CONTROLS OF EXTENSION ON CLIMACTIC ARC MAGMATISM: EBBETTS PASS-CARSON PASS AREA, SIERRA NEVADA (CA)

Ancestral Cascades arc volcanism in the central Sierra Nevada occurred in three Miocene pulses, at about 16-13 Ma, 11-9 Ma, and 7-6 Ma. Our new work in the Carson Pass to Ebbetts Pass area shows that range-front faults clearly controlled the positions of volcanic centers during the third magmatic pulse. These are the biggest volcanic centers in the central Sierra, and the only ones with silicic volcanic rocks, other than the pulse 2 Little Walker Caldera of the Sonora Pass area. They include the Markleeville Center southeast of Carson Pass and the Ebbetts Pass Center east of Ebbetts Pass.

The 8 km diameter Markleeville Center consists of andesites and dacites that formed within the Hope Valley graben. The next fault to the east of the Hope Valley graben, which we name the Grover Hot Springs fault, extends southward to Ebbetts Pass, where it forms an overlapping right (releasing) step with the Noble Canyon fault to the west. Activity on the Grover Hot Springs and Noble Canyon fault overlapped in time as well as space, resulting in tilting of the intervening block, which shed landslides into a paleo-canyon, exhuming basement on the ramp. Then the ramp acted as a sediment transfer path for granitic boulders that were funneled into the paleo-canyon.

The Ebbetts Pass Center straddles the Noble Canyon fault, the Grover Hot Srings fault and the Silver Mountain fault. Its largest feature is a 10 km diameter volcano, sited on the Grover Hot Springs fault, that consists of radially-dipping basaltic andesite lava flows and overlying scorea fall deposits with internal slide and slump features; the core of this volcano is intruded by dacite plugs. Rhyolites also form part of the Ebbetts Pass Center; the volcano rests upon the only rhyolite ignimbrite we have found within the Sierra Nevada Ancestral Cascades arc, and the only ultra-welded one of any composition. At the same stratigraphic level off the south flank of the volcano, the rhyolites include lava flows up to 200 m thick. The Ebbetts Pass center also includes two-pyroxene dacites. The assemblage at Ebbetts Pass may thus represent a transition between arc magmatism and Basin and Range bimodal magmatism.