Determining the growth rate of topographic relief using in situ-produced 10Be: A case study in the Black Forest, Germany

To determine how topographic relief in mountainous regions evolves through time we present a new approach that uses in situ-produced cosmogenic Be-10 to quantify (1) spatially averaged denudation rates of small watersheds and (2) local denudation rates of the ridge crests bounding these basins. The technique is applied to two catchments in the Black Forest, a forested mountain range with a local relief of a few hundred meters, which is typical for ranges in central Europe. Both the Acher and the Gutach catchments expose predominantly Carboniferous granite, and only minor amounts of high-grade gneiss and Triassic sandstone. The latter occurs on ridges defining the eastern boundaries of the catchments, above a regional unconformity. In the Acher and northern Gutach watersheds denudation rates of subcatchments derived from Be-10 concentrations in stream sediment range from 52 to 87 mm/ka and 59 to 91 mm/ka, respectively. In contrast, grus samples from the ridge crests bounding both watersheds yield lower denudation rates of 34 to 59 mm/ka. The differences in denudation rates for sample pairs from individual subcatchments and adjacent ridge crests reveals that topographic relief is growing at a mean rate of 24 12 mm/ka (with the exception of the flat southwestern part of the Gutach catchment, where catchment-wide denudation rates are similar to the rate of ridge crest lowering). The inferred rates of denudation and relief growth are consistent with erosion rates calculated from the known thickness of Triassic to Lower Jurassic sediments, which were once present above the regional unconformity but have been largely eroded during the exhumation of the Black Forest. The onset of exhumation similar to 19 Ma ago is constrained by thermal modelling of apatite fission track data, which suggest a cooling rate of similar to 3 degrees C/Ma. Combined with a geothermal gradient of 30 to 40 degrees C/km this cooling rate yields an average exhumation rate of 75-100 mm/ka for the modelled apatite fission track data, which is comparable to spatially averaged denudation rates derived from cosmogenic Be-10. Our new approach may help to determine whether tectonically active mountain ranges are in a topographic steady state, in which rates of rock uplift and denudation are equal, or if such a dynamic equilibrium has not yet been attained. (C) 2009 Elsevier B.V. All rights reserved.