Coastal deformation and sea-level changes in the northern Chile subduction area (23°S) during the last 330 ky

Abstract

The Nazca-South American plate boundary is a subduction zone where a relatively complex pattern of vertical deformation can be inferred from the study of emerged marine terraces. Along the coasts of southern Peru and northern Chile, the vertical distribution of remnants of Pleistocene terraces suggests that a crustal, large scale uplift motion is combined with more regional/local tectonic processes. In northern Chile, the area of Hornitos (23°S) offers a remarkable sequence of well-defined marine terraces that may be dated through U-series and aminostratigraphic studies on mollusc shells. The unusual preservation of the landforms and of the shell material, which enabled the age determination of the deposits, is largely due to the lengthy history of extreme aridity in this area. The exceptional record of late Middle Pleistocene to Late Pleistocene high seastands is also favoured by the slight warping of two distinct fault blocks that have enhanced the morphostratigraphic relationships between the distinct coastal units. Detailed geomorphological, sedimentological and chronostratigraphic studies of the Hornitos area led to the identification, with reasonable confidence, of the depositional remnants of sea-level maxima coeval with the Oxygen Isotope Substages 5c, 5e, 7 (probably two episodes) and the isotope stage 9 (series of beach ridges). The coastal plain, at the foot of the major Coastal Escarpment of northern Chile, appears to have been uplifted at a mean rate of 240 mm/ky in the course of the last 330 ky. From the elevation of the older terraces and late Pliocene shorelines, it can be inferred that these steady vertical motions were much more rapid than during the Early Pleistocene.