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. 5–6 kbar 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. 85 Ma on syntectonic phengite which are interpreted as representing cooling synchronous with mica crystallization during the main compressive deformational event. The 78–81 Ma U–Pb zircon crystallization ages of crosscutting plutonic and hypabyssal rocks and 40Ar/39Ar amphibole age of ca.79 Ma from lamprophyric dikes within the fold-thrust belt constrain an upper age limit of the ophiolite tectonic emplacement deformation.This work was supported by Initiation Fondecyt grant 11075000 to M.C., and the international collaboration grants of the GRICESCONICYT (José Munhá and M.C.) and the BMBF-CONICYT (H.-J.M. and F.H) programs. This research paper is a contribution of Anillo Project ACT-105 of CONICYT and PBCYT

Constraints on deformation of the Southern Andes since the Cretaceous from anisotropy of magnetic susceptibility

“…Complete closure of the Rocas Verdes marginal basin and collision between the magmatic arc and the South American continental margin was achieved in the Late Cretaceous (Halpern and Rex, 1972;Bruhn and Dalziel, 1977;Cunningham, 1994;1995;Calderón et al, 2012), and produced initial uplift of continental basement…”

“…39 Ma, suggesting that the metamorphic Paleozoic basement was completely exposed by that time . Apatite and zircon fission-track and (U-Th-Sm)/He data (Gombosi et al, 2009) (Calderón et al, 2012).…”

Petrological vestiges of the Late Jurassic-Early Cretaceous transition from rift to back-arc basin in southernmost Chile: New age and geochemical data from the Capitán Aracena, Carlos III, and Tortuga ophiolitic complexes

“…Thus, the age of the ophiolites is indirectly constrained as Tithonian to by biofacies associations in the overlying sedimentary successions (Fuenzalida and Covacevich, 1988;Suárez et al, 1985). The ophiolites underwent seafloor-type hydrothermal metamorphism (Stern et al, 1976;Elthon and Stern, 1978) and were subsequently buried and obducted onto the margin of South America during the ensuing phases of Andean orogenesis in the mid-Cretaceous (Katz and Watters, 1966;Dott et al, 1977;Dalziel, 1981;Cunningham, 1994;Fildani et al, 2003;Fosdick et al, 2011;Calderón et al, 2012). Granitoids consisting of hornblende-biotite tonalite, granodiorite, and restricted monzonite and monzodiorite (Katz and Watters, 1966;Godoy, 1978;Allen, 1982) intruding the ophiolite complexes are of earliest Late Cretaceous age (Hervé et al, 1984Cunningham, 1994;Calderón et al, 2012).…”

“…The base of this unit is composed of metagabbro and amphibolite. These rocks are exposed in two northsouth trending steeply dipping thrust sheets located in the central and western portions of the Sarmiento Cordillera (Rapalini et al, 2008;Calderón et al, 2012). The rocks are foliated and metamorphosed in a discontinuous way; parts of the rocks preserve original textures and structures.…”

“…The Rocas Verdes ophiolites (nomenclature of Stern and de Wit, 2003) host critical information about back-arc basin evolution at the southwestern convergent margin of South America, which has a reconstructed basin configuration analogous to the present-day Sea of Japan marginal basin (as suggested by Dalziel et al, 1974;Wilson, 1991;Fildani and Hessler, 2005;Romans et al, 2010) and/or the Gulf of California (Alabaster and Storey, 1990). The Rocas Verdes ophiolites are particularly well suited for examining the petrological transition from rift to back-arc basin that preceded the mid-to Late Cretaceous basin closure and orogenesis in the Patagonian Andes (Dalziel, 1981;Cunningham, 1994;Fildani et al, 2003;Klepeis et al, 2010;Calderón et al, 2012).…”