RE-INTERPRETATION OF THE RANGE-BOUNDING FAULTS OF THE BLACKTAIL MOUNTAINS, SW MONTANA: IMPLICATIONS FOR CENOZOIC TECTONIC HISTORY

The Blacktail Mountains are one of a group of fault-bounded, Archean basement-cored ranges of the northern Rocky Mountain foreland in SW Montana. The range is elongated NW-SE and its topographically prominent NE margin is bounded by two major faults, the moderately NE-dipping, highly silicified Jake Canyon fault (JCF) and the sub-vertical Blacktail fault (BF) lying several hundred meters to several km to the NE. Tysdal (1988) interpreted the JCF as a Laramide reverse fault (with possible minor, post-Laramide normal reactivation) and the BF as a Cenozoic normal fault that cuts the JCF in the subsurface. Tysdal and others (1990) constrained the last significant movement on the JCF by correlation of an unaltered, 48 Ma basalt overlying brecciated and mineralized footwall rocks near the mouth of Jake Canyon with an undated, but similar basalt on the hanging wall to the SW. They interpret the basalt to have flowed down ~1800 ft of relief and across the JCF.

We believe that the JCF must have experienced significant post-Laramide normal movement for three reasons. 1) Preservation of ~1800 ft of relief between the two correlative basalt exposures for 48 Ma would be highly problematic given the active Cenozoic tectonics the area has experienced. 2) If the present ~3000 ft of maximum relief along the NE front of the range is entirely the result of down to the NE movement on the BF, then the 1800 ft of relief across the JCF must have been preserved by post 48 Ma burial by the Eocene-Miocene Renova and Miocene-Pliocene Six Mile Creek Fms. Subsequently, this relief would have to have been exhumed in virtually the same topographic configuration. There is, however, no apparent geological evidence for complete burial of the Blacktail range during the Cenozoic. 3) Evidence for significant Cenozoic offset along the BF is confined to the northern half of the range where a 44 Ma rhyolite plug is exposed in the footwall of the fault. South of Ashbough Canyon, the morphology of the mountain front changes considerably and we suggest that Cenozoic offset was distributed between the BF and JCF.