Low temperature thermochronology of the southern East Greenland continental margin: Evidence from apatite (U-Th)/He and fission track analysis and implications for intermethod calibration

The southern coast of East Greenland is characterized by high topographic relief and deep fjords, but the evolution of the landscape and the low-temperature thermochronology of the region is not well understood. Here we present apatite fission-track and (U-Th)/He ages that suggest several important features of the long-term geomorphic history of this region, but which also illustrate an important discrepancy in the thermal histories derived from each technique. Apatite from bedrock of the southern coast of East Greenland between 62°N and 67°N has fission-track ages ranging from 60 to 840 Ma and (U-Th)/He ages ranging from 21 to 250 Ma. The ages generally increase with elevation and distance from the coast, and fission-track analyses show significant differences in thermal histories for the different regions. In the Kangertittivatsiaq area (c. 66-67°N) apatite fission track data and models suggest two separate periods of slow cooling: prior to c. 200 Ma and between c. 160 Ma and the late Cenozoic (more recently than c. 20 Ma), each of which was followed by a period of rapid cooling and inferred exhumation. Apatite He data in the Kangertittivatsiaq region, including crystal-size-age correlations in low-elevation samples, are most simply interpreted as recording an incision event of at least 1.5 km later than 20 Ma near the coast. This may have been caused by glacial erosion. The (U-Th)/He data also indicate an earlier phase of rapid exhumation at c. 250 Ma. In the Skjoldungen/Kap Møsting area (c. 62-64°N) approximately 200 km south of the Kangertittivatsiaq, apatite fission-track data suggest slow exhumation from c. 200 Ma into the Neogene followed by fast exhumation. The similarity of fission track ages (200 Ma) at sea level in the fjords in the Skjoldungen area (c. 62-64°N) do not suggest tilting in the hinterland related to the breakup of the East Greenland continental margin. Furthermore, the Cenozoic fission track ages and modeling fission track data suggest that pre-breakup basins may have covered the outer coast. Despite the broadly similar topographic implications of the fission-track and (U-Th)/He data, the thermal histories derived from these systems are inconsistent. Fission-track data require thermal histories that predict He ages younger than observed, and He data require thermal histories that predict fission-track ages older than observed. Similar discrepancies have also been observed in other settings characterized by long-term low-temperature thermal histories, and may reflect changes in annealing or diffusion behavior (or both) that either develop or become more apparent in such cases.