Large‐Volume and Shallow Magma Intrusions in the Blackfoot Reservoir Volcanic Field (Idaho, USA). Issue 11 (11th November 2021)

• Record Type: Journal Article
• Authors: Hastings, M. S.; Connor, C. B.; Wetmore, P.; Malservisi, R.; Connor, L. J.; Rodgers, M.; La Femina, P. C.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-11
• Subjects: gravity -- volcanic field -- inversion -- volcanotectonic -- Basin and Range -- silicic eruptions; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB022507
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: The Blackfoot Reservoir volcanic field (BRVF), Idaho, USA, is a bimodal volcanic field that has hosted silicic eruptions during at least two episodes, as recently as 58 ka. Using newly collected ground and boat‐based gravity data, two large negative anomalies ( − 16 mGal) are modeled as shallow ( < 1 km) intrusions beneath a NE‐trending alignment of BRVF rhyolite domes and tuff rings. Given the trade‐off between density contrast and model volume, best‐fit gravity inversion models yield a total intrusion volume of 50 − 120 k m 3 ; a density contrast of − 400 kg m − 3 results in two intrusions, each ∼ 9 km × 4.5 km and about 0.5 km thick, with cumulative volume of 100 k m 3 . A network of 340 ° − 360 ° trending faults lies directly above and on the margins of the mapped gravity anomalies. Most of these faults have 5 − 10 m throw; one has throw up to ∼ 50 m. We suggest that the emplacement of shallow sill‐like intrusions produced this fault zone and also created a ENE‐trending fault set, indicating widespread ground deformation during intrusion emplacement. The intrusions and silicic domes are located 3 − 5 km E of a regional, 20 mGal step in gravity. We interpret this step in gravity as thickening of the Upper Precambrian to lowermost Cambrian quartzites in the Meade thrust sheet, part of the Idaho‐Wyoming Thrust Belt. Silicic volcanism in the BRVF is a classic example of volcanotectonic interaction, influenced by regional structure and creating widespread deformation. We suggest volcanic hazard assessments should consider the possibility of large‐volume silicic eruptions in the future. Plain Language Summary: On Earth, gravity anomalies occur where there are significant, subsurface, lateral density variations. We map two gravity anomalies located in the Blackfoot Reservoir volcanic field, Idaho, a site which has experienced explosive volcanic eruptions as recently as 58, 000 years ago. Our numerical models of the gravity anomalies indicate that they are caused by two saucer‐shaped intrusions, magma bodies that likely fed eruptions at the surface and triggered fault displacement. Although these magma bodies have cooled, they have large volumes and the timing of the emplacements suggest that large‐volume explosive volcanic eruptions are possible in this volcanic field in the future. Key Points: Large‐amplitude gravity anomalies are mapped in a combined ground and boat‐based gravity survey in the Blackfoot Reservoir volcanic field, Idaho (BRVF), adjacent to young (1.5 Ma, 58 ka) topaz rhyolite domes and tuff rings within a Quaternary basaltic volcanic field Best‐fit 3D inversion of the gravity data, constrained by density contrast estimates and excess mass calculations, indicates the presence of two bodies with thick sill‐like shapes in the uppermost crust, with cumulative volume of ∼ 100 k m 3 and volume uncertainty in the range 50 − 120 k m 3 Extensive volcanotectonic interaction is suggested by comparison of mapped gravity with fault distribution. The western edges of the gravity anomalies coincide with normal faults with vertical displacements that range from 5 − 10 m (maximum 50 m)