Jose Luis Antinao Rojas
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Ph. D. Thesis
Quaternary Landscape Evolution of the Southern Central Andes of Chile Quantified Using Landslide Inventories, 10Be and 36Cl Cosmogenic Isotopes and (U-Th)/He Thermochronology
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The Andes Mountains – the largest subduction orogen in the world – are a unique natural laboratory suited to test interactions at different timescales between tectonic and surface processes. However, Andean Landscape Evolution still remains relatively understudied. This lack of knowledge will probably change during the next few years with growing understanding of Andean rates and styles of geomorphic processes.
           Long term (106 a) landscape evolution of the high relief Cordillera Principal, in the Chilean southern Central Andes (SCA), is strongly marked by the previous tectonic history of the orogen. Exhumation rates determined from apatite (U-Th)/He low-temperature thermochronology are on the order of ~1 mm a-1 since 2-3 Ma. The exhumation history is consistent with a segmented orogenic architecture in this region, with inherited pre-Cenozoic structures and domains that define not only the distribution of tectonic processes but also erosional styles in response to stress and deformation. Late Pliocene acceleration of exhumation from values of 0.3-0.4 mm a-1 has probably occurred along with surface uplift of Cordillera Principal, creating a high elevation area still present today that displays the highest Quaternary erosion rates in the SCA. Quaternary sediment production rates from landslide inventories on a long term (106 a) basis (0.16 mm a-1) are similar to transport rates determined at intermediate timescales (0.25-0.5 mm a-1 over 103-104 a) with the help of terrestrial cosmogenic nuclide 36Cl inventories in stream sediment. The cosmogenic nuclide study confirmed that landslides dominate denudation providing a distinctive isotopic signature to the different grain sizes of the stream sediment. At short (10 a) timescales, total fluvial transport rates, including a modeled bedload component, are similar (0.25 mm a-1) to long- and intermediate-term production and transport rates. I conclude that during the Quaternary, occurrence of large landslides and increased activity of fluvial processes during glacial stages have removed efficiently mass from Cordillera Principal, in a waxing orogen. However, variability in erosion over long term timescales and the difference with inferred Quaternary exhumation indicates that a macroscale steady state has not been reached.
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Pages: 254
Supervisor: John Gosse