Why are the Appalachians high? New insights from detrital apatite laser ablation (U-Th-Sm)/He dating

Abstract

The timing of surface uplift across the Appalachian Mountains, Eastern USA, remains a controversial topic. Thermal history modelling, geomorphological analysis, stratigraphic analysis, and river profile analysis have all suggested the region experienced surface uplift during the late Cenozoic. This is in contrast with many low-temperature thermochronology studies, which consistently present Mesozoic dates and typically infer the landscape to be the remnant of a much older mountain belt that has been slowly exhumed following the end of rifting in the Late Triassic-Early Jurassic. Resolving a regional exhumation history of the Appalachians through traditional low-temperature thermochronology methods is difficult due to the spatial constraints of bedrock sampling, while traditional detrital river sampling has limitations underpinned by analytical issues and grain selection bias. We aim to resolve the exhumation histories of large areas across the Appalachians using detrital laser ablation apatite (U-Th-Sm)/He dating (LA-AHe). LA-AHe is a new dating approach that produces a high number of apatite (U-Th-Sm)/He ages from modern river sands to infer the low-temperature (70–40°C) exhumation history of the bedrock across a catchment. Supplementary apatite and zircon U-Pb dates were obtained to study sediment transport in each catchment and their respective bedrock geological histories. LA-AHe results show most dates to be Cretaceous (69%), with only a small Cenozoic fraction (6%), while thermo-kinematic modelling of data from four catchments can be explained by protracted Mesozoic-Cenozoic exhumation with rates ∼31–20 m/Myr. The lack of Cenozoic dates does imply the magnitude of exhumation has not been significant enough to greatly affect the apatite (U-Th-Sm)/He system, though models incorporating enhanced late Cenozoic exhumation between 2.1–1.4 km are consistent with data from three catchments. These results have implications for the study of elevated topography across passive margins and ancient orogens and highlight how laser ablation apatite (U-Th-Sm)/He dating can be effectively used to study the change of landscapes over geological timescales.