The interaction between active crustal faults and volcanism: A case study of the Liquiñe-Ofqui Fault Zone and Osorno volcano, Southern Andes, using magnetotellurics

The relationship between subduction in convergent margins, crustal structures and magmatism is crucial to understand processes such as the type and frequency of volcanic activity in these areas. Although it is well known that the release of fluids by the subducting slab is the main cause of the arc volcanism in areas like the Central and Southern Andes, details such as the timing and pathways of magma ascent and storage are still not well understood. A key factor that needs to be better studied in the Southern Volcanic Zone of the Andes of Chile is the role of large tectonic features in fluid transport and magma ascent processes, such as the Liquiñe-Ofqui Fault Zone, a N-S strike-slip crustal structure parallel to the main volcanic arc. In this study, we focus on Osorno volcano, a stratovolcano composed mainly by products of basaltic to basaltic-andesite composition with minor dacites and with historical Hawaiian-Strombolian eruptions. Through the measurement of magnetotellurics and the use of 3-D modeling tools and petrologic constraints, two magmatic reservoirs have been inferred, which suggest a complex magmatic system with reservoirs of different depths and compositions. The shallowest magmatic reservoir (4–8 km) has a dacitic composition, while the deepest one (7–15 km), has an andesitic composition instead. The shallow reservoir is located 2 km to the E of the volcano and the deepest one is located 10 km to the E. Considering that the Liquiñe-Ofqui Fault Zone is located 20 km to the E of the volcano, we suggest that eruptions of Osorno volcano are associated with the ascent of deep crustal basaltic magma enhanced by this master fault, re-activating the inferred reservoirs and the volcano.