Abstract

Cenozoic subsidence rates in southern California basins were as high as 700 m/m.y. in Eocene-Miocene time, but they accelerated to rates greater than 1,000 m/m.y. after the Miocene. These extraordinary rates are observed only in basins clustering about the big bend in the San Andreas fault as it crosses the Transverse Ranges. Highest rates are found in the Ventura (2,000 to 4,000 m/m.y.), Los Angeles (1,330 to 1,500 m/m.y.), and Ridge (2,000 m/m.y.) basins. Lower rates, still in excess of 1,000 m/m.y., are found in the South San Joaquin, Cuyama, and Santa Monica basins. A higher degree of geochronological calibration in the Ventura basin permits the dating of the acceleration of subsidence rates there at about 4 ± 1 m.y. ago, the same time that the Gulf of California began to open. The geographic distribution suggests that the very high rates of subsidence are a Poisson effect of horizontal displacement on the San Andreas fault at the big bend, where it is under compression. The date of acceleration may indicate the age of formation of the big bend from an earlier, straight San Andreas trace, but more likely it reflects a strengthening of the lithosphere as it cools from the effect of overriding the East Pacific Rise about 15 m.y. ago.

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