Chronology of polyphase extension in the Windermere Hills, northeast Nevada
Fission-track and 40 Ar/ 39 Ar dating and chemical correlation of volcanic strata exposed in the Windermere Hills and northern Pequop Mountains, northeast Nevada, indicate a protracted, polyphase history of Tertiary (late Eocene‐late Miocene) extension along the northern margin of a major Cordilleran metamorphic core complex. Early extension is recorded by a west-tilted half graben filled with early Oligocene (34.79 ± 0.18‐39.18 ± 0.12 Ma) sedimentary rocks in the eastern Windermere Hills above the lowangle Black Mountain detachment fault. The early Oligocene half graben conformably overlies a widespread suite of late Eocene (39.18 ± 0.12‐ 40.38 ± 0.06 Ma) calc-alkaline volcanic rocks, reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare-up. These strata are cut by east-west‐striking normal faults, which are exposed along, and parallel to, the northern margin of the metamorphic complex. Available age data (e.g., between 14.93 ± 0.08 and 34.79 ± 0.18 Ma) permit the interpretation that the east-west‐ striking faults formed at the same time as, or after, large-magnitude unroofing of highgrade rocks. We interpret the east-west‐ striking faults to accommodate differential uplift of greenschist-grade metamorphic rocks in the upper crust, above a lateral ramp in a west-northwest‐directed mylonitic shear zone. Subsequent extension in the Windermere Hills is defined by deep, rapidly filled half grabens of middle Miocene (<7.42 ± 2.0 to 14.93 ± 0.08 Ma) age that unconformably overlie older faults and synextensional deposits. These are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume (e.g., the Yellowstone hotspot) located in southeastern Oregon at this time.