STRUCTURAL EVOLUTION OF THE ALES COAL BAS IN (NEW DATA AND INTERPRETATION)

A derailed structural analysis of the Ales coal basin, made from plans of underground workings, boreholes and exposures in opencast mines, h as revealed the presence of two major types of fault structure which c ontrolled the deposition of the coal measures: horsetail structures an d positive flower structures. Various sub-basins separated by highs ha ve thus been individualized. It is difficult to accept a contemporaneo us movement for all these structures within a single stress field. A c hronology for the formation of the major structures has been establish ed from a systematic study of large faults sealed within the basin and from an analysis of tilting and sedimentary wedges, of progressive di scordances, of juvenile sedimentary deformation and of the sequential formation of the sub-basins. On the basis of this chronology, it is po ssible to propose a succession of local stress states that pass progre ssively from extension to compression. The conclusions are compared wi th other structural models that have been proposed for this basin and are discussed in relation to models proposed for the entire French Mas sif Central during the late Hercynian.