Basement and Cover-Rock Deformation During Laramide Contraction in the Northern Madison Range (Montana) and Its Influence on Cenozoic Basin Formation

Abstract

Two major Laramide fault systems converge in the northwestern Madison Range: the northwest-striking, southwest-vergent Spanish Peaks reverse fault and the north-striking, east-vergent Hilgard thrust system. Analysis of foliation attitudes in basement gneiss north and south of the Spanish Peaks fault indicates that the basement in thrusted blocks of the Hilgard thrust system has been rotated by an amount similar to that of the basement-cover contact. Steeply dipping, north-striking breccia zones enclosing domains of relatively undeformed basement may have permitted domino-style rotation of basement blocks during simple shear between pairs of thrusts. In most places along the Hilgard thrust system, a large basement overhang, produced by thrusting of Archean blocks above rocks as young as Late Cretaceous, overlies a tight footwall syncline. This tight folding is largely concentric and was accommodated by flexural slip, resulting in severe crowding in synclinal hinges that resulted in observed or inferred features such as bedding-plane slip, imbricate and out-of-syncline thrusting, and hinge collapse. The north-striking Madison normal fault system, a zone of Tertiary and Quaternary valley-forming normal faults, is approximately parallel to the Hilgard thrust system. In some places, normal faults are reactivated thrusts on which large basement overhangs of the Hilgard thrust system were dropped back into the Madison Valley and covered by Tertiary basin-fill deposits, leaving only the rocks of the footwall synclines exposed. In other places, both the thrusted Archean blocks and the near-isoclinal footwall synclines are well preserved. This paired fault system (the Madison normal fault system and the Hilgard thrust system) of the northern Madison Range is strikingly similar to other paired systems in southwestern Montana along and adjacent to the western margins of the Ruby Range, Snowcrest Range, Greenhorn Range, Tobacco Root Mountains, and Bridger Range. Such systems may be the result of collapse of the crestal zones of large Laramide basement uplifts (arches) during Tertiary extension. No hydrocarbon discoveries have been made in this unique structural province. However, petroleum exploration here has focused on basement-cored anticlines, both surface and subthrust, related to the two major Laramide fault systems and on the fault-bounded blocks of Tertiary rocks within the post-Laramide extensional basins. The interplay of the two Laramide fault systems during both Laramide shortening and Tertiary extension has produced a variety of possible structural traps in the Madison Range that have not yet been thoroughly investigated.