Two orthogonal systems of lineaments reflect recurrent structural movement in the basement rocks of the Rocky Mountains and adjacent plains--the area of the middle Phanerozoic Cordilleran continental shelf. The shelf lay between the Transcontinental arch and the Cordilleran continental margin (or miogeocline) from Arizona and New Mexico to Montana and North Dakota. Major tectonic features that affected middle Phanerozoic sedimentation on this shelf were the Big Snowy trough and the Williston basin, the Ancestral Rocky Mountains, and the Zuni uplift. Some of the boundaries of these and other structural elements in the shelf area seem to be related to an orthogonal system of two perpendicular sets of lineaments, one trending northeast and the other northwest. The first para lels the northeast-trending cordilleran miogeocline in Nevada, and the other parallels the northwesterly continuation of the miogeocline north of Idaho. The two sets are roughly parallel to the Transcontinental arch and the margin of the Canadian shield, respectively. A second, less important orthogonal system is oriented north-south and east-west.

Differential vertical movements of rectangular basement blocks bounded by the fracture systems were apparently propagated upward through the strata, forming a variety of structures. Movements of the blocks at different times in the Phanerozoic influenced erosion, deposition, and lithofacies of the sediments and thus influenced the distribution of sedimentary minerals and petroleum deposits and later igneous intrusions, volcanism, and hydrothermal ores. The lineaments appear to have been avenues for passage of hydrothermal solutions, so their identification should help guide exploration for mineral deposits. Tectonic movements in the Phanerozoic seem to have been mainly along the northeast and northwest system of lineaments, but the north-south and east-west system also influenced the ormation of Laramide structures and the present landscape in the Rocky Mountains. Deformation associated with the two systems is probably related to events at the North American plate margin or to incipient continental fragmentation of the plate.