STRUCTURAL INVERSION OF THE SOUTHERN MADISON-GRAVELLY ARCH BY THE HEBGEN-RED CANYON NORMAL FAULTS, SW MONTANA

The Madison Range in SW Montana is characterized by geologically young tectonic inversion from shortening (Paleogene) to extension (late Pliocene-Quaternary). On August 17, 1959 the area experienced the largest recorded earthquake in the Intermountain West (Ms 7.5, intensity X) that ruptured two SW-dipping normal faults, the Hebgen-Red Canyon normal faults. These faults occupy an overlap zone between Laramide contractile structures (Ancestral Madison-Gravelly Arch) and extensional structures at the eastern margin of the northern basin-and-range province (intermountain seismic belt). A proposed geometric model based on new geological mapping, coupled with balanced cross-sections, suggests that lateral ramps in the Laramide Beaver Creek-Wells-Rapids-Divide (BWRD) thrust system played an important role in the development of subsequent Neogene-Quaternary extensional structures. Lateral-oblique thrust ramps mark relatively sharp boundaries between structural highs and lows of the BWRD thrust surface, compartmentalizing the up-dip portions of Neogene-Quaternary normal faults to produce a distinct pattern of en echelon extension at the south end of the Madison Range (i.e., en echelon perched basement wedges). We further propose that 1959 “reactivation” of the BWRD thrust system was a relatively shallow phenomenon in which the main rupture, which nucleated deep in the BWRD footwall, propagated up-dip to reactivate leading-edge imbricate splays of the BWRD thrust system and the overturned limb of an associated Laramide fold on Kirkwood Ridge. This model supports the “dual-basin” model Fraser et al. (1964). Lastly, the Hebgen-Red Canyon normal fault system is interpreted to be in the early stages of crustal extension as evidenced by near-surface fault reactivation, small amounts of stratigraphic separation across the Hebgen and Red Canyon faults, and the narrow width of the Hebgen Lake Basin. Back-thrusts and the Cache Creek-Cabin triangle zone are an integral part of the Laramide structural geometry, but are not (as yet) extended.