Aeromagnetic Mapping of Fault Architecture Along Lagos–Ore Axis, Southwestern Nigeria
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Open Geosciences 2020; 12: 376–389 Research Article Olagoke Peter Oladejo, Theophilus Aanuoluwa Adagunodo*, Lukman Ayobami Sunmonu, Moruffdeen Adedapo Adabanija, Charity Adaeze Enemuwe, and Patrick Omoregie Isibor Aeromagnetic mapping of fault architecture along Lagos–Ore axis, southwestern Nigeria https://doi.org/10.1515/geo-2020-0100 deformation, which is associated with folds of geological received July 30, 2019; accepted March 17, 2020 features. The faults, as depict by the UC map, reveal ff – Abstract: A seismic wave is released when there is sudden di erent depth ranges of 500 2250 m at the western side – displacement on a fault plane. The passage of this wave and 1,500 1,250 m at the northwestern area of the study. along the fault plane or within the lithosphere could result Since it has been on record that September 11, 2009, earth in ground shaking or vibration at the surface of the Earth. tremor of magnitude 4.4, with the epicenter at Allada, To provide a geophysical explanation to this phenomenon, Bennin Republic, 128 km west of Lagos, Nigeria occurred the high-resolution aeromagnetic data of the sedimentary within the study area, it can be inferred that the established terrain and part of the Basement Complex of Southwestern geologic fault architecture could be responsible for the - Nigeria were processed and interpreted to provide fault hazard and be part or synthetic to the Ifewara Zungeru fault architecture of the area, which could serve as conduit for in Nigeria. the passage of seismic energy in the study area. High- Keywords: magnetic intensity, tectonic deformation, resolution aeromagnetic data along the Lagos–Ore axis are fault architecture, tremor, aeromagnetic data processed for fault mapping in the study area. The reduced- to-equator (RTE) residual aeromagnetic data used were enhanced using the total horizontal derivative (THD) and upward continuation (UC) filtering techniques on Oasis 1 Introduction Montaj 6.4.2 (HJ) software. The resultant maps were overlaid and compared with the plotted RTE residual Fault occurs when there is a mechanical break or generic maps for relevant interpretations. Varying signatures of discontinuity within the rock volume. It could also be termed magnetic anomalies are grouped into high (57.9–89.1 nT), as the displacement in a rock volume by fracturing along a intermediate (38.2–57.9 nT),andlow(4.0–38.2 nT) mag- planar interface. A fault initiates or propagates when there is netic intensities, which are associated with contracting equity between the stress and strength of a rock. Some of the basement rocks features. The obtained lineaments from the mechanisms that produce high stress within the crust fl THD reveal areas of various deformations such as brittle, include uid pressure; lithostatic; geothermal occurrence; ’ which is associated with faults/fractures, and ductile tectonic forces as regard to lithospheric plates movement; geologic processes such as volcanic activities, folding as well as salt intrusion; and other forces by extraterrestrial bodies * Corresponding author: Theophilus Aanuoluwa Adagunodo, [1]. The fault architecture is related to the fault size, Department of Physics, Covenant University, Ota, Nigeria, orientation, shape, and its interconnectivity [2].Itcould e-mail: [email protected], also be termed as the overall distribution of fault’s [email protected] displacement into subfaults. Olagoke Peter Oladejo: Department of Physics, Emmanuel Alayande Any event that occurs suddenly in nature could be College of Education, Oyo, Nigeria fi Lukman Ayobami Sunmonu: Department of Pure and Applied termed as natural disaster. Such events are wild re, solar Physics, Ladoke Akintola University of Technology, Ogbomoso, flare, sinkhole, cyclone, drought, thunderstorm, tornado, Nigeria flood,volcaniceruption,landslide,hurricane,tsunami,and Moruffdeen Adedapo Adabanija: Department of Earth Sciences, earthquake. Of all these events, earthquake has been Ladoke Akintola University of Technology, Ogbomoso, Nigeria regarded as the greatest disaster [3].Itcomesswiftly, Charity Adaeze Enemuwe: Department of Physics, Covenant [ ] University, Ota, Nigeria without giving uncertainty room for its occurrence 4 .This Patrick Omoregie Isibor: Department of Biological Sciences, phenomenon is so scary as a result of its destructiveness to Covenant University, Ota, Nigeria lives, properties, and economy. Generally, earthquake is Open Access. © 2020 Olagoke Peter Oladejo et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 Public License. Aeromagnetic mapping of fault architecture along Lagos–Ore axis, Nigeria 377 believed to occur within tectonic margins, but record has structures and some human activities have aided the showed that it could also occur outside the tectonic mechanisms that are responsible for the seismic events in margins, as in the case of Nigeria [5]. Nigeria [3].Theyearsofoccurrenceoftheseeventsare1933, Nigeria is situated on a seismic inactive plate, but the 1939, 1964, 1984, 1985, 1987, 1990, 1994, 1997, 2000, 2006, rate at which tremors are being recorded (via instrumental 2009, 2016, and 2018 [7,8]. In recent time, these events have and/or historical platforms) from 1933 across the country till begun to produce a scary outcome, which could be likened the recent one that occurred in Federal Capital Territory, to a detonation of a small fission bomb as reported by [9].If Abuja, in 2018 had proved that Nigeria is no longer safe from this event is not properly monitored, seismic activities with earthquake occurrence(s), which requires further investiga- greater magnitude than those recorded earlier (magnitude tions. Nigeria resides on the eastward of the Atlantic Ocean, of up to 4.5) could be triggered in the region [10].According which is attached to the passive margins (Figure 1) toward to the literature, some locations that have been affected by the south and southwestern regions of the country. tremors so far include Saki, Igbogene, Kwoi, Sambang Dagi, Geologically, there have been consistent openings or move- Dan Gulbi, Kano, Kombani Yaya, Lupma, Abuja, Yola, ment in the Atlantic margins since the Jurassic (201.3 – 145 Gembu,Abeokuta,Oyo,Ibadan,Ogbomoso,Akure,Sha- Mya) to present [6]. Unlike the active margins (such as the gamu, Ijebu-Ode, Lagos, Okitipupa, Warri, Ile-Ife, Ijebu- Pacific), which is associated with devastating earthquake Remo, Ohafia, Kundunu, Obi, Akko, Kurba, Jerre, Benin, occurrences as a result of subduction tectonics, passive Edo, Jushi-Kwari, Akure, and Abomey-Calavi [3,6,11,12]. margins (such as Atlantic) are believed to be stable and Their studies had shown that more than half of the seismic quiet; and due to this, little awareness and preparation have events in Nigeria occurred in southwest ([SW];Figure2). been made for earthquake occurrences along the passive This shows that southwestern region of Nigeria is more margins, especially Nigeria and other countries in the coastal vulnerable to tremor occurrence compared to other zones. region of West Africa. However, the epicenters of these earthquakes with their For about 85 years of seismic activities in Nigeria, there locations are presented in Table 1. was no record of loss of life nor damage of properties, but of Critical review of the previous works has revealed that recent, damage to properties were observed. The geological tremor occurrence in Nigeria could be attributed to regional Figure 1: Global distribution of passive margins (Modified after [33]). 378 Olagoke Peter Oladejo et al. stress, presence of weakened zones in the crust, and transfer represented in the study area. Lagos lies on Dahomey of stress from plate boundaries [3,6–8,11–25].TheWest (Benin) basin – one of the Nigerian sedimentary terrains, African Craton and Congo Craton bounded the Nigerian while Ore is situated on the Basement complex of SW Basement Complex in the west-northwest and east-south- Nigeria (Figure 3a). east directions, respectively. This could have deduced the Dahomey basin is located in the SW Nigeria covering stress acting in this orientation, because records of tremors Ondo, Ogun, and Lagos states. Dahomey Basin encapsu- in Nigeria have been along this orientation [26,27].Regional lates the onshore and offshore zones of this region. The stress could cause intraplate earthquakes on preexisting onshore includes the region where the Tertiary and faults within the crust that are of low strength. When the Cretaceous sedimentary rocks are revealed along the intrusive rock (magma) strength is weaker than the host, quarries and roadcuts [44]. This basin is bounded at the this could generate a localized stress within the body of west of Niger–Delta basin and the Basement rocks to rock. Rock intrusions in the sedimentary terrains of Nigeria the north. Both basins are of course low lying, as expected could have resulted to sufficient stress capable of causing of a basin. The Dahomey basin extends beyond Nigeria tremors. As reported by Short and Stauble [28],stress and, like the Niger Delta, seems to have some oil deposits, generated as a result of upwelling magma deforms the crust, although the former is far less explored than the latter. The which leads to faults. Over (geologic) years, these faults are Continental basin of the former is not as extensive, and the covered by sediments that have formed the midplate seabed slopes away relatively steeply from shore; while in structures, which are subjected to tectonic compressional the central