Rheological control on the development of shear zones: A case study of the Ophiolitic assemblages in Central Tierra del Fuego. Argentina.

CERREDO, M. E.; MENICHETTI, M.; REMESAL, M.; TASSONE A.

San Salvador de Jujuy

Congreso; XVII Congreso Geológico Argentino; 2008

CGA-AGA

The deepest rock assemblages of the Middle Jurassic-Lower Cretaceous Rocas Verdes Basin (RVB) in Argentine Tierra del Fuego are embodied within the Lemaire Formation (LM) which outcrops along a discontinuous WNW belt from Sierra de Alvear to Isla de los Estados. The Lower Cretaceous turbidite sequences of Yahgán Formation complete the infill of RVB. LM was characterized as a submarine volcanic-sedimentary complex that includes sedimentary rocks (conglomerates, chert, and black radiolarian and carbonaceous mudstones); acidic volcanic and volcaniclastic rocks and basaltic facies (Hanson and Wilson, 1991). The mostly siliceous volcanic rocks of the LM are related to the widespread extensional stage which involved extra-Andean Patagonia along with the southernmost Andes and Antarctic Peninsula (Feraud et al, 1999; Pankhurst et al., 2000) and resulted in the opening of the ensialic back-arc RVB partly floored by oceanic crust in southernmost South America (Dalziel, 1981). Post-Albian closure of the back-arc RVB during the Andean Orogeny produced a thick-skinned fold-and-thrust belt of the Fuegan Andes. From the Palaeogene onwards, EW sinistral wrench tectonics affected the region as a component of the relative motion between South America and the Antarctic Peninsula. This strike-slip activity is well documented from the Carbajal valley to the Canal de Beagle region south of the Magallanes-Fagnano transform fault system (Menichetti et al., 2008 and references therein). In central Tierra del Fuego, rock exposures in Sierra de Alvear (between Carbajal Valley and Paso Garibaldi) show dismembered fragments of the crust and deep infilling of RVB. Several thrust slices are separated by mylonitic rocks characterized by a strong deformation within the ductile regime and lower greenschist facies assemblages. WNW-ESE trending large folds, low-angle to bedding-parallel thrusts/décollements, and asymmetric chevron folds and moderately steeply SSW dipping thrusts characterize the major fold-and-thrust belt system. In spite of the structural complexity and the dramatic microstructural reorganization imposed by Andean contraction, the sedimentary/igneous record still preserve relic primary features which allow to partially reconstructing RVB crust. A fairly complete oceanic upper crustal section including gabbros, pillow-lavas and breccias, hyaloclastites and sea bottom sedimentary rocks may be integrated from fragmentary exposures. The lowermost levels are represented by highly deformed siliceous tuffs and ignimbrites. Abysal sedimentation is represented by black shales with sparse siliceous oozes and occasionally with interlayers of bedded chert horizons. Thin, discontinuous levels of black shales are also found interbedded within partially exposed pillow-lavas, pillow-breccias to hyaloclastite sequences. Two groups of basaltic to basaltic-andesitic rocks are distinguished within the oceanic igneous component of RVB crust: a plagioclase-phyric (to aphyric) vesicle-rich facies, generally displaying a profuse vein (calcite-quartz-opaque mineral) network and an ophitic facies (either porphyric or aphyric) vesicle-poor facies. Primary texture and mineralogy of different lithologies have been severely modified by a former sea-floor metamorphism, later overprinted by oriented greenschist facies associacions related with Andean tectonics. The localization of shear zones is clearly controlled by rock rheology. Phyllosilicate-rich hyaloclastites and black shales are the locus of highest strain deformation zones, whereas massive ophitic rocks are relatively undeformed. Highly porous, vesicle-rich and intensely veined basaltic to andesitic rocks, instead, often display mylonitic texture.