IANIGLA   20881
INSTITUTO ARGENTINO DE NIVOLOGIA, GLACIOLOGIA Y CIENCIAS AMBIENTALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Inferences of Late Pliocene-Pleistocene rock avalanches clustering in the Central Andes (32º LS).
Autor/es:
MOREIRAS S.M.
Lugar:
Göttingen. Alemania
Reunión:
Congreso; LateinAmerika Kolloquuim; 2009
Resumen:
In the Argentinean Central Andes (at 32° 30´ S - 69° 20´ W) preserved huge Late Pliocene- Middle Pleistocene rock avalanches let to know about the behaviour of this extreme events, as well as, evidence the regional neotectonic activity. The Tigre Dormido rock avalanche (TD) mobilised 1.7x109 m3 of debris material with a velocity of 100 m/s (H/D 1.55). It ran off 10 km from the source area reaching the Mendoza river valley being dammed up and descended topographically 2,000 meters (H/D=0.22). Stratigraphic studies reveal the TD deposit, as well as, fine paleo-lake deposits were eroded by an outwash. Based on relative dating techniques (soil development, rock varnish development on blocks, weathering degree measured on dacite blocks) and topographic relations this drift was correlated with the Uspallata Glaciation. This cold period was assigned to at least Early Middle Pleistocene age as the corresponding terminal moraine is younger than an ash level dated by fission track 360±70 Ky (Espizúa, 1993). The Tigre Dormido rock avalanche (TD) mobilised 1.7x109 m3 of debris material with a velocity of 100 m/s (H/D 1.55). It ran off 10 km from the source area reaching the Mendoza river valley being dammed up and descended topographically 2,000 meters (H/D=0.22). Stratigraphic studies reveal the TD deposit, as well as, fine paleo-lake deposits were eroded by an outwash. Based on relative dating techniques (soil development, rock varnish development on blocks, weathering degree measured on dacite blocks) and topographic relations this drift was correlated with the Uspallata Glaciation. This cold period was assigned to at least Early Middle Pleistocene age as the corresponding terminal moraine is younger than an ash level dated by fission track 360±70 Ky (Espizúa, 1993). The Placetas Amarillas rock avalanche (PA) covers an area of 7 km2 with an estimated volume of 1.6x109 m3. This event also shows two separate episodes and the ending debris flow ran 10 km (H/D=0.11) descending 2000 m topographically (Moreiras, 2005d, 2006b). This extraordinary event also generated a paleo-lake evidenced by a 38 m thick lacustrine sequence where layers of sands and silts are intercalated. Lower sediments are rich in organic matter, while relict diatomite levels are observed on the top of the lake sequence. Diatomite deposits, rich in carbonate and with a high content of volcanic glass, were analysed determining cosmopolitan species that do not indicate climate conditions (Moreiras, 2004b, 2005d, 2006b). Stratigraphically, this rock avalanche is overlain by three ash layers interbedded in alluvial fans. The middle level was dated by Ar39/Ar40 method 350±80 Ky. Whereas, the Piedras Blancas rock avalanche (PB) had a volume ~9.6x108 m3 and an H/D relation equal to 0.2. Dammed paleo-lake generated by this event reached 30 m thick sequence of fine sediments with a maximum level of 2677 m a.s.l. that covered ~0.75 km2. Three greyish white volcanic deposits interbedded in alluvial fans overlaid, as well, the PB deposit and associated paleo-lake sequence. The middle ash level could be correlated by geochemistry analyses with the previously dated 350±80 Ky located 4 km far (Moreiras, 2006c). Besides, teeth material of an old horse was found in the impounded paleo-lake related to the PB. The mammal specie was identified as Hippidion devillei that lived in South America from Upper Pliocene to Upper Pleistocene when it then became extinct (Cerdeño et al, 2009). Due to clustering of these extraordinary paleo-events and according to involved rock avalanche volumes, a simultaneous seismic triggering mechanism with magnitude >6 is proposed. Field observations match with this assumption. Although hesitated liquefaction structures were recognised in the lake sequence related to PB, disconnected lake outcroppings appear strongly folded. Hence, this work’s findings extend the paleo-seismicity of this region being affected historically by Ms>7 earthquakes linked to the Nazca Plate flat-slab subduction at this latitude. Thus, Quaternary activity of regional faults is suggested as generally liquefaction features and folds are associated with nearby seismic source. Nevertheless, forcing paleo-climate conditions are not underestimated. Even through evidences of local warm period is lacking, rock avalanches are previous to a glacial period and the existence of paleo-lakes suggests a humid period. Thus, horse’s rests may implicate that this areas were non-glaciated during this period at this altitude (2650 m a.s.l.). Future effort in numerical dating and regional paleo-climate proxies are required. Hence, this work’s findings extend the paleo-seismicity of this region being affected historically by Ms>7 earthquakes linked to the Nazca Plate flat-slab subduction at this latitude. Thus, Quaternary activity of regional faults is suggested as generally liquefaction features and folds are associated with nearby seismic source. Nevertheless, forcing paleo-climate conditions are not underestimated. Even through evidences of local warm period is lacking, rock avalanches are previous to a glacial period and the existence of paleo-lakes suggests a humid period. Thus, horse’s rests may implicate that this areas were non-glaciated during this period at this altitude (2650 m a.s.l.). Future effort in numerical dating and regional paleo-climate proxies are required.
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