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Present address: Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544.

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.

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