Impact of glaciations and denudation on geodetic uplift and seismicity in the Alps: A rheological control ?

Abstract:

Normal faulting, uplift and low convergent rates are observed in the Western Alps. Until recently, occurrence of normal faulting in weakly convergent mountain ranges has been attributed to the conversion of potential energy of the topography into seismic motion in the upper crust and viscous flow in the lower crust. However, recent insights indicate that gravitational mountain collapse may not be the only process responsible for extension in the core of mountain belts: -1. Leveling and vertical GPS measurements show that the elevated topography of the western and central Alps is currently uplifted at rates of 1-2 mm/yr while gravitational collapse would lead to a downward motion; -2. There is no correlation between mountain core extension and average mountain topography. Champagnac et al. (2009) have proposed an elastic plate model in which present-day uplift in the Alps occurs in response to mountain scale unloading by denudation. Following this first approach, we have developed a numerical model that demonstrates a causal link between topography denudation, crustal uplift and extensional strain in an alpine-type mountain belt (Vernant et al. 2013). A key aspect of the seismotectonic field observations in the Alps is that seismic strain and geodetic uplift are occurring in relatively narrow areas, while our mechanical model predicts a broader uplift. Also, geodetic uplift rates appear to be larger than the maximum value associated to Holocene denudation rates. Using a 2D Finite Element Modeling, we explore the possibility that localized uplift may be associated to re-activation of pre-existing tectonic structures. These weak zones are accounted for using low frictional faults and/or low viscosity zones. We also investigate the possibility that large uplift rates may partly contain a rebound associated to Holocene deglaciations cycle.