Cleavage–fold relationships in the Lower Cambrian Tubize Group, southeast Anglo-Brabant Fold Belt (Lembeek, Belgium)

Abstract

Little is known about the cleavage-fold relationships in the Lower Palaeozoic Brabant Massif, primarily because of the scarcity of suitable exposures. Moreover, to date, folds have only been described in the Ordovician and Silurian sequences along the southern extremity of the basement. However, excavation works for the construction of the TGV(railway)-track south of Brussels created an opportunity to study cleavage-fold relationships in the Lower Cambrian terrigenous series (Tubize Group) in a more central part of the Brabant Massif. The structural features observed seemed inconsistent with the suspected regional trends. Primarily, a divergent cleavage fan was observed in a region thought to have a regular cleavage attitude. The symmetrical but divergent disposition of the cleavage with regard to the fold hinges is explained by flexural folding of a pre-existing bedding-parallel compaction fabric. Cleavage development and folding are considered synchronous. The divergent cleavage fan reflects local strain variations. Also the steeply plunging hinge lines of the open, subangular folds are seemingly inconsistent with the regional trend characterized by subhorizontal fold hinges. Taking into account the structural position of the fold assemblage in the subvertical limb of a large-scale, upright, isoclinal fold structure, the fold assemblage is interpreted as an incongruous parasitic feature. The steeply plunging hinge lines are considered to be caused by fold hinge rotation during progressive coaxial deformation. Although this incongruous fold assemblage with its divergent cleavage fan is localized, it provides important information on both local and regional deformation circumstances in this part of the Anglo-Brabant Fold Belt.

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

“…This fold type has been described by Sintubin et al (1998) along a discontinuous temporary outcrop section of 1.15 km length in Lower Cambrian turbidites of the Tubize Formation at Lembeek (outcrops Lembeek 1-7 in Fig. 2).…”

“…The cleavage-fold relationship points to a cogenetic development of cleavage and folding. The divergent cleavage fanning is attributed to flexural folding (Sintubin et al 1998), compatible with the presence of subhorizontal striations on the fold limbs and with the geometry of small-scale folds in the hinge zones.…”

“…On the basis of aeromagnetic data, and the original description by Legrand (1967) of the Asquempont fault, Sintubin (1997bSintubin ( , 1999 and Sintubin et al (1998) suggested that the Asquempont fault in the Sennette valley (Fig. 1, F8 in Fig.…”

“…Apparently a division in fold style exists between the Ordovician-Silurian southern part and the Lower Cambrian core of the massif (Sintubin 1997a(Sintubin , 1999Sintubin et al 1998;Debacker 2001). In the Ordovician-Silurian outcrop areas, subhorizontal to gently plunging folds occur (Malaise 1873;Fourmarier 1914Fourmarier , 1921Mortelmans 1953;Legrand 1967;Sintubin 1997aSintubin , 1999Debacker 1999Debacker , 2001Debacker , 2002Debacker et al 1999Debacker et al , 2001Debacker et al , 2003Belmans 2000).…”

“…In the Ordovician-Silurian outcrop areas, subhorizontal to gently plunging folds occur (Malaise 1873;Fourmarier 1914Fourmarier , 1921Mortelmans 1953;Legrand 1967;Sintubin 1997aSintubin , 1999Debacker 1999Debacker , 2001Debacker , 2002Debacker et al 1999Debacker et al , 2001Debacker et al , 2003Belmans 2000). In contrast, the poorly exposed Lower Cambrian core of the massif appears to be characterized by an overall steepness of bedding and cleavage, with steeply plunging to reclined folds (Fourmarier 1921;Vander Auwera 1983;Sintubin 1997aSintubin , 1999Sintubin et al 1998).…”