The Late Cenozoic Geodynamic Evolution of the Central Segment of the Andean Subduction Zone

摘要

The presented model of the Late Cenozoic geodynamic evolution of the central Andes and the complex tectonic, geological, and geophysical model of the Earth's crust and upper mantle along the Central Andean Transect, which crosses the Andean subduction zone along 21° S, are based on the integration of voluminous and diverse data. The onset of the recent evolution of the central Andes is dated at the late Oligocene (27 Ma ago), when the local fluid-induced rheological attenuation of the continental lithosphere occurred far back of the subduction zone. Tectonic deformation started to develop in thick-skinned style above the attenuated domain in the upper mantle and then in the Earth's crust, creating the bivergent system of the present-day Eastern Cordillera. The destruction of the continental lithosphere is correlated with ore mineralization in the Bolivian tin belt, which presumably started at 16°S and spread to the north and to the south. Approximately 19 Ma ago, the gently dipping Subandean Thrust Fault was formed beneath the Eastern Cordillera, along which the South American Platform began to thrust under the Andes with rapid thickening of the crust in the eastern Andean Orogen owing to its doubling. The style of deformation in the upper crust above the Subandean Thrust Fault changed from thick- to thin-skinned, and the deformation front migrated to the east inland, forming the Subandean system of folds and thrust faults verging largely eastward. The thickening of the crust was accompanied by flows at the lower and/or middle crustal levels, delamination, and collapse of fragments of the lower crust and lithospheric mantle beneath the Eastern Cordillera and Altiplano-Puna Plateau. As the thickness of the middle and lower crustal layers reached a critical thickness about 10 Ma ago, the viscoplastic flow in the meridional direction became more intense. Extension of the upper brittle crust was realized mainly in gliding and rotation of blocks along a rhombic fault system. Some blocks sank, creating sedimentary basins. The rate of southward migration estimated from the age of these basins is 26 km/Ma. Tectonic deformation was accompanied by diverse magmatic activity (ignimbrite complexes, basaltic flows, shoshonitic volcanism, etc.) within the tract from the Western Cordillera to the western edge of the Eastern Cordillera 27-5 Ma ago with a peak at 7 Ma; after this, it began to recede westward; by 5 Ma ago, the magmatic activity reached only the western part of the Altiplano-Puna Plateau, and it has been concentrated in the volcanic arc of the Western Cordillera during the last 2 Ma.