Evidence for the subduction and underplating of an oceanic plateau beneath the south Peruvian margin during the late Cretaceous: structural implications

Abstract

uring late Cretaceous times (Santonian to Maastrichtian), the southern Peruvian and northernmost Chilean Andes show a series of tectonic, magmatic and sedimentological features, which appear to be specific to this area when compared with northern and southern parts of the Andes: a gap in the magmatic activity in the Coastal Range and the Western Cordillera between 84 and 70 Ma; huge overthrusting faults involving the Precambrian basement in the Arequipa region; and the syntectonic filling of the Cuzco and Sicuani foreland continental basins. These particular features may be regarded as consequences of the subduction and underplating of a paleo-oceanic plateau (the “Mollendo ridge”) which would have occurred in this area between ±85 and ±70 Ma B.P. with an almost N-S direction of convergence between the Farallon and South American plates. This inferred “Mollendo ridge” would have been an eastern equivalent of the present-day Manihiki plateau of the western Pacific Ocean. The proposed model provides new elements for the interpretation of the subsequent structural and magmatic evolution of the central Andes; it permits the explanation of the particular location and the petrochemical and metallogenetic features of the Andahuaylas-Yauri batholith. The segment of abnormally thick crust created by the underplating of the “Mollendo ridge” and by overthrusting tectonics will act as an accumulation point for further Cenozoic deformation along the Andean margin; then the subduction of the “Mollendo ridge” would mark the outset of the development of the oroclinal bending of the central Andes (the Arica elbow) and of the associated Altiplano and its anomalous thick crust.