The puzzle of axis-normal magnetic lineations in folded low-grade sediments (Bude Formation, SW England)

Abstract

AbstractA single upright, open anticline from sub-greenschist facies sedimentary rocks of the Bude Formation (Cornwall, UK) was sampled in order to investigate the kinematic relationships between fold development and anisotropy of magnetic susceptibility (AMS). The mean magnetic susceptibility of these samples is 0.25 × 10−3 SI, suggesting low concentrations of ferromagnetic phases. AMS ellipsoids have a mean corrected anisotropy degree of 1.03 and a mean shape parameter of −0.54 (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. This is discounted, however, on the basis of broad correlation between the orientation of AMS and AIRM (anisotropy of isothermal remanence) ellipsoids. The prolate shapes and axis-normal orientation of Kmax axes contrast markedly with the widely observed relationship of AMS ellipsoids in folds, which are typically oblate and have Kmax parallel to the fold axis. 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). 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 sub-horizontal stretching at a late stage or post-dating fold formation. Such a situation is not inconsistent with superimposed southward-directed thrusting simple shear that has been suggested in this area to account for variations in fold attitude on a regional scale. It is more likely, however, that the fabric reflects post-orogenic extension, with the fold occupying a position in the immediate hanging-wall of a major northward dipping normal fault. In either case, the AMS fabrics around the fold record only the last increments of deformation in this area, with earlier primary and fold-related fabrics being entirely obliterated.