Geological cross-sections in the Fuegian Andes

MENICHETTI M.; TASSONE A.; CERREDO M.E.; ESTEBAN F.

19-20 de noviembre. Santiago. Chile

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

GEOSUR

Seriate cross sections through the Fuegian Orogen have been constructed by integrating original superficial field data with the geometries of the deep structures deriving from on-shore and off-shore geophysical scientific and industrial exploration. In the internal part of the Fuegian Andes the principal stacks of the thick-skinned complex includes two south-dipping duplexes,  with a sole thrust rooted in the high grade metamorphic rocks of the Upper Palaeozoic to the Lower Tertiary, outcropping in the Cordillera Darwin. Towards the east, in the Argentinean sector of Tierra del Fuego, these compressional structures merge in thick mylonitic shear zones formed in greenschist metamorphic conditions. The sole thrusts merge northward onto the Jurassic volcanoclastic sediments and leads the edge of the Cordillera in the Magallanes fold-and-thrust belt. The geometry of the thrust complex is an upright, south plunging monocline of moderate tilted sedimentary cover strata, as well as the related thrust faults and chevron folds that affected both the Upper Jurassic and the Cretaceous rocks. Two main coaxial populations of macro-and mesoscale structures, WNW-ESE trending, can be identified: (1) large folds and low-angle to bedding-parallel thrusts/décollements and (2) asymmetric chevron folds and moderately steeply SSW dipping thrusts that form part of the major fold-and-thrusts belt systems. The shallow sole thrust structures are well exposed in the Cordillera and they show at least two main detachment levels, one of which is localized in the Lemaire Formation, and the others in the Cenozoic clastic sediments. The thrust surfaces are characterized by brittle shear zones few tens of meters thick where S/C tectonites are very well developed. In the central part of the Fuegian Cordillera, several thrust slices consist of mylonitic rocks characterized by a strong deformation within ductile regime. The thrust surfaces display a regional continuity along the northern side of the Fuegian Andes with a cumulative shortening estimated of tens of kilometers. The area in the north slope of the Cordillera is affected by the northernmost system of left lateral E-W strike slip faults. The releasing step-over geometry and the kinematics pattern of these structures  show a dominant transtension with the formation of elongated pull-apart basin of the Lago Fagnano.  The external domain of the Cordillera comprises the fold-and-thrust belt where the Cenozoic sediments of the Magallanes basin are involved in the shortening. The shallow structures are tight folds, with imbricate stepped thrust surfaces. On the northern end of the section the structures became wider and  the detachment surface is located in the Neogene marl levels. The deeper structures  of the Magallanes basin show an evident step in the top of the basement rocks with an offset of a few kilometers. The transtensional nature of these faults produces a southward stepped foredeep, which is clearly displayed in many seismic lines in  front of the Cordillera, and affects both the Magallanes and Malvinas basins. The age of the deformation is Eocene, well constrained within seismic reflectors in the Atlantic Ocean off-shore. The shallow geometries of the structures  in the foreland pertain to the thin-skinned fold-and-thrust belt reveal a progressive migration of the deformation front and its consequent influence on the sedimentation depocentre in the foreland. The orogenic shortening of the Fuegian Andes, including the Cordillera and the Magallanes fold-and-thrust belt, calculated with simple geometrical assumptions, spans for a few hundred kilometers, with a significant left-lateral wrenching component.