Structural constraints on palaeomagnetic rotations south of the Arica Bend, northern Chile: implications for the Bolivian Orocline

Abstract

Palaeomagnetic data from the Central Andean fore-arc appears to support the Bolivian Orocline model, with clockwise rotations to the south, and counter-clockwise rotations to the north of the Arica Bend, respectively. However, counter-clockwise rotations in northernmost Chile extend south of the Arica Deflection, consistent with Mesozoic rotations in southern Peru. This suggests that the Bolivian Orocline model needs to be redefined. We present a palaeomagnetic study of 140 specimens from 15 sites, sampled in Middle Jurassic sediments and volcanics of the Camaraca Formation and in the Upper Jurassic-Lower Cretaceous Cuya Dykes, of northernmost Chile (19.2dgS, 70.3°W). The calculated pole (81.4°S, 354.7°E, α 95 = 6.7°), when compared with the Upper Jurassic pole for cratonic South America (89.0°S, 217.1°E, α 95 = 4.6°), suggests that the sampled area has undergone a small counter-clockwise rotation of approximately 10°. The change from counter-clockwise to clockwise rotations occurs between 19°S and 23°S and may correspond to a known bend in the Peru-Chile trench. These rotations were probably the result of in-situ block rotation in response to shear. This shear is the result of stresses in the subducted lithosphere, with sinistral shear to the north (counter-clockwise rotations), and relatively larger dextral shear to the south (clockwise rotations) of the bend in the trench, respectively. The amount of shear is reduced in the area of the bend due to contortion of the subducting slab. These stresses resulted due to the shape of the Andean margin rather than as a result of oroclinal bending. The Arica Bend is interpreted as an ancient feature which may have evolved from a hot-spot generated triple-junction rift system. The Velasco Alkaline Province of eastern Bolivia possibly formed the failed-arm of this triple-junction.