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Journal of the Geological Society; 1998; v. 155; issue.1; p. 105-114;
DOI: 10.1144/gsjgs.155.1.0105
© 1998 Geological Society of London

Article

Syntectonic magnetization of the mid-Palaeozoic Sierra Grande Formation: further constraints on the tectonic evolution of Patagonia

A. E. RAPALINI

Laboratorio de Paleomagnetismo ‘Daniel Valencio’, Departamento de Ciencias Geologicas, FCEyN—Universidad de Buenos Aires, Pabellon 2, Ciudad Universitaria, 1428, Buenos Aires, Argentina (E-mail: rapalini{at}tango.gl fcen.uba.ar)

A palaeomagnetic study was carried out in the Silurian-Devonian clastic sedimentary rocks of the Sierra Grande Formation, exposed in northeastern Patagonia (41.6°S, 65.3°W). Thirteen sites (n=88) were located on opposite limbs of a syncline-anticline structure. Stepwise thermal demagnetization permitted the identification of a very stable magnetic component of reversed polarity carried by hematite. Stepwise performance of the fold test yielded negative results both in situ and after 100% bedding correction, but positive after partial unfolding (19%). This indicates a syntectonic origin for the isolated magnetization. A pole position was computed for the partially (19%) corrected remanence: SG3: 77.3°S, 310.7°E, {delta}p=7.7°, {delta}m=6.6°, N=13. Its position is coincident with late Early to Late Permian palaeomagnetic poles from South America, suggesting that age for the previously undated folding of the Sierra Grande sequence and therefore for the main tectonic event that affected the northern boundary of Patagonia. Palaeozoic palaeomagnetic poles from Patagonia obtained to date agree with those from Gondwana of Devonian or younger age, suggesting that Patagonia did not undergo important displacements relative to South America since those times. This and the Permian age of deformation determined in this study invalidates tectonic models involving collision of a far-travelled Patagonia with Gondwana in the mid- or Late Palaeozoic.

Key Words: Patagonia • Palaeozoic • Gondwana • palaeomagnetism




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