HEBGEN LAKE – RED CANYON NORMAL FAULT SYSTEM, MT: AN EXAMPLE OF EARLY-STAGE STRUCTURAL INVERSION

The collapse of the crest of the Laramide Madison-Gravelly arch along the Madison Range normal fault system, forming the modern Madison Valley in southwest Montana, is a regional structural pattern that typifies the overlap zone of the easternmost Basin and Range structural province with the Cretaceous-Paleogene Laramide and Sevier orogens. At the south end of the Madison-Gravelly arch, Quaternary extension has stepped eastward from the core of the basement arch to reactivate the leading-edge Laramide thrust structures that subtend the Beaver Creek and Divide thrust plates, analogous to the interaction of range-bounding normal faults with the Hilgard thrust system and at the north end of the Madison Range. The Hebgen and Red Canyon normal faults overlap a left-stepping displacement transfer zone, in which shortening was transferred eastward from the Beaver Creek plate to the Kirkwood plate (Tysdal, 1986); this spatial overlap of SW-dipping Laramide thrusts created the template for later extension along SW-dipping normal faults. However, the degree to which the Hebgen and Red Canyon normal faults interact with (i.e., reactivate or exploit) older Laramide fault zones has been interpreted several ways by previous workers, particularly at depth (e.g., Fraser et al., 1964; Witkind, 1969; Doser, 1985; Tysdal, 1986). Based on balanced structural cross-sections, this author interprets the Hebgen and Red Canyon normal faults to reactivate older SW-dipping Laramide thrusts and merge at depth, resulting in overlapping, en echelon perched basement wedges. In addition, it is proposed that the Hebgen-Red Canyon area is in the initial stages of extensional development and thus offers an unparalleled opportunity to study the detailed structural geometry of early-stage fault reactivation and inversion tectonics. Evidence of early-stage extension includes, but is not limited to: 1) juxtaposition of Paleozoic formations in the hanging wall and footwall of the two normal faults; 2) the narrow width of the Hebgen Lake valley; and 3) restored structural cross-sections that show small amounts of separation across the two faults.