Abstract
The Lewis overthrust emplace the Precambrian meta-sedimentary Belt Series over Cretaceous sandstones and shales. A thin (3-10 feet thick) shale shear zone separates the thrust plate from the footwall. Most of the deformation associated with the thrusting was taken up within the shear zone. Dikes of the shear zone shale were injected into the overlying plate. Fragments of plate rock suspended in the shear zone appear to have been pried loose by coalescing dikes. These features suggest that, at the time of overthrust movement, the shale behaved as a fluid medium which acted to reduce the coefficient of sliding friction on the fault. The Hubbert-Rubey (1959) hypothesis that abnormally high pore pressure reduces the critical shear stress for overthrust movement without reducing the coefficient of sliding friction appears inapplicable to the Lewis overthrust. The highly fractured plate rock in the vicinity of the shear zone create a permeability through which abnormally high pore pressure would escape. Shale lubrication provides a more tenable mechanism for reducing the critical shear stress for movement of the Lewis overthrust. ...
Wilson, Raymond Carl (1971). The mechanical properties of the shear zone of the Lewis overthrust, Glacier National Park, Montana. Doctoral dissertation, Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -181559.