Palaeomagnetism of the Cordillera de la Costa, northern Chile: evidence for local forearc rotation

Palaeomagnetic studies of Lower Jurassic to Lower Cretaceous volcanic, plutonic and sedimentary rocks from the Cordillera de la Costa of northern Chile, indicate that there has been no significant latitudinal movement of northern Chile since early Cretaceous times. A palaeomagnetic pole for the Cordillera de la Costa (63°S, 187°E; α95 = 7.7°) was derived from combining the results of three studied units, the La Negra Formation (Jurassic volcanics); Jurassic/Lower Cretaceous granodioritic plutons; and the E1 Way Formation (Hauterivian marine carbonates) together with the Coloso Formation (Lower Cretaceous red beds). Comparison with a Cretaceous reference pole for the stable shield area of South America (87°S, 235°E; α95 = 2.4°) indicates that 24 ± 7° of clockwise rotation has taken place. Possible mechanisms for the rotation of the Andean forearc in this area have been examined in relation to the kinetics of the subducting Nazca and over-riding South American plates. The Cordillera de la Costa has been subjected to both compressional and extensional tectonics since the Lower Cretaceous. Major eastwardly directed thrusting in mid-Cretaceous and early Tertiary times, possibly associated with a period of oblique subduction of the Nazca plate, contributed to the accretion of the Cordillera to the main forearc; rotation may have occurred during this process. In Oligo-Miocene and later Tertiary times, extensional faulting (possibly associated with roll-back of the Nazca plate) led to the reactivation of the Atacama fault zone and the formation of oblique extension faults in the Cordillera de la Costa. Structural rotation associated with this phase of extension could equally explain the observed palaeomagnetic results. © 1988.