Tectonic Evolution of the Patagonian Andes

RAMOS, V.A.; GHIGLIONE, M.C.

The Late Cenozoic of Patagonia and Tierra del Fuego, Developments in Quaternary Science 11

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Año: 2008; p. 57 - 71

Throughout the Cenozoic, the southern Andes have had a distinctive feature when compared to the central Andes. Since the early definition of Windhausen (1931) and Feruglio (1949–1950), the presence of a continuous batholith belt that starts at nearly 39 S and ends in Cape Horn Islands in the southernmost tip of the Andes at about 52 S was evident (Fig. 1). These granitic rocks of Andean age are not exposed in a continuous belt further north in the Principal Cordillera. Dessanti (1972) used that feature to define the northern limit of the Patagonian Cordillera as a different geological province – the criterion used in later regional studies by Leanza (1958, 1972), Sua´rez (1976), Haller and Lapido (1980), Ramos et al. (1982) and Ramos (1999). However, the presence of a batholith in the backbone of the cordillera was taken as a natural fact, without questioning why it was there, what controls its continuity and why it is not a common feature in other segments of the Andes. The objective of this chapter is to focus on the importance of the different geological and tectonic processes that uplifted the Andes at these latitudes, and how the climate has had an important role not only in carving the landscape, but also in controlling the uplift mechanisms through time. This interaction between climate and tectonics can be addressed in three distinct topics: 1. The present structural volume of the Patagonian Cordillera is relatively small when compared with the central segments of the Andes. This difference has been attributed to the sediment fill of the trench, which lubricates the friction in the subduction channel and therefore produced less coupling between the continental upper plate and the subducted slab. This sediment supply is related to the dominant wet winds from the southwest, which produce the rain shadow and asymmetric erosion in the Patagonian Cordillera. 2. The continuous batholith belt is also a consequence of uplift and climatic interaction. As soon as the uplift of the Patagonian Cordillera initiated in Miocene times, most of the western slope of this part of the cordillera was subjected to an extreme erosional gradient as a consequence of rain shadow. As a result, the magmatic arc was deeply  eroded and the batholith was denudated. 3. The Patagonian Cordillera has an anomalous isostatic rebound, near to 20 mm/yr along the axis of the cordillera, which has been associated with the last glacial maximum (LGM) deglaciation. This anomalous behavior is related to a hot upper mantle, generated as a consequence of several episodes of ridge subduction, which favors the important isostatic rebound. Because of that, this segment of the Andes records the first glaciations during the Late Cenozoic. Prior to the discussion of these topics a brief summary of the geologic and tectonic framework of the Andes at these latitudes will be presented. This description does not aim to be a comprehensive review of the geology of this region, but it will focus on its main characteristics in order to show the effective control of climate in the tectonic history of this segment of the Andes.