THE PRECORDILLERAN FAULT SYSTEM OF CHUQUICAMATA, NORTHERN CHILE - EVIDENCE FOR REVERSALS ALONG ARC-PARALLEL STRIKE-SLIP FAULTS

The Chilean Precordillera, situated between the Longitudinal Valley an d the Western Cordillera of Northern Chile, was the site of the Andean magmatic are from the late Cretaceous to the Eocene-Oligocene boundar y. Magmatism came to an end during the Incaic tectonic phase (38 Ma), which caused are-normal shortening and the development of longitudinal dextral strike-slip faults (Precordilleran Fault System). This magmat ic are tectonism is also related to the formation of the Chuquicamata porphyry copper ore deposit as well as of other important deposits of this type in the Precordillera. Structural investigations in and aroun d the Chuquicamata open-pit mine have shown that wrench tectonics dete rmined the kinematics of the area. The N-S-striking West Fissure, whic h separates a 35-Ma-old non-mineralised pluton to the west from a cent ral late Paleozoic basement ridge containing the mineralization, becam e 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 Me sozoic-Cenozoic sediments. This fault bears structures indicating dext ral movements, which probably are of an age that is similar to the myl onites (34.8 Ma) in the western pluton. The dextral movements preceded the sinistral shear. Thus, the fault system of Chuqicamata displays a reversal of are-parallel shear movements. According to the orientatio n of quartz veins in the mineralized body, it is presumed that the sen se of displacement of these strike-slip motions reversed, when mineral ization 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 s ubduction cannot be used as an argument for are-parallel sinistral she ar movements.