Determining Relief Evolution in the Alps with Muon Paleotopometry

The proposed project uses a novel approach to quantify the rates and timing of topographic relief change over hundreds of thousands to million-year timescales using muon paleotopometry. Understanding causes and rates of topographic change in our solar system is important because of feedbacks between relief and planetary climate, sediment fluxes to basins or oceans, and geohazard frequency including landslides and tsunamis. In the past three decades, a major international effort has been placed on the understanding of how climate controls isostasy, mountain and generates uplift and relief. Isostasy is a lithospheric process that can be difficult to distinguish from other lithospheric processes. For instance, the creation of deep valleys by streams or glaciers in the Alps led to a reduction in the average mass and density of the upper lithosphere. Just as the density differences between ice and seawater causes icebergs to float, the lithosphere maintains an isostatic equilibrium above the mantle. When the average density of the lithosphere is reduced, peaks of the lithosphere will rise until a new equilibrium has been reached. It is hypothesized that most relief was generated in the Western Swiss Alps as glacial incision became regionally significant during the Pleistocene. While isostasy has been included in numerical models for decades, validation measurements in nature have lagged behind. Modern techniques (e.g. thermochronology) alone cannot be used to determine the specific cause of relief (i.e., was it due the tectonic stresses or simply isostatic adjustments during incision). Alternative hypotheses have been proposed for the nature of this relief-including how a subduction of a lithospheric slab under the Alps tens of millions of years ago may have altered the its buoyancy.The project aims to determine the dynamics of Alps topography with measurements of cosmogenic muon-produced isotopes. The record may extend beyond the past 8 million years, depending on the rate of erosion. The paleotopometry method will be applied along a near horizontal transect in the Gotthard Base Tunnel to determine if the Alps relief is related to mantle-driven rock uplift or Pleistocene glacial incision. The research is complemented by a parallel study at a second Swiss Alps location (GTS - Grimsel Test Site) to calibrate and improve the new field of paleotopometry. Muon paleotopometry is a newly-conceived method to ‘image‘ the time-dependent crustal thickness and ancient relief above a sample datum over an isotope-specific monitoring duration (10^5-10^7 years). Relief changes will be determined with multiple muogenic terrestrial cosmogenic nuclides (TCNs) sampled between 100 and 250 m depth. A proof of concept has already been completed in rocks collected in an underground mine in the Colorado Plateau of North America. This paleotopometry study will be the first in Europe. It will deepen the understanding of glacial and stream incision of the Alps, and improve the tecontic understanding by extending the history of incision of continental crust in the Alps. Through further calibration of the technique at the GTS, the successful completed project will establish a new method that has a broad applicability, within the geosciences community among other i.e. tectonics, geochronology and geomorphodynamics.