THE NEOGENE FOREARC BASIN OF NORTHERN AND CENTRAL CALIFORNIA REVISITED

The modern and active forearc basin of northern California was established during the Neogene and consists of the Eel River basin and its offshore equivalents to the north. The Late Mesozoic and early Tertiary forearc basin consisted of the fill of the Great Valley of northern California and possibly the Hornbrook basin of southern Oregon. The transition from the older to the younger forearc system is not yet fully understood, partly because Neogene and older forearc deposits have been uplifted and stripped away during northward migration of the Mendocino triple junction. During triple-junction migration, pre-Neogene convergent-margin tectonics were progressively replaced by strike-slip tectonics related to the San Andreas fault system, the older forearc-basin fill was uplifted and eroded, and new areally restricted strike-slip basins were formed. In the 1980s, we proposed that the Neogene forearc basin originally extended southeastward across the Coast Ranges from the Eel River basin through the Petaluma, Livermore, and several other remnant basins into the southern San Joaquin basin. The implications of this concept will be reviewed in the light of newer data.