Geological constraints on the emplacement mechanism of the Parinacota debris avalanche, northern Chile

The Holocene Parinacota Volcanic Debris Avalanche (ca. 8,000 years B.P.) is located in the central Andes of northern Chile. The avalanche formed by the sector collapse of a major stratovolcano adjacent to a lake basin in a single, catastrophic event. The deposit has an estimated volume of ca. 6 km(3), a run-out of over 22 km. and covers more than 140 km(2) of the surrounding terrain. The values of the Heim coefficient (approximate to0.08) and the ratio A/V-2/3 = (ca. 50), where A is the area covered and V the volume of the deposit, indicate high mobility of the avalanche debris in transport. Two avalanche units can be distinguished. The lower unit consists mainly of blocks of rhyodacitic lavas and domes and pyroclastic flow deposits, and glacial, fluvial and lacustrine sediments. The upper unit consists of a coarse-grained breccia with little matrix, largely composed of andesite blocks, which are angular with little or no rounding by abrasion. The avalanche displays pronounced hummocky topography, in which hummock volume and amplitude, as well as maximum block size within individual hummocks, tends to decrease with transport distance and towards the lateral margins of the avalanche deposit. Some surfaces of individual breccia blocks are covered by tens to thousands of small-scale impact marks, indicating that neighbouring blocks were vibrating and colliding without significant shearing motion. Most of the deformation and shearing was, instead, accommodated in a basal layer of wet, structureless sediments incorporated into the avalanche debris from the inundated lake basin. We propose that the ancestral Parinacota stratovolcano collapsed because of loading of underlying fluvioglacial and lacustrine sediments. The edifice disintegrated during collapse along existing fractures into large rock domains (volumes from 10 to greater than 1x10(6) m(3)), which were transported with little internal deformation, and then fragmented into hummocks of breccia as they were deposited. The decrease of hummock volume with distance suggests that material that travelled further broke up and had an initial greater kinetic energy.