THE RISE AND DEMISE OF THE NEVADAPLANO: INVESTIGATING THE MECHANISMS AND TIMESCALES OF CRUSTAL THICKENING AND COLLAPSE IN THE NORTH AMERICAN CORDILLERA

The Nevadaplano is inferred from several proxies to be a major Late Cretaceous orogenic plateau comparable to the Altiplano or Tibet. Structures to thicken the crust to >55 km are sparse to absent in the Basin and Range, which requires further investigation of the mechanism and timing of crustal thickening. Using an integrated structural-thermobarometric-thermochronologic approach rooted in geological mapping and field observations, we obtain estimates for the timing, rate and magnitude of crustal thickening and collapse of the Nevadaplano. Our focus is in the Toano and Pilot ranges (TPR) in the Sevier fold-thrust belt hinterland of northeastern Nevada. The stratigraphy of the TPR is identical to that exposed in the nearby Ruby-East Humboldt metamorphic core complex. Geological mapping in the TPR reveals shortening strain (~20%) was complete prior to emplacement of the Late Jurassic Silver Zone Pass pluton. RSCM thermometry and classical thermobarometry indicate regional greenschist facies metamorphism at ~425 ℃ and 3.5-4 kbar for the lower Cambrian section, with local amphibolite facies metamorphism at >550 ℃. ⁴⁰Ar/³⁹Ar muscovite, biotite and phlogopite cooling ages from the Toano Range indicate protracted post-Jurassic cooling was interrupted by a regional Late Cretaceous thermal pulse coeval with intrusion of the 80 Ma Toano Springs pluton. ⁴⁰Ar/³⁹Ar hornblende and biotite cooling ages from late Eocene plutons in the Toano and Pilot ranges suggest emplacement at ambient crustal temperatures below biotite closure temperature (~350 ℃). Exhumation of these ranges from ~15 km-depth in the middle Miocene was accommodated by ~50% extensional strain along brittle detachment faults. The TPR record Middle Jurassic shortening and episodic magmatic heating in the Late Jurassic, Late Cretaceous and late Eocene. We found no evidence for large magnitude thrust burial of the upper curst during the Late Cretaceous, suggesting limited thickening in the hinterland during that period. Exposures of upper crustal rocks metamorphosed at pressures >6 kbar are anomalous in the Basin and Range, and possibly record non-lithostatic conditions. Our observations of <5 km of Middle Jurassic crustal thickening in northeastern Nevada requires any additional thickening to be accommodated in the lower crust.