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| Titel |
Correlations between morphometric parameters and catchment wide denudation rates in catchments affected by crustal bending |
| VerfasserIn |
Gerold Zeilinger, Florian Kober, Kristina Hippe |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043609
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| Zusammenfassung |
| We analyzed the morphological parameters of catchments incising the Bolivian
Altiplano and the Eastern Cordillera. The correlation of mean slopes and mean relief
in the subbasins and their respective erosion rates are not instantly recognizable.
However, there is a trend that the subbasins with high erosion rates are located
close to the Cordillera, whereas subbasins with low erosion rates are located in
immediate vicinity of the Altiplano. This observation led us to a more detailed
analysis of the subbasins and their river networks in order to investigate the feedback
mechanism between erosion rates and a surface morphology possibly affected by crustal
response.
Our test area, the La Paz drainage system is sourced on the very low relief Altiplano and links
this region with the Subandean zone by cutting across the eastern high Cordillera. The
catchment, with a total drainage area of 4850 km2, is shaped by a combination of feedback
mechanisms involving erosion and crustal bending. Cross-cutting relationships between dated
strata and incised valleys indicate that incision in the Rio La Paz headwaters postdates 2.8
Ma. The volume of about 3950 km3, which has been evacuated since then by the Rio La Paz
drainage system to the Rio Beni relates to an average erosion rate of 290 mm/ky.
This is in a similar range as the catchment-wide erosion rates determined by using
terrestrial cosmogenic nuclides (TCN) derived from sediments being currently
exported from the Rio La Paz basin. The method integrates for a timescales ranging
from 1-10ky. Our results show an erosion rate of 230 mm/ky for the entire Rio
La Paz basin (sample from basin outlet) and erosion rates from ca. 100 up to 600
mm/ky in the subbasins. In contrast, on the Altiplano west of the Rio La Paz drainage
divide, erosion rates are one to two orders of magnitude lower than in the Rio La Paz
catchment.
We observed that the higher erosion rates correlate unexpectedly with a lower hypsometric
integral for the subbasins in the Rio La Paz catchment. Further analysis concentrates on
parameters derived from the channel network. Particularly parameters like Stream-gradient
index (SL), Steepness index (ks) and Specific Stream Power (SSP) reveal the focus of erosion
within the studied catchments. These spots of enhanced erosion coincide in general with the
parts of the rivers, where mixed channel or bedrock incision is observed, and mainly where
the channel length profiles show knickpoints. A spatial analysis of the geological properties
detects those knickpoints induced by structures (faults and folds) and changes in
lithology.
We will demonstrate that the TCN results from the interior parts of the Rio La Paz
catchments correlate only to certain extend with the surface morphology within the
catchment. However, including the erosion rates and morphometric parameters from the
catchments on the Altiplano, the correlation spanning data from both landscapes is
obvious. This implies that the effects of feedback mechanisms between erosion
and lithospheric deformation are substantial at the scale of individual structures,
where flexural feedback mechanisms between erosion and rock uplift influences the
morphometry of catchments and channel morphologies. When the dominant type of
erosion processes (e.g. headwater expansion by landsliding and / or fluvial incision) is
possibly influenced by tectonics then it can also cause the spatially variable erosion
rates and different surface morphologies and consequently the partly inconclusive
correlations. |
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