RECURRENCE OF EXPLOSIVE ERUPTION IN THE ANDEAN FORELAND AT 42º SOUTH

SALANI, FLAVIA M.; LURO, SILVINA E.

Jogyakarta

Congreso; Cities on Volcanoes 8; 2014

IAVCEI

In Eastern Patagonia at 42° south, the Andean foreland is characterized by a number of Paleocene and Neogene volcanic structures related to the extensional environment of North Patagonian Massif. Western and nearest to the orogenic front the region between 68º and 69º W, show a sequence of recurrent explosive events with activity starting during the Triassic with a small volume of rhyolitic and rhyodacitic ignimbrites and during the Jurassic with ignimbrites and andesitic breccias. On top of this setting two less voluminous but better preserved Cenozoic events are noticeable: The Paleocene Catan Lil eruptive center that generated rhyodacitic to andesitic pyroclastic flows, the Catan Lil Ignimbrite, with 21 km of length and up to 15 m in thickness. It is a crystal rich type thought to represent moderately explosive eruptions. During the Miocene the Rucu Luan Ignimbrites and surges were the initial explosive event linked partially to the opening of a caldera, followed by the extrusion of acidic lavas that evolved later to basaltic compositions. The pyroclastic sequence has thicknesses of up to 60 meters and reaches 15 km in length. There seems to be a general cycle starting with acid explosive sequences being followed by less evolved compositions with relatively minor explosivity. This pattern applies both to the Mesozoic and Cenozoic events. The recurrence ratios and causes for the variation of eruptive types are under consideration. The nearest villages, Gastre and other minor settlements, have 1600 permanent inhabitants within the reach of the Catan Lil Center. In the area there is going to start to operate one of the largest silver deposits in the world. Although the reactivation possibility for the volcanism is remote proper consideration has to be given to all accounts in order to build accurate volcanic hazard maps.