PALEOMAGNETIC AND 40AR/39AR DATA FROM REMAGNETIZED MAFIC DIKES AND HOST GRANITE ROCKS IN THE SOUTHWEST SILVER PEAK RANGE, NEVADA, TERTIARY STRUCTURAL EVOLUTION OF THE CENTRAL WALKER LANE

Paleomagnetic data from lower plate intrusive rocks and upper plate volcanic rocks in the Silver Peak Range (SPR), central Walker Lane (CWL), suggest that late Neogene displacement transfer has been accommodated by differential motion among tectonic blocks. The present-day stress field in the CWL is related to displacement transfer from the Owens Valley-Furnace Creek fault systems to transtensional structures of the CWL with strain accommodated via tilting and vertical axis rotation. Block geometries, boundaries, and stability of the strain field since the mid-Tertiary are currently unclear. New ⁴⁰Ar/³⁹Ar age determinations from mafic dikes and granite rocks and paleomagnetic data that bear on the Late Cretaceous (K) to recent structural evolution of the SPR. Eleven ⁴⁰Ar/³⁹Ar age estimates all yield somewhat disturbed age spectra that reflect alteration and 39Ar recoil. Seven mafic dike groundmass concentrates yield saddle-shape age spectra with integrated apparent ages between 85 and 80 Ma and 4 granite biotite age spectra yield double-hump spectra with integrated values between 92 and 78 Ma. Fifty two sites (each in a separate dike, with 7 to 10 samples per site) were established with 40 sites fully demagnetized; 31 yield consistent, in situ, normal polarity results. The excluded sites either had mean directions greater than 2s from the overall group mean (5 sites) or were indistinguishable from the Quaternary field direction (4 sites). In situ results provide an overall group mean (D=35.8° , I=53.7° , a₉₅=4.23°) that is discordant to a late-K expected field (~341° , 69° ). Mafic dikes were emplaced into late Jurassic granite country rock and no reference to the paleohorizontal is currently available. Twenty five sites in a gently tilted sequence of Miocene pyroclastic rocks resting nonconformably on the granite are currently being demagnetized and should provide a minimum estimate of vertical axis rotation (if any) in the region. We tentatively interpret the data to indicate about 50° clockwise vertical axis rotation of part of the SW SPR. The ⁴⁰Ar/³⁹Ar age data suggest that dike remanence acquisition occurred in the late-K. The near uniform distribution of site mean data may reflect a pervasive reheating event related to late-K magmatism to the north that remagnetized rocks and reset Ar systematics.