Thermal demagnetization studies on the old red sandstone of the Anglo-Welsh Cuvette

Abstract

Thermal demagnetization of samples from folded and flat-lying sites of the Old Red Sandstone of the Anglo-Welsh cuvette shows that the natural remanence is composite. There is a weak component possessing high thermal stability, which can be shown to be of pre-Hercynian age by application of the fold test and which can only be detected by heating to above 600°C. There is also a second and dominant component thermally less stable, which is almost entirely responsible for the natural remanence and which represents a secondary magnetization imposed on the rocks at some later date. The direction of the more stable component agrees with the direction of the natural remanent magnetization of the Lower Old Red Sandstone lavas from the Midland Valley of Scotland. It is therefore suggested that the paleomagnetic pole position (140°E, 10°S) obtained from the Lower Old Red Sandstone lavas is the best estimate of the Devonian paleomagnetic pole. That previously suggested at 159°E, 30°N, which was deduced from the remanent magnetization of the Old Red Sandstones of the Anglo-Welsh cuvette, is shown to be appropriate to a later paleomagnetic field. The discrepancy between the revised Devonian pole for Britain and Devonian data from other parts of Europe, notably Russia, is noted.