PALEOINTENSITY RESULTS FROM THE PERMO-CARBONIFEROUS (KIAMAN) REVERSEDSUPERCHRON - THE GREAT-WHIN AND MIDLAND-VALLEY SILLS OF THE NORTHERN UNITED-KINGDOM

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 Carboniferou s quartz-dolerite complex. Palaeodirectional analysis confirms that th e reversed polarity characteristic remanence carried by samples from b oth sills is consistent with the Permo-Carboniferous reversed superchr on. Palaeomagnetic results suggest that the two sills may not be exact ly 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 wa s calculated for the Great Whin Sill, with a corresponding VDM value o f 5.9 x 10(22) A m(2). The palaeointensity value calculated from the M idland Valley Sill was much lower, 13.0+/-0.5 mu T, with a correspondi ng VDM value of 3.3 x 10(22) A m(2). Detailed rock magnetic analyses a nd 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 v ariations 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 b een 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.