Abstract
Paleomagnetism and anisotropy of magnetic susceptibility (AMS) reveal pyroclastic flow patterns, stratigraphic correlations, and tectonic rotations in the Miocene Stanislaus Group, an extensive volcanic sequence in the central Sierra Nevada, California, and in the Walker Lane of California and Nevada. The Stanislaus Group is a useful stratigraphic marker in regard to the post-9 Ma uplift of the Sierra Nevada and transtensional tectonics within the central Walker Lane. In ascending order, the Stanislaus Group consists of the Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation with alkali- silica compositions ranging from basaltic trachyandesite to trachyte. The stratigraphy of the Eureka Valley Tuff is refined by detailed geologic mapping of the reference section located at Tollhouse Flat, California. We measured directions of remanent magnetization at 32 sites within the Stanislaus Group. The Table Mountain Latite has a distinctively shallow reversed-polarity direction of I=-26.0\deg and D=162.8\deg at sampling sites in the Sierran foothills, although the unit mainly consists of normal-polarity flows near the Sierran crest. The Tollhouse Flat Member of the Eureka Valley Tuff has a reversed-polarity magnetization (I=-62.8\deg, D=159.9\deg). The By-Day Member (I=52.4\deg, D=8.6\deg) and Upper Member (I=27.9\deg, D=358.0\deg) of the Eureka Valley Tuff overlie a polarity transition within the Eureka Valley Tuff. The Dardanelles Formation also has normal polarity. We measured vertical-axis rotations of sites in the Walker Lane and Sweetwater Mountains by establishing a virtual geomagnetic reference pole for the Tollhouse Flat Member of the Eureka Valley Tuff in the relatively stable Sierran block. We infer clockwise vertical- axis rotations of approximately 8.5\deg to 30\deg in the central Walker Lane north of Mono Lake, California, and to the east in the Anchorite Hills, Nevada. No significant vertical axis rotations relative to the central Sierra Nevada were indicated within the Sweetwater Mountains, California. The AMS results from 19 sites show that the Eureka Valley Tuff flowed outward from its proposed source area, the Little Walker Caldera, and the flow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane.