SEISMIC ACTIVITY RECORDING IN THE MESSINIAN CARBONATE PLATFORMS OF NORTHWESTERN AFRICA

Numerous synsedimentary deformational structures are observed in the M essinian platforms from North Africa (Morocco, Algeria). These structu res were identified in for to five levels of the platforms and are esp ecially well developped in the post-reefal sediments. Two kinds of str uctures occur: 1) brittle structures with mainly normal faults; 2) vis coplastic structures with synsedimentary folds, diapiric structures an d slumpings. The lithology and the sedimentology of the deformed sedim ents as well as the geometry of the structures that are fossilized by the upper beds imply that the deposits were reworked by superficial an d sudden events. Such structures may result from seismic activity thro ugh differently compacted levels. In still water-rich sediments, the s tructures depend on several mechanical constraints as differential rew orking, constant lateral stresses, etc. Some of these structures (slum pings, drag-folds) are however influenced by gravity with slidings on slopes, immediately after the seismic event. In early lithified sedime nts, seismic waves induced brittle deformations. The evidence of a Mes sinian seismic activity in the Western Mediterranean area underlines a slight, but permanent, tectonic activity that may have locally contro lled the sedimentation and the distribution of some facies.