GEOLOGIC AND THERMOCHRONOLOGIC CONSTRAINTS ON THE STRUCTURAL EVOLUTION OF THE SHAWAVE-NIGHTINGALE HORST BLOCK, NORTHWESTERN NEVADA

Geologic and thermochronologic constraints on the timing of initiation and amount of extension across the Shawave Nightingale horst block (SNHB) in northwestern Nevada provide insight into several outstanding questions related to the thermal and structural evolution of the northern Basin and Range Province (NBR). Of specific interest are (1) the timing of northwestward progression of extension in the NBR between 15 Ma and 12 Ma, (2) the temporal links between extension and regional magmatism, (3) the role of strain partitioning in the NBR as the Mendocino Triple Junction (MTJ) migrated northwestward since the late Miocene, and (4) how the region factors into the contemporary plate boundary strain budget.

The SNHB is a synclinal accommodation zone formed at the transition from west-tilted fault blocks to the east and east-tilted blocks to the west. It is bounded to the east by the ~30˚E dipping Granite Springs Valley fault (GSVF) and to the west by the west dipping Nightingale Mountain fault. Coupled apatite fission track and (U-Th)/He thermochronology indicate that fault slip initiated by ca. 14.5 Ma and was continuous through ca. 8.5 Ma. Active normal slip on the Granite Springs Valley fault (GSVF) is evidenced by a 47 km long, Holocene scarp that steps east off the ancestral GSVF. Surface erosion coupled with tectonically-enhanced stream incision exposed a thick (> 2.7 km) section of differentially west-tilted (11˚-39˚), lacustrine-alluvial strata deposited syn-tectonically in the hanging-wall block of the ancestral GSVF. Through U-Pb dating of the uppermost tephra layer and 40Ar/39Ar dating of a basalt flow, we constrain deposition of the lacustrine sequence to have occurred between ca. > 11.4 + 0.9 Ma and 9.0 + 0.4 Ma. Tilt-fanning and conglomerate units within the hanging wall sequence document at least three extensional pulses; the first began by 11.4 Ma and was followed by two smaller events that initiated by 9.0 Ma; all events predate passage of the MTJ at this latitude.

Detailed mapping of volcanic strata across the horst block documents three episodes of volcanism spanning ~7 m.y. and coincident with (1) the onset of Miocene extension in the central and eastern northern Basin and Range province to the east (~19-15 Ma), (2) the onset of extension across the SNHB (~14.3 - 12.5 Ma), and (3) at 11.4 + 0.94 Ma during relatively rapid slip on the GSVF.