Abstract

Wrench faults are near vertical fractures characterized by Important lateral slip components. They form under regional compression when the greatest and least principal pressures (axes) lie In the horizontal plane. With the aid of conceptual models (block diagrams) it is shown that since faults are finite and confined within an essentially incompressible crust, vertical separation and an important component of vertical slip must accompany lateral movement along wrench faults. Thus separation and slip along wrench faults may vary importantly, a geometric condition considered singularly characteristic. The reversal of fault dip along a fault line (“propeller faulting”), the development of belts of en echelon parafolds (“drag folds”) or pinnate tension fractures, or abrupt stratigraphic change across the fault zone, also are considered indicative of wrench faulting.

In the central Rocky Mountain foreland area many of these wrench fault features can be recognized along major west-northwest (left wrench) and northeast (right wrench) trending fault zones. Considering the nearly north-south orientation of primary folds and of the low angle thrust faults of the disturbed belt, therefore, it appears that a single north-northeast–south-southwest direction of principal horizontal stress (PHS) can account for all of the Laramide structural features of the central Rocky Mountain foreland. An idealized tectonic diagram and a regional tectonic map of the central Rocky Mountain area are presented to show graphically the writer’s interpretation of the various faults and folds of this area according to the wrench fault concept. These diagrams portray the genetic relationships between first and second order wrench fault zones and other structural features.

A check list of characteristic features is also presented as an aid in identifications of other wrench fault zones in the Rocky Mountain Area. Such identifications are considered economically important as the structures around and along wrench fault zones commonly are the habitat of mineral deposits.