active tectonics, incision rate and sedimentation in the subandean belt inferred from the caracteristics of the Pilcomayo river
Abstract
Simple rationales have generally been used to derive a resultant macroscopic relationship and to relate channel river properties, to the rate of fluvial incision into bedrock. The fluvial shear stress model seems a robust model for the active foothill environment. To use this model, we have therefore collected the parameters of the Pilcomayo river: a) granulometry from field work ; b) drainage area from DEM ; c) slope from 1/50000 topographic maps and GPS measurements ; d) width from field work and topographic maps ; e) water discharge from filled work and bibliography. A comparison between the calculated shear stress and the structure of the subandean zone shows that the shear stress increases at the hanging-wall of 3 thrust faults. These faults show field evidence of recent thrust activity, and we use the ratio of Shield stress through the tertiary sediments at the hanging-wall of these faults to estimate the ratio of activity of these faults. It is found that half of the motion occurs along the Aguarague structure. Considering a 10 mm/yr. mean shortening rate through the subandean belt, the maximum uplift rate is about 6 mm/yr. and the erodability coefficient of the Tertiary is about 40 mm/yr. The shield stress also abruptly increases at the transition between subandean zone and interandean zone, suggesting that this latter develops like a mid-crustal in sequence stack. In the Eastern cordillera, the long term incision is low (1200 m for 3 Ma, i.e. 0.4 mm/yr.) and river presently incises fluvial sediments (Puente Sucre) deposited after 100 000 yr. BP.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....5112B