ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) DATA FROM MIOCENE IGNIMBRITES, WESTERN GREAT BASIN, NEVADA, AND THEIR BEARING ON THE TECTONIC EVOLUTION OF THE SOUTHERN WALKER LANE DISPLACEMENT TRANSFER SYSTEM

Mid-Miocene, regionally extensive ignimbrites in the southernmost Walker Lane area include the Tuff of Stonewall Flat (TSF), Timber Mountain Group (TMG), and Tuff of Mount Dunfee (TMD). Our new, high precision ⁴⁰Ar/³⁹Ar sanidine age determinations for each of the three ignimbrites (TSF, 7.62 ± 0.01 Ma; TMG, 11.55 ± 0.012 Ma; TMD, 16.31 ± 0.009 Ma) clarify their eruption history. Paleomagnetic and magnetic anisotropy data from these rocks bear on their mechanism of emplacement and effects of Neogene tectonism attending the formation of a major right lateral displacement transfer system. AMS data collected from 277 sampling sites, from north Goldfield to west of Beatty, are of high quality and allow interpretations of local/regional transport directions, based on imbrication fabric orientations. AMS data confirm that each of these ignimbrites was sourced from calderas to the east and southeast. The TSF, which has experienced minimal vertical axis rotation in the study area, yields a transport direction to the NW (e.g., site GP233, with a K3 trend of 315.2° and plunge of 54.7° (confidence angles (CA) = 3.8/3.5) and a source direction (K1 trend) of 183.9° and plunge of 25.0° (CA = 15.8/3.5, based on n=27 specimens). Tuffs of the TMG, which have experienced variable magnitudes of rotation, also yield general transport directions to the WNW (e.g., site GP227, with a K3 trend of 308.0° and plunge of 64.4° (CA = 9.2/7.0) and a source direction of 202.2° and plunge of 7.4° (CA = 22.6/5.7, based on n=20 specimens). The oldest ignimbrite studied, the TMD, yields a transport direction to the WSW (e.g., site GP34, with a K3 trend of 232.9° and plunge of 67.9° (CA = 15.9/10.1) and a source direction of 99.9° and plunge of 15.5° (CA = 26.9/11.4, based on n=24 specimens). Rock magnetic data and petrographic inspection show that the primary magnetic phase is magnetite. Anisotropy of remanence data will clarify the domain state of the grains and will be used to evaluate effects of changes in welding state and crystal content on magnetic fabrics. The magnetic fabrics in these ignimbrites are consistent with estimates of clockwise vertical axis rotation in deformation attending the formation of the right lateral displacement transfer system defining the southern Walker Lane.