Linking Tectonics and Cratonic Vertical Motions: Deep‐Time Thermochronology Reveals >1,000 Myr of Low‐Temperature Conditions in Southeastern Finland

Abstract

The Fennoscandian Shield potentially hosts the longest continuous habitable conditions for Earth's deep biosphere. Billion‐year equilibrium freeboard conditions of cratons and the longevity of deep biosphere microbial communities are directly related through the thermal histories of rocks that host these communities. Thermochronology can constrain deep‐time histories by reconstructing time‐temperature paths as proxies for sedimentary processes. We present new zircon (U‐Th)/He dates (ZHe, n  = 59) and apatite (U‐Th‐Sm)/He dates (AHe, n  = 4) from a borehole in southeastern Finland crystalline basement. Negative relationships between date and effective uranium ([eU] = [U] + 0.238*[Th]) in zircons and inverted AHe and ZHe dates indicate prolonged accumulation of radiation damage at low temperatures. We combine these data with previous apatite fission track (AFT) and Ar‐Ar multi‐domain diffusion (MDD) measurements to reconstruct thermal histories since 1,860 Ma. Inverse thermal history modeling requires three distinct reheating episodes bracketed by cooler temperatures since cratonization: (a) reheating to 100°C–170°C between 1,400 and 1,100 Ma; (b) reheating to 80°C–100°C sometime between 900 and 600 Ma; and (c) well‐constrained reheating to 50°C–75°C between 340 and 250 Ma while temperatures never exceeded 80°C in the Paleozoic. The absence of magmatism since the Mesoproterozoic means that periodic reheating must result from multiple kilometers of sedimentary burial and denudation. We suggest that far‐field tectonism drove long‐term vertical variations at our shield location and that these results may apply to all of southern Finland. Although our data confirm a habitable deep biosphere setting since ∼1,000 Ma, the Fennoscandian Shield's thermal history is more complex than previously characterized.