COMPARING SPATIAL VARIATIONS IN PRECIPITATION AND EROSION INDEX TO DIFFERENCES IN LONG TERM EXHUMATION ALONG THE EUROPEAN ALPS

The pattern of average annual precipitation in the Alps, derived from 50 years of measurements from a dense rain-gauge network, reveals variations of more than twice the mean value over regions 40-100 km in scale. Using this data and a 1 km DEM to derive stream discharge, the spatial variation in stream power per unit area and shear stress were calculated over the Alps and neighboring regions. These indices of erosion rate vary by more than 60% on a spatial scale of tens of kilometers, indicating that, to the extent that the pattern of precipitation and the slopes in the range are relatively constant over significant timescales, erosional exhumation of the Alps should vary spatially on this scale.

At the broadest scale, the erosion index in the western Alps exceeds that in the eastern Alps by 35% on average. The transition between these two regions coincides with the transition between the more deeply exhumed western Alps and the eastern Alps where overlying nappes are largely preserved, suggesting that differences in the extent of exhumation along the Alps are consistent with a long-term precipitation decrease from west to east. More locally, large variations in precipitation define three distinct regions 60-100 km in diameter. The Lepontine region receives nearly three times the mean annual precipitation while the Mont Blanc area to the west and the Campo Nappe region to the east each get only 60% of the average. The deeply exhumed Lepontine Dome exposes basement rocks that were subjected to Tertiary amphibolite facies metamorphism. Fission-track cooling age data exist for the Lepontine Dome and the Mont Blanc area and indicate young cooling ages in the region with a high erosion index, and older cooling ages in the region of low erosion index to the west. Collectively, these data suggest that spatial variations in erosion were important in the development of the observed variation in the rate and total amount of exhumation.

Finally, this preliminary analysis leads us to speculate on the impact of intra-orogen variations in erosion on the structural and metamorphic development of mountain ranges. Recent studies of the syntaxial corners of the Himalaya suggest that significant feedbacks can arise between erosion, metamorphism and deformation; we suspect that similar localized feedbacks may also be manifested in other mountain ranges.