Anisotropy of magnetic susceptibility (AMS) and diamagnetic fabrics in the Durness Limestone, NW Scotland

AMS fabrics in the Durness limestone show principal axes with orientations that are counterintuitive to, but symmetrical, with the regional tectonic axes (X, Y, Z) where X is the stretching axis and Z is the shortening axis. In the field, cleavage (XY) is nearly NS and nearly vertical. Low-field susceptibility measurements of 57 cores with positive bulk susceptibility (κ > 0) have a nearly vertical maximum susceptibility (κMAx) that is similarly oriented to the regional extension axis (X) but with intermediate susceptibility approximately parallel to the regional EW shortening axis. We explain this fabric as the blending of an oblate subhorizontal bedding with a north–south feeble tectonic AMS fabric, parallel to the regional N–S vertical cleavage. The 79 diamagnetic (κ < 0) cores reveal a similar AMS fabric when the orientations of the maximum and minimum axes are exchanged to produce a paramagnetic-compatible fabric.

► Diamagnetic AMS fabrics are counterintuitive since “maximum” susceptibility (most negative) aligns with the tectonic shortening axis.
► To interpret AMS in terms of tectonic axes, maximum and minimum AMS axes should therefore be exchanged.
► The Durness limestone reveals blends two diamagnetic fabrics that yield a counterintuitive E–W “foliation”.