Abstract

Ollagüe is an active stratovolcano located in the Central Andes of Northern Chile, which evolved in three stages from Middle Pleistocene (ca. 1.2 Ma) to Upper Pleistocene (ca. 150 ka), and partially collapsed to the west at ca. 600-400 ? ka. The collapse generated a well-preserved debris avalanche deposit on its western fl ank which partially inundated the Carcote and Ollagüe Salar basins at minimum velocities in distal areas ranging between 20 and 35 ms-1. The deposit shows a characteristic hummocky morphology with a general tendency of hummock volume to decrease with distance. The deposit can be divided into several distinct facies according mainly to hummock morphology, composition and internal structures. Large and high-slope hummocks are mainly formed of a breccia with predominant lava fragments, whereas small-volume and low-angle hummocks consist essentially of pyroclastic deposits and/or saline sediments. The remaining hummocks (of limestones, gypsum, etc.) were partially incorporated by the avalanche during its transport and/or deposition, forming 30 percent by volume of the avalanche deposit. In initial stages the rock mass slid on top of unconsolidated alluvial-pyroclastic deposits, which reduced basal friction, thus delaying deposition until as far as 9 km from origin. As the fl ow continued, inundating the Salar de Carcote basin, it started to incorporate ductile material from the underlying units, mainly saline, alluvial and pyroclastic deposits. These deposits were partially deformed during avalanche emplacement, generating folds and faults. More consolidated substrata units such as limestone layers were also partially incorporated, folded and/or faulted during avalanche emplacement. Unconsolidated pyroclastic and saline deposits were injected into new open-block fractures at late stages of emplacement, partially infi lling the volcanic breccia. Thickness of the deformed substrata units, as well as the density of folding and faulting, increased with fl ow, showing that the deformation front migrated towards the western edge of the avalanche fl ow, driven by the large rock mass coming from the volcano.