Development of normal faults during emplacement of a thrust sheet: an example from the Lewis allochthon, Glacier National Park, Montana (U.S.A.)

Abstract

Geologic mapping in southern Glacier National Park, Montana, reveals the presence of widespread, E-dipping normal faults within the basal portion of the Lewis allochthon. The displacement along the normal faults increases downward from less than 1 m at the highest exposure, 200–300 m above the Lewis Thrust, to a maximum of 200 m near or at the Lewis Thrust. The normal faults are located below discrete, bedding-parallel shear zones associated with mesoscopic structures characterized by NE- or SW-trending striations on bedding surfaces and asymmetric E-verging folds. These shear zones lie directly below the E-directed Brave Dog Fault, a major bedding-subparallel fault within the Lewis allochthon. The shear zones are interpreted to have formed during the development of the Brave Dog Fault. Striations on the Brave Dog Fault, normal faults and shear surfaces in the shear zones are consistent with the transport direction of the Lewis Thrust. The kinematic compatibility of the normal faults with the Lewis Thrust, the concentration of the normal faults along the basal part of the Lewis plate, and the increase in displacement along them toward the Lewis Thrust, all suggest that their development was synkinematic with eastward emplacement of the Lewis allochthon. The normal faults may have formed as Riedel shears (R) that accommodated a bulk, simple-shear strain within the thrust plate between the simultaneously moving subhorizontal Brave Dog and Lewis faults.