LASER ABLATION (U-TH-SM)/HE DATING OF DETRITAL APATITE IN THE APPALACHIANS: A NEW ANALYTICAL TOOL FOR RESOLVING REGIONAL LANDSCAPE EVOLUTION

Detrital thermochronology is a valuable technique for understanding the topographic evolution of mountainous regions. Fission-track and (U-Th-Sm)/He dating of detrital apatite and zircon are widely used to resolve exhumational and tectonic histories of river catchments. However, the application of apatite (U-Th-Sm)/He (AHe) in detrital studies is hindered by issues with the conventional ‘whole-grain’ measurement method, including bias during grain selection and the high cost of analysis that limit the number of measurements that can be obtained. Laser ablation AHe dating (LA-AHe) is a new innovative dating technique that can measure a much larger number of grains quickly (n>100) and at lower analytical costs. Moreover, the technique essentially removes grain selection bias and allows for the simultaneous collection of U-Pb ages and chemical composition.

We present new LA-AHe measurements from 6 river catchments along the Appalachian Mountains of the eastern USA, between New Hampshire and North Carolina. A total of 493 single grain LA-AHe ages and 450 apatite U-Pb ages were obtained. The majority of LA-AHe ages are Cretaceous, consistent with previous AHe studies from the Appalachians. Catchments underlain by crystalline bedrock show little age dispersion while those underlain by sedimentary strata exhibit much higher levels, highlighting underlying limitations to the detrital AHe method. Apatite U-Pb ages are consistent with each catchment’s metamorphic or stratigraphic histories and contextualise the LA-AHe results. Finally, catchment-wide thermal histories derived using Pecube show that LA-AHe ages can be explained through the protracted erosion and exhumation (<0.05 mm/yr) of a post-orogenic elevated landscape since 250 Ma. This work highlights the potential for LA-AHe analysis in detrital studies and the information it can provide as part of geomorphological studies.