Latnmag Leters, Volume 3, Special Issue (2013), PB05, 1-5. Proceedings Montevideo, Uruguay PB05 - 1/5 PALEOMAGNETIC STUDY OF THE LATE MIOCENE LAS ARCAS FORMATION, PAMPEAN RANGES, NORTHWESTERN ARGENTINA. Lucía I. Domínguez 1* , Cecilia M. Spagnuolo 12 , Sergio M. Georgieff 12 , Lucía M. Ibáñez 13 1 Universidad Nacional de Tucumán, Facultad de Ciencias Naturales e Instituto Miguel Lillo. Miguel Lillo 250, 4000-Tucumán, Argentina 2 Consejo Nacional de Investigaciones Científicas y Técnicas de la República (CONICET) Argentina 3 Fundación Miguel Lillo. Miguel Lillo 251, 4000-Tucumán, Argentina ABSTRACT A paleomagnetic study was carried out in the Late Miocene Las Arcas Formation of northwestern Pampean Ranges (26° 45.757’ S, 66° 2.189’ W) in order to contribute with paleomagnetic information to the knowledge of the deformation of the area. The sampled formation is composed of fuvial red sandstones and pelites that were recently dated, near the top, in 6.88 My (Georgieff et al. 2013). It was sampled a stratigraphic section of 340 m thick from bottom to top of the unit. Of 17 sites analyzed, 14 carried a primary magnetization that was used to calculate a paleomagnetic pole (D = 176.2°, I = 34.0°, A 95 = 6.9°) that indicates a rotation of 0.3° ± 5.8°. This result suggests that the area did not undergo rotation since Late Miocene. Keywords: Cenozoic, Santa María valley, block rotation Introduction The Pampean Ranges consists of uplifted basement ranges bounded by high-angle thrusts and alternating Cenozoic basins (Ramos, 1999). One of these basins is the Santa María Valley, an elongated SSE-NNW tectonic depression, more than 100 Km long and about 30-20 Km wide, located in the provinces of Catamarca, Tucumán and Salta (Bossi et al, 2001) (fg. 1A). The sedimentary succession of this valley lies on a peneplain surface of crystalline basement rocks; it begins whit Paleogen continental sedimentites overlain in discordance by the Mio-Pliocene Santa María Group (Ruiz Huidobro, 1960; Galván, Ruiz Huidobro, 1965). A paleomagnetic study was carried out in the Jujuil Creek (26° 45.757’ S, 66° 2.189’ W) over the fuvial red sandstones and pelites of the Santa María Group, known as Las Arcas Formation. The top of this unit was recently dated in 6.88 Ma (Georgieff et al. 2013). The rotations observed in the Central Andes were explained by two different models, one involves Late Cenozoic oroclinal bending (Bolivian Orocline) and the other explains deformation by small-block rotations (Somoza et al. 1996). The study succession is located in the Transition Segment (26°- 30° S) (Ramos, 1999), a zone of contact between the Puna and the Pampean Ranges that is of interest for the understanding of the kinematics of the Andean Ranges. In this transition zone the angle of the subducted slab of the Nazca plate shallows, the continental lithosphere slims and the amount of horizontal shortening decreases. There are also important lineaments perpendicular to the range, like the Tucumán Transfer Zone, a right-lateral transpressional zone that form the southern bound of the Puna (de Urreiztieta et al. 1996). This work intends to contribute whit paleomagnetic information to the knowledge of the history of deformation of the area. Methodology The sampled interval encompasses Las Arcas Formation; 31 sites were determinate along a 340 m thick stratigraphic section. The sampled rocks vary between very fne sandstones to siltstones. Three to four hand samples were collected at most of the sites. In sites 27 and 30 the cores were obtained with a portable electric