INFLUENCE OF THE COLORADO MINERAL BELT ON THE NORTHWEST MARGIN OF THE DENVER BASIN

An abrupt change in structural style and apatite fission-track cooling history occurs between Golden Gate and Van Bibber canyons north of Golden, Colorado. Southward, east-vergent thrusts and AFT ages > 100 Ma characterize the Front Range margin; northward, southwest-vergent back thrusts and Laramide (76 - 45 Ma) AFT ages prevail. The transition occurs within 3.5 km and no major faults cross the range margin. However, a N35°E line located ~30 km southeast of the axis of, and parallel to, the trend of intrusions in the northern Colorado Mineral Belt, separates old AFT ages from Laramide AFT ages. Thus, the abrupt transition can be explained as the intersection of a northeast-trending, Laramide, synmagmatic thermal/structural upwarp of the ~110°C isotherm with the north-trending range margin.

This Laramide synmagmatic upwarp explains several features within the Denver Basin. The basin margin changes from north-northwest to northerly across the transition and the Golden thrust veers northeast and disappears. The Denver Basin axis is deflected northeastward 60 km by growth of the Boulder-Greeley arch during late Cretaceous-Paleocene sedimentation; decollement faulting off this arch produced a wide zone of tilted fault blocks within which Laramie Formation coals accumulated. The Wattenberg gas field and thermal anomaly are located on trend with the edge of the synmagmatic upwarp.

Subsurface AFT ages of Cretaceous Dakota (~ 8 Ma) and Sussex (17 - 25 Ma) sandstones are the same, within error, across the Wattenberg vitrinite reflectance anomaly, indicating that the vitrinite reflectance pattern formed prior to 25 Ma.