The low-grade Canal de las Montañas Shear Zone and its role in the tectonic emplacement of the Sarmiento Ophiolitic Complex and Late Cretaceous Patagonian Andes orogeny, Chile

Abstract

The Canal de las Montañas Shear Zone (CMSZ), southern Patagonian Andes (51–52°S), is a low-grade mylonite belt generated from felsic ignimbritic, pelitic and basaltic protoliths of the Late Jurassic–Early Cretaceous Rocas Verdes basin. The different types of rock fabrics across the CMSZ are thought to be associated with relatively intermediate and high strain conditions, characterized by the development of a narrow western belt of S–C´-type mylonites and phyllonites interpreted as the metamorphic sole thrust of the Sarmiento Ophiolitic Complex. Highly strained rocks of the CMSZ display a reverse, continent-ward tectonic transport, with a minor dextral component of shearing. Transitional pumpellyite–actinolite and upper greenschist facies metamorphic conditions at ca. 5–6kbar and 230–260°C indicate that the primary shearing event occurred in a subduction zone setting. In-situ 40Ar/39Ar laserprobe chronology yielded ages of ca. 85Ma on syntectonic phengite which are interpreted as representing cooling synchronous with mica crystallization during the main compressive deformational event. The 78–81Ma U–Pb zircon crystallization ages of cross-cutting plutonic and hypabyssal rocks and 40Ar/39Ar amphibole age of ca.79Ma from lamprophyric dikes within the fold-thrust belt constrain an upper age limit of the ophiolite tectonic emplacement deformation.