Tectonic rotations in the Late Palaeozoic continental margin of southern South America determined and dated by palaeomagnetism

Abstract

SUMMARY Detailed palaeomagnetic studies of Late Palaeozoic rocks exposed in the UspallataCalingasta Valley region, located at about 32s 69.5W in the Argentine Andean Chain, have been recently accomplished. Previous results are discussed in Valencio & Vilas (1985) and Rapalini et al. (1989). The most recent studies were carried out mainly on the thick sequences of late Early Permian to Late Permian rhyolites and ignimbrites assigned to the Tambillos and Horcajo Formations. Both formations yielded palaeomagnetic pole positions (P15: 319.6E, 78.9S7 N = 16, k = 33.5, AgS = 6.5 and P17: 264.8E, 72.4S, N = 26, k = 6.4, Ag5 = 12, respectively) concordant with the Late-Palaeozoic path of South America. Preliminary palaeomagnetic results from the Middle Carboniferous Hoyada Verde Formation, exposed in the same region, suggest that these rocks carry a synfolding magnetization acquired during the Late Carboniferous, as the palaeomagnetic pole position computed from partially corrected remanence directions (C6: 356.2E, 41.9S, D1 = 8.3, 02 = 6.0) agrees with the Late Carboniferous poles from cratonic areas of South America. This concordant position of C6 definitely rules out the possibility of a Late Palaeozoic allochthony of this section of the Argentine Andean Chain. The discordant positions previously found in Late Carboniferous and early Early Permian rocks of this region are interpreted as caused by large clockwise crustal block rotations that occurred not later than the late Early Permian. A tectonic model is proposed to explain these rotations which suggests that they were associated with strike-slip displacements parallel to the western continental margin of South America caused by an oblique subduction of the Proto-Pacific Plate during the Early Permian. Many tectonic and geologic observations fit this model. The concordant position of C6 also suggests that a complex pattern of crustal block rotations should have developed in this continental margin. Other palaeomagnetic data suggest that similar rotations may have taken place in other areas of the Argentine Andean Chain in the Early Permian.