Geophysical Methods in Geological Applications

#### **Chapter 4**

## Tectonic Collision, Orogeny and Geothermal Resources in Taiwan

*Chao-Shing Lee, Lawrence Hutchings, Shou-Cheng Wang, Steve Jarpe, Sin-Yu Syu and Kai Chen*

#### **Abstract**

The recent tectonic evolution of Taiwan created ideal conditions for geothermal resources: heat, water and permeability. We examine heat flow measurements, seismic tomography, seismicity, hot spring distribution, tectonic history, geology, and volcanism described in previous studies to understand the relation between tectonics and geothermal potential in Taiwan. Taiwan is the youngest tectonically created island on earth. The island formed as a result of the transition from subduction of the Eurasian Plate under the Philippine Sea plate to active collision. Collision results in orogenic mountain building. The geology of the island is primarily an accretionary prism from the historic subduction. This active orogeny creates unusually high geothermal gradients by exhumation of the warmer material from depth and by strain heating. As a result, temperatures reach up to ~200 degree C. Volcanoes in the northern tip of Taiwan provide an additional source of heat. Favorable fluid flow from meteoric waters and permeability from seismicity and faulting results in exploitable geothermal systems near the surface. These systems can potentially provide geothermal power generation throughout the whole island, although there are currently only two geothermal power plants in Taiwan.

**Keywords:** geothermal, Taiwan tectonics, Taiwan heat

#### **1. Introduction**

Most of the energy consumed in Taiwan is produced by imported fossil fuels. The development of renewable energy is a priority, both to move the country toward energy independence and to combat global climate change. Power produced from geothermal energy is an important component of a renewable energy portfolio because it is continuously available, as opposed to wind and solar, which are intermittent in availability.

The presence of numerous hot springs and high measured heat flow throughout Taiwan are indications that significant geothermal energy resources are present, even though only two geothermal power plants are currently operating in the country. **Figure 1** shows the distribution of hot springs. The hot springs and high heat flow at the northern tip of Taiwan are associated with known volcanoes, but, interestingly, thousands of hot springs are also found in many other areas from the north to the south where no volcanism is found. The potential for geothermal

#### **Figure 1.**

*Hot springs of Taiwan. Light beige indicates the central range, with foothills to the east and the Longitudinal Valley (a long green strip) east of the foothills. From [1, 2] the blue box identifies the area of the highest heat flow in Taiwan.*

energy from these sources is perhaps more relevant for Taiwan than the limited volcanic area, as their spatial distribution is much wider and thus can perhaps serve a much larger area.

In this chapter we examine previous studies to show the relation between evolutionary tectonics and the potential for geothermal resources in Taiwan. We show how this evolution created ideal conditions for geothermal resources; heat, water and permeability. We examine heat flow measurements, seismic tomography, seismicity, hot spring distribution, tectonic history, geology, and volcanism described in previous studies to understand the relation between tectonics and geothermal potential in Taiwan.

#### **2. Evolutionary tectonics**

Prior to 6–9 Ma, sea floor spreading occurred in the South China Sea and the lithosphere within the latitudes of Taiwan was subducted eastwards beneath the Philippine Sea Plate along the Manila Trench [3]. Convergence was at a 5.6 cm/year with respect to Eurasia [3, 4]. The convergence created an accretionary prism of varying ages, a result of off-scraping sediments and rocks from the under-thrust plate and depositing sediments and volcanic ashes from volcanic arcs on the overriding plate [5, 6]. The sediments are a result of marine sedimentation occurring on

**Figure 2.**

*Major tectonic features near Taiwan (modified from [10]). Large red triangles indicate direction of subduction. Small red triangles along the north of the island are the volcanoes of the active volcanic front of the Ryukyu arc [11]. Volcanoes to the southeast are part of the Luzon arc.*

the South China Sea oceanic crust during the Miocene [5, 7]. Taiwan is an accretionary prism from the previous subduction [8]. About 6–9 Ma sea floor spreading ceased in the South China Sea leaving the Eurasian plate subducting eastward under the Philippine Sea plate. At the same period the Luzon arc acquired a significant enough topographic expression to resist subduction and start to collide with the Eurasian plate. The sediment strata began to show evidence of plate collision in early Pliocene and rising of Taiwan, about 5 Ma [8, 9]. Active subduction continues today north and south of Taiwan (**Figure 2**).

The Luzon volcanic arc (LVA) is an intra-oceanic volcanic arc which belongs to the Philippine Sea plate (PSP). Today the LVA is in contact with Coastal Range of Taiwan, along the Longitudinal Valley in south western Taiwan [3, 4, 12]; **Figures 2** and **3**. Fission track dating in the Central Range indicates a gradual rising until about 2 Ma when it began to accelerate [2, 4] and caused the Central Range to experience rapid uplift of the roughly 40 km wide range for most of the length of Taiwan; the highest mountain uplift in the world, as much as ~26 mm/year. Other regions experiencing present-day uplift in Taiwan at rates of ∼0.2–25.8 mm/yr. include the Hsuehshan Range, the Central Range, and the southern part of the Western Foothills [14, 15] **Figure 4**. An uplift rate of 22.9 mm/yr. occurs in the central northern portion of the Central Range, and the southern part of the Coastal range displays uplift rates of ∼1.3–25.8 mm/yr., the largest in Taiwan. The uplift rates decrease rapidly toward the north and diminish gradually toward south [17].

A number of studies agree oblique collision results in the evolution through time of Taiwan mountain building visible as a continuum from the present-day Manila

#### **Figure 3.**

*From [13]. Thick red lines are faults. Triangles point to subduction, plain line is the Philippine fault hypothesized to extend into Taiwan. Description of other feature are available in [13] the Taitung (1) and Leyte (2) geothermal prospects are located along the left-lateral strike-slip Philippine fault [13]. Black dots shown the location of the Luzon volcanic arc.*

**Figure 4.** *Provinces (from [16]).*

*Tectonic Collision, Orogeny and Geothermal Resources in Taiwan DOI: http://dx.doi.org/10.5772/intechopen.101504*

subduction system to the south (before collision), through middle Taiwan (collision) and northeast of Taiwan, across the southern Okinawa Through and Ryukyu subduction system (post-collision) [3, 4, 10]. Also, the age of metamorphic geology increases from the west to the east across Taiwan [18].

There are many thrust faults and folded anti- and syn-clines roughly perpendicular to the convergence. Active seismicity and highly fractured zones create permeability. The 1999 M = 7.8 Chi-Chi earthquake occurred on a thrust fault in the Central Range and demonstrates one way stress buildup from collision is released [19, 20]. The Longitudinal Valley is oblique to the convergence and hosts a left-lateral strike-slip fault, which may be an extension of the Philippine Fault that runs through the Luzon and several smaller islands of the Philippines (**Figure 3**). The saturated fractured geology creates the pathways for hot water and steam to come up to the surface making the Taitung and Leyte geothermal prospects (**Figure 3**). Sibuet et al. [13] connects this fault with the Longitudinal left-lateral fault. In Taiwan, most geothermal prospects are located along this key fault.

#### **3. Geology**

Seven provinces of the Taiwan orogen can be recognized (**Figure 4**): (1) the Coastal Range, along the northern extension of Luzon arc, consisting of fore arc sedimentary units and andesitic volcanic rocks; (2) the Longitudinal Valley, filled with young sediments, being the plate boundary between the Eurasian and Philippine Sea Plates; (3) the eastern Central Range, consisting of pre-Tertiary (Tailuko belt) and Miocene (Yuli belt) metamorphic complex rocks; (4) the western Central or the Backbone Range (capped by Miocene slates), (5) the Hsuehshan Range (Eocene-Miocene slates); situated in the northern half of the island and tapers off toward the south, (6) the Western Foothills, the fold-and-thrust belt, composed of clastic Oligocene-Pleistocene sedimentary rocks, and; (7) the Coastal Plain, containing younger sediment deposits [16, 21]. Longitudinal Valley represents the tectonic suture zone separating metasedimentary sequences of the Central Ranges from the accreted sedimentary and volcanic arc rocks of the eastern Coastal Range.

Continental crust of the Chinese continental margin colliding with the Luzon Volcanic Arc has deformed Miocene to Quaternary sedimentary marine stratigraphy into easterly-dipping fold and thrust belts of the Western Coastal Plains, Western Foothills and the Hsuehshan Range in western Taiwan Island. To the east a metamorphosed continental margin sequence has been exhumed along westerly dipping faults as the Central Ranges. Along the eastern side of Taiwan Island, the Coastal Range represents the northern extent of the Luzon Volcanic Arc, which has been accreted onto the eastern margin of the exhumed metamorphic rocks. The geology of the accretionary prism of the eastern Central Range is primarily made up of a huge sequence of deep-sea turbidities (Miocene Lushan Formation). It consists of mudstones, siltstones, and sandstones [16, 22].

#### **4. Heat**

In Taiwan, in the mature collision zone, the heat flows are high. Heat flow in the mountains is mostly between 80 and 250 mW/M2 [23]. Overall, the high heat flow in the mountainous regions is interpreted as a result of rapid uplift and exhumation of the warmer material from depth [24–26]. However, several researchers in orogenic zones have shown that uplift alone is not sufficient to account for the high heat flow [27–30]. Theoretical and geological considerations suggest that viscous

heating is a cumulative process that may explain the heat deficit in collision orogens; where severely deformed rocks over a short time span cause viscous heating and can account for this deficit and explain further up-warping of the isotherm [31, 32]. Whereas radiogenic heat production can be inferred from measured concentrations of radioactive elements and heat flow in stable regions of the lithosphere, the contribution to heating by deformation can potentially be measured only in actively deforming lithosphere where it may not be easily separated from other sources of heat. The strain heating and upwarping together, create favorable geothermal gradients in the Central mountains of Taiwan. The shallowest isotherm in the Central Range may also account for the aseismic zone, likely due to too hot and pliable crust unable to sustain enough stress to generate earthquakes [8]. Other sources near surface heat may include groundwater circulation, topographic effects, and higher radiogenic heat production rates in the continental crust. Complex deformations in lower crust and upper mantle following the collision might have also affected the thermal structures in this region [33].

Heat flow gives valuable insight into evaluating the tectonics of a region, and the geothermal gradient is the primary initial indicator of a viable geothermal resource. Heat flow is derived from the geothermal gradient and thermal conductivity, which are typically obtained from temperature measurements in pre-existing wells or laboratory samples. Heat flow is generally described in units of milli Watts per meter squared (mW/M2 ), and the geothermal gradient is described as degrees Celsius per kilometer (o C/km).

**Figure 5** shows heat flow measurements from [25]. They studied heat flow in Taiwan using previous values obtained from [27, 33] and added many values

**Figure 5.** *Heat flow measurements from [33]), re-plotted from [25].*

obtained off-shore of Taiwan. Chi and Reed [25] points out that there is still debate whether the heat flow data from some of the "geothermal wells" are representative of the regional heat. However, they also point out that several studies are able to fit the high heat flow pattern by thermal modeling using different crustal kinematic models [33–35]. Within Taiwan, and along the central range, heat flow reaches over 300 mW/M2 , whereas the worldwide average is about 50 mW/M2 .

Chi and Reed [25] identified a dramatic difference in heat flow between the subduction zone to the south, where values are near world averages, and the collision zone in Taiwan. At a latitude of ~20.5°N (not shown in **Figure 5**), in the subduction zone, heat flow decreases even further from about 75 to 40 mW/M2 from the trench to the upper slope domain of the accretionary prism. To the east in the fore arc basin, heat flow values are ~25 mW/M2 (**Figure 5**). The heat flow pattern along this transect is consistent with the three in situ heat flow measurements farther to the south at ~19°N [33, 35]. Heat flows in the satellite basins in the arc region are ~50 mW/ M2 . Farther to the north in the initial collision zone (**Figure 5**), the continent-ocean boundary (COB) enters into the trench near 21.2°N. Hwang and Wang [36] have collected 12 thermal probe measurements along a transect from continental shelf (117°E, 22.8°N) to continental slope (118.1°E, 19.3°E) that is 220 km west of and parallel to the trench. Chi and Reed [25] treat this data set as the initial condition before the Chinese passive margin enters into this convergent boundary. Hwang and Wang [36] found that heat flows are ~80 mW/M2 in the continental slope and decrease to 70 mW/M2 in the abyssal plain. Chi and Reed [25] also found slightly increased heat flows once the incoming sediments were scraped off and incorporated into the toe of the accretionary prism, where intensive dewatering occurs. High geothermal

#### **Figure 6.**

*a. Temperature gradient map of Taiwan showing nine EGS regions with anomalous high temperature gradients: (I) Tatun; (II) Chingshui-Tuchang; (III) Lushan; (IV) Juisui-Antung; (V) Wulu-Hungyeh; (VI) Chihpen-Chinlun; (VII) Paolai; (VIII) Kuantzuling; (IX) Hsinchu-Miaoli. b; potential Mwe of the nine zones. From [37].*

gradients (40–80°C/km) and heat flows (50–105 mW/M2 ) were found in a thick basin near the toe in this region east of the COB (**Figure 2**). This might be a result of intensive dewatering in this basin, which covers a circular region with a diameter of 60 km centered at 119.8°E, 21.6°N. Seismic reflection data show conjugate fault plane reflections within the basin, even though the displacements across the faults are small, suggesting possible fluids within the fault zones. Away from the toe in the initial collision zone, the heat flows decrease toward the arc as the sediments stack thicken, especially from lower slope domain to the upper slope domain. Heat flows ranging from 30 to 60 mW/M2 and increasing toward the arc are identified in the back-thrust domain.

Taiwan has a very good geothermal gradient. The average land heat flow in the world is about 30 mW/M2 . If there is a local heat flow value greater than the average, then there is a good potential for geothermal development [37]. Song and Liu [37] identified nine major geothermal resource regions based on anomalous geothermal gradients >35°C/km (**Figure 6**): (I) Tatun volcanic area in the north; (II) Yilan Plain along the Lishan Fault, it extends southwest to Mount Lu, covering Jiaoxi, Qingshui, Tuchang, Lushan and other geothermal areas; (III) For regions with abnormal geotemperature gradients higher than 35o C/km Lushan; (IV) Ruisui-Antong; (V) Wulu-Hongye; (VI) Zhiben-Jinlun; (VII) Baolai; (Viii) Guanzailing; (IX) Hsinchu-Miao Li. These nine high anomalous areas of geothermal gradient are the potential areas of geothermal development. Taiwan is currently conducting small-scale exploration to potentially exploit these areas.

#### **5. Geothermal reservoirs**

Organic geothermal reservoirs generally are permeable zones with fluid flow and heat. Taiwan has about 2500 mm of rain each year [38], sufficient for replenishing reservoirs. Engineered geothermal reservoirs only require heat, and fluid and permeability are created artificially. Heat is the important source for geothermal development in both cases. In Taiwan, orogenic reservoirs are typically located at a shallow depth of 2–3 kilometers in the Central range and volcanic zone where the geothermal gradient is greater than 30o C/km. The development depth of enhanced geothermal away from the Central range is usually greater than 3000 m where the geothermal gradient is approximately 30 mW/M2 . Exploration is necessary to determine whether there is an organic reservoir or a candidate for an engineered system. Within areas outlined in **Figure 6**, there are 108 geothermal potential locations for geothermal development that are located at a shallow depth of 2–3 kilometers due to high geothermal gradients [39]. Except for the Tatun and Keelung Volcano Groups in the north, the main high-gradient areas are located in the metamorphic rock belts.

The Tatun volcanic group is located in the northernmost part of Taiwan, mainly composed of more than 20 Pleistocene andesite volcanoes such as Tatun Mountain. At Tatun, there is a classical lava cone, and there are many hot springs, fumaroles, sulfur pores [40, 41], and other indications of intense geothermal activity. The main shallow geothermal reservoirs are located between the Jinshan and Kanjiao faults. The temperature is about 200-290o C [40]. Deep geothermal wells have encountered a high temperature of 293<sup>o</sup> C, which is the highest temperature currently reached in Taiwan [25].

Organic geothermal reservoirs can be roughly divided into two types: hot water type and steam type. Engineered geothermal system (EGS) is another type of reservoir. Organic geothermal energy development is limited by hydrothermal production capacity. In geothermal fields or reservoirs, abundant and high-temperature

#### *Tectonic Collision, Orogeny and Geothermal Resources in Taiwan DOI: http://dx.doi.org/10.5772/intechopen.101504*

geothermal water and well-developed fissure structures are required. The hot water type geothermal system is based on the presence of hot water in the reservoir. The water phase controls the reservoir pressure, and its temperature is the highest. Temperatures can range from less than 100o C to 370o C, but geothermal system above 200o C are optimum for power generation. The vapor type of reservoir is formed by the high temperature heat source supplying heat and the low permeability of the rock formation. Reservoirs can evolve from water type to steam type if the amount of hot water extracted is more than the amount of that replenished from groundwater. In the vapor reservoir, two phases of hot water and vapor coexist, and the vapor phase controls the storage layer pressure. The hot water phase flows in small pores due to high surface tension, while the vapor phase escapes through larger conduits in the upper geothermal system. Currently, steam type reservoirs only account for 10% of global geothermal production.

Engineered geothermal system typically create permeability between two wells by hydro-fracking or controlled fracturing from injection of cold water. **Figure 7** shows a scheme of an engineered system. This approach has been largely unsuccessful to date. This is because one, the fluid is not heated sufficiently in the short distance traveled, and two there has been difficulty creating a large enough fracture system, which would help in solving the first problem. Because the general gradient is about 30o C/km, so the development depth of enhanced geothermal is usually greater than 3000 m.

According to the geothermal research report [42], if 2% of the thermal energy stored in the 3–10 km rock layer of the earth's crust can be obtained the heat energy produced by it is as high as 2.8 × 10 5 EJ, which is about 2800 times the

**Figure 7.** *Engineered geothermal reservoir.*

total energy consumption of the United States in 2005. There is a huge geothermal potential for engineered geothermal systems deep underground in Taiwan. If it can be successfully developed, it will be an important independent energy source in Taiwan.

Two geothermal power plants, a total of 6 MWe, have started to produce electricity in Taiwan at the end of 2021. Two other small geothermal plants, 1 and 2 MWe, are being developed in indigenous areas, and are planned to operate by the end of 2022. The government has established a short-term geothermal goal to reach 200 MWe by 2025 with expansive exploration, test drilling, and power plant management. Further, the government is investing in developing engineered geothermal systems to reach long-term goals. It is hoped Taiwan may reach the GWe-level geothermal power production by 2035, thereby reducing the carbon dioxide emission as part of a global-village member.

#### **6. Methods for exploration**

Geophysical methods for exploring for orogenic geothermal reservoirs include the gravity, aeromagnetic, electro-magnetic (magnetotelluric), and tomographic imaging. The gravity method models the spatial distribution of rocks with different densities at depth to match the measurements of the acceleration of gravity at

#### **Figure 8.**

*Gravity showing low density rocks with geothermal potential at RF1, RFL1, RF2 and RFL2 along faults. Square in sub-figure shows the area, which is the same as the square showing high heat flow in Figure 1.*

different positions on the surface. **Figure 8** shows contours of gravity in mgals in southern Taiwan [43]. Geothermal reservoirs typically occur at low gravity values as seen below RF1, RFL1, RF2 and RFL2 where the gravity contours show significantly lower values than surrounding rocks.

An aeromagnetic survey records the magnetic field from an air plane flying over the area of interest. Magnetic field measurements are typically at 500 m intervals and the survey area is crisscrossed in parallel and perpendicular directions. Typically, a helicopter is flown over at a height of about 500 m above topography. In addition, magnetometers are used to continuously measure the geomagnetic field at a chosen base station on the ground. Normally, correction for the International Geomagnetic Reference Field (IGRF) involves removing it in order to show only magnetic anomalies related to geology. Resolution can be as high as 100 m from the surface to 10 km depth [44], **Figure 9** shows typical results over a geothermal reservoir. In this case, the linear features of the magnetic susceptibilities (H1, H2, and H3) were interpreted as being separated by large linear vertical faults. Although there is no direct evidence of a geothermal reservoir in this particular aeromagnetic survey, the delineation of structures such as faults can aid in the search process.

Electromagnetic methods can be either passive, utilizing natural ground signals (e.g. magnetotellurics) or active, where an artificial transmitter is used either in the near field (as in ground conductivity meters) or in the far field (using remote high powered military and civil radio transmitters as in the case of VLF and RMT methods). Magnetotelluric (MT) surveys estimate the Earth's electromagnetic impedance by measuring naturally occurring electromagnetic waves in a very broad frequency range. They typically record the full component MT data (i.e., Ex, Ey, Hx, Hy and Hz) induced by natural primary sources and measured relatively uniformly at 1000 m

#### **Figure 9.**

*Magnetic susceptibilities. From [44].*

#### *Earth's Crust and Its Evolution - From Pangea to the Present Continents*

intervals across the area of interest. Frequency range is generally from 10 kHz down to 0.01 Hz and can be even lower when sounding duration is as long as five days. Interpretation is based upon inversion of the MT data to derive resistivity values. The final 3D model used for interpretation is the one with the lowest root mean square misfit. **Figure 10** shows typical MT results at three depths in a reservoir in southern Taiwan. The low resistivity values (green) can indicate the location of a reservoir. The underground resistivity is very low resistance layer of 10 Ω-m. When the rock layer is subjected to hot water, the resistivity of the formation will be significantly reduced. Further abnormally low (below 100 Ω-m) area may indicate a heat source.

A recently developed seismic method utilizes passive earthquake sources and dense recording networks to image reservoirs, and has shown promise in seismically

**Figure 10.** *Resistivity values at three depths. From [45].*

*Tectonic Collision, Orogeny and Geothermal Resources in Taiwan DOI: http://dx.doi.org/10.5772/intechopen.101504*

active areas [46, 47]. Inexpensive recorders and automated data processing makes this possible in a short amount of time at a minimal cost [48]. The propagation energy from the earthquakes passes through the geology to recording systems at the surface. Tomographic inversion is used to back project these recordings to provide the images at depth [49]. The primary information comes from propagation of first arriving *P-* and *S-*waves and their pulse widths. These provide *P-* and *S-*wave velocity and *Qp* and *Qs* attenuation structure throughout the volume [46]. Attenuation measures the energy lost as waves propagate through the geology.

Further, the values of *Vp* and *Vs* throughout the volume can be used to calculate Poisson's ratio, and Bulk, Young's, Lambda and shear moduli throughout the

**Figure 11.** *Shear modulus. Low values likely indicate fractures. From [43].*

**Figure 12.** *Qs. Low values (orange) likely indicate saturation. From [45].*

volume. These proprieties are utilized in the context of rock physics relationships to identify the effects of fluids, fractures, porosity, and permeability on seismic velocity [46]. Following [46], several typical interpretations of porosity, permeability and saturation can be made from observable microearthquake data. Comparisons are made relative to normal geology at similar depths and temperatures, meaning geology that has a monotonic increase in velocity and *Q* as a function of depth, and saturation, porosity and temperature that is considered average for the geologic condition of the study area [46].

**Figure 11** shows a cross-section of a likely reservoir in southern Taiwan [43]. Black dots are micro-earthquakes. Low values of shear modulus (red) indicate a soft geology [43], likely due to fractures and may indicate the location of the reservoir. Almost no earthquakes occur in the soft geology. This shows how faulting can create permeability. The faults create pathways for fluids, and highly fracture the geology. Here, the conjugate faulting matches the flanks of anticlines caused by the orogeny deformation [43].

**Figure 12** shows *Qs* for the same volume [45]. The low values of *Qs* can indicate the existence of water. Shear wave propagation is not affected by liquid, so this figure is independent of the shear modulus. Together, the fractures and saturation are a good indication of a reservoir. Blue lines indication the location of possible faults, which may provide pathways for water.

#### **7. Induced seismicity**

During EGS geothermal reservoir creation or organic stimulation, rocks may slip along pre-existing fractures and produce microseismic events. Researchers have found these microseismic events, also known as induced seismicity, to be a very useful diagnostic tool for accurately pinpointing where fractures are reopened or created, and characterizing the extent of a reservoir. In almost all cases, these events occur in deep reservoirs and are of such low magnitude that they are not felt at the surface. Although induced seismicity data allows better subsurface characterization, GTO also understands public concern. With this in mind, US DOE led an effort to create a protocol for addressing induced seismicity associated with geothermal development, which all US DOE-funded EGS projects are required to follow [50]. This work was informed by panels of international experts and culminated in an International Energy Agency- accepted protocol in 2008. The protocol was updated in early 2011 to reflect the latest research and lessons learned from the geothermal community. In addition, in June 2012, the US National Academy of Sciences (NAS) issued Induced Seismicity Protocol in Energy Technologies [51]. The report found that geothermal development, in general, has a low potential for hazard from induced seismicity. The NAS report cited the DOE Induced Seismicity Protocol as a best practice model for other subsurface energy technologies.

**Figure 13** is a demonstration of induced seismicity due to cold water released by gravity flow over a month period of time in four wells at the Geysers, California geothermal reservoir [52]. The white dots are the earthquakes, including the natural and human-induced earthquakes. *Vs* tomography results are shown as backdrop. In **Figure 13a**, the well at far left, water release was increased just prior to the month. For the well second from left, water was released only for two days at the end of the month. Seven earthquakes occurred after water release was started and no earthquakes occurred prior. For the well third from left water was released at a low rate during the month. The seismicity was relatively little near the bottom of the well. The well at the far right is not in hot geology, and no earthquakes are observed.

*Tectonic Collision, Orogeny and Geothermal Resources in Taiwan DOI: http://dx.doi.org/10.5772/intechopen.101504*

#### **Figure 13.**

*Demonstration of induced seismicity due to the water release in hot geology.*

**Figure 13b** shows the effect of water injection at four wells for the month immediately following. The well on the far left has ceased generating earthquakes. The well second from left has created many induced earthquakes due to the ongoing release of water. The third well from the left has increased water flow and shows an increase in seismicity at the bottom of the well. The well at the far right continues to show no earthquakes.

The case study presented above illustrates that by continuously monitoring seismic activity and modifying water injection accordingly, the occurrence of induced seismicity can be mitigated.

#### **8. Discussion and conclusions**

Power produced from geothermal energy is an important component of a renewable energy demand because it is continuously available, as opposed to wind and solar, which are intermittent in availability.

The recent tectonic evolution of Taiwan created ideal conditions for geothermal resources; heat, water and permeability. The island formed as a result of the transition from subduction of the Eurasian Plate under the Philippine Sea plate to active collision. The Central Range has been the most dramatic manifestation of this collision. It has experienced the highest mountain uplift in the world, as much as ~26 mm/year. This active orogeny creates unusually high geothermal gradients by exhumation of the warmer material from depth and by strain heating. As a result, temperatures reach up to ~200 degree C. Volcanoes in the northern tip of Taiwan provide an additional source of heat. Favorable fluid flow from meteoric waters and permeability from seismicity and faulting results in exploitable geothermal systems near the surface. These systems can potentially provide geothermal power generation throughout the whole Island.

The average land heat flow in the world is about 30 mW/M2 . If there is a local heat flow value greater than the average, then there is a good potential for geothermal development if permeability and fluids also exist. Song and Liu [37] identified 11 major geothermal resource regions based on anomalous geothermal gradients >35°C/km (**Figure 6**). These 11 high anomalous areas of geothermal gradient are

the potential areas of geothermal development. Hot springs are prevalent throughout the 11 areas (**Figure 3**), indicating permeability and fluids. These resource areas occur either along the central range with the high rate of seismicity that sustained the 1999 M = 7.8 Chi-Chi earthquake (II, III, VII, **Figure 6**), along the Philippine fault within the Longitudinal Valley (IV, V, VI); both of which provide ample permeability for hot fluid circulation. Area I is within the Tatun volcanic area and permeability and fluid flow has been observed from exploratory wells [41, 53]. Area VII has high heat flow and hot springs, but no further confirmation of permeability has been identified. Taiwan has about 2500 mm of rain each year [38], sufficient for replenishing reservoirs.

In Taiwan, orogenic reservoirs are typically located at a shallow depth of 2–3 kilometers in the Central range and volcanic zone where the geothermal gradient is greater than 30o C/km. The development depth of enhanced geothermal away from the Central range is usually greater than 3000 m where the geothermal gradient is approximately 30 mW/M2 . Exploration is necessary to determine whether there is an organic reservoir or a candidate for an engineered system. Taiwan is currently conducting small-scale exploration to potentially exploit potential geothermal resources [39].

### **Author details**

Chao-Shing Lee1 , Lawrence Hutchings2 \*, Shou-Cheng Wang1 , Steve Jarpe3 , Sin-Yu Syu1 and Kai Chen1

1 National Taiwan Ocean University, Keelung, Taiwan

2 Lawrence Berkeley National Laboratory, Berkeley, California, USA

3 Jarpe Data Solutions Inc., Prescott Valley, AZ, USA

\*Address all correspondence to: ljhutchings@lbl.gov

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Tectonic Collision, Orogeny and Geothermal Resources in Taiwan DOI: http://dx.doi.org/10.5772/intechopen.101504*

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#### **Chapter 5**

## Geodynamics of Precambrian Rocks of Southwestern Nigeria

*Cyril C. Okpoli, Michael A. Oladunjoye and Emilio Herrero-Bervera*

#### **Abstract**

The geodynamics of the Southwestern Nigeria Precambrian Basement Rocks were studied with aim of understanding the evolution of rocks globally. Magnetic carriers of Precambrian Basement rocks samples collected from 110 locations were prepared for rock magnetism, optical microscopy and Scanning Electron Microscopy (SEM). The Natural Remanent Magnetisation (NRM) of the remagnetised and unmagnetised rocks are strong (0.3–1.7 A/m -< 0.5 A/m) showed northwesterly direction with moderate inclination and weak NRM with westerly shallow direction respectively. Primary and secondary NRMs are carried by maghemite, and the remagnetised and unmagnetised rocks revealed a higher coercivity for alternating field demagnetisation (<20 mT – < 10 mT median destructive field). Optical microscopy revealed maghemite, poor titanomagnetite, titanomaghemite lamellae >30 pm and finer maghemite/magnetite grains finer than 10 pm. X-ray Diffratometry (XRD) and SEM results implied NW remanence in the remagnetised rocks reside in the fine poor-maghemite during the alteration of hornblende to actinolite while the coarse-grained maghemite in both rocks carries the W remanence of a thermoremanent magnetisation acquired in the Pan – African times. Global cold collision geodynamics resulted in the generation of ultra-high pressure metamorphic complexes and remagnetisation and True Polar Wander drifts of the paleomagnetic pole move towards the equator.

**Keywords:** remagnetised, unremagnetised, pan-African, tectonometamorphism, NRM, orogenesis

#### **1. Introduction**

Geodynamics of the Precambrian is a fascinating and contentious topic that is now preventing us from better understanding how the Earth evolved over time. The dearth of raw data related to this tectonic regime is largely to blame for the current controversy and lack of consensus on Precambrian geodynamics. Geodynamics is the study of how the interior and surface of the Earth change through time. A timedepth diagram (**Figure 1**) that spans the whole history and interior of the Earth can be used to show this process schematically. For a systematic characterisation of geodynamic interactions, data points characterising the physical-chemical condition of the Earth at different depths 0 to 6000 km, for discrete times in geological time, are shown in **Figure 1**. (ranging from 0 to around 4.5 billion years ago).

#### **Figure 1.**

*Data availability for restricting the geodynamic connection for the earth is depicted in a simplified time–depth diagram. (modified after [1]).*

Geophysical data measurements, unfortunately for geodynamics, provide systematic coverage of the current Earth structure and the geological record recorded in rocks formed near the Earth's surface (usually within a few tens of kilometres). As a result, Precambrian geodynamics remains a controversial topic. It's also worth mentioning that four key Precambrian Earth evolution topics are among the top ten questions defining 21st-century Earth sciences [2]:

1st "What happened during Earth's "dark period" of the first 500 million years?" This period is critical for understanding planetary history, particularly how the Earth's atmosphere and seas formed, yet scientists know little about it because few rocks from this age have been preserved."

2. "How did life begin?" Remaining records of geological examinations of rocks and minerals could be used to identify where, when, and in what and what form life first arose."

3. "How does the Earth's interior function, and how does it impact the surface?"

Earth's magnetic field was formed by the continual movement of the mantle and core.

How and when continents formed and were preserved throughout billions of years, as well as their future evolution.

In this study, we will concentrate on the last questions, which have improved dramatically over the last decade.

This progress has been fueled by an increase in the quality and quantity of geological, geochemical, petrological, and geochronological data for Precambrian rock complexes, as well as the ongoing development of analogue and numerical models for early Earth dynamics [3, 4]. Volcanism, seafloor building, and mountain formation are all aided by mantle convection, which has an impact on surface conditions.

Scientists, on the other hand, are unable to exactly characterise these motions or calculate how they differed in the past, making it impossible to comprehend the past and predict the Earth's future surface environment.

How did the Earth's plate tectonics and continents form?

Despite the fact that plate tectonic theory is widely accepted, scientists are still perplexed as to why Earth has plate tectonics and how closely it is tied to other planet features such as water content, continents, oceans, and life. Modelling has become increasingly important in generating new goods due to the shortage of empirical constraints (**Figure 1**).

Indeed, as Benn et al. [3] point out, one of the unique aspects of Precambrian geodynamics is that there is no thriving global geodynamics paradigm, and early Earth lithosphere tectonics differs from modern-day plate tectonics, which

#### *Geodynamics of Precambrian Rocks of Southwestern Nigeria DOI: http://dx.doi.org/10.5772/intechopen.104668*

we can integrate and evaluate using our ever-growing set of observational and analytical data.

Several new major results have been obtained to address this particular challenge since Benn et al. [3]'s wide summary of Archean geodynamics, based primarily on merging geochemical, geological, petrological, and geophysical data sets. "This very concise, up-to-date synthesis of Precambrian geodynamics was motivated by analogue and numerical model results."

This research integrate modern paleomagnetic remanence, rock magnetism and optical microscopy and concepts (Plate tectonics and subduction, Orogeny and collision), petrology (metamorphic parageneses and relation with deformation), and geochronology. Data from regional literature including geophysical data are also abundantly used for synthesis and re-interpretation. The most important achievements include: the paleomagnetic studies and geodynamics on plate tectonics and subduction and orogeny and collision.

#### **2. Regional settings of Precambrian geodynamics**

The southwestern Nigeria granitoids is within the basement complex domain that was reopened in the Pan- African time of the Neoproterozoic period. This province was located around East Saharan, southeast Congo craton and west of West African craton (**Figure 2**), and has a long stretch from Hoggar to Brazil, which ranges from 4000 km to an extensive orogen in hundreds of kilometres [7]. The Trans-Saharan fold belt runs north-southerly, and the reopening of this belt was due to East Saharan, Congo and West African cratons continental collision about 790 and 500 Ma [6, 8, 9]. Granitoids, growth of thrust-nappe, medium- to highgrade metamorphism, parallel orogen tectonics typifies this belt [10]. The Hoggar separated into Air, Eastern and Central polycyclic; but now called the Pharusian belt plus Laouni terrain Algeria (LATEA) microcontinent [11]. Aggregation of twenty-three micro terranes constitute eastern and polycyclic central Hoggar in the northern province [5], while in the southern block (Dahomeyide), we have the Aïr-Hoggar composed of various continental oblique collisions [12].

Nigerian sector evolved by profuse magmatism in late Neoproterozoic times at the culmination of prior basin made up of depleted Archaean crust [13]. The Nigerian part of the Dahomeyide was separated into the eastern (granulite

**Figure 2.** *Regional geological map of trans-Saharan metacraton/shield (modified after [5, 6]).*

facies) and western (greenschist to amphibolites facies) domains based on some petrological attributes [14]. The southwestern Precambrian granitoids consist of migmatite-gneisses, schists, granites and dykes [15]. Pan-African granitoids and unmetamorphosed dykes are assigned Neoproterozoic isochron [8, 16–20]. The Archean crust characterised the supercrustals, which later interpreted to be deposited in diverse proto ocean floors [12, 15].

Pan-African belt evolution was by Plate tectonism, which led to the active margin colliding with the Pharusian belt and passive continental margin of the West-African craton about 600 Ma [7, 21–23]. The existence of basic to ultra-basic rocks thought to be remnants of mantle diapers or paleo-oceanic crust is part of this fact, and they have complex ophiolitic characteristics. Geochronological studies have examined major magmatic complexes with their isochron ages varying from 557 ± 8 to 686 ± 17 Ma (Rb/Sr. whole rock); 640 ± 15 Ma (U–Pb), which were determined in these complexes. Deformation of migmatite-gneisses and post-tectonic uplift typifies the Pan-African fold belt in southwestern Nigeria; which consist of polycyclic orogen of Liberian (2700 ± 200 Ma), Eburnean (2000 ± 200 Ma), Kibaran (1100 ± 200 Ma), and Pan-African (600 ± 150 Ma) [7, 24]. For the Liberian and Eburnean, the International Geological Time Scale (2002) has followed the following ages: "Paleoarchean to Mesoproterozoic (3600 to 1600 Ma)", "Mesoproterozoic to Neoproterozoic (1600 to 1000 Ma)", "Neoproterozoic to Early Paleozoic (1000 to 545 Ma)", and "Neoproterozoic to Early Palaeozoic (1000 to 545 Ma [25].

Ferre *et al.* [6] studied the northeastern Nigeria Pan-African continental collision, which resulted in high grade - high pressure and temperature (HP-HT) metamorphism up to granulite facies, migmatisation in supracrustal units of the same tectonism as the southeastern Nigeria domain [26, 27]. The extensive Archean crust of northern Nigeria was modified and remelted during the Pan-African tectonometamorphic episode. The Pan-African nappe system rejuvenates older polyorogenic times [15, 28].

Precambrian basement rocks into four units: Migmatite-Gneiss (migmatites, gneisses, granite-gneisses); Schist zones (schists, phyllites, pelites, quartzites, marbles, amphibolites); Pan African Granitoids (granites, charnockite, granodiorites, diorite, monzonites, gabbro) and Undeformed Acid and Basic dykes (muscovite, tourmaline, pegmatites, aplites, syenites, basaltic, dolerites and lamprophyre dykes). They occur as a small medium-grained rock with massive hills. This charnockites is made up of orthopyroxene, clinopyroxene, hornblende, plagioclase, alkali feldspar, magnetite, quartz and zircon. In some places, granite, porphyritic, augen gneiss, banded gneiss can be seen as low-lying outcrops and large hills (**Figure 3**).

#### **2.1 Gondwana configuration**

Although different models exist for the absolute position of Gondwana [30] as well as the relative positioning of cratons can be done with small margins of error (**Figure 4**; [32]). The formation of Gondwana is often presented as a merger of East Gondwana (Antarctica, Australia, and India) with West Gondwana (those currently in Africa and South America). However, evidence, especially from the eastern Gondwana cratons, indicates that it was not a simple unification of two halves but rather a poly-phase amalgamation of cratons during the waning stages of the Proterozoic, as a result, that Gondwana was created [33].

The Congo and West Africa cratons form part of West Gondwana and are connected through the Borborema Province in northern Brazil (**Figure 4**). This province was essentially an assemblage of several terrains and comprised reworked Mesoproterozoic- Neoproterozoic metasedimentary rocks and

*Geodynamics of Precambrian Rocks of Southwestern Nigeria DOI: http://dx.doi.org/10.5772/intechopen.104668*

**Figure 3.** *Geological map of southwestern Nigeria (modified from NGSA, 2010 [29]).*

#### **Figure 4**

*Gondwana map with its cratonic nuclei positions (adapted after* **[***31***]).** *RP -Rio de la Plata craton; SF- Sao Francisco craton.*

Archean-Palaeoproterozoic crystalline basement [34]. Reworking is the result of Neoproterozoic continent-continent collision, which caused extensive deformation, migmatisation, granitisation and intrusive plutons. Geochronological constraints for the different stages of deformation in the Borborema Province are provided by U-Pb radiometric ages of the granitoid plutons [35]. Ages for zircons from syn-tectonic I-type granitoids and zircons from migmatitic gneisses show that deformation started ca. 625 Ma and peaked at about 600 Ma [36]. Post-tectonic alkaline granitoids mark the final orogenic stage, and U-Pb zircon ages show that deformation had ceased around 570 Ma [36]. The Borborema Domain was correlated, predominantly based on Sm-Nd model ages and U-Pb zircon ages of Archaean-Palaeoproterozoic basement rocks in conjunction with Neoproterozoic structural tectonic data, with the

Central African fold belt (**Figure 4**) and with the Nigerian Shield (**Figure 4**) in NW Africa [35].

The Central African fold belt demonstrate a poly-stage geodynamic evolution of nappe emplacement onto the Congo Craton northwardly [37]. Geochronological constraints reveal a history of individual orogenic stages broadly coeval with those of the Borborema Province: high-pressure metamorphism with granulite facies typified for syn-tectonic calc-alkaline and S-type granitoids and migmatisation occurred at 640–610 Ma, as well as post-collisional phase of exhumation and late-tectonic calc-alkaline to sub-alkaline granitoid emplacement was dated at 610–570 Ma [37]. The exact nature of the continental landmasses involved was still enigmatic. The belt could be entirely the consequence of the collision of the Congo Craton with the ill-defined Saharan Metacraton [38].

Neoproterozoic intrusions within the Nigerian Shield show a history very similar to that of the Borborema Province [34]. Combined structural data and U-Pb ages suggest that an early deformational phase took place at 640–620 Ma, peak metamorphism and syn-tectonic granitoids are positioned between 620 and 600 Ma and a post tectonic phase from 600 to 580 Ma (geochronological data synthesised by [34]). Geochronology of these plutons shows that the continental collision evolved diachronously between 620 and 580 Ma [5]. The Nigerian Shield and the Tuareg Shield were parted from the West African Craton by the Dahomeyide and Pharusian belts (**Figure 4**). Peak metamorphism in the Dahomeyides occurred approximately at 610 ± 2 Ma [39] to 603 ± 5 Ma [40] premised based on radiometric dating of U-Pb obtained from gneisses from granulite-facies peak metamorphic zones. The postcollisional exhumation was dated by 40Ar-39Ar muscovite ages of 587 ± 4.3 and 581.9 ± 2.4 Ma [39], which corresponds with rutile ages of 576 ± 2 Ma, which represent regional cooling below 400°C [40]. The collision of Island Arc with the West African Craton around 620 and 580 Ma, simultaneous with the height of the tectonic events in the Tuareg Shield to the east [5]. The Borborema domain evolved synchronous with the Central African Fold Belt, the Nigerian Shield (Dahomeyide Belt), and the Tuareg Shield (Pharusian Belt) strongly implies that this part of West Gondwana had amalgamated by 600 Ma, and all tectonic activity had ceased by 570 Ma.

Archaean to Mesoproterozoic granite-gneiss-migmatite complexes, greenstone belts and metasedimentary and metavolcanic units are caught up in the Brasilia Belt involving the Sao Francisco Craton and Magmatic Arc of Goias [41]. Observed data from the Paraguay-Araguaia belt that flanks the Goias Arc's western side imply that the collision of Sao Francisco/Goias with Amazonia slightly post-dates the Brasilia event at ca 550 Ma [42, 43]. Biotite and muscovite ages around 530 Ma from Archaean basement gneisses may record late-orogenic cooling in the Araguaia belt (K-Ar) [43].

#### **3. Materials and methods**

We make use of the following instrument to conduct measurements on some Precambrian basement rocks of southwestern Nigeria:

#### 1.**Alternating frequency, thermal demagnetisations and Spinner magnetometer for Paleomagnetic analysis**.

Granite biotite granite gneiss, banded gneiss, Augen gneiss, porphyritic granite, syenite were selected based on their mineralogy, magnetic susceptibility, and natural remanent magnetisation (NRM). A combination of alternating field (AF) and thermal (TH) demagnetisation methods were employed. Their primary and secondary multi-remanence constituents were measured using the equipment.

These techniques were used because constituent minerals obtained through different mechanisms have different coercivity spectra and blocking temperatures. The coercivities of magnetic minerals are involved in AF demagnetisation. The alternating field method entails exposing the specimen to increasing amounts of AF, with the waveform being sinusoidal and decreasing in magnitude linearly with time. It was used to extract remanence from grains whose coercivities were less than the peak demagnetising area. The alternating magnetic field is a quick treatment procedure likened to the thermal demagnetisation method. Test of the natural remanent magnetisation in determining the rock material is not a superimposition of several magnetic constituents, and this was done by isolating the components of stable magnetisation (CRM).

The specimens were heated to a temperature below and near ferromagnetic mineral Curie temperatures in steps of 30 and 50°C during step-by-step thermal (TH) demagnetisation and then cool in a zero magnetic field at room temperature. It gives magnetic grains blocking temperatures (Tb) lower than the temperature used to strip a portion of their normal remanent magnetisation. Step by step, temperature ranges were measured, and residual magnetisation and susceptibility were calculated. The basic measurement of NRM yields the remanent magnetisation recorded in rocks (declination, inclination, and total intensity). In the present study, the samples were AF demagnetized in 14 steps following a sequence 2.5, 5, 7.5 10, 12.5, 15, 17.5, 20, 25, 30, 40, 60, 80 and 100 mT respectively. The thermal demagnetisation was done on some selected samples in a sequence of 50°C, 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, 400°C, 420°C, 440°C, 460°C, 500°C, 530°C, 550°C and 570°C respectively.

As soon as alternating field and thermal demagnetisation are treated, the rock specimen directions are studied to isolate magnetic constituents. In this paleomagnetic study, stereographic and orthogonal projections were adopted. Stereographic projection direction was characterised, magnetisation vector unit length tip was measured, the same sphere diameter aligned with the southern pole. They are the contact site with the equator plane sphere, usually referred to as a small open circle. The geographic directions of the north, east, south and west were defined. Magnetic declination ranges from 0° (N direction) to 360° clockwise, and from 0° at the edge of the equator plane to 90° at the midpoint. AF and thermal datasets were analysed with AGICO's Remasoft 3.0 program [44] and Demagnetisation analysis in excel DAIE-v2015 program [45]. Fisher [46] 's statistics were employed to measure mean orientations.

#### 2.**Kaiser 785 nm Micro-Raman Microprobe system and Renishaw 830 nm inVia micro-Raman to determine iron oxides.**

Granite; biotite granite gneiss; Augen gneiss and banded; banded gneiss; porphyritic granite; syenite, and amphibolite rock samples were hammered bits and pieces and selected with a solid permanent magnet in the laboratory because of their mineralogy and magnetic susceptibility in order to determine the magmatic effect of maghemite. Tiny, unpolished grains of different iron titanium oxides concentrations were affixed on carbon tape attached to a glass slide for Raman spectra measurements. In addition to optical images, micro-Raman spectroscopy of various excitation wavelengths was used. At the University of Hawai'i, various instruments were used to capture Raman spectra. Spectra with 785 and 830 nm - Kaiser Optical Systems' micro-Raman system and Renishaw in Via microspectroscopy were used for the study. The system consists of a 785 and 830 nm Invictus diode laser, a Kaiser Holospec/Renishaw spectrometer, a spectral range of 150–3300 cm–1, a Leica microscope with imaging capabilities, as well as an Andor CCD camera. The laser

light and Raman pulse are sent to the microscope and spectrometer via a 100 meter optical fibre. A 50 objective lens fixed on the microscope in backscattering geometry was used to focus the laser spot and observe the signal. The spectrometer and microscope are fixed through optical mirrors of different wavelengths and were operated using a PRIOR workstation (via WiRE 3.2 software). The spectra were imported into MATLAB 7.4.0 and Grams/AI v8.0 for normalising statistical analysis, background interference in each spectrum, as well as baseline diffraction patterns, i.e. correction and peak fitting using Gaussian and Lorentzian geometries. Background correction was done using sixth-order polynomials in both cases. Principal component analysis (PCA) and significant factor analysis (SFA) were employed to determine the principal components. Specimen were stored to avoid artefacts, and laser power had below 0.7 mW to prevent the destruction of the specimen; neutral density filters had a constant power of 675 μW; acquisition time was 60 s; spectrometer calibration before acquiring Raman spectra; and cyclohexane standard protocols were used [47, 48].

#### 3.**Scanning electron microscopy (SEM) (JEOLJSM-5900LV) and X-ray diffractometry (XRD) for magnetic mineralogy.**

Gneiss, granite, biotite-granite-gneiss, charnockite, and granite were used to describe the ferromagnetic minerals based on their mineralogy and magnetic susceptibility. The thin polished sections were studied using SEM. Thus, SEM and XRD were employed in the Institute of SOEST-HIGP (Manoa, Hawaii, USA) to constrain the mineralogy of accessory minerals. SEM was used for imaging, qualitative analysis (equipped with an "EDS") and quantitative analysis (when equipped with an "EDS/WDS"). SEM and EMPA were applied to characterise the specimen for Mineral identification; compositional information, microstructures/deformation and compositional evolution of minerals.

4. We modelled the tectonometamorphism episodes in the Precambrian era to picture the evolution of the Precambrian rocks of southwestern Nigeria and relate them to present-day orogenesis.

#### **4. Results and discussions**

#### **4.1 Paleomagnetic results**

For samples of the same site, CO-23c subjected to thermal treatments have secondary remagnetisation averagely 70% at 300–500°C the remainder of the signal was washed up to 570°C (**Figure 5d**). **Figure 5**(d-i) demonstrated weak magnetic coercivity, unstable remanent directions and abrupt changes in intensity in Zijderveld curve plotting not directly to the origin due to tectono-metamorphic episodes. Up to 500°C, the rest of the samples retains >50% of the magnetisation is lost and it It was difficult to isolate the ChRMs (e.g., CO-23c, CO-018,CO-37a, CO-74 N and CO-100A in **Figure 5a**-**e**). The unblocking temperature revealed two distinct elements, one with natural polarity against N and the other towards NNW and NE. The second specimen has a low unblocking temperature and was thoroughly cleaned up to 300°C, while the northerly specimen reported magnetisation up to intermediate unblocking temperatures (580°C), which is referred to as characteristic remanent magnetisation (*CRM*). Thus, regardless of NRM decrease for the first 300 samples, a large percentage of the samples were treated to remove secondary remanence [49]. Compared to the ferrimagnetic one, the samples have a low paramagnetic effect, as shown by the similarities of the curve before and

#### **Figure 5.**

*(a-ce) alternate field and thermal demagnetization showing the Zijderveld orthogonal vector diagram of unremagnetised Precambrian rocks and (d-c) remagnetised rocks.*

after slope correctionCO-23c, CO-018,CO-37a, CO-74 N and CO-100A samples were decomposed into two overlapping modules with medium destructive fields (MDF) ranging from 30 to 40 to 60–70 mT, and a third higher coercivity segment (*MDF* ~ 467 mT).

Determining the time of growth during folding is difficult because syntilting results observed in incremental tilt tests do not give a unique result. The formations of new minerals (maghemites) were demonstrated in most of the granitoids invoked for many syntilting CRMs. The remanence-carrying Fe oxide grains may have rotate during folding and tectonometamorphic episodes as a result of syntilting, which would alter the original magnetic direction. The rotated directions were not related to the ambient field during folding. Shear strain during flexural flow folding could cause a prefolding magnetization to be rotated into a syntilting configuration. Folds with different geometries and tilted thrust sheets all have the same magnetic characteristics and are probably caused by the same remagnetization events. The determined tilt test results however suggest that the CRM is pre-tilting in both the thrust sheets and a fold with a fault-bend fold geometry and syntilting in folds with a fault propagation fold geometry that probably experienced higher strains. A primary remanent magnetisation should theoretically require more stress than most rocks have been subjected to during deformation, be partially reversible and have the greatest effect on the low-coercivity. Developing a better understanding of remagnetization processes and use of palaeomagnetism for its studies, the preponderance of multi-domain and pseudo-Single domain magnetic phases and presence of maghemite suggest that the type 1 magnetite has been modified during the orogenesis. They are correlated to their respective bedding tilt orientation base on correlation fold test. The tectonic correction brought the site direction in its geographic coordinates; the same rotation was applied to the mean geographic coordinate to produce forward corrections of the mean. The remagnetised component of the granitoids was interpreted as partial thermoremanentmagnetisation (pTRM) overprint acquired during the tectonometamorphic episodes 600 ± 150 Ma (Pan –African orogeny) associated with tectonic accretion along southwestern Nigeria Precambrian shield. The overprints directions were likening to the reverse magnetization. The intermediate degree of unfolding at peak concentration could be due to subtle amounts of component mixing, diachronous magnetization acquired

during a short time interval, syn-folding magnetisation acquisition, local structural differences within the fold.

Precambrian rocks of southwestern Nigeria with an intermediate unblocking temperature of 100—400°C witnessed perfect dispersed clustering in geographic coordinates after tilt correction, suggested remanence imprint after folding (**Figure 5a**-**c**).). The corresponding imprint on the paleomagnetic pole situated at 85.1°N, 183.0°E with α95 = 10.1° (*dp* = 12.7, *dm* = 8.0) in geographic coordinates, are very similar to the Basement system of Precambrian poles of southwestern Nigeria. The majority of the sites were unable to isolate the intermediate temperature component due to its remagnetisation ((**Figure 5d**). Therefore, this overprint was considered to be remagnetisation in Pan-African times. In Nigeria's Eastern basement complex, NE Brazil, Central Cameroon, and much of the west Gondwana crystals provinces, different remagnetisation has been observed.

Available geologic model isochron ages, tectono-metamorphic history, and crustal evolution model that support accretional model for Paleoproterozoic and Neoproterozoic rocks with consistent older model isochron age in support of significant involvement of Archean felsic crust in their orogeny and suggested that southwestern Nigeria's tectono-metamorphic history and crustal Nigerian active margins and Trans Saharan belt utilised U–Pb geochronological to infer the magmatism that occurred from 670 to 545 Ma ([26] and this study) for the overriding plate of Benino-Nigerian Shield. The age of ultra-high pressure metamorphic eclogites from the passive margin of the West African Craton subducted to mantle depths recently restricted the timing of crustal deformation to 600–150 Ma. The lower plate (West African Craton) and upper plate (Benino-Nigerian Shield) both experienced east-plunging continental subduction, which pushed the passive flanking margin to >90°, which suggested that granitoids subducted between 670 and 610 Ma. Both plates crystallised the Pharusian oceanic plate, while igneous rocks associated with arc magmatism include the hornblende-biotite granodioritic gneiss, dated at 610–694 Ma. Pan-African granites older than 610 Ma predate nonsubduction-zone deformation. The Benino-Nigerian basement complex was formed by continental arc, according to a geochemical dataset and Sr-Nd isotopic kinematics application to the 670–610 Ma migmatite-gneisses.

Results exhibited preliminary records of exsolved maghemite in silicate, plagioclase and pyroxene minerals in the southwestern Nigeria Precambrian gneiss and granitoids. These iron oxides are seen in the pyroxene and plagioclase minerals showing good magnetic stability (**Figure 6a**). **Figure 6a** (i-iii) shows the site mean directions of the study area. Site mean direction for component i clustered around mean Dm = 325.6° lm = 28.4°(N = 12, α95 = 9.8, k = 10.93), which resulted to paleomagnetic pole located at 7.38°N. 5.57°E (A95 = 9.8, K = 12.9). Site mean direction for

#### **Figure 6.**

*(a) Site mean directions of the study area (b) spline apparent pole wandering APW of Africa pole and (c) VGP reconstruction of Africa databasefit (lat = 0.0, long = 151.60, angle = 27.50) with Africa, taking account of the opening of the South Atlantic.*

component ii Dm = 0.6° lm = −38.4° ((N = 7, α95 = 14.1, k = 15.24), which resulted to paleomagnetic pole located at 7.24°, N 5°E (A95 = 12.5, K = 13.4). Site mean direction for component iii is Dm = 225.8° lm = 26.4° (N = 4, α95 = 16, k = 17.93), which resulted to the paleomagnetic pole located at 8.17.5°N 4.21°E (A95 = 15, K = 13.9). Thus, site mean directions of the study area were accomplished as a reliable recorder of the old geomagnetic fields. **Figure 6b**-**c** were employed to demonstrate the approximate tilt estimates, when distinct geological criteria demonstrated folding and faulting cannot be correlated to the unit. They used to evaluate the pole position for the SW Nigerian shield, errors due to (1) the large elliptical confidence was defined for the pole position, (2) recognised tilt events, and (3) the streaked great circle distribution of reversed, normal and mixed Remanent magnetization (RM) directions towards the Pan African events must be examined. Because the pole was poorly constrained, the data can accommodate several possible interpretations. First, the unit may have gained an RM orientation during long-term cooling compared to steady African cooling, and magnetic acquisition in the gneiss may have taken place over long stretches of time, during which reversals in the Earth's magnetic field direction occurred. Both polarities may have been recorded in individual sample owed to varying blocking temperatures. Perhaps, the field direction changed slightly during this period, then a deflection along a great circle towards the polarity which the earth's field sustained for the shortest length of time, would be observed. Such a deflection, where changes in the field direction are represented by angular limits of mean directions from Van Der Voo, [50] and percentages represent the relative time the earth's field spent in each polarity. These results in conjunction with demagnetization data suggest that the geomagnetic field was represented by both polarities (reversed dominating) during the magnetization of the southwestern Nigeria granitoids. The evident polar wandering path of Precambrian rocks in southwest Nigeria was caused by the effects of the mantle and superplumes. The acquired conjugate poles lie towards SW-NE at 304.8°E and 61.8°S directions (d*p* = 5.4, d*m* = 10.7); which was relatively at mean direction of 305.1°E and 64.5°S (d*p* = 2.3, d*m* = 4.5).

The Precambrian Basement rocks of southwestern Nigeria demonstrated low coercivities and low unblocking temperatures especially in the coarser grains.. No younger geological event occurred in the study area. The mafic syenite dykes witnessed in the study area occurred during the last phase of deformation. The tectonometamorphic history is interpreted as remagnetisation due to Neoproterozoic Pan-African event which is correlated to Brasilian 650 ± 150 Ma. The remagnetisation was not directly attributed to tectonic stress but to fluid, chemical and viscous effects, The remagnetisation phenomenon is due to new mineral growth, whose chemical remanent magnetization (CRM) swamps that of magnetically softer earlier grains. The progressive demagnetization, along the great circle path from initial towards pressure-vessel field. Only the initial remanence and the demagnetization fields determine the remagnetisation paths

#### **4.2 Tectonic implications**

The area has been tectonically moved as a result of post-magnetization causing the Nigerian shield magnetic direction to be deviated from an original primary direction. At least three explanations for the observed SW Nigeria paleopole position exist, these are: present position acquired magnetic remanence; NW thrust to after RM acquisition brought the SW craton to its present position, or the southwestern Nigeria acquired its magnetization and was transported to the northeast >1000 km by a left-lateral transcurrent fault system, and was later thrust to the northwest to the present location of the body. The magnetic fabric exhibited by the SW Nigeria granitoids closely

approximates the mineral fabric, suggesting that both were acquired during the four deformational (D1, D2, D3 and D4) events which have affected the body. Results indicate that the RM was acquired during magnetite recrystallization or cooling from metamorphic temperatures (~600°C) to maghemite. RM acquisition in the gneiss seems to have happened over a comparatively long period of time, with reversed, mixed and normal polarities represented in the magnetic signature of the unit.

The introduction of hydrothermal fluids occurred at a temperature well above the Pb–Pb closure, which corresponds to the age of the magnetic pole in southwestern Nigeria (~ 571 Ma). Granitoids emplaced over 700 Ma were not reliant on high-level hydrothermal emplacement in unmetamorphosed southwestern Nigerian Precambrian rocks, implying a Pan-African episode. Thus, the Pb–Pb date, which is younger than the 620 Ma U–Pb acquired from a deformed syenite, can provide the thrust a better constrain age. As a result, 620 Ma was proposed as the magmatic rock's crystallisation period rather than the tectonic event's age. The 571 Ma periods, on the other hand, proposed retrograde metamorphism at the nappe's base. The metamorphism ranges from amphibolite-granulite and retrograde greenschist facies towards north to base of nappe respectively in Nigeria.

#### **5. Discusssion on Precambrian geodynamics in relation to paleomagnetism**

#### **5.1 Plate tectonics and subduction**

It's worth mentioning that thermomechanical numerical experiments were only recently employed to study the onset and patterns of Precambrian plate tectonics and subduction [51–53]. Van Thienen et al. [54] presented one of the most wellknown attempts to employ numerical modelling to analyse global tectonic processes of the early Earth).Since of the considerably different temperature and viscosity conditions that existed beneath the early Earth's mantle, and because currentday geodynamics cannot be easily projected back to the Earth's early history. Van Thienen et al. [54] used computational thermochemical convection models with partial melting and a basic mechanism for melt segregation and oceanic crust formation to investigate an alternative set of dynamics that may have been active in the early Earth. They are: Small scale convection involving the lower crust and shallow upper mantle; Large-scale resurfacing processes in which the entire crust sinks into the (ultimately lower) mantle, forming a stable reservoir of incompatible elements in the deep mantle and segregating melt builds a fresh crust on the surface and excessive melting and crustal growth due to the intrusion of lower mantle diapirs into the upper mantle at a high excess temperature (about 250 K). This allows plumes in the Archean upper mantle to have substantially higher excess temperatures than previously thought possible based on theoretical considerations. Various geodynamical theories have predicted a dense enriched layer at the mantle's base [55, 56]. Massive scale sinking of the thick basaltic/eclogitic crust induced by decompression melting of mantle peridotite, according to Van Thienen et al. [54], may have developed such a layer over a brief period early in the Earth's mantle's history. The large-scale crustal sinking model described by Van Thienen et al. [54] might thus be considered an alternative (or predecessor) to Albarede & Van der Hilst's proposed subduction model (2002).Large-scale sinking appear like subduction and could be a Precambrian forerunner to modern plate tectonics.

In contrast to other studies, O'Neill et al. [57] presented an alternate explanation for crustal growth episodicity by providing paleomagnetic evidence for periods of rapid plate motions matching to observed peaks in crustal age distribution. The Nd

*Geodynamics of Precambrian Rocks of Southwestern Nigeria DOI: http://dx.doi.org/10.5772/intechopen.104668*

and Sr. isotope ratios of many juvenile terrains [58] support the idea of increased plume activity associated with these overturns and provide a model for their cratonization. Superplumes and other ideas have been presented to explain this episodicity [59]. Plate-driven episodicity arises naturally in response to the early Earth's high mantle temperature, and hence can explain quick pulses of plate motion and crustal formation without the need for mantle overturn events [57]. To assess the possibility of subduction in the hotter Precambrian Earth, Van Hunan and van den Berg [53] employed 2D thermomechanical models with a single subduction zone enforced by a weak fault (**Figure 7**).

In contrast to Davies [61], instead of focusing on changes in crustal thickness spanning 10 to 22 km, this model ignores early upper mantle depletion; caused by rising mantle temperatures, that decided plate tectonics' viability on a hotter Earth. Numerical results revealed no Ultrahigh-Pressure Metamorphisms (UHPM) or blueschists in most of the Precambrian: early slabs were too weak to provide a mechanism for UHPM and exhumation. Due to the lower viscosity and higher degree of melting, a hotter, fertile mantle would have resulted in a thicker crust and a thicker depleted harzburgite layer in the oceanic lithosphere, according to van Hunan and van den Berg [53]. A thicker lithosphere may have been a significant stumbling block to subduction, and Earth in the Precambrian may have been characterised by a different mode of downwelling [62] or "sub-lithospheric" subduction [53], though the conversion of basalt to eclogite may greatly relax this limitation [54, 63]. The natural reduced viscosity of the oceanic lithosphere on a hotter Earth would lead to increased Slab breakoff (**Figure 7**) and crustal detachment from the mantle lithosphere has occurred in some situations. Hence, lithospheric weakness may limit the feasibility of present plate tectonics on a hotter Earth. By merging knowledge from geochemical data and numerical models, Halla et al. [51] used inferences from van Hunan and van den Berg's [53] numerical study to constrain early Neoarchean (2.8–2.7 Ga) plate tectonics. Sizova et al. [60] employed a two-dimensional (two-dimensional) petrological–morphological model. To investigate the dependence of tectonic-metamorphic and magmatic regimes at an active plate margin on upper-mantle temperature, crustal radiogenic heat production, and lithospheric weakening, a thermomechanical numerical model of oceanic–continental subduction (**Figure 7**) was used to conduct

#### **Figure 7.**

*Modelling the evolution of an active continental margin with high-resolution numerical models for various mantle temperature differences (T) above current values (modified after [60]).*

a series of high-resolution experiments. Based on their testing, the scientists observed a first-order change from a "no-subduction" tectonic regime to a "pre-subduction" tectonic regime, and then to the current mode of subduction (**Figure 7**). The first transition is gradual and occurs between 250 and 200°C over current upper-mantle temperatures, whereas the second transition is abrupt and occurs between 1 and 2°C above current upper-mantle temperatures. The change to the current plate tectonic regime occurred at 3.2–2.5 Ga, according to the link between geological evidence and model results. Convergence does not result in self-sustaining one-sided subduction in the "pre-subduction" tectonic regime (upper-mantle temperature 175–250°C above the surface), but rather two-sided lithospheric downwellings and shallow underthrusting of the oceanic plate beneath the continental plate (**Figure 7b**).

#### **5.2 Orogeny and collision**

Interpretations of geological, petrological, and geochemical data from Proterozoic and Archean orogenic belts revealed that the Precambrian had different tectonic kinds of orogeny than the present-day Earth [64–66]. Accretionary and collisional orogens are the two forms of Precambrian orogens [66–68]. When the oceanic crust is subducted along active continental margins, accretionary orogens form ([66] and references therein). Precambrian accretionary orogens make a significant contribution to continental expansion compared to Phanerozoic accretionary orogens due to their high rates of juvenile crust growth [66]. Several post-Archean accretionary orogens are terminated by continent-continent collisions during supercontinent formation [66]. The average terrain lifespan during the Archean is 70–700 million years, 50–100 million years during the pre-1 Ga Proterozoic, and 100–200 million years in later orogens [66]. When continents collide, collisional orogens emerge; they initially arose in the Proterozoic, but had little impact on continental growth [67, 68]. The appropriateness of studies of current collisional orogens to the Precambrian is yet unknown, given the impact of a warmer continental crust and a higher mantle on the geodynamic regime earlier in Earth history [69, 70]. Extreme ultrahigh-pressure (UHP) metamorphic rock complexes are generated and exhumed by Phanerozoic collisional orogenic systems, which also create clockwise metamorphic P–T routes. About a thousand high-pressure (HP)-ultrahigh-pressure (UHP) metamorphic terrains have been discovered around the world, the most of which are Phanerozoic in age. One is Neoproterozoic, while the other is Neoarchean to Paleoproterozoic in age [1, 71]. The lack of UHP metamorphic complexes in the Precambrian geological record indicates that another type of orogenesis predominated earlier in Earth's history [64, 65]. Based on field results, Precambrian orogens differ greatly from current orogens. At high apparent geothermal gradients, Precambrian orogenesis made important contributions to crustal growth and magmatism [64, 66]. Four orogen categories was proposed by Chardon et al. [67] and Cagnard et al. [72] recently attempted to define Precambrian accretionary orogens using first-order structural and metamorphic traits, which represent the state of the continental lithosphere in these convergent settings involving enormous juvenile magmatism (**Figure 8**).

#### **5.3 Micro-Raman spectroscopy**

**Figure 9** revealed Raman spectra were observed within the white matrix, pyroxene, opaque mineral pockets and diverse places around the mineral matrix. Maghemite Raman shift peaks are recorded at some points within the biotite granite gneiss, and thin section petrography of all the rock units in the study area shows the abundance of quartz, microcline and plagioclase as the major minerals that dominate the rock samples with other minor components such as hornblende, muscovite

*Geodynamics of Precambrian Rocks of Southwestern Nigeria DOI: http://dx.doi.org/10.5772/intechopen.104668*

#### **Figure 8.**

*Proposed classifications for Precambrian orogens Chardon et al. [67] (a) and Cagnard et al. [72] (b). (a) Orogen construction possibilities [67]: LM1 = stiff upper mantle lithosphere; LM2 = ductile, lower viscosity, lower lithospheric mantle; C = crust; LM = lithospheric mantle; LM1 = stiff upper mantle lithosphere; LM2 = ductile, lower viscosity, lower lithospheric mantle. (b) Schematic orogenic cross sections depicting the evolution of distinct orogenic styles over time. [72].*

**Figure 9.** *Micro-Raman spectroscopy of some Precambrian rocks.*

and the opaque minerals. Plagioclase, quartz, and microcline minerals were found to make up to 70% of the volume percentages of the rock in thin sections, with plagioclase being the most dominant mineral, followed by quartz microcline being the third most dominant mineral. The results of 830 nm Raman microspectroscopy of biotite granite gneiss grains have 398.8 cm–1 and 663 cm–1 indicating weak spectra while that of 785 nm have 714.8 cm–1, 720.4 cm–1 and 764.48 cm−1 indicating strong peak bands respectively (**Figure 9**) [47, 73–75].

Raman shifts are: 398.8 cm−1, 521 cm−1, and 714.8 cm−1 signals in P4332 mode, parallel swinging of the two tetragonal centres corresponds to polymorph Fe-O bond stretching, cubic P4332 structure, and suggested change in the tetragonal symmetry. This originates from Fe2−, Fe3− and O2− which are the bond stretching in the [γ- Fe2O3] cubic P4332/tetragonal P41212 having transformation phase of magnetite spinel of α- Fe2O3 and γ- Fe2O3 polymorphs [76]. Kaiser microprobe of 785 nm recorded Raman peaks at 522.67 cm−1 and 714.8 cm−1, 720.4 cm−1, 764.48 cm−1 for maghemite mineral grains observed in biotite granite gneiss.

Low-temperature shortage causes oxidation of magnetite in the magnetic moment, resulting in a reverse spinel structure with both Fe2− and Fe3− ions in tetrahedral positions (A) and octahedral (B) sites configuration were all factors that contributed to the presence of maghemite in the biotite granite gneiss [76]. The main minerals in granite are quartz, microcline and plagioclase, while minor minerals include hornblende, muscovite, and opaque minerals. Under plane-polarised light, the quartz mineral in the rock samples was colourless, and it appears as subhedral prismatic crystals. Microcline is typified by polysynthetic twinning in two directions (cross-hatched), one according to albite law, and the other according to pericline law (monoclinic orthoclase/sanidine transformed to microcline), whereas its polysynthetic twinning distinguished plagioclase according to albite law. Biotite is brown, yellowish-brown and reddish-brown in the thin section. It is pleochroic, occurring as plates and laths and showed elongation along the foliation plane.

Several granite grains with variable colours were subjected to two excitation wavelengths of Raman microprobe, disregarding configuration and cause of the 521 cm–1 peak. The Raman mode at 519.1 cm−1 and 521 cm−1 corresponds to polymorph Fe-O bond stretching, cubic P4332 structure, which described the tetragonal distortion symmetry. The Raman shifts of 519 cm–1, 522.67 cm–1, 663 cm–1 and 714.8 cm−1 are laser excitation wavelengths and not fluorescence induced. These bands appeared only for dark or opaque granite crystals when excited with Raman microprobe (785 and 830 nm) laser, even if the clear samples are less intense. Since the points are similar to those determined for maghemite lattice modes, and these spectra correspond to translational lattice modes in maghemite geometry [73].

Scanning electron microprobe mineralogical compositions of granitoids from the basement complex of Southwestern Nigeria were studied using scanning electron microscopy (SEM). Gneiss, granite, biotite granite gneiss, banded gneiss and charnockite predominantly recorded maghemite/magnetite, ilmenite, pyrite and poor (titano)magnetites, with differences in titanium (Ti), grain sizes content and configuration, respectively (**Figure 9**). Examinations of polished sections of samples from the southwestern Nigerian granitoids (**Figure 10A**-**F**) revealed: grains of maghemite and magnetite (light grey); titanomagnetite (grey) magnetite (light grey); magnetite and titanomagnetite (striations of light grey); maghemite and titanomagnetite (grey); ilmenite and pyrite observed between (white and grey) respectively.

Scanning electron microscope serves as a proxy in determining the diverse magnetic phases in iron titanium oxides present in the selected rock samples. Studies showed larger altered (titano)magnetite grains in the gneiss, titanomagnetite in granite, phases of titanomagnetite and magnetite in biotite granite gneiss with evidence of dehydration, maghemite and titanomaghemite in banded gneiss, ilmenite in charnockite and pyrite as seen in granite gneiss (**Figure 10** a-d). The Fe-Ti-O grains indicated transformations of spinel rods, low-temperature oxidation reaction, precipitation of crystalline rock phases (exsolution) and dehydration due to tectonic-metamorphic episodes observed in studied rock samples. The abundance of precursor magnetite was susceptible to transformation than the smaller magnetite grains resulting in the pronounced formation of maghemite and titanomaghemite in the rock samples. This result correlates with the arguments of Carporzen *et al*. [77] that suggested tectono-metamorphism-related temperatures in the rock assemblage and heating of magnetite.

*Geodynamics of Precambrian Rocks of Southwestern Nigeria DOI: http://dx.doi.org/10.5772/intechopen.104668*

#### **Figure 10.**

*SEM results of magnetic minerals (JEOLJSM-5900LV). (a) Altered magnetite (maghemite) of gneiss (scale:10 pm), (b) titano-magnetite of granite (scale:10 pm). (c) Titano-magnetite and magnetite from dehydration of biotite in biotite granite gneiss (d) titanomaghemite and maghemite of banded gneiss (e) ilmenite of charnockite (f) pyrite of Charnockite.*

#### **Figure 11.**

*XRD of some Precambrian basement rocks the study area (a) granite (b) gneiss (c) Charnockite.*

#### **5.4 X-ray diffractometry**

The magnetic minerals in the selected rock samples were investigated further using the XRD data (**Figure 11** a-c) to unravel the mineralogical phases found in the granite, gneiss and charnockite. It demonstrates mainly the silicate and pseudomorphs of magnetite phases. **Figure 11** a-c showed pronounced spectra of silicate phase while the smaller spectra are pseudomorphs of magnetite (magnetite and maghemite). These results are consistent with that of the Raman spectroscopy, SEM and temperature dependence.

#### **6. Discussion of results**

The Precambrian rocks witnessed remagnetisation due to four phases of tectonometamorphic episodes. Rocks like diorite, biotite granite gneiss and syenite were not remagnetised and recorded normal and reversed polarities. The documentation of preliminary paleomagnetic, geochronological and microstructural datasets for Precambrian granitoids from the Southwestern Nigeria basement complex, located on the Pan African nappe system covered the Southwestern area of Nigeria, promoted extensive greenschist hydrothermal metamorphism in the underlying cratonic basement, according to research conducted on the northern edge of the Congo craton. The tectonometamorphism has resulted in widespread remagnetisation of the granitoids in southwestern Nigeria. On amphibole grains from Precambrian rocks, 206Pb/207Pb techniques were used to date both metamorphism and magnetic resets at 571.6 Ma. The normal and reverse polarities found in late Neoproterozoic granitoids are coeval with the paleopole at 304.80E and 61.80S (DP = 5.4, d*m* = 10.7) meet the fifth criteria of Van der vool. These pole and specific primary poles of the Congo craton propose an elbow-shaped apparent polar wandering path ranging from 593 to 547 Ma at the Pan African tectonic metamorphism [23, 26, 42]. Raman spectroscopy revealed the presence of maghemite iron oxide minerals in most of the rocks. SEM results showed maghemite, magnetite, titanomagnetite, ilmenite and pyrite, while XRD recorded pseudomorphs of magnetite. Two-sided lithospheric downwellings and shallow underthrusting weakened the plates. When the upper mantle temperature rises over 250°C, a "no-subduction" zone emerges, in which small deformable plate pieces move horizontally. The degree of lithospheric weakening caused by the intrusion of sub-lithospheric melts into the lithosphere controls the tectonic regime. At upper-mantle temperatures of 175– 160°C, a reduced melt flow leads in less melt-related weakening and more strong plates, stabilising the present subduction type even at high mantle temperatures.

#### **7. Conclusions**

Remagnetization was prevalent in the Precambrian era. Reactivation of the Pan-African tectonics on the migmatite gneiss protolith (Eburnean granitic pluton) was not affected in numerous sites. The rocks demonstrated primary and secondary remagnetisation (normal, reversed and mixed polarities) and stability established by representative rocks of biotite granite gneiss, granite gneiss and syenite. Geochemical and isotopes parameters have revealed that the Paleoproterozoic/ Eburnean orthogneiss and the granite plutons represent the same lithospheric source Paleoproterozoic/Eburnean source became molten all through the Pan-African event. The paleomagnetic pole positions of some Precambrian rocks in southwestern to the orogenic events revealed actual polar wander paths towards the equator during the assemblage of the Rodinia supercontinent. The Raman spectra of maghemite through estimation and observations of analogous wavenumbers of magnetite pseudomorphs revealed its atomic origin. The individual specimens of biotite granite gneiss, granite and charnockite have maghemite at strong peak spectra 519, 521, 522.00 cm−1 and 1285.5 and weak shoulder Raman spectra 398.8, 663, 710 and 717 cm−1 with 830 and 785 nm infra-red Raman spectroscopy. SEM revealed evidence of magnetite and titanomagnetite and dehydration of biotite in biotite granite gneiss. Study area dominated by plume tectonics and lithospheric delamination. Numerical models suggest that the transition occurred at mantle temperatures 175–250°C higher than present-day values triggered by stabilisation of rheologically strong plates of continental and oceanic type. Widespread development of modern-style (cold) collision on Earth started during Neoproterozoic at 600–800 Ma decoupled and is thus the onset of modern-style subduction. The cold collision created favourable conditions for the generation of ultrahigh-pressure (UHP)metamorphic complexes in southwestern Precambrian rocks.

*Geodynamics of Precambrian Rocks of Southwestern Nigeria DOI: http://dx.doi.org/10.5772/intechopen.104668*

### **Author details**

Cyril C. Okpoli1,3\*, Michael A. Oladunjoye2 and Emilio Herrero-Bervera3

1 Faculty of Science, Department of Earth Sciences, Adekunle Ajasin University, Ondo State, Nigeria

2 Department of Geology, University of Ibadan, Ibadan, Nigeria

3 Paleomagnetics and Petrofabrics Laboratory, School of Ocean and Earth Science and Technology (SOEST), Hawaii Institute of Geophysics and Planetology (HIGP), Honolulu, Hawaii, USA

\*Address all correspondence to: cyril.okpoli@aaua.edu.ng

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 6**

## Exploring the Application of Potential Field Gravity Method in Characterizing Regional-trends of the Earth's Sequence System over the Sokoto Basin, NW, Nigeria

*Adamu Abubakar and Othniel K. Likkason*

### **Abstract**

In this chapter some preliminaries evaluation were outline briefly based on exploring the use of potential gravity field method in characterizing regional trends of Earth's sequence system interms of gravitational potentials and fields, viz-a-viz., the background, instrumentation, theoretical model, gravity data reduction, geological framework, Bouguer reduction density, density acceptable models, free-air anomaly and interpretation of Bouguer gravity anomaly. Therefore, application of potential field method (gravity) explore these relationships by focusing on the formation and fill of a continental rift basin in characterizing regional trends of the Earth's system interms of data processing, interpretation and Earth's modelling. The study was carried out with the aim to understand and characterized the structural styles and regional trends of the Earth's sequence beneath the Sokoto Basin and its surroundings. Results from the Bouguer gravity anomaly revealed gravity high of characteristic feature on the Bouguer map with a strong positive oval shape of causative bodies anomaly (> 14.0 mGal) having E-W trend. On contrary a number of gravity minima (43.31 mGal) and maxima (39.54 mGal) can be found ENE, N-W parts which are almost defining locations of deep basinal areas. The anomalous features shows negative as well as the lineaments pattern are virtually oriented in the NW-SE, ENE and E-W trends.

**Keywords:** potential field, gravity method, regional trends, Earth's characterization, Sokoto Basin, interpretation model

#### **1. Introduction**

Earth scientists explore and investigate the structures of the Earth using diverse means, such as tectonic mapping, solid minerals, groundwater and hydrocarbon or for the harvest of geologic structures. Earth scientists may be interested in the determination of, for example, the thickness of sedimentary sequence, depth to basement structures and delineation of fractures (shallow and deep plate sources) for appropriate use in resource evaluation. For example, the identification and mapping of geometry, scale and nature of basement structures are critical in understanding the influence of basement during rift development, basin evolution

and subsequent basin inversion. From regional gravity data, information such as tectonic frame work and other aforementioned information can be obtained. The geophysical information invariably combined with geological data are essential for a better understanding of the subsurface and characterizing regional trends of the Earth's structures. The use of gravity, can powerfully lead to a better detection and geological interpretation of structural features and has the potential of constraining quantitative details and reducing the ambiguity of geological interpretation. Geophysical method involving gravity are commonly used in the structural interpretation of sedimentary basins because of their better spatial resolution [1]. Potential field gravity method has proved very effective for providing useful information known to guide various exploration campaigns, be it regional studies, economic mineral or oil and gas exploration [2, 3].

Meaningful reconnaissance and detailed geological information have been generated by the analyses of gravity data for defining basin's tectonic framework, gravity survey is the primary method in geophysical exploration as a regional and local structural mapping tool [4–12]. The effectiveness of gravity survey depends on the existence of a significant density contrast between altered rocks or structures and their host rocks. Moreover, gravity survey not only reflects the shape of major granitoids, but also a correspondence between the tectonic lineaments and regional fault systems [12]. The present chapter guide and explore on the use of the acquired gravity data in characterizing regional trends of the Earth system in some parts of the sedimentary terrain of Africa (i.e. the Sokoto Basin of Nigeria, The Agnes of Egypt as well as Kenya). It's evidence that the gravity method depends on the different earth materials which have different bulk densities (mass) that bring out variations in the measured gravitational field. The variations can be interpreted through the use of enhancement techniques to determine the density, geometry and depth which causes the gravity variations in gravitational field. The Earth's gravitational field anomalies results from lateral variations of subsurface materials density and the distance from the measuring instruments, the general problem in geophysical surveying is the ambiguity in data interpretation of the subsurface geology. This arises because many different geologic configurations could reproduce similar

#### **Figure 1.**

*Illustrations showing the relative surface variation of Earth's gravitational acceleration over geologic structures, after [13].*

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

**Figure 2.** *Typical linear drift curve (middle curve) which is a combination of instrument drift and earth tidal variations.*

observed measurements (**Figure 1**). The method can infer location of faults, permeable areas for tectonic movement. It is however, more commonly used in determining the location and geometry of Earth's system characterisation (**Figures 1** and **2**).

#### **2. Instrumentation**

The Lacoste and Romberg model gravity meter was used in data acquisition for this study. It has an advantage of repeatability of 3 mGal (980,000,000 mGal is the Earth's gravitational field) reading and is one of the preferred instruments for conducting gravity surveys in industry. It has a reading precision of 0.01 mGal and a drift rate less than 1 mGal per month (model G569 manual). Measurements were also made along designated areas to further check the behaviour of the instrument.

**Figure 3.** *The La Coste Romberg gravity meter.*

All necessary routine checks on level adjustments and sensitivity of gravimeter were carried out as described in gravimeter manual (**Figure 3**).

#### **3. Gravity data reduction**

It's understood, the Earth's is slightly irregular oblate ellipsoid which means that the gravity field at its surface is stronger at the poles than the equator. The density distribution is irregular, particularly in an inelastic crust, which causes gravity to vary from expected value as the measurement position changes. Therefore, the variations are expressed as gravity anomalies. Mapping the gravity anomalies gives an understanding the structure of the Earth's [1, 5]. It's therefore essential to identify the reasons gravity varies and that it can be corrected while using gravity method in exploring and characterizing regional trends of the subsurface [5]. In this present survey, correction for the tide was not made because loops were closed at interval of about 2 hours or less. Also, since the area is relatively flat, there was no need considering excess mass or mass deficiency, hence terrain correction was not carried out. The results of gravimeter measurements are gravity differences between an arbitrary reference point and a series of field stations. The measured values at each station have some influences which completely mask the desired effect if they were not removed. Therefore before gravity measurements may be useful in possible indications of subsurface conditions (The observed gravity differences must be corrected for those various large influences). The objective of data is to remove the known effects caused by predictable features that are not of the target. The remaining anomaly is then interpreted in terms of subsurface variations in density. Each known effect is removed from observed data. The various corrections are described below:

#### **3.1 The latitude correction**

Both the rotation of the earth and its slight equatorial bulge produce an increase in gravity with increase in latitude (**Figure 4**). Therefore it becomes necessary to apply latitude correction for stations at different latitudes. The value of gravity increases with the geographical latitude [5]. With advance of Earth's rotation, the Earth's is not spherical but is flattened at poles thus the distance factor causes the '*g*' value to increases from equator to pole by 6.6 Gals because the surface is closer to the centre at the poles (**Figure 4**) [1, 14]. The formula for latitude effect is the 1967 Gravity Reference System (GRS67) whose approximation is of the form:

**Figure 4.** *Earth's rotation bulging at the equator.*

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

$$\log \theta = 978,171.261 \left( 1 + 0.005278895 \sin^2 \theta + 0.000023462 \sin^4 \theta \right) \text{ mGal} \tag{1}$$

Where *θ* is the latitude of the station concerned in degrees.

#### **3.2 The free air correction**

Free air anomaly is obtained from the difference between the measured or absolute gravity of a station, *gobs* at the topography surface and its theoretical gravity, *glat*, extrapolated from the reference ellipsoid and correcting it for the free air effect. The final result of the free air anomaly is given as:

$$
\Delta\_{\text{gy}A} = \mathcal{g}\_{obi} - \left( \mathcal{g}\_{lat} - \frac{d\mathcal{g}}{ds} h \right) \tag{2}
$$

where *dg ds* is the vertical gradient and it is the station elevation above mean sea level in meters and its value is 0.3086 Gal m�<sup>1</sup> [1, 5]. In practice the value of 0.3086 mGal/m is the only value used after deriving from Eq. (2) thus, assumed that the Earth's is spherical and non-rotating. Finally, the correction considers only elevation differences relative to a datum and does not take into account that the mass between the observation point and datum as the station were suspended in free-air, not sitting on land (**Figure 5**). These serve as the reason that the correction termed as free-air correction (**Figure 5**). In general the datum used for gravity surveys is sea level and gravity decreases 0.3086 mGal for every meter above sea level [1, 5].

#### **3.3 Bouguer correction**

This is the difference between the observed gravity and the theoretical gravity at any point on the earth corrected for the mass of materials between the point and the datum plane (**Figure 6**), its value 0.04188 *ρ*, where *ρ* is the density of the slab [4, 5, 8, 11, 15]. Bouguer correction is applied in the opposite sense of free air that is it is subtracted when the station is above the datum plane and vice-versa. Bouguer correction accounts for gravitational of the mass above sea-level datum (**Figure 6**).

**Figure 5.** *Free air correction.*

**Figure 6.** *Bouguer correction.*

The equation for Bouguer gravity at a point after all the necessary preceding corrections have been applied can be written as:

$$\mathbf{g}\left(\mathcal{B}\_{\text{grav}}\right) = \mathbf{g}(obs)\text{-BC} \tag{3}$$

Where BC is the Bouguer correction.

$$\mathbf{g}\left(\mathbf{BA}\right) = \mathbf{g}\left(\mathbf{B}\_{grav}\right) \mathbf{-g}\left(\mathbf{Theoretical}\right) \tag{4}$$

Hence the Bouguer anomaly is determined using the expression:

$$
\Delta \mathbf{G}\_{BA} = \mathbf{g}\_{abs} - \mathbf{g}\_{lat} + \frac{d\mathbf{g}}{ds}h - 2\pi\rho G h \tag{5}
$$

Where *πρ* is the assumed crustal density value which is 2.67 � <sup>10</sup><sup>11</sup> kgm<sup>3</sup> or Bouguer density and *G* is the universal gravitational constant. The term 2 Gh in the Bouguer correction which is the additional attraction exerted on a unit mass by a slab of rock material of density between a station and reference datum-mean sea level (m.s.1).

#### **3.4 Bouguer gravity anomaly**

A Bouguer gravity anomaly is the difference between the observed acceleration of an object in free fall (gravity) on surface of the Earth's, and the corresponding value predicted from a model of the gravitational field. If the attraction due to the effect of material between the plane of observation and the m.s.l. (known as the Bouguer correction (Bc)) is subtracted from the free-air anomaly, the corrected gravity field is called the Bouguer gravity anomaly and is given by:

$$\mathbf{g}\_B = \mathbf{g}\_{obs} + d\_{\mathbf{g}L} + d\_{\mathbf{g}FA} \mathbf{-} d\_{\mathbf{g}B} \mathbf{-} d\_{\mathbf{g}T} \tag{6}$$

Where *gobs* = station readings; *dgL* = latitude correction; *dgFA* = free air correction; *dgB* = Bouguer correction; *dgT* = terrain correction. Putting in numerical values we have:

$$\mathbf{g}\_B = \mathbf{g}\_{obs}\mathbf{-g}\_0 + \mathbf{0.3086H}\mathbf{-0.0419 \text{ rH-}d\mathbf{g}\mathbf{T}}\tag{7}$$

#### **3.5 Drift correction**

In the reduction of gravity data, the removal of drift which occurs as a result of elastic creep in the spring of the instrument is very necessary. The instrumental

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

drift of the gravimeter used in this survey was removed using a Geosoft computer Algorithm routine of [13]. It is assumed that there is a linear relationship of the drift with time as given by the drift rate which is expressed as:

$$\mu = \frac{(\mathbf{g}\_2 - \mathbf{g}\_1) - (R\_2 - R\_1)}{t\_2 - t\_1} \tag{8}$$

Where *g*<sup>1</sup> and *g*<sup>2</sup> are absolute gravity values at the two end stations of a loop while *R*<sup>1</sup> and *R*<sup>2</sup> are tie observed reading (converted to milliGal) at times *t*<sup>1</sup> and *t*2, respectively at those stations.

If the sums station is reoccupied, then *g*<sup>2</sup> – *g*<sup>1</sup> = 0 therefore Eq. (10) becomes; repeated computation for loops continued until all observations are referred to an initial time the drift correction for any intermediate station referred to the initial time *tO* thus becomes:

$$
\mu = \frac{(R\_2 - R\_1)}{t\_2 - t\_1} \tag{9}
$$

With the assumption that drift of the instrument is a linear function of time over a short time interval, it was ensured that all observations in a day were tied to the same time origin during a day's work and the repeat observations at the same station after drift correction was equal to the former. Drift correction was done separately for each altimeter height value using free same cascade drift model. The absolute elevation for each of the stations were determined for each altimeter using the height of the Bench Mark No BM15 to which they were tied. Due to the characteristic behaviour of instrument [15], the field values recorded front tie altimeters for each station were varying. The observed gravity value at the detailed station is given by:

$$\mathbf{G}\_{obs} = \mathbf{g}\_1 \mathbf{-K} \left[ (R\_o - R\_1) \mathbf{-} \mu (t\_o - t\_o) \right] \text{ mGals} \tag{10}$$

Where *g*<sup>1</sup> is the absolute gravity value at fee first base station, *K* is the meter constant. *Ro* and *Rs*, *t*<sup>1</sup> and *to* are the readings and times at the first base station and detail nation, respectively.

#### **4. Geological framework**

The Sokoto Basin is the Nigerian sector of the larger Iullemmeden Basin which spans parts of Algeria, Benin Republic, Niger Republic, Mali and Libya [16]. The study area falls within the Sokoto basin and lies between Latitudes 3:30 E–5:30 E and longitudes 11: 00 N–13:00 N. It is geographically located in the semi-arid with a zone of savannah-type vegetation as part of the sub-Saharan Sudan belt of West Africa with an elevation ranging from 250 to 400 m above sea level (**Figure 7a** and **b**). The area enjoys a tropical continental type of climate. Rainfall is concentrated in a short-wet season, which extends from April to October [17]. Mean annual rainfall is about 800–1000 mm while the mean annual temperature ranges from 26.5 to 40°C. Night temperatures are generally lower. The highest temperature occurs between April and July, the lowest in August (during the rainy season). An average nature of 40% low humidity during the wet season reaches a maximum of 80%, explain the dry nature of the environment in the area of study (**Figure 8**), which is in agreement of a sharp contrast to a humid environment when compare in the southern parts of Nigeria. The Sokoto Basin is predominantly a gentle undulating plain with an average

**Figure 7.** *(a, b) Geological map of Nigeria showing the "Sokoto Basin" and the study area [17].*

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

**Figure 8.** *Regional gravity anomaly map of the study area.*

elevation varying from 250 to 400 m above sea level. The plain is occasionally interrupted by low mesas and other escarpment features [18–20]. The sediments of the Iullemmeden Basin were thought to accumulate during four main phases of deposition as follows (**Figure 7b** and **Table 1**):



#### **Table 1.**

*Stratigraphic successions in the Sokoto Basin, after [20].*


The sediments dip gently and thicken gradually towards the northwest with maximum thicknesses attainable towards the border with Niger Republic.

#### **5. Accuracy of the Bouguer gravity anomaly in Earth's system characterization**

The computed Bouguer anomalies could have several errors introduced to it. Errors could be as a result of incompleteness of the formulae used and the correctness of the numerical values of the constants occurring in them [13, 15]. The calibration factor of the modern Lacoste and Romberg gravimeter depends only on the quality of the measuring screws and the lever system. Errors which could arise from the calibration factor is thought to be negligible because, the calibration factor does not change perceptibly with time, which eliminates the need for frequent checks of calibration. At each station, errors could arise from several sources. These include: errors in elevation determination (eh), errors in terrain effect (et), errors in base value (eb) errors is assumed which recommended that the most likely to in situ densities of subsurface rock lies between the dry and the saturated densities. The summary of the results for the various rock types identified in the area are shown in **Table 2**.

#### **5.1 Bouguer reduction density**

The objective of gravity survey is to detect subsurface density variations. Observed/measured gravity value at the station includes all kinds of attraction. Remove the effect of attraction except that of subsurface density anomaly (**Figure 9a**). There are three methods of selection of Bouguer reduction density; one is a "traditional" or standard density with which most regional maps have traditionally been reduced using a value of 2.67 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> (**Figure 9b**). The second is by determining a Bouguer reduction density which minimizes the correlation between the computed Bouguer anomaly and topography. This method is widely used in areas of rugged topography [21] and which was originally suggested by [11]


#### **Table 2.**

*Summary of rock densities, after [14].*

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

**Figure 9.** *(a, b): (a) Bouguer correction for subsurface density variations. (b) Regional gravity map at upward continue of 1 km after [14].*

and [22]. This second method was not used in this chapter because the area is relatively flat. The third method is to measure the density of representative rock samples just as described and characterized them interms of earth system evolution. The fact is that it is usually difficult to obtain a suite of rock samples that is truly representative [2, 23]. Therefore in order to ensure consistency and compatibility

with after regional gravity map in adjacent areas, the standard density value of 2.67 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> was used for reduction in this survey purpose.

#### **5.2 Densities acceptable for models**

The proper density values used for gravity interpretation depend upon the depth of formation in relation to the water table, which will in turn depend on whether the climate is arid or moist. The age and depths of sediments depend on how long they are buried. If the period is long enough, the sediments usually consolidate and lithify, resulting in a reduction in porosity and increase in density. Limestones and sandstones which are found in the study area increase in density by infiltration of cements without volumetric change [17, 18]. Clays and shales which are compacted clays are the most highly compressible of all sedimentary rocks and they therefore show the greatest amount of compaction. If sandstone and limestones on the other hand are subjected under the same environment, they experience smaller density change [18–20].

From the **Table 2**, it can be seen that the range for the rock density in some part of the Sokoto basin (i.e. Argungu, Shanga Kalambaina, Dange), area is from about 1.5 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> to 4.5 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> and their respective mean densities approximately agree with the published values for similar rock types from other places [1, 8]. Considering the results from Telford et al. for example, the mean densities for limestone, clay, shales, and laterites are (3.45, 2.43, 2.50 and 2.66) 10<sup>3</sup> kgm<sup>3</sup> , respectively and from the table, the same set of rocks have their density values ranging from 1.76 to 5.30 103 kgm<sup>3</sup> . Since limestone and laterites are the dominant rocks in the study area, density values within this range were used *w* that of the sediments during interpretation. Generally, the common rocks of the basement are gneisses, granites, phyllites and quartzite, their densities range from 1.67 to 2.01 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> and their average densities are (2.80, 2.64, 2.74 and 2.77) <sup>10</sup><sup>3</sup> kgm<sup>3</sup> as mentioned above, respectively [8]. Therefore the average density of the earth crust (2.67 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> ) was then used as that of the basement.

#### **5.3 Free-air anomaly map and topography**

Free-air correction essentially takes care of the vertical decrease of the gravity with increase of Elevation and no account of the materials between the station and the datum plane taken. The variation amounts to 0.3086 mGal/m. The relationship between the free-air anomaly and heights was investigated and explained in the previous Section 5.1 above. The result of the free air anomaly are shown in **Figure 10** below. The free-air anomaly map indicates values ranging from a maximum of 11.8 mGal to a minimum of 41.2 mGal and a contour interval of 2 mGals was used for the map. A careful study of the map reveals that major linear pattern is generally in NE-SW direction with exception of few anomalies located at the S-W trend of the area.

#### **5.4 Interpretation of Bouguer anomaly map**

The Bouguer gravity map (**Figure 11**) comprises various low and high anomalies extend in the NW-SE, ENE and E-W trends as consequence with fold patterns in the southeastern part of Iullemmeden basin (Sokoto Basin in particular). These alternated anomalies are primarily due to the density contrast between the sedimentary blanket and some portion of the crystalline basement in Taloka formation. Sokoto Basin gravity high is a very characteristic feature on the Bouguer map with a strong positive oval shape anomaly (> 14.0 mGal) having S-E trend. The

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

**Figure 10.** *Free air gravity anomaly map over Sokoto Basin after [14].*

**Figure 11.** *Bouguer gravity anomaly map over Sokoto Basin after [14].*

structural trend pattern from the map trending in the S-E direction is associated with deep basinal area of the causative (anomalous) body determined from the gravity survey are found to range from (�43.31 mGal) in the west corner of (**Figure 11**) fold patterns, and (�39.54 mGal) in the N-W part. The Bouguer gravity anomalous shows negative values and the structural lineaments patterns trending E-W (major trends) and NW-SE, ENW (minor trends) (**Figure 12**). The deep basinal of the causative (anomalous) body are fall within Gwandu, Kalambaina, Dange, Gamba, Wurno and Taloka formation represents two complementary different events; an older event probably of Continental intercalaire and pre-Cretaceous ages which caused major folding and faulting of NE-SW and ENE trends (**Figure 12**), respectively [17, 18].

#### **5.5 Regional-residual gravity separation**

In the present study, a purely analytical method was used with (Geosoft Oasis montaj V.8.4.3) in which matching of the regional by a polynomial surface of low order exposes the residual features as random errors. A first order polynomial surface was considered adequate for estimating the regional effect. Regional-residual separation process was applied to gravity data-set in order to estimate the amplitude of the regional background. Upward continuation was used to separate a regional gravity anomaly resulting from deep sources from the observed gravity (Bouguer anomaly) (**Figure 11**). The regional field (**Figure 9b**) is a plane dipping gently in a NW-SE direction with a gradient of about 1 mGal/km. The regional effect correspond to low frequencies therefore the anomalies are usually of long wavelength showing a gradual change in value while the residual anomalies which are due to local effects may show larger variations [6, 7]. There are several methods of removing the unwanted regional, some approach is entirely graphical while others are analytical. In some cases the graphical methods are incorporated in the analytical methods. The regional gravity values shows the negative entirely and are found to range from a maxima of �21.9 mGal to a minima of �59.3 mGal (**Figure 9b**).

**Figure 12.** *Rose diagram, structural trends pattern.*

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

**Figure 13.** *Residual gravity map at upward continue of 1 km.*

The residual anomaly at any point is then calculated as the difference between the observed Bouguer anomaly *gB* and the regional effect *g* at that point and this is expressed as:

$$\mathbf{g}\_{res} = \mathbf{g}\_{OB}\mathbf{-g} \tag{11}$$

The residual anomalies at all the points were gridded and contoured through the application of (Geosoft Oasis montaj V.8.4.3). The resulting map (**Figure 13**) shows the gravitational effect of the near surface and local structures in the study area and the values was found to range from �19.5 mGal (minima) and 16.3 mGal (maxima). The larger features generally show up as trends which continue smoothly over very considerable areas, and they are caused by the deeper heterogeneity of the earth's crust superimposed on these trends, but frequently camouflaged by them, lie the smaller, local disturbances, which are secondary in size but primary in importance. These are the residual anomalies, which may provide the direct evidence for reservoir—type structure or mineral bodies.

#### **6. Gravity data modelling/advanced processing**

This process is aimed in modelling the source of the gravity signal measured at the surface. This can be done through processing of


As the Bouguer gravity value represents the effect of crustal and upper mantle density variations, the gravity anomalies were used to study the entire lithosphere. The 2D modelling (**Figure 14**). In quantitative interpretation of gravity data, the

#### *Earth's Crust and Its Evolution - From Pangea to the Present Continents*

**Figure 14.**

*2D regional crustal modelling of the Sokoto Basin, after [14].*

objective is to estimate a subsurface structure whose calculated gravity effect satisfactorily approximate the observed gravity field measured on the surface. The magnitude of gravity anomaly caused by any structure depends directly on its volume times its density contrast. Secondly, the amplitude of the anomaly decreases as the depth of the structure causing it increases. If the shape of the structure is irregular or diffused, the observed gravity will be predictable to reduce in sharpness and in magnitude. Quantitative interpretation, generally barely unique or specific as it is always based on geologic implications. Thus, sufficient and adequate information about the geology of the study area becomes necessary for a meaningful interpretation. The study area falls into the sedimentary basin in the northwestern part of Nigeria, and the particular sediments found from the surface (Gwandu formation) which is of the Eocene age have average density of about 2.74 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> considering the lithologic sequence downward to depth of about 45 m. Underlying it with a slight unconformity are the Sokoto groups (Gamba, Kalambaina and Dange formation) which are of Paleocene age which have average density of about 2.43 <sup>10</sup><sup>3</sup> kgm3 . These deposits extend to the depth of about 80 m (**Figure 14**) [18]. Below this occurs continental deposits (fluvial) which were of lower cretaceous or pre-Maastrichtian age. The estimated density value has a first density contrast of 2.4 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> , and the second of 2.92 103 kgm<sup>3</sup> , with respect to the average density of the basement (2.48 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> ) used. Therefore almost all the gravity lows in the study area were accounted for by the thickening of the sediments. In the interpretational procedures, the gravitational effect of any assumed initial model is calculated and compared with the observed effect. Changes are made as necessary on the presumed model in order to get a better fit. The common changes usually involve volume, shape and density contrasts. This process is repeated within geologically realistic limits until a new structure whose calculated effect best fits the observed effect was obtained. This approach is referred to as forward modelling (**Figure 14**). Profile 2 was chosen atleast to cross one major causative (anomalous) bodies identified earlier for interpretation (**Figure 13**). The computer program used for quantitative gravity interpretation of this profile 2. This *Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

**Figure 15.** *(a, b) 2D density interpretation model.*

interpretation reveals the prominent Gwandu formation and the Sokoto groups (i.e. Gamba, Kalambaina and Dange formation) of sedimentary in-fills have a common origin. The profile runs in the E-W direction and cuts across the causative (anomalous bodies) while modelling, an intrusion lie with density contrast of 2.43 <sup>10</sup><sup>3</sup> kgm<sup>3</sup> introduced in part of Gwandu formation (**Figure 14**) at about 20–45 km along the profile before a fit of the computed with the observed was obtained in uppermost part of (**Figure 14**). While the low gravity at the western side of the profile was accounted for by thickened sediments which has high density contrast of 2.92 103 kgm3 . The maximum and minimum depths to the top of the sediments in-fills along this profile are 40 and 80 km, respectively. The body it has inward dipping walls and the dips are 45° and 55° on its western and eastern flanks, respectively has been calculated from the GYS-System.

#### **6.1 2D density interpretation**

In indirect interpretation, the Earth's sequence model whose theoretical anomaly can be computed simulates the causative body of a gravity anomaly characterization. The shape of the body can be altered until the computer anomaly closely matches the observed anomaly (**Figure 15a** and **b**).

#### **6.2 Gravity contribution to conceptual model**

Gravity methods are good in structural mapping in potential exploration for: Earth's system characterization interms of (imaging the lithospheric structures, dense material in shallow crust), fractures/faults (gravity gradients/slopes), help to identify potential drilling sites, help to identify potential recharge areas, etc.

#### **7. Conclusion**

The chapter were able to explore the potential application of gravity method for Earth's system exploration interms of regional trend characterization in African tectonic evolution settings. The strength of the potential gravity field method lies in the adequate density mass distribution of gravitization effect within the crustal materials of the Earth in the light of measurable gravity field over them. The Earth's gravitational field, that is the Earth's shape and global force, is itself complex.

Advanced data processing, analysis, interpretation and modelling provides the means of characterizing the Earth's regional trends and with such a representation; it is possible to predict the Bouguer anomalies and other densities acceptable for models. The knowledge of the free-air anomaly of the Earth enables the gravity anomaly to be determined over a survey area from measurements of the gravitational field strength. The method were applied to real field measurements of Bouguer gravity data over the Sokoto Basin, Nigeria. The working data were corrected for Bouguer reduction density variation using regional-residual separation model. In particular, the major anomalies of the regional and the observed Bouguer gravity field exhibits majorly trending in the E-W, and NW-SE directions adjacent to the main structural fold patterns of (**Figures 9b** and **11**) in the northwestern parts. The anomaly field which is the summary of the regional field was further processed to obtain the residual gravity anomaly (**Figure 13**). The regional models show that the crustal structure in the study area consists of normal continental crust, which is divided into lower and upper by the Conrad of a nearly constant depth. The density effect for the sedimentary formations is necessary valuable and extremely critical to interpret the deeper effect of Sokoto Rima groups. Also, it may occur on the edges, in other gravity filtering enhancements which significantly influence the structural fittings. In order to overcome the edge effect, the modelled length is slightly enlarged outside the limits.

#### **Acknowledgements**

I acknowledge the words of encouragement support of my chairman supervisory team for my ongoing Ph.D. Program (Prof. O.K. Likkason) of Physics Program, ATBU, Bauchi, Nigeria as well as members'supervisory team (Prof. A.S. Maigari & Dr. S. Ali) of ATBU Bauchi, Nigeria. Indeed I'm grateful to the support intervention for Petroleum Technology Development Fund, PTDF, Abuja, Nigeria. Finally, I also wish to acknowledge the reference text citation made within context of this work.

#### **Author details**

Adamu Abubakar<sup>1</sup> \* and Othniel K. Likkason<sup>2</sup>

1 Department of Applied Geophysics, Federal University Birnin Kebbi, Kebbi State, Nigeria

2 Department of Physics, Abubakar Tafawa Balewa University, Bauchi, Nigeria

\*Address all correspondence to: adamu.abubakar35@fubk.edu.ng; talk2adamuabubakar35@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Exploring the Application of Potential Field Gravity Method in Characterizing Regional… DOI: http://dx.doi.org/10.5772/intechopen.102940*

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### Section 3
