The Influence of Laterally Migrating Topographic Features on Low-temperature Thermochronometric Ages

Abstract:

Despite the simplified notion of erosion as a top down process, landforms evolve by both vertical and horizontal migration of surface features. Low-temperature thermochronometry has the potential to resolve laterally migrating features if vertical relief is the same scale as the closure depth of the thermochronometer. In actively eroding landscapes this is about 2 km for (U-Th)/He thermochronometry. The issue of migrating surface topography has been addressed at a large scale in the cases of mountain divide migration and plateau retreat, but not for glacial and fluvial valley formation. Here we show that valley walls will have distinct ‘C’ shaped patterns of cooling age vs. elevation in valleys formed by retreating escarpments. The other end-member, a prograding escarpment will exhibit a distinct ‘7’ shaped age-elevation profile. Our schematic model simulates propagation of an escarpment in a valley for a range of vertical rock velocities and horizontal escarpment velocities and cooling ages are predicted for a range of isotherms. High temperature isotherms are essentially flat and the exhumation rate determined from the cooling age-elevation profile is exactly equal to the vertical rock velocity regardless of the horizontal velocity of the migrating feature. Very low temperature isotherms are highly perturbed by the overlying topography and the magnitude and direction of the horizontal rock velocity towards the surface greatly influence the shape of the cooling age-elevation profile. We show that valleys carved by headward erosion have relatively older low temperature cooling ages at low elevations, characterized by a ‘C’ shaped cooling age-elevation profile. This pattern of cooling ages is also expected for landscapes that have undergone a decrease in topographic relief, highlighting the potential of misinterpretation of low-temperature thermochronometric data. Traditional analysis of thermochronometric data collected along vertical transects has focused on evaluating the resulting age distribution in terms of exhumation rate and changes in topographic relief. We show its potential use for reconstructing headward valley erosion through geologic time.