PLIO-PLEISTOCENE UPLIFT OF WESTERN CORDILLERA PRINCIPAL FROM SPATIAL AND TEMPORAL DISTRIBUTION OF LARGE ROCKSLIDES, SOUTHERN CENTRAL ANDES OF CHILE

The Southern Central Andes (SCA, 32-35° S) are transitional between flat slab and steep subduction zones. This section of the Andes also records a north-south transition in terms of climate, relief and elevation of the orogen.

Large rockslides, common in mountain areas with high relief and structural and lithological discontinuities, have been hypothesized as limiting relief generation in areas of active tectonics. The SCA are an ideal area to test this idea. An inventory of large rockslides for the Chilean side of the SCA is presented and compared with existing lithological, structural and shallow seismicity data. The inventory was extensively field-checked and a relative chronosequence was developed, allowing a regional assessment of the temporal and spatial distribution of rockslides.

More than 500 landslides were found, spanning an age range between Pliocene to Recent. Landslides have a spatially heterogeneous distribution, and appear clustered along major structural features rather than distributed evenly. Most of the landslides appear in the Western Cordillera Principal (WCP). Younger landslides (Late Pleistocene-Holocene) dominate the northwestern (Upper Choapa, Rocin rivers) and eastern areas (Yeso, Maipo rivers), whereas the oldest ones (Pliocene-Middle Pleistocene) appear on the western and southwestern areas (mountain front of Santiago and Rancagua). There is a descent in the amount of landslides towards the south of the study area, suggesting either that conditions for large rockslides are not prevailing there since at least Middle Pleistocene times or that an increase in fluvial denudation due to increased precipitation towards the south is eroding landslide deposits faster. The eastern area shows a relationship between shallow seismicity, clustering of relatively young landslides, and presence of two major faults bounding by the east the WCP.

The spatial and temporal distribution suggests continuous uplift of WCP during Pleistocene to Holocene times along the Pocuro-San Ramon reverse fault system, that separates WCP from the Central Depression on the western side, and the Diablo-Laguna Negra fault system, that bounds the WCP from the Aconcagua fold-and-thrust belt, on the east. The uplift has been compensated by incision of major rivers and, in turn, large-scale landsliding.