Relict karst landscapes record fluvial pre-maturity at the margin of the Eastern Alps

Relict karst landscapes record fluvial pre-maturity at the margin of the Eastern Alps

by Rob Watson, University College Dublin, Dublin, Ireland

Viewed here from halfway up a sheer cliff-face accessed by rock climbing, the Paleozoic of Graz is an area of ancient limestones occurring in the transition zone between the lofty peaks of the Eastern Alps and the sunny wine-country of the Pannonian Basin. The area is one of few in the Alps to have escaped glaciation during the Quaternary, and so surfaces of the landscapes of the past few million years that were obliterated by glaciers in other areas are preserved here. Termed 'relict landscapes', these are the stepped green areas and ridge crests that you can see descending from left to right here, towards the valley of the river Mur (visible in the top right corner), one of the major drainage routes of the Eastern Alps.

The oldest 'steps' in the landscape are the highest ones, and the descent towards the river is indicative of the scale of surface uplift that has occurred in the region. This surface uplift was caused by a combination of rock uplift caused by as-yet uncertain tectonic processes, and the incision of the Mur river triggered by this rock uplift. The landscape presented here is a classic demonstration of the concept of 'fluvial maturity', whereby the ancient land surfaces (green flat bits) are initially uplifted, and then dissected by fluvial incision to create the steep-sided valleys.

Working out the relative contributions of tectonically-forced uplift and fluvial incision to the overall relief of a landscape is a complex conundrum which was attempted in the Paleozoic of Graz by Thomas Wagner and colleagues. They did this by dating cave passages which were formerly level with the Mur valley and have since uplifted relative to the valley floor. The highest of these caves occur at around 500m above the present Mur valley floor and are dated at 5 ma, meaning that the Mur has incised at around 100 m/myr during this time.