New insights into the deformation of a Middle Pleistocene glaciotectonised sequence in Norfolk, England through magnetic and structural analysis

AbstractNorth Norfolk is a classic area for the study of glacial sediments with a complex glaciotectonic deformational history, but the processes leading to the formation of some structures can be ambiguous. Anisotropy of magnetic susceptibility (AMS) analyses, providing quantitative fabric data, have been combined with the analysis of visible structures and applied to the Bacton Green Till Member, exposed at Bacton, Norfolk. Thermomagnetic curves, low temperature susceptibility and acquisition of isothermal remanent magnetism (IRM) reveal that the magnetic mineralogy is dominated by paramagnetic phases. The magnetic foliation is parallel to fold axial planes and weakly inclined to bedding, whilst the magnetic lineation is orientated parallel to stretching, indicated by the presence of stretching lineations and the trend of sheath folds. Variations in the orientation of the magnetic lineation suggest that the Bacton section has been subject to polyphase deformation. After subaqueous deposition, the sequence was overridden by ice and glaciotectonically deformed which involved stretching initially north–south, then east–west. These results show that AMS can be used to detect strain in three dimensions through a glaciotectonite where paramagnetic mineralogy is dominant. This approach therefore provides further support to the use of AMS as a fast, objective and accurate method of examining strain within deformed glacial sediments

Laccolithic, as opposed to cauldron subsidence, emplacement of the Eastern Mourne pluton, N. Ireland: evidence from anisotropy of magnetic susceptibility

<p>The structural evolution and emplacement of the Eastern Mourne pluton was investigated using anisotropy of magnetic susceptibility (AMS) measurements (carried out on 112 oriented block samples) and structural data from the host rocks. From these new data cauldron subsidence, as the emplacement mechanism, is disputed and evidence for an alternative, laccolithic style model involving inflation is presented. This includes deflection and uplift of host-rock bedding close to contacts and the magnetic fabric pattern, which has a gentle dome geometry, even close to contacts. The magnetic lineations usually plunge down-dip near the external margins but otherwise have a general SSW–NNE trend that diverges northward. This suggests a northward-directed inflow direction. The model for the emplacement of the Eastern Mourne pluton is a laterally fed laccolith, emplaced south to north. The eastern margin is interpreted as a faulted contact facilitating the inflation of an asymmetrical ‘breached’ laccolith. </p

Mesozoic-Cenozoic exhumation and volcanism in Northern Ireland constrained by AFTA and compaction data from the Larne No. 2 borehole

There have been many studies into the post-Palaeozoic exhumation history of the Irish Sea basin system, which is thought by some to be the locus of Cenozoic exhumation in the British Isles. Few studies, however, have sought to constrain the history of Mesozoic–Cenozoic vertical motions in Northern Ireland, where the geological record of this time period is comparatively complete. Post-Triassic rocks are missing from large parts of the Irish Sea, but sediments of Lower Jurassic, Upper Cretaceous and Oligocene age are found in Northern Ireland, in addition to the Paleocene flood basalts of the Antrim Lava Group. Here we present apatite fission-track analysis (AFTA) and sedimentary rock compaction data from the Larne No. 2 borehole, NE Northern Ireland, which penetrated a c. 2.9 km thick Permian–Triassic succession intruded by Palaeogene dykes and sills. We show that the preserved section was more deeply buried by up to 2.45 km of Upper Triassic–Lower Jurassic sediments that were removed during exhumation episodes beginning during the mid-Jurassic and early Cretaceous. Our results suggest limited early Palaeogene exhumation, which is consistent with the preservation of Upper Cretaceous Chalks beneath the Antrim Lava Group. They also indicate deeper burial of the preserved section by up to 1.3 km prior to late Cenozoic exhumation. This additional section could include a substantial thickness of Paleocene basalt, which provides a likely explanation for the anomalously low porosities of the Chalk in Northern Ireland.Simon P. Holford, Paul F. Green, Richard R. Hillis, Jonathan P. Turner and Carl T.E. Stevenso