Geomorphic evidences of recent tectonic activity in the forearc, southern Peru

Abstract

As the Andean forearc is not concentrating as much tectonic shortening as the foreland (since Middle Miocene) and as GPS measurements can not inform on the long-term deformation but rather describe the elastic response of the Andean forearc (Nazca-South American convergence), little is known about the active deformation in the Central Andes Pacific lowlands. However, geomorphic evidences of recent tectonic activity are observed from the Coastal Cordillera to the piedmont of the Western Cordillera. In this paper we analyze this Quaternary tectonic activity in the southern Peruvian forearc, from 17°S to 18°30'S. Examination of aerial photographs and satellite data, and focused field work not only confirms that there is recent tectonic activity but also has revealed the presence of additional active structures that should be taken into account in the description of Andean deformation. In response to active tectonics, these tectonic structures affected very young terraces and Quaternary pediments in the southern Peruvian forearc. We discuss some of the strong geomorphic signatures, such as active fault traces, scarplets, sag ponds, river terraces and some major and minor landslides, which are indicative of active tectonics in this area. Mapping of fault trace geometry and identifying recent surface offsets are used to determine the key places where active tectonics can be involved in the deformation of the forearc, either through normal faulting, strike-slip faults or thrust faults. Among those major tectonic features, some are likely due to seismic crustal activity (along the ongoing Andean tectonic processes) and some to relaxation processes of the stress imposed on the outer forearc area after each major subduction earthquakes.