Paleozoic-early mesozoic tectonic evolution of the Southern Patagonian Andes as revealed by structural fabric of low-grade metasedimentary rocks

Abstract

Along the southern Patagonian Andes, the succession of very low- to low-grade metasedimentary rocks is uncomfortably overlain by the Middle-Upper Jurassic volcanic rocks (Fig. 1; e.g. Giacosa et al. 2012). This angular unconformity is a ubiquitous stratigraphic element of the Andes between ~46°-50°S, depicting a complex tectonic activity in pre-Middle Jurassic times, i.e. pre-Andean times. Since the scarce of structural data about the fabric of these metasedimentary rocks, our aim in this contribution is to present new structural data to yield some insights about the Paleozoic-early Mesozoic tectonics of this segment of the southwestern Gondwana margin.Our structural results about the Río Lácteo Formation (47.8°S; Late Devonian-early Carboniferous?), the Bahía de la Lancha Formation (49°S; early Carboniferous) and the Nunatak Viedma Unit (49.4°S; Late Triassic), are integrated with available U-Pb zircon detrital ages (Hervé and Fanning, 2001, 2003; Augustsson et al. 2006; Suárez et al. 2019), and post-metamorphic fission tracks cooling ages in zircons determined in metamorphic complexes defined in Chilean territory (Thomson and Hervé, 2002). All together, lead us to identify:?Westward younging of the maximum depositional ages.?Two-main pre-Middle Jurassic deformation phases during the Permian (Gondwanide phase) and during the Late Triassic-Early Jurassic (Chonide phase). ?Main inland vergence of the deformation tectonic structures.These results drive us to discuss the pre-Andean tectonic evolution of the southern Patagonian Andes in the context of the Paleozoic-Early Mesozoic continental growth produced by the long-lived Terra Australis accretionary orogen (Cawood 2005) implanted along the Gondwanide margin from Australia to South America.