CHICKENS AND EGGS: EVIDENCE FOR GLACIATION-INDUCED RELIEF ENHANCEMENT IN THE EASTERN ALPS

Digital geomorphic parameters have been computed for the Eastern Alps and have been evaluated by statistical methods based on a mid-resolution Digital Elevation Model. Results prove that glaciation-induced relief enhancement took place in most of the area: highly increased sediment evacuation, increase in valley spacing and valley deepening are traceable in the formerly glaciated areas by sediment budget calculations and applying numerical methods. The isostatic equilibrium seems to be not yet attained: recent vertical crustal movements (up to 1.5 mm/a uplift) strongly correlate with the area of glaciation. In certain parts of the Eastern Alps remnants of paleosurfaces (recognized in the last century) suggest that parts of the Northern Calcareous Alps and the Gurktal Alps were less affected by the relief enhancement, while the Hohe and Niedere Tauern and the westernmost Eastern Alps can be characterised by isostatically-enhanced strong surface uplift in combination with the orogeny-dictated rock uplift background.

The Last Glacial Maximum (LGM) nunatak pattern was also evaluated to delineate regions which were already mountainous in pre-LGM times. They are separable from those which have been strongly modified by the glaciation. Although the relief enhancement modified considerably the ruggedness of the elevated areas, the geomorphic domains (outlined by the slope angle distribution) correlate rather with the long-term uplift pattern (derived from apatite fission track geochronology) than the local lithology and the glaciation.

Based on these observations we conclude that (1) the long-term (>100 ka) behaviour is largely determined by the orogenic processes and the uplift attains 0.3 mm/a, and (2) the glaciation increased the effectiveness of the orogenic forces by postponing the effect of the trend: during and after the short deglaciation period this delay starts to compensate by isostatic rebound caused by the melting ice and the increased volume of evacuated sediments.