THE NEOGENE STRATIGRAPHIC AND PALEOGEOGRAPHIC EVOLUTION OF THE MEJILLONES PENINSULA (NORTHERN CHILE) AND ITS RELATIONSHIP WITH LONG-TERM, SUBDUCTION-RELATED EXTENSIONAL TECTONICS

The Mejillones Peninsula has been undergoing east-west extension since early Miocene. The Miocene to Pliocene extension and the concomitant extension and regional uplift during Pleistocene have been linked to subduction erosion and subcrustal accretion of material removed at the trench, respectively. Extension has resulted in the formation of two halfgraben basins bounded by N-S trending, east-dipping normal faults: the Caleta Herradura-Pampa del Aeropueto basin (CHPA) to the west and the Pampa de Mejillones basin (PA) to the east. Detailed study of lateral and vertical relationships between the various Neogene units and of these with the substrate has enabled us to develop a stratigraphic framework that can be used to reconstruct the history of subsidence and faulting in the Mejillones Peninsula. During the Miocene only the northernmost portion of the CHPA basin was flooded by the sea. This area was a north-facing narrow embayment where the shallow-marine Caleta Herradura Fm was deposited. During early Pliocene, as the rifting continued and the fault segments propagate southward, the sea flooded also the central part of the CHPA basin and the Cerro Bandurrias Mb was laid down. At this time the central part of the PA basin and, likely, the southernmost portion of the CHPA basin were still subaerially exposed. As the fault segments continued to grow throughout the late Pliocene, the PA and CHPA basins were completely flooded and became interconnected. The shallow marine Cuesta del Burro Mb was deposited. Both the Cerro Bandurrias-Cerro Moreno fault block and Morro Mejillones fault block became separated from the mainland. During Pleistocene, because of the regional uplift, both footwall and hangingwall of basin-bounding faults were uplifted. The central portion of the CHPA basin became subaerially exposed, and a series of marine terraces and sets of coeval beach ridges were formed in footwall and hangingwall areas, respectively.