The significance of fission-track ages of apatite in relation to the tectonic history of the Front and Sawatch Ranges, Colorado

Fission-track ages of apatite in the Mount Evans area of the Front Range show that the Late Cretaceous 100 °C isotherm, which was 4 km below the sea floor, is now at an altitude of 3.5 km. Assuming a geothermal gradient of 25 °C per kilometre, the projected level of the latest Cretaceous sea floor, represented by the Fox Hills Sandstone, is at an altitude of 7.5 km, about 3.2 km above the top of Mount Evans, and the structural relief from that projection of the Fox Hills on the crest of the Front Range uplift to the deepest part of the Denver Basin to the east is 6.5 km.

Ages of apatite in Precambrian rock along the Elkhorn fault on the west margin of the Front Range uplift show that in the area of maximum stratigraphic displacement the fault must have a very low dip and that the east margin of the Paleocene South Park basin must have been nearby.

Ages in the Sawatch Range suggest that significant uplift occurred during or after Eocene time. Ages on the east side of the range reflect development of the Arkansas Valley graben and concomitant uplift of the east margin of the modern Sawatch Range in Miocene time.