Laramide Foreland Thrust Faulting in Southwestern Montana: ABSTRACT

Abstract

Two major schools of though exist on the nature of Rocky Mountain foreland (Laramide) deformation: (1) advocates of the classical fold-thrust model of R. R. Berg, including variations and plate-tectonics settings by J. K. Sales, J. D. Lowell, A. W. Bally, and others; and (2) advocates of the classic upthrust or drape-fold model of J. J. Prucha and others, with recent vigorous support based on field and model studies by D. W. Stearns and his students. The first school concentrates on lateral-compressive End_Page 532------------------------------ deformation and the second vertical, or nearly vertical tectonics, as the principal deformation mechanism. The main battleground involves the floored by Archean crystalline rocks. This province includes much of southwestern Montana, west of commonly recognized limits of Laramide foreland deformation and south of the Helena salient of the Cordilleran overthrust belt. In this area, west-northwest of Yellowstone National Park, thrust faults and other structural features indicative of compressive deformation are widespread and have been mapped in the Snowcrest Range by M. R. Klepper and in the Greenhorn Range by J. B. Hadley. These two ranges expose the Snowcrest structural terrane: the complexly deformed steeper limb of the asymmetric Laramide Blacktail-Snowcrest massif. This massif, like the Madison-Gallatin uplift farther east, has been broken and stretched by Tertiary extension faults, principally subparallel and behind the major range-front thrusts and probably listric to these thrusts. Current geologic and gravity studies in the southwestern part of the Snowcrest structural terrane extend the zone of Laramide foreland thrust faulting into the southwest Montana reentrant of the overthrust belt. Here foreland thrust faulting is chiefly Late Cretaceous in age, and thrust faulting occurs along the Greenhorn lineament, a zone of crustal weakness active since Paleozoic time within the northwestern part of the Wyoming province. The presence of foreland thrust faulting has broad implications for oil and gas potential of the southwestern Montana and adjacent foreland. End_of_Article - Last_Page 533------------