BIASING OF DETRITAL MINERAL RECORDS WHEN ERODING THROUGH LAYERED STRATIGRAPHY (Invited Presentation)

Detrital mineral records represent primary means by which to interpret tectonic and climatic change at a range of temporal and spatial scales. Standard applications share an underlying set of assumptions that (1) the target mineral is uniformly concentrated throughout the source area and (2) any spatial variations in erosion rate (often assumed negligible) are related to the tectonic or climatic history. Both of these assumptions can be violated during the erosion of layered stratigraphy. Contrasts in rock strength and the geometry of contacts between different units can produce significant spatial variations in erosion rate, even when landscape development is driven by constant uplift or baselevel fall. Additionally, concentrations of minerals of interest often vary between different stratigraphic units. We explore the implications of erosional landscapes developed in layered stratigraphy on detrital mineral records through a suite of numerical models. We use a modified version of the CHILD landscape evolution model, which supports numerous rock units of arbitrary orientation, varying erodibilities, and target mineral concentrations within a landscape. We couple these synthetic landscapes to models of zircon U-Pb ages, modified versions of CRONUS to model ¹⁰Be production in quartz, and Pecube to compute U-Th/He and fission track ages for both apatite and zircon. Our results show variable degrees of biasing of detrital records depending on the technique and setting. Distributions of detrital U-Pb zircon ages are significantly influenced by both spatial variation in erosion rates and variable zircon concentration between units, with the degree of biasing directly related to the degree of erosion rate variation. Simulated catchment average erosion rates from ¹⁰Be accurately record spatial variations in erosion rate, and thus not the uplift rate, but results can be significantly influenced by variable quartz concentration. Detrital thermochronometers are the most complicated as the thermal history is perturbed by the evolving topography, leading to complicated age-elevation relationships, which are incompletely sampled due to variability in erosion rates. Variable mineral concentrations further exacerbates the potential for significant biasing of detrital thermochronometric records.