UNDERGRADUATES MAP FORCEFULLY EMPLACED WESTERN NEVADA PLUTON AND ADJACENT REGIONAL STRUCTURES

Well-exposed, 1.6 km-long pluton margins in Copper Mountain, in the S. Koegel Hills 100 km E of the Sierra Nevada batholith, contain map- and outcrop-scale regional structures as well as unrelated local structures produced by forceful intrusion of a 155-158 Ma granitic pluton. Three sophomores paid as research assistants accompanied the first author and spent 3 weeks mapping in western Nevada at 1:4000- to 1:24,000-scale. Students described rock types, measured S1, S0, and fold orientations, located contacts, discussed alternative hypotheses, and drafted figures. Field methods were taught during fieldwork. Units mapped include: a) Upper Triassic unmetamorphosed dark gray laminated limestone and sparse interstratified carbonate diamictite, b) gray foliated marble, c) Middle Jurassic granite, d) Jurassic amphibole-feldspar porphyry dikes, and e) 2 Tertiary igneous units. Laminated deep-marine limestone contains ammonites characteristic of Middle Norian Columbianus Zone (identifications by N.J. Silberling). Granite pluton has been dated at 158 and 155 Ma (K-Ar on biotite; Ekren and Byers, 1978). Several alternative sequences of events were tested: 1) pluton emplacement produced foliation in marble (S1) and spaced cleavage in limestone (S1), 2) pluton emplacement caused marble foliation (S2) that cross-cut earlier spaced cleavage in limestone (S1), and 3) foliation in marble (S1) and spaced cleavage in limestone (S1) define a strain gradient that predated pluton intrusion. Alternative 3 best fits the data and is part of this sequence of events: 1) Regional D1 deformation produced D1 folds and a strain gradient, 2) Granite pluton intruded, cross-cutting and reorienting S1, 3) Amphibole-feldspar porphyry dikes cross-cut the granitoid and S1, 4) NE- and NW-trending folds correlated to Luning-Fencemaker thrust belt deformation folded S1, 5) Cenozoic normal and strike-slip faults of the central Walker Lane cross-cut earlier structures. Thus at this location regional and pluton-caused deformation events are unrelated. Coupled with timing constraints from the nearby Sand Springs Range, Copper Mountain relations constrain the high-strain regional D1 event to have occurred between 202 and 158 Ma: no older than Sinemurian - Pliensbachian, age of the youngest map unit deformed by D1 in the Sand Springs Range, and before 158 Ma, age of the Copper Mountain granite.