Miocene to Recent geological evolution of the Lazufre segment in the Andean volcanic arc

Abstract:

The volcano-tectonic setting in which the InSAR-detected Lazufre deformation is developing is particularly relevant in the evolution of this Andean volcanic arc segment (25-26°S). Through regional mapping techniques, a comprehensive field control in addition to geochronological sampling, various volcanic units comprising stratovolcanoes, volcanic complexes, ignimbrites and caldera structures are distinguished. The Lazufre intumescence is located above the overlying block of the NE trending Middle Miocene, Pedernales-Arizaro overthrust. This area comprises an Upper Miocene (8-4 Ma) basal unit of andesitic-dacitic volcanoes and lava fields, upon which nine volcanic complexes of similar composition, including Caletones de Cori Ignimbrite and Escorial Volcano, Lastarria, Cordón del Azufre and Bayo volcanic complexes, were emplaced in several pulses between 3.5 Ma and Holocene times. Coalescing Lazufre structure, immediately to the SE, we have discovered the Miocene (9.8 Ma) Los Colorados caldera. This caldera is 30 km in diameter and sourced the homonymous dacitic ignimbrite of about 500 km³. The caldera scarp was formed in Paleozoic rocks, Miocene dacitic-rhyolitic ignimbrites and ~16 and 10 Ma volcanoes. A 6.9-6.8 Ma andesitic-dacitic volcano ridge formed by Abra Grande, Río Grande and Aguas Calientes stratovolcanoes, from NE to SW, is nested on the caldera floor. Lavas of early stages of Cordón del Azufre and Bayo complexes were shed into the NW part of the caldera. The coalescing structure formed by the Lazufre intumescence and Los Colorados caldera is conjugate at about 30° to the Pedernales-Arizaro overthrust, and has a NW-SE orientation, parallel to the Archibarca lineament. A SE to NW migration of volcanism is observed along this structure at least since the Middle Miocene. We proposed that, since Miocene, tectonic spaces with no surficial fault displacements and conjugated to the main compressive structures within the upper crust, have been created as a result of tensional stresses. Subsequently, the so increased lithostatic gradient could play a major role in the vertical traction of magma rising, favoring crustal assimilation processes. The available geochronological data indicate that the deformation that preceded the Los Colorados caldera occurred in a maximum period between 13 and 10 Ma.