REPEATED INTRACONTINENTAL TECTONISM IN THE COLORADO FRONT RANGE: FILLING CRITICAL GAPS IN THERMAL HISTORIES WITH TITANITE AND ZIRCON (U-TH)/HE THERMOCHRONOLOGY

The Colorado Front Range, the easternmost and largest Laramide uplift of the southern Rocky Mountains, is a classic example of intracontinental tectonics – as is its counterpart in Ancestral Rockies time. Since 1.8 Ga, the area has been affected by at least four major tectonic events, occurring in the Proterozoic (~1750 Ma and ~1400 Ma), Pennsylvanian (~300 Ma), and Laramide (~65 Ma). Thus, the Proterozoic basement of the Front Range records a complex, protracted thermal history punctuated by multiple episodes of uplift and erosion. Yet why this area was repeatedly affected in this manner despite its intracontinental setting remains debated. Understanding of this problem is hindered by limited information about the ~1 byr history between the Proterozoic and Ancestral Rockies. Time-temperature constraints in the 300-150 °C range are required to address this problem, but their absence here – as well as their paucity in thermochronology datasets more generally – is due to the dearth of thermochronometers with sensitivities to these temperatures.

We acquired titanite and zircon (U-Th)/He data for Front Range basement rocks to both test the relative temperature sensitivities of these minerals and decipher a cryptic portion of the region’s history relevant for understanding intracontinental tectonism. Samples were collected from an ~50 km transect across the Front Range in Big Thompson Canyon and Rocky Mountain National Park. Titanite He dates (closure temp. ~250-200 °C) range from ~1000 to 600 Ma, whereas zircon He dates (closure temp. ~180-150 °C) are 200 to 60 Ma. Titanite and zircon date-eU correlations reflect the influence of radiation damage on He retentivity and provide additional constraints on the time-temperature path. These results record protracted cooling in the Neoproterozoic, and suggest Laramide temperatures were warm enough to cause He loss from the low eU zircons. In conjunction with existing fission-track, ⁴⁰Ar/³⁹Ar, and U-Pb data, these new data allow for construction of a near-complete thermal history for the Front Range, spanning approximately 1.7 Ga. The utility of titanite and zircon (U-Th)/He data for deciphering the critical ~300-150 °C temperature gap in this region highlights their promise for providing new insights into the tectonic history of other complex mountain belts.