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How is the Altiplano affected by the Rio
La Paz drainage system?
G. Zeilinger (1), F. Schlunegger (2), A. Kounov (3)
(1)
Institute of Geosciences, University of Potsdam, Karl-Liebknecht-Str. 24/25,
D-14476 Golm/Potsdam,
(2) Institute of Geological Science, University of Bern, Baltzerstrasse 1 -
3, CH - 3012 Bern,
(3) Geological-Paleontological Institute, University of Basel,
Bernoullistrasse 32, CH-4056 Basel
It has been proposed that focused incision might initiate elastic and/or
non-elastic rebound in association with an increase in the depth of
dissection. Uplift is then localized and recorded as permanent deformation,
which in turn, exerts a positive feedback on erosion. The Eastern Cordillera
of the Bolivian Andes represents such a situation of focused erosional
unloading. The Altiplano is an almost closed basin with a distinct watershed
defined by theWestern Cordillera and Cordillera Real. However, the Rio La
Paz drainage basin with a total area of 4850 km2 is the only prominent
system that is sourced on the Altiplano. The Rio La Paz cuts across the
Cordillera Real south of the 6438 m-high Illimani, connecting significant
portions of the Altiplano with the Amazon Basin. Newly dated 8 - 10 Ma
Apatite fission track ages from the Nevada Illimani, indicate that major
exhumation occurred during the Quechuan phase of deformation
(11 - 5 Ma). The Rio La Paz system removed approximately 3950 km3 of rocks.
We anticipate that focused erosion in this particular situation must
modulate the general pattern of rock uplift where the Rio La Paz cuts the
Cordillera. We use morphometric data in combination with channel
morphologies to illustrate and quantify feedback mechanisms between focused
erosion and rock uplift. This feedback explains why all drainages beyond the
watershed disperse their waters to the Altiplano. It also explains the
presence of the highest peaks adjacent to the location where the Rio La Paz
cuts across the Cordillera Real. However, it remains unclear through which
process, and at what time, the La Paz drainage system breached the formerly
internally drained Altiplano basin initiating these feedback mechanisms.
