TERMINUS OF THE MONTANA DISTURBED BELT AND THE ORIGIN OF THE HELENA STRUCTURAL SALIENT

The eastern margin of the overthrust belt in western Montana is interrupted across the Big Belt and Castle Mountains in west-central Montana by an east-trending salient, termed the Helena structural salient. Historically the salient was considered an eastern embayment of the Middle Proterozoic Belt basin but was subsequently identified as a structural salient. Newly completed geologic mapping suggests that the concept of a thrust salient requires revision. The Montana disturbed belt, frontal part of the overthrust belt, flexes from a generally north-trend to a southeast trend south across the Scapegoat-Bannatyne arch. Southeast from Wolf Creek, Montana, structures rise up plunge in the northern Big Belt Mountains; tight, faulted folds expose progressively older rocks. There the disturbed belt terminates along a west-northwest-trending zone in which the folds and faults are detached along evaporite deposits in Upper Mississippian strata from gently west-dipping older strata of the continental autochthon. Termination of the disturbed belt and its detachment zone coincide spatially with the northwest-trending high-angle fault boundary of Late Proterozoic age between Early Proterozoic crystalline rocks on the north and a thick succession of Middle Proterozoic Belt Supergroup strata on the south. At the east end of the salient, Belt strata of the Greyson and Spokane Formations have been traced kilometers east of their originally perceived wedge-edges or salient margin. Along exposed faulted segments of the south margin of the salient, Belt strata show no evidence of depositional thinning against adjacent older crystalline rocks. The salient can be considered as a crustal segment that was translated eastward by right-slip along high-angle faults on the north, and by left slip and reverse movement on the south; low-angle thrust faulting is limited to the western part of the salient. Faults bounding the salient had a long history of complex recurrent movement.