VILAMA CALDERA: ONSET OF LARGE ERUPTIONS DURING IGNIMBRITE FLARE-UP IN PUNA-REGION, AND ITS INDICATIONS OF STRESS FIELD ORIENTATION

The central Andean plateau hosts an extensive back-arc ignimbritic province, erupted from several collapse calderas during 10-2 Ma, immediately after its main stage of uplift. The Vilama ignimbrite marks the onset of large ignimbrite eruptions in the Puna at ~8.4 Ma. The eruption, one of the largest of the world, involved the discharge of 2100 km³ (~1680 km³ DRE) of dacite magma. The related caldera is a subeliptical NW-SE structure (~ azimuth of 290°) of ~35 x 15 km, with central coordinates of 22°24'S - 66°57'W. Caldera collapse is thought to have been symmetric, during which ignimbrites with varying grades of welding were erupted. Resurgence caused bending of part of the intracaldera deposits, building a central dome. Late (postcaldera) eruptions channelled mainly through inferred structural borders. The high magma viscosity (crystal-rich, high-K dacites), and the large erupted volume require an external driving force to initiate and sustain the eruption. That “external force” was the mechanical failure of the roof of the pre-eruptive magma chamber. If we accept that the configuration of the Vilama caldera in map view reflects the shape of the underlying magma chamber, its elongation may be due to: a) stretching of the reservoir under regional WNW extension, or b) control by pre-existing WNW-ESE structures. The location of the Vilama caldera in the ~NW Lípez lineament suggests that the control of distensive WNW-ESE structures, related to a counterclockwise strike-slip movement of faults belonging to the former, would be the most likely explanation for the elongated shape of the Vilama magma chamber.