Pliocene lahar deposits in the Coastal Cordillera of central Chile: Implications for uplift, avalanche deposits, and porphyry copper systems in the Main Andean Cordillera

Abstract

Lahar deposits occur within a shallow marine sedimentary succession of the Pliocene La Cueva Formation in the Coastal Cordillera of central Chile (33 degrees 40'-34 degrees 15'S). Provenance studies of the abundant volcanic material in the lahar deposits suggest that they derive from denudation by mass wasting of Oligocene-Miocene volcanic rocks on the western slopes of the Main Andean Cordillera at the same latitude. Pliocene rock debris deposits preserved in the region of El Teniente (similar to 34 degrees S) and scattered along the westernmost part of the Andes of central Chile indicate catastrophic erosive events related to the rapid uplift of the cordilleran block. This rock debris was deposited by avalanches and transformed further downslope into lahars by dilution with stream water. Lahars were channeled along the ancient drainage system that reached a shallow Pliocene sea at the site of the present Coastal Cordillera. The exceedingly rapid exhumation of active porphyry systems during the Early Pliocene in this part of the Andes may have played a role in affecting hydrothermal processes, brecciation, and diatreme, formation at the porphyry systems of El Teniente and Rio Blanco-Los Bronces.