The state of stress in an overriding plate situated above a flat slab: The Andes of central Peru

Abstract

In the Andes of central Peru that are situated above a flat dipping slab, analysis of both structural data collected in the field and available focal mechanisms show that the Quaternary and Recent state of stress is characterized by N‐S trending extension in the Western Cordillera and along the Pacific Coast, whereas E‐W trending compression prevails in the sub‐Andean Lowlands and at the contact between the Nazca and South American plates. The Eastern Cordillera deformations suggest an intermediate state of stress (i.e., strike‐slip faulting) characterized by E‐W trending compression and N‐S trending extension. Thus major fault zones striking NW‐SE, i.e., parallel to the central Peruvian Andes, are normal with a sinistral component in the Western Cordillera (e.g., Cordillera Bianca) and reverse sinistral in the Eastern Cordillera (e.g., Cordillera de Huaytapallana). This state of stress displays similarities with that of southern Peru and may be also interpreted as an effect of compensated high topography. In this model the vertical stress σzz increases with the topography and σHmax is considered as fairly constant, trending E‐W roughly parallel to the convergence. On both edges of the Andes, tectonics being compressional, σzz is σ3 and σHmax is σ1; in the High Andes, σzz becomes σ1, then σHmax is σ2, allowing σHmin striking N‐S to be σ3. Along the Pacific Coast, tectonics is not compressional as expected, and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep Peru‐Chile trench. However, a conspicuous difference occurs in the Andes of central Peru in respect to the southern Peru Andes: compressional strike‐slip tectonics is observed in the Eastern Cordillera. Indeed, the mean elevation of the Eastern Cordillera is 3.7 km, whereas that of the Western Cordillera is 4.2 km, allowing an intermediate state of stress to occur. Therefore in central Peru a lower mean Andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern Peru. Superficial deformations related to the subduction of the aseismic Nazca ridge are restricted to the coastal domain opposite to this ridge. They support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting.