ABSTRACT Volcanic rocks from the Ebbetts Pass (ca. 6–4.6 Ma) and the Sierra Crest–Little Walker (ca. 12–9 Ma) volcanic centers provide a test of how structural setting can influence magma storage and transport. Both volcanic centers lie within well-exposed pull-apart basins within the ancestral Cascades arc. Prior studies show that highly potassic lavas at the Sierra Crest–Little Walker volcanic center erupted during periods of higher rates of Walker Lane transtensional faulting. In contrast, the Ebbetts Pass volcanic center, while erupting similarly evolved volcanics, has few high-K lavas, and the volcanics appear to have erupted during higher transtensional extension rates. Here, we present new mineral composition data (clinopyroxene, olivine, and plagioclase) that reveal contrasts between the Ebbetts Pass and Sierra Crest–Little Walker plumbing systems. We found that the high-K volcanics of the Sierra Crest–Little Walker volcanic center are the only volcanic materials to be erupted from a very wide range of depths, from just above the mantle to the shallow upper crust (~0.5–9 kbar). In contrast, every other eruptive unit in the Sierra Crest–Little Walker volcanic center records storage depths that are restricted to the upper half of the upper crust (<2 kbar). The Ebbetts Pass volcanic center also differs from the Sierra Crest–Little Walker volcanic center in that volcanics there record cooler temperatures and a much more restricted range of pre-eruption temperatures. For example, clinopyroxenes span a 250 °C temperature range (875–1125 °C) at the Sierra Crest–Little Walker volcanic center but just a mere 50 °C range (950–1000 °C) at the Ebbetts Pass volcanic center. For olivine and plagioclase, temperatures at the Ebbetts Pass volcanic center are not only more restricted than those at the Sierra Crest–Little Walker volcanic center, but they cluster only at the low-temperature range of that exhibited by some of the Sierra Crest–Little Walker volcanics. Plagioclase thermometry shows a trend of decreasing temperature with time, while olivine temperatures are similar across the Sierra Crest–Little Walker and Ebbetts Pass volcanic centers. We infer this to mean that the magmas entered the system equally hot, but that cooling was greater at the Ebbetts Pass volcanic center. Perhaps our most significant finding is, unlike other large, intermediate-composition volcanic centers, the Ebbetts Pass volcanic center is not transcrustal. We also measured olivine profiles for diffusion time scales, but we found no evidence of a contrast in such time scales between the two volcanic centers. Fieldwork and age dates show that the transtensional basin that holds the Ebbetts Pass volcanic center experienced subsidence rates 50% higher than the basin that holds the Sierra Crest–Little Walker volcanic center system (3000 m/m.y. vs. 2000 m/m.y.), indicating that higher transtensional strain rates were active in the Ebbetts Pass volcanic center. We posit that larger transtensional strain rates encouraged recharge magmas at the Ebbetts Pass volcanic center 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. However, we acknowledge that magmatic flux could be a significant limitation in interpreting these findings, as varying magmatic input over time might influence magma storage.

Abstract Day 4 of the Field Forum is dedicated entirely to the magmatic evolution of the Tuolumne Intrusive Complex. The Tuolumne Intrusive Complex is mostly located in Yosemite National Park and is superbly exposed due to a series of glaciations that started 2–3 million years ago and ended ca. 10,000 yr B.P. one of the many reasons why it has become one of the most studied intrusions in the world. We will be driving from Mammoth Lakes to Yosemite National Park taking Hwy 395 north and then Hwy 120 west crossing the entire Tuolumne Intrusive Complex before we start off at its western margin. On this side, the Tuolumne Intrusive Complex intruded into the rocks of the Yosemite Valley Intrusive Suite, Sentinel, and Yosemite Creek granitoids (Day 3). Today we will make stops along Hwy 120 while traversing the Tuolumne Intrusive Complex from west to east ( Fig. 4-1) looking at exposures near the road and discussing field data, structural geology, geochronology, and geochemistry from the Tuolumne Intrusive Complex…

Abstract To expand our evaluation of the Mesozoic Sierran arc, we need to consider the volcanic section that overlies the intrusive parts of the arc seen in Days 1–4. While doing so, we can discuss evidence for links between the magmatic and volcanic components of the arc, the nature of regional sedimentation and tectonism that accompanied active volcanism, and the challenges of estimating volume rates of volcanic activity in ancient arc systems. The most complete exposures of the volcanic section of the Mesozoic arc in the central Sierra occur in several pendants near the eastern topographic divide of the Sierra Nevada Mountains. These volcanic sections are exposed in the Saddlebag and Ritter Range pendants. We have completed new 1:24,000–1:10,000 scale mapping in four areas within these eastern pendants, which from north to south are: (1) Eagle Creek pendant around Twin Lakes, (2) the Virginia Canyon area, (3) the Saddlebag Lake pendant, and (4) the northern Ritter Range pendant…

Abstract Our goals today are several-fold. We have now spent five days examining different parts of the Mesozoic Sierran arc, and hopefully discussions are already under way attempting to integrate both the shared and distinct characteristics of these individual magma plumbing systems and synchronous tectonics. We will briefly continue these discussions below. Our main focus will be to consider the arc as a whole and introduce a number of new regional data sets related to the tectonic and magmatic components of this arc. By the end of the day, we hope that our discussions have evolved to a consideration of the overall petrologic evolution of the arc, the tectonic and magmatic arc tempos, and their potential links. Without an airplane, or satellite, or Hollywood earth coring machine, it is difficult to take you to field locations where we can observe large sections of the arc. Instead, as we travel west back across the arc, we have selected a number of scenic overview stops, where we will introduce and discuss these new data sets while looking at gorgeous views of the arc…

Abstract We begin our journey through the Mesozoic Sierran arc with an examination of the Guadalupe Igneous Complex, a layered Jurassic pluton that intrudes into largely oceanic materials in the Foothills Terrane of the Western Metamorphic belt (Figs. 1-1 and 1-2). The Guadalupe Igneous Complex is an intuitively pleasing target to begin with because of its outboard (western) location and because it consists of some of the most mafic (>8% MgO gabbros) and felsic (high-silica and high-K 2 O granophyres and rhyolites) igneous units that we will see on this trip and thus raises some longstanding petrologic questions about the connections between mafic and felsic granitoids in arcs. It is also an exciting objective because of the preservation of its likely feeder zone (the Hornitos pluton), internal layering, and capping volcanics (Fig. 1-2)…