Magnetic fabrics and fold development in the Variscan belt of SW England

Mark W. ANDERSON & Antony MORRIS

Department of Geology, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

manderson@plymouth.ac.uk

A frequently observed relationship between the orientation of anisotropy of magnetic susceptibility (AMS) ellipsoids in folded low grade metamorphic rocks is an alignment of Kmax along the intersection of an axial planar cleavage and primary foliations (e.g. bedding). This relationship is interpreted to represent progressive overprinting of primary depositional/compactional fabrics (Kmin perpendicular to bedding) by a tectonic fabric (Kmin perpendicular to cleavage). Such composite fabrics are usually oblate in shape with Kmax typically aligned perpendicular to regional transport directions.

Sandstone and shale beds have been sampled around a single upright, open anticline from the sub-greenschist facies Bude Formation (Cornwall, UK) in order to investigate further the kinematic significance these relationships. The mean magnetic susceptibility of these rocks is 0.2 x 10-3 SI, suggesting low concentrations of ferromagnetic phases. The mean corrected anisotropy degree is 1.04 with a mean shape parameter of -0.5 (prolate). Kmin and Kint define a girdle distribution striking sub-parallel to the fold axial plane, with Kmin tending to cluster around the fold axis. Kmax axes from both limbs of the fold define a cluster with a mean azimuth perpendicular to the fold axis. This arrangement of Kmax and Kmin could represent an inverse magnetic fabric of composite primary/tectonic origin. However, this is discounted on the basis of close correlation between the orientation of AMS and anisotropy of isothermal remanence (AIRM) ellipsoids. A further discrepancy with the commonly observed AMS fabrics in folds is the strongly prolate shape of the ellipsoids.

The consistency of Kmax orientations irrespective of position within the fold clearly points to a fabric of tectonic origin. Prolate ellipsoids with long axes perpendicular to the fold hinge line are indicative of superimposed stretching at a late stage or post-dating fold formation. Such a situation is consistent with superimposed simple shear documented by the variation in fold attitude on a regional scale, with Kmax parallel to the regional transport azimuth. For this to be the case, the AMS fabric must represent only the last increments of strain in this area, with earlier primary and fold-related fabrics being entirely obliterated.