Palaeomagnetic Studies in the British Caledonides—IV Lower Devonian Lavas of the Strathmore Region, Scotland

Abstract

Mean palaeomagnetic directions after dip‐correction at 61 sites in lavas, ignimbrites and sediments in the Lower Devonian sequence of the Strathmore region, show a principal clustering around an axis D= 42°/222° E, I=–49°/+49° which are regarded as normal and reversed (N and R) polarity groups respectively. The frequency distribution of site mean directions is not simply bipolar, however, having subsidiary maxima around D= 76°, I=+29° (‘Group A’) and D= 162°, I=–35° (‘Group B’). It is shown that the population can be subdivided into N and R Fisherian distributions (34 sites altogether) plus Groups A and B (18 sites together) and a random background of nine ‘unclassified’ sites. This analysis leads to a best estimate of the mean N and R directions corresponding to a palaeomagnetic pole 5° N 140° E (D = 5°, dχ= 9°). The primary (Lower Devonian) origin for the stable remanent direction, at least in the N, R, A and B groups is demonstrated by fold tests, a conglomerate test, igneous contact tests and by the recognition of a magnetic polarity stratigraphy consisting of at least eight N and R zones and at least five zones of ‘A’ or ‘B’ polarity. These latter groups are believed to record preferred intermediate directions of the geomagnetic field during polarity transitions. Remanent intensity is similar in the lavas in all polarity groups; it is believed to be the product of oxidation during cooling of the lavas. The more usual situation of lower intensity in rocks formed during polarity transitions is perhaps observed in intermediate polarity samples of the Highland Border Ignimbrite and may indicate an original TRM in these rocks. A composite calculation of the mean N and R direction after AF cleaning based on this and previous studies in the eastern part of the Midland Valley of Scotland (50 sites altogether) gives D= 41°, I=–40° (α95= 6°); the corresponding palaeomagnetic pole is 4° N, 140° E (I=4°, dχ=7°). Copyright © 1973, Wiley Blackwell. All rights reserved