A Review of the Geology, Structural Controls, and Tectonic Setting of Copahue Volcano, Southern Volcanic Zone, Andes, Argentina

Abstract

Copahue Volcano lies in the Southern Volcanic Zone of the Andes Mountains, although its geology and local structural controls differ from nearby active volcanic centers. Most of its geology is substantially older than active volcanoes at these latitudes, as the postglacial component is relatively minor. The basement of Copahue Volcano, represented by the Agrio Caldera products and its basal sections, accumulated in extensional depocenters when the arc narrowed from a broad geometry on both sides of the Andes to its present configuration. Initial stages comprise early Pliocene basaltic-andesitic eruptions associated with extensional (transtensional?) processes that ended with the formation of a series of rhombohedral calderas that emitted important amounts of ignimbrites in latest Pliocene-early Pleistocene time. Copahue Volcano concentrates the Pleistocene activity of one of these calderas, the Agrio Caldera, before the emplacement and development of the Present arc front to the west. Volcano morphology reflects this particular evolution, looking more degraded than Antuco, Callaqui and Lonquimay volcanoes located immediately to the west in the arc front. Most of Copahue’s volume is early Pleistocene in age, showing a thin resurfacing cover in synglacial (>27 ka) and postglacial times. A synglacial stage occurred mainly to the east of Copahue Volcano toward the caldera interior in a series of independent, mostly monogenetic centers. Postglacial eruptions occurred as both central and fissural emissions reactivating the old Pleistocene conduits. Its particular geological record and eastern longitudinal position indicate that Copahue was probably part of the late Pliocene-Pleistocene arc mostly developed in the axial and eastern Andes. Narrowing and westward retraction of the arc front, proposed in previous works for the last 5 Ma at 38°S, could have been the result of the eastward migration of the asthenospheric wedge during slab steepening. Reasons for this long-lived eruptive history at Copahue volcano could be related to the particular geometry of the active Liquiñe-Ofqui dextral strike-slip fault system that runs through the arc front from south to north when penetrates the retroarc area at the latitude of Copahue volcano. This behavior could be due to the collision of the oceanic Mocha plateau at these latitudes, as recently proposed. This jump and related deflection would have produced local transtensional deformation associated with abundant emissions of syn- and post-glacial products that could have partially resurfaced this long-lived center.