A new look at Belgian aeromagnetic and gravity data through image-based display and integrated modelling techniques

Abstract

Digital processing and image-based display techniques have been used to generate contour and shaded-relief maps of Belgian aeromagnetic data at a scale of 1:300000 for the whole of Belgium. These highlight the important anomalies and structural trends, particularly over the Brabant Massif. North and vertically illuminated shaded-relief plots, enhanced structural belts trending west-east to northwest-southeast in the Brabant Massif and west-east to southwest-northeast in the core of the Ardennes. The principal magnetic lineaments have been identified from the shaded-relief plots and tentatively correlated to basement structures. Most short lineaments are correlated with individual folds while the more extensive lineaments are correlated with large scale fault structures. Magnetic highs within the Brabant Massif are attributed to folded sediments of the Tubize Group. The magnetic basement in the east of Belgium is sinistrally displaced to the north by an inferred deep NNW-SSE crustal fracture. The Bouguer anomaly map of Belgium identifies the Ardennes as a negative area, and the Brabant Massif as a positive area, with the exception of a WNW-trending gravity low in its western part. The southern margin of the Brabant Massif is defined by a steep gravity gradient coincident with the Faille Bordiere (Border Fault). Trial modelling of the gravity and magnetic data, carried out along profiles across the Brabant and Stavelot massifs, has identified probable acid igneous intrusions in the western part of the Brabant Massif, and a deep magnetic lower density body underlying the whole Ardennes region, which is thought to be a distinctive Precambrian crustal block.

A new geological map of the outcrop areas of the Brabant Massif (Belgium)

“…Interpretation of the latter has been very helpful for the Cambrian core of the massif where magnetic and lithologic contrasts between the black shale in the Mousty Formation, the magnetite-bearing siltstones in the Tubize Formation and the quartzite in the Blanmont Formation are particularly important. Aeromagnetic data used are from Chacksfield et al (1993) and Everaerts (2000). The Bouguer gravity data and the filtered gravity data are taken from Chacksfield et al (2004).…”

Transitional geometries between gently plunging and steeply plunging folds: an example from the Lower Palaeozoic Brabant Massif, Anglo-Brabant deformation belt, Belgium

“…Within the aeromagnetic high, several more aeromagnetic lineaments are present, oriented subparallel to the Asquempont lineament (De Vos et al 1993a;Belgian Geological Survey 1994). Like the latter, these have been interpreted as steep faults or shear zones (Chacksfield et al 1993), with a dextral to reverse displacement (Everaerts et al 1996;Sintubin 1997bSintubin , 1999Van Grootel et al 1997;Sintubin et al 1998;Mansy et al 1999). Whereas the reverse movement is mainly based on the erroneous equation of the Asquempont lineament with the Asquempont fault as described by Legrand (1967), the dextral sense of movement is based on the step-like, lobe-shaped morphology of the southern margin of the aeromag- (a), demonstrating the obliqueness between the approximate tectonic shortening direction (Sintubin 1999;Debacker 2001) and the normal to the steep NE margin of the low-density body, probably resulting in a dextral transpression.…”

“…In the SW part of the Brabant Massif a large, NW-SE-trending negative Bouguer anomaly occurs (Jones 1948;De Meyer 1983;Chacksfield et al 1993;De Vos et al 1993a;Lee et al 1993), modelled as a steep-sided low-density body with a roof at between 1 and 2 km depth in the shallowest parts ( Fig. 7; Everaerts et al 1996;De Vos 1997).…”

“…7). It also forms the southwestern limit of the aeromagnetic high of the Brabant Massif, attributed to the magnetite-bearing Lower Cambrian Tubize Formation and possibly older deposits (De Vos et al 1992;Chacksfield et al 1993;Lee et al 1993;Everaerts et al 1996). Within the aeromagnetic high, several more aeromagnetic lineaments are present, oriented subparallel to the Asquempont lineament (De Vos et al 1993a;Belgian Geological Survey 1994).…”

“…Many claim these reflect folds (Chacksfield et al 1993;Mansy et al 1999); others see them as parallel to the cleavage trajectories (Sintubin 1999), or as reflecting faults or shear zones (Sintubin 1997b;Debacker 1999). Because aeromagnetic anomaly highs (lows) are caused by the presence of rocks with a high (low) magnetic susceptibility, and magnetite in the Lower Cambrian Tubize Formation occurs in bedding-parallel zones (de Magnée & Raynaud 1944;Vander Auwera & André 1985;De Vos et al 1992;Everaerts et al 1996), the aeromagnetic maps should essentially reflect bedding and possible faults or shear zones bringing into contact rocks with different magnetic susceptibilities (see de Magnée & Raynaud 1944).…”