The Caviahue caldera revisited, preliminary results

PETRINOVIC, I. A.; MARTI, J; AGUIRRE-DÍAZ, G.; GEYER,A.

Congreso; ISC 2010; 2010

IAS

The depression of Caviahue has been previously studied by several authors and two competing hypothesishave been proposed to explain its origin and related deposits. On one side, it has been suggested an origin as acollapse caldera, while on the other side, it has been interpreted as a tectonic depression with associated volcanism.One of the main problems concerning the origin of this depression, which remains still unresolved is the apparentlack of correspondence between the intra-caldera and extra-caldera deposits (> 160 km3 and 7 km3, respectively).In this contribution, we present an alternative explanation of the origin of the Caviahue depressionbased on a revision of previous data and detailed field work, which regards it as a passive collapse caldera developedunder a strong structural control in the framework of a pull-apart local tectonic setting. The origin wepropose allows to satisfactorily explain the volume correspondence between caldera deposits and depression andto interpret the subsidence history in a simpler manner. Our results indicate that both intracaldera and outflow faciescorrespond to different pyroclastic flow deposits which may be interpreted as correspond to a caldera-formingsequence. Following a pre-caldera eruptive episode with localized strombolian activity represented by scoriaconesand low-volume lavas, a catastrophic opening of major conduits along the main fault borders of a pullapartstructure fed large volume pyroclastic density currents that generated proximal and distal facies. Interbeddedvolcaniclastic deposits indicate short time gaps between explosive events or the existence of syn-depositionalreworking. No plinian fall deposits preceded the emplacement of pyroclastic flows, thus suggesting that thecaldera-forming eruption immediately developed into massive proportions, as it occurs in many other large collapsecalderas with a similar strong tectonic control. The caldera eruption continued with the emplacement ofhigh degree and low aspect ratio ignimbrites inside and outside the caldera, forming the top of the caldera-formingsequence. After this caldera-forming episode subsidence in the Caviahue depression continued at a muchslower rate controlled by the opening of the pull-apart structure and allowing the deposition of continental sedi -ments. Post-caldera volcanism has continued till present concentrated at the western border of the caldera givingrise to the construction of the Copahue stratovolcano.