Sophie E. Baker

Piedmont aggradation has a large impact on denudation rates of mountain ranges because it controls the base-level for erosion upstream. Trends in fluvial aggradation and incision across piedmonts can also reveal differential rock uplift resulting from tectonic activity. The subject of this thesis is the Quaternary aggradation and incision history of a piedmont reach of the Rio Diamante, a river which drains a 2500 km² area of the southern central Andes. The catchment is located within a latitudinal zone of the Andes (~33-37^oS) which is transitional in terms of morphology and foreland tectonics. North of 33^oS, compressional deformation and uplift is active across a wide zone of the foreland, whereas south of ~35^oS, crustal shortening has been confined to a narrow mountain belt to the east of which lies in an area of extension and back-arc volcanism. The study reach of the Rio Diamante, at 34.5^oS, flows eastwards between the mountain front and the San Rafael Block, which is the southernmost expression of shortening east of the main Andean cordillera. Unlike the proximal reaches of the adjacent piedmont rivers, the Rio Diamante has carved a deep canyon into the Neogene syn-orogenic sedimentary rocks and occupies an area where the Quaternary tectonic activity has not been extensively studied. The aim of the project is to determine, from the terrace record, the timing, rate and driving force for the incision and to elucidate any differential incision along the reach related to differential rock uplift.

Five fill and Strath terraces were mapped, correlated and surveyed along the ~35 km reach. Geochemical correlation of ignimbrite deposits interbedded with one of the fill terrace deposits to a previously dated ignimbrite, as well as eight cosmogenic nuclide ages of terrace surfaces, has allowed the chronology of the terraces to be constrained. The ages are consistent with a synchronous relationship between fill terrace deposition and glaciation in the cordillera. Terraces are tentatively correlated with oxygen isotope stages 16 (Qt1), 12 (Qt2), 4 (Qt3) and 2 (Qt4 and Qt5).

Overall, the terrace long profiles indicate that incision and aggradation has taken place almost uniformly across the piedmont. In detail, minor differential incision is apparent from the Qt2 strath, which is folded into an open anticline culminating mid-reach. Although no terraces younger than Qt2 (~450 kyr) are obviously folded, down-valley variations in terrace width and the sinuosity of the modern channel suggest that minor deformation has persisted into the Late Quaternary.

The long-term rate of bedrock incision along the reach is ~0.1-0.5 m/kyr and the shorter term rate (since OIS 2) is ~2 m/kyr. While quaternary rock uplift in the piedmont area is one possible cause for bedrock incision, some of the incision may have resulted from base-level fall due to subsidence on the eastern side of the San Rafael Block. A decline in the amount of sediment being delivered from upstream may have also contributed. Studies of the rates and controls of piedmont geomorphic processes such as this will contribute to the understanding of the topographic and tectonic transition that characterises the southern central Andes, especially regarding the importance of north-south erosional gradients stemming from along-strike variations in climate.

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Pages: 267
Supervisor: John Gosse