Late Neogene Basaltic Flare-up: 330-vent NE-trending Belt—170-km-long—Continuous through Central Sierra Nevada–Long Valley–Adobe Hills–Mina Deflection–to ­­Candelaria

Abstract:

Ongoing mapping and Ar-dating of precaldera volcanics around Long Valley—and far SW and NE of it—led us to appreciate the continuity and vent density of a Late Neogene strip of mafic vents that extends from the Central Sierra across Long Valley, Adobe Hills, Huntoon Valley, Teels Marsh, to Candelaria. A Pliocene pulse of mafic volcanism has long been recognized in the southern Sierra and to its east—Kern Plateau, Kings River, Coso Range, Panamint Valley, Saline Range—but all are discrete fields with 7–43 vents each. By contrast, our basaltic belt is 170 km long, 35–50 km wide, has at least 300 Pliocene vents (principally 3.7–3.1 Ma), and is clearly bounded on its sides and ends. Most Pliocene units are trachybasalt or basaltic trachyandesite; only a few are strongly alkalic. About 40 are trachyandesite and trachydacite (58–68% SiO₂), notably a cluster around Long Valley Caldera. Units in the San Joaquin drainage—mostly glaciated remnants—include at least 10 of Late Miocene age (12–5 Ma), but ~60 are Pliocene. About 100 Neogene units around Long Valley include ~15 vents partly buried by Pleistocene rhyolites of Glass Mountain; and others may be concealed by caldera fill and outflow Bishop Tuff. About 85 mafic Pliocene units scatter from Cowtrack Mountain across the Benton Range and Adobe Hills to the Nevada state line. The basaltic strip continues 55 km farther NE into Nevada and has at least 60 vents; and we anticipate more as mapping and dating advance.

The mafic-vent strip crosses at high angles: (1) many range-bounding normal faults (most of which postdate the basalts); (2) transtensional faults of the Eastern California Shear Zone–Walker Lane Belt; (3) the “0.706 line” of initial Sr-isotope ratios; (4) the translation vector of the Sierran microplate; (5) the present-day geodetic velocity field; and (6) the ESE trend of the Miocene “ancestral” Cascade arc through the Bodie Hills. Only the 60-km-long NE third of the Neogene strip is within the Mina Deflection. As the strip has no regard for crustal structure, the flare-up was evidently triggered by mantle processes concentrated beneath a NE-elongate well-circumscribed region. This requires rethinking the lithospheric “delamination” model popularized for the southern Sierra two decades ago.