Quaternary tectonic and recent seismic crustal movements in the arauco peninsula and its environs, central chile

An outer part of the outer arc in the arc-trench system is a zone where great earthquakes have occurred periodically and remarkable crustal movements have been recorded both at the great earthquake and at the inter-earthquake times. For comparing these short-term historical crustal movements with long-term Quaternary ones, the Arauco Peninsula in the Pacific coast of central Chile is one of the most suitable land on the earth to be studied. This is the reason why we investigated the Quaternary crustal movements in and near the Arauco Peninsula by means of geomorphology and Quaternary stratigraphy. The Arauco Peninsula, which lies in the continental shelf zone bordering the Coastal Range of central Chile, consists of five gemorphic surfaces: Las Nochas, Buena Esperanza, Cañete, Lower terraces, and Holocene lowland surfaces in a descending order. The former three are marine terraces, and the latter two are partly marine and partly alluvial in origin. The topographic features and surface-making deposits of these surfaces show that each of the surfaces has been built during various periods of marine transgression, probably caused by an interglacial or an interstadial and a post-glacial uplift of sea level. Among these surfaces, the widest Cañete Surface, most probably formed in the last great interglacial or Sangamon Interglacial, shows composite features of two modes of crustal movements: a landward tilting and an upwarping with a NW-SE trending axis. NW-SE trending upwarping of the latter is also indicated by heights of elevated shorelines of the Buena Esperanza and Las Nochas Surfaces, and even by heights of erosional surfaces in the Nahuelbuta Mountains, a part of the Coast Range, lying east of the Arauco Peninsula. The erosional surfaces in the mountains, which were probably built in the late Tertiary, imply that a N-S trending upwarping has taken place during the Quaternary, superposing the above mentioned NW-SE trending one. Two islets, Santa Maria and Mocha in the continental shelf zone, off the northwestern and southwestern coasts of the Arauco Peninsula, consist of marine terraces. These marine terraces show a landward tilting during the late Quaternary. Radiocarbon datings for Holocene terrace deposits in Mocha Island indicate that the mean rate of uplift has been 55 cm per 100 years for the last several thousands years. The mode of landward tilting during the late Quaternary in the shelf zone isjust the same as that of co-earthquake crustal movements in the historical age, and the amount of late Quaternary tilting is most likely to have been formed by the accumulation of repeated co-earthquake tiltings; while the mode of the N-S trending Quaternary upwarping in the Nahuelbuta Mountains is rather reverse to that of co-earthquake movements, and thus the amount of the Quaternary upwarping of the mountains would have been formed by the accumulation of inter-earthquake movements. Based on this, the terms, a zone of co-earthquake deformation and a zone of inter-earthquake deformation are proposed to the shelf zone and the Coast Range respectively.departmental bulletin pape