| Erosion is a fundamental process for exhumation of rocks during the late stages of orogeny.
Lithology, relief, climate and tectonics have been alternatively invoked as the most important
controlling factors (Harrison, 1994; Vannay et al., 2004; Koppes & Montgomery,
2009).
Fission track analysis on detrital minerals allows to investigate both long- (106 yr) and
short-term (103-105 yr) exhumation and erosion patterns in orogenic belts, the former being
recorded by the age of fission-track populations, the latter by their relative size, provided that
apatite fertility in the source rocks and the effects of hydraulic sorting are taken into account
(Malusà et al., 2009).
Our analyses are focused on two transects across the Western Alps, in the Aosta and Arc
Valleys, draining in opposite directions a comparable stack of tectonic units. Available
fission-track data on bedrock apatite indicate increasing long-term exhumation
rates from the axial sector of the belt towards the European foreland. Major age
jumps across the main tectonic discontinuities detected in the field testify for a
primary tectonic control on long-term erosion rates (Malusà et al., 2005). Spatial
distribution of bedrock age data made it possible to infer provenance of detrital
grains.
Our study shows that the focus of short-term erosion is located in the External Massifs, thus
reproducing the long-term erosion pattern derived from bedrock fission-track data.
Geomorphological analysis and distributions of pluviometric data show no clear correlation
between local morphology, climate and erosion. Tectonics is instead invoked as the main
driving force controlling both short-term and long-term erosion rates in the Western
Alps.
Harrison, C.G., 1994. Rates of continental erosion and mountain building. Geol. Rundsch.,
78, 431-447.
Koppes, M., Montgomery, D.R., 2009. The relative efficacy of fluvial and glacial erosion over
modern to orogenic timescales. Nature Geoscience, 2, 644-647.
Malusà, M.G., Polino, R., Zattin, M., Bigazzi, G., Martin, S., Piana, F., 2005. Miocene to
Present differential exhumation in the Western Alps: insights from fission tracks
thermochronology. Tectonics, 24, 1-24.
Malusà, M.G., Zattin, M., Andò, S., Garzanti, E., Vezzoli, S., 2009. Focused erosion in the
Alps constrained by fission-track ages on detrital apatites. Geological Society, London, Spec.
Publ., 324, 141-152.
Vannay, J.C., Grasemann, B., Rahn, M., Frank, W., Carter, A., Baudraz, V., Cosca, M.,
2004. Miocene to Holocene exhumation of metamorphic crustal wedges in the NW
Himalaya: evidence for tectonic extrusion coupled to fluvial erosion. Tectonics, 23,
1-24. |