Paleogene pull-apart basins on Atlántic margin of de Tierra del Fuego. Argentina.

ALEJANDRO TASSONE; MARCO MENICHETTI; EMANUELE LODOLO.

19-20 de noviembre. Santiago. Chile.

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

GEOSUR

The Atlantic Fuegian continental margin is occupied by the Magallanes and Malvinas foreland basins. The former lies partly onshore and extends towards the central-eastern Tierra del Fuego offshore, whereas the Malvinas basin lies totally offshore to the E of the Magallanes basin. This southernmost region of South America is characterized by prominent WNW-ESE to W-E morphostructural lineations which cut across Tierra del Fuego island and continue eastwards in the continental margin. These structures belong to the set of sinistral transform faults that distribute the deformation along the South-America/Scotia plate boundary (Lodolo et al., 2003). Three main fault systems are distinguished from north to south: the Magallanes-Fagnano (MFS), the Carbajal-Lasifashaj (CLS) and the Beagle Channel Fault System (BCS). The strike-slip faults are superposed on the main Late Cretaceous compressional structures, and generated complex geometries; restraining bends and overlapping step-over geometries characterize few sectors of the strike-slip faults with pop-ups, pressure ridges and uplifted slivers of crust (Lodolo et al., 2003). The major structural arrangement of the deformed belt and the southernmost Malvinas basin is recovered from offshore N-S seismic sections: the fold-and-thrust belt is characterized by a sequence of N-verging thrusts and folds that affect the sedimentary pile up to the Middle Eocene strata, and shows part of the crystalline basement of the Fuegian Cordillera-Isla de Los Estados platform partially outcropping at the sea floor. The southernmost areas of both basins limiting with the Magallanes fold-and-thrust belt is presently poorly known. We analyze new acquired multichannel seismic data, combined with available industrial seismic sections and other existing bibliographic information with the aim of characterizing the sedimentary setting, structural and tectonic relationships between the Paleogene pull-apart basins associated with the above mentioned major transform fault systems. The seismic sections located in the inner sector of the Magallanes fold-and-thrust belt show extensional basins bounded by high-angle normal faults. These faults reach 0.7 s TWT, involve Cretaceous units of the inner fold-and-thrust belt and bound isolated 5-10 km wide depocenters filled with Paleogene units affected in turn by normal faults. These basins are inferred to be related to transtensional movements likely linked to extensional overlaps among fault segments. This interpretation is supported by the existence of similar onshore structures located at the same latitude, i.e. in the Carabajal valley (Caminos et al., 1981), and along the northern shoreline of the Beagle Channel (Cunningham, 1993; Caminos et al., 1981), where evidence of sinistral strike-slip was reported. Therefore, two main phases (Paleogene and Neogene) of strike-slip tectonic activity may be inferred from the sedimentary record of some restricted and confined basins found on both the outer and inner deformation belt. The older phase generated wrench basins in the inner sector of the deformation belt, and the second one is represented by transtensional systems (Lodolo et al., 2002, 2003, 2006; Tassone et al., 2005) that cut across the outer fold-and-thrust belt and delineates the actual South America-Scotia plate boundary, producing a series of pull-apart basins along its trace.