Fracture systems in the younger granite rocks around fobur, northern Nigeria: product of residual stress?

Abstract

Outcrop-scale fractures and associated veins found in the Younger Granite rocks around Fobur, Northern Nigeria were studied in relation to similar structures in their host rocks (Migmatites-gneisses -quartzite complex). Fractures and veins attitude (strike and dip) data were collected across the study area and subjected to graphical and stereographic analyses. The result revealed a predominant N-S strike with variations in the NE-SW and NW-SE directions; and others trending in the E-W direction (dip joints). Field observations indicated neither deformation of joints planes nor distortion of joint directions which confirmed that the jointing occurred in an anorogenic setting. Nevertheless, the joint structural trend in the Younger Granite rocks replicated those found in the surrounding orogenic basement rock exposures. This could reflect the likely existence in the host Migmatite-gneiss-quartzite complex imposed residual stresses of the Pan African event that were not fully relieved. These ancient stress systems possibly helped to orient the fractures and other structures in a later emplaced Younger Granite complex in a fashion similar to the enclosing basement complex rocks.

KEY WORDS: Residual Stress, Fractures, Younger Granites, Fobur, Nigeria