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

<p>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. </p

Volumetric matrix strain related to intraformational faulting in argillaceous sediments

<p>Soft-sediment deformation involves complex interactions between discrete fracturing and diffuse bulk strain, described in terms of volume change and shear strain in a critical state mechanics framework. This study reports on a mesoscale normal fault zone, intraformational in Oligocene argillaceous sediments from the Boom Formation (Belgium), containing several metre-scale normal fault strands. They form either discrete fault planes or decimetre-wide shear zones with internal fabric. The faults have been subjected to microtectonic and petrophysical analysis. Small but significant changes occur in the porous network of the argillaceous matrix approaching a fault or shear zone, indicating compactional strain in both footwall and hanging wall. Internal compaction associated with faulting is put forward as a ductile–brittle feedback mechanism in the kinematics of intraformational fracture systems. Small differential stress induced by compaction and minor regional tectonic forces (differential uplift and tilt) and subsequent gravitational forces (downslope shear stress) induce small shear bands in nearly critically stressed weak mud. Shear banding is accompanied by layer-parallel shortening and bulk volume loss. This provides an additional extension of endogenous origin, accommodated by further deformation. This ductile–brittle feedback mechanism eventually leads to commonly observed intraformational collapse structures called polygonal fault systems. </p