THE EOCENE COMPRESSIVE TECTONIC EVENT IN PERU: A RESULT OF FLAT SUBDUCTION

The most important compressive tectonic event in the development of the Central Andes of Peru and north Bolivia took place during the Eocene (43-32 Ma). It affected the forearc basin of Talara (NW Peru) and was responsible for most of the tectonic shortening in the Western Cordillera of north and central Peru, and in the Altiplano of south Peru. Tectonic shortening coincided with important uplifts, subsequent erosion, synorogenic sedimentary basin fills, and emplacement of intrusive and volcanic rocks. In the NE-SW-trending Talara basin, the early Eocene transtensional system became transpressive in the late Eocene, originating deformation of the basin fill by SE-verging thrusts. In northern Peru, the Eocene event began at 43 Ma and was responsible for the deformation of the Western Cordillera and the Marañón imbricate thrust system located further to the east.

In the Western Cordillera of central Peru, large east-verging thrusts originate most of the tectonic shortening. At the same time, synorogenic basins developed, including an important volcanism exceeding 1 km thicknesses.

In southern Peru, the strongest Eocene deformation took place along the NE margin of the Western Cordillera and in the Altiplano. It includes NE-verging thrusts and important uplifts, with continental red beds filling sedimentary basins leading to thicknesses exceeding 5 km, progressive unconformities and growth strata. Thrusting and sedimentary filling coincided with the emplacement of subvolcanic stocks, porphyry-type mineralizations and volcanism. Forearc basins resulted from tectonic erosion after Eocene tectonism.

In Bolivia, the strongest shortening of the Bolivian orocline took place along the Altiplano-Eastern Cordillera boundary via the Calazaya nappe and most probably also during the Eocene.

The long-lived Eocene compressive tectonic event (also known as Inca 1) took place between 43 and 32 Ma. It was the strongest deformation event during the development of the Peruvian Andes, and probably also for the whole Central Andes. It coincided with a period of very fast subduction rate and hence with a flat slab setting.