Observation and interpretation of fault activity in the Rochefort cave (Belgium)

Abstract Very recent fault movements have left their imprint on the karstic morphology in caves of the Belgian Ardenne (Western Europe). To quantify these movements and to understand their possible causes, they have been measured since 1997 in the Rochefort cave where this recent geological activity is clearly visible. Specific extensometers adapted to the underground conditions have been developed to measure very slow fault movements. In the Rochefort cave, the movements correspond to a normal mechanism along N070°E striking faults with a small sinistral strike-slip component. Ten years of measurements at the “Fontaine-Bagdad” fault, which shows the most significant displacement in the order of 0.3 m, evidences continuous relative movements of 0.04 mm/year and 0.01 mm/year in the fault dip and strike direction respectively. If the geological displacement has been continuous, the total displacement could have occurred since the end of the last glacial period. Microtectonic analysis from the recent striated faults suggests that the maximal principal horizontal stress is oriented N030°E. This direction is similar to that of the post last glacial period compressive stress resulting from the glacial isostatic adjustment in this part of Europe, but is perpendicular to the direction derived in Western Europe from earthquake fault plane solutions. We provide evidence that the fault movements cannot be caused by gravity due to the surface topography or the vacuum in the cave. We propose that these fault slips are an expression of the deformation of the lithosphere due to the compressive strain acting since the end of the last glacial period in the peripheral bulge of the flexure resulting from the unloading of the large ice sheet in Fennoscandia.