The Precordilleran fault system of Chuquicamata, Northern Chile: evidence for reversals along arc-parallel strike-slip faults

Abstract

The Chilean Precordillera, situated between the Longitudinal Valley and the Western Cordillera of Northern Chile, was the site of the Andean magmatic arc from the late Cretaceous to the Eocene-Oligocene boundary. Magmatism came to an end during the Incaic tectonic phase (38 Ma), which caused arc-normal shortening and the development of longitudinal dextral strike-slip faults (Precordilleran Fault System). This magmatic arc tectonism is also related to the formation of the Chuqicamata porphyry copper ore deposit as well as of other important deposits of this type in the Precordillera. Structural investigations in and around the Chuqicamata open-pit mine have shown that wrench tectonics determined the kinematics of the area. The NS-striking West Fissure, which separates a 35-Ma-old non-mineralised pluton to the west from a central late Paleozoic basement ridge containing the mineralization, became a sinistral fault along with other parallel faults in the area. The central part is separated from similar Paleozoic rocks to the east by the Messabi-Este fault and a narrow faulted and sheared syncline of Mesozoic-Cenozoic sediments. This fault bears structures indicating dextral movements, which probably are of an age that is similar to the mylonites (34.8 Ma) in the western pluton. The dextral movements preceded the sinistral shear. Thus, the fault system of Chuqicamata displays a reversal of arc-parallel shear movements. According to the orientation of quartz veins in the mineralized body, it is presumed that the sense of displacement of these strike-slip motions reversed, when mineralization started at about 32 Ma. During this time the stress field must have changed fundamentally. The Incaic Phase dextral transpression is supposed to have been induced by the oblique vector of plate motion. The following sinistral transtension corresponds to a time of reduced convergence rate and possibly reduced plate coupling. As, however, the vector of plate motion remained unchanged during that time, oblique subduction cannot be used as an argument for arc-parallel sinistral shear movements.