LATERAL EXPANSION OF THE MIOCENE VOLCANIC FRONT OVER THE PAMPEAN FLAT SLAB SEGMENT, SOUTHERN CENTRAL ANDES

By the Upper Oligocene (~ 24 Ma) an Andean type arc was developed in the Southern Central Andes. In the Valle del Cura-El Indio belt these volcanic activity was represented by calc-alkaline volcanics of Doña Ana Group (~ 18–25 Ma). Its volcanic front was located in Chilean territory, although its volcanic flows are also located in Argentine slope along the international border. The arrival of the east-west trending segment of the Juan Fernández ridge under the El Indio-Valle del Cura belt, by the Middle Miocene, resulted in a pronounced shallowing of the subduction zone over the Pampean flat slab. One of the main consequence was the concomitant migration of the volcanic activity toward the east.

Cerro de las Tórtolas Formation volcanism represent the lower to mid–early late Miocene peak of andesitic and dacitic magmatism; the volcanic front had migrated and was now located on the western margin of the Valle del Cura, in Argentine territory. Although its lava flows spread out in the Chilean slope, its outcrops are mostly developed in the Argentine area, where its main eruptive centers are located. These Miocene volcanics where geographically restrictive to the west of Colangüil Cordillera that mark out eastern Valle del Cura limit. However, new Cerro de las Tórtolas outcrops were recognized outside this limit. They are located in La Coipita area, in the northern extreme of Valle del Cura, but to the east of Valle del Cura river, on the Colangüil Cordillera. They are orthpyroxene and clinopyroxene-bearing porphyric andesites, with subordinate amphibole; as shown by the older Cerro de las Tórtolas volcanic rocks. Petrographical features and stratigraphical relations suggest their assignment the Lower to Middle Miocene unit, thus extending the Miocene volcanic front toward the east.

As the volcanic front gradually migrated toward the east during the Upper Miocene, as consequence of the gradual flattening of the subduction angle, the andesitic volcanism ended in the Valle del Cura as well as along the flat-slab segment; although Miocene volcanic activity was still developed further east of the volcanic front, in Western Precordillera, as a result of interaction of slab-derived fluids and the asthenospheric wedge in a shallow subduction regime.