The Otumpa faulting (Gran Chaco plain, Argentina): the farthest megastructure due to the Central Andean flat slab subduction

ROSSELLO, EDUARDO ANTONIO; BORDARAMPÉ, CRISTÓBAL P.; PERI, VERÓNICA GISEL

Montpellier, Francia

Congreso; Subduction Zone Geodynamics Conference; 2007

Géosciences Montpellier

Introduction. The Otumpa faulting is an active megastructure located into the Gran Chaco plain on the northernmost portion of the Argentine pampas (Figure 1). The Gran Chaco is a huge plain with monotonous topography with altitudes ranging from 200 to 40 m.a.s.l. representing the farthest foreland features of the Andean Cordillera. Being controlled by the Paraná river base-level, only a few permanent rivers such as Bermejo, Salado and Pilcomayo flowing eastwards are found in this flat region. From satellite images and DEMs associated into a GIS platform it is possible to recognize the Otumpa faulting trending NNE-SSW on the eastern flank of the homonymous hills into the Santiago del Estero province (Figure 2). From subsurface viewpoint, oil seismic data (Figure 3) show the Otumpa faulting to be continuous to the north up to the western boundary of the Las Breñas Basin considered a Paleozoic half-graben depocenter. Besides, its SW extreme is associated with the eastern boundary of the Sierras Pampeanas (Sierra Chica of Córdoba and Ambargasta hills). Regarding the seismicity of this portion of the South America plate the position of the Otumpa faulting coincides with the easternmost boundary of the 500 to 800 km in depth hypocenters related to the subducted Nazca plate (Figure 4). CONCLUSION. From all the available data up to present, the Otumpa faulting should be considered a megatranspressional structure with dextral strike-slip compound produced by the Neogene reactivation upon extensional discontinuities related to the origin of Paleozoic depocenters such as Las Breñas Basin (Figure 5). The Otumpa hills represent the western uplifted flank that produced changes in the drainage pattern, particularly on the Salado River middle course. The Otumpa faulting extends for more than 600 km in the considered area (Figure 6). Finally, the Otumpa faulting is regarded as a first-order structure verging towards-east representing the most external Andean deformation front affecting a significantly thick intracontinental portion of the South America plate (Figure 7).