Plio-Quaternary kinematics and geometry of the Calama-Olacapato-El Toro fault zone across the Puna Plateau, Argentina

The Calama-Olacapato-El Toro (COT) fault zone transversally crosses the Andean chain and the Puna Plateau at about 24° S. We describe the field geological structural data collected along this fault zone where it crosses the Puna Plateau (Argentina). The investigated area goes from the Chile-Argentina border to a few km east of the San Antonio del Los Cobres village, along an ideal NW-SE strip that follows the surface trace of the COT fault zone. Available radiometric dating and local stratigraphic controls enabled us to date the main tectonic events. Fault kinematics has been reconstructed through the individuation of striated fault planes and the offset of surface deposits and landforms. A series of main faults have been detected in the area, aligned along the known surface trace of the COT lineament. Our observations suggest that it consists of a series of discrete NW-striking fault segments partially overlapping laterally. The strike range of the various fault segments is in the order of 5-20°. The surface trace is rectilinear suggesting a vertical dip at least in the uppermost crust. Kinematics is characterised by oblique movements with a dominant left-lateral strike-slip component. A vertical component is usually present in the form of relative uplift of the northeastern tectonic block. These faults offset Pliocene or Pleistocene volcanic deposits suggesting a Plio-Quaternary activity of the Puna Plateau segment of the COT fault zone. In some places along the Puna Plateau, open fissures have also been observed along the COT fault zone. These fissures are only partially filled and are not located along or close to slopes. Moreover they affect “hard” rock deposits. Thus, they cannot be interpreted as due to local gravity deformations or “soft” sediment compaction. Taking into account also the low erosion rate of the area, we believe that these fissures might be interpreted as an evidence of prehistoric/historic surface faulting. We carried out also paleoseismic investigations along a trench crossing the main fault trace, which suggest that here faulting occurred through multiple events with surface ruptures. Further westward in the Chilean region, field evidence of Plio-Quaternary motions disappears, and the COT lineament should be older in age there. We will also present other structural data depicting the kinematics and geometry of the secondary faults detected along the COT fault zone, as well as the paleostress tensors reconstructed by numerical inversion of the striated fault plane data