Chronology of polyphase extension in the Windermere Hills, northeast Nevada

Fission-track and Ar-40/Ar-39 dating and chemical correlation of volcanic s trata exposed in the Windermere Hips and northern Pequop Mountains, northea st Nevada, indicate a protracted, polyphase history of Tertiary (late Eocen e-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) sedi mentary rocks in the eastern Windermere Hills above the low-angle Black Mou ntain detachment fault. The early Oligocene half graben conformably overlie s 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 extensio n 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.; Availab le 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 high-grade rocks. We interpret t he east-west-striking faults to accommodate differential uplift of greensch ist-grade metamorphic rocks in the upper crust, above a lateral ramp in a w est-northwest-directed mylonitic shear zone. Subsequent extension in the Wi ndermere Hills is defined by deep, rapidly filled half grabens of middle Mi ocene (<7.42 +/- 2.0 to 14.93 +/- 0.08 Ma) age that unconformably overlie o lder faults and synextensional deposits. These are the youngest half graben s in the region and are inferred to be initiated by extensional stresses im parted to the base of the lithosphere by a laterally spreading mantle plume (e.g,, the Yellow-stone hotspot) located in southeastern Oregon at this ti me.