POST-SNOWBALL EARTH DEVELOPMENT OF THE GREAT UNCONFORMITY IN THE SOUTHERN CANADIAN SHIELD

The Great Unconformity erosion surface between Paleozoic sediments and Archean-Proterozoic basement has been related to multiple events including Snowball Earth and Rodinia supercontinent assembly, but the full extent, timing, and synchroneity of erosion remains unknown. In this study, we combine new thermochronologic data and existing geologic data below the unconformity in southeastern Ontario, Canada, to assess the synchroneity of exhumation on the Canadian Shield and North America more broadly. In this region, the unconformity is defined by Middle Ordovician carbonate atop Archean-Proterozoic basement. We obtain zircon and apatite (U-Th)/He thermochronology dates from seven samples below the unconformity. Zircon analyses display a range of dates that correlate with crystal radiation damage spanning 960 ± 20 Ma to 37.5 ± 0.9 Ma. Apatite analyses are generally consistent across samples and show dates between ~200 and 300 Ma regardless of radiation damage level, indicating Phanerozoic temperatures were hot enough to reset apatite He accumulation. Additional temperature constraints independent from thermochronology are preserved in the region: 1) paleomagnetic studies suggest the c. 590 Ma Grenville dikes were emplaced into bedrock with ambient temperatures 184 ± 40°C corresponding to depths of ~6.5 km, 2) petrologic relationships suggest the 577 ± 1 Ma Callander Complex was emplaced 6-12 km deep, and 3) conodont alteration index classification limits Phanerozoic temperatures to ≤100°C. Combined, the thermochronologic and geologic data provide a time-integrated thermal history for the region that requires substantial exhumation between c. 580 and 470 Ma, well after the c. 717-635 Ma Snowball Earth glaciations. This timing is significantly younger than has been inferred for Great Unconformity development at some North America locations but is consistent with previous interpretation of thermochronology datasets for the central Canadian Shield. This timing overlaps with emplacement of the Central Iapetus Magmatic Province, possibly linked to impingement of a mantle plume with eastern Laurentia. Ediacaran-Cambrian exhumation of the Canadian Shield also coincides with development of the Transcontinental Arch and provenance changes in sedimentary successions on the Laurentian margins.