NOT ALL LARGE, INTERMEDIATE COMPOSITION VOLCANIC SYSTEMS ARE TRANSCRUSTAL: EVIDENCE FROM THE SIERRA CREST – LITTLE WALKER & 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 of how structural setting influences magma storage and transport. Both centers lie within well-exposed pull-apart basins in the ancestral Cascades arc. Prior studies show highly potassic lavas at the SCLW erupted during periods of higher Walker Lane transtensional faulting rates. In contrast, the EPVC, while erupting similarly evolved volcanics, has few high-K lavas and erupted during higher transtensional extension rates. Here, we present new mineral composition data (clinopyroxene, olivine, and plagioclase) that reveal contrasts between the EPVC and SCLW plumbing systems.

High-K volcanics of the SCLW are the only units to erupt from a wide range of depths, from just above the mantle to the shallow upper crust (ca. 0.5-9 kbar). In contrast, other SCLW units record storage depths restricted to the upper half of the upper crust (< 2 kbar). The EPVC volcanics differ by recording cooler and more restricted pre-eruption temperatures. For example, SCLW clinopyroxenes span a 250°C range (875-1125°C), while EPVC clinopyroxenes span just 50°C (950-1000°C). Similarly, olivine and plagioclase temperatures at the EPVC cluster only at the low-temperature range of SCLW volcanics. Plagioclase thermometry shows a trend of decreasing temperature over time, while olivine temperatures are similar across both centers. This suggests magmas enter equally hot but cool more extensively at the EPVC.

Perhaps most significantly, unlike other large, intermediate volcanic centers, the EPVC is not transcrustal. Olivine profiles for diffusion timescales reveal no contrast between the centers. Fieldwork and age dates show the transtensional basin holding the EPVC experienced subsidence rates 50% higher than the SCLW basin (3,000 m/My vs. 2,000 m/My), indicating higher transtensional strain at the EPVC. We posit that larger strain rates caused recharge magmas at the EPVC to bypass viable lower and middle crust staging regions. These findings imply transtensional strain profoundly influences volcanic center development, though varying magmatic flux over time may also affect magma storage.