『Abstract
To determine how topographic relief in mountainous regions evolves
through time we present a new approach that uses in situ-produced
cosmogenic 10Be 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 10Be
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 〜19 Ma ago is constrained by thermal modelling
of apatite fission track data, which suggest a cooling rate of
〜3℃/Ma. Combined with a geothermal gradient of 30 to 40℃/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
10Be. 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.
Keywords: cosmogenic nuclides; denudation; topographic relief;
fission track analysis; Black Forest; central Europe』
1. Introduction
2. Geological setting
3. Denudation rates derived from cosmogenic 10Be
3.1. Sampling strategy and analytical procedures
3.2. Quantification of denudation rates
4. Apatite fission track analysis
4.1. Sample preparation
4.2. Thermal modelling of apatite fission track data
5. Discussion
5.1. 10Be-derived denudation rates
5.2. Growth rate of topographic relief
5.3. Denudation rates integrated over 〜107 years
5.4. Implications for landscape evolution
6. Conclusions
Acknowledgements
References