KINEMATIC ANALYSIS OF LATE PLEISTOCENE FAULTING IN THE BIMODAL BLACKFOOT LAVA FIELD, IDAHO, USA: EVIDENCE OF DIKE-INDUCED FAULTING?

New mapping in the Blackfoot Lava Field (BLF) of southeastern Idaho was completed to investigate whether a system of fault scarps, termed the Blackfoot Rift Zone (BRZ), records dike emplacement or typical Basin & Range faulting. The bimodal BLF consists of late Pleistocene olivine tholeiite flows, cinder cones, and three rhyolite domes including one dated at 58 ka.

The BRZ is about 5 km wide and 20 km long, trends 350°, and has a central zone of slightly elevated topography elongated parallel to its length. More than 50 normal fault scarps are defined by linear rubble slopes, vertical cliffs, and numerous monoclines. Fault scarps define an en echelon geometry along strike and a simple graben or half-graben geometry across strike, that becomes a more complex series of multiple nested graben in the central BRZ. The floors of some graben contain either tension cracks or chains of sinkholes in the loess layer capping the BLF. Vertical offset on individual scarps is typically 1-10 m and locally as much as 50 m. Most faults are vertical at the surface, likely due to reactivation of near-surface cooling joints, but at depth are inferred to dip moderately. Assuming a subsurface fault dip of 60°, the BRZ accommodated 40-100 m of horizontal extension with the amount increasing toward the center from the north and south ends. Although no eruptive vents were observed, a rhyolite dome flows over a large fault scarp and is cut by a smaller scarp, evidence of coeval faulting and silicic magmatism at 58 ka. Additionally, many basaltic eruptive centers in the BLF have been cut and dissected by later faulting.

If the BRZ merely records Basin & Range faulting, then it would be atypical considering its structural style, spatial location (in the middle of a broad valley), and extremely high extension rate (1-2 m/ka). Instead, our preferred interpretation is that the BRZ is a volcanic rift zone that records the emplacement of multiple dikes. Using previously published models that relate rift geometry to dike geometry, we estimate the dikes have a cumulative width of 25-75 m. Depth to dike top is more difficult to infer due to the complexity of graben geometries, with estimates ranging from 60-4000 m. Future work will attempt to fit subsurface dike geometry to the observed surface deformation using numerical modeling.