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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. 

Ancient magnetization(s), often recorded by hematite (Fe₂O₃), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations.