BASEMENT AND COVER-ROCK DEFORMATION DURING LARAMIDE CONTRACTION IN THE NORTHERN MADISON RANGE (MONTANA) AND ITS INFLUENCE ON CENOZOIC BASINFORMATION

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, Ana lysis 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 th at of the basement-cover contact. Steeply dipping, north-striking brec cia zones enclosing domains of relatively undeformed basement may have permitted domino-style rotation of basement blocks during simple shea r between pairs of thrusts. In most places along the Hilgard thrust sy stem, a large basement overhang, produced by thrusting of Archean bloc ks above rocks as young as Late Cretaceous, overlies a tight footwall syncline, This tight folding is largely concentric and was accommodate d by flexural slip, resulting in severe crowding in synclinal hinges t hat resulted in observed or inferred features such as bedding-plane sl ip, imbricate and out-of-syncline thrusting, and hinge collapse. The n orth-striking Madison normal fault system, a zone of Tertiary and Quat ernary valley-forming normal faults, is approximately parallel to the Hilgard thrust system, In some places, normal faults are reactivated t hrusts on which large basement overhangs of the Hilgard thrust system were dropped back into the Madison Valley and covered by Tertiary basi n-fill deposits, leaving only the rocks of the footwall synclines expo sed. In other places, both the thrusted Archean blocks and the near-is oclinal footwall synclines are well preserved. This paired fault syste m (the Madison normal fault system and the Hilgard thrust system) of t he northern Madison Range is strikingly similar to other paired system s in southwestern Montana along and adjacent to the western margins of the Ruby Range, Snowcrest Range, Greenhorn Range, Tobacco Root Mounta ins, and Bridger Range, Such systems may be the result of collapse of the crestal zones of large Laramide basement uplifts (arches) during T ertiary extension. No hydrocarbon discoveries have been made in this u nique structural province. However, petroleum exploration here has foc used on basement-cored anticlines, both surface and subthrust, related to the two major Laramide fault systems and on the fault-bounded bloc ks of Tertiary rocks within the post-laramide extensional basins. The interplay of the two Laramide fault systems during both Laramide short ening and Tertiary extension has produced a variety of possible struct ural traps in the Madison Range that have not yet been thoroughly inve stigated.