Abstract

The Los Angeles basin has undergone three stages of development related to complex plate interactions within the evolving San Andreas transform system: transrotation (16–12 Ma), transtension (12–6 Ma), and transpression (6–0 Ma). Timing of these stages correlates with microplate-capture events along the continental margin, and is expressed in changes in subsidence rates and provenance within the Los Angeles basin.

The Modelo Formation and related units were deposited in the northern part of the Los Angeles basin at bathyal depths during late Miocene time. The northern Los Angeles basin was segmented into three subbasins, in each of which coarse sediment was deposited as submarine fans (Puente, Tarzana, and Simi). A fourth fan system (Piru) formed in the Ventura basin, just north of the Los Angeles basin. The Puente, Tarzana, and Piru fans were derived from the San Gabriel block, which consists primarily of crystalline basement and lesser volcanic and sedimentary components. Sandstone within the Puente fan reflects unroofing of the central and eastern San Gabriel block. The Tarzana fan was derived primarily from the central San Gabriel block, and the Piru fan was derived primarily from the western San Gabriel block, which is distinctly characterized by Ca-rich plagioclase derived from Proterozoic anorthosite and related bodies. The lack of Ca-rich plagioclase in the other fans eliminates the western San Gabriel block as a possible source area, and confirms differentiation of the Ventura basin from the Los Angeles basin by late Miocene time. The Simi fan was derived from locally uplifted Cretaceous and Paleogene strata; sandstone composition reflects the recycling of these sediments.

Subsidence and provenance analyses are consistent with the following paleogeographic and paleotectonic reconstruction. Beginning at approximately 16 Ma, transrotation of the Western Transverse Ranges induced extension and thermal subsidence of the Los Angeles basin area. A second pulse of extension and thermal subsidence occurred when motion began along the San Gabriel fault at 12 Ma. Right slip of 60–70 km occurred along the San Gabriel fault, which produced transtension in the Los Angeles basin area and deposition of the Puente, Tarzana, Simi, and Piru fan systems. At 6 Ma, transform motion was transferred to the San Andreas fault; transpression has dominated the Los Angeles basin since 6 Ma, including rapid uplift, flexural subsidence due to tectonic loading, and rapid sedimentary filling. The rapid subsidence and filling and the sudden switch between transtension and transpression in the Los Angeles basin are typical of strike-slip basins in general. However, initiation of the Los Angeles basin by transrotation reflects the uncommon process of microplate capture along the rapidly evolving California margin.

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