NON-STANDARD THRUST RELATIONSHIPS ASSOCIATED WITH INTERCUTANEOUS BACKTHRUSTS IN A SYSTEM OF STACKED TRIANGLE ZONES; EXAMPLES FROM THE SOUTHEASTERN FLANK OF THE FRONT RANGE, COLORADO

Thrusting along the southeast flank of the Colorado Front Range between Colorado Springs and Boulder involves triangle zone detachments at multiple stratigraphic levels. Principal among these are detachments near the top of crystalline basement, in the lower and upper parts of the Cretaceous Pierre Shale, as well as breaks to the erosional or perhaps free surface. The master floor thrust works its way up through the stratigraphic column, utilizing then abandoning successive tip zones as it works its way toward the free surface. The roof thrusts often juxtapose younger rocks over older with up to 3000 meters of stratal attenuation, and have typically been mistaken for forethrusts. Outcrop and subsurface data reveal that such non-standard roof thrust relationships are caused by backthrusts within the intercutaneous wedge. The result is a complex system of anastomosing forethrusts and backthrusts that transfer displacement along strike and conspire to build the mountain front.