CONICET | Buscador de Institutos y Recursos Humanos

In the Andean margin between ~38-47º S (Chile Triple Junction), both the Main and Coastal cordilleras exhibit a southward decrease in topography and amplitude, features that are inversely correlated with a widening of the Pacific oceanic platform. These characteristics have been explained either by variable constructive processes, such as reduction of shortening and crustal thickness, or by intensification of surficial destructive processes due to an increase in erosion rates. Both hypotheses are at a certain extent complementary and could explain variable topography through the Southern Central Andes. However, the relative contribution of each mechanism (tectonic-climate feedback) in the morphological evolution of this Andean section remains debated. In particular, between ~41°- 43°S, the Chiloé Island, a basement-cored topographic high, represents the last continuous expression of the Coastal Cordillera. This basement unit exposes metamorphic rocks of a Late Paleozoic accretionary complex, intruded by Paleogene igneous rocks and unconformably covered by late Oligocene-early Miocene marine - continental sedimentary rocks. The Chiloe Island is surrounded by thick fore-arc marine basins (> 4 km infilling), the Ancud basin in the submerged Central Valley to the east, and the Chiloe Basin in the oceanic platform to the west. Previous thermochronological data had identified an initial fore-arc uplift and exhumation during Cretaceous ? Early Paleogene (~100-70 Ma) followed by sedimentation in fore arc basins (e.g., Arauco Basin). Even though a sedimentary gap is inferred until the early Miocene, the enhanced glacial erosion could have a role in limiting the available sedimentary record. Then, the Eocene?Oligocene time period is only represented by few plutonic-volcanic rocks emplaced under an extensional regime associated to a strong slab roll back. Finally, during the early-middle Miocene, tectonic inversion of Paleogene volcano-sedimentary depocenters has been proposed. In this study, new thermochronological data (AFT and (U-Th)/He) combined with previously published data sets distributed around the continental and Chiloé island zones are presented. These data indicate an Eocene?Oligocene exhumation ages (~40 - 30 Ma), which had not been recognized before in the area, indicating that the Coastal ranges were shaped at the time of either a extensional-related uplift or part of a recently proposed contractional episode that affected the eastern Andean slope to the south. In order to understand the unsteady growth of the fore-arc relief and to establish time/intensity evolution stages, we present a geodynamic model incorporating the obtained ages to the regional geological framework during Eocene to Oligocene times. The ultimate aim of this study will be to discuss the geological differences between the fore-arc and the Main Andean Cordillera deformational evolution.