Style and Timing of Frontal Structures, Thrust Belt, Central Utah

Abstract

The Cordilleran fold and thrust belt in central Utah may be divided into a western belt of Precambrian to lower Mesozoic strata shortened above ramp-style thrust faults and an eastern belt of folded middle to upper Mesozoic rocks. Shortening in the eastern foldbelt occurred above a bedding-plane thrust fault system that terminates within a thick section of Jurassic shale, siltstone, and anhydrite. This thrust fault system probably failed to reach the synorogenic erosion surface. Deformation associated with the thrust system takes the form of ramp anticlines in the west and a regionally extensive triangle zone at the eastern extent of the shortening. The ramp structures indicate that the thrust system rose from deeper structural levels and became bedding parallel within th Jurassic section. The Middle Jurassic section was approximately doubled in thickness by duplexing within the triangle zone. Shortening across the triangle zone is approximately 50% (5 km or 3 mi). Demonstrable synthrusting deposits within the region are late Early to Late Cretaceous in age (late Albian to late Campanian). The synorogenic rocks were folded above the triangle zone and are now overlain unconformably by strata of Tertiary age. Tertiary clastics onlap structures in the foldbelt and were not deformed by the deformation within the triangle zone. The age of synorogenic deposits and structural relations of postorogenic strata indicate that deformation was complete by the close of the Cretaceous or early Paleocene, and support a thrust mechanism for much of the folding in the region.