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.