Detrital geochronology of unroofing magmatic complexes and the slow erosion of Oligocene volcanoes in the Alps

Tectonic reconstructions and quantitative models of landscape evolution are increasingly based on detailed analysis of detrital systems. Since the definition of closure temperature in the 1960s, mineral ages of low- temperature geochronometers are traditionally interpreted as the result of cooling induced by erosion, whose rate is a simple, unique function of age patterns. Such an approach can lead to infer paradoxically high erosion rates that conflict with compelling geological evidence from sediment thickness in basins. This indicates that tectonic and landscape models that solely interpret mineral ages as due to cooling during exhumation may not be valid. Here we propose a new approach that takes into account the effects of both crystallization and exhumational cooling on geochronometers, from U–Pb on zircon to fission tracks on apatite. We first model the mechanical erosion of an unroofing magmatic complex and the resulting accumulation and burial of the eroded units in reverse order in the basin. Detrital mineral ages follow a regular pattern downsection. Some mineral ages, such as e.g. U–Pb ages of zircons, cluster around the “magmatic age”, i.e. the crystallization of the magma. Its value is constant along the stratigraphic column in the sedimentary basin; we refer to this behavior as “stationary age peak”. Some other mineral ages, such as e.g. apatite fission-track ages, are often younger than the magmatic age. When they vary smoothly with depth, they define a “moving age peak”, which is the only possible effect of undisturbed cooling during overburden removal, and can therefore be used to calculate an erosion rate. The predictions of our model were tested in detail on the extremely well-studied Bregaglia (Bergell) orogenic pluton in the Alps, and on the sedimentary succession derived from its erosion, the Gonfolite Group. The consistency between predicted and observed age patterns validates the model. Our results resolve a long- standing paradox in quantitative modelling of erosion–sedimentation, namely the scarcity of sediment during apparently fast erosion. Starved basins are the observational baseline, and modelling must be tuned to include a correct analysis of detrital mineral geochronology in order to reconcile perceived discrepancies between stratigraphical and geochronological information. In addition, our data demonstrate that volcanoes were active on top of the growing Oligocene Alps. This study illustrates rigorous criteria for detrital mineral geochronology that are applicable to any geological setting, including magmatic arcs and collision orogens, and provides fundamental interpretive keys to solve complex puzzles and apparent paradoxes in geological reconstructions.