EarthRef.org Reference Database (ERR) -- Thomas et al. 1995

Fifty Thellier palaeointensity determinations were made from the Great Whin Sill (GWS) of northern England and the Midland Valley Sill (MVS) of central Scotland, which form the major part of a late Carboniferous quartz-dolerite complex. Palaeodirectional analysis confirms that the reversed polarity characteristic remanence carried by samples from both sills is consistent with the Permo-Carboniferous reversed superchron. Palaeomagnetic results suggest that the two sills may not be exactly contemporaneous. The MVS was intruded rapidly some time during the emplacement of the GWS, which was itself emplaced over a considerable time period. The MVS may well represent the northernmost expression of the instrusive activity. A mean palaeointensity of 22.9+/-2.6 mu T was calculated for the Great Whin Sill, with a corresponding VDM value of 5.9 x 10(22) A m(2). The palaeointensity value calculated from the Midland Valley Sill was much lower, 13.0+/-0.5 mu T, with a corresponding VDM value of 3.3 x 10(22) A m(2). Detailed rock magnetic analyses and stepwise thermal demagnetization studies reveal that the remanence carrier in both sills is magnetite of mixed domain state. Multidomain grains are less important in the Midland Valley Sill and thus greater confidence is attached to palaeointensity results from this sill. VDM values from this and other studies of the Permo-Carboniferous reversed superchron (P-CRS) indicate that the dipole strength showed similar variations to that witnessed for the past 5 Ma. There is no conclusive support in favour of either the weak or strong held models that have been suggested for superchrons. More palaeointensity results are needed from the termination of the P-CRS and the late Permian/early Triassic in order to understand how the dipole field evolved during the P-CRS and its relationship to the so-called Mesozoic dipole low.