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

Abstract

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 singlephase 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.

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

“…As demonstrated by Debacker (2001) and Debacker et al (2004b), type A folds also occur in the Cambrian core, gradual transitions exist between the type A and type B folds, and both fold types formed during the same progressive deformation event, the Brabantian deformation event (see also Debacker, 2012).…”

“…The Cambrian core of the Brabant Massif is characterised by an overall steepness of bedding and contains steeply plunging folds (type B folds of Debacker (2001) and Debacker et al (2004b)) whereas the Ordovician-Silurian rim is characterised by subhorizontal to gently plunging folds (type A folds). As demonstrated by Debacker (2001) and Debacker et al (2004b), type A folds also occur in the Cambrian core, gradual transitions exist between the type A and type B folds, and both fold types formed during the same progressive deformation event, the Brabantian deformation event (see also Debacker, 2012).…”

“…Several types of syncleavage folds have been distinguished, ranging from steeply plunging folds associated with subvertical to steeply dipping cleavage (type B folds sensu Debacker, 2001;cf. Sintubin et al, 1998) in the Cambrian core of the BM, to more widespread subhorizontal to gently plunging folds, associated with a steep to moderately north dipping cleavage (type A folds sensu Debacker, 2001) (Sintubin, 1997(Sintubin, , 1999Debacker, 2001;Debacker et al, 2003Debacker et al, , 2004bDebacker et al, , 2005bDebacker, 2012).…”

“…In the few cases where a basal Intepreted shear zone at depth (Sintubin, 1999;Debacker et al, 2004b) Magnetic high N N 5 km conglomerate is visible it is thin (10 cm maximum) and reduced to some small debris derived from the nearest substrate (Herbosch & Verniers, 2015, p. 25).…”

“…The Central Brabant Unit (blue hues on the new maps) is exclusively formed by the lower Cambrian Blanmont, Tubize and Oisquercq formations (Figs 4, 9, 10), whereas the Senne-Dyle-Orneau Unit (green and purple hues on the new maps) includes all younger formations. The Central Brabant Unit is characterised by the common to frequent occurrence of steeply plunging folds (Type B folds of Debacker et al, 2004b) (Figs 9, 10, 11, 12;e.g. Debacker, 2012).…”