ALBERT

Description: Numerous factors control the topographic evolution of mountain belts. Crustal thickening, rock uplift rate, and denudational forcing doubtlessly interact, but the feedback mechanisms amongst these are disputed, because they operate over entirely different time scales. Cosmogenically-derived denudation rates cover time-scales of 10(3) to 10(5) years, providing a denudational tool that allows us to shed light on interactions between long-term topography-forming processes and short-term factors destroying topography. Prerequisites for the application of this method in presently uplifting and fast-eroding mountain belts like the Central Alps are an investigation of appropriate watershed sizes for systematic sampling. Denudation rates in Maggia tributaries of various sizes reveal that the trunk stream yields statistically the same denudation rate (0.85 ± 0.14 mm/yr) as the tributaries (0.74 ± 0.14 mm/yr). Therefore, sampling of large watersheds is a feasible approach. Denudation rates of watersheds from the Central Alps are amongst the highest ever measured in similar complex settings, ranging in mean from 0.27 ± 0.05 mm/yr for the Alpine foreland to 1.42 ± 0.4 mm/yr for the high crystalline Central Alps. The measured cosmogenic denudation rates are in good agreement with post-LGM lake infill rates; they are significantly higher than recent denudation rates from river loads. We attribute this discrepancy to differences in methodology and integration time scale. We will show that denudation is high in areas of high altitude and high relief. Furthermore, our data shows that denudation rates are low in areas of low rock uplift, and are high in areas of high rock uplift, respectively. It appears that rock uplift and denudation are intimately linked. It follows that either crustal thickening is generating rock uplift; the mountain belt reacts with erosional unloading. Alternatively, high precipitation and glaciers, most pronounced at high altitude, result in high denudation rates at these sites. Topography then would respond by increasing rock uplift.

Description: An excellent view on long-term denudation processes is provided by the study of alluvial samples using the cosmogenic nuclide method. In situ-produced 10Be is obtained from quartz and measured by accelerator mass spectrometry. The production of 10Be in quartz by secondary cosmic rays in the Earth's surface is a function of latitude and altitude of the sampling point, and the produced nuclide concentration is inversely proportional to the erosion rate. Therefore, measurements of in-situ produced 10Be are a suitable tool to determine spatially-averaged denudation rates. We first performed a detailed erosional study in the Maggia Valley, southern Switzerland, to design an appropriate sampling strategy and to test the method on different catchment sizes. In the Maggia Valley, sediment from second-order tributaries of the Maggia, ranging in size from 2- 46km2, are generally yielding lower erosion rates (between 380 and 840mm/kyr) than the main stream itself. The trunk stream of the Maggia and large, U-shaped valleys (67- 683km2) are eroding at 600 to 2000mm/kyr. This is probably due to the incorporation of glacial sediment, which has been deposited during LGM and is now being remobilized. This sediment has long been shielded from cosmic rays, resulting in a low nuclide concentration and high erosion rates, respectively. We then proceeded to sample medium-scale catchments from Luzern (Central Switzerland) to Bellinzona (Southern Switzerland) in order to map out large-scale patterns of uplift and erosion. In the Central Alps, some of the highest erosion rates ever measured with cosmogenic nuclides for a presently uplifting mountain range have been recorded. In this area, erosion rates range from 480 - 2110mm/kyr. The sampled catchments feature considerable relief (from 620m in the north Alpine Drainage Basin to 1640m in the Reuss valley) and are partly glaciated (up to 18% in a Rhone tributary). Unlike many other environments, we do not recognize a connection between slope gradient, topographic relief, and erosion rate in the Central Alps of Switzerland. The lack of correlation between topography and denudation points at a possible topographic steady state condition. The observed agreement of denudation rates with rock uplift rates within a factor of two is supportive of this assumption. Recent vertical movements relative to the benchmark at Aarburg easily exceed 1000mm/kyr in the rear of the Central Alps and diminish to about 200mm/kyr in the North Alpine Drainage Basin. In other words, we believe that erosion rates are at present balanced by rock uplift rates in the Central Alps of Switzerland, maintaining steady-state topography. Recent climatic parameters such as temperature and precipitation and even the extent of glaciation do not seem to play a major role with respect to Alpine erosion.

Description: The Central Alps of Switzerland represent a mountain belt in which an exceptional wealth of geophysical data allows the unique test of tectono-geomorphic models. Levelling measurements show that the Central Alps are uplifting today with 0.5-1.6 mm/yr (Kahle et al. 1997). Here we present a North-South denudation rate transect through the Swiss Central Alps from a study of in situ-produced cosmogenic 10Be in river-borne quartz. Denudation rates range from 0.1 to 1.5 mm/yr. They yield a mean of 0.27+/-0.14 mm/yr for the Alpine foreland, where integration times are 2-8 ky, and of 0.9+/-0.3 mm/yr for the high crystalline Central Alps, where integration times are 0.5-1.5 ky. The measured cosmogenic nuclide-derived denudation rates are in good agreement with post-LGM lake infill rates and significantly higher than recent denudation rates from river loads. We attribute this discrepancy to differences in methodology and integration time scale. Our new rates are in the same range as denudation rates from apatite fission tracks that record denudation 3-5 Ma ago. Denudation rates correlate with hill slope in the Mittelland catchments, but they are independent of slope in the high Alps. We interprete this to mean that high Alpine landscapes are at threshold hillslope, where slopes cannot increase any further before failure occurs. In general, denudation rates are high in areas of high relief and high altitude. Importantly, good spatial agreement exists between denudation rates and recent rock uplift rates. Since all of the mentioned parameters are also highest where crustal thickness is largest, a major driving force for rock uplift and concomitant denudation is likely to be isostatic. However, given that crustal thickening of the Alps has all but ceased, the relief-forming events that set the rate of denudation and uplift must be presented by transient perturbations, such as increased erosional response to climate cycling. Thereby a quasi-steady state has been achieved in which rock uplift is balanced by denudation. This means that although the landscape is never in steady state on the short term, the conditions for long-term steady state are fulfilled nevertheless. Kahle, H., et al. (1997). Recent crustal movements, geoid and density distribution: Contribution from integrated satellite and terrestrial measurements. In O. Pfiffner (Ed.), Results of the National Research Program 20 (NRP 20), pp. 251-259, Birkhaeuser Verlag, Basel.

Description: A north-south traverse through the Swiss Central Alps reveals that denudation rates correlate with recent rock uplift rates in both magnitude and spatial distribution. This result emerges from a study of in situ–produced cosmogenic 10Be in riverborne quartz in Central Alpine catchments. As a prerequisite, we took care to investigate the potential influence of shielding from cosmic rays due to snow, glaciers, and topographic obstructions; to calculate a possible memory from Last Glacial Maximum (LGM) glaciation; and to identify a watershed size that is appropriate for systematic sampling. Mean denudation rates are 0.27 ± 0.14 mm/a for the Alpine foreland and 0.9 ± 0.3 mm/a for the crystalline Central Alps. The measured cosmogenic nuclide-derived denudation rates are in good agreement with post-LGM lake infill rates and are about twice as high as denudation rates from apatite fission track ages that record denudation from 9 to 5 Ma. In general, denudation rates are high in areas of high topography and high crustal thickness. The similarity in the spatial distribution and magnitude of denudation rates and those of rock uplift rates can be interpreted in several ways: (1) Postglacial rebound or climate change has introduced a transient change in which both uplift and denudation follow each other with a short lag time; (2) the amplitude of glacial to interglacial changes in both is small and is contained in the scatter of the data; (3) both are driven by ongoing convergence where their similarity might hint at some form of long-term quasi steady state; or (4) enhanced continuous Quaternary erosion and isostatic compensation of the mass removed accounts for the distribution of present-day rock uplift.