Tracking ancient unconformity development with martite (U-Th)/He thermochronometry

Unconformities in the rock record reflect change(s) in tectonics, climate, and/or sediment routing systems. Pinpointing when sub-unconformity rocks reached the near-surface environment (<1 km depth) remains a challenge that inhibits assignment of causality. We addressed this problem with a new approach using (U-Th)/He analyses of martite (hematite pseudomorph after magnetite). Martitization resets the (U-Th)/He system and links rocks to residence in the near surface. Here, we applied this tool to document the timing of unconformity development in deep time. We integrated martite (U-Th)/He (martite He) and electron backscatter diffraction (EBSD) data to constrain the minimum timing of martitization in crystalline basement directly below a major nonconformity in the Colorado Front Range, western United States. Martite comprises hematite crystallites with no remnant magnetite and exhibits crystallographic orientation relationships that suggest martitization occurred by oxidation in the near surface. Individual martite He dates range from 1042 ± 24 Ma to 127 ± 8 Ma (n = 52). Martite He dates are consistent with martitization and associated basement residence in the near surface prior to the Cryogenian, potentially as early as ca. 1040 Ma. Thermal history models show that our spectrum of martite He dates reasonably reflects partial He loss from variably sized He diffusion domains during Phanerozoic burial and reheating. Our work highlights the antiquity of unconformity development in the Colorado Front Range. We suggest paired martite He-EBSD analysis as a new forensic tool for quantifying spatiotemporal patterns of low-temperature alteration, paleotopographic evolution, and unconformity development.