Paleomagnetic constraints on the geometry and timing of deformation at Yucca Mountain, Nevada

Abstract

Paleomagnetic data from Miocene ash flow sheets indicate that southern Yucca Mountain has undergone about 30° of clockwise vertical‐axis rotation since emplacement of the Tiva Canyon Member of the Paintbrush Tuff at about 13 Ma. Declinations of remanent magnetization from 32 sites in the reversely magnetized Tiva Canyon Member display a systematic southward increase over the 25 km north–south extent of Yucca Mountain. Probable errors in estimating structural attitudes are a major source of uncertainty in determining the amount of rotation between any two sites. Analysis of tilt‐corrected directions of remanent magnetization from the Tiva Canyon sites indicates that structural attitudes determined from outcrop patterns on geologic maps are generally more accurate than field measurements of attitudes of compaction foliation or contacts. Attitudes determined from map patterns apparently minimize effects of (1) initial dips, (2) poorly defined compaction foliation, and (3) tilting that occurred after compaction but before blocking of remanence; however, such attitudes cannot accurately represent tilting related to unmapped local structures. Rotations implied by data from three older ash flow sheets, the Topopah Spring Member of the Paintbrush Tuff (seven sites), and the Prow Pass (four sites) and Bullfrog (three sites) Members of the Crater Flat Tuff are generally consistent with the amount and sense of rotation documented by data from the Tiva Canyon Member. Uncertainties in the declinations of tilt‐corrected site mean directions from these older units preclude evaluating whether older units have undergone more rotation than the Tiva Canyon Member.