Cenozoic evolution of the Shawave-Nightingale horst block, Northwestern Basin and Range, Nevada, U.S.A

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 link between extension and regional magmatism, (3) the role of strain partitioning eastward into the NBR as the Mendocino Triple Junction (MTJ) migrated northwestward, 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 crustal blocks to the east and east-tilted blocks to the west. It is bounded to the east by the &sim30°E dipping Granite Springs Valley fault (GSVF) and to the west by the &sim35°W dipping Nightingale Mountain fault. Field data and coupled apatite fission track and (U-Th)/He thermochronology constrain &sim10.5 km of normal fault slip initiated by ca. 14.5 Ma. The apparent AHE thermochronology age and paleodepth data for the structurally deepest sample indicate that it was uplifted from a depth of &sim4 km to within 1--2 km of the earth's surface by 8.6 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. A thick (> 2.0 km) section of differentially west-tilted (11°--38°), syn-tectonic alluvial and lacustrine strata are exposed in the hanging-wall block of the GSVF. Age constraints from U-Pb age dating of the uppermost tephra layer and 40Ar/39Ar age dating of a basalt layer, indicate that the deposition of the lacustrine sequence began ca. &ge 11.5 Ma and continued through 9.0 Ma. Tilt-fanning of strata and conglomerate units mapped within the hanging wall sequence document a minimum of three extensional pulses the first began by 11.5 Ma and was followed by two smaller events that initiated by 9.0 Ma. Together, geologic and thermochronologic data indicate that extension was ongoing through ca. 9.0 and likely continued through to the present day at a mean rate of &ap 0.8 km/my. In contrast to modest amounts of extension to the north (5--25%) and moderate to large amounts of extension to the east and south (40--200%), the total amount of extension across the SNHB documented here is 20--40%. When considered in light of the timing of initiation and duration of extension, the SNHB study area is interpreted to represent a transitional stage in extension that documents the northwestward progression in the kinematic evolution of the Northern Basin and Range Province.Detailed mapping of volcanic strata across the horst block documents three episodes of volcanism spanning &sim7 m.y. and coincident with (1) the onset of Miocene extension in the central and eastern northern Basin and Range Province (&sim19--15 Ma), (2) the onset of extension across the SNHB (&sim14.3--12.5 Ma), and (3) at 11.5 Ma during the peak of slip on the GSVF.The timing of initiation of extension predates passage of the MTJ to its present day latitude. However, the geologic data suggest that normal fault slip on the GSVF may have rejuvenated ca. 9.0 Ma co-eval with the initiation of right lateral shear within the adjacent northern Walker Lane (NWL) associated with the northern migration of the MTJ. Present day plate boundary strain is partitioned as extension within the SNHB study area and as dextral shear in the adjacent NWL.