A magnetotectonic study of the Hercynian Montagne Noire (France)

Abstract

A paleomagnetic study of eleven sites of Devonian limestones sampled along the flanks and hinge of a polyphase Hercynian recumbent fold in the Montagne Noire reveals well defined directions of characteristic remanent magnetization (ChRM). The directions have good within‐site precision but a large between‐site scatter, which fails to be reduced by a classical tilt correction. This is readily understood if widespread remagnetization occurred between the two major Hercynian phases of folding (A and B), which are known to have shaped the Montagne Noire. This remagnetization appears to be an instance of a continent‐wide chemical event that wiped out much of the magnetic memory of Devonian and lower Carboniferous rocks in Laurasia, Baltica and northern Gondwana, possibly due to groundwater circulation following the major Hercynian tectonic phases. Comparison of observed and predicted Hercynian magnetic directions allows one to determine, through a fairly simple geometric construction, the tectonic elements of the later phase B. It is found that different rotations affected the flanks and hinge zone of the folds generated by phase A, thus demonstrating how a pre‐existing structure can control the development of deformation in a later tectonic phase. The effects of the non cylindrical phase B can next be removed to reveal the broad tectonic features of phase A deformations. The reversed (Kiaman) polarity of the ChRM allows a determination of the age of phase B, which is found to be about 300–310 Ma and of the age of the remagnetization itself at about 310–320 Ma. The search for the age of remagnetization included paleomagnetic measurements at two lower Permian sites from the nearby Lodève basin. These provide new data for the Permian pole of Europe and point out the limitations of some earlier studies, due to insufficient magnetic cleaning. Finally, the principal shortening directions of the two phases can be determined from the magnetic data and are found to have rotated counterclockwise by 30° between phase A and phase B. This feature, already observed in the collision of India against Eurasia, may be related to the progressive suturing of former subduction zones during the Hercynian collision of Gondwana against Laurasia. This study demonstrates the potential of remagnetizations for the solution of rather complex tectonic problems. We propose the term “magnetotectonics” for this rapidly expanding field of studies, in which the methods of paleomagnetism and structural analysis are closely associated.