ANISOTROPY OF MAGNETIC SUSCEPTIBILITY DATA BEARING ON THE VOLCANIC SOURCE OF THE CANDELARIA HILLS REGIONAL TUFFS, CENTRAL WALKER LANE, WEST-CENTRAL NEVADA

Anisotropy of magnetic susceptibility (AMS) of mid-Tertiary volcanic rocks in the Candelaria Hills, Nevada, were investigated to identify the source areas of four regional tuffs in this tectonically important part of the central Walker Lane. The four tuffs include three previously recognized ignimbrites (Candelaria Junction (CJ), Belleville (BV), and Metallic City (MC) tuff) and one composite tuff (UT). Recent ⁴⁰Ar/³⁹Ar age estimates on CJ and MC, of 23.7 Ma and 25.8 Ma, respectively, constrain the age of the regional tuffs and define a relative eruptive progression. The BV tuff lies stratigraphically between CJ and MC. The stratigraphic position of UT is variable.

AMS data were collected from 57 sites with 56 yielding interpretable results. Interpretation of AMS patterns is not straightforward due to Neogene to recent tectonic activity that has fragmented the tuffs into many tectonic blocks along strike-slip and oblique normal faults and paleomagnetic evidence suggests modest (20° to 30°) clockwise vertical axis rotation of parts of the area. As an initial evaluation, we have applied a uniform regional tectonic (rotation) correction of 25° to all AMS data. The AMS data reveal a systematic pattern of K1 axes that define at least three transport directions. CJ yields a NNW-SSE flow direction and BV yields a NW-SE flow direction. MC yields two spatially variable flow directions (west: N-S direction, east: WNW-ESE direction). UT yields variable flow directions, but a general NE to NW pattern. We hypothesize that a buried source region is located SSE of the Candelaria Hills, possibly in the Columbus Salt Marsh area. The source of UT is difficult to infer, although a local source in the Candelaria Hills is permissible. Overall, the distribution of K1 axes precludes a source lying to the east or west. Speed and Cogbill (1979) proposed a volcanic source located in the Candelaria Hills. Based on AMS data we find a local source unlikely. The high dispersion between some tuffs may reflect variable rotation between blocks or different source regions. The definition of a fixed or stable block within the area may allow for specific tectonic corrections to be applied to each structural block, thereby refining our interpretation presented here; ongoing paleomagnetic investigations are currently under way to evaluate this possibility.