New evidence for active deformation off the Annaba region (easternmost Algerian margin): Estimates of Quaternary shortening and slip rates from folding analyses

From the first marine investigations off the region of Annaba (easternmost part of Algeria) made after the MARADJA2 survey in November 2005, a set of large active faults and folds was discovered near the foot of the margin. This active thrust system resumes a previous passive margin and creates growth strata deposition on the backlimbs of large Quaternary folds, resulting in the development of perched basins at the foot of the margin since less than 1 Ma. The system forms a set of overlapping fault segments verging towards the Algerian basin. From the seismic line analysis (high-resolution and Chirp data), and using sedimentation rates obtained from coring and extrapolated, a shortening rate of about 0.9 mm/yr is estimated. If we consider a single fault dipping at about 45° (hypothesized from the Boumerdès 2003 rupture zone), a fault slip rate of about 1.3 mm/yr can be postulated. Considering recent kinematic studies, this fault-related fold system could accommodate a significant part of the present-day submarine shortening rate between the European and African plates, predicted by Stich et al. (2006) to be 1.5 mm/yr from GPS modelling. This faulting is apparently active at a faster rate than the similar faulting observed at the foot of the margin NW of Algiers. This system of faults off Annaba represents a major threat for the region. Indeed, the location of this system changes drastically the seismic hazard assessment of the region, since no large historical earthquake is reported there since 3 centuries at least.