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

Landsat thematic mapper (TM) analysis, aerial photograph interpretation, an d field studies of the semiarid Puna Plateau, adjacent Eastern Cordillera, and Sierras Pampeanas of Argentina (lat 24 degrees-28 degrees S) have revea led the existence of at least 55 rock-avalanche deposits with volumes large r than 10(6) m(3) that formed by the collapse of entire mountain fronts. Th e spatial distribution of landslide deposits is not random, but it clusters along mountain fronts bounded by active faults. Inspection in the field re veals five principal controls on the distribution of these events. The sour ce area of the rock avalanches has two topographic constraints: (1) vertica l relief contrasts between the breakaway zone and the mountain front must e xceed 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. Stru ctural controls are very important. Rock avalanches are controlled by plana r 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 reactiv ation of former transfer faults with strike-slip kinematics, The trigger me chanism 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 corres pond to a more humid interval in southern South America.