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Seismic Evidence for Significant Melt Beneath the Long Valley Caldera, California, USA

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Seismic Evidence for Significant Melt Beneath the Long Valley Caldera, California, USA University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln USGS Staff -- Published Research US Geological Survey Seismic evidence for significant melt beneath the Long alleV y Caldera, California, USA Ashton F. Flinders David R. Shelly Philip B. Dawson David P. Hill Barbara Tripoli See next page for additional authors Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub Part of the Geology Commons, Oceanography and Atmospheric Sciences and Meteorology Commons, Other Earth Sciences Commons, and the Other Environmental Sciences Commons This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Ashton F. Flinders, David R. Shelly, Philip B. Dawson, David P. Hill, Barbara Tripoli, and Yang Shen Seismic evidence for significant melt beneath the Long Valley Caldera, California, USA Ashton F. Flinders1*, David R. Shelly1, Philip B. Dawson1, David P. Hill1, Barbara Tripoli2, and Yang Shen3 1U.S. Geological Survey, California Volcano Observatory, Menlo Park, California 94025, USA 2Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA 3Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA 3 4 ABSTRACT gases (CO2, He/ He), have led to the perspective A little more than 760 ka ago, a supervolcano on the eastern edge of California (United that the caldera-forming magma reservoir is now States) underwent one of North America’s largest Quaternary explosive eruptions. Over this near wholly crystallized (Hildreth, 2017). This ~6-day-long eruption, pyroclastic flows blanketed the surrounding ~50 km with more than 1400 assessment holds crucial implications for the km3 of the now-iconic Bishop Tuff, with ashfall reaching as far east as Nebraska. Collapse of interpretation of ~30 yr of ongoing uplift cen- the volcano’s magma reservoir created the restless Long Valley Caldera. Although no rhyolitic tered on the post-caldera resurgent dome, and eruptions have occurred in 100 k.y., beginning in 1978, ongoing uplift suggests new magma the long-term hazard of the volcano (Hill and may have intruded into the reservoir. Alternatively, the reservoir could be approaching final Montgomery-Brown, 2015). crystallization, with present-day uplift related to the expulsion of fluid from the last vestiges Is uplift at Long Valley driven by the exsolu- of melt. Despite 40 years of diverse investigations, the presence of large volumes of melt in tion of magmatic fluids from the last few ves- Long Valley’s magma reservoir remain unresolved. Here we show, through full waveform tiges of melt from the caldera-forming reser- seismic tomography, a mid-crustal zone of low shear-wave velocity. We estimate the reservoir voir; i.e., “second boiling” (Hildreth, 2017)? Or contains considerable quantities of melt, >1000 km3, at melt fractions as high as ~27%. While could the uplift be related to the intrusion of new supervolcanoes like Long Valley are rare, understanding the volume and concentration of magma (Battaglia et al., 1999)? While North melt in their magma reservoirs is critical for determining their potential hazard. America’s other two Quaternary supervolcanoes, Valles and Yellowstone, likely contain signifi- INTRODUCTION Long Valley’s caldera-forming supereruption cant quantities of melt (Huang et al., 2015; Steck Volcanoes capable of explosive caldera- was followed by 120 k.y. of subPlinian activity et al., 1998), Long Valley remains enigmatic. forming supereruptions are exceedingly rare, yet and 400 m of resurgent dome uplift (Hildreth, are arguably the most globally catastrophic natu- 2017). While minor eruptions continued to 100 Geophysical Overview ral process on the planet. In a single eruption, ka, there have been no eruptions on this resur- The search for a modern magma reservoir these volcanoes can erupt >1000× the volume gent dome in 500 k.y. (Hildreth, 2017). This beneath Long Valley led to more than 20 geo- that erupted from Mount St. Helens (Washing- quiescence, along with a perceived migration physical studies over the past 40 yr, most com- ton State, USA) in 1980 (1 km3) (Crosweller et of volcanism westward toward compositionally monly based on local-earthquake tomography al. 2012). Of the 13 Quaternary-active supervol- distinct systems (Hildreth, 2017), a post–ca. 300 (LET) of P-wave and/or S-wave traveltime canoes in the world, three are in the continental ka decrease in high-temperature hydrothermal (VP, VS) or S-wave attenuation. While these stud- United States: Long Valley (California), Valles activity, and the current absence of magmatic ies agree there is likely no melt within the upper (New Mexico), and Yellowstone (Wyoming) (Crosweller et al. 2012). 120˚W119˚W 118˚W At approximately 767 ka, Long Valley vol- cano in eastern California (Fig. 1) erupted >1400 km3 of rhyolitic ash and pyroclastics in an ~6-day-long Plinian eruption (tephra equivalent; Hildreth and Wilson, 2007). With the exception Figure 1. The 127 seismic of the 639 ka eruption of Yellowstone, this is stations (green triangles) Walker Lane and 11 regional earth- North America’s most recent supereruption quakes (Mw >4; focal (Crosweller et al. 2012). Unlike Yellowstone, 38˚N spheres) used to image Long Valley has no deep-mantle hotspot source, the crustal shear-wave and regional mafic volcanism is relatively young velocity beneath the Long Valley Caldera (outlined in (<4 Ma) (Bailey et al., 1976). Tectonically, the A A' Sierra Nevada yellow) in California. Red volcano sits in a right-stepping offset in the dex- line shows location of the tral Walker Lane shear zone (Fig. 1). The region cross section in Figure 2. is bounded by the Sierra Nevada to the west and Inset: Location of the study the Basin and Range to the east (Fig. 1), with area (red box) in the west- ern United States. volcanism focusing at Long Valley at ca. 2.5 Ma at a left-stepping offset in the Sierra range-front 37˚N fault system (Bailey et al., 1976). 50 km *E-mail: [email protected] GEOLOGY, September 2018; v. 46; no. 9; p. 799–802 | GSA Data Repository item 2018290 | https:// doi .org /10 .1130 /G45094 .1 | Published online 2 August 2018 ©GEOLOGY 2018 The Authors.| Volume Gold 46 |Open Number Access: 9 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 799 ~6 km (Romero et al., 1993; Foulger et al., 2003; independent of earthquake locations and provide depth) low velocity is indicative of volcanoclas- Seccia et al., 2011; Lin, 2015) their ability to sensitivity to seismic structure deeper than previ- tic caldera infill, with the primary reservoir low- resolve a deeper magma reservoir (>8 km) is ous LET studies at spatial resolutions higher than VS zone extending from ~5 km depth, near the limited by the focal depths and raypaths of local teleseismic body-wave studies. In later iterations, base of down-dropped Paleozoic metasedimen- earthquakes. Beneath the caldera, LET studies we supplement these data with Rayleigh-wave tary roof (Bailey et al., 1976), to more than 20 can, at most, resolve to depths of ~6–10 km. records from 11 large regional earthquakes (Fig. km (Fig. 2D). On average, this zone is ~20% Despite this limitation, these studies have found 1; Tables DR1 and DR2). slower (2780 m/s) than expected for a mid- evidence for a low VP and VS or high S-wave crustal granite (3460 m/s) near its solidus tem- attenuating zone near the base of their mod- RESULTS perature (350 MPa, ~680 °C; Evans et al., 2016; els (Romero et al., 1993; Foulger et al., 2003; We image a spheroidal low shear-wave veloc- Ji et al., 2002). The magnitude of the reduced Seccia et al., 2011; Lin, 2015). However, given ity (VS) zone (30 km in diameter) that underlies velocity and the vertical extents agree with what the proximity to the relatively low-temperature the entire caldera (Fig. 2). Near-surface (<3 km has been observed teleseismically (Dawson et base (100 °C) of the Long Valley Exploration Well (Sorey et al., 2000), low VP/VS, and low resistivity, many suggest these anomalies reflect 120˚W119˚W 118˚W magmatically derived fluids and/or hydrother- A B 15 km mal alteration (Romero et al., 1993; Lin, 2015; Peacock et al., 2016). The best evidence to date for a deep magma reservoir comes from limited lower-resolution teleseismic studies, which show an 8–30% 10 km reduction in V between 7 km and 20 km depth P 38˚N (Dawson et al., 1990; Steck and Prothero, 1994; Weiland et al., 1995). The most recent of these studies imaged two isolated systems: a mid-to- A A' 20 km upper crustal reservoir at ~10–18 km depth, and C a second, deeper, low-velocity zone, centered at ~25 km depth (Weiland et al., 1995). This type of multilevel storage is similarly observed at the Valles (Steck et al., 1998) and Yellowstone calderas (Huang et al., 2015), and is supported 37˚N by the latest perspectives on the transcrustal development of large silicic systems (Cashman 50 km et al., 2017). D A Caldera A' METHODS 0 To address the longstanding uncertainties of Long Valley’s magma reservoir, we solved for −10 the caldera’s crustal shear-wave velocity struc- ture using three-dimensional (3-D) full-wave- Depth (km) −20 form tomography. We invert for traveltime dif- −30 ferences between source and forward-modeled −50 km 0 km 50 km seismograms using a multi-step iterative process Shear Wave Velocity (m/s) that includes 3-D finite-difference wave propa- 2500 2800 3100 3400 3700 gation simulations and the calculation of 3-D A A' finite-frequency sensitivity kernels (Flinders and E Exploration Well (LVEW) Shen, 2017).
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