Deformation of shale and dolomite in the Lewis thrust fault zone, northwest Montana, U.S.A.

The Lewis thrust fault zone at Marias Pass, northwest Montana, is an example of a fault zone in which hanging-wall dolomite and footwall shale deformed at relatively shallow levels (~7 km). Fabric in the fault zone depends on the rock type. Deformation of dolomite involved coalescence and widening by cataclasis of fractures, formation of anastomosing cataclasite zones that isolate less deformed clasts, and rounding and reduction in size of clasts to produce random-fabric cataclasite. Whereas dolomite deformed by progressive widening of cataclasite zones, shale deformation localized along ultracataclasite zones and slip surfaces that bound shale duplexes. Fault rocks that include both footwall shale and hanging-wall carbonate are characterized by isoclinal, intrafolial folds and a foliation that is defined by alternating shale- and carbonate-rich bands, elongate lenses of carbonate, and preferred orientation of phyllosilicates. Calcitization and subsequent solution of hanging wall rocks incorporated in the shale contributed to the development of this planar fabric. Lenses of hanging-wall carbonate were isolated in footwall shale by the emplacement of shale tongues into the hanging wall along mesoscopic faults. Displacement on the Lewis fault was accommodated by deformation of both dolomite and shale. Grain-size reduction of dolomite, mixing of dolomite and shale, and calcitization of dolomite in the fault zone may have enhanced diffusional processes in the carbonate and thereby weakened the fault zone.