Abstract: Cooling Histories Along the High Plains-Southern Rocky Mountains Boundary as Deduced from Apatite Fission-Track Thermochronology

Shari A. Kelley, Charles E. Chapin

Proterozoic rocks comprising the topographic escarpment west of Golden, Colorado, yielded apatite fission-track ages ranging from 100 to 267 Ma. A sample at the east base of Pikes Peak yielded a fission-track age of 171 +/-9 Ma. Thus, the Front Range-High Plains boundary owes its topographic relief to Cenozoic erosion and exhumation of Laramide structure rather than late Cenozoic faulting, as is commonly assumed. In contrast, samples collected at the south end of the Front Range, along Phantom Canyon Road, yielded Laramide fission-track ages (57-67 Ma) at lower elevations and a fossil partial annealing zone at an elevation of approximately 2700 m (8850 ft), above which fission-track ages are 200 Ma or older, except near the Cripple Creek volcanic center. This suggests that Proterozoic rocks along the High Plains boundary were thrust laterally over the Denver basin above low-angle faults (as indicated by seismic reflection lines), whereas in the core of the Front Range vertical uplift was sufficient to expose the base of a partial annealing zone (approx. 60-140°C) that was at a depth of 3-4 km prior to 67 Ma.

Proterozoic rocks at high elevations in the Wet Mountains have fission-track ages between 110 and 300 Ma, but the top of an Eocene fossil annealing zone with fission-track ages of about 50 Ma is present near the base of the eastern escarpment. Miocene uplift and erosion accompanied by westward tilting of the Range has been strong enough to expose rocks which were 2-2.5 km beneath the range in the early Eocene.

Fission-track dating throughout the southern Rocky Mountains reveals that differences in cooling history exist between ranges and even between segments of the same range. For example, Proterozoic rocks on the broad, thrust-faulted bench of the southeastern Sangre de Cristo Range east of the Pecos River (as well as in the Pedernal Hills to the south) yield fission-track ages between about 12 and 28 Ma, in contrast to Laramide cooling ages in the high part of the Sangre de Cristo Range.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994