2000 GSA Annual Meeting -- Reno, Nevada

Abs. No. 50990

GEOLOGY OF THE CENTRAL EGAN RANGE AND ROBINSON (ELY) PORPHYRY COPPER DISTRICT, NEVADA: ANATOMY OF AN EOCENE EXTENSIONAL ACCOMMODATION ZONE

Author(s): SEEDORFF, Eric, Specialty Product Systems, 6336 Oracle Rd., Suite 326, PMB 387, Tucson, AZ 85704, seedorff@earthlink.net; MAHER, David J., 7920 Royal Ct., Tucson, AZ 85704

Keywords: Robinson, extension, accommodation, Eocene, structure

A new geologic map has been compiled for five, 7.5'-quadrangles of the central Egan Range. The new map is based on detailed remapping of the Robinson district and vicinity, located in the center of the map. In the surrounding region, previous maps were revised based on a new understanding of the distribution, geometry, and crosscutting relationships of a complex array of extensional faults, as deciphered at Robinson. The nature of contacts (i.e., depositional, intrusive, normal or reverse fault) between rocks as previously mapped was reevaluated. Also, geometric constraints were used to identify which fault segments--among numerous possibilities--are actually dismembered pieces of the same fault, offset by younger faults. The dips of a pre-Eocene unconformity, Eocene lacustrine rocks, and pre-tectonic rhyolite tuffs vary: 45 degrees west in the northwestern part of the map, versus 65 degrees east in the southeast. A large Mesozoic syncline is overturned westward where exposed in the northwestern part of the map area but overturned eastward in the southeast. Generally, the area has sets of early, closely spaced, westerly dipping faults cut by more widely spaced, easterly dipping faults, complicated by additional fault sets that have other orientations. The combined fault movement produced 400% east-west extension. North of the map area the Tertiary structure is dominated by several sets of down-to-the-east normal faults and westward tilting, whereas to the south there are down-to-the-west faults and eastward tilting. The new map covers the area where these two structural regimes overlap. This constitutes an extensional accommodation zone, where both fault sets are observed, as well as fault sets of numerous other orientations. Hence, variable orientations of the syncline, unconformity, and Eocene strata are newly interpreted to result from the sum of tilting produced by movement on the different sets of Tertiary normal faults, the dominance of which varies across the accommodation zone.


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