ZIRCON (U-TH)/HE DATA REVEALS DEEP-TIME THERMAL HISTORIES OF CRATONS AND THE GREAT UNCONFORMITY SURFACE

Craton interiors preserve a record of Earth’s 4-D evolution that can be used to assess interactions between surface dynamics and deep-Earth processes over billion-year time scales. The burial and exhumation history of these settings reveals the timing and magnitude of surface responses to long-term, long-wavelength vertical motions caused by asthenosphere-lithosphere interactions and supercontinent amalgamation and break-up. Multiple cycles of burial and exhumation produce complex time-temperature (t-T) histories in cratons that can be constrained by low-temperature thermochronology. Previous thermochronology studies have focused on apatite (U-Th)/He and fission-track dates in cratons, but both systems have relatively low temperature sensitivities and record exhumation of only the upper few kilometers of crust, with t-T histories largely confined to the Phanerozoic. Here, we exploit the radiation damage-He diffusivity relationship in the zircon (U-Th)/He system to extend the t-T histories of craton burial and exhumation to over a billion years. Samples come from two intracratonic locations: ~1.8 Ga granitoids from southeastern Sweden (Fennoscandian Shield), and ~1.4 Ga granites and rhyolites from the Ozark Plateau of southeastern Missouri. In these locations, deep-time thermochronology reveals the billion-year t-T paths of Precambrian crystalline basement rocks at or just below the Precambrian-Cambrian (Great Unconformity) boundary surface. Our Fennoscandian Shield zircon (U-Th)/He data constrain two episodes of burial and exhumation: burial to ~220°C between ~940 Ma and ~850 Ma followed by exhumation from 850 to 500 Ma, and burial to ~150°C between 370 Ma and 220 Ma. In the Ozark Plateau, zircon (U-Th)/He data record exhumation from a maximum reheating (burial) temperature of ~260°C at ~850-680 Ma, whereas complementary apatite (U-Th)/He and fission-track data record a younger exhumation episode at ~225-150 Ma. Results from both locations identify phases of denudation related to formation of the Great Unconformity and coincide with Rodinia and Pangaea supercontinent break-up, which has implications for understanding surface responses to the supercontinent cycle, and potentially the effects of increased intracratonic erosion at ~850 Ma on CO₂ drawdown and snowball Earth cooling.