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Rapid Eocene extension in the Robinson district, White Pine County, Nevada: Constraints from ⁴⁰Ar/³⁹Ar dating

¹ Department of Geological Sciences, University of California, Santa Barbara, California 93106-9630, USA
² Specialty Product Systems LLC, 6336 North Oracle Road, Suite 326, PMB 387, Tucson, Arizona 85704-5480, USA
³ 7831 Wade Springs Drive, Tucson, Arizona 85743, USA
⁴ BHP Tintaya S.A., Avenida San Martín 301, Urbanización Vallecito, Arequipa, Peru
⁵ 7920 North Royal Court, Tucson, Arizona 85704, USA
⁶ 3055 Natalie Street, Reno, Nevada 89509-3873, USA
⁷ Department of Forestry and Geology, 735 University Avenue, University of the South, Sewanee, Tennessee 37383-1000, USA

Precise ⁴⁰Ar/³⁹Ar ages on pretectonic, syntectonic, and posttectonic rhyolites from the Robinson mining district, which is within a highly extended domain in east-central Nevada, indicate that extreme extension (400%) occurred in fewer than 900 k.y. during the Eocene. The duration of extension is tightly delimited by an Eocene lacustrine unit that was horizontal prior to extension and by younger rhyolitic units. The rhyolites represent at least five eruptive episodes, including the preextensional and synextensional rhyolite of White Hill emplaced between 37.6 and 37.4 Ma, and the postextensional rhyolite of Garnet Hill emplaced at 36.7 Ma, although some of it is perhaps as old as 37.1 Ma. Thus, the duration of extreme extension at Robinson was between 0.9 and 0.3 m.y., comparable to or shorter than other highly extended areas in western North America. The extensional domain in east-central Nevada is a composite feature, produced by localized, episodic, rapid extension that began in early Eocene time and continued to at least middle Miocene time.

Key Words: Ar/Ar • geochronology • extension • accommodation zones • rhyolite • tilting

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