Thesis

The Stanislaus Group in a Beheaded Drainage: Tectonics at the Margin of the Sierra Nevada Microplate

West of Bridgeport Valley near the Sierra Nevada crest, the Little Walker Caldera erupted Late Miocene Stanislaus Group lavas (Table Mountain Formation) and ignimbrites (Eureka Valley Tuff). Remnants of these rocks are now distributed from the western Sierra Nevada foothills across the range and into the Walker Lane. This wide distribution is attributed to the lavas flowing down paleochannels, and provides an excellent marker for post-emplacement deformation in the region. Priest (1978) documented a thick section of these lavas along Flatiron Ridge and other peaks surrounding Buckeye Canyon, including four members in stratigraphic order: Lower, Large Plagioclase, Two-Pyroxene, and Upper Member. 40Ar/39Ar geochronology indicates these Table Mountain Formation Lavas erupted from 10.4-9.5 Ma. Lithologically similar lavas have been identified near Rancheria Mountain, geochemical and paleomagnetic data support this correlation. The lavas flowed down a now-beheaded late Miocene drainage, supporting a westward shift of the Sierra Nevada crest since the Late Miocene.

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