| Keywords |
Cerro del Azufre, Cerro Inacaliri, Cerro Pabellón, Apacheta, volcanic evolution, tectonic evolution, geology, geothermal field |
| Abstract |
Cerro Del Azufre – Cerro de Inacaliri Volcanic Chain (AIVC) is located at Central Volcanic Zone of The Andes range and at the NW edge of Altiplano-Puna Volcanic Complex, close to the Chile - Bolivia border. In this volcanic chain, Cerro Pabellón geothermal field (formerly known as Apacheta) lies and the homonymous geothermal power plant (the first one in Chile and South America) has been built. Within this NW-SE chain, the Graben Pabelloncito or Apacheta stands out, which is a first order structure in this segment of the Andes. On the other hand, the rocks of the AIVC area account for a complex geological history, with multiple stages of volcanic and tectonic activity. The stratigraphy observed, in the exploration and exploitation boreholes, added to the surface geological mapping, has allowed us to develop a better conceptual geological model. Six major geological stages have been identified, associated to major changes of both, volcanic and tectonic activity: 1. Stage 1 (Oligocene - Middle Miocene): Deposition of fine detrital sediments of volcanic origin in a continental basin. 2. Stage 2 (Middle Miocene – Late Miocene): Eruption of andesitic-dacitic lavas and emplacement of several ignimbrites. 3. Stage 3 (Late Miocene – Late Pliocene): Deformation of the older units in the form of a NW-SE anticline, originated on a deep fault. Development of a first NW-SE chain of volcanos beside the anticline. Contemporaneously, deposition of volcanoclastic sediments and ignimbrites in a depocenter just to the NE of the anticline. 4. Stage 4 (Late Pliocene - Early Pleistocene): Starts the volcanic activity of the current AIVC, with approximate orientation NW-SE. At the same time, the anticline would have collapsed at the hinge zone, giving rise to the development of the current NW-SE structure of Pabelloncito or Apacheta graben, and its syn-tectonic filling. 5. Stage 5 (Early Pleistocene – Middle Pleistocene): In the AIVC, destruction of ancestral Apacheta Volcano summit, at the same time as the deposition of an ignimbritic unit, which culminates with the Aguilucho Ignimbrite. Extrusion of Chanca Dome, Cerro del Azufre Domes and Cachimba Domes. 6. Stage 6 (Middle Pleistocene to Holocene): The localized extension and subsidence of the graben goes on. Extrusion of Cerro Aguilucho volcano and the youngest lavas from Cerro Del Azufre. Extrusion of the unnamed domes of summit and SW flank of Apacheta Volcano, the youngest domes of Chanca domes complex, Chac-Inca domes and Cerro Pabellón dome, as the last volcanic buildings of this area. Deposition of debris avalanches, moraines and rock glaciers. The geometry of the border faults of the graben and the volcanic history of the AIVC, are evidence to support the existence of a shallow magmatic chamber located under Apacheta – Aguilucho complex. Both, volcanic and tectonic evolution of the area, have created especially favorable geological conditions (as the existence of a magmatic heat source, a permeability of fractured rocks and an important clay cap), to make possible the very existence of the Cerro Pabellón Geothermal System. |