The central Los Angeles basin represents the deepest part of a basin that apparently resulted from rapid and prolonged lithospheric thinning owing to extension between rotating blocks. Subsidence in this tectonic setting began about 18 Ma and presumably reflects isostatic adjustment to the thinning of the buoyant crust. Sediment starving in the period immediately following the initiation of rapid subsidence resulted in a deep water-filled basin that reached water depths in excess of 2 km during Pliocene time. Sedimentation accelerated immediately following the widespread extrusion of andesitic and basaltic volcanics about 16 Ma. Maximum tectonic subsidence, which may require 50% to 75% of lithospheric thinning under the central deep, is about 3 km depending on assumptions. This amount of thinning can be used to estimate the maximum time-temperature history of basin sediments. The pattem of subsidence is best explained by a model of crustal rotation between right-slip faults that results in both extension in the early development of the basin and compression in the later phase of basin development.
Figures & Tables
Active Margin Basins
“The most distinctive characteristic of the Los Angeles basin“The most distinctive characteristic of the Los Angeles basin is its structural relief and complexity in relation to its age and size” (Yerkes et aI., 1965, p. AI6); however, its very complexity caused no small amount of discussion in designing and naming this volume of the AAPG World Petroleum Basin Memoirs. (See the Foreword for a discussion of the scope of these memoirs.) The series coordinators decided early that the Los Angeles basin should be included in the World Petroleum Basins project because of its interesting geology and importance as a hydrocarbon producer. Initially, the Los Angeles basin was considered for a convergent-margin volume, presumably in recognition of the late-stage shortening that has taken place in the Los Angeles region of southern California. There is little doubt, however, that the Los Angeles basin has formed and deformed within the evolving San Andreas transform system (Atwater, 1970, 1989; Campbell and Yerkes, 1976; Blake et al., 1978; Engebretson et al., 1985; Wright, this volume). There is also little doubt among those who have worked in the area that the initial subsidence of the Neogene Los Angeles basin was caused by extension (Yeats, 1968; Crowell, 1974, 1976, 1987; Wright, this volume). The series coordinators decided, therefore, that to portray the Los Angeles basin as a model for basins formed in convergent-margin settings would be misleading.
The title of this volume, Active Margin Basins, is a compromise, but, like many compromises, this title falls short of completely describing its subject