Paleomagnetic directional dispersion produced by plastic deformation in a thick miocene welded tuff, southern Nevada: Implications for welding temperatures

Abstract

Two voluminous rhyolitic to quartz‐latitic ash flow sheets, the Tiva Canyon and Topopah Spring members of the Paintbrush Tuff, were erupted from the Miocene Claim Canyon cauldron in southern Nevada. Although lithologically similar, these units differ greatly in their recording of the ancient geomagnetic field. The reversely magnetized Tiva Canyon Member yields remanent directions that are well grouped both within and between sites. The normally magnetized Topopah Spring Member, however, yields remanent directions that vary both laterally and vertically. Remanent directions from a 100‐m‐thick section of nonwelded to partially welded Topopah Spring are well grouped (precision parameter, k > 200) and give a mean direction (D = 293°, I = 69°) that is probably a good approximation of the geomagnetic field at the time of emplacement and cooling. In three 300‐m‐thick sections of more densely welded tuff, inclinations from the upper parts of the Topopah Spring are steep (70°), but flatten with depth to about 20°, and then steepen abruptly near the base of the unit. In general, the directional dispersion increases with the thickness of the section (k ∼ 30 for sections that are about 300 m thick). Much of the directional variation in the Topopah Spring can be attributed to an inclination error produced by subblocking temperature rotation of the magnetic carriers during welding. The possibility of subblocking temperature deformation in an ash flow sheet is enhanced by (1) high‐blocking temperature spectra of the primary magnetic phases, (2) relatively low emplacement temperatures, and (3) great stratigraphic thickness. Thermal demagnetization results from the Topopah Spring Member indicate that welding may have continued to temperatures as low as 475°C in the basal vitrophyre of the ash flow sheet. Remanent directions from the Tiva Canyon Member were unaffected by the welding process because this unit possesses significantly lower blocking temperatures and does not attain the great thicknesses of the Topopah Spring.