Thrusting in the Northwestern Montana Overthrust Belt

White, Brian

Structural analysis at 33 stations d istributed throughout a portion of the northwestern Montana overthrust belt provides a basis for distinguishing four major domains of individually distinctive movement geometry. The domains are each restricted to discrete intervals of strata; progressively westward domains contain successively older stratigraphic sequences; and thrusting within each more westerly domain of older strata succeeded thrusting within the next easterly domain of younger rock. The o rig in o f the domains is viewed as the consequence o f sequential gravitational sliding within the westward-thickening sequences, as uplift in the west successively steepened multiple slid e zones towards the east. Small scale strike-slip fractures are symmetrically disposed with respect to the mean horizontal movement direction at each station, and their formation both preceded and followed development of major thrust faults. Origin of the strike-slip fractures is considered to have resulted from tangential extension around major or minor movement salients (Crosby, 1969; Crosby and Link, 1972). The early strike-s lip fracture s are distinguished by contained striations that lie parallel to bedding planes, probably indicating that bedding-parallel sheets underwent initial lateral translation upon basal decollements, prior to formation of major thrusts which cut across strata. The later strike-slip fractures succeeded formation of major thrust faults, as internally-thrusted allochthonous masses advanced farther towards the craton. Thick zones of intensely deformed strata beneath the Lewis Overthrust plate and below thick Paleozoic carbonates in the Sawtooth Range contain structures concordant with those in the overlying rocks. These are examples of sub-plate flowage zones, after Kehle (1970). Sequential gravitational sliding by successive steepening of slide zones does not necessarily preclude that some other overthrust mechanism may be active on a larger scale, including either of the contrasting models that have been presented for this region by Mudge (1970), and by Price and Mountjoy (1970).