Timing and Tectonic Setting of Tertiary Low-Angle Normal Faulting and Associated Magmatism in the Southwestern United States

Major episodes of Tertiary low-angle normal faulting in the southwestern United States south of latitude 37°N moved northward with time, occurring about 25 million years ago in southern Arizona, 20 million years ago in the Mojave Desert region, and 10 to 15 million years ago in the Las Vegas area. In most areas, faulting and associated volcanism occurred when the Mendocino triple junction lay to the west. This correspondence probably resulted from the unstable configuration of the triple junction. East-west to northeast-southwest horizontal stretching, as plates pulled apart at the triple junction, prevented development of a hole in the lithosphere at the triple junction. The regional strain history predicted by this model is consistent with observed Tertiary low-angle faults, strike-slip faults, and folds.