Abstract

The Mesozoic evolution of the Andean Cordillera of southern Patagonia is recorded in two formations that are now part of the fold-thrust belt: the Zapata Formation and the Punta Barrosa Formation. Extension in the Ultima Esperanza region began in the Late Jurassic with deposition of the marine volcaniclastic Tobífera Formation and eventually resulted in the full-fledged oceanic Rocas Verdes Basin. The Zapata Formation was deposited over a time span of ∼50 m.y. in an irregular basin ultimately bordered by an Early Cretaceous arc to the west. It is characterized by interbedded shale and siltstone mostly deposited in shallow water and, in areas of oceanic crust emplacement, also as deep-water hemipelagic deposits. Sediments of the Zapata Formation were derived initially from local oceanic upwarps and Tobífera highs and later from the ande-sitic cover of the juvenile arc and/or exhumed oceanic crust. The paucity of sandstone in the Zapata Formation in the Ultima Esperanza region indicates a highly irregular basin partitioned by prominent horsts, with sand deposition confined to sub-basins closer to the arc. Changes in depositional regimes and sediment dispersal patterns related to the onset of Andean contraction and formation of the Magallanes foreland basin are recorded by sediments of the overlying Punta Barrosa Formation. This formation records the evolution of a fold-thrust belt on the basis of the multimodal mineralogical and geochemical character of its sandstone and shale. The presence of an arc is indicated, but nearby Andean metamorphic terranes are more significantly represented in Punta Barrosa sediments. Stratigraphic details across the Zapata and Punta Barrosa Formations indicate that deformation and development of a proto–Andean Cordillera in southern South America was initiated in the late Mesozoic and involved conspicuous crustal shortening.

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