A klippe of unmetamorphosed Devonian carbonate rocks rests on the Harrison Pass intrusive body south of Toyn Creek in the central Ruby Mountains, Elko County, Nevada. This klippe and other klippen of Carboniferous strata—first mapped by R. P. Sharp (1942)—that rest on lower Paleozoic strata are believed to represent a once-continuous thrust sheet that developed after emplacement of the intrusive body. This intrusion of coarse-grained granodiorite to quartz monzonite is exposed over an area of about 45 square miles.

Potassium-argon and lead-alpha age determinations on four samples of the intrusive body establish Oligocene or younger age for the thrusting. Potassium-argon age determinations on biotites from the four samples range from 29 to 36 m.y. with a possible analytical error of ±10 percent. Lead-alpha age determinations (all with a possible analytical error of ±10 m.y.) on zircon showed 40 m.y. for three of the samples and 30 m.y. for the fourth. Lead-alpha and potassium-argon dates on a fifth sample collected by R. R. Coats also fall within these ranges. The agreement of the radiometric dates indicates a lack of thermal activity subsequent to the emplacement of the intrusion and establishes a maximum age of Oligocene for the thrust faulting.

The present distribution of thermally metamorphosed Paleozoic rocks in the Ruby Mountains seems to preclude the possibility that the thrust plate originated within the Ruby Mountains. Sharp suggested a western source with displacement of from 7 to 10 miles.

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