Quaternary normal and reverse faulting and the state of stress in the central Andes of south Peru

Abstract

Field studies in the Andes of southern Peru show that in the High Andes and Pacific Lowlands, Quaternary and Recent faults are normal. This extensional tectonics postdates compressional deformations of Pliocene‐early Quaternary age. In the sub‐Andes the observed deformations are compressional; they affect early Quaternary deposits. Some of the faults separate Quaternary deposits from the bedrock and thus are clearly of tectonic origin and not landslide effects. Striations on the fault planes indicate N–S trending extension in the High Andes and Pacific Lowlands. The total amount of crustal stretching is small, probably of the order of 1% during the last 1–2 m.y. In the sub‐Andes, folds and faults affecting Neogene and early Quaternary deposits result from N–S shortening. Nevertheless, it is supposed that this N‐S shortening is of early quaternary age. The present‐day compression probably strikes E‐W, judging from focal mechanisms in the sub‐Andes of central Peru, southern Bolivia, and northwest Argentina. Data from structural analysis of faults and from earthquake focal mechanisms allow us to surmise the state of stress in the Andes of southern Peru. The High Andes and Pacific Lowlands, subjected to N‐S trending extension, are bounded by two zones of E‐W trending compression: the sub‐Andes to the east, and the contact between the convergent Nazca and South America plates to the west. In our model the maximum horizontal compressive stress trajectory σ Hmax is roughly parallel with the E‐W convergence between the two plates; σ Hmax corresponds to σ 1, in the sub‐Andes and to σ 2 in the High Andes. The latter situation is caused by the elevated mass of the High Andes, where σ zz (the vertical stress) is inferred to be σ 1. Thus the third principal stress axis, being orthogonal to the other two axes, it is oriented N‐S, allowing extension to occur in that direction. On the other hand, in the sub‐Andes σ zz is σ 3, and horizontal E‐W shortening occurs. The state of stress in the Andean continental crust above the 30° dipping slab appears to be different from that in the Andes of Central Peru situated above the flat subducting segment. In this region, compressional deformantion affect a wider part of the Cordillera.