The Mahelma fault: a secondary structure of the Sahel anticline?

The Mahelma fault, located upon the Sahel anticline at the tip of the major Sahel fault, is often suspected to be a secondary structure. Yet, to date, the Mahelma fault has displayed no definitive evidences for faulting activity and no unambiguous typology. This paper reviews new data, especially geological and subsurface data, acquired through the Algiers west microzoning project. The data indicate that (1) the Mahelma fault is an antithetic high-angle reverse fault, (2) the interpreted dip is around 80° toward the SSE and thus implies a North-verging fault, and (3) the fault zone has been observed in several fault sites and allowed direct observation of fault displacements and fault throws. (4) Displacements directly related to the Mahelma fault affect clearly young sediments including consolidated Quaternary sands. We suggest that the Mahelma fault forms an active backthrust originating from pre-existing normal fault with a possible linkage and interaction with the major Sahel forethrust. We conclude from the foregoing that the Mahelma fault most likely ruptures in concert with earthquakes on the Sahel main fault. The results also point the activity of the Sahel fault-related fold and hence complicate earthquake hazard analyses.