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

Fluid flow, alteration and polysulphide mineralisation associated with a low-angle reverse shear zone in the Lower Palaeozoic of the Anglo-Brabant fold belt, Belgium

In the Lower Palaeozoic rocks of the Brabant Massif (Belgium), a recently discovered polysulphide mineralisation is related to a low-angle reverse shear zone. This shear zone has been attributed to the main early Devonian deformation event. Data from boreholes and outcrops allow a detailed investigation of the alteration pattern and palaeofluid flow along this shear zone. Macroscopic observations of the mineralogy and quantitative changes in the phyllosilicate mineralogy indicate that this shear zone is characterised by an envelope of intense sericitisation and silicification. In addition, chloritisation is associated with this alteration. The alteration zone may reach a thickness of 250 m. Ore mineralisation occurred synkinematically and is spatially related to the shear zone, The mineralisation consists of pyrite, marcasite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, stibnite and smaller amounts of tetrahedrite and other sulphosalts, It is concentrated in quartz-sulphide veins or occurs diffusely in the host rock. The mineralising fluids have a low-salinity H2O- CO2-CH4-NaCl-(KCl) composition and a minimum temperature of 250-320 degreesC. The delta(18)O values of quartz vary between + 12.3parts per thousand and + 14.5parts per thousand SMOW, and deltaD compositions of the fluid inclusions in the quartz crystals range from - 65parts per thousand to - 35parts per thousand V-SMOW. The deltaD and the calculated 6180 values of the mineralising fluids fall in the range typical for metamorphic fluids and partly overlap with that for primary magmatic fluids, The delta(34)S values, between + 4.7parts per thousand and + 10.6parts per thousand CDT, fall outside the interval typical for I-type magmas. Important migration of likely metamorphic, fluids, causing a widespread alteration and a polysulphide mineralisation along a low-angle shear zone, has, thus, been identified for the first time in the Caledonian Anglo-Brabant fold belt