HOW FAR DOES THE WILLIAMS RANGE THRUST NEAR DILLON, COLORADO OVERHANG CRETACEOUS ROCKS?

The Laramide Williams Range thrust defines the western structural boundary of the Colorado Front Range and places Paleo- and Mesoproterozoic igneous and metamorphic rocks over mostly Cretaceous shale and sandstone. The low-angle to nearly horizontal, north-northwest-striking, west-directed thrust can be traced from near Breckenridge to just north of Kremmling. Although timing of thrusting is poorly constrained, the thrust is probably continuous to the south with the Late Cretaceous to early Eocene Elkhorn thrust. The Williams Range thrust is exposed in a thrust window near Keystone and is intersected in a water tunnel under the Continental Divide, where Proterozoic rocks lie structurally above Late Cretaceous Pierre Shale. These relationships indicate at least 9 km of horizontal displacement; however, several indirect lines of evidence suggest that displacement was considerably greater. First, upper-plate Proterozoic rocks along the western margin of the Front Range are pervasively fractured, in part along the broad Loveland-Berthoud Pass fault zone of suspected Laramide age. The fractures appear randomly oriented and have short trace lengths and in many places the host rocks are strongly oxidized and altered. Rolling topography and widespread landsliding in these fractured regions reflect the weak nature of the underlying bedrock. Second, an abrupt drop in residual magnetization intensity to the west across a north-northwest-trending line appears to coincide approximately with the boundary between strongly fractured rocks to the west and relatively intact rocks to the east. Between the latitudes of Dillon and Kremmling, this line is approximately 20-25 km east of the trace of the Williams Range thrust (about 4 km west of Silver Plume). These relationships suggest the following model: A flexure in the thrust surface, from relatively steep at depth to gentle nearer the surface, occurs approximately where the thrust intersects the base of the Cretaceous rocks in the footwall block, at least 20 km east of the present thrust trace. The brittle hanging-wall Proterozoic rocks became pervasively fractured as they moved and “bent” through the flexure. The lower magnetization can be attributed to widespread oxidation of the fractured rock and the presence of a wedge of low-magnetization sedimentary rocks in the footwall block.