ALBERT

Description: Over the past two decades, thermochronological studies have greatly increased our knowledge of the Cenozoic evolution of the Colorado Plateau (western United States). There has been particular interest in the southwestern part of the plateau, leading to debate regarding the timing of uplift and fluvial incision along the Colorado River system. We here combine apatite fission track (AFT) and apatite (U-Th)-He (AHe) analyses as well as zircon U-Pb dating to investigate the much less studied northeastern Colorado Plateau, particularly the Uncompahgre Plateau and the Unaweep Canyon, which has a very unusual drainage pattern in two opposite directions. We obtained 12 AFT ages from the Uncompahgre Plateau: 3 from the top of the basement of the plateau reveal Laramide ages (65–63 Ma), and 6 samples from the Unaweep Canyon (35–27 Ma) and 3 from the northeastern plateau margin (33–17 Ma) underwent complete thermal resetting in the late Eocene to Oligocene. Thermal history modeling of top basement samples reveals Late Cretaceous heating to temperatures of at least 90 °C, implying sedimentary burial to ~3 km, followed by cooling throughout the latest Cretaceous to Eocene. However, AHe ages (38–31 Ma) indicate minor reheating to 40–80 °C for these samples in the late Eocene to Oligocene. Zircons from the La Sal Mountains laccolith gave an Oligocene U-Pb crystallization age of 29.1 ± 0.3 Ma. AFT ages from the laccolith range from 33 to 27 Ma, confirming rapid cooling of this shallow subvolcanic intrusion. This late Eocene to Oligocene magmatism caused thermal resetting of most of the AFT (except top basement) and AHe ages from the Uncompahgre Plateau, even though samples were collected as much as 60 km away from the intrusion. Canyon samples also underwent an increase in cooling rates in the past 5–10 m.y. This Miocene–Pliocene cooling event is interpreted as regional uplift of the Colorado Plateau associated with canyon incision.