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Stratigraphy, paleomagnetism, and anisotropy of magnetic susceptibility of the Miocene Stanislaus Group, central Sierra Nevada and Sweetwater Mountains, California and Nevada

¹ Geology Department, California State University, Sacramento, California 95819, USA
² U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA
³ 420 Chaucer Street, Palo Alto, California 94301-2201, USA
⁴ Geology Department, California State University, Sacramento, California 95819, USA
⁵ Earth Sciences Department, University of California, Santa Cruz, California 95064-1077, USA

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 (Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation) is a useful stratigraphic marker for understanding the post–9-Ma major faulting of the easternmost Sierra Nevada, uplift of the mountain range, and transtensional tectonics within the central Walker Lane. The Table Mountain Latite has a distinctively shallow reversed-polarity direction (I = –26.1°, D = 163.1°, and ₉₅ = 2.7°) at sampling sites in the foothills and western slope of the Sierra Nevada. In ascending order, the Eureka Valley Tuff comprises the Tollhouse Flat Member (I = –62.8°, D = 159.9°, ₉₅ = 2.6°), By-Day Member (I = 52.4°, D = 8.6°, ₉₅ = 7.2°), and Upper Member (I = 27.9°, D = 358.0°, ₉₅ = 10.4°). The Dardanelles Formation has normal polarity. From the magnetization directions of the Eureka Valley Tuff in the central Walker Lane north of Mono Lake and in the Anchorite Hills, we infer clockwise, vertical-axis rotations of 10° to 26° to be a consequence of dextral shear. The AMS results from 19 sites generally show that the Eureka Valley Tuff flowed outward from its proposed source area, the Little Walker Caldera, although several indicators are transverse to radial flow. AMS-derived flow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane.

Keywords: ash-flow tuff • Miocene • California • paleomagnetism • magnetic anisotropy