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 (-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 km3; a density contrast of -400 kg m-3 results in two intrusions, each similar to 9 km x4.5 km and about 0.5 km thick, with cumulative volume of 100 km3. A network of 340 degrees-360 degrees 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 similar 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.

Automated summary

The Blackfoot Reservoir volcanic field in Idaho, USA, has experienced silicic eruptions during two episodes, with the most recent one occurring about 58,000 years ago. New gravity data suggests the presence of shallow intrusions and fault zones, indicating volcanotectonic interactions and regional deformation.