195-14 Thermometric, Geochronologic, and Petrologic Evidence for an Oligocene Thermal Pulse in the Sangre de Cristo Range, Southern Colorado

Session: Evolution of Orogenic Belts Through Time: Insights from Sedimentation, Deformation, Magmatism, and Metamorphism (Posters)

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Abstract:

The Sangre de Cristo Range in southern Colorado preserves an exceptional record of Cenozoic tectonics including Rio Grande Rift extension. However, the tectonomagmatic setting prior to rifting and during early rifting remains poorly constrained. We present new geochronologic, thermometric, and petrologic data from the range that documents a widespread thermal pulse during the Oligocene which likely modified the crustal architecture and influenced the style and timing of early rift-related deformation. Raman spectroscopy of carbonaceous material (RSCM) from 36 samples of Pennsylvanian sedimentary rock across the range reveals significant reheating. On the western range front, metamorphic aureoles surrounding Oligocene intrusions reached peak temperatures of ~650° C, with inter-aureole temperatures as low as ~340° C. In contrast, the eastern flank records peak reheating up to ~500° C within aureoles and baseline temperatures consistently <300° C. Peak metamorphic assemblages in metapelites within aureoles commonly contain staurolite, andalusite, and biotite with less common garnet and fibrolite, consistent with RSCM peak temperatures and contact metamorphism at ~2-3 kbar. These results suggest elevated and spatially variable heat flow in the Oligocene, with consistently higher temperatures on the western flank. Abundant secondary muscovite and chlorite along this same trend and structural level suggest hydrothermal fluid flow was regionally significant and likely associated with magmatism and metamorphism. New monazite U-Pb geochronology in Pennsylvanian metasedimentary rocks reveals localized pulses of metamorphism and hydrothermal activity from ~35-25 Ma. At Garner Creek, monazite U-Pb and muscovite ⁴⁰Ar/³⁹Ar ages overlap at ~35 Ma; monazite from Mosca Creek records ~32-31 Ma growth, and Crestone Creek samples record ~26-25 Ma growth. Together, these results define a regional Oligocene thermal pulse that both preceded and overlapped the onset of rifting at ~30 Ma. This elevated heat regime significantly modified local geothermal gradients and weakened the crust, facilitating and influencing the nature and location of early Rio Grande Rift deformation. Early rift deformation was characterized by the development of top-SW mylonitic shear zones localized within and around Oligocene intrusions along the western flank of the range, where strain was likely concentrated by thermal softening. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11061

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