Abstract
The Darwin Plateau, in the southwestern Great Basin of California, is underlain by upper Cenozoic basalt flows, pyroclastic rocks, and alluvial deposits. Many of the volcanic units are laterally extensive and can be correlated across the study area, using hand-sample characteristics and magneto-stratigraphy. The age of the section, from potassium-argon dating and magnetic polarities, is largely 5.3 to 5.7 m.y., although the oldest basalt flow is 7-8 m.y. old. Structural, radiometric, and sedimentologic data all suggest that basin-range crustal extension was underway locally by 7-8 Ma, and the earliest high-angle normal faulting predates 5.8 Ma. This chronology is consistent with westward migration of tectonism in the southwestern Great Basin. Additionally, structural data suggest two extension directions, the current west-northwest-east-southeast direction and an earlier west-southwest-east-northeast one. The change in extension direction occurred after 5.7 Ma. Local and regional data also indicate that the maximum compressive and intermediate stresses in the latest stress regime have been approximately equal in magnitude. Paleomagnetic data do not indicate significant rotation of the Darwin Plateau between the many large strike-slip faults in the area.
