Waiting for the snow to melt…
Over the past two decades, thermochronological studies have greatlyincreased our knowledge of the Cenozoic evolution of the Colorado Plateau(western United States). There has been particular interest in the southwesternpart of the plateau, leading to debate regarding the timing of uplift andfluvial incision along the Colorado River system. We here combine apatite fissiontrack (AFT) and apatite (U-Th)-He (AHe) analyses as well as zircon U-Pbdating to investigate the much less studied northeastern Colorado Plateau,particularly the Uncompahgre Plateau and the Unaweep Canyon, which has avery unusual drainage pattern in two opposite directions.We obtained 12 AFT ages from the Uncompahgre Plateau: 3 from the top ofthe basement of the plateau reveal Laramide ages (65–63 Ma), and 6 samplesfrom the Unaweep Canyon (35–27 Ma) and 3 from the northeastern plateaumargin (33–17 Ma) underwent complete thermal resetting in the late Eoceneto Oligocene. Thermal history modeling of top basement samples revealsLate Cretaceous heating to temperatures of at least 90 °C, implying sedimentaryburial to ~3 km, followed by cooling throughout the latest Cretaceous toEocene.However, AHe ages (38–31 Ma) indicate minor reheating to 40–80 °C forthese samples in the late Eocene to Oligocene.Zircons from the La Sal Mountains laccolith gave an Oligocene U-Pb crystallizationage of 29.1 ± 0.3 Ma. AFT ages from the laccolith range from 33 to27 Ma, confirming rapid cooling of this shallow subvolcanic intrusion. Thislate Eocene to Oligocene magmatism caused thermal resetting of most of theAFT (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 past5–10 m.y. This Miocene–Pliocene cooling event is interpreted as regional upliftof the Colorado Plateau associated with canyon incision.
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