TECTONIC IMPLICATIONS OF LATE CENOZOIC SEDIMENTATION FROM THE COASTAL CORDILLERA OF NORTHERN CHILE (22-24-DEGREES-S)

Late Cenozoic sediments from northern Chile (22-24 degrees S) are exposed on the coastal plain between the Coastal Cordillera to the east and tilted fault blocks of the Mejillones Peninsula to the west. During the mid-Miocene to Pliocene (?up to mid-Pleistocene) a shallow marine basin developed unconformably over basement. Sedimentation was initially restricted to a small half-graben on the western margin of the Mejillones Peninsula during the Miocene. Expansion of the basin during the early Pliocene resulted in widespread shallow marine sedimentation across the study area. Alluvial, aeolian and beach sediments were restricted to the basin margins, where sediment was supplied from the Coastal Scarp (a major cliff-line bounding the western margin of the Coastal Cordillera) and isolated (islands) fault blocks. Areas of restricted clastic input were characterized by carbonate deposition. Marine planation surfaces or terraces (0.5-600m elevation) and associated palaeo-sea cliffs cut into Miocene-Pliocene sediments and basement rocks, developed along the coastline of northern Chile in the ?late Pliocene to late Pleistocene. A similar age for a number of late Pleistocene terraces (125 000 years) now at different elevations, suggests that they were cut during interglacial highstands (oxygen isotope stage 5). Variations in terrace elevation are attributed to faulting. Regional scale uplift (over hundreds of kilometres) of shallow marine sediments beneath an extensive pre-mid Pleistocene shoreline is considered to be related to variations in subduction zone geodynamics. The favoured hypothesis is that of aseismic ridge subduction.