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Internal Deformation of Thrust Sheets, Sawtooth Range, Northwest Montana

LAGESON, D. R., Montana State University, Bozeman, MT

A popular conception in the geological community is that sedimentary rocks of the Cordilleran foreland fold-and-thrust belt are unmetamorphosed and lack significant internal deformation. This idea has been incorporated by many workers into regionally balanced cross sections and palinspastic restorations that yield approximately 50% shortening. This amount of shortening is based entirely on plane strain, rigid body translations, and rotations of strata above a regional decollement that becomes structurally and stratigraphically deeper to the west.

Detailed field studies in the Sawtooth Range (Disturbed belt) of northwestern Montana challenge the popular model of thrust belt internal deformation and shortening. A variety of deformation fabrics exist at the microscopic and mesoscopic scales, including pervasive axial-planar pressure-solution cleavage, boudinage, pencil structures, deformed oolitic and pelletal carbonate grains, distributed shear zones, and a host of extension and shear fractures. These features demonstrate that these rocks have experienced substantial internal finite strains. The internal strains are primarily the result of layer-parallel longitudinal shortening, coupled with pure and simple shear. It is clear that volumetric changes have accompanied distortions within the rocks, violating the plane-strain assump ion of balanced cross sections. Furthermore, different types of internal strain are compartmentalized as a function of ductility and lithology, resulting in distinct "lithotectonic zones" within the stratigraphic section.

These observations are important for the following reasons. (1) The amount of total shortening across the fold-and-thrust belt has been traditionally underestimated, producing errors in palinspastic facies maps. (2) Internal deformational fabrics are important controls on reservoir heterogeneity and permeability. (3) Recognition of internal deformation requires the reevaluation of dynamic and thermochemical models for foreland thrust belt formation.

 

AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)