Late Pliocene-Middle Pleistocene huge rock avalanches in the Argentinean Central Andes: its implications.

MOREIRAS S.M.

Oslo, Noruega

Congreso; 33th International Geological Congress; 2008

The preservation of three huge ancient rock avalanches in the Plata range (32° 30´ S - 69° 20´ W) gives the opportunity of enhancing our understanding of these extreme processes. According to geomorphological studies, the Tigre Dormido rock avalanche (TD) transported 1.7x109 m3 of rock debris with a velocity of 100 m/s (H/D 1.55) running 10 km until it reached the Mendoza river valley, which then became dammed up, generating a paleo-lake. Secondly, the Placetas Amarillas rock avalanche (PA) ran 9 km descending 2000 m in altitude and covered ~6.8 km2. This event of 1.6x109 m3 in volume also generated a paleo-lake which is evidenced at present by 32 m-sequence of fine sediments. Lastly, the Piedras Blancas rock avalanche (PB), caused a ~0.7 km2 dammed paleo-lake, had a volume ~9.6x108 m3 and an H/D ratio equal to 0.2. Chrono-stratigraphic techniques used on other Quaternary deposits allow us to establish their Late Pliocene-Middle Pleistocene age. Firstly, the TD rock avalanche deposit is partially covered by a relic outwash related to the Uspallata Glaciation which is assigned to the Middle Pleistocene. Terminal moraine of this glaciation is younger than an ash level dated 360±70 Ky (fission track). Whereas, the PA and PB rock avalanches are overlain by three ash layers interbedded in alluvial fans. A dating of 350±80 Ky was determined for the middle ash level (Ar39/Ar40). In addition, bones of an ancient horse identified as Hippidion devillei were found in the impounded paleo-lake related to the PB rock avalanche. This mammal species lived in South America from the Upper Pliocene to Upper Pleistocene when it then became extinct. In accordance with similar ages obtained for this cluster of rock avalanches and the intensive deformation of the paleo-lake dammed by the PB rock avalanche, where liquefaction features were also observed, a quasi-simultaneous seismic shaking is assumed. Moreover, this shaking should be related to an M>6 earthquake consistent with such mobilised volumes and liquefaction features. This is further supported by historical M>6 earthquakes recorded at this latitude linked to the Nazca Plate flat-slab subduction. Hence, our findings provide further back up for the neotectonic activity of this region and extend the record of high magnitude paleo-earthquakes for the Western Central Argentina region. Furthermore, Quaternary activity of regional faults is suggested as general liquefaction features are associated with a nearby seismic source. Nevertheless, the causal effects of paleo-climate conditions should not be underestimated. Even though evidence of a local warm period is lacking, rock avalanches pre-date the Uspallata glacial period and the existence of paleo-lakes may indicate a wetter period. Thus, the Hippidion deveillei remnants could allude to non-glaciated areas during this period at this altitude (2650 m a.s.l.). Accordingly, further work in numerical dating and regional paleo-climate proxies are required.