A fault and stress field analysis in a coal mine (Gardanne, Bouches Du Rhone, France)

Abstract

This paper develops an interpretation of faulting in the mining district of the Arc basin where research has been conducted since 1978 in the underground workings (coal winning is carried out by the Houillères de Bassin du Centre et du Midi: H.B.C.M.). Mining takes place in an upper Cretaceous series lying ahead of an overthrust sheet (Etoile sheet) forming the southern limit of the Arc basin. With a few exceptions the observations and measurements have been taken in the seam named Grande Mine, which is the only bed being worked at present. A chronological account of faulting is put forward. Normal faulting occurred prior to the overthrusting which brought about reverse faulting. Continued compression (NNE-SSW) led to strike-slip faulting. The last compressive phase again induced strike-slip faulting along a few major faults: this was a nearly E-W trending compression. The stress field pertaining to each period is investigated and an attempt is made to calculate the values of the principal stresses. These findings on the stress history fit what is already known about the sequence of geological events in the region. The present-day stress field (determined through in situ measurements) can be, at least in one part of the coal field, said to result directly from the tectonic conditions. It is a major cause of rock bursts in the mine workings.