THE MADISON RANGE AND MADISON VALLEY, MONTANA – LARAMIDE CONTRACTION AND UPLIFT SET THE STAGE FOR CENOZOIC BASIN FORMATION

The Madison Valley in southwestern Montana, a prominent physiographic feature in the Basin-and-Range structural province, formed along Neogene-Quaternary extensional faults that were controlled by older contractional structures. The valley is bounded on the east by the west-dipping Madison Range fault zone (MRFZ), east of which, along the west side of the Madison Range, a complex array of generally north-striking, east-directed Laramide thrusts (Hilgard thrust system, HTS) place high-grade Archean rocks over a thrust-imbricated, overturned footwall syncline that contains a thick Mesozoic through Cambrian section, with a preponderance of Cretaceous rocks. Paleomagnetic data on dacite dikes that intrude thrust surfaces indicate that Laramide contraction in the region was essentially complete by ~69 Ma. The HTS occupies the eastern leading edge of the large basement-cored Gravelly arch and forms the east side of a “perched basement wedge,” bounded on the west by the MRFZ and on the east by the HTS. These relationships suggest that the leading edge of the basement uplift was thrust eastward during the Cretaceous over the mostly Mesozoic section and underwent rotational “sagging” into the softer sedimentary rocks, producing extension and normal faulting in the axial crest region of the arch. Structural inversion occurred as the crest region later collapsed during post-early-Eocene extension, forming the east-tilted Madison Valley half graben. Recent detailed mapping and seismic data across the Madison Valley support this concept, in which many features of the MRFZ were inherited from the earlier Laramide geometry. Similar inheritance of Laramide structures by Cenozoic normal faults, producing perched basement wedges and structural inversion, is a common feature in the Basin-and-Range province and other extensional tectonic settings. An example from the San Luis valley (northern Rio Grande Rift) in southern Colorado is compared.