Matrix strains along normal fault planes in the Campanian White Chalk of Belgium: structural consequences
Significant matrix strains were associated with normal faulting in the Campanian chalk of the Mons basin. These transformations, mainly due to pressure solution, located against the fault planes, brought about systematic changes in the porous network: a reduction in volume against the fault planes, a change in access diameter and an increase in heterogeneity of the material. The measurements of other physical properties (elastic waves velocity, capillary rise, permeability) of the chalk provide good evidence for estimating the width of the transformed zone: it ranges between 100 and 150 mm. These transformations cannot be detected when the medium is observed at the grain scale: the most visible evidences of transformation are limited in a 50 mm thick fringe. The transformation may have occurred either in a closed or in an open system. If we follow the first assumption we can estimate the change in volume, and therefore the change in horizontal dimension of the faulted blocks: it is small compared to the horizontal extension. The second assumption is supported by a few evidences: tilted and striated rostra of belemnites along fault planes. In that case the geometrical change can be estimated too and reveals that the horizontal shortening is in the same range as the horizontal extension. It means that the extensional deformation of the Mons basin might have been partially hindered by chemically induced matrix strains.