NEW KINEMATIC ANALYSIS OF LATE PLEISTOCENE FAULTING IN THE BLACKFOOT RIFT ZONE, IDAHO, USA

To determine whether a kinematic link exists between normal faulting and rhyolite eruption in the Blackfoot Rift Zone (BRZ) of southeastern Idaho, a detailed topographic analysis was completed using DEM, LIDAR, and field measurements. The BRZ is a north-striking rift within the bimodal Blackfoot Lava Field, which consists of olivine tholeiite flows and scoria cones as well as NE trending rhyolite domes and maars (China Hat dome set). A 58 ka rhyolite dome flows over one fault scarp, while another fault cuts the dome, constraining faulting at ~58 ka. Given the large offsets of these fault scarps, it is likely they formed through numerous dike events.

A series of 33 east-west elevation profiles spaced 500 m apart were collected from a USGS National Elevation Dataset (NED) DEM to provide kinematic interpretations for the entire BRZ. Spatially restricted LIDAR data was used to validate fault scarp offset measurements made using the NED. Cumulative fault throw on each transect was converted to an estimate of horizontal extension, using fault dips that range between 70 - 50°. Maximum east-west horizontal extension is estimated at 75 – 200 m depending on assumed fault dip. Cumulative extension increases linearly from the southern end of the BRZ to the Blackfoot Reservoir. Though the northern end of the BRZ is underwater, extrapolation of a linear relationship between cumulative cross-section fault throw and rift zone width provides an estimate of 75 – 200 m cumulative extension beneath the reservoir.

The China Hat dome set overlaps the area of greatest horizontal extension in the BRZ. Additionally, these domes are aligned with what appears to be an area of structural accommodation between large offset (40-50 m) antithetic faults bounding the BRZ. Finally, previous work found that magmatic mafic enclaves occur within these rhyolite domes, implying that a mafic dike intruded the China Hat magma chamber prior to its eruption. These data indicate a strong kinematic link between extension, mafic dike injection and rhyolite eruption in the BRZ. Such a conclusion is consistent with observations in active volcanic rift zones elsewhere in the world, where mafic dike injection caused the eruption of silicic volcanoes. Such structural controls on rhyolite eruption also have implications for potential future geothermal exploration in the region.