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 Camp anian chalk of the Mons basin. These transformations, mainly due to pressur e solution, located against the fault planes, brought about systematic chan ges in the porous network: a reduction in volume against the fault planes, a change in access diameter and an increase in heterogeneity of the materia l. The measurements of other physical properties (elastic waves velocity, c apillary rise, permeability) of the chalk provide good evidence for estimat ing the width of the transformed zone: it ranges between 100 and 150 mm. Th ese transformations cannot be detected when the medium is observed at the g rain scale: the most visible evidences of transformation are limited in a 5 0 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 secon d 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 ran ge as the horizontal extension. It means that the extensional deformation o f the Mons basin might have been partially hindered by chemically induced m atrix strains. (C) 1999 Elsevier Science B.V. All rights reserved.