COMPOSITE MAGNETIC FABRIC IN WEAKLY DEFORMED BLACK SHALES

The anisotropy of magnetic susceptibility (AMS) of weakly deformed sed iments of the subalpine Terres Noires (French Alps) is due to phyllosi licates (matrix contribution) and detrital titanomagnetites (ferromagn etic contribution) of variable grain size. Various rock magnetic and a nisotropy techniques have been carried out to understand the origin of the scattering of maximum and minimum susceptibility axes. We demonst rate the major contribution of the matrix, both to susceptibility and AMS anisotropy, even when mean matrix susceptibility is weak. This sug gests that phyllosilicates are better oriented within the foliation pl ane and that intrinsic anisotropy of the detrital titanomagnetic grain s is probably weaker than expected. Also, the separation of the two ma gnetic subfabrics of three selected sites by coupling together low-fie ld AMS, HFA (high-field AMS) and AAS (anisotropy of anhysteretic reman ence) demonstrates the existence of a competition between two magnetic lineations and an interchange of the intermediate and minimum AMS axe s as a result of fine titanomagnetite. This is the first evidence of t he so-called 'intermediate fabric' as predicted by theory.