Gravity Anomalies Reveal Volcano-Tectonic Interaction in an Active Distributed Volcanic Field, Blackfoot Reservoir Volcanic Field (ID)

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Intrusion geometries are particularly difficult to deduce in distributed volcanic fields that are common throughout the western US and in a wide variety of tectonic settings. Here we use new gravity data collected in the Blackfoot Reservoir volcanic field (BRVF), Idaho, to identify two distinct, roughly circular gravity anomalies located adjacent to an alignment of five late Quaternary rhyolite domes and tuff rings. The negative gravity anomalies, > 10 mGal in amplitude, are bounded on their west margin by a network of N-S to NNW-trending normal faults. Our preliminary models indicate that the gravity anomalies are best-fit with a shallow (<1 km) and 1 km thick low density rock. Using the densities of the domes, found using Parasinis’ method and field samples yields a density contrast of -850 kg/m3. We estimate a volume of the two gravity anomalies of 50 km3 and mass deficit of ~ 4.0 x 1013 kg, with overall sill geometries. The intrusions are emplaced in relatively high density metasediments, leading to a negative density contrast. Remarkably, the two sills are separated by only 1 km. We suggest that Basin and Range faults were reactivated at the time of intrusion of the sills, leading to a maximum throw on the faults of 50 meters. This volcano-tectonic interaction is important for several reasons. First, the BRVF mostly consists of mid-Quaternary basalts which include ~23 vents between area of the rhyolite domes and 40 km SW into Gem Valley, a prominent NNW-trending graben. The presence of these basalts indicate that dikes accommodated strain in the region during the Quaternary. As basaltic volcanism waned ~ 1Ma, strain was no longer accommodated by dikes. We suggest the rhyolite domes played a similar role in strain accommodation, largely by deformation at the west end of the sills. Second, and earthquake swarm has occurred 25 km S/SE of the domes and sills from 2017-present. This earthquake swarm is indicative of continued extension to the south, which must be accommodated either by earthquakes or intrusions and or volcanism in the north. As there is no sign that volcanism has ceased, we suggest that that strain accommodation near the reservoir could be through seismic or igneous activity.

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Presented at the AGU Fall Meeting on December 12, 2019 in San Francisco, CA