Abstract Fission-track dates of zircon and apatite from porphyry and country rock in and near the Aspen mining district of central Colorado form clusters at about 52 and 30 Ma and reveal some complications in the thermal history of the rocks. Zircons from three varieties of Late Cretaceous porphyry and from both altered porphyry in the mining district and less altered and unaltered porphyry outside the district have fission-track dates of about 52 Ma. Apatite fission-track dates in the area are about 30 Ma. These dates furnish no definitive evidence for the age of the ore deposits. Fission-track dates of zircon from Proterozoic granite and Penn-sylvanian sandstone near the mining district are much older, do not significantly reflect a Tertiary thermal event, and show that the Aspen mining district is not marked by a regional paleothermal anomaly. In that respect the Aspen district differs from the Leadville, Gilman, and Tennessee Pass districts.

Apatite fission-track ages from basement rocks in the Wasatch Mountains between Ogden and Bountiful, Utah, range from 5 m.y. near the Wasatch fault along the west margin of the mountains to 94 m.y. on the crest of the range and show a correlation with altitude within individual fault blocks. Analysis of these ages yields an uplift rate of 0.4 mm/yr for the last 10 m.y. for the fault blocks having the most sustained rapid uplift. A variety of intermediate ages of apatite from the crest of the range and at lower altitudes is interpreted as representing fault blocks with rocks having mixed ages due to various degrees of tilting and varying but lesser amounts of uplift.

Abstract Fission tracks that are present in apatite crystals recovered from sedimentary and crystalline basement rocks can be used to determine the thermal (tectonic) history of a sedimentary basin. Because fission tracks in apatite usually record the time when the rock temperature cooled below 100° C, one can use the apparent apatite fission-track ages as a function of depth, to determine the amount and duration of an uplift event.