Abstract

The Precambrian polycyclic basement terrains of Oban and Obudu in southeastern Nigeria contain a plethora of fractures, which on analysis fall into only four sets or trends. The most prominent fracture set in Oban Massif is the NNW-SSE, with a trend of 150⁰-160⁰ from the north. Others are NNE-SSW, E-W and NW-SE sets. The most prominent fracture set in Obudu is the NW-SE which trends 140-150⁰ from north. Minor sets occur in the NNE-SSW, E-W and ESE-WNW directions. While E-W fractures are interpreted as ‘ac’ tensile or extension fractures, NE-SW and NW-SE fractures are shear fractures. The orientation difference of 10⁰ between the major facture sets in Oban and Obudu is interpreted as the amount of fracture induced angular rotation between the two basement blocks, leading to the formation of the Ikom-Mamfe embayment. The most frequently occurring fracture length in both areas is ~ 2km, while the fracture frequency decreases exponentially as fracture length increases. This is depicted by the exponential relationship n(l) = n_oe^-kl. There appears to be compensation between fracture length and frequency, such that directions of long fractures have lower density and vice versa. Whenever fracture lineaments less than 5km are ignored in the Nigerian basement, it could represent over 60% loss of data in the analysis.

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