Paper No. 27-5
Presentation Time: 2:50 PM
LATE MIOCENE TO QUATERNARY SILICIC VOLCANISM IN THE WALKER LANE: AN EXAMPLE FROM THE GREENWATER RANGE, CALIFORNIA
Late Miocene to Quaternary silicic volcanism is an important feature of the Walker Lane in Nevada and California. Within and adjacent to the Walker Lane, there are 12 compositionally variable volcanic centers with silicic volcanism < 10 Ma, seven of which are < 6 Ma (Steamboat Springs, Mono-Inyo, Long Valley, Coso, Latest Bodie and Aurora, Silver Peak, and Greenwater Range). An example of a < 6 Ma silicic center in the Walker Lane is in the Greenwater Range, on Death Valley National Park's eastern boundary. Dacite to low-silica rhyolite (Greenwater rhyolite-GR) erupted from volcanic domes just to the east of the Greenwater road, in the northern part of Greenwater Canyon, and in the Black Mountains near Dante's View. GR volcanism, including pyroclastic eruptions and volcaniclastic accumulation, occurred between 6.7 and 5.1 Ma (U-Pb zircon). We dated seven GR domes and flows, and four have dates between 5.8 and 5.9 Ma, suggesting that a significant amount of GR erupted in about 100,000 years, although this volcanic system was active for about 1 m.y. Pyroclastic flows and surge and the accumulation of volcaniclastic material accompanied the formation of GR domes and flows. This series of events produced a stratigraphy with GR interbedded with the volcaniclastic section. New U-Pb dates of pyroclastic-flow deposits document five explosive events occurring at 6.7, 6.1, 5.9, 5.7, and 5.5 Ma. The first of these events may be related to the formation of a caldera or volcanic sag in the central Greenwater Range. Basalt flows dated at 4 Ma (40Ar/39Ar date) cap the GR. Busby (2013) recognized the association of volcanic centers with the Walker Lane and suggested that transtensional tectonic environments control volcano locations. Many volcanic centers are at stepovers or releasing bends. The Greenwater range lies in a similar tectonic regime between two segments of the Death Valley fault system, the Furnace Creek and Black Mountains faults. Aside from volcanoes related to subduction and active rift zones (Rio Grande, Snake River Plain), the concentration of < 10 Ma silicic centers in the Walker Lane is greater than any other area of the western US. This fault zone is not only an area of increased seismic risk but also a place of increased risk of future explosive volcanism.