Diethard Sanders
Talus accumulation in detachment scars of late Holocene rock avalanches, Eastern Alps (Austria): rates and implications

Geo.Alp 9, 2012, p. 82-99

Abstract
In mountain ranges, under the present interglacial conditions, active scree slopes tend to cluster within a certain range of altitude, probably because of an altitudinal/climatic span with maximum efficiency of frost cracking (‚talus window‘). In the Northern Calcareous Alps, large active scree slopes within the detachment scars of two late Holocene rock avalanches accumulated at mean rates of 7–18 mm/a over a time interval of ~1.7–3.8 ka. A plot of sedimentation rates as a function of time interval (Sadler plot) shows: (a) that the two scree slopes accumulated at comparatively high rates, and that (b) the rate of deposition must have been much higher (up to ≥1 meter/a) shortly after rock avalanching, but then diminished. The apex of each scree slope is located close to 1400 m a.s.l., i. e. some 400–1000 meters lower in altitude than the apices of most other scree slopes of the Eastern Alps. At the time of rock avalanching, the mountain flank laterally adjacent to the scree slopes was forested up to its crest, as it is today. Climatic data (1971- 2000) of stations ranging from 498–3105 m a.s.l. in altitude suggest that the intense scree production is not readily explained by the annual number of ice days (ID; days with T < 0°C). The annual number of freeze-thaw days (FTD), in contrast, remains nearly constant from station Haiming (695 m a.s.l.; 130 FTD) up to Obervermunt (2040 m a.s.l.; 125 FTD). Thus, scree production may have been mainly controlled by FTD or by, both, FTD and ID compensating in effect each other with increasing altitude. In addition, processes unrelated to freezing probably significantly contributed to scree production, such as ‚scraping‘ off scree by heavy rainfalls and snow cascading or avalanching down cliffs, spalling of rock by increased pore-water pressure, and rock cracking/loosening by thermal stress fatigue well-above the freezing point.
I suggest that the prevalence of presently-active scree slopes in a certain altitude range (slope apex between ~1800 to ~2600 m a.s.l.) of the Eastern Alps results from: (a) the gross topography across the orogen, with internides (Central Alps) the highest, (b) medium-scale morphology produced mainly by glacial erosion (cirques and glacially-carved valleys flanked by cliffs), (c) post-glacial climb of vegetation, superposed with (d) an optimal combination of all processes (irrespective of their total altitude range) capable to (i) liberate scree from cliffs, and (ii) at a combined rate high enough to sustain a sizeable, active scree slope. This interpretation does not invalidate, but embeds the concept of the ‚frost-cracking window‘. In the Northern Calcareous Alps, observations on the long-term (here: > 10 ka) and shortterm (here: tens of years) dynamics of talus accumulations indicate that the role of vegetation for scree shedding from cliffs (technically, rock flanks with dip ≥45°) has been underestimated. In the Alps, the scarcity of presently active talus below about 1600–2000 m a.s.l. mainly results from slope stabilization by vegetation rather than by lack of processes capable to liberate scree from bare rocky slopes. The two scree slopes described herein indicate that, under certain geological circumstances, rapid talus accumulation in comparatively low topographic position is possible also under interglacial climatic conditions.


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