Geophysical mapping of the inland extension of Deep Ocean Fault zones, south-western Nigeria

Abstract

Research activities close to the south western edge of the coastal line has shown inland extension of deep oceanic features identified as regional fault zones. This research work focused on mapping of inland extension of Chain Fault zones into the south-western part of Nigeria using both satellite images and aeromagnetic intensity map over a 55 km by 55 km Ifon Basement-Sedimentary area. The area is a transition zone between the south-western basement complex and sedimentary basin of the south-western Nigeria. Interpretations of magnetic field over the area revealed intra-sedimentary features identified as fault zone which form an alignment and inland extension pattern with the offshore Chain fault zone. On the account of the qualitative and quantitative interpretation of the acquired aeromagnetic data, the Landsat-ETM+ multi-spectral bands and Digital Elevation Model (DEM) of the topography of the area, the evidence of inter-sedimentary tectonic events are recorded in the geomorphic characteristics of the study area's terrain in relation to the underlying influence of geology and structural trends revealed by extracted lineaments. From geophysical and geomorphologic interpretations, the observed geomorphic characteristics show three physiographical areas: the northern uplifted block (connection of a series of hills cut by deep-sided valleys), the central SW-NE trending subsided block (or graben) and the southern uplifted block. This geomorphic pattern of a series of flat and highly undulating terrain broken by a series of deep valleys and hills is believed to be related to rifting arising from the influence of the transform fault zones, which subdivide the area into smaller units. The rift valleys in the area are long, deep valleys bounded by parallel faults, or fractures. In addition, the predominant SW/NE, SE/NW and E-W trending linear structures mapped as fracture/faults cut the basement and penetrated the overlying Cretaceous and Tertiary sediments in the study area.