Eruption of alkali basalts during crustal shortening in southern California

Abstract

The common occurrence of alkali basalts in continental rifts has led to the widespread assumption that late Cenozoic alkali basalts of the western United States indicate an extensional tectonic regime. However, field relations of late Cenozoic alkali basalt flows and cinder cones in the Mojave Desert area, including flows that bear mantle xenoliths, indicate that many were erupted in contractional or transpressional tectonic settings. These observations call into question models of extensional magmatism in which lithospheric extension facilitates transport of magmas to the surface and suggest that an association of alkali basalts and coarse clastic sedimentary rocks need not indicate rifting.