REFINING THE PRE- AND SYN-RIO GRANDE RIFT COOLING HISTORY ACROSS THE SANGRE DE CRISTO RANGE AND ALVARADO FAULT THROUGH APPLICATION OF MID- TO LOW-TEMPERATURE THERMOCHRONOMETRY

The complex geologic and thermal history of the Sangre de Cristo Range in southern Colorado includes Pennsylvanian-Permian Ancestral Rocky Mountains sedimentation and deformation, Late Cretaceous-early Paleogene Laramide shortening, mid-Cenozoic magmatism, and late Cenozoic extensional exhumation associated with the Rio Grande rift. Previous thermochronometry studies have relied primarily on apatite fission track (AFT) and (U-Th)/He (AHe) dating, which yield Oligo-Miocene ages likely related to extensional exhumation. Here, newly collected AFT and AHe data combined with zircon (U-Th)/He (ZHe), zircon fission track (ZFT), and K-feldspar ⁴⁰Ar/³⁹Ar dating from samples across the Sangre de Cristo Range near 38° N refine the timing and magnitude of Rio Grande rift exhumation, and further define the pre-Rio Grande rift cooling history, including the cryptic slip history of the east-dipping Alvarado fault that bounds the eastern flank of the range. Across most of the range AHe and ZHe dates are ~19–10 Ma, with the stratigraphically highest Jurassic sample interpreted to represent the base of a Cenozoic ZHe partial retention zone. New and published ZFT ages range from the early Paleozoic to the middle Cenozoic. Thermal models based on these data and geologic constraints indicate most of the Sangre de Cristo Range was at 200–250 °C at the onset of late Oligocene rifting and underwent an episode of abrupt cooling in the Middle Miocene, ~10–15 Myr after the initiation of the Rio Grande rift. These models are consistent with a K-feldspar ⁴⁰Ar/³⁹Ar multiple diffusion domain model from a sample of Proterozoic granite along the western flank, indicating inception of rapid cooling at ~14 Ma with very little cooling between the Late Cretaceous and late Oligocene. We report Late Cretaceous to early Paleogene AHe ages and scattered Paleozoic to Jurassic ZHe ages in Proterozoic basement rock east of the Alvarado fault and Miocene AHe ages in Phanerozoic strata west of the fault. These data and thermal models suggesting very slow late Paleozoic cooling and rapid Laramide-age cooling east of the fault support the hypothesis that the Alvarado fault initiated as a Laramide reverse fault that was reactivated as a normal fault during Miocene extension.