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Tectonique, érosion et évolution du relief dans les Andes du Chili Central au cours du Néogene

Abstract : The structural evolution related to the uplift of the Andean Cordillera between 33 and 35ºS is well known in the eastern Argentinean flank of the belt, in which it has been developed an east vergent thrust system. In turn, deformation in the Chilean flank of the chain has not been very well constrained and several discrepancies exist in order to explain uplift and Central Depression development.
This study shows that in the Andes of Central Chile, compressive deformation consisted in a tectonic inversion accommodating ca. 16 km shortening between 22 and 16 Ma. This shortening is much lesser than that accumulated in the eastern flank of the chain (1/5 of total shortening), in which more than 70 km of shortening has been accommodated between 16 and 4 Ma.
Compressive tectonics and Andean uplift in Central Chile were diachronic. Surface uplift mainly occurred between 8 and 4 Ma, that is, several million years after deformation occurred in the Chilean flank of the belt. In fact, surface uplift essentially was the result of deep thickening because of an east-vergent detachment fault located at 10 km depth beneath the Principal Cordillera, which connects to the Benioff zone at 60 km depth through a ramp. This “ramp-flat” structure and its geometry are both controlled by lithospheric rheology. Moreover, this structure would have been essential during Andean evolution because it controls strain and stress transference from the subduction zone toward the continent, and because the flat segment facilitates shortening transfer at deep from higher deformed zones at surface. Therefore, this deformation mode in “simple-shear” would be the responsible for forearc uplift not only in Central Chile, but also along most of the Andean margin.
Erosional response to uplift has been very slow. Using fission-tracks determinations and the ages of volcanic layers that recorded the relief evolution, incision in the highest sectors of the Principal Cordillera occurred between 2 and 6 m.y. after surface uplift. Lithological contrasts, which control erosional upstream propagation, favored the development of a valley parallel to the coastline due to the river piracy exerted by great rivers over headwaters of streams that crossed through higher resistance lithology. This process was analyzed using numerical algorithms and analytic solutions of the stream power-law, evidencing the physical feasibility of this model. Therefore, the Central Depression would not be a tectonic feature sensu stricto, but the result of the development of a lateral drainage network strongly controlled by the lithologic distribution and being the final stage of the erosive response to the Late Miocene high and rapid surface uplift.
The major episode of uplift, which occurred after 10 Ma, was not an isolate phenomenon occurring in Central Chile, but it has been registered along most the Andean Cordillera. Even though at this time the plate convergence rate diminished, the absolute velocity of South America increased, thus it seems that absolute plate velocity of continents is the first-order process on mountain building as actually it has been proposed.
Finally, the Central Chile Andes constitutes a transitional region between two climatic and morphologic regions: to the north (dryer), constructional process rules, whereas destructional processes do it to the south (more humid). This contrast contributes to the absence of the Central Depression north of 33ºS, even though the influence of the subduction geometry, which became flat to the north, would also contribute to this evolution.
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Submitted on : Monday, February 18, 2008 - 10:35:29 AM
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Marcelo Farias. Tectonique, érosion et évolution du relief dans les Andes du Chili Central au cours du Néogene. Tectonique. Université Paul Sabatier - Toulouse III, 2007. Français. ⟨tel-00256817⟩

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