Active oblique ramp faulting in the Southern Tunisian Atlas

The Gafsa fault is the longest and most active structure of the fold-and-thrust belt achieving southeastward propagation of the Atlas belt of Eastern North Africa onto the Saharan platform. The Gafsa fault is a 75-km long dextral-oblique basement fault ramp that poses a sizable challenge in earthquake hazard assessment because the post-Paleozoic sedimentary cover is decoupled from its basement above the basement fault. In this study, we combine seismic lines interpretation, tectonic geomorphology and paleoseismological investigations to assess the level of seismic hazard of this fault and evaluate its role in the geodynamic framework of the Central Mediterranean. We show that despite a moderate instrumental and historical seismicity, the fault has produced M ≥ 6 earthquakes with a return period of ca. 500–5000 years during the Late Quaternary. The latest large event having produced a surface rupture on the fault occurred around 8000 yr BP, suggesting an M ≥ 6 earthquake is overdue on the fault. The fault has a minimum reverse component of slip rate of 0.21–0.34 mm/yr over the past 50 Ka. The occurrence of M ≥ 7 paleoearthquakes on the fault may be suspected but not established. Such very strong earthquakes would require transient coseismic linkage of the buried basement fault with the overlying listric fault ramping off the décollement layer. The level of seismic hazard may be underestimated on the Gafsa fault. Indeed, given the geometry of the basement–cover fault system, a number of earthquakes generated in the basement would have led to coseismic surface folding instead of to surface rupture. The Gafsa fault is a major structure accommodating eastward extrusion / spreading of the Atlas belt onto the Saharan and Pelagian plateforms above the retreating Ionian lithospheric slab. ► First paleoseismological data are provided on the longest fault of southern Tunisia. ► Despite a low seismicity, the fault produces strong earthquakes every 500–5000 yr. ► Potentially very strong events and coseismic folding occurred on the fault. ► The fault accommodates eastward extrusion of the Atlas belt.