Estimate of Displacement in Major Zone of Tear-Thrusting in Fold and Thrust Belt, Southwest Montana: ABSTRACT

Abstract

The most pronounced deflection of the marginal fold and thrust belt in Montana occurs in the northern Tobacco Root End_Page 986------------------------------ Mountains where a zone of imbricated thrusts follows the southern margin of the eastward-trending Helena embayment of the Belt basin. Movements along major thrusts north and south of this east-trending zone are nearly pure dip-slip. Movements within the zone are demonstrably oblique with nearly equal components of dextral and reverse slip. This suggests that the fault zone may be described as being one of tear-thrusting. Accumulated net slip along the tear-thrust zone may be estimated by summing estimates of net slip along the individual faults which comprise the zone and by examining displaced isopach lines for rocks deposited prior to faulting. Summing net-slip estimates for individual faults along several sections though the fault zone indicates that the horizontal components of net slip is between 15 and 30 km. Right separation of isopach lines for a limestone member of the Kootenai Formation (Cretaceous) indicates a horizontal displacement which is somewhat larger (between 30 and 60 km). Displacements of this magnitude cannot be accounted for by folding and minor thrusting in the rocks north of the tear-thrust zone and strongly suggests one or more zones of detachment within the section above Arc ean basement rocks. Estimates of net horizontal displacement along this fault zone can be very useful in interpreting anomalous isopach trends such as those seen in the Flathead Sandstone (Cambrian) and Maywood Formation (Devonian). End_of_Article - Last_Page 987------------