Utilizing Zircon (U-Th)/He And 40ar/39ar Thermochronology To Refine The Thermal History Of The Great Unconformity Within New Mexico And Colorado

The Great Unconformity beneath Paleozoic strata represents 100-1000 m.y. of missing rock record that formed through extensive weathering and erosion. Despite its global extent, the timing of Precambrian crystalline rock exhumation prior to Phanerozoic sediment deposition remains relatively unresolved in many locations. I utilized zircon (U-Th)/He (ZHe) thermochronology in order to constrain the long-term thermal history of Precambrian rocks in Colorado and New Mexico, and test my hypoThesis that these units below the Great Unconformity experienced multiple cooling and reheating episodes resulting in a compound surface representing multiple unconformities. In addition to a synThesis of previously reported data in the area, three new samples have been collected within New Mexico and Colorado for ZHe analysis from Santa Fe, Los Pinos Mountains, and Tres Piedras. Results for the ZHe dates range from 31.6-835.06 Ma in Santa Fe, 79.62-863.86 Ma for Los Pinos Mountains, and 47.34-364.73 Ma for Tres Piedras. HeFTy software was then utilized to create new forward and inverse thermal history models from the compiled ZHe data. Forward models for the samples resulted in ZHe date vs. eU curves that did not correspond to the data. Two inverse models were then completed for each sample and included a gradual cooling and reburial scenario in order to constrain the long-term thermal history of each location. For the Santa Fe, Tres Piedras, and Sandia Mountains samples the gradual cooling scenario is more likely to have occurred. For the Los Pinos Mountains and Front Range, CO locations both the reburial and gradual cooling model remain a possibility. The inverse model results indicate that two scenarios are possible for the five samples analyzed. Scenario one would be that the gradual cooling model occurred for all five locations. While scenario two would be that only the Santa Fe, Tres Piedras, and Sandia Mountains samples experienced the gradual cooling model while the Los Pinos Mountains and Front Range locations underwent the reburial model. Overall the thermal histories recorded in the inverse models link the cooling to near-surface temperatures to the breakup of supercontinent Rodinia. These new data provide new and important insight into the timescales and processes of continental exhumation during assembly and break-up of supercontinents