Paleomagnetic and 40Ar/39Ar geochronologic data bearing on the structural evolution of the Silver Peak extensional complex, west-central Nevada

Abstract

Abstract The Silver Peak extensional complex, located in the Silver Peak Range of west- central Nevada, is a displacement-transfer system linking the Furnace Creek–Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volcanic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths. Paleomagnetic investigation of Miocene–Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal- axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the extensional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual- polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (declination D = 329°, inclination I = 37°, α95 = 4.3°, number N = 30 sites; in situ) (α95 = the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity results that are interpreted to indicate modest clockwise vertical-axis rotation (D = 021°, I = 57°, α95 = 4.3°, N = 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization directions that are north-directed with positive inclinations of moderate steepness, similar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical-axis rotation (D = 032°, I = 53°, α95 = 8.8°, N = 10 sites). The apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remagnetized shortly after deposition. Of eight 40Ar/39Ar determinations from mafic dikes in the lower plate, five groundmass concentrates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain excess Ar. If rapid cooling and Ar retention below ∼250 °C are assumed, the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10.5 Ma. The 40Ar/39Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar isotopic age information from lower-plate granitic rocks and indicate rapid cooling of the lower-plate assemblage from well above 300 °C to 100 °C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side-up tilt and clockwise rotation and the younger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system.