EVIDENCE ON THE DEEP-STRUCTURE OF THE ANGLO-BRABANT MASSIF FROM GRAVITY AND MAGNETIC DATA

Gravity and aeromagnetic data from Britain, Belgium and the southern N orth Sea have been compiled to provide coverage of the greater part of the Anglo-Brabant Massif. Colour pseudo-relief maps of the gravity an d magnetic fields highlight important anomalies and trends which provi de new information on the structure of the massif and its margins. Wit hin the massif, prominent SSE-trending geophysical lineaments define t he margins of distinctive blocks within the upper crust. These are cro ss-cut on the northeastern margin of the massif by prominent ESE- and SE-trending magnetic and gravity lineaments. The possible history and origin of the more prominent geophysical anomalies and lineaments are considered. Integrated modelling of the potential field data has been carried out along the BIRPS MOBIL-7 seismic reflection line to provide an interpretation of crustal structure across the northeast margin of the massif constrained by all three datasets. The principal features of the model are a non-reflective, non-magnetic upper crust, interpret ed as the Caledonian fold-thrust belt, overlying a heterogeneous middl e-lower crust with laterally varying reflectivity, magnetization and d ensity. ESE-trending magnetic anomalies along the northeast edge of th e massif are explained in terms of an irregular mid-crustal magnetic l ayer with a susceptibility comparable to that of the Tubize Group in t he Brabant Massif. The top of this body is coincident with prominent d ipping mid-crustal reflectors observed on the seismic reflection profi le and its overall geometry is compatible with mid-crustal imbrication inferred from the seismic data.