Abstract

The Turrialba volcano has a complex graben on its summit with an approximate strike of N50°E, a length of 3.5 km and a maximum width of 2.8 km. The boundaries of the graben are a complex system of left-lateral, strike-slip faults with normal component, several craters oriented N45°E and a collapse caldera oriented to the NE of 6 km long and 2.5 km wide. Two secondary cones are on its SW flank with N30°E orientation; all these elements show a NE volcanotectonic trend along 9 km. The analysis of secondary faults presents on its flanks, the morphoneotectonics and seismicity, together with morphovolcanic features, suggests a volcanotectonic model more complex than those proposed in the literature for this volcano. We propose the existence of a regional left-lateral strike-slip fault, oriented SW-NE, which forms a flower structure at the surface, generating a complex graben bounded by the Elia and Ariete faults. The transtensive stress favored the formation of the craters and escarpments, whereas the transpresive stress produced a dome fold structure and highly fractured areas. The zones of weakness and possible folding favored the formation of a sectoral collapse toward the NE, with the associated development of one or several volcanic debris avalanche deposits. A similar mechanism could be the trigger of the Angostura volcanic debris avalanche 17 ka ago, from the Coliblanco avalanche caldera on the SW flank of Turrialba. The Ariete and Elia faults have a seismic potential of Mw 6.2 and 6.0, respectively. The generation of new eruptive volcanic vents and sector collapses along these faults can also occur in the future.

Keywords: Volcanic graben; avalanche caldera; flower structure; morphoneotectonics; seismicity; pull-apart basin; Turrialba volcano