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

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%) occured 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.