Large slope failures in the La Paz basin, Bolivian Andes

Thursday, 18 December 2014: 8:45 AM
Nicholas J Roberts, Simon Fraser University, Burnaby, BC, Canada, Reginald L Hermanns, Geological Survey of Norway, Trondheim, Norway, Bernhard Rabus, 3vGeomatics, Vancouver, BC, Canada, Marco-Antonio Guzmán, Universidad Mayor de San Andrés, Instituto de Investigaciones Geológicas y del Medio Ambiente, La Paz, Bolivia, Estela Minaya, Observatorio San Calixto, La Paz, Bolivia and John J Clague, Simon Fraser University, Department of Earth Sciences, Burnaby, BC, Canada
The La Paz basin in the eastern Bolivian Andes has been a hotspot for large-scale, deep-seated gravitational slope deformation during the Holocene. In less than 2 Ma, a network of steep-sided valleys up to 800 m deep formed in sediments of the Altiplano Plateau and underlying basement rocks. We characterize the distribution, extent, mechanisms, and modern activity of large-scale failures within this landscape using optical image interpretation, existing geologic maps, synthetic RADAR interferometry (InSAR), and field investigation. Deposits of nearly 20 landslides larger than 100 Mm3 occur within the basin. Most failures have occurred in weakly lithified Late Miocene to Pliocene sedimentary rocks and include earth flows, translational and rotational landslides, and plug flows. Failures in underlying tectonized Paleozoic sedimentary rocks include bedding-parallel rockslides. The largest failure is the 3 km3 Achcocalla earth flow (ca. 11 ka BP), which ran out ~20 km. Other dated events span the period from the early Holocene to nearly the Colonial historic period. InSAR results show that many large slope failures, including the Achocalla earth flow, are currently moving at rates of a few centimeters to a few decimeters per year. Rapid deposition, shallow burial, and rapid incision of the basin fills produced steep slopes in weak geologic materials that, coupled with groundwater discharge from the valley walls, are the primary controls on instability. In contrast, the Altiplano surface has changed little in 2 Ma and the adjacent slopes of the Cordilleran Real, although steep, are relatively stable. Of the over 100 landslides that have occurred in the city of La Paz since the early twentieth century, most are at the margins of large, deep-seated prehistoric failures, and two of the most damaging historic landslides (Hanko-Hanko, 1582; Pampahasi, 2011) were large-scale reactivations of previously failed slopes. Improved understanding of large, deep-seated landslides in the La Paz basin provides insights into evolution of the Central Andes and the geologic contribution to the exceptionally high landslide risk in the modern city of La Paz.