MESOZOIC TECTONICS AND METAMORPHISM IN THE PEQUOP MOUNTAINS AND WOOD HILLS REGION, NORTHEAST NEVADA - IMPLICATIONS FOR THE ARCHITECTURE ANDEVOLUTION OF THE SEVIER OROGEN

The Pequop Mountains-Wood Hills-East Humboldt Range region, northeast Nevada, exposes a nearly continuous cross section of Precambrian to Me sozoic strata representing middle to upper crustal levels of the Mesoz oic hinterland of the Sevier orogen. These rocks preserve the transiti on from unmetamorphosed Mesozoic upper crust to partially melted middl e crust. Integration of new structural, metamorphic, and U-Pb thermoch ronologic data from the Wood Hills and Pequop Mountains, coupled with a regional tectonic reconstruction, reveals substantial Cretaceous met amorphism, contraction, and extension in the Sevier hinterland in nort heast Nevada. We report two phases of contraction not previously recog nized that are accommodated by top-to-the-southeast thrust faults, the Windermere and Independence thrusts. Contraction was succeeded by two phases of extension along west-rooted normal faults, the Late Cretace ous Pequop fault and Tertiary Mary's River fault system. The earliest phase of thrust faulting resulted in as much as 30 km of crustal thick ening and an estimated minimum of 69 km of shortening along an inferre d fault called the Windermere thrust, The timing of this thrusting eve nt is bracketed between Late Jurassic (ca. 153 Ma) and Late Cretaceous (84 Ma), Relaxation of crustal isotherms following and perhaps during thrusting resulted in Barrovian-style metamorphism of footwall rocks, and partial melting of metapelite at deep levels. Peak metamorphism w as attained ca. 84 Ma, and by this time hinterland crustal thickening had reached a maximum. During 84-75 Ma another minor pulse of shorteni ng and thickening along the Independence thrust was followed by partia l exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the Pequop fault. Thus the interval from 84 to 75 Ma in northeast Nevada marks a fundamental, and apparently permanent, chang e from horizontal contraction to extension in the upper to middle crus t in the hinterland. Final exhumation of the metamorphic rocks was acc omplished by the Tertiary Mary's River fault system. Our data indicate that much of the metamorphism and some of the contraction in the Sevi er hinterland in northeast Nevada, which was previously thought to be largely Late Jurassic, is actually Cretaceous in age. Furthermore, the data indicate that widespread metamorphism of the middle crust is a b yproduct of tectonic burial, and that hinterland and foreland thrust f aulting were coeval, suggesting that thrust faults in the Sevier oroge n do not form a simple foreland younging sequence.