High-precision 40Ar/39Ar geochronology and volumetric investigation of volcanism and resurgence following eruption of the Tshirege Member, Bandelier Tuff, at the Valles caldera

Abstract

Despite recognition as a significant volcanic threat to the southwestern United States, the post-caldera history of Valles caldera is poorly constrained. This study provides 49 new high-precision 40Ar/39Ar sanidine ages and volume estimations that reveal insights into eruptive timescales and resurgence. Caldera collapse and deposition of the Tshirege Member of the Bandelier Tuff at 1231.9 ± 1.3 ka was followed by eruption of 34 volcanic units from vents on the resurgent dome and along the caldera ring-fracture. New ages indicate post-caldera dome complexes are characterized by dominantly polygenetic and minor monogenetic emplacement. The Deer Canyon eruptions immediately followed caldera formation between 1231.6 ± 4.1 ka and 1212.0 ± 5.6 ka, for a total lifespan of 19.6 ± 6.9 kyr. Similarly, Redondo Creek eruptions occurred between 1199.9 ± 5.6 ka and 1186.8 ± 2.9 ka, a lifespan of 13.1 ± 6.3 kyr. Cerro del Medio is the longest-lived dome complex, erupting during a 24.5 ± 5.5 kyr lifespan between 1158.0 ± 3.1 and 1133.5 ± 4.6 ka. Following Cerro del Medio, Cerros del Abrigo erupted between 1010.1 ± 1.7 and 997.7 ± 1.7 ka. These longer-lived dome complexes are located within structurally complex zones of the caldera, suggesting the dense network of faults facilitate magma transport to the surface, producing long-lived eruptions. Conversely, the six dome and vent complexes that erupted between 932.5 ± 1.1 ka and 68.9 ± 1.0 ka have shorter eruptive lifespans than early post-caldera eruptions. Three of the complexes—San Antonio Mountain, South Mountain, and the East Fork Member—are polygenetic with lifespans ranging from 4.9 to 5.4 kyr with measurable repose periods. Dating of Cerro Santa Rosa 2, Cerro San Luis, and Cerro Seco domes yields a suite of indistinguishable ages, constraining rapid emplacement of each dome complex between 804.7 ± 2.0 ka and 800.3 ± 2.7 ka. The post-caldera volcanic units are grouped into a total of 17 temporally distinguishable eruptive events emplaced during 93.9 kyr of intra-dome volcanic activity. The eruptive frequency is 1 event per 5.5 ka for when Valles caldera enters periods of dome activity. A combination of new and published volumes of the dome and vent complexes indicate that these eruptions range from 0.8 km3 to 14.1 km3. In contrast, the average eruptive flux for each volcanic complex has nearly systematically increased from 0.05 ± 0.02 to 2.66 ± 0.75 km3/kyr during post-caldera evolution. Using new ages of the Tshirege Member, a syn-resurgent lava, and the oldest post-resurgent eruption, resurgence durations at Valles caldera are constrained between 73.9 ± 3.4 and 41.9 ± 6.4 kyr. Average magmatic flux during resurgence is between 0.52 ± 0.02 and 0.92 ± 0.14 km3/kyr, similar to pluton-filling rates, suggesting that resurgence is partly caused by significant decreases in magmatic flux following higher fluxes that sustain caldera-forming magma bodies. The eruptive frequency during dome activity and increasing flux has important implications should Valles caldera enter a new volcanic phase.