COMPOSITE MAGNETIC FABRIC IN WEAKLY DEFORMED BLACK SHALES

The anisotropy of magnetic susceptibility (AMS) of weakly deformed sediments of the subalpine Terres Noires (French Alps) is due to phyllosilicates (matrix contribution) and detrital titanomagnetites (ferromagnetic contribution) of variable grain size. Various rock magnetic and anisotropy techniques have been carried out to understand the origin of the scattering of maximum and minimum susceptibility axes. We demonstrate the major contribution of the matrix, both to susceptibility and AMS anisotropy, even when mean matrix susceptibility is weak. This suggests that phyllosilicates are better oriented within the foliation plane and that intrinsic anisotropy of the detrital titanomagnetic grains is probably weaker than expected. Also, the separation of the two magnetic subfabrics of three selected sites by coupling together low-field AMS, HFA (high-field AMS) and AAS (anisotropy of anhysteretic remanence) demonstrates the existence of a competition between two magnetic lineations and an interchange of the intermediate and minimum AMS axes as a result of fine titanomagnetite. This is the first evidence of the so-called 'intermediate fabric' as predicted by theory.