Late Cenozoic geology of La Paz, Bolivia, and its relation to landslide activity

La Paz lies in a deeply incised valley on the Bolivian Altiplano. It has experienced frequent damaging historic landslides and numerous, much larger, prehistoric landslides. I documented the Neogene and Pleistocene lithostratigrahic and magnetostratigraphic framework of La Paz, produced an inventory of recent (1995-2014) landslides, and characterized ongoing (2008-2011) slow ground motion using radar interferometry (InSAR). The upper part of the sediment sequence beneath the Altiplano is glacial in origin and fines distally away from the Cordillera Real. It records at least 15 late Pliocene and Early Pleistocene glaciations, most of which predate the oldest known North American continental glaciation. The plateau surface formed by ca. 1.0 Ma, but most likely before ca. 1.8 Ma. After that the headwaters of the Amazon River extending westward through the Cordillera Real incised the underlying sediments. The poorly lithified fill sequence is exposed in steep slopes, promoting instability. Between 1995 and 2014, La Paz experienced 43 discrete landslides and slow ongoing landslides at 13 additional locations. Landslides were most frequent late in the rainy season and generally happened after particularly wet periods weeks in length, indicating a strong hydro-meteorological control. The margins of several landslides coincide with buried culverted streams, indicating that this engineering practice reduced slope stability. InSAR results show that about one-third of slopes in La Paz are moving at rates up to ~20 cm/a. They also identify previously unknown landslides, detect hectare-scale movements of as little as ~0.5 cm/a, and indicate several distinct failure mechanisms. Many recent landslides correspond with large, creeping paleolandslide deposits, indicating that the reduced residual strength and modern activity of these deposits influences the localization of recent failures. My findings highlight aspects of slope instability in La Paz that can be used to reduce risk. Future failures are most likely to happen in previously displaced fine-grained sediments, particularly the slowly moving paleolandslides south and east of the city centre. Several key areas require detailed ground-based monitoring, particularly during the rainy season when cumulative precipitation thresholds are exceeded. The practice of burying river channels should be re-assessed, and a survey of existing culverted channels conducted.