USING DETRITAL APATITE U-TH/HE THERMOCHRONOMETRY TO UNDERSTAND APPALACHIAN LANDSCAPE EVOLUTION

Along their length, the modern Appalachian Mountains display significant variations in the relationship between relief, mean elevation, and bedrock geology making it difficult to describe the landscape as uniformly decaying. Curiously, long-term and short term erosion rate estimates suggest slow and steady erosion of the orogen following rifting in the Triassic, however, the sedimentary record from Atlantic passive margin basins is characterized by unsteadiness including a pulse of rapid sediment deposition in the Miocene. To address spatial variations in the timing and pace of landscape evolution we use apatite U-Th/He thermochronometry (AHe) on detrital samples collected from large rivers in New England and smaller nested drainages in the Blue Ridge Mountains of western North Carolina. Nearly all cooling ages from over 200 single-grain analyses are pre-Cenozoic, which agree with previous estimates of slow long-term erosion rates and place limits on the amount of exhumation these rugged areas experienced related to the observed Miocene sediment pulse. These datasets also highlight several aspects of the AHe technique that must be considered when using detrital samples. First, not all apatite grains survive the weathering and transport processes involved in becoming detrital sediment, and as a result, grains that do survive may not reflect a true synoptic perspective of the bedrock within the drainage. Second, the closure temperature reflected by an AHe age is a function of both the thermal history and the concentration of radiation damage within the individual grain. Thus, at slow-cooling rates characteristic of the Appalachians, the range of U and Th concentrations observed within a detrital sample will directly impact the population of ages and any interpretations of long-term average erosion rates estimated from it. Despite these two caveats, we illustrate that useful low-temperature cooling history information can be derived from detrital AHe samples in slowly cooling regions through forward modeling and comparison to existing bedrock data.