VOLCANOLOGY AND STRUCTURE OF THE LEVIATHAN PEAK ARC VOLCANIC CENTER, ANCESTRAL CASCADES ARC, MONITOR PASS REGION, CENTRAL SIERRA NEVADA CALIFORNIA

The Ancestral Cascades arc in the central Sierra Nevada is composed of Early Miocene to Pliocene volcanic rocks and hypabyssal intrusions, emplaced in a rift to oblique rift tectonic setting. New geologic mapping, petrography, geochemistry and ⁴⁰Ar/³⁹Ar geochronology in the Monitor Pass region allows correlation with regional magmatic and structural events. This work builds on mapping by John et al. (1981)¹ and unpublished geochemistry and ⁴⁰Ar/³⁹Ar geochronology provided by David John (written communication). Major volcanic events include: (1) 11.2 Ma dacite lavas and block-and-ash-flow tuffs at the northern edge of the map area, related to the ca. 11 Ma Diamond Valley volcanic center² immediately to the northwest; (2) 8.2 Ma quartz-phyric crystal-rich rhyolite lavas, block-and-ash-flow tuffs, and tuffs in the newly-defined Leviathan Peak volcanic center. These are the likely source of detrital zircon that provide a maximum depositional age on the “Miocene Zoo”³ in the lower reaches of the Mokelumne paleochannel, 160 km to the west in the Sierra foothills. The Leviathan Peak volcanic center also includes large 6.8 Ma andesite hypabyssal intrusions, immediately preceding the onset of volcanism at the Ebbetts Pass volcanic center 25 km to the southwest, at 6.4 Ma⁴. The magmatism in both areas destroyed the Mokelumne paleochannel at this latitude; (3) ca. 4.7 Ma silicic plugs, which represent satellites of silicic plugs and dikes emplaced in the Ebbetts Pass volcanic center at 4.6 – 4.5 Ma⁴.

This stratigraphy is used to construct cross sections to estimate displacement on range front normal faults, which mainly step down to the east, although one full graben (Bagley Valley) is present. The range front faults include seismically active faults (Antelope Valley and Slinkard Valley faults), as well as another fault we propose is active, named herein the West Bagley Valley fault, which is largely mantled by Quaternary avalanche deposits. These discoveries shed insight into the volcanic and tectonic evolution of the Monitor Pass Region.

¹John et al., 1981, Reconnaissance geologic map of the Topaz Lake 15’ quadrangle, CA and NV; U.S.G.S. Open-file report, 81-273, scale 1:62,500.

²Wesoloski, C, 2018, unpubl. MS Thesis, UC Davis.

³Swarner et al., 2022, GSA Annual Meeting Abstract.

⁴Busby et al., 2018, Geosphere, 14-5, 2068-2117.