First Palaeogene sedimentary rock palaeomagnetic pole from stable western Eurasia and tectonic implications

Abstract

A palaeomagnetic investigation of lower Eocene (ca. 52 Ma) London Clay Formation cemented mudstones from Sheppey (SE England) has yielded a mean direction of Dec. = 1.1, Inc. = 43.2°, where N = 9, a95 = 6.8° and K = 58.5.This apparently high-quality direction (Q-factor = 5) has an associated palaeopole of 178.6°E, 63.7 N, where A 9 5 = 6.8°. The data represent the first pole from post Jurassic stable Eurasia rocks outside of the European North Atlantic Igneous Province (NAIP), of which most results have been obtained from NW Britain and the Faroe Islands. The data can in part be used to constrain the position of Palaeogene Eurasia, in particular the zero-offset declination implying negligible rotation of western Eurasia since the early Cenozoic. This is in contrast with data derived from the European NAIP, which imply small to moderate clockwise rotations for this part of the plate. The inclination angle may provide less useful information as it appears to be anomalously shallow when compared with that associated with the NAIP derived poles. In an attempt to understand the shallowing, we re-examined data from Palaeocene-Eocene sediments recovered in several boreholes (bathyal sediments in DSDP Hole 550, four cores through fluvio-delatic to middle shelf sequences in the London area, and one borehole sequence from East Anglia). In all cases, the sediments show systematic inclination shallowing similar in magnitude to that reported from Sheppey. Tectonic and geomagnetic explanations can be discounted; sediment compaction appears to be the likely cause. In light of the current controversy surrounding the 'stable Asia shallow inclination problem', the result reinforces the suggestion that tectonic modelling needs to he done carefully when the supporting data are based exclusively on palaeomagnetic studies of sedimentary rocks.