LATE CENOZOIC GEOMORPHOLOGIC SIGNALS OF ANDES DEFORMATION IN A UPLIFTING AND CLIMATIC CHANGES CONTEXT (26°S-28°S)

We consider remarkable examples of large (~10000 km2) and local-scale (~100 km2) landscape forms related to Neogene geomorphologic configuration of the forearc region of the Southern Atacama Andes. The landscape forms and the large-scale sedimentological variations of the Atacama Gravels are studied in order to reconstruct the Neogene geomorphologic evolution of the forearc region. The main physiographic units of the landscape are formed before the Neogene and most of the present-day altitude of the Precordillera was reached before that time. Five episodes of Neogene geomorphologic evolution can be differentiated: (1) the development of an deep incised drainage system cutting the uplifted Precordillera (up to 2000 meters of vertical incision) and crossing the entire forearc; (2) the infills of the deep incised valleys by up to 400 m of Atacama Gravels deposits. This infill started in the Early Miocene with the development of fluvial depositions and finished in the Middle Miocene with flood plain and playa related depositions. We propose that playa related deposition occurs in a endorreic context related to tectonic activity of the Atacama Fault System and Coastal Cordillera uplift. However, the upward sedimentologic variation in the Atacama Gravels evidences a progressive aridification of the climate. (3) Reopening of the valley network to the Pacific Ocean in the Middle Miocene can be related to base level changes due to the tectonic activity, and (4) deposition in the Upper Miocene of up to 200 meters of alluvial Atacama Gravel in the Central Depression in response to the increasing of the E-W regional slope. The tilting of the inner forearc region during the Upper Miocene produce some hundred of meters of uplift accommodated in the Precordillera for the last 10 Ma. (5) In response to this tilting, a new vertical incision (up to 800 meters in the Precordillera) allows the development of the present-day canyon that crosses the forearc. We propose that climate aridification is an important factor controlling the Early-Middle Miocene landscape evolution. We have identified the effects of the Middle-Upper Miocene slow tectonic deformation: the Neogene Andean uplift is accommodated in the forearc region by a tilting of the inner forearc, but a generalizated relative uplift of the entire forearc region must by also considered.