QUATERNARY PHREATO-MAGMATIC VOLCANIC PROCESSES IN SOUTHERNMOST SOUTH AMERICA

In southernmost South America along the northern margin of the Strati of Magellan there are numerous Quaternary volcanic centers indicating major magmatic activity. The predominant magmatic features are maars which are considered typical indicators of phreato-magmatic volcanism. These maars are mainly located in a 150 km zone that extends from the Patagonian Cordillera to the Atlantic Ocean. Within this zone, the westernmost maars are older, as determined by absolute and relative age data. The generation of this magmatism is thought to be related to the close interaction of several geologic processes acting simultaneously. The deposition of a thick Cretaceous-Tertiary stratigraphic column originated the Austral Basin which in turn provoked a thinning of the continental crust. The thin crust facilitated the penetration of the upper mantle magma directly into the fracture zones without significant petrographic changes. The fractures of the continental crust, predominantly oriented NW – SE, were formed as a consequence of continuous and recurrent penetration of the oceanic Pacific crust into the continental Atlantic crust in southern South America. This plate interaction provoked intense and continuous regional tectonic activity characterized by orogen-parallel, transcurrent left lateral thrust faults. The magma then ascended through the openings provided by the orthogonal extensional fractures. Extensive glacial coverage to the west contributed the water that fed the phreatic levels. The volcanic activity was also affected by the uplift of the continental crust associated with glacio-isostatic adjustment that resulted from the loss of mass from the receding glaciers. The phreatic levels increased in depth as the crust rebounded over time. The sizes of the maars appear to be associated with the depths at which the magma encountered the phreatic levels. This relationship is confirmed as the maars with the larger diameter are the younger ones.