The use of jointing to infer deformation episodes and relative ages of minor Cretaceous intrusives in the western part of Ikom – Mamfe basin, southeastern Nigeria

•The intrusives have been affected by mostly brittle deformation.•The intrusives reveals two episodes of deformation from fracture studies.•Extension fractures are dominant in these intrusives.•Mineral veinlets in the intrusives are mostly emplaced in ‘ac’ extension fractures.

The most fundamental joint sets in a region are those oriented normal to the fold axis (‘ac’- extension fractures) and those oriented parallel to the fold axis (‘bc’- tensile fractures). Four sets of joints were observed in deformed igneous intrusives of the study area, representing two phases of Cretaceous deformation. In the Ikom –Mamfe basin, the stronger (‘ac’- extension) fractures are oriented NW – SE and NNE – SSW while the weaker sets were oriented NE – SW and ESE – WNW, representing the first and second phases of Cretaceous deformation respectively. The study of the growth of the ‘ac’- and ‘bc’ fracture sets observed on the intrusives at different locations reveals that generally the ‘ac’- fracture set has a dominant percentage frequency over the ‘bc’- fracture set. The high percentage frequency of the ‘ac’-sets in locations with insignificant or absence of the ‘bc’- set implies that the ‘ac’- extension fractures are the first to initiate and probably propagated with greater ease during any episode of compression, than the ‘bc’- tensile fractures. Mineralized veins hosted by the intrusives are also more pronounced in the ‘ac’-sets than the ‘bc’- sets. Also because of the extensive development of deformation structures in the intrusives, this study proposes the idea of syn-tectonic, rather than post-tectonic magmatic emplacement for the nine intrusives studied.