Abstract

Basement-involved foreland deformation in the Rocky Mountains of the conterminous U.S.A. occurred during three major Phanerozoic events: the Pennsylvanian Ancestral Rocky Mountain orogeny the latest Cretaceous through Paleogene Laramide orogeny and the post-Laramide Rio Grande orogeny. The origins of these orogenies and their relationships to each other and to earlier Precambrian deformations are important unknowns in the geosciences.

To test the tectonic controls of the Phanerozoic orogens, a geographic information system (GIS) structural database was created within ARCVIEW™ to quantify the structural trends in the Rocky Mountains in an accurate and unbiased manner. Rocky Mountains structural map data (e.g., faults, folds, and Precambrian contacts) were entered into this database from maps covering Wyoming, Colorado, northern New Mexico, southeastern Utah and northeastern Arizona. Faults and folds were divided into four subsets determined by the youngest rocks involved in the structures. These subsets bracket the three major Phanerozoic orogens (RIO for post-Paleogene, LAR for post-Pennsylvanian through Paleogene, ARM for Cambrian through Pennsylvanian, and PRE for pre-Phanerozoic). The data were queried by a variety of new programs within ARCVIEW™ that calculate vector mean line orientations and length-weighted rose diagrams over geographically limited subsets.

The orientations of Rio Grande orogeny faults, represented by the RIO dataset, support regional extension in two directions and/or reactivation of pre-existing faults with bimodal orientations. The ARM dataset was designed to represent the Ancestral Rocky Mountain orogen, but it is highly contaminated by structures that formed during later events. Although parallelism between ARM and LAR structures supports hypotheses proposing analogous tectonic histories for these orogens, more kinematic studies of Ancestral Rocky Mountain structures arc needed before definitive conclusions can be drawn. The Laramide orogen, represented by the LAR dataset, is the most extensive and best recorded Phanerozoic event in the Rocky Mountains. LAR fault, fold and arch orientations show the primary influences of (1) NE-directed transpression, probably related to plate convergence directions, and (2) the boundaries of the Laramide province itself. The resulting fabric of Laramide deformation was further complicated by secondary effects including reactivation of pre-existing faults and basement contacts as well as the impingement by the adjoining Cordilleran thrust belt.