia regions by the San Andreas fault system. The middle-upper Miocene Punchbowl Formation has been considered the oldest Neogene strata of the Vincent Gap region. The present study documents that strata southeast of the main exposure of the Punchbowl Formation, though aerially restricted, are temporally extensive; together with the Punchbowl Formation, they comprise a sedimentary record that spans from ~25 to ~6 Ma, includes equivalents of the Vasquez, Tick Canyon and Mint Canyon formations of the Soledad region, and relates to three sequential tectonic stages in southern California. Uppermost Oligocene-lower Miocene strata are closely correlative with the Plush Ranch, Vasquez and Diligencia formations of the Tejon, Soledad and Orocopia regions, respectively; they formed during extension induced by triple-junction instability. Interbedded 25 Ma volcanics near the base of these strata are chemically and chronologically similar to those of the Plush Ranch, Vasquez and Diligencia formations. Middle Miocene strata beneath the Punchbowl Formation are equivalent to the Tick Canyon Formation of the Soledad region, and document exhumation of the Pelona Schist. Sandstone petrofacies, conglomerate composition and detrital-zircon age data provide compatible but distinct provenance information; using all three in combination results in a more complete understanding of the provenance of each of these units. The results of this study imply that transrotation of the western Transverse Ranges and accompanying basement exhumation extended farther inboard than generally thought, adjacent to if not across the future trace of the San Andreas fault. Data from the Vincent Gap also reveal that the middle-upper Miocene Punchbowl Formation was likely part of a large drainage system, with the Mint Canyon Formation of the Soledad region representing a tributary of this system that joined downstream, and the Caliente Formation of the Tejon region representing the confluence of the two.