Gravity Anomalies in Blackfoot Reservoir Volcanic Field, Idaho constrain intrusion geometries, intrusive:extrusive volume ratio, and volcano-tectonic interaction.

Abstract:

Intrusive:extrusive (I:E) volume ratios are difficult parameters to estimate within a volcanic field due to the uncertainties in constraining the volume of buried volcanic features. Values on I:E ratios range from 1:1 to 200:1 with no discernible correlation to the petrotectonic setting. Here we present 3D gravity models that constrain the intrusive geometry of a shallow magmatic plumbing system beneath the Blackfoot Reservoir in southeastern Idaho. The models were constrained by ~1400 marine and ~500 terrestrial gravity data that show clear separation between two distinct anomalies with amplitudes of ~25 mGals. Model results show an intrusion volume of ~200 km3 with the top of the magmatic system emplaced at a depth of ~200-500 meters, while the total estimated extrusive volumes of the rhyolitic domes are ~1.15 km3. This yields an I:E of ~170:1 for the Blackfoot Reservoir Volcanic Field (BRVF), similar to estimates in the Ninetyeast Ridge and Coso Volcanic Field. Additionally, a network of diffuse normal faults span the volcanic field and accommodate a maximum extension of 725 meters. We suggest that emplacement of the sill complexes reactivated Basin and Range normal faults near the Blackfoot Reservoir. This volcano-tectonic interaction is important for several reasons. First, the BRVF comprises Quaternary basaltic fields including ~23 volcanic vents that are believed to be ~100 - 25 ka and the 3 rhyolitic domes south of the reservoir dated at ~58±7 ka. The presence of these basalts indicate that dikes accommodated strain in the region during the Quaternary. As basaltic volcanism waned ~100 - 60ka, 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, an earthquake swarm occurred 25 km S/SE of the domes and sills and is still active to present. This earthquake swarm is indicative of continued extension to the south, which must be accommodated either by earthquakes, intrusions, volcanism, or aseismic slip.