TECTONIQUE DU FRONT ...

TECTONIQUE DU FRONT VARISQUE EN HAINAUT ET DANS LE NAMUROIS

ABSTRACT.
Tectonics of the variscan front in the provinces of Hainaut and Namur (South Belgium). The coal fields of the Hainaut and Namur Provinces are made up of 2 distinctive tectonic units. The upper unit stretches from Douai in the West to Huy in the East and comprises the Denain massifs, which stretch from France into Belgium and become the massif of Masse, followed by the massifs of Chamborgneau, Ormont and Malonne. It is shown that these massifs, though slightly separated one from the other, form actually one big massif, which was named "Great Superficial Massif". It lies on a parautochtonous unit made up of an imbricate fan, which is separated by inverse faults of easily determinable throws.
The sole fault that separates those 2 units was called "Great Hainaut fault". It passes over a footwall of cataclastic rocks. For a long time, it has been called "faulted zone" or even "overthrust sheet".
However, in our opinion this zone represents the surface of a fossil peneplain. Indeed, no Westphalian C is known in the parautochtonous massifs. Therefore, erosion and peneplanation must have taken place before the emplacement of the "Great superficial massif".
The "Great superficial massif" is made up of Westphalian C, B and A, of Namurian, Dinantian, upper and middle Devonian, Silurian and Ordovician formations. This "Great superficial massif" originated from the North and was originally deposited on the Brabant massif. From there, it underwent a gravitational slip into the "Flenu trough" and covered the parautochtonous unit. It was then in turn partly overthrusted by the Midi massif set in motion towards the North.
This slip into the Flenu trough is caused, on the one hand by the uplift of the Brabant massif and on the other hand by the dissolution of evaporites in the deeply buried Dinantian. The latter must have contained halite as shown by the brecciated anhydrite, indicative for early collapse.
This explains why the "Great superficial massif" does not include lower Devonian formations and that the coals have not evolved much as they have not undergone any deep burial.
The sole fault of the "Great superficial massif" seems linked to the Midi fault. One must write "seems" since the intersection between both faults is not exposed as the coal mining kept a distance from this intersection. However, one can reasonably locate it between the overturned strata, which characterise the southern front of the "Great superficial massif" and the moderately dipping strata of the parautochtonous massifs.

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