Palaeomagnetic signature of slow post-orogenic cooling of the north-east Highlands of Scotland recorded in the Newer Gabbros of Aberdeenshire

Abstract

Consideration of previous work and new data show a systematic distribution of the directions of high coercivity and high blocking temperature magnetization in the Newer Gabbros of Aberdeenshire. The site-mean directions show a non-Fisherian distribution with the corresponding pole positions distributed along a published early Palaeozoic apparent polar wander path for Great Britain. The mean of the distribution closely agrees with the mean direction previously reported and widely quoted. There is a close correlation of the site-mean directions with the position of the sampling sites with respect to the contours (‘chrontours’) of the K-Ar coolingage. Sites where stable NRM has SE declinations and shallow (+20°) inclinations have the oldest chrontour ages (∼ 468 Myr). The mean palaeomagnetic direction progressively shifts and the sites with the youngest chrontour ages (∼ 428 Myr) have SW declinations with moderately steeper (+45°) inclination. The relative age of magnetization, determined from the position of the corresponding pole on the apparent polar wander path, is consistent with the relative chrontour age. The gabbros have therefore recorded the shift of the north-east Highlands with respect to the palaeomagnetic pole during the slow cooling of the terrain. The slow cooling is presumably related to uplift and erosion, and the resulting downward migration of isothermal surfaces. A previously published suggestion that all magnetizations with southerly declinations and moderately downward inclination are the resultant of normal and reversed magnetizations with shallow inclination is shown to be incorrect.