FOLDING AND BRITTLE DEFORMATION IN JURASSIC ROCKS OF THE BOULONNAIS REGION (FRANCE), LITHOSTRUCTURAL INFLUENCE AND PALEOZOIC INHERITANCE

The Jurassic sedimentary rocks (Bajocian to Tithonian) in the Boulonna is area (NW France) uncomformably overlie a structured Palaeozoic base ment. The Jurassic rocks are composed of three litho-structural units: 1) the Bajocian and Bathonian carbonates form a thin brittle unit; 2) the Callovian and Oxfordian clays form a thick weakly folded unit 3) the Kimmeridgian and Tithonian layers of alternately competent and inc ompetent rocks form both a faulted and a folded unit. Each unit deform s specifically. The mapping of the Jurassic rocks in the Boulonnais ar ea was completed in the submarine western prolongation from the interp retation of seismic profiles. The structural pattern of the Boulonnais is marked by faults striking N110-120, N090 and N030, and by folds wi th axes trending N090 and N120. The submarine structure is marked by n umerous E-W faults and folds. The analysis in the field and the paleos tress computation reveal both brittle deformation and folding, from wh ich we interpret a relative chronology of tectonic events. The compari son between the Jurassic and Palaeozoic deformations makes the similar ity of their major directions obvious. A structural Inheritance of the Palaeozoic basement is this noteworthy when the Jurassic cover deform s: shown by various types of folding. Nevertheless, not all of the pal eozoic faults were reactivated during Mesozoic and Cenozoic tectonics.