COCORP profiling across the Rocky Mountain Front in southern Wyoming, Part 1: Laramide structure

Abstract

COCORP deep seismic-reflection profiles have been recorded showing significant crustal deformation of the North American craton; profiles were made across the Laramie Mountains, the Wyoming section of the Front separating the southern Rocky Mountains from the Great Plains, and eastern margin, in Wyoming and Colorado. These mountains are underlain by a series of westerly dipping seismic reflectors traceable as deep as 10 to 12 km, arranged en echelon with various dips (20° to 50°), which in some cases can be traced to the surface position of faults flanking the Laramie Mountains. Although other interpretations are possible, the reflectors are thought to be thrust faults, whose distribution and orientation suggest that the mountains were uplifted by horizontal crustal shortening during the Laramide orogeny. These inferred thrusts apparently do not generate such pronounced fault-zone reflections as those seen on COCORP lines recorded across the Wind River Mountains in Wyoming, probably because fault displacements were much smaller. Differential movements along the thrusts may explain variations in the character of the Rocky Mountain Front in the region of the COCORP lines. On the basis of morphological continuity, the Front in Colorado may also have been uplifted by crustal shortening with variations of structural style caused by adjustments to Precambrian and Ancestral Rockies structures. COCORP data have now been recorded across two mountain ranges lying on the eastern and western margins of the region of Laramide basement uplifts in Wyoming. Both ranges (Laramie and Wind River) apparently are underlain by moderately dipping thrust faults. Probably, crustal shortening by lateral compression was the dominant cause of the Laramide basement uplifts. These mountain-building compressional stresses were transmitted to the interior of the continent, 1,000 to 1,500 km from an active plate margin.