USING DEEP-TIME THERMOCHRONOLOGY TO RECOVER MISSING GEOLOGIC HISTORIES ACROSS UNCONFORMITIES: A PROOF OF CONCEPT USING SAMPLES BELOW NEOPROTEROZOIC STRATA IN THE UINTA MOUNTAINS, UTAH

Unconformities, or gaps in the rock record, have typically limited our understanding of Earth’s history. However, geologic processes such as erosion and burial leave a thermal imprint on the rocks below unconformities that can be recovered using thermochronology. Thermochronology uses the temperature-sensitive diffusive loss of radiogenic daughter products within a mineral to obtain information about a sample’s temperature and time evolution, or its thermal history. In the Uinta Mountains of northeastern Utah, Paleoproterozoic metamorphic rocks are nonconformably overlain by Neoproterozoic sedimentary rocks of the Uinta Mountain Group (UMG), resulting in a nearly 1 Gyr interval of missing time. The UMG is a valuable record during early stages of rifting of Rodinia along the western margin of Laurentia where, elsewhere, this early rifting is rarely preserved. In our work, we use the unconformity as an independent thermochronologic constraint: at the time of deposition of the overlying rock, the underlying rock must have been at the surface and at cool, near-surface temperatures. As deposition ensued, we expect that the underlying rocks were heated. To evaluate whether the thermal record of this burial is preserved in the underlying rock and extract information about the pre-depositional history of the region, we sampled basement rocks in depositional contact with the overlying Uinta Mountain Group in a well-characterized structural context for thermochronologic analysis. Here, we present new zircon (U-Th)/He data to constrain the thermal history of each sample. Despite the complexity of multiple episodes of deformation within the study area, these preliminary data and thermal history models support an episode of heating ca. 800-700 Ma, corresponding with deposition of the UMG. Ongoing work will evaluate the spatial heterogeneity of this thermal record within the study area and thus the ability of deep-time thermochronology to fill in the knowledge gaps left by unconformities, particularly in other localities where the sedimentary record of Neoproterozoic rifting is not preserved.