REGIONAL INTERACTIONS AMONG CRUSTAL TECTONICS, STRUCTURES AND MAGMA TRANSFER ASSOCIATED WITH THE QUATERNARY BLACKFOOT VOLCANIC FIELD, SE IDAHO

The Blackfoot volcanic field (BVF) is a Quaternary volcano-tectonic complex located adjacent to the Eastern Snake River Plain (ESRP) segment of the Yellowstone-Snake River Plain volcanic track. Eruptions of primitive to moderately evolved olivine tholeiite occurred from vents scattered over an area of 1375 km², extending ~30 to 80 km SE of ESRP. Basaltic eruptions produced ~100 km³ of lavas that infill several late Miocene to late Quaternary B&R grabens to maximum depths of at least 230 m. Three clusters of topaz rhyolite lava domes (erupted between ~1.5 and 0.057 Ma) dot the center of the field. Within the northeastern Basin and Range, BVF is located within a unique transition in extension direction. To the south and east, east-west extension defines the eastern Great Basin (~2.8 mm/yr; Bennett et al., 2003) but to the north, extension in central Idaho and southwestern Montana is northeast-southwest oriented (~2.5 mm/yr; Payne et al., 2012). In southeast Idaho and westernmost Wyoming, coincident with the northern margin of the Wyoming salient of the Cordilleran fold-thrust belt, active normal faults display two prominent en echelon patterns: north-striking normal faults are right-stepping, whereas northwest-striking faults are left-stepping. This pattern is exceptionally expressed as scarps cutting intrabasin basalt flows near the center of BVF (the Blackfoot Rift Zone of Polun et al., 2011). Analogous to en echelon normal fault traces in Iceland (Grant and Kattenhorn, 2004), we propose that oblique extension is also occurring in the BVF as a consequence of the transition in extension direction. Deflections of normal fault traces also roughly define a northeast trend, hinting at a pre-existing basement weakness. These structures may have facilitated formation of steeply-dipping crustal conduits that enable magma transfer adjacent to the eastern Snake River Plain. Continued crustal weakening due to robust magma intrusion and storage also results in fault slip reorientation near the center of BVF. Structural controls on magma emplacement may have implications for understanding the emplacement and size of a shallow high-T geothermal resource associated with this volcanic system.