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South America & Antarctica |
Department of Geology, University of Leicester, Leicester, LE1 7RH, U.K.
As a result of back-arc extension along the Andean margin of southern Chile, marginal basins formed during both Mesozoic and Tertiary times. Crustal extension was limited but was accompanied by tholeiitic basalt or bimodal (acid-basic) volcanism rather than the calc-alkaline magmatism normally characteristic of the Andean margin.
The basement in southern Chile is composed of highly deformed metasediments, part of a major accretionary wedge built out from the Pacific margin in late Palaeozoic to early Mesozoic times in front of the late Palaeozoic magmatic arc now situated in Argentina. In the mid-Jurassic the locus of calc-alkaline magmatism moved W to near the present continental margin, and a narrow marine basin developed behind the magmatic arc from the late Jurassic to the early Cretaceous. The formation of the basin was marked by extensive silicic volcanism, probably a product of melting of the metasedimentary basement, followed by bimodal acid-basic volcanism in the early to mid-Cretaceous and the emplacement of a major batholith. Extension was less than in the contemporaneous ‘rocas verdes’ back-arc basin in southernmost Chile in which new mafic crust was formed. The basin was closed and uplifted during compression in the mid-Cretaceous. However, in the late Cretaceous a new basin, floored by mafic crust, opened within the calc-alkaline arc. Toleiitic volcanism and plutonism continued in this basin during the Tertiary while alkaline volcanism occurred in the back-arc regions. Compression in the Miocene, accompanied by further calc-alkaline plutonism, closed the basin and resulted in deep levels of the Patagonian batholith being juxtaposed with the volcanic sequences of the basin.
