The Middle Niger Basin in Nigeria Has a Rift Origin, as Revealed by the Inversion and Modeling of Gravity and Magnetic Data

The NW‐SE oriented Middle Niger Basin, which is a Campano‐Maastrichtian inland basin has been the subject of debate among geoscientists regarding its origin and development. This paper aims to unravel the basin's origin and evolution by using high‐resolution magnetic, gravity, and topographical data. The integration of aeromagnetic and gravity data provides a means to assess the influence of igneous intrusions during the basin's evolution. The study presents a new perspective on the origin of the basin, suggesting a rift origin where orthogonal extension played a crucial role in its evolution. The research also investigates the relationship between intrusive rocks and the formation of the basin, finding these rocks are solely located within the basement. The magnetic and gravity maps reveal anomalies associated with intrusive structures within the Precambrian crystalline basement. This is a common feature shared with rifted basins on a global scale.


Competing Theories on the Origin of the Middle Niger Basin
The NW-SE trending Middle Niger Basin is structured like an elongated ellipse and consists predominantly of Campano-Maastrichtian sediments specifically sandstones, siltstone, and ferruginous sandstones (Adeleye, 1973;Ojo, 1990) Due to its proximity to and potential link with the Sokoto Basin, as well as its promising prospect as a commercial source of minerals and hydrocarbons, the Middle Niger Basin has generated significant interest among geoscientists, policy makers and stakeholders in the exploration business (Kogbe et al., 1983;Ojo, 1990;Ojo & Ajakaiye, 1976;Rahaman et al., 2019;Salawu et al., 2020Salawu et al., , 2021)).However, despite this interest, the tectonic origin and evolutionary history of the Middle Niger Basin continue to be a subject of debate (Kogbe et al., 1983;Ojo, 1990;Ojo & Ajakaiye, 1976;Rahaman et al., 2019;Salawu et al., 2020Salawu et al., , 2021) ) till date.The evolution of the Middle Niger Basin has been interpreted through three primary tectonic models: the simple sag, orthogonal rift, and strike-slip models.Braide's (1990Braide's ( , 1992) ) strike-slip model suggests a pull-apart origin for the basin, attributed to transtension, and supported by the presence of the "Gboko transform fault" and its linear northeastern boundary (Whiteman, 1982).Kogbe et al. (1983) propose a rift origin based on Landsat imagery without explicitly distinguishing between transcurrent and orthogonal rift types.They hypothesize that the formation of the basin was constrained by NE-SW oriented fractures within the adjacent crystalline basement.Ojo (1990) expands on the rift model by evaluating aeromagnetic data, which indicates the presence of basic rocks at depths of 4-6 km.Conversely, Likkason (1993) proposes a mantle plume near the Benue and Niger Rivers' confluence as the primary mechanism for the development of the Middle Niger Basin.Drawing upon folding within the adjacent Benue Trough, Adeleye (1976) proposes a third or so-called simple sag model, attributing the basin's formation to folding within the adjacent Benue Trough during the Santonian.Ojo andAjakaiye (1976, 1989) further support this hypothesis by analyzing gravity data, revealing the basin's margins to be possibly unfaulted, while Whiteman (1982) describes the basin as a shallow cratonic basin formed post-Santonian.
To thorough understand the connection between magmatic intrusions and the rifting processes that shaped the basin, an in-depth examination of the basement structures beneath the basin is important.In this context, we propose that basement modeling offers a more promising approach for unraveling the origin of the basin.This study therefore presents the outcomes of a detailed analysis of potential field and topographical data from the Middle Niger Basin, shedding novel insights into its formation and evolution.By integrating potential field and remotely sensed data, we delve into the underlying framework of the basin, including the Precambrian basement across various crustal levels.This strategy yields new knowledge that significantly enhance our understanding of the basin's formation and evolutionary processes.
Our findings challenge prevailing theories (Adeleye, 1976;Braide, 1990;Ojo & Ajakaiye, 1976, 1989) and emphasize the need for further investigations into the underlying structures beneath the basin.While the rift model is still plausible, the regional evidence from this study does not support the alternative hypotheses.We advocate for future exploration endeavors in the Middle Niger Basin to be anchored in the novel knowledge presented here, with focus on deepening our knowledge of its origin.Further research into the basement structures beneath the Middle Niger Basin is central to advancing our understanding of the influence of magmatic intrusions on rifting and providing a comprehensive explanation for the basin's origin.

Geological Settings
The Middle Niger Basin is situated in west-central Nigeria, stretching from Kontagora in Niger State to regions marginally beyond Lokoja in the south.The basin is bounded by the basement complex in the northeast and southwest, while it converges with the Sokoto and Anambra Basins in the northwest and southeast, respectively (Figure 1).The Nigerian basement (Figure 1) lies in the Trans-Saharan Belt and is situated northwest of the Congo Craton and east of the West African Craton (Ajibade et al., 2011) and to the southwest of the Saharan metacraton (Liégeois et al., 2013).The basement shows an orogenic system with preserved history of metamorphism, deformation, and magmatic events of the Pan-African multi-staged orogenic event.These tectonic events developed the prominent N-S and NE-SW oriented shears that defined main structural elements (e.g., foliations, lineations, folds and faults) of the entire region (Dada, 2008).In terms of rock assemblages, the basement consists of: (a) Migmatite-gneiss complex, (b) Probable Neoproterozoic age schist belts (metavolcanics and metasedimentary rock units), and (c) Pan-African granitoids (Dada, 2008).
The Middle Niger Basin (Figure 2) on the other hand is recognized as the NW extension of the Anambra Basin (Akande et al., 2005).Upper Cretaceous sedimentary rocks with NW-SE orientation mainly fill the basin and were deposited because of faulted basement block movement, basement fragmentation and subsidence.The northern part of the basin is often called the Bida sub-basin while the southern part is called Lokoja sub-basin.Both sub-basins lie unconformably on the basement and consists of immature (texturally and mineralogically), massive, clast to matrix reinforced conglomerate that fines upwards into conglomeratic-sandstone, sandstone with small grains, siltstone with subordinate-claystone.The Enagi Formation rightly overlays the sandstones of the Bida and Lokoja and is comparable to the Patti Formation within southern segment of the Middle Niger Basin, which sequentially is covered with the Batati and Agbaja Formations (Adeleye, 1974;Akande et al., 2005).The basin's structural elements include NW-SE striking fault systems at its contacts with the adjacent basement complex terrain (Kogbe et al., 1983).There are no major morphological elements, fractures, or intrusive rocks in the basin (Salawu et al., 2020(Salawu et al., , 2021)).However, the basement of the basin is associated with major lineaments, some of which include the horizontal continuation of the NNE-striking Kalangai-Zungeru-Ifewara sheared zone (Figure 1) created through the Pan-African orogenic events (Salawu et al., 2020).

Topographic, Gravity, Magnetic Data
An approximately 30 m (1-ARC) spatial resolution topographic data (Figure 3a) that was acquired by the Shuttle Radar Topography Mission (SRTM) was used in this work.Major remotely sensed shaded-relief map (Figure 3b) Earth and Space Science 10.1029/2023EA003025 SALAWU ET AL.
have been generated based on SRTM data enhancement using the hill shading processing method.The interpretation of the generated shaded relief map reveals morphological patterns in the investigated sedimentary basin.Moreover, topographic data investigation is very effective when combined with the interpretation of potential field data set.The magnetic data used for this research were acquired from the Nigerian Geological Survey Agency and represents a portion of the main Nigeria's magnetic data sets collected between 2004 and 2009.The magnetic data set was collected at 80 m terrane clearance with northwestern-southeastern flight lines.These lines were space out at 500 m then tied at 2,000 m.The diurnal variation effects were accounted for and removed from the acquired aeromagnetic data, resulting in the production of magnetic anomaly data for the study area (Figure 4a), which was obtained by subtracting the geomagnetic field (IGRF) from the data.
The final data set used with the topographic and magnetic data is the global EIGEN-6C4 satellite gravity data.The gravity data was downloaded from the International Center for Global Earth Models (ICGEM) website (http:// icgem.gfz-potsdam.de/calcgrid)at a grid step of 0.1°.The EIGEN-6C4 model is a reconstruction of the Earth Gravity Model 2008 (EGM 2008), which is based on the GRACE (Gravity Recovery and Climate Experiment) satellite missions, GOCE (Gravity Field and Steady State Ocean Circulation Explorer) observations and Danmarks Tekniske Universitet (DTU) ground data.The satellite gravity data has the order of 2159 and degree of 2190 with spatial resolution of about 9 km.Subsequently, the satellite gravity data was corrected using terrain and Bouguer plate corrections by subtracting Bouguer plate attraction from the data.All masses above the sea level were accounted for by using the Bouguer plate corrections in Equation 1 to the gravity data.2πGρH = 0.0419H (1)

Methods
A suite of digital processing techniques was used to enhance features on the magnetic and remotely sensed data of the basin to investigate its origin and evolution.The Oasis Montaj software was used to interpret potential field data sets to localize structural features while the ArcMap software was used to enhance remotely sensed data to reveal topographical variations.

Enhancement of SRTM Data
Multidirectional hill shading technique was applied to the topographic data (digital elevation model) of the region in ArcGIS Pro software.The multidirectional technique uses a combination of different sources of light to reveal the hill shaded terrain.The use of multiple light azimuth angles enhance the understanding of topographic patterns, which would remain concealed with single light shaded-relief map.This approach fosters a more realistic representation of the topography, striking a balance between underexposed and overexposed terrains within the SRTM image.

Reduction-To-Pole Filter
Since magnetic anomalies are not always exactly above their respective magnetic sources, interpreting magnetic anomaly data recorded at lower latitudes is more difficult.This is because the earth's field and source magnetization directions are not vertical in low-latitude locations.The reduction-to-pole (RTP) filter was used in this study to create a map of anomalies that have been moved on their sources when applied to low-latitude magnetic data.The application of the RTP technique on the total magnetic intensity (TMI) anomaly map (Figure 4a) was achieved using declination: 2.082°and inclination of 4.978 degree of the geomagnetic field.The declination and inclination were selected at the center of the study area (latitude: 9°30'N and longitude: 6°00'E).Furthermore, correctional amplitude inclination of 80°was employed to stabilize the reduced-to-pole filter output that is often unstable at lower latitudes like the investigated region.Figure 4b reveals the resultant RTP magnetic map of Sokoto and Middle Niger Basins and adjacent crystalline basement terrane.

Enhancement of Aeromagnetic Data
The processing routine of the resulting RTP aeromagnetic map was achieved using horizontal gradient magnitude (HGM) procedure to map structural features for the tectonic framework.Recent magnetic investigation of the basin by Salawu et al. (2021) showed the lack of volcanic rock units in the Middle Niger Basin.Hence, applying HGM technique to RTP data can give valuable estimates of magnetic basement contacts irrespective of magnetic latitudes (Phillips, 2000).The HGM technique is defined as (Phillips et al., 2007): Where M is the RTP aeromagnetic data.The RTP-HGM map, was utilized to delineate contacts in the study area, which will reveal the basin's boundaries.

2-D Modeling
Aeromagnetic and gravity crustal modeling was implemented to investigate the crustal structural elements variation of the basement of Middle Niger Basin.Profiles A-Aʹ, B-Bʹ, and C-Cʹ, which extends from the crystalline basement through the sedimentary basin, crossing various magnetic and gravity anomalies, were selected for crustal modeling.Modeling of aeromagnetic data along profiles A-Aʹ and C-Cʹ, and the aeromagnetic responses of models was achieved using the techniques of Talwani and Heirtzler (1964) and Talwani et al. (1959) while the models were produced based on susceptibility parameters of Stanton et al. (2021) as shown in Table 1.This is necessary to get minimum RMS error with maximum fit between the theoretical and observed magnetics models generated.Due to the magnetic technique depth limitations, which cannot effectively image structures deeper than the Curie isotherm depth, typically around 20 km (Fairhead, 2023).Therefore, profile B-Bʹ was selected for joint magnetic and gravity modeling of crustal structures since it shows various gravity anomalies along its length.The integration of gravity data with magnetic data in modeling profile B-Bʹ aids in constraining the vertical extent, size and differentiation of various intrusive igneous bodies since magmatic rocks lack magnetic signatures below the Curie depth.The joint magnetic and gravity modeling of profile B-Bʹ was constraint by density values of sedimentary and volcanic rocks in adjacent Benue Trough (Table 1) as reported by Ajayi and Ajakaiye (1986).Furthermore, the density values from Stanton et al. (2021) were used for the upper crust, lower crust, and the mantle in profile B-Bʹ (Table 1) while we used top basement depth values ranging from 1 to 2 km in the simulation (Ojo, 1990;Salawu et al., 2020Salawu et al., , 2021)).According to Bouzid et al. (2008), Brahimi et al. (2018) and Deramchi (2016), 20 km may be employed to estimate top of the lower crust in the Trans-Saharan Belt.

Inversion of Gravity Data
We used a gradient based and Gauss-Newton method for the 3-D inversion of the satellite gravity data to reveal the subsurface density variation across study area.The 3-D inversion algorithm is implemented in the SimPEG framework (Cockett et al., 2015) using the L p norm, compact inversion procedure.We construct a 3-D mesh with 9 km × 9 km horizontal cell width, and 2 km × 2 km vertical cell width with vertical cell size increasing from 850 m in the top, to 49,150 m at the base of the grid, resulting in 62,275 cells.The 3-D mesh covers the entire Bouguer gravity data of the region.The inverse problem has two essential features, which are the data misfit, and regularization procedure.The data misfit can be defined as the metric that estimates the misfit between predicted and observed data.For model Ф d (m), the misfit data is given in Equation 3 (Cockett et al., 2015): The observed data is given as d obs , the forward model, which generates the predicted data is given as Diagonal matrix with features equivalent to W dii = 1/ɛi is given as W d and ith data point standard deviation is ɛi.
The model in this study used ɛ of 0.01 mGal.Earth and Space Science 10.1029/2023EA003025 The regularization procedure is the second crucial aspect, which is a metric designed to evaluate the consistency of the model with prior knowledge and expectations.Numerous models can adequately fit the data.Hence, we employed the model that possessed the anticipated characteristics and was harmonious with past knowledge.One model can be selected from a multitude of models by determining the norm, or length, of each model (Miller et al., 2017).For this investigation, two-phase 3-D inversion method was implemented.First, L 2 -norm "smooth" constraint was used for the inversion of the Bouguer gravity anomaly data.Next, we compacted the resulting smoothed models with a L p -norm, where 0 ≤ p ≤ 2. L 2 -norm models produces bodies, which have boundaries that are poorly defined (Miller et al., 2017).Our geological knowledge of igneous intrusions as a finite body with distinct boundaries (examples presented therein from modeling of magnetic data), are well signified mathematically by a L p -norm, which generates a more compact bodies with sharper boundaries.
The iterative re-weighted least squares optimization procedure, which is scaled is used to solve the inverse problems and express a combined approach, which is given as (Miller et al., 2017): where β is the Tikhonov or tradeoff parameter.To run the inversion, we use lower density contrast bound of 3.0 g/cm 3 .The upper bound is set at 3.0 g/cm 3 to account for the positive gravity anomalies.

Architecture of the Middle Basin as Revealed by Magnetic Data
Figure 4b shows the Reduced-to-Pole (RTP) magnetic anomaly field, ranging from 60 to 140 nT.These magnetic anomalies are distinguished by low frequency and high amplitude, providing valuable insights into the magnetic properties of the basin.In stark contrast to the surrounding basement terrane, which shows higher frequency and amplitude magnetic anomalies, the basin stands out as a region with unique magnetic characteristics.The basin's magnetic anomalies, with their low-frequency components, indicate the presence of substantial intrusive structures within the bedrock of the basin (Ojo, 1990).The pattern of magnetic anomalies prevalent in the Middle Niger Basin exhibits a sharp contrast with the high frequency magnetic anomalies that dominate the surrounding crystalline basement (Figure 4b).We attribute the presence of these abrupt transitions in anomaly pattern as the basin's boundaries (Figure 4b).
Moreover, the magnetic structural features observed within the study area, as revealed in the RTP-HGM (Reduced to Pole-Horizontal Gradient Magnitude) map, reveal distinct horizontal gradient values.These values are represented in magenta color in Figure 5, highlighting the intricate nature of the basin's magnetic structure.In contrast, the broader structural framework of the basin is characterized by lower horizontal gradient values, shown in shades of blue in Figure 5. Distinctive magnetic characteristics emerge along the boundaries of the Middle Niger Basin where it interfaces with the adjacent crystalline basement terrane.At this interface, discernible variations in horizontal gradient magnitude become apparent, ranging from 0.03 nT/m to 0.15 nT/m.These variations are interpreted as a series of fractures with a NW-SE orientation.These fracture sets are also vividly showcased in the multidirectional shaded-relief map (Figure 3b), where they manifest as steep topographic gradients along the margins of the basin.As a result of this observation, it can be inferred that the structural configuration of the basin is significantly influenced by an array of fractures along its margins.
The structural control of the sedimentary basin is unveiled through an analysis of the gradient of the aeromagnetic data and shaded-relief map.As Figure 3b illustrates, there is a distinct magnetic contrast along the edges of the basin, suggesting the presence of basin boundary fractures.Notably, the basin's edges are characterized by a transition from lower (0.03 nT/m) to higher (0.15 nT/m) HGM (Horizontal Gradient Magnitude) values.These transitional HGM values at the basin's boundaries serve as compelling indicators that the basin is unequivocally a rift basin.In tandem, the Bouguer anomaly map in Figure 6, shows positive gravity anomalies across the basin, consistent with findings from gravity investigations conducted in the East African Rift Valley (Kogbe et al., 1983).

Basement Configuration as Revealed by Crustal Modeling Techniques
Crustal modeling techniques have improved our understanding of the magnetic and gravity anomalies in Figures 4a and 6, shedding light on the distribution of magnetized intrusions in the basement of the Middle Niger Earth and Space Science 10.1029/2023EA003025 SALAWU ET AL.
Basin.Figures 7-9 offer further insights into the basement's structure and boundaries, revealing sedimentary thickness ranging from about 200 m near the basement terrane to about 1000 m within the basin.Additionally, the crustal models highlight the presence of magnetized rock units in the basement, particularly intrusive basic rocks associated with crustal extension processes (e.g., Ball, 1980;Ju et al., 2022;Ojo, 1990).These intrusions likely originated because of lithospheric fracture reworking, which generated partial melting in the low-velocity zone with the fractures allowing the magma to rise into the upper crust and might have turned the basement into an ancient rift zone.These intrusions contributed to the observed magnetic lows in the Middle Niger Basin (Ojo, 1990).Consequently, a depth slice at 10 km (Figure 10) reveals irregularly shaped bodies with varying densities, primarily in the central part of the basin, that align with the shallow magnetized bodies in the aeromagnetic and gravity models (Figures 7-9).Figure 11 clearly displays two extensive magmatic intrusions surrounding the Middle Niger Basin, both to the north and south.These intrusions are mainly found within basement fractures, particularly within the Kalangai-Zungeru-Ifewara shear zone (Figure 10).

Tectonic Implications of Magnetic and Gravity Anomalies in the Middle Niger Basin
Magnetic anomalies in the Middle Niger Basin, arising from non-igneous rock units, exhibit lower amplitudes compared to the metamorphic and igneous rocks in the basement, which might possess higher concentrations of magnetic minerals (Ojo, 1990).High-amplitude magnetic anomalies, displayed as an oval pattern in the Reducedto-Pole (RTP) map, are identified as originating from the basin's basement.This interpretation is corroborated by the low-frequency characteristics of these anomalies, indicative of the presence of igneous intrusive features within the basin's basement.Studies conducted by Kogbe et al. (1983), Salawu et al. (2020Salawu et al. ( , 2021) ) provided additional supporting evidence for this interpretation, collectively confirming the distinctive nature of the Middle     Niger Basin as an extensional sedimentary basin.This uniqueness is attributed to its formation within an extensional tectonic setting, where basement faults are rejuvenated because of the ascent of plutons.As magma intrusion continues and the segments inflate, they fuse to form throughgoing intrusions that incorporate vertical fractures.This process yields variation in thickness, which translates into amplitude anomalies due to intrusive  Earth and Space Science 10.1029/2023EA003025 igneous effects.We observe these elongated amplitude anomalies on the gravity data of the Middle Niger Basin in map view (Figure 6).
The orientation of these basement structures is thus critical in predicting primary fractures, which typically develop in response to the extension of sedimentary basins (Gunn, 1997a).In the study area, fractures identified in the Horizontal Gradient Magnitude (HGM) map predominantly displaying a NE-SW trend.The parallel alignment of the Benue Trough with this NE-SW trend aligns with the primary orientation of faults within the Nigerian basement.In contrast, the Middle Niger Basin aligns itself along NW-SE fractures, representing a secondary orientation of lineaments in the region.It is established that intrabasin faults originating within an orthogonal extensional setting tend to adopt a strike orientation parallel to the rift orientation and perpendicular to the direction of maximum extension, often characterized by normal faulting (Withjack et al., 2002).The HGM map and the multidirectional shaded-relief map of the study area both show evidence of this faulting pattern, indicating that the Middle Niger Basin has undergone mainly orthogonal extensional tectonics.The structural features observed in the aeromagnetic data thus closely reflect orthogonal rifting in the Middle Niger Basin.
The aeromagnetic data of the Middle Niger Basin reveals that it differs from the Benue Trough in its orientation, trending NW-SE instead of NE-SW, and in its tectonics.The Middle Niger Basin has predominantly undergone orthogonal extension, while the Benue Trough has undergone wrench tectonics (Fairhead, 2023).On the other hand, there is general agreement about the geometry of the boundary fractures aligning with the orientations of the Middle Niger Basin and the Benue Trough.The magnetic data suggest that the Middle Niger Basin's scale and orientation (trending NW-SE compared with NE-SW oriented Benue rift; Figure 2) have been influenced by underlying secondary Pan-African basement fractures, which exerted a critical influence on rift localization.

Pan-African Intrusive Rocks in the Middle Niger Basin
A thorough examination of gravity anomalies within the Middle Niger Basin reveals a noteworthy correlation between areas of elevated gravity values and the boundaries of the basin (Figure 6).This correlation primarily stems from the presence of intrusive rocks within the crystalline basement, a geological characteristic previously documented by the Nigerian Geological Survey Agency (Geological Map of Nigeria; NGSA, 2020).The occurrence of reduced gravity anomalies within the Middle Niger Basin is therefore interpreted as evidence for the existence of lower-density intrusive bodies within the basin's bedrock(see Figure 6).In addition, the presence of a thicker sedimentary cover within the basin intensifies this effect.Considering the intrusive nature of these rocks into the bedrock of the basin, the relative timing of the plutons is proposed to be Pan-African.A common consequence of these intrusions in the Middle Niger Basin is the possible reactivation of older basement fractures, which could have led to more basement fracture and, ultimately, the creation of the weak zone that accommodate the formation of the sedimentary basin.This is supported by the magnetic and gravity data of the region, which suggests that the intrusions are predominantly situated within the basin.The intrusion of the magma into the basement of the basin was aided by pre-existing cracks and fractures, which are probably older than Pan African structures in the study area.As a result, the new magma pulses migrated through these pre-existing cracks and fractures, forming dikes, sills, and plutons.The magma cooled and crystallized, forming plutons of Pan African age.

Formation of the Middle Niger Basin
The initiation of the formation in the Middle Niger Basin can be traced back to rifting associated with the break-up of Gondwana with the basin developing because of this extension (Bumby & Guiraud, 2005).After the onset of rifting, magmatic intrusions were emplaced culminating in extensive magma when the rifting was maximum.The magmatism can be considered to have occurred within broad Pan-African lineaments, which represent reactivation of earlier fractures.The lithospheric fracture reworking induced partial melting in the low-velocity zone.This process causes the melt to rise, and, in turn, aids the rift process.The uprising of hot magma can further cause the crust to thin and stretch, leading to the formation of a rift basin.In stable continental interiors, rifting and further episodes of magmatic intrusion is often accommodated by orthogonal extensions, which result in the formation of narrow and long intracratonic basins.These basins are typically bounded by graben and horst structures as the extensional forces that created the basin acted in perpendicular directions, meaning they were oriented at right angles to each other.Rifting and further episode of injection of magma into the basement thus mark the initial stage of basin formation for the Middle Niger Basin.Based on the current orientation of the Middle Niger Basin, these orthogonal extensional forces were most likely oriented in the NE-SW direction.Our two-dimensional magnetic model reveals the presence of intrusive rock bodies beneath the central part of the basin.This observation aligns with previous research conducted by Ojo and Ajakaiye (1976), Rahaman et al. (2019), and Salawu et al. (2020, 2021), who collectively suggested that volcanic rocks are absent from the sedimentary formations of the Middle Niger Basin.Intra-basement igneous intrusive rocks, contemporaneous with or younger than the sedimentary formations in the basement, can contribute to crustal extension, a process often associated with basin formation (Gunn, 1997a).The depth and horizontal extent of the magmatic intrusions, as inferred from the gravity data in Figures 10 and 11, closely correspond to the basic to ultra-basic rocks previously modeled by Ojo (1990) within the basement of the basin.These high-density bodies are primarily situated at the center of the basin, reinforcing our model's consistency with the thermodynamic prerequisites for a rifted basin.During the opening of South Atlantic, the orthogonal thermal expansion of the lithosphere would have reactivated preexisting fractures, facilitating the injection of magma into the basin's basement (Figure 12), thereby contributing to the subsidence of the Middle Niger Basin's basement.
Generally, the weak zones in the crust exert a major control on magmatic intrusions, which led to thermal weaking of surrounding host rocks during magma emplacement (Magee et al., 2014).Also, the magmatism can influence the reactivation of older basement fractures (Magee et al., 2014).The NW-SE trending basin boundary structures were likely generated by this process, as observed from the magnetic data.The resulting structural template commonly involves that the igneous intrusion can generate further expansion of the crust that helped the formation of the Middle Niger Basin, with existing Pan-African fractures serving as conduits for plutons or intrusions.These plutons can produce significant magnetic anomalies, typically exhibiting elliptical or circular shapes and preferentially occurring at depths beneath sedimentary basins (Gunn, 1997a).Similar anomalies have been identified within the Middle Niger Basin (see Figure 4b), resembling those generated by sources beneath Australia's Bass and Canning Basins (Gunn, 1997a).It is essential to emphasize that the formation and evolution of the Middle Niger Basin and the Australia's Bass and Canning Basin are not directly comparable.In the Middle Niger Basin, the plutons were emplaced after the crustal extension, with their final morphology influencing the overall structuring of the Middle Niger Basin.
The formation of plutons in the Middle Niger Basin can have a significant impact on the geological evolution of the region.Plutons can influence the stratigraphy, structure, and tectonic activity of the crust.Moreover, plutons can provide a source of valuable mineral deposits.Additionally, groundwater can further modify the pluton by transporting heat, dissolving minerals, and influencing the rate of crystallization.Gunn (1997b) investigated intrusion processes linked to crustal extension and demonstrated that the injection of successive igneous intrusions along the axis of the extension often coincides with the initiation of crustal extension.However, this scenario does not appear to be the case for the Middle Niger Basin, as the intrusions in the basin's basement are not the primary cause of the extension.Rather, these plutons were formed after the onset of crustal extension.We therefore propose that during the early stages of crustal extension, the presence of intrusions facilitated the transfer of strain into the host rocks, creating new fractures or reactivating the pre-existing ones.These scenarios seem plausible since weak zones within the crust can typically exert a major control on magmatic intrusions, leading to thermal weakening of surrounding host rocks during magma emplacement.The voluminous igneous intrusions at boundary of the Middle Niger Basin extending to depths of 43 km in the north and 31 km in the south of the basin as per the 3D gravity inverted model (Figure 11), are controlled by and emplaced within basement fractures that can be directly linked to Pan-African orogenic events.Similar emplacements of voluminous igneous intrusions within basement fractures have been noted in the Borborema Province of northeast Brazil, and these are associated with the Brasiliano Orogeny (Caxito et al., 2020).The Borborema Province can be correlated with the Trans-Saharan Belt (Caxito et al., 2020), which also features large igneous intrusions emplaced within the Tuareg Shield in shear zones active during the Pan-African Orogeny (Liégeois et al., 2003).
In addition to revealing and characterizing the morphologies of intrusions that gave rise to fractures along the boundaries of the Middle Niger Basin, these faults may play a significant role in future surface investigations concerning the transition of the basin to the adjacent basement.In the 3D gravity inverted model (Figure 11), the two gravity sources surrounding the basin's basement borders have been identified as intrusions that intrude within the Kalangai-Zungeru-Ifewara shear zone and may have influence the formation of secondary fractures, which aligned with the basin's orientation.It is suggested that lithospheric fracture reworking during the opening of the Atlantic Ocean could have led to partial melting in the low-velocity zone, allowing magma to ascend through fractures and created the gravity sources in the basement of the Middle Niger Basin (Liégeois et al., 2005).Comparable magmatic intrusions have been confirmed in the nearby Benue Trough (Ajayi & Ajakaiye, 1986).Faults and intrusions have played pivotal roles in governing rifts (e.g., Gunn, 1997aGunn, , 1997b;;Kogbe et al., 1983) their examination is imperative when investigating the genesis of sedimentary basins.The Middle Niger Basin is bounded by system of NW-SE linear faults, a characteristic feature of rift structures, and analogous NE-SW trending fractures are discernible at the boundary of the neighboring Benue Trough.This suggest that Middle Niger Basin underwent rifting and subsidence during the Cretaceous, contemporaneously with the Benue Trough, with the genesis of the two basins associated with the rifting attributed to the opening of the Atlantic Ocean.Oblique-slip and strike-slip faults are commonly encountered in rifted sedimentary basins resulting from oblique extension (Withjack et al., 2002).Furthermore, the potential field data does not reveal any extensional or transcurrent components of transtensional rifting.As a result, it is unclear how oblique rifting of the Middle Niger Basin developed.Therefore, it is proposed that further seismic investigations should be undertaken in the central region of the basin to delineate the regional stress fields and their influence on the basin's development.

Age, Evolution, and Structure of the Middle Niger Basin in Comparison to Other Rifted and Intra-Basement Sedimentary Basin in Africa
Sedimentary basins worldwide are typically classified based on their geometry into foreland, rift valley, and rhombohedral basins, while their kinematics can be categorized as extensional, flexural or strike-slip basins (Ju et al., 2022).Extensional sedimentary basins have been recognized globally, predominantly in the Cenozoic and Mesozoic eras, with prominent occurrences in the Middle East, West China, America, Russia, Africa, and other regions around the world (Ju et al., 2022;Suo et al., 2014).In the Mesozoic era, the fragmentation of Africa's continental crust gave rise to the West and Central Africa Rift Systems (Fairhead, 2023).These rift systems, spanning from Nigeria to Kenya, are associated with extensional tectonics that played a pivotal role in the formation of the sedimentary basins in the region (Fairhead, 2023).Consequently, these regions hold significant importance in the global comprehension of extensional events that led to the creation of a series of rifted basins in Africa.
The Trans-Saharan mobile belt in Northwest Africa encompasses the Tuareg and Benino-Nigerian Shields, the Gourma and Dahomeyides belts in Mali, Togo, and Benin (Caxito et al., 2020).These extensive regions within the Pan-African mobile belt are dominated by Early Cretaceous rifted sedimentary basins, which were formed due to extensional events.The Middle Niger Basin stands as one of the most notable basins within the Trans-Saharan mobile belt, offering a structural setting and evolutionary history that can be directly compared to other basins in Africa.For instance, the separation of Africa from Northeast Brazil during the Cretaceous rifting event resulted in the subsidence and formation of the Benue Trough in Nigeria, characterized by NE-SW trending faults along its margin (Benkhelil, 1989;Benkhelil et al., 1998).Comparatively, the Benue Trough shows clear structural similarities with the Middle Niger Basin (Kogbe et al., 1983), as both are fault-bounded sedimentary basins.While there is a consensus on basin boundary fault geometry, there remains discussion regarding extension orientation and the significance of strike-slip faulting in the Benue Trough.This switch in orientation style between the Middle Niger Basin and the Benue Trough is attributed to a proposed change in the orientation of basin geometry resulting from the opening of the southern Atlantic Ocean.Notably, the reactivation of earlier-formed Pan-African shear zones (manifested as the NE-trending Chain and Charcot oceanic fracture zones) appears to have directly controlled the location of the failed Benue rift, achieved through dextral transtensive conditions (Fairhead, 2023).The magnetic data (HGM map; Figure 5) provides exceptions to the rule that established the Benue Trough (wrench tectonics), revealing that secondary basement fractures may have contributed to the formation of the Middle Niger Basin.
The formation of the Middle Niger Basin is attributed to the extensional regime that followed the Cretaceous rifting episode and responsible for the creation of the Benue Trough (Benkhelil, 1989;Obaje, 2009).It is noteworthy that the Benue Trough and Middle Niger Basin share structural trends along their margins that align with the orientation of these basins.The NE-SW trending Charcot and Chain oceanic faults, which extended into continental fault zones within the Benue Trough suggest significant sinistral strike-slip motion within the trough, converting horizontal displacement into an extensional sedimentary basin (Fairhead, 2023).Furthermore, the Trans-Saharan Mobile Belt extends approximately from south to north, from the Nigerian Atlantic coast to Algeria.The northern segment of the NE-SW trending Benue Trough transitions into a major NW-SE oriented extensional sedimentary basin known as the Termit Basin in Niger Republic.
The Termit Basin primarily shows extensional characteristics but also features a sinistral transtensional tectonic regime and NE-trending transpressional anticlines (Fairhead, 2023).Both the Termit Basin and Middle Niger Basin share similarities in fault dimension and basin orientation.The NW-SE trending Middle Niger Basin forms the southern extension of the Benue Trough, running parallel to the NW-SE oriented Termit Basin, the northern extension of the trough.This parallelism implies that both the Termit Basin (approximately 300 km in length) (Wang et al., 2022) and the Middle Niger Basin (roughly 350 km in length) (Rahaman et al., 2019) share a common origin.Both sedimentary basins exhibit NW-SE fractures at their margins, aligned with their orientations.Proterozoic basement fractures are believed to have influenced the formation of the Termit Basin during the Cretaceous sedimentary basin formation (Wan et al., 2014), a proposition that can also be applied to the Middle Niger Basin based on magnetic data.The reactivation of these basement structural trends during the Cretaceous opening of the Atlantic Ocean and regional NE-SW extension.Provides a suitable explanation for the formation of the Middle Niger Basin (Kogbe et al., 1983).Igneous intrusions in the center of the Middle Niger Basin, as seen in Figure 11, exhibit a predominantly NW-SE orientation.The emplacement of plutonic rocks in the basement of the Middle Niger Basin is geometrically controlled by NW-striking fractures, which are also observed at the edges of the basin.Similarly, the Ténéré Basin, representing the northwestern continuation of the Termit Basin, was formed by the reactivation of preexisting faults during the extensional episode associated with the Cretaceous opening of the South Atlantic (Ball, 1980).These faults also govern the emplacement of magmatic intrusions in the Ténéré Basin as well as in the Middle Niger Basin, suggesting that these rifted sedimentary basins in the Trans-Saharan mobile belt share similar features.These sedimentary basins in the Trans-Saharan mobile belt can be linked to the multiphase breakup of the Gondwana supercontinent during the Phanerozoic, a process generally associated with extensional tectonics (Bumby & Guiraud, 2005).This extensional tectonic framework is also associated with the development of rifted sedimentary basins outside the Trans-Saharan and generally within Africa (Bumby & Guiraud, 2005).
Beyond the Trans-Saharan mobile belt, in central and east Africa, Mesozoic rifting resulted in the formation of a discontinuous sequence of sedimentary basins characterized by NW-trending normal faulting in northern Kenya and southern Sudan (e.g., Ebinger & Ibrahim, 1994;Reeves et al., 1987).Most of the Phanerozoic tectonism and magmatism within the African plate, arising from the multi-phase breakup of the Gondwana supercontinent, is believed to have occurred within large-scale fracture zones representing the reactivation and exploitation of Neoproterozoic Pan-African sutures (Bumby & Guiraud, 2005).Rifting associated with the Gondwana supercontinent's breakup began in the Late Carboniferous with the formation of the Karoo sedimentary basins in southern Africa and the gradual opening of the Neotethys (Bumby & Guiraud, 2005).
Although, the breakup of the supercontinent, leading to the separation of West and East Gondwana, initiated in the Early-Middle Jurassic and resulted in the opening of the central Atlantic Ocean (Bumby & Guiraud, 2005), the South Atlantic progressively opened northwards from the Late Jurassic to Early Cretaceous.Intraplate extension gave rise to the West, Central, and East African rift systems, signifying an aborted breakup within the African plate itself (Guiraud & Maurin, 1992).The Middle Niger Basin serves as a notable example of an extensional basin within the West African rift system.Rifting of the East African margin continued until the present day, marking the final stage of Gondwana rifting that developed between the Late Eocene and Early Miocene, leading to the formation of sedimentary basins such as the Red Sea, Dead Sea, and Gulf of Aden (Bumby & Guiraud, 2005).The Middle Niger Basin share common characteristics with the West Africa Rift, including rift basins in Nigeria, western Chad, and Niger, as well as the Central Africa Rift, housing rift sedimentary basins and strikeslip shear faults in Cameroon, southern Chad, and the Central African Republic.In addition, orthogonal extensional basins exist in Western Sudan, South Sudan, and Kenya.These sedimentary basins developed under dextral transtensive conditions associated with the opening of the Atlantic Ocean and are distinguished by the faults along their edges like the Middle Niger Basin.
It is important to recognize that faults are prominent features of extended continental crust and play a crucial role in accommodating deformation through the reactivation of pre-existing faults (Magee et al., 2014).These geological structures significantly influence the long-term, large-scale structural evolution of rifted sedimentary basins.The strike-slip faults generated during the Pan-African orogenic events in the Trans-Saharan and Oubanguides Belts are believed to have directly influenced the locations of the West and Central African rift systems (Bumby & Guiraud, 2005).Therefore, a transtensional or pull-apart origin for the Middle Niger Basin cannot be ruled out due to the direction of the aeromagnetic data acquisition, which aligns with the orientation of the basin and may not accurately delineate NW-SE trending shear zones in the region.This suggests that in addition to the delineated NW-SE trending faults at the margin of the Middle Niger Basin, other potential NW-SE trending strike-slip fractures at the center of the basin may exist, not yet identified, which may have taken up the extensional movement of the Middle Niger Basin rift system through lateral shear.

Conclusions
The magnetic and gravity anomaly maps of the Middle Niger Basin indicate the presence of low frequency anomalies in the region.These anomalies are typically massive, elliptical or circular in shape and were created by fault-related intrusions.Magnetic models were generated along two selected profiles spanning these anomalies using the 2-D forward modeling approach.The results show that, despite differences in basement topography, depths to the top of intrusions in the basement along the magnetic profiles are typically between 2 and 4 km.The presence of intrusions in the basement of the Middle Niger Basin has been confirmed through joint magnetic and gravity modeling.The findings indicate that the magnetic and gravity anomalies in the Middle Niger Basin are likely caused by rocks with high magnetic susceptibility or intra-basement intrusive rocks.The inverted gravity data supports the results of the magnetic 2-D models and provides density values for the inferred intrusions, suggesting that the intrusive bodies or rocks may be basic igneous rocks.The presence of deeper fractures within the basement below the Middle Niger Basin would imply the existence of intrusions in the basement.This could possibly signify a rifted formation for the basin.

Figure 1 .
Figure 1.Simplified geological map of Nigeria and its environs showing the study area, which is indicated with square box in west-central Nigeria (Modified after Thieblemont, 2016).The map shows the sedimentary basins and volcanic provinces of the basement complex terrain of Nigeria.KZI-Kalangai-Zungeru-Ifewara sheared zone.

Figure 2 .
Figure 2. (a) Generalized geologic map of the study area in central western half of Nigeria (modified from Geological Map of Nigeria, 2020).(b) Mesozoic tectonic map of Northern Africa that shows the Kandi Shear Zone, sedimentary basins that are indicated with green color and the fractures of the West and Central Africa Rift System.The white color arrows show the direction of crustal extension, and the black arrows show the strike-slip directions.The yellow color shows the Afar and Cenozoic East Africa Rift System (Modified from Fairhead, 2023).

Figure 3 .
Figure 3. (a) Digital elevation model map of the study area showing the elevation variation of the Middle Niger Basin.The boundaries of the Sokoto, Anambra and Middle Niger Basins are redrawn from (Geological Map of Nigeria, 2020).(b) The multidirectional shaded-relief map of the study area produced from the SRTM-DEM of the region in panel (a).

Figure 4 .
Figure 4. (a) Aeromagnetic anomaly map of the study area showing Sokoto and entire Middle Niger Basins and its surrounding basement complex terrane.At lowlatitude regions such as the studied area, geologic magnetic sources are usually characterized by magnetic low anomalies.The continuous thick black lines represent magnetic profiles.The A-Aʹ, B-Bʹ and C-Cʹ represents the profile lines used for crustal modeling of the study area.(b) Reduced-to-pole aeromagnetic anomaly map of the Sokoto and entire Middle Niger Basins and its surrounding basement complex terrane.

Figure 5 .
Figure 5.The color shaded-relief horizontal gradient magnitude map produced from the reduced-to-pole aeromagnetic anomaly map in Figure 7.

Figure 6 .
Figure 6.Bouguer gravity anomaly map of the Middle Niger Basin and surrounding basement complex terrane derived from EIGEN-6C4 data.

Figure 7 .
Figure 7.The 2-D forward magnetic model for profile A-Aʹ established from the total magnetic intensity anomaly map in Figure 4.

Figure 8 .
Figure 8.The magnetic and gravimetric 2-D forward model for profile B-Bʹ established from the gravity map and the total magnetic intensity anomaly map.

Figure 9 .
Figure 9.The 2-D forward magnetic model for profile C-Cʹ established from the total magnetic intensity anomaly map in Figure 4.

Figure 10 .
Figure 10.Depth slice and cross sections from the inversion model of the gravity data of the study area.The depth slice (plan view map) is at 10 km below the surface, the profile A-Aʹ represent the E-W cross section and the profile B-Bʹ represent the N-S cross section through the inversion model produced from the gravity data.The outline of sedimentary basins is shown in green color.

Figure 11 .
Figure 11.The 3-D view of the inverted gravity data of the study area, illustrating the magmatic intrusions beneath the Middle Niger Basin.

Figure 12 .
Figure 12. (a-c) The evolution of the Middle Niger Basin has been characterized by linear volcanism associated with the basement of the basin directly producing magnetic and gravity anomalies.(d) Evolutionary model for the crustal development of the volcanic system of the Middle Niger Basin Rift.Development of the Middle Niger Basin and volcanic activity mainly within the basement of the basin.After magmas were produced down in the upper mantle, during adiabatic ascent the volcanics were stored at the lower crust before reaching the basement of the Middle Niger Basin.Diagram not to scale.

Table 1
Stanton et al. (2021)ty Values for Each Layer in the 2-D Forward Aeromagnetic Models Adopted FromStanton et al. (2021)