The Pacific-North America plate boundary does most of its work in California, but its reach extends deep into Arizona. In Geosphere, SES graduate student Timothy Brickey and co-authors reconstruct how the south Dome Rock Mountains basin, in the lower Colorado River corridor of west-central Arizona, recorded that distant tectonic drama over more than 30 million years.
Working with SES faculty Paul Umhoefer and Christine Regalla, along with collaborators at the U.S. Geological Survey and NAU, Brickey combined new geologic mapping with zircon uranium-lead dating of six volcanic marker beds, the timestamps that let the team put ages on each chapter of the basin’s history.
The story unfolds in three acts. From about 35 to 24.4 million years ago, rivers filled an internally drained mountain basin with sediment. A pulse of volcanic rocks was then emplaced just before the ground itself began to pull apart: after 23.3 million years ago, coarse conglomerates piled up on the hanging wall of a growing half graben as the listric south Dome Rock Mountains fault slipped 3 to 7 kilometers, the local expression of the great Basin and Range extension. Finally, sometime after 12.8 million years ago, the region’s deformation switched character entirely, from stretching to dextral shear, as the modern sideways-sliding plate boundary asserted itself.
The result is a precisely dated record of how plate-boundary deformation played out more than 100 kilometers from the boundary itself, in the rocks of a range that many Arizonans drive past on Interstate 10 without a second look.
The study is part of Brickey’s graduate research in structural geology and basin analysis.
Reported by SES Research Highlights for the School of Earth and Sustainability.