PRE-DEVONIAN, BRABANTIAN DEFORMATION WITHIN THE SOUTHERN CONDROZ INLIER (RUISSEAU DES CHEVREUILS, DAVE, BELGIUM)

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Influence of slump folds on tectonic folds: an example from the Lower Ordovician of the Anglo-Brabant Deformation Belt (Belgium)

Although it is generally accepted that buckle folds will not develop in a perfectly planar layer without the presence of some irregularity or perturbation at which the folds initiate, there are very few cases in which individual natural folds can be linked to specific irregularities. Within the Lower Ordovician Abbaye de Villers Formation, Anglo-Brabant Deformation Belt, metre-scale tectonic folds occur, of which the position and, to a certain extent, the geometry appear to be controlled by slump folds and related features. The metre-scale tectonic folds, interpreted as parasitic structures on the limb of a large-scale host fold, occur only within a stratigraphic level affected by slumping. In this level, tectonic antiforms tend to form superimposed on antiformal slump folds and on zones of abrupt, slump-related thickness increase, and tectonic synforms on synformal slump folds and on zones of abrupt thickness decrease. The rather irregular 3D geometry of sedimentary sequences suggests that many more similar cases should exist in which folds can be linked to specific irregularities. However, possibly it is also this abundance of irregularities in sedimentary sequences, in combination with fold and outcrop scale, that makes it difficult to attribute a particular fold to a particular perturbation

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

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A regional, S-dipping late Early to Middle Ordovician palaeoslope in the Brabant Massif, as indicated by slump folds (Anglo-Brabant Deformation Belt, Belgium)

Within the upper Lower to Middle Ordovician directly north of the Quenast plug (Senne valley, Brabant Massif) numerous pre-cleavage folds and associated detachments occur. These folds and the associated detachments are attributed to slumping. On the basis of a combination of a) the mean axis method, b) the separation arc method, c) the axial-planar intersection method and d) the fold hinge azimuth and interlimb angle method a SSE-dipping palaeoslope is inferred from these Slump features. This is fully compatible with the palaeoslopes inferred by two previous studies on slump folds in other upper Lower to Middle Ordovician outcrop areas of the Brabant Massif. Combined, the results indicate that during the late Early to Middle Ordovician a regionally persistent S-dipping palaeoslope existed within the southern part of the Brabant Massif, with an along-strike length of at least 30 kill. Slumping is attributed to slope failure due to seismic loading by normal faulting related to the separation of Avalonia from Gondwana. In this respect, the regional S-dipping palaeoslope may correspond to bedding tilted by antithetic (N-dipping) lystric faults, or to synthetic (S-dipping) normal fault scarps