Structural and lithological controls on large Quaternary rock avalanches (sturzstroms) in arid northwestern Argentina

Landsat thematic mapper (TM) analysis, aerial photograph interpretation, and field studies of the semiarid Puna Plateau, adjacent Eastern Cordillera, and Sierras Pampeanas of Argentina (lat 24 degrees-28 degrees S) have revealed the existence of at least 55 rock-avalanche deposits with volumes larger than 10(6) m(3) that formed by the collapse of entire mountain fronts. The spatial distribution of landslide deposits is not random, but it clusters along mountain fronts bounded by active faults. Inspection in the field reveals five principal controls on the distribution of these events. The source area of the rock avalanches has two topographic constraints: (1) vertical relief contrasts between the breakaway zone and the mountain front must exceed a threshold of 400 m, and (2) the slope inclinations must be steeper than 20 degrees, Rock avalanches are restricted to three types of lithology: granites, low-grade metamorphic rocks, and coarse elastic sediments. Structural controls are very important. Rock avalanches are controlled by planar structures such as bedding planes, exfoliation joints, minor faults, and cleavage that all dip toward the valley. In addition, major slide clusters occur along mountain fronts that underwent Quaternary reverse-fault reactivation of former transfer faults with strike-slip kinematics, The trigger mechanism for the majority of these landslides is interpreted to be seismic, although the ages of some major slides are about 30 ka, and they may correspond to a more humid interval in southern South America.