QUATERNARY TEPHROCHRONOLOGY HELPS TO DEFINE CONDITIONING FACTORS AND TRIGGERING MECHANISMS OF ROCK AVALANCHES IN NW ARGENTINA

Microprobe analyses of glass shards from tephra were used to define 9 Quaternary tephra layers in NW Argentina which were found in stratigraphic association with rock-avalanche deposits (Hermanns et al., 2000). In recent years, 3 new tephras were characterized by microprobe analyses of glass shards. In addition, XRF analyses of two tephras (the Alemanía and the Buey Muerto ashes), which have identical glass chemistry and lie in several outcrops only a few dm stratigraphically apart, were performed. These analyses show that both tephras have identical composition but were deposited a few hundred years apart. The ages of some of these tephra described earlier were better defined by 40Ar/39Ar ages, 14C dates of organic material, or 10Be surface exposure ages of landslides in stratigraphic association with these tephra (Hermanns, et al., 2000). The tephra layers have been used to constrain the temporal distribution of rock avalanches in NW Argentina. This distribution shows that these large collapses concentrated in deeply incised valleys during climate periods characterized by enhanced precipitation and run-off (Hermanns et al., 2000). In addition they could also be related to paleoseimic activity. In the Tonco valley, four landslides, one with a 10Be surface exposure age of 7,820 +/- 830 a, lies in direct contact on top of the un-redeposited Paranilla ash, showing the coeval age of these landslides. If the landslide had been triggered by rainfall, it is suggested that this same rainfall would have redistributed the ash layer before collapse. Therefore it is presumed a seismic trigger led to the collapse of several millions of cubic meters of bedrock. Nearby soft sediment deformation structures within lake deposits with a new 14C age of 7,500 +/- 70 a ‘BP CAL' support this interpretation.

Hermanns, R.L., et al., 2000, Tephrochronologic constraints on temporal distribution of large landslides in NW Argentina, Journal of Geology, v. 108, p. 35-52.