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
Highlights
Fractures are the most ubiquitous structures in the crust of the earth, occurring in a wide range of rock types and tectonic settings
Much work has been done on the structures of the Younger Granites, especially with respect to ring dykes, cone sheets, volcanic cauldrons, and high level granite plutons (Turner, 1989), very little is known about the patterns of fractures and the stress fields that produced them
This paper examines the structural attitudes of fractures in the Younger Granite rocks around Fobur and their course of emplacement
Summary
Fractures are the most ubiquitous structures in the crust of the earth, occurring in a wide range of rock types and tectonic settings. Geological Setting The Younger Granites of Jos Plateau, Nigeria, are made up of alkaline feldspar granites in association with rhyolites, minor gabbros and syenites They occur mainly as sub-volcanic intrusive complexes of ring dykes and related annular and cylindrical intrusions. Each cycle was marked by deposition of supra crustal sediments and volcanics, succeeded by deformation and metamorphism, basement reactivation and emplacement of granitic intrusions These developments are products of four orogenies which affected the Nigerian basement during the Precambrian. The deformed basement appeared to have exerted a measure of structural control in the emplacement of the Younger Granites of Nigeria. Their emplacement is completely anorogenic and of Jurassic age (160-170ma) (Badejoko, 1988; Ogunleye et al, 2005; Jacobson et al, 1963) This is in sharp contrast with the calc-alkaline Older Granites of the basement that are Pan African in age (≈650ma) and orogenic in origin (Van Breeman and Bowden, 1973)
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