COASTAL DEFORMATION AND SEA-LEVEL CHANGES IN THE NORTHERN CHILE SUBDUCTION AREA (23-DEGREES-S) DURING THE LAST 330 KY

The Nazca-South American plate boundary is a subduction zone where a r elatively complex pattern of vertical deformation can be inferred from the study of emerged marine terraces. Along the coasts of southern Pe ru and northern Chile, the vertical distribution of remnants of Pleist ocene terraces suggests that a crustal, large scale uplift motion is c ombined with more regional/local tectonic processes. In northern Chile , the area of Hornitos (23 degrees S) offers a remarkable sequence of well-defined marine terraces that may be dated through U-series and am ino-stratigraphic studies on mollusc shells. The unusual preservation of the landforms and of the shell material, which enabled the age dete rmination of the deposits, is largely due to the lengthy history of ex treme aridity in this area. The exceptional record of late Middle Plei stocene to Late Pleistocene high seastands is also favoured by the sli ght warping of two distinct fault blocks that have enhanced the morpho stratigraphic relationships between the distinct coastal units. Detail ed geomorphological, sedimentological and chronostratigraphic studies of the Hornitos area led to the identification, with reasonable confid ence, of the depositional remnants of sea-level maxima coeval with the Oxygen Isotope Substages 5c, 5e, 7 (probably two episodes) and the is otope stage 9 (series of beach ridges). The coastal plain, at the foot of the major Coastal Escarpment of northern Chile, appears to have be en 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 shoreli nes, it can be inferred that these steady vertical motions were much m ore rapid than during the Early Pleistocene. Copyright (C) 1996 Elsevi er Science Ltd