Characterisation of late Cretaceous NE-SW sinistral wrench faults in the Upper Benue Trough (Nigeria) using microtectonic and aeromagnetic data

Abstract

The northern section of the Cretaceous Benue Trough in Nigeria presents several major faults zones striking N 30°E to N 60°E cross-cutting both sedimentary and Precambrian basement rocks in the area. The Mesozoic deposits in the area are also deformed by the NE-SW to N 80° E trending assymetric large scale folds (Lamurde and Jarawa anticlines, Dadiya syncline). A detailed microtectonic study of the deformed tectonic zones has been carried out, mainly along the major faults zones (Gombe, Burashika, Kaltungo and Teli areas) on the sedimentary basin and on the Pan-African basement (Plateau and Gurukusa areas) surrounding the basin. At each microtectonic station a significant population of striated planes and their associated slickensides were measured. Analysis of the brittle microfaults using a computer-aided method resulted in the definition of the tectonic paleostress tensors responsible for their formation. A first brittle compression characterized by a maximum compressive stress σ 1 predominantly trending N 15° E was mainly identified along the NE-SW striking major fractures. This particular brittle tectonic event is believed to be related to the sinistral Cretaceous reactivation of Pan African NE-SW transcurrent faults. In general all the estimated stress tensors close to strike-slip tensor are characterized by a significant deflection of the direction of the maximum compressive stress σ 1 ranging from N 180° E to N 60°E. By comparing these data with microtectonic and mathematical models on strike-slip faults, the characterization of the NE-SW megafractures as sinistral strike-slip faults is particularly demonstrated. A second N 150° E compressive brittle event is also observed. The two tectonic events are documented as prevailing at the end of the Cretaceous. A structural interpretation of the total magnetic field intensity map of the study area confirms the predominance of NE-SW trends in the area. Many of these trends appear as deep magnetic discontinuities which obviously have their source in the basement. A number of these trends coincide with mapped faults (e.g. Gombe, Teli, Kaltungo and Burashika fractures). In the northeastern part of the investigated area, magnetic elongated zones demonstrate a tectonic control of the Cretaceous sediments by structurates in the basement.