ABSTRACT

At the intersection of the Montana Disturbed Belt and the Scapegoat-Bannatyne trend the orientation of the structural grain of the Montana Disturbed Belt shifts from nearly north-south to northwest-southeast. The thrust faults in this part of the Montana Disturbed Belt have adjusted to a southward change in the strike of the top of the Precambrian crystalline basement from nearly N-S to NW-SE. The Scapegoat-Bannatyne trend was a focus of more intense deformation in the region of thrust fault adjustment because periodic tectonic activity along the trend weakened the overlying sedimentary rocks. In the area of Smith and Weasel creeks (T19N,R8W, Lewis & Clark County) the shift in structural grain formed a structural salient. Thus, the change in orientation of the thrust faults in this part of the Montana Disturbed Belt reflects the influence of: (1) the Precambrian crystalline basement configuration, and (2) a narrow northeast-trending Precambrian crystalline basement fracture system, the Scapegoat-Bannatyne trend.