SEDIMENT FLUX FROM LANDSLIDE INVENTORIES AND SUSPENDED SEDIMENT RECORDS IN THE SOUTHERN CENTRAL ANDES: SIGNATURE OF A TECTONICALLY ENABLED, CLIMATICALLY DRIVEN, TRANSPORT-LIMITED SYSTEM

The spatial distribution of large (>0.1 km²) Pliocene to recent landslides in the Chilean Cordillera Principal (32-34.5°S), Southern Central Andes, was compared to the distribution of lithology, topography, large scale structures, and shallow seismicity. Variographic analysis showed spatial correlation between landslides, geological structures and shallow seismicity. A relative chronosequence of the landslides was calibrated and combined with the use of statistical relationships for landslide area distributions and seismically-triggered landslide populations to provide a maximum sediment flux from landslides. Long-term equivalent denudation rates range between 0.7 (Late Pleistocene to recent) to 1.8 mm/a (Holocene to recent), while short term (30 a) estimates from seismicity records are 0.7 mm/a.

Three rivers draining from the Andes to the Pacific (Maipo, Aconcagua, Cachapoal, with similar catchment areas) were analyzed for sediment transport using records from the Water Agency for up to 30 years of observations. The flux derived from analysis of suspended sediment data, including estimates for total bedload transport, does not exceed 0.6 mm/a of equivalent denudation. Although depending on the assumptions made for estimation of bedload contribution, this result suggests that most sediment transfer is enabled by tectonically-generated slides, and that streams mostly remobilize sediment, instead of eroding through bedrock. The comparison suggests that the system is currently transport–limited, a condition that is reflected in field observations of streams of order >1. The transport-limited condition is consistent with an increased sediment transport during high flow conditions, particularly pluvial and snowmelt events during ENSO years.

The larger Holocene estimate for landslide-derived flux is partly attributed to the slope-destabilizing effect of glaciation in the high (2000-6500 m a.s.l.) altitude areas. Higher sediment output coupled with increased stream power can account for the voluminous post-glacial fan and terrace deposits that are being incised now in the adjacent Aconcagua, Santiago and Rancagua basins.