Transversal segmentation of the southernmost Andes: interaction between mid-late Jurassic rifting and wrench tectonics.

GHIGLIONE, M.; SUE, C.; GONZÁLEZ GUILLOT, M.; BUJALESKY, G.; BURLANDO FARÍAS, J.

Neuquén

Congreso; 18 Congreso Geológico Argnentino; 2011

To date, there is a general agreement on that deformation history of the Fuegian Andes involved late Paleozoic to early Mesozoic compression, Jurassic extension, and a Cretaceous – Paleogene overall compressive regime followed by a Neogene to Recent phase of strike-slip deformation (i.e. Diraison et al., 2000; Menichetti et al., 2008). However, there is a current debate about the exact timing of each tectonic phase, and how each phase affected the previous ones. Furthermore, a late Paleocene-early Eocene phase of extensional deformation related to the opening of Drake Passage was recently proposed (Ghiglione et al., 2008) as well as a Neogene rifting system showing extension parallel to the orogen (Diraison et al., 1997).New data from the Fuegian fold-thrust belt north of Fagnano Lake – río Irigoyen allowed a detailed reconstruction of the interaction between mid-late Jurassic faulting and Neogene strike-slip deformation. The presented detailed mapping and serial structural cross-sections define lateral variations in the structural style and a transversal segmentation of the fold-thrust felt. The folds in some segments are symmetric, display synclinal sinking, changes in bed thickness, intermediate detachment levels and have a high slip to propagation ratio denoting a detachment mechanism for their formation. In others, the anticlines are asymmetrical, northward verging, their bed thickness is maintained, and conform a fault-propagation fold geometry. This passage from detachment folds in some segments to fault-propagation folds, although they shear a similar shortening rate, may be denoting lateral changes in the rheological properties of the detachment level (Ghiglione et al., 2010). There are also changes in the basement depth of each segment, reflecting differential extensional faulting during the Mesozoic rifting.The structural mapping allows to separate the Fuegian fold-thrust belt north of the Magallano-Fagnano fault system in discrete fault-bounded blocks that were reactivated by strike-slip tectonics. Regarding the presence of widespread Quaternary scarps that may be reflecting the presence of active faulting, our mapping allows to distinguish which lineaments are truly the expression of active strike-slip deformation. This last is in partly confirmed by previous paleomagnetic studies (i.e. Maffione et al., 2009) and will have implications for future studies trying to understand the evolution of the Patagonian Orocline (Rapalini, 2007). The new data allows the interpretation that there may be a continuum of deformation between different Cenozoic tectonicphases and a synergetic transition took place from Middle-Late Eocene compression to Oligocene-Neogene transcurrence.