Syn-extension rotations in the Permian St Affrique basin (Massif Central, France): paleomagnetic constraints

• Published in: Earth and planetary science letters Vol. 115; no. 1; pp. 29 - 42
• Main Authors: Cogné, J.P., Van Den Driessche, J., Brun, J.P.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.03.1993; Elsevier
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Geophysics: general, magnetic, electric and thermic methods and properties; Internal geophysics; Aveyron; Central Massif; France; Europe; magnetite; synsedimentary tectonics; isothermal remanent magnetization; Permian; magnetic declination; rotation; Montagne Noire; Stephanian; normal faults; chronostratigraphy; Lodeve Basin; Paleozoic; Tilted blocks; extension tectonics; correlation; thermal demagnetization; Upper Carboniferous
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/0012-821X(93)90210-Z

We present the results of a paleomagnetic study of the Permian sedimentary basin of St Affrique (southern Massif Central, France). This study was devised to test for tectonic rotations in the Montagne Noire area during the Stephano-Permian extension in the Hecynian chain and to complete a preliminary study [1] in which we suggested an counterclockwise rotation of St Affrique basin with respect to the neighbouring Lodève basin. This rotation was attributed to the domino-like rotation of the basement of St Affrique (i.e., the Paleozoic Monts de Lacaune imbricate units) as expected from the tectonic interpretation of the Montagne Noire area made by Van Den Driessche and Brun [2,3]. However, the rotation of the basin during Permian should induce declination differences between the basal and the upper part of the stratigraphic sequence. We therefore conducted vertical sampling of the basin throughout the Permian sequence. Two sites of Stephanian age gave no results, but twelve sites from the lower to the upper Permian have been sampled in the St Affrique basin and did provide results. Paleomagnetic analysis using thermal procedures confirms our previous analysis and allowed us to demonstrate three magnetization components. The low temperature A component is unblocked by 300°C and conforms to the present-day dipole field direction. An intermediate temperature component (B) has also been isolated, with unblocking temperatures of 300° to 550°–600°C, and this component was possibly carried by magnetite. Between 600° and 680°C a third component (C) may be suspected at some sites, but it has not been isolated with confidence. At a site showing a tilted-block geometry produced by syn-sedimentary normal faulting (site p7), it is shown that the B component is an early magnetization acquired during faulting of the succession, and we are therefore able to give it a stratigraphic age. The mean directions of the B component, which are computed at the stratigraphic stage level, show the following counterclockwise deviations from the reference magnetic field direction for stable Europe: 21° ± 4.5° for the upper Autunian, 10° ± 7° for the Saxonian, 10° ± 9° for the lower Thuringian and 4° ± 6° and 3.5° ± 4.5° for two successive stages of middle/upper Thuringian age. We thus consider that (1) the comparison of the lowest units of St Affrique and Lodève basins [1], and (2) the evolution in paleomagnetic declination during the Permian within the St Affrique basin itself provide evidence, in space and time, of a rotation of the basin during its formation. We attribute this rotation to the domino-like rotation of its basement (the Monts de Lacaune imbricate units) during the Stephano-Permian extension in the Hercynian chain [2,3].