NOT ALL LARGE SILICIC VOLCANIC SYSTEMS ARE TRANSCRUSTAL: EVIDENCE FROM THE SIERRA CREST-LITTLE WALKER AND EBBETTS PASS VOLCANIC CENTERS, CENTRAL SIERRA NEVADA, CALIFORNIA

Volcanic rocks from the Ebbetts Pass (EPVC; ca. 6-4.6 Ma) and the Sierra Crest-Little Walker (SCLW; ca. 12-9 Ma) volcanic centers provide a test on how structural setting can influence magma storage and transport. Both of these volcanic centers lie within well-exposed pull-apart basins, within the ancestral Cascades arc. Prior studies show that highly potassic lavas at the SCLW erupted during periods of higher rates of Walker Lane transtensional faulting. In contrast, the EPVC, while erupting similarly-evolved volcanics, lacks the high-K lavas of the SCLW and appears to have erupted during higher transtensional stress rates. Here, we present new mineral composition data (clinopyroxene, olivine, and plagioclase) that reveal contrasts between the EPVC and SCLW plumbing systems. We find that high-K volcanics of the SCLW are the only volcanics to be erupted from a very wide range of depths, from just above the mantle, to the shallower upper crust (ca. 0.5 – 9kbar). In contrast, every other eruptive unit in the SCLW records storage depths that are restricted to the upper half of the upper crust (< 2kbar). The EPVC also differs from the SCLW in that volcanics there record cooler temperatures, and a much more restricted range of pre-eruption temperatures. For example, SCLW clinopyroxenes span a 250°C-temperature range (875-1125°C), but just a mere 50°C range (950-1000°C) at the EPVC. And for olivine and plagioclase, temperatures at the EPVC are not only more restricted than SCLW, but cluster only at the low-temperature range of that exhibited by some of the SCLW volcanics. But especially intriguing is that apparently, unlike other large volcanic centers (such as Lassen), the EPVC is not transcrustal. Field work and age dates show that the transtensional basin that holds the EPVC experienced subsidence rates 50% higher than the basin that holds the SCLW system (3,000 m/my vs. 2,000 m/my), indicating that higher transtensional stresses were active in the EPVC. We posit that larger transtensional stresses encouraged recharge magmas at the EPVC to quickly transit otherwise viable lower and middle crust magmatic staging regions. This implies that transtensional stresses may exert a profound control on volcanic center development.