Structure of the Pasqui pyroclastic cone from indirect interpretation of gravity data

Abstract

Irazú volcano shows several structures on its southern and western flank that are associated with ancient vents of lava and pyroclastic material outside of its summit caldera. A set of these type of structures is related to an extensional deformation zone that includes pyroclastic cones such as Nochebuena, Gurdián, Dussan-Quemados and Pasquí. This last structure is an eroded cone with lava flows which shows record of activity within the last 40 000 years. In this work, we study the internal structure of the Pasquí pyroclastic cone by creating density model from gravity data. The Bouguer anomaly data reduced to an altitude of 2,389 m, show a spatial correlation of the Pasquí cone with a local negative gravity anomaly with a short wavelength and an amplitude of approximately 5 mGal. This anomaly is indicative of the presence of material with a lower density than its sorroundings. From the density models, we infer the presence of pyroclastic material associated with the cone with a thickness of at least 300 m. This material of relative lower density, contrasts with the surroundings of the cone composed of andesitic and andesitic-basaltic lavas of the Reventado Formation and lavas originated by fissures from vents such as the Dussan-Quemados complex.