A crustal magnetic model of Britain obtained by 3D inversion

D Beamish,T.C Pharaoh,D.I Schofield

doi:10.1016/j.tecto.2021.228982

Abstract

The national baseline aeromagnetic survey of Britain allows a uniform assessment of the shallow and deep magnetic properties of the British tectonic terranes. The most significant is that associated with destruction of early Palaeozoic oceanic lithosphere across the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Here a formal 3D inversion of a continuous swathe of the data is considered. The study provides a uniform volumetric whole crust assessment extending for over 1000 km. Normally a 3D inversion of magnetic data is controlled using a variety of constraints however this is not appropriate at the crustal scale due to our increasingly imprecise knowledge of lithology at increasingly greater depths. The main crustal interface encountered occurs at the Curie isotherm depth. We demonstrate the behaviour of introducing different magnetic crustal depths and suggest the crustal ‘magnetic depth’ of our models can be independently constrained using global or regional studies of the deep geotherm. Static magnetic data have no inherent depth resolution. Here an empirical ‘1D depth’ weighting and a more formal ‘3D distance’ weighting are assessed. The inversion procedure is regularised to provide stable models appropriate to the data and their errors. To gain confidence when using such a ‘geologically-unconstrained’ inversion, we compare our 3D inversion results with an existing geologically-constrained 2.5D profile inversion across northern Britain. A surprising agreement in the 3D susceptibility magnitudes is observed. The chosen study area traverses 10 British terranes and images their tectonic fabric by way of non-magnetic zones (i.e. susceptibilities <0.0001 to 0.001 SI) and magnetic zones displaying geological relevance and tectonic significance at deeper crustal levels. Here we discuss the more significant 3D model features which, by virtue of a continuous crustal-scale assessment and fitting the data with a high degree of fidelity, provide additional structural insights.

Highlights

The remarkably varied surface geology of Britain reflects an varied crustal structure
Since the 1990's, most authors have considered the Avalon Composite Terrane, to south of the Solway Line (SL), to incorporate the Leinster-Lakesman Terrane (LLT), which includes the Lake District and North Pennines (Fig. 1a); the Caledonides of Southern Britain (CSB); and a nucleus of smaller terranes accreted to Gondwana in Neoproterozoic time, of which the Charnwood Terrane
A subset of the baseline TMI (Total Magnetic Intensity) data, comprising a NW-SE swathe used for the 3D inversion is identified in Fig. 1b, within the wider rectilinear data set

Summary

Introduction

The remarkably varied surface geology of Britain reflects an varied crustal structure. Significant crustal reworking and further accretion affecting southernmost Britain occurred during the Variscan Orogeny, ending at about 300 Ma. The Laurentian terranes include crust generated in Archaean (HT and NAT in part), Palaeoproterozoic - Mesoproterozoic (NHT, CHGT in part and ?MVT), Neoproterozoic (CHGT) and early Palaeozoic time (MVT and SUT) (Fig. 1a and Table 1). The Laurentian terranes include crust generated in Archaean (HT and NAT in part), Palaeoproterozoic - Mesoproterozoic (NHT, CHGT in part and ?MVT), Neoproterozoic (CHGT) and early Palaeozoic time (MVT and SUT) (Fig. 1a and Table 1) They were accreted to the Lau rentian margin by early Silurian time, with late orogenic sinistral strikeslip modifying terrane boundaries and internal structure well into Devonian time (Dewey and Strachan, 2003). Since the 1990's, most authors (following Bluck et al, 1992) have considered the Avalon Composite Terrane, to south of the SL, to incorporate the Leinster-Lakesman Terrane (LLT), which includes the Lake District and North Pennines (Fig. 1a); the Caledonides of Southern Britain (CSB); and a nucleus of smaller terranes accreted to Gondwana in Neoproterozoic time, of which the Charnwood Terrane

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