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Detailed field studies of 12 small basement-cored folds in the Rocky Mountain foreland from southwestern Montana to northern New Mexico indicate that there was considerable variation in the degree of deformation of the basement rocks during Late Cretaceous and Paleocene folding. This variation may be characterized by two end-member styles of basement behavior (mode 1 and mode 2). In mode 1 folds basement deformation is confined to a narrow zone of cataclasis adjacent to a single fault, the cover rocks are significantly thinned on the forelimb of the fold and have a small carbonate to clastic rock ratio (<0.2) in the lower 300 m (1000 ft) of section, the basement-cover contact on the forelimb is a fault, and the interlimb angle is 60° or less.

In mode 2 folds basement deformation occurs in a broad zone between the principal fault and the anticlinal hinge surface, which is a fault in several structures. The basement deformation occurs as slip on sets of closely-spaced fractures, as flexural slip on preexisting foliation oriented subparallel to bedding, as axial surface-parallel slip on foliation, or as pervasive cataclasis. The cover rocks in mode 2 structures maintain nearly constant thickness through the fold, have a carbonate to clastic rock ratio that is relatively high in the lower 300 m (1000 ft) of section (>0.4), and are in stratigraphic (as opposed to fault) contact with the basement on the forelimb. The axial surface penetrates the basement, the interlimb angle is >90°, and backthrusts are common.

Most existing folds will have characteristics of mode 1 and mode 2 to varying degrees; for example, a basement-cover interface that is part fault and part stratigraphic contact on the forelimb, an intermediate interlimb angle (60°–90°), moderate thinning of cover rocks on the forelimb, and deformed forelimb basement with an intermediate thickness.

The style of basement-cored folds depends partly on the nature and orientation of prefolding basement fabric and the competence of the cover rocks. Well-foliated basement rocks that have foliation oriented subparallel to bedding or that have foliation in a “favorable” orientation for hinge-surface-parallel slip produce mode 2 folds, as do cover-rock sections with high carbonate to clastic rock ratios. Relatively isotropic basement rocks with low carbonate to clastic ratios produce mode 1 folds. Other factors that probably control the style are degree of influence of earlier faulting and the taper of the hanging-wall basement wedge; however, observations of the 12 folds in this study are inconclusive regarding the importance of these factors. Confining pressure and temperature are important only insofar as they determine the overall mechanical behavior of the basement and the cover rocks. Total variation in overburden (2.5–5 km) during initial deformation has not permitted basement behavior to deviate from the brittle field.

In progressive deformation of mode 1 structures, a relatively competent basement block is forced into relatively incompetent cover, resulting in no significant basement deformation. In mode 2 structures a relatively incompetent basement block is forced against a relatively competent cover. The basement deforms by generation of an anticlinal hinge surface that migrates away from the fault. Faults can propagate into the cover along the synclinal hinge, across the forelimb, or along the anticlinal hinge surface. In the latter two cases fault-dip changes can produce backthrusts.

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