Abstract

In the upper part of the Benue Trough of Nigeria, several major NE–SW lineaments are essential components in the structure of both the Pan-African basement and the Cretaceous sediments. In order to estimate their influence on the geodynamics of the Trough, detailed structural analyses have been carried out on the fault rocks of one of them, the Kaltungo lineament. It cross-cuts a basement inlier emerging from Lower to Middle Cretaceous sediments. This inlier of granitic rocks exhibits a wide fault zone trending N50E which displays an anastomosing network of both ductile and brittle shear zones. The former are expressed by mylonitic bands generated by a Pan-African dextral wrenching on a regional scale; the latter are represented by fault rocks of the cataclasite series related to a sinistral wrenching which occurred during the Mesozoic. The fault zone extends into the cover where it is seen as a thin crush zone characterized by conjugate strike-slip and reverse faults. From a computer dynamic analysis of microfaults, an earlier compression (N–S shortening axis) was found only within the basement, whereas a second one (N140E shortening axis) appears in both sediments and basement. The first palaeostress field corresponds to cataclasite genesis which preceded or coincided with the formation of pull-apart basins along the sinistral Kaltungo wrench fault, which initiated the Upper Benue Trough during the Lower Cretaceous. Sedimentary basins and uplifted zones (Kaltungo Inlier) were formed on releasing and restraining oversteps of such a fault. The above statement may be extended to other similar faults at the scale of the entire trough. The second palaeostress field corresponds to the Late Cretaceous compression responsible for the large scale folding in the sediments and the formation of fault-breccia along the sinistral Kaltungo lineament.

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