Large-Volume and Shallow Magma Intrusions in the Blackfoot Reservoir Volcanic Field (Idaho, USA)

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 ((Formula presented.) mGal) are modeled as shallow ((Formula presented.) 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 (Formula presented.) (Formula presented.); a density contrast of (Formula presented.) kg (Formula presented.) results in two intrusions, each (Formula presented.) km (Formula presented.) km and about 0.5 km thick, with cumulative volume of 100 (Formula presented.). A network of (Formula presented.) trending faults lies directly above and on the margins of the mapped gravity anomalies. Most of these faults have (Formula presented.) m throw; one has throw up to (Formula presented.) 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 (Formula presented.) 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.