Tephrochronologic constraints on temporal distribution of large landslidesin northwest Argentina

Two morphologic settings in the northwestern Argentine prone to giant mount ain-front collapse-deeply incised narrow valleys and steep range fronts bor dering broad piedmonts-were analyzed through detailed investigations of fos sil landslides and related fluvio-lacustrine sediments. Nine different rhyo dactic tephra layers were defined by geochemical fingerprinting of glass, m orphology of pumice, stratigraphic relationships, and mineralogy. The age o f three tephra could be determined either directly by Ar-40/Ar-39 dating or relatively by C-14 dating of associated sediments: Paranilla Ash (723 +/- 89 ka), Quebrada del Tonco Ash (similar to 30 ka), and Alemania Ash (simila r to 3.7 ka). These units permit correlation of several spatially separate landslide deposits. Landslide deposits in narrow valleys were generated in the late Pleistocene between 40 and 25 Ita and in the Holocene since ca. 5 ka and correspond to periods characterized by increased humidity in subtrop ical South America. In contrast, the age of large landslides in piedmont re gions is significantly greater but moro difficult to define by tephrochrono logy. However, selected deposits from this second environment have cosmogen ic nuclide exposure ages of 140-400 ka. Because of the large distance of th e collapsed mountain fronts from eroding streams and because of important Q uaternary displacement along the mountain-bounding faults, we suggest that strong, low-frequency seismic activity is the most likely trigger mechanism for most of the landslides in this environment.