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Focussed erosion and
possible flexural accommodation: A case study from the eastern edge of the
Altiplano.
G. Zeilinger (1), F. Schlunegger (2) and Guy Simpson (3)
(1) Institute of Geosciences, University of Potsdam, Karl-Liebknecht-Str.
24/25, D-14476 Golm/Potsdam (zeilinger@geo.uni-potsdam.de)
(2) Institute of Geological Science, University of Bern, Baltzerstrasse 1 -
3, CH - 3012 Bern (fritz.Schlunegger@geo.unibe.ch)
(3) Geological Institute, ETH Zürich, Haldenbachstr. 44, HAD F 1, CH - 8092
Zürich (guy.simpson@erdw.ethz.ch)
It has been accepted that erosional unloading by focused incision might be
accommodated by flexural rebound, which in turn, potentially exerts a
positive feedback on erosion. We will argue that the Eastern Cordillera of
the Bolivian Andes represents such as situation of focused erosional
unloading. This is the case because the Rio La Paz and the Rio Consata that
originate on the Altiplano cut across the Eastern Cordillera immediately
adjacent to the highest peak of the Cordillera Real of Bolivia (e.g., the
6438 m-high Illimani). The La Paz system has removed approximately 3950 km3
of material since the Late Miocene at the latest and has incised into the
Cordillera Real by up to 3500 m. We anticipate that this focused erosion in
this particular situation must modulate the general pattern of rock uplift.
We identified two locations of enhanced surface erosion. These are the
headwaters where landsliding has resulted in headward expansion of the
drainage divide into the Altiplano, and the segment where the La Paz river
cuts across the Cordillera Real and where fluvial incision has resulted in
partial exposure of bedrock on the channel floor. We use morphometric data
of these locations to illustrate the effects of such a feedback mechanism
between erosion and crustal bending. Moreover, we quantify these effects
with a flexural feedback model. This model explains why all drainages beyond
the watershed disperse their waters to the Altiplano. It also provides an
explanation for the presence of the highest peaks just next to the location
where the La Paz River cuts into the bedrock across the Cordillera Real.
The effects of feedback mechanisms between erosion and lithospheric
deformation has been partly addressed in a global sense, but the dynamics
and implications of this relationship in smaller compartments within an
orogen are largely unresolved at the scale of individual structures. Our
study implies that the effects are substantial at the scale of individual
structures, where flexural feedback mechanisms between erosion and rock
uplift influences the morphometry of mountain belts and channel
morphologies.
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