Deformation of a foreland carbonate thrust system, Sawtooth Range, Montana

Abstract

An analysis of mesoscale faulting associated with thrust-belt development was completed along a transect crossing several foreland thrust sheets of the central Sawtooth Range, Montana. At a regional scale, a balanced cross section of the central Sawtooth Range, constrained by well, seismic, and surface data, indicates a minimum of 60% horizontal shortening of the upper Paleozoic carbonate bank sequence, accommodated by a forward-developing thrust system. Subsurface data delineates a 4° westward-dipping Precambrian basement and westward thickening of the Precambrian Belt Supergroup. Major decollements within the central Sawtooth Range are present at the base of the Devonian Jefferson Formation, in the lower Mississippian Allan Mountain Limestone, and in the Cretaceous Colorado Group. Within the Diversion, French, Norwegian, and Beaver thrust sheets of the central Sawtooth Range, deformation has been partitioned into arrays of meter- to decimetersized faults that constitute brittle deformation zones (BDZs). BDZ thickness is directly related to fault displacement but is not related to fault trajectories or subsequent foreland imbrication. The ratio of BDZ thickness to displacement is nearly constant, with BDZ thicknesses ranging from 65 m to 200 m for faults whose displacements range from 2 km to 5 km. The BDZs of each thrust sheet can be separated into three regions defined by differences in fault diversity, fault intensity, and fault kinematics. Proportionally, the locations of boundaries between regions within the BDZs are remarkably constant for each of the four thrust sheets studied, indicating that the BDZs widen uniformly during thrust sheet emplacement. BDZ bases are characterized by an interlocking network of faults that exhibit great diversity and accommodate mesoscale cataclastic flow. BDZ bases are dominated by oblique-slip faults, whereas the middle and upper regions of BDZs are dominantly dip- or strike-slip faults. In all regions of a BDZ, localized zones of contractional, extensional, or more commonly, transport-parallel strike-slip faults bounded by bedding-plane detachments are present. Fault length/ m2 of outcrop decreases logarithmically to zero from a maximum of 4 m/m2 across the French and Norwegian BDZs. Zones of intense faulting alter this general pattern of decreasing fault intensity in the Diversion and Beaver thrust sheets.