Mammoth Mountain and its mafic periphery--A late Quaternary volcanic field in eastern California

Abstract

The trachydacite complex of Mammoth Mountain and an array of contemporaneous mafi c volcanoes in its periphery together form a discrete late Pleistocene magmatic system that is thermally and compositionally independent of the adjacent subalkaline Long Valley system (California, USA). The Mammoth system fi rst erupted ca. 230 ka, last erupted ca. 8 ka, and remains restless and potentially active. Magmas of the Mammoth system extruded through Mesozoic plutonic rocks of the Sierra Nevada batholith and extensive remnants of its prebatholith wall rocks. All of the many mafi c and silicic vents of the Mammoth system are west or southwest of the structural boundary of Long Valley caldera; none is inboard of the caldera's buried ring-fault zone, and only one Mammoth-related vent is within the zone. Mammoth Mountain has sometimes been called part of the Inyo volcanic chain, an ascription we regard inappropriate and misleading. The scattered vent array of the Mammoth system, 10 × 20 km wide, is unrelated to the range-front fault zone, and its broad nonlinear footprint ignores both Long Valley caldera and the younger Mono-Inyo range-front vent alignment. Moreover, the Mammoth Mountain dome complex (63%-71% SiO 2 ; 8.0%-10.5% alkalies) ended its period of eruptive activity (100-50 ka) long before Holocene inception of Inyo volcanism. Here we describe 25 silicic eruptive units that built Mammoth Mountain and 37 peripheral units, which include 13 basalts, 15 mafi c andesites, 6 andesites, and 3 dacites. Chemical data are appended for nearly 900 samples, as are paleomagnetic data for ~150 sites drilled. The 40 Ar/ 39 Ar dates (230-16 ka) are given for most units, and all exposed units are younger than ca. 190 ka. Nearly all are mildly alkaline, in contrast to the voluminous subalkaline rhyolites of the contiguous long-lived Long Valley magma system. Glaciated remnants of Neogene mafi c and trachydacitic lavas (9.1-2.6 Ma) are scattered near Mammoth Mountain, but Quaternary equivalents older than ca. 230 ka are absent. The wide area of late Quaternary Mammoth magmatism remained amagmatic during the long interval (2.2-0.3 Ma) of nearby Long Valley rhyolitic eruptions. Recent Unrest Geophysical unrest beneath Mammoth Mountain and in adjacent parts of the Sierra Nevada and Long Valley caldera has generated concern among residents, stakeholders, and geoscientists since at least 1980, when 4 magnitude 6 earthquakes shook the area. Extensive monitoring for three decades has documented numerous earthquake swarms, ground uplift and deformation, changes in hydrothermal systems, and emission of magmatic CO 2 at several sites semiencircling

Tomographic image of a seismically active volcano: Mammoth Mountain, California

“…Figure shows a simplified version of the basement geology near Mammoth Mountain (modified from Hildreth et al [], Figure ). Mammoth Mountain consists of a complex of 25 trachydacitic domes that were emplaced from 100 ka to 50 ka.…”

“…Map of the basement geology in the region of Mammoth Mountain (modified from Hildreth et al [], Figure ). Mammoth Mountain is shaded orange and peripheral vents are shown as stars.…”

Three‐dimensional electrical resistivity model of the hydrothermal system in Long Valley Caldera, California, from magnetotellurics

“…The second is thought to have begun about 40 ka under the west moat, coincident with the onset of silicic magmatism at Inyo Craters [Sorey et al, 1991]. However, Hildreth et al [2014] have shown that a magma body was in place under the west moat in the late Pleistocene that is unrelated to the Inyo eruptions, demonstrating that the heat from the Inyo eruptions did not initiate the current hydrothermal system. The active shallow hydrothermal system has been well characterized in the near surface (<2 km) through geophysical and geochemical analysis [e.g., Sorey et al, 1991].…”

Late Quaternary Eruption Record and Probability of Future Volcanic Eruptions in the Long Valley Volcanic Region (CA, USA)

“…LVVR includes the 32 km × 18 km Long Valley caldera, which collapsed at ~770 ka during eruption of the Bishop Tuff (Bailey, ; Crowley et al, ; Hildreth & Mahood, ; Mark et al, ; Sarna‐Wojcicki et al, ). Quaternary vents younger than 180 ka are associated with the Mammoth Mountain system, with a 3,400‐m above sea level composite dome volcano at its core, on the southwest topographic rim of the caldera (Hildreth et al, ; Hildreth & Fierstein, ; Mahood et al, ), and the Mono‐Inyo Craters volcanic chain and Mono Lake Islands, a nearly linear array of vents stretching north of the caldera for ~45 km (Bailey, ; Gilbert et al, ; Kelleher & Cameron, ; Sampson & Cameron, ).…”