Abstract Sediment 2005

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09-12-2007

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Geologist crushing rocks!

Nanga Parbat

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The Rio La Paz drainage system and its effect on the Altiplano

Gerold Zeilinger (1), Fritz Schlunegger (1), Alexander Kunov (2), Guy Simpson (3)

(1) Institute of Geological Science, University of Bern, Baltzerstrasse 1 - 3, CH - 3012 Bern
(2) Dept. of Geological Sciences, University of Cape Town, University Avenue 13, SA - 7700 Rondebosch
(3) Geological Institute, ETH Zurich, Sonneggstrasse 5, CH - 8092 Zurich



The Altiplano developed as an intramontane basin during the orogeny of the Andes and is today the second highest plateau on Earth. It represents a closed basin with a distinct watershed defined by the Western Cordillera and Cordillera Real. However, the analysis of the SRTM digital elevation revealed two locations where the Altiplano, in its function of a sediment trap, is not limited by the Cordillera Real, but where two drainage systems cut into the Altiplano: 1) the Rio La Paz drainage system with a total area of 4850 km2 and an eroded volume of approximately 3950 km3 and 2) the Rio Consata drainage system with an area of 2300 km2 and an eroded volume of approximately 2380 km3.

We focus on the Rio La Paz, but the Rio Consata system displays similar geomorphic and geologic features. Field work in Rio La Paz system in combination with morphometric analyses of a digital elevation model raises two major questions concerning the development of this drainage system: 1) why is the Rio La Paz cutting the Cordillera Real between the high peaks of Nevada Illimani (6439 m) and Huara (5148 m) and 2) why is the dispersal direction of the drainages beyond the watershed of the Rio La Paz in the opposite direction (Fig. 1), i.e. perpendicular to the drainage divide (pattern pointing radially away from the watershed between drainage system and the Altiplano)?

Cross-cutting relationships between dated strata and incised valley indicate that incision in the headwaters of the Rio La Paz in the Cordillera Real postdates 5 Ma. Preliminary apatite fission track ages of the Cordillera Real close to the Nevada Illimani of around 8 - 10 Ma indicate that the major exhumation occurred during the Quechuan phase of deformation (11 - 5 Ma). The uplift of the high peaks next to the river gorge of around 1000 m (relatively to the mean elevation of the Cordillera) could have been caused by crustal bending (flexural feedback to erosion), which accounts for ca. 400 m, and due to enhanced local erosion, which is likely to have initiated an additional uplift (high peaks) in an area of decreasing mean elevation. The maximum vertical exhumation later than 10 Ma was not significant enough to be recorded by apatite fission track.

Undoubtedly, the feedback mechanism between erosion and crustal bending plays an important role in controlling the overall morphometry and dispersion direction of the drainages beyond the watershed (Fig. 1). A further possible control for initiation and location of the main valley systems might be filling of the originally closed system of the Altiplano and overflow, causing the rivers to erode re-incise into the previously depositional realm. Our ongoing research will address these hypotheses in order to better understand the active mechanisms.
 

Figure 1: 3-dimensional view (Landsat 7 draped on SRTM elevation, 2 x vertical exaggeration) of the Rio La Paz drainage system. The drainage divide between the Rio La Paz system and the Altiplano is marked as thick line, the course of potential flow paths are shown as thin lines.

 

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