Offshore mud volcanoes and onland faulting in southwestern Africa: neotectonic implications and constraints on the regional stress field

Recently discovered mud volcanoes in the Orange Basin, offshore southwestern Africa, denote the existence of neotectonic faults in the submerged continental shelf. Interpretation of seismic lines perpendicular to the trend of the alignment of the mud volcanoes shows flower structures, diagnostic of strike-slip faulting along a N/NNW direction. Analysis at the regional scale of onland neotectonic features in southwestern Africa shows that recent faulting occurred both in central Namibia and Namaqualand, South Africa and that it created both N/NNW- and NW-trending lineaments. It is proposed that the newly discovered offshore neotectonic activity and the onland structures described in this paper represent the structural expression of the same stress field. These structures form a set of conjugate transtensive faults, which constrain the regional horizontal greatest compressive stress in a NW/NNW direction. Such stress orientation, also supported by in situ stress measurements, defines the so-called Wegener stress anomaly, the predominant present-day stress field of southwest Africa. The Wegener anomaly is incompatible with the stress orientation required by plate-scale tectonic constraints, mainly in the form of recently published GPS motion values for the African plate.