Geometry and kinematics of the “Herbeumont shear zone” in the High-Ardenne slate belt (Belgium)

Abstract

On the geological maps of the High-Ardenne slate belt (Belgium) an EW-trending, weakly S-dipping thrust fault is classically drawn, bordering the Eifel depression to the south. The ‘type locality’ of this fault lies in a – now completely overgrown – railway section at Herbeumont. This fault is indeed known as the ‘Herbeumont fault’. A detailed structural and microstructural analysis of an outcrop area on the east bank of the Semois river, south of the railway section, allowed us to complement the geometrical and kinematic context of the classical ‘Herbeumont fault’. The rocks exposed are predominantly fine-grained siliciclastic rocks belonging to the Lower Devonian.

Besides the bedding (S0), which is often very difficult to discern, three types of tectonic foliation can be distinguished: a primary cleavage (S1) at small angle to the bedding, a shear band foliation (SBF = S2) and an axial-planar, crenulation cleavage (S3) of late, shear-related folds affecting all pre-existing structural features. The pervasiveness – or even the mere presence – of these foliations is highly dependent on lithology, layer thickness and competence contrast within the multilayer sequence, even to the degree that a strongly developed shear band foliation can easily be mistaken as bedding. Other typical structural features include folded, boudinaged layers and folded, foliation boudins, as well as chevron folds.

All these structural features can be fitted in a kinematic model of a single, polyphase progressive top-to-the-north shearing event, occurring in the late stages of the Variscan orogeny. The outcrop area studied thus offers an excellent look into a low-grade metamorphic shear zone – the ‘Herbeumont shear zone’ – that developed in mid-crustal, brittle-plastic deformation conditions in a slate belt. Evidence of purely brittle thrust faulting – the ‘Herbeumont fault’ – could, however, not be identified in the outcrop area studied.