Bimodal Holocene flank eruptions at Lanín composite volcano, Southern Volcanic Zone of the Andes: Characterization of their deposits and controls on the vents array

Abstract

The Pleistocene to Holocene Lanín composite volcano, in the Southern Volcanic Zone of the Andes, has summit and flank eruptions of Holocene age. We present here a characterization of flank vents and their deposits produced during the most recent eruptions of the Lanín volcano. Volcanic facies analysis indicates four unreported Holocene eruptive units sourced from flank vents. The associated deposits have basaltic, basaltic andesitic and trachytic compositions, and were formed mainly by low-volume effusive and mainly Strombolian-like explosive eruptions. Pyroclastic deposits were identified at the NE, E and SW volcano flanks, and correspond to concentrated and diluted pyroclastic density current deposits, in addition to non-welded to welded fall deposits. Basaltic lava flows of pahoehoe morphology were produced in a purely effusive episode. An analysis of the distribution of Lanín's flank vents (dyke's orientation and vent alignments) along fault-kinematic analysis in basement rocks were carried out with the aim of assessing the basement anisotropies that may have controlled the location and orientation of dykes, eruptive fissures and vent alignments. Contrary to expectations, the Liquiñe-Ofqui Fault System does not constitute a first-order control on the orientation of dykes and vents alignments in Lanín. The vents arrangement may be partly influenced by pre-Andean crustal scale fault zones, such as the Mocha-Villarica Fault Zone, and local basement structures. The predominance of flank vents over summit eruptions strongly suggests that the load of the volcanic edifice is a controlling factor.