DETRITAL SANIDINE POPULATIONS INDICATE PROBABLE MIDDLE MIOCENE EXTENSION, UPLIFT, AND EXPOSURE OF THE RUBY MOUNTAINS-EAST HUMBOLDT METAMORPHIC CORE COMPLEX, NORTHEAST NEVADA, USA

Terrestrial basins record major tectonic events; obtaining precise ages for these deposits is critical but challenging. In the absence of directly datable horizons (e.g., tephra), detrital zircon dating can reveal maximum deposition ages (MDAs) for strata. Assessing how close the MDA is to the depositional age can be difficult. We use precise ⁴⁰Ar/³⁹Ar dates and coupled K/Ca of detrital sanidines (DS) to (1) assess source regions, (2) evaluate detrital mineral ages that are, or are not, expected in a deposit, and (3) better constrain the depositional age and uncertainty of strata based on observed MDAs. The pre-/syn-kinematic Clover Creek basin adjacent to the Ruby Mountains-East Humboldt Range metamorphic core complex (REHR) contains 3.5 km of strata, mostly deposited during initial normal-fault exhumation of the REHR. Precise dating of the deposits is crucial to resolve the timing of faulting. Less than 300 m of 40–≤29 Ma parallel volcanic rocks accumulated in a paleovalley before faulting. Fault initiation is marked by 500 m of overlying, undated megabreccia shed from growing fault scarps. The upper 2.5 km was deposited rapidly ~15.6 to 15.3 Ma. Other constraints reveal that ductile shearing in the REHR footwall occurred in the Oligocene and regional extension initiated post-17 Ma. All except 13 Mesozoic DS from 6 samples of syn-megabreccia deposits (n=729) and 2 middle Miocene deposits (16 Ma; n=201) are ≤45 Ma, matching the regional igneous record. Precise DS dates and K/Ca uniquely tie some DS to specific sources in the 37–19 Ma ignimbrite flareup across Nevada. The MDA of syn-megabreccia samples requires deposition ≤20.1 Ma. No syn-megabreccia or middle Miocene DS fall between 20–17 Ma because the nearest felsic volcanic sources of that age were ≥400 km to the south. Although the MDA allows the megabreccia to be 20 Ma, we interpret that it was deposited ≤17 Ma in a fault-bounded basin based on equal tilting of 40–15 Ma rocks, provenance analysis, the reworked character of deposits, and the lack of 20–17 Ma felsic volcanic rocks to provide DS. A ≤17 Ma megabreccia age overlaps other records of regional extension, whereas a 20 Ma age does not correlate with any known major event and would imply slow Eo-Oligocene deposition, rapid megabreccia deposition at 20 Ma, a hiatus, then resumption of extension and rapid deposition 16–15 Ma.