Transfer and transform fault zones in continental rifts: examples in the Afro-Arabian Rift System. Implications of crust breaking

Abstract

The concepts of transfer and transform fault zones are applied to the Afro-Arabian Rift System. Transfer fault zones are described in the Gulf of Suez and in Lake Malawi, where they trend respectively north-northeast and northeast. The intracontinental transform fault zones of Tanganyika-Rukwa-Malawi and Aswa are characterized by northwest trending “en echelon” faults, (possibly associated with basement folds and pull-apart basins) that link the principal arms of the East African Rift System. In each case the typical direction of extension is approximately parallel to the fault zones. Comparison between transfer and transform fault zones leads to the conclusion that the former are due to reactivation of local supra-crustal ancient distontinuities while the latter rework steeply dipping deep lithospheric boundaries. Asymmetric polarity of rift basins separated by transfer fault zones alternates frequently with a spacing of approximately 70 km. During formation of typical half-graben shaped rift-segments, dip-slip reworking of frontal thrust ramps may have linked high level normal faults with lower crust detachment zones, giving the rift asymmetry. However, the frequent change in polarity of this asymmetry shows that it is an epiphenomenon and therefore a fundamental lithospheric discontinuity must have initiated rifting. Such preexisting boundaries within the lithosphere may be deep steep dipping suture zones corresponding to ancient orogenic belts.