The continental borderland and adjacent land areas of southwestern California, Baja California, and the Gulf of California area were the site of extensive Cenozoic volcanism. Radiometric dates and stratigraphic relations indicate major mid-Miocene volcanism (Jacumba, California basalts 18.7 ± 1.3 m.y.; Tijuana area of Mexico, 14.3 ± 2.6 m.y.; Channel Islands and Los Angeles basin area, California ~15.5 m.y., Luisian stage). High-alumina basalts of both tholeiitic and alkalic types are predominant; andesites and dacites are less voluminous but are locally important. These silicic rocks appear to be differentiates of the basalts. A Pliocene episode of volcanism is characterized by abundant alkalic basalts and low-alumina tholeiites and by lesser amounts of high-alumina tholeiite (Santa Rosa basalts, Murietta, California, 8.3 ± 0.5 m.y.). Scattered outpourings of alkalic and tholeiitic basalt, andesite, and rhyolite continued in the region until latest Pleistocene time. The mid-Miocene volcanism appears to be synchronous with the postulated initial stages of crustal dilational which formed the Gulf of California. All of the late Cenozoic volcanism shows close spatial relations to fault zones of the area, and a genetic relationship between faulting, regional dilation and volcanism is inferred. The compositional trend of the sequence of volcanic rocks over the past 20 m.y. may be generalized as follows: (1) high-alumina basalts of both alkalic and tholeiitic types, (2) high-alumina tholeiite, (3) low-alumina alkalic basalt, (4) low-alumina tholeiite. Andesites and dacite appear as differentiates of steps (2) and (4). The complete sequence is not represented in any one area but may apply to the whole region. The sequence suggests a progressive increase to the depth of magma separation in the mantle and may be related to the progressive change in thermal gradients and in crustal and mantle stresses as the borderland was disrupted.

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