Thermal and structural history of Paleozoic and Proterozoic rocks in the vicinity of Molas Pass, northern Needle Mountains, southwestern Colorado
— S. A. Kelley and Karlstrom K.E.

Apatite fission-track (AFT) data collected along an elevation traverse at Molas Pass in the Needle Mountains of southwestern Colorado record Early to Middle Miocene cooling of this region. A younger thermal pulse that is likely related to Late Miocene magmatism in this region is superposed on the simple cooling trend. High elevation samples from Pennsylvanian to Permian sedimentary rocks, west of Molas Lake, have AFT ages that range from 19.3±3.6 Ma to 13.6±2.8 Ma; the ages decrease systematically with decreasing elevation. In contrast, AFT ages from Proterozoic rocks at lower elevation to the east of Molas Pass do not correlate with elevation and appear to have been affected by a localized, fault-related Late Miocene hydrothermal event. Thermal history modeling of the age and track-length data reveals that the fission-track system in all samples was totally reset prior to 20 Ma as a result of heating during stock emplacement at 26.6 Ma along the southern margin of the Silverton caldera located just a few kilometers to the north. Burial of the area by volcanic and volcaniclastic rocks during formation of the San Juan volcanic field also contributed to thermal resetting. The lag in time between caldera formation at 26.6 Ma and cooling of the higher elevation samples at 19 to 14 Ma and the track length distributions (mean lengths of 13.1±2.8 to 13.9±2.3 μm) suggest that the section was cooling as a result of exhumation as opposed to relaxation of isotherms following volcanism.