Influence of the cleavage/bedding angle on magnetic and phyllosilicate fabrics:
examples from the Anglo-Brabant belt, Belgium

Timothy N. Debacker & Manuel Sintubin

Structural Geology & Tectonics Group, K.U.Leuven, Redingenstraat 16, 3000 Leuven, Belgium

timothy.debacker@geo.kuleuven.ac.be

Because of the often fine-grained lithologies and the common scarcity of strain markers, performing strain analyses in slate belts may be difficult. As an alternative, one may use markers such as the phyllosilicate preferred orientation (X-ray pole figure goniometry) and the anisotropy of the magnetic susceptibility or AMS. However, because of the influence of a large number of factors, interpreting the results of these analytical methods is not straightforward. One of the influencing factors is the angle between cleavage and bedding.

The Brabant Massif, the southeastern part of the Anglo-Brabant belt, is a single-phase deformed, low-grade metamorphic slate belt consisting of a steep Cambrian core surrounded by Ordovician-Silurian sequences. In the southern part of the Cambrian core, the transition between steeply plunging folds, considered typical for the steep core, and gently plunging folds, considered characteristic for the peripheral sequences, occurs in the homogeneous mudstones of the Lower Cambrian Oisquercq Formation. Mica and chlorite show a similar degree of preferred orientation. Mica is always aligned along the cleavage, whereas chlorite is aligned along the bedding. Clear intersection pole figure patterns emerge for samples with large cleavage/bedding angles. In contrast, flattening fabrics only become apparent for samples with very small cleavage/bedding angles. For both mica and chlorite, the degree of preferred orientation is higher for samples with small cleavage/bedding angles (flattening fabrics) and lower for samples with large cleavage/bedding angles (intersection fabrics). The magnetic fabric shows prolate susceptibility ellipsoids for samples with large cleavage/bedding angles and oblate susceptibility ellipsoids for samples with small cleavage/bedding angles. In the former case, the short axis of the susceptibility ellipsoid (k3) is oriented perpendicular to bedding, in the latter case perpendicular to bedding, to cleavage or takes up an intermediate position. The long axis of the susceptibility ellipsoid (k1) is always parallel to the cleavage/bedding intersection. The shape parameter T shows an almost linear relationship with respect to the cleavage/bedding angle, independent of the fold orientation. Similar results are obtained from compact turbidite intervals from the Silurian southern rim of the Brabant Massif. There, the shape changes of the susceptibility ellipsoid across the folds is compatible with the shape changes of locally occurring calcitic nodules, being oblate in the fold limbs and prolate in the fold hinges.

On the basis of Fry-analyses and X-ray pole figure goniometry, Giese et al. (1997) concluded prolate strains within the Silurian southern rim and oblate strains in the Cambrian core of the Brabant Massif. The present results show that such a generalisation may be rather deceptive. Nevertheless, the preliminary AMS results do suggest that for similar cleavage/bedding angles, the shape parameters (T) within the Silurian rim are slightly lower than those within the Cambrian core.

Giese, U., Katzung, G., Walter, R. & Weber, J. 1997. The Caledonian deformation of the Brabant Massif and the Early Palaeozoic in northeast Germany: compared. Geological Magazine 134, 637-652.