EVIDENCE OF VOLCANO-ICE INTERACTION IN THE EVOLUTION OF THE COPAHUE VOLCANO (ARGENTINA-CHILE)

BÁEZ, ALEJANDRO D.; BÁEZ, WALTER A.; SOMMER, CARLOS A.; CASELLI, ALBERTO T.

Cuiabá, Matto Grosso

Simposio; VIII Simpósio de Vulcanismo e Ambientes Associados; 2022

Sociedade Brasileira de Geologia

Interactions between volcanic activity and ice in all its forms, including meltwater derived from volcanic heating (glaciovolcanism), produce distinctive diagnostic features including landforms, lithofacies, and fracture patterns in lavas. Evidence of this type of interaction is a useful paleoclimate proxy to establish the temporal and spatial distribution of glaciers in the past. For this reason, in recent years many researchers have put emphasis on glaciovolcanism around the world, including volcanoes in mid-latitude continental arcs. The Copahue volcano is an active volcano of intermediate composition located in the central sector of the Andean Southern Volcanic Zone, about 30 km east of the main arc. The beginning of its activity is classically attributed to the lower Pleistocene, although more recent dating indicates an upper Pleistocene age. Previous stratigraphic schemes divided the evolution of the volcano into three stages: pre-, sin-, and post-glacial. However, the evidence of glaciovolcanism in most of the stratigraphic sequence that constitutes the Copahue volcano, makes these stratigraphic criteria ineffective in correctly representing its evolution, which would be strongly influenced by the glacial advances of the upper Pleistocene. Consequently, a new stratigraphic scheme is proposed based on the recognition of constructive and destructive main phases and adopted a basic non-genetic nomenclature. According to this, the evolution of the Copahue volcano can be divided into two main sequences separated by a major erosive phase. The Ancient Sequence (S1; units A-C) consists of an initial subaerial effusivestage followed by a major glaciovolcanic stage linked to a thick ice sheet. The Young Sequence (S2; units D-I) consists of mainly effusive activity during periods of glacial advance and retreat recorded by an alternation of ice-confined lavas associated with volcaniclastic deposits and unconfined subaerial lavas. The products generated during the historical activity of the volcano are also included in the S2. Although in previous works there are mentions of glaciovolcanism in the Copahue volcano, the influence of glaciations in its evolution has been scarcely analyzed. New field observations on the eastern flank of this volcano are presented here, allowing a better understanding regarding the mechanisms and processes involved in the interaction of volcanism with glacial systems. The glaciovolcanic successions located at the volcano include volcanic activity beneath an englacial lake associated with the development of a lava-fed delta, ice-confined lavas associated with volcaniclastic deposits generated by syn-eruptive sub-glacial meltwater streams, glaciovolcanic sheet-like sequences and subglacial lavas. This evidence indicates two glaciations in the area during the late Pleistocene, a main one characterized by a thick ice sheet (>300 m), and a second probably less extensive. However, new geochronological data are necessary to establish more precisely the glaciations recorded in the evolution of the Copahuevolcano.