Structural evolution of the foredeep Magallanes and Malvinas basins in the Tierra del Fuego and the Southern Atlantic ocean.

MARCO MENICHETTI; E. LODOLO; A. TASSONE; J. FLORES; A. COMINGUEZ

19-20 de noviembre. Santiago. Chile

Simposio; International Geological Congress on the Southern Hemisfere (Geosur 2).; 2007

GEOSUR

The Magallanes and Malvinas basins are two juxtaposed perisutural basins located in front of the external margin of the southernmost Andean Cordillera in the Tierra del Fuego region. These basins, both developed on continental crust, are structurally separated by the N-S-trending Dungeness Arch, a crystalline basement high located offshore. Part of the Magallanes basin lies onshore, while the Malvinas basin extends completely offshore to the E of Dungeness Arch. Both basins are filled by a 5-km-thick siliciclastic sedimentary succession, spanning from Jurassic to Holocene. The analysis of a set of multichannel seismic reflection profiles acquired in the southern Atlantic off the Tierra del Fuego Island document a multi-stage evolution of the two basins and their deformed southern margin, represented by the Magallanes fold-and-thrust belt. Five seismic units, separated by angular unconformities, have been differentiated and calibrated with exploratory wells and previous stratigraphic interpretations. These units record different tectonosedimentary phases during the basin evolution. The basins developed in front of the Fuegian Andes, and since the Late Cretaceous their southern margin underwent a significant compressional deformation that originated the present-day Magallanes fold-and-thrust belt. The history of the Magallanes and Malvinas basins are similar but diachronic due the differential thrusts propagation that progressively migrated northward, and which involved the foreland in the deformational process. The propagation of the thrust systems toward the foreland shifted progressively northeastward the sedimentary depocenters and reduced the thickness of the clastic wedges. During the early evolution of the foredeep, several Jurassic extensional structures were reactivated and possibly inverted as strike-slip faults. The basins subsidence, at least until the Late-Middle Eocene unconformity, has been mainly controlled by several transtensional faults, which run broadly parallel to the compressional front. In the system foredeep/chain of the Fuegian Andes of the Tierra del Fuego, the subsidence and the development of the peripheral foreland basins system have been conditioned by severe faulting and by topographic load over the continental lithosphere of the South America. Several sedimentary features which outcrop along the Tierra del Fuego Atlantic coast can be easily correlated with the structures observable on the off-shore seismic profiles. In the Magallanes basin stratigraphic succession, at least four syn-tectonic angular unconformities can be recognized from the Late Cretaceous, the Palaeocene, the Eocene to the Lower Miocene. Several seismically-triggered sand intrusions in the Late Cretaceous, the Late Palaeocene and the Middle Miocene mark important the tectonic events. The syntectonic angular and progressive unconformities constrain the timing to the thrusts propagation in the frontal part of the Fuegian Andes.