Speaker
Prof.
Yves Quinif
(University of Mons)
Description
The relations between the karstification and the sismotectonic events can be divided in two great chapters. (1) During karstogenesis, there are relations between tectonic conditions and possibility of karstogenesis. (2) After active evolution of endokarst, we can use the geometry of the endokarstic features and the endokarstic deposits to characterize and date the sismotectonic phenomena.
I. Karstification and geological evolution
The knowledge of many karstic features proves that karstification happens during some steps of the geological history and not only as geomorphological phenomena which takes place during the last million years. To make this synthesis, we need to examine the condi-tions and the modes of the karstification.
In the frame of the karstogenesis by ghostrock, the tectonic environment is particularly important. This type of karstification is the same that occurs during the pedogenesis. The weathering of the minerals of the bedrock is sequential following the kinetics of the chemical reactions. In the case of the carbonates, the weathering dissolves preferentially the micritic part of the rock. The sparitic part (fossils, veins, a.s.o.) and the insoluble parts (clay minerals, quartz, oxides, a.s.o.) stay in place. In a first step, this evolution happens at constant volume. The residual alterite occupies bodies like vertical corridors from the top of the massif, or underground bodies: the “pseudoendokarsts”. The tectonic conditions play an important role. The initial permeability is realized in joints in extension because of the tectonics, as well as the permeability of the residual alterite. The genesis of speleological caves happens when a hydrodynamical potential appears generally by tectonic surrection.
II. Recent tectonics events and modification of the endokarstic geometry
The connected voids of a karstic system (galleries, pits, chambers) constitute a well know geometry. One can thus characterize perturbations to this geometry, like tectonics events like the action of a fault. If the gallery is always active, one must take into account the erosion processes. In Belgium, a fine example is the Lorette active faults and the setting up of an underground laboratory in the Lorette cave. A team with the ROB and the universities of Mons and Namur has collaborated to this project. Some normal faults deform galleries. Extensometers were installed and operate since 15 years. Those measurements have proved a present continuous slip of this fault zone (0,04 mm/y) which correspond to the geological observations.
III. Recent tectonics events and modification of the endokarstic deposits
Deformation of underground deposits, essentially the speleothems, also provides infor-mation on recent tectonic events: rupture of a column, apparition on columns of tension gashes, displacement of a stalactite in comparison with the associated stalagmite or formation of a bending in a stalagmite. Dating of broken speleothems by earthquakes gives the possibility to have the interval, in which the break occurred, especially for the soda straws breccia in flowstones. In conclusion, the consequences of tectonic phenomena in caves are proved and are tool for studying those phenomena. A new way of researches is the consequence of earthquakes on the collapses in great rooms, like in the “dome” chamber in the Han-sur-Lesse cave.
Primary author
Prof.
Yves Quinif
(University of Mons)
