NEW INSIGHTS INTO LARAMIDE CONTRACTION, OLIGOCENE METAMORPHISM, AND RIO GRANDE RIFT EXTENSION IN THE SANGRE DE CRISTO RANGE (SOUTHERN COLORADO) FROM MID- AND LOW-TEMPERATURE THERMOCHRONOLOGY

The Sangre de Cristo Range in southern Colorado records a complex geologic history that includes Late Cretaceous-Eocene Laramide contraction, Oligocene magmatism and contact metamorphism, and Neogene Rio Grande rift extension. We present new thermochronometric data (K-feldspar ⁴⁰Ar/³⁹Ar, zircon and apatite fission track + (U-Th)/He) and thermal modeling that provide insight into this polyphase history. The eastern flank of the Sangre de Cristo Range is bound by the E-dipping Alvarado fault, which juxtaposes Proterozoic crystalline rocks in the hanging wall against predominantly Pennsylvanian-Permian strata in the footwall, whereas the western flank is bound by the active W-dipping Sangre de Cristo normal fault system and the extensional San Luis basin. Our data indicate that the Alvarado fault initiated as a reverse fault during the early stages of the Laramide orogeny and was subsequently reactivated as a normal fault in the Early Miocene. Raman spectroscopy of carbonaceous material and metamorphic mineral assemblages in the Pennsylvanian Minturn Formation record widespread heating across the range to ~350–650°C during Oligocene magmatism. This reheating event was relatively short lived and in most areas did not fully reset Paleozoic zircon fission track dates. The Miocene onset of rapid cooling associated with extensional exhumation postdated the Oligocene reheating event by ~7–14 Myr, indicating that this magmatism was unrelated to development of the range-bounding fault systems. The onset of extensional exhumation systematically youngs westward across the range from ~20–17 Ma along the eastern flank, ~17–14 Ma near the crest of the range, and ~13–11 Ma along the western flank. We attribute this pattern to an earlier onset of normal slip on the Alvarado fault, with extension shifting to the Sangre de Cristo fault system at ~13 Ma. Late Oligocene to Early Miocene mylonitic and brittle-plastic shear zones that record the earliest stages of extension in the Sangre de Cristo Range were not associated with significant cooling.