VOLCANISM DUE TO TRANSTENSION AT THE BIRTH OF THE SIERRA NEVADA MICROPLATE: SIMILARITIES TO ONGOING CONTINENTAL LITHOSPHERIC RUPTURE AT NEARBY LONG VALLEY

High-K volcanic rocks of the Stanislaus Group record the onset of transtensional calving of the Sierra Nevada microplate off the western edge of the Nevadaplano at 11–9 Ma, in the axis of the Ancestral Cascades arc - not delamination. This magmatic event was immediately preceded by development of a deep erosional unconformity, which records uplift at the onset of transtension. The high-K volcanic rocks were deposited within a complicated paleocanyon network that drained the Nevadaplano. In the largely unfaulted Sierra Nevada block, the volcanic rocks are preserved as erosional remnants on modern Sierran ridges (through topographic inversion). Along the Sierra Nevada range front, the volcanic rocks show growth relationships with range-front faults, and vent positions and types were controlled by faults.

The type section of the Stanislaus Group on the Bald Peak-Red Peak ridgeline shows the following stratigraphy: (1) Table Mountain Latite trachyandesite (latite) and basaltic-trachyandesite (shoshonite) lava flows; (2) Tollhouse Flat Member of the Eureka Valley Tuff (EVT) trachydacite welded ignimbrite; (3) newly-recognized trachydacite lava flow, Lava Flow Member of the EVT; (4) By-Day Member of the EVT, trachydacite welded ignimbrite; (5) Upper Member of the EVT, trachydacite unwelded ignimbrite; and (6) Dardanelles Formation, aphyric basaltic-trachyandesite (shoshonite) lava flow. Down-paleocanyon (west) of the type section lies a landmark ridge called The Dardanelles; however, we find that it is not capped by Dardanelles Formation, as previously reported. Instead, it is capped by trachydacite Lava Flow Member of the EVT, which thickens between the Red Peak-Bald Peak ridge and The Dardanelles, from 35 m to greater than 70 m (top eroded). Silicic lava flows were not recognized in the Stanislaus Group of the Sierra prior to our work.

Trachydacite ignimbrites of the Stanislaus Group vented from the Little Walker Caldera, which we have interpreted to lie at a releasing stepover along the range front faults. We map small trachydacite intrusions that emanate NW from the caldera along the faults. These features are similar to those of the nearby Long Valley Caldera – Inyo Mono chain, where continental lithosphere continues to be ruptured along a releasing bend, although no longer above a subducting slab.