THE DISPARATE RECORD OF UPPER-CRUSTAL EXTENSION AND MID-CRUSTAL EXHUMATION IN AND AROUND THE RUBY-EAST HUMBOLDT METAMORPHIC CORE COMPLEX, NORTHEASTERN NEVADA, USA

The record of Cenozoic extension in NE Nevada has been intensely studied yet remains controversial, particularly with respect to unroofing of the Ruby-East Humboldt (RMEH) metamorphic core complex. Studies of metamorphic rocks in the RMEH indicated major decompression—inferred to result from extension—that began as early as Late Cretaceous, peaked in the Eocene to early Miocene, and was followed by Miocene and younger Basin and Range extension. Together, these processes unroofed rocks from depths up to 9-10 kb equivalent. In contrast, studies of Cenozoic surficial deposits and brittle faults in NE Nevada indicate only minor Eocene extension (~40-38 Ma), with major upper-crustal extension (that accomplishes map-view widening) taking place ~16-12 Ma. Eocene sedimentary and volcanic rocks are deposited on unmetamorphosed upper Paleozoic rocks that yield Mesozoic low-temperature cooling ages, and no topographic barriers prevented 45–40 Ma ash-flow tuffs from flowing eastward down paleovalleys from NE Nevada into Utah. The presence of ≤1 km of late Eocene sedimentary—especially lacustrine—deposits, a low-angle (≤15°) angular unconformity between ~40 and 38 Ma rocks, and ENE-striking normal faults attest to a minor episode of NW extension ~40-38 Ma. The lack of angular unconformity between ~38 and 16 Ma rocks, and the overall absence of sedimentary rocks and faults of that age, indicates little if any surface deformation ~38-16 Ma. Most demonstrable upper-crustal deformation in NE Nevada took place ~16-12 Ma, as indicated by thick (≥4 km) sedimentary deposits in half-graben basins, tilting and fanning of dips in the synextensional sediments, and low-temperature cooling ages that indicate rapid cooling of the southern Ruby Mountains and other ranges at that time. Gravitational potential energy alone or with thermal weakening by Eocene magmatism was therefore insufficient to drive major upper crustal extension, which did not occur until regional tectonic boundary conditions permitted it in the middle Miocene. Miocene extension may have unroofed RMEH rocks from ~15 km depth. The earlier decompression must have taken place with either little or no concurrent surface deformation, or in such a way that upper-crustal extension was so highly localized that no record of it is preserved.