PALEOMAGNETIC DATA BEARING ON THE SPATIAL EXTENT OF THE NEOGENE WALKER LANE BELT TRANSFER ZONE

The southern Walker Lane Belt (WLB) transfer zone presently links the Eastern California Shear Zone to the Central Nevada Seismic Belt and northern WLB and initially formed in the mid-Miocene as a broad, distributed zone of deformation, with defined boundaries on the northern and western margins. The extent of the system south and east is only inferred. Paleomagnetic data from the northern transfer zone suggest that early development of the transfer system involved modest-magnitude (about 20 to 30 degrees) clockwise vertical axis rotation. To better define the regional development of the WLB transfer zone, new data ( 171 sites) have been obtained from Neogene ash flow tuffs, lava flows, and shallow-level intrusions near its inferred southern and eastern boundaries (e.g., central San Antonio Range, Goldfield area, eastern Slate Ridge , and Monte Cristo Range. Data from 32 sites from the Goldfield area, near the inferred southern boundary, show a mean of D = 025.8°, I = 60.6°, α95 = 4.3°, discordant from the expected mid-Miocene field (D = 358.8°, I = 58.3°, α₉₅ = 5.0°) and indicating ~27 degrees of clockwise rotation. The central San Antonio Range, near the inferred eastern boundary, shows ~23 degrees of clockwise discordance (D = 021.5°, I = 53.9°, α95 = 5.3°, N = 21 sites). The eastern Slate Ridge area includes regionally extensive ash-flow tuffs from the Miocene Timber Mountain caldera complex, and data from these rocks (D = 041.2°, I = 55.5°, α95 = 5.8°; N = 22 sites) show ~ 50 clockwise discordancy relative to results from identical units exposed to the east that have not been rotated. Basalt flows at Thunder Mountain, east of the San Antonio Range, indicate no appreciable rotation (D = 355.8°, I = 64.4°, α95 = 11.1°, N = 5 sites). These new data suggest that the displacement transfer system, is locally characterized by greater clockwise rotation and was larger in areal extent than previously thought. We hypothesize that the displacement transfer system involved development of three detachment systems underlying a NW-SE oriented region of ~15,000 km², from east of Death Valley to past the northern end of the White Mountains including the southern WLB. These new data are consistent with the transition from a more diffuse zone of displacement transfer to considerably more localized deformation (forming the Mina Deflection) at about 3 Ma.