Lower-Crustal Normal Faulting and Lithosphere Rheology in the Atlas Foreland

Earthquakes beneath the foreland basins of the Andes and Tibet are rare but follow a simple pattern, with normal faulting from 0 to 20 km depth and reverse-faulting from 30 to 50 km depth. The switch in faulting style with depth suggests that the stresses generated by foreland flexure are large enough to break faults, with opposite senses of horizontal strain either side of a neutral surface in the mid-crust. In this study, we document a 31 km-deep Mw 5.2 normal-faulting earthquake in the forelands of the Algerian Atlas Mountains near Biskra. The Biskra earthquake is of interest, as it indicates that the lower crust of the Atlas forelands is in extension at the same depth that the Tibetan and Andean forelands are in compression. In order to match the gravity anomalies and the depth of normal faulting near Biskra, we find that models of lithospheric flexure require the neutral surface to be >35 km deep and at least the top 5–10 km of the lithospheric mantle supports elastic bending stresses. The differences in the pattern of earthquakes between the forelands of Tibet, the Andes and the Algerian Atlas can be explained by differences in the buoyancy forces acting between these mountain ranges and their lowlands that place the forelands into varying amounts of net compression. Our results suggest the upper mantle beneath cratonic foreland lithosphere may therefore support bending stresses of the order of 10s of MPa, likely because it is cool and the strain rates associated with bending are low.