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.

The distribution of Laramide strain in the Precambrian basement rocks of four small Rocky Mountain foreland folds was controlled by lithologies and orientations of preexisting foliation in the faulted forelimbs. Features of brittle deformation that developed in the basement were faults, sets of parallel, conjugate, or anastomosing fractures, zones of penetrative grain cracks and intergrain slip without grain-size reduction, and local zones of cataclasis or incipient mylonitization. In the London Hills anticline of Montana, foliation in amphibolite and gneiss was nearly parallel to bedding in cover rocks prior to folding. The foliation in the forelimb was rotated and deformed by layer-parallel slip between a forelimb thrust and a fault in a diabase dike located at the hinge zone of the anticline. In the Sheephead Mountain anticline of Wyoming, the forelimb fault cut foliation in quartzofeldspathic gneiss at a high angle. Penetrative brittle deformation occurred in the forelimb as a wide zone of fractures parallel to a forelimb thrust at the basement-cover contact, but the basement rocks were not folded. In the Gnat Hollow anticline-syncline, Colorado, foliation in interlayered granite, schist, and gneiss dipped about 20° more steeply than the forelimb thrust. Brittle deformation was confined to the fault zone and a small region in the core of the forelimb anticline. In the Romero Hills anticline, New Mexico, foliation in the basement was parallel to thrusts that cut both basement and cover rocks. Slip on foliation surfaces near the thrusts was pervasive and simple shear on foliation was distributed across the backlimb and forelimb of the structure. In comparing the four structures, preexisting foliation surfaces were most active during Laramide deformation where they paralleled forelimb faults, and were least active where foliation was at a high angle to forelimb faults or was not rotated into the forelimb orientation. If the angle between faults and foliation is about 15° or more, or if foliation is not nearly parallel to cover-rock layers, preexisting foliation appears to have exerted little influence on Laramide strain patterns in the foreland folds.

The Scarface thrust of the western Madison Range, Montana, is a 17° west-dipping Late Cretaceous thrust that places Archean gneisses over a complexly folded panel of Phanerozoic sedimentary rocks. The Archean-Cambrian contact on the footwall of the Scarface thrust is nearly vertical, and both bedding in the cover and foliation in the gneisses near the contact were rotated by 38° during folding. Paleozoic rocks up section in the footwall are overturned, with an axial surface that dips less than 10° west. The Scarface thrust is locally folded over lower Paleozoic rocks on the footwall. Folding was produced by post-Scarface thrust movement on a minor east-dipping splay fault that follows bedding in Devonian rocks. Of the two dominant shear fracture and fault sets in the basement (strikes and dips of N52°W, 47°NE; N20°W, 50°SW), the northeast-dipping set is parallel to foliation and reflects a strong influence of foliation on basement deformation. Intergranular fractures nucleated at the tips of biotite grains. Narrow zones of cataclasis containing shredded biotite formed along the intergranular fractures. Advanced stages of deformation were accompanied by formation of thicker zones of wavy, foliated cataclasites defined by dark seams of comminuted biotite, feldspar, and quartz. The recumbent footwall syncline is superimposed on the west limb of a large, more open syncline in Paleozoic and Mesozoic rocks. We are unable to resolve which fold formed first. Faulting sequences are also equivocal. The Scarface thrust may have been emplaced as a shallowly dipping sheet, or it may have been steeper initially and rotated during movement on the structurally lower Beaver Creek thrust.