Quaternary tephrochronology helps define conditioning factors and triggering mechanisms of rock avalanches in NW Argentina

Abstract Ten previously defined Quaternary tephra layers from northwestern Argentina are described here in terms of electron microprobe analyses of glass shards. In addition, XRF analyses of three tephras (Alemania, Buey Muerto and El Paso ash) were performed. The Alemania Ash and Buey Muerto Ash occur within 1 m of each other stratigraphically in several outcrops and exhibit identical glass chemistry. As well, XRF analyses show that both tephras have identical composition. Based on stratigraphic relations and age determinations, these tephras can clearly be separated as two individual layers. Tephra ages were defined relatively by AMS 14 C dates of organic material, by 10 Be surface exposure ages of landslides in stratigraphic association with these tephra layers, or directly by 40 Ar/ 39 Ar ages of mineral separates or fission-track dating of zircons. Ages of these tephra layers constrain the temporal distribution of large landslides (rock avalanche type) in northwestern Argentina. Temporal distribution shows that these large collapses concentrated in deeply incised valleys during climate periods characterized by enhanced precipitation and run-off. In addition, in the Tonco valley, tephra characteristics also suggest earthquake triggering of coeval landsliding. In this valley, four landslides, one with a 10 Be age of 7820±830 yr, lie in direct contact atop un-redeposited Paranilla ash, showing the coeval age of these landslides. The age of the dated landslide deposit coincides with the age of soft sediment deformation structures in nearby lake deposits with an AMS 14 C age of 7430–7570 cal yr BP.