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The East Pacific Rise reached California 24-29 m.y. ago north of the Murray fracture zone and 18-20 m.y. ago south of it, on the basis of correlations of offshore magnetic anomalies with the Heirtzler geomagnetic time scale. This rearranged Paleogene paleogeography and established Neogene oil basins, first north, then south of the Transverse Ranges. South San Joaquin basin, central Coast Range basins, and the northern edge of the Ventura basin originated in Zemorrian-Arikareean time, 22.5-26 m.y. ago. Los Angeles basin and southern California borderland basins originated in late Saucesian time, over 15.3 m.y. ago.
Basin initiation was accompanied by volcanism, with K-Ar dates averaging 23 m.y. in central California and 15-16 m.y. in southern California. The rift basins were silled, favoring deposition of sapropelic "Monterey" shale alternating with reservoir-quality sands from adjacent highlands. High heat flow accompanying the East Pacific Rise caused early generation and accumulation of Neogene oil from these sapropelic shales, and accumulation of some and perhaps all of the oil in Paleogene reservoirs. The basins underwent almost continuous deformation because of Pacific-North American relative plate motion, resulting in early formation of anticlinal and stratigraphic traps. After disappearance of the East Pacific Rise heat source, the lithosphere cooled, increased in density, and subsided. eformation of basins that were generally subsiding prevented uplift and breaching of many early-formed traps.
Comparable relations with the East Pacific Rise are found in several untested basins in the southern California borderland, Colorado delta and northern Gulf of California, and southern Baja California.
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