Colgan, Joseph A.¹, Henry, Christopher D.², and John, David A.¹, (1) U.S. Geological Survey, 345 Middlefield Road, MS-901, Menlo Park, CA 94025-3561, (2) Nevada Bureau of Mines and Geology, MS 178, University of Nevada, Reno, Reno, NV 89557
The modern Sierra Nevada and Great Basin were likely the site of a high-elevation orogenic plateau well into the Cenozoic, supported by crust thickened during Mesozoic shortening. Although crustal thickening at this scale can lead to extension, the relationship between Mesozoic shortening and subsequent formation of the Basin and Range is difficult to unravel because it is unclear which of the many documented or interpreted episodes of extension were the most significant for net widening and crustal thinning. To address this problem, we integrate geologic and geochronologic data that bear on the timing and magnitude of Cenozoic extension along a ~200 km east-west transect south of Winnemucca, Battle Mountain and Elko, Nevada. Pre-Cenozoic rocks in this region record east-west Paleozoic and Mesozoic compression that continued into the Late Cretaceous. Little to no tectonism and no deposition followed until intense magmatism began in the Eocene. Eocene and Oligocene ash-flow tuffs flowed as much as 200 km down paleovalleys cut as deeply as 1.5 km into underlying Paleozoic and Mesozoic rocks in an otherwise low-relief landscape. Eocene sedimentation was otherwise limited to shallow lacustrine basins in the Elko area; extensive, thick clastic deposits are absent. Minor surface extension related to magmatism locally accompanied intense Eocene magmatism, but external drainage and little or no surface deformation apparently persisted regionally until about 16–17 Ma. Major upper crustal extension began across the region ca. 16–17 Ma, as determined by cross-cutting relationships, low-temperature thermochronology, and widespread deposition of clastic basin fill. Middle Miocene extension was partitioned into high-strain (50–100%) domains separated by largely unextended crustal blocks and was over by 10–12 Ma. Bimodal volcanic rocks erupted during middle Miocene extension can be found across most of the study area but are volumetrically minor outside of the northern Nevada rift. The modern physiographic basins and ranges were formed during a distinctly different episode of extension that began after about 10 Ma and has continued to the present. Late Miocene and younger faulting is characterized by widely-spaced, high-angle normal faults that cut both older extended and unextended domains. Major widening of the Basin and Range at this latitude thus took place during a relatively brief interval in the middle Miocene, and the lack of major shortening west of the Sierra Nevada at this time suggests that the plate margin played an important role in allowing extension to occur when it did, as rapidly as it did. The onset of extension ca. 16-17 Ma was coeval with both Columbia River flood-basalt volcanism and the hypothesized final delamination of the shallow Farallon slab that was beneath the western United States in the early Tertiary, but it is unclear if these events were necessary prerequisites for extension, simply coincidental, or themselves consequences of rapid extension and/or reorganization of the plate boundary.