MIDDLE TO LATE TERTIARY COOLING OF THE GORE AND WESTERN FRONT RANGES, CENTRAL COLORADO, FROM APATITE FISSION-TRACK DATA

Apatite fission-track (AFT) data from Proterozoic crystalline rocks and Paleozoic sedimentary rocks of the mountains flanking the Blue River graben (western Front Range to the east and the Gore Range to the west) indicate significant Neogene cooling. AFT ages are between 5 and 40 Ma and become older with elevation in both mountain blocks. Ages from the western Front Range are significantly older than ages from similar elevations in the eastern Gore Range, suggesting asymmetrical uplift, erosion, and cooling. For example, AFT ages from 3,000 m altitude at Ute Pass in the western Front Range are about 25 Ma, whereas samples at 3,000 m altitude in the eastern Gore Range have an apatite age of about 9 Ma. Apatite ages from the western Gore Range are also older (about 22 Ma at 3,000 m) than those obtained at similar altitudes on the east side of the range. Most apatite track lengths are greater than 14 µm. These long lengths, and the AFT ages, indicate that total track annealing was followed by relatively rapid cooling in the Neogene. The AFT data suggest that all of the rocks that flank the Blue River graben were at temperatures >110°C well into middle, and in some cases, late Tertiary time. For a normal geothermal gradient of 20-30°C, a stratigraphic reconstruction indicates that the basement rocks in the northern, lower, part of the Gore Range should yield Laramide (early Tertiary) or older AFT ages. However, the AFT data indicate that an elevated Neogene gradient has removed evidence of these older ages. Generally accordant drainage divides in the Gore and western Front Ranges cut the AFT cooling isochrons. If such accordance is related to an erosion surface, the AFT data require that it be Pliocene or younger, which seems unreasonable. This accordance is probably due to geomorphic processes acting over time on rocks of similar resistance to erosion. In a regional context, the AFT ages along the flanks of the Blue River graben are significantly younger than AFT ages farther to the east in the central and eastern Front Range where the apatite ages suggest Laramide cooling below 110°C. These data support the interpretations of previous workers, that the Front Range was tilted eastward in Neogene time and that at the 3700 m summit of Red Mountain in the western Front Range 1.6 km of rock has been removed since 28 Ma (average erosion rate of 57 m/Ma).