THE LATE GREAT UNCONFORMITY OF THE CENTRAL CANADIAN SHIELD

The Great Unconformity is a widespread surface separating Precambrian rocks from overlying Phanerozoic sedimentary sequences. The causes and implications of this feature, and whether it represents a singular or global event, are much debated. Here, we present new apatite (U-Th)/He (AHe) thermochronologic data from the central Canadian Shield (southern Hearne province, Trans-Hudson Orogen, southwestern Superior craton) that bear on when the Precambrian basement was exhumed to the surface before deposition of overlying early Paleozoic sedimentary sequences that mark the Great Unconformity. AHe data (N=57) from 11 samples across a broad region define the same date-eU pattern, implying a common thermal history. Higher eU (25-100 ppm) apatites form a distinct “plateau” of reproducible dates at 510 ± 49 Ma (mean and 1σ uncertainty), while lower eU (<25 ppm) apatites define a positive date-eU trend with younger dates. The data patterns, geologic context, and thermal history modeling point toward relatively rapid cooling and erosion of the entire area (~400,000 km²) between 650 Ma and deposition of early Paleozoic sedimentary rocks, followed by modest reheating during later burial. The timing of this Neoproterozoic erosion event suggests either the Snowball Earth glaciations or plume/rift activity along the eastern margin of Laurentia as the cause. Geologic and thermochronologic data provide a western bound on the footprint of this post-650 Ma erosion signal, and additional future constraints on its spatial extent may help discriminate among its causes. Post-650 Ma development of the Great Unconformity erosion surface in the central Canadian shield contrasts with evidence for its pre-717 Ma formation in the Colorado Front Range, supporting the notion of multiple Great Unconformities with differing ages and origins.