Morphotectonic segmentation of an active forearc, 37°–41°S, Chile

Abstract

Active forearc regions are characterized by seismo- and morphotectonic segments that record recurrent seismic activity and subduction-related tectonic deformation on different timescales. However, it is not well known on which timescales morphotectonic forearc segments retain their tectono-geomorphic identity, how they are related to earthquake-rupture zones, and what governs their long-term evolution. The forearc of the active convergent margin of south-central Chile (37°–41°S) is located within the rupture zone of the 1960 Chile M w 9.6 megathrust earthquake. In this region, we combine geomorphological, sedimentologic, and morphometric DEM analysis with compiled structural and geophysical data to reconstruct the morphotectonic evolution of the forearc. The southern Chile forearc comprises three segments (Nahuelbuta, Toltén and Bueno segments) that record a differential structural and geomorphic evolution. Whereas the Toltén segment appears to be quasi-stable, the other two segments record pronounced ongoing Quaternary uplift. The Nahuelbuta and Bueno segments contain the N–S oriented Coastal Cordillera, characterized by numerous tectonically controlled drainage anomalies and evidence of sediment rerouting. The morphotectonic segmentation appears to have persisted over timescales of 10 5 to 10 6 years. Importantly, these segments do not correlate with earthquake-rupture zones that exhibit a short-term variability over 10 2 to 10 3 years. Thus, earthquake-rupture zones are transient, second-order phenomena, which do not determine the long-term morphotectonic forearc segmentation. Overall, the differential forearc evolution and segmentation is influenced by a complex interplay of inherited upper-plate structures and physical properties of the subducting oceanic Nazca plate.