Metamorphism and deformation of the Tobifera Formation in the Cordillera Riesco, Region of Magallanes, Chile: Evidences for its tectonic evolution

Abstract

The Tobífera Formation (150 Ma) was in part deposited contemporaneously with the Sarmiento Ophiolitic Complex during the rifting stage of the Rocas Verdes basin. Later, the Cretaceous Zapata, Punta Barrosa and Cerro Toro Formations were deposited conformably on top of the Tobìfera. In Cordillera Riesco, it is composed of siliceous tuffs, siliceous welded tuffs, quartzose black shales and siltstones. It is affected by a mylonitic deformation that developed different ductile reverse kinematic indicators, with a minor dextral component which suggest a reverse- dextral movement of the fault zone. The metamorphic mineralogy indicates that the greenschist facies conditions were reached for the matrix forming minerals, and prehnite-actinolite facies in mineral associations in retrograde cross-cutting veinlets. Geothermobarometric calculations indicate that this mylonite zone reached 6.3 ± 1.1 kb and 399 ± 64°C. These values suggest that these rocks reached a depth between 19 and 26 km, maybe during a reverse subduction episode in the area. Deformed clasts of Tobífera Formation in conglomerates of Cerro Toro Formation indicate the exhumation of the Tobífera Formation before or during the late Cretaceous. The present configuration of the structures in Cordillera Riesco is possibly determined by westward rotations occurred in the Magallanes fold and thrust belt.