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

Although many studies have dealt with markedly different fold orientations and associated cleavage–fold relationships within individual, single-phase deformed fold belts, there are very few descriptions of possible gradual, transitional fold geometries. The Lower Cambrian steep core of the single-phase deformed Brabant Massif contains steeply plunging, west-facing folds with a Z-shaped asymmetry, whereas the Ordovician–Silurian southern rim consists of gently plunging, upward facing folds. A gradual transition is observed between these end-member orientations, in a NW–SE-trending zone 1–1.5 km wide, in which the folds appear to be strongly curvilinear and locally downward facing. The structural geometries within this transition zone are described in detail and the geometric changes analysed in the light of the fold transition. The strongly variable fold orientations are tentatively attributed to a bulk oblate tectonic strain. The transition zone overlies an aeromagnetic lineament, classically interpreted as a dextral shear zone. The steeply plunging folds, the transition zone and the aeromagnetic lineaments are all attributed to a local dextral transpression, in which deformation is partitioned both vertically and laterally. The results indicate that within zones of heterogeneous transpression, the different deformation domains are not necessarily always fault bounded.