ABSTRACT: Oblique Thrusting in Laramide Foreland Arches

MOLZER, Philipp C. and ERIC A. ERSLEV, Colorado State University, Ft. Collins, CO

East-west Laramide arches provide a critical test of Laramide tectonic hypotheses. Models invoking multiple stages of compression frequently assume slip perpendicular to Laramide arch axes. Models explaining the variable arch orientations with one compression direction often invoke high-angle wrench faulting.

To test these hypotheses, slickensides were measured on minor faults (N>1200) from the east-trending Bridger Range (eastern Owl Creek Mountains) and Casper Mountain. They are subhorizontal with a northeast trend. Dip-slip faults are predominately shallow-dipping thrusts with northeast-southwest transport directions. Two sets of high-angle, strike-slip fractures form a conjugate set about a northeast shortening direction. Subhorizontal slickensides on faults with normal separation (e.g., Boysen fault) show that many of these are actually wrench structures formed during oblique thrusting of the range. Another set of purely dip-slip normal faults paralleling the range front formed in addition to wrench faults at Casper Mountain.

Preliminary results from north-trending sections of Front Range and Beartooth mountains indicate complementary right-lateral oblique slip. Comparison of map-scale fault patterns with patterns from sandbox experiments and other wrench systems also support the oblique-slip hypothesis. These data suggest a single northeast-trending Laramide principal stress orientation. East- and north-trending arches probably formed above lateral ramps that merged into the master Laramide detachment in the lower crust.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)