Rifting evolution of the Malvinas basin, offshore Argentina: New constrains from zircon U–Pb geochronology and seismic characterization
Résumé
The Malvinas basin, emplaced offshore southern Patagonia, is an asymmetric extensional basin formed by NNW-striking faults. In this contribution we present a review and updated structural framework for the Malvinas basin, together with two new U–Pb zircon ages for the synrift series. By extrapolation from the neighboring Austral basin, the Middle Jurassic Serie Tobífera unit has typically been interpreted as the synrift infill of the Malvinas basin and correlated to the V2 event of the Chon Aike Magmatic Province. A tuff sample recovered from Serie Tobífera in a well in the Malvinas basin, confirmed this correlation and yielded a crystallization age of 169.6 ± 2 Ma (U–Pb in zircon). Furthermore, a volcanic breccia obtained from a deeper section within the synrift, yielded a Late Triassic age (215 Ma, U–Pb in zircon). This is the first Triassic age in the Malvinas basin and allowed correlation of these deposits with the El Tranquilo basin in the Deseado Massif (onshore Patagonia). The synextensional emplacement of the retroarc Chon Aike Magmatic Province is associated with trenchward migration of the coetaneous volcanic arc. To explain the rotational extension observed in the Chon Aike structural fabric at continental plate-scale, we follow the differential rollback model for the Jurassic evolution of southern Patagonia and introduce a vertical slab tear between Patagonia and Antarctica. The slab tear model provides a border condition for the differential rollback and explains the lateral displacement between Eastern and Western Gondwana during the Mozambique Channel opening. Moreover, the asthenospheric window opened by the slab tear would have produced a thermal anomaly that triggered the opening of the Weddell Sea in the Late Jurassic between Eastern and Western Gondwana.