Crustal Structure of Nigeria and Southern Ghana, West Africa from P-Wave Receiver Functions Tectonophysics

TECTO-126947; No of Pages 11 Tectonophysics xxx (2016) xxx–xxx

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Crustal structure of Nigeria and Southern Ghana, West Africa from P-wave receiver functions

Ofonime Akpan a,b,⁎, Andrew Nyblade c,d, Chiedu Okereke b, Michael Oden b, Erica Emry c, Jordi Julià e a Centre for Geodesy and Geodynamics, Toro, Nigeria b Department of Geology, University of Calabar, Calabar, Nigeria c Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USA d School of Geosciences, The University of the Witwatersrand, Johannesburg, South Africa e Departamento de Geofísica & Programa de Pós-Graduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grando do Norte, Natal, Rio Grande do Norte, Brazil article info abstract

Article history: We report new estimates of crustal thickness (Moho depth), Poisson's ratio and shear-wave velocities for eleven Received 20 November 2015 broadband seismological stations in Nigeria and Ghana. Data used for this study came from teleseismic earth- Received in revised form 1 February 2016 quakes recorded at epicentral distances between 30° and 95° and with moment magnitudes greater than or Accepted 2 February 2016 equal to 5.5. P-wave receiver functions were modeled using the Moho Ps arrival times, H–k stacking, and joint Available online xxxx inversion of receiver functions and Rayleigh wave group velocities. The average crustal thickness of the stations Keywords: in the Neoproterozoic basement complex of Nigeria is 36 km, and 23 km for the stations in the Cretaceous Benue Nigeria Trough. The crustal structure of the Paleoproterozoic Birimian Terrain, and Neoproterozoic Dahomeyan Terrain Ghana and Togo Structural Unit in southern Ghana is similar, with an average Moho depth of 44 km. Poisson's ratios Neoproterozoic for all the stations range from 0.24 to 0.26, indicating a bulk felsic to intermediate crustal composition. The crustal Paleoproterozoic structure of the basement complex in Nigeria is similar to the average crustal structure of Neoproterozoic terrains Crustal structure in other parts of Africa, but the two Neoproterozoic terrains in southern Ghana have a thicker crust with a thick Suture zones maﬁc lower crust, ranging in thickness from 12 to 17 km. Both the thicker crust and thick maﬁc lower crustal section are consistent with many Precambrian suture zones, and thus we suggest that both features are relict from the collisional event during the formation of Gondwana. © 2016 Elsevier B.V. All rights reserved.

1. Introduction 1981), as well as continental and global models of crustal structure (Mooney et al., 1998; Tedla et al., 2011; Tugume et al., 2013; Laske In this paper we report the ﬁrst seismological estimates of crustal et al., 2013). structure in Nigeria and Ghana using broadband data from the Nigeria Data from eleven broadband seismic stations have been used to ob- and Ghana national seismic networks. In spite of the prominent role tain new point estimates of crustal thickness, Vp/Vs ratios, and crustal Nigeria and Ghana play in supplying the world with petroleum and shear-wave velocities in two different tectonic regions of Nigeria, the other natural resources, very little is known about crustal structure Precambrian basement complex and the Cretaceous Benue Trough, within these countries as it relates to the geologic development of key and three different tectonic regions in Ghana, the Birimian and Daho- tectonic features, such as the Cretaceous Benue Trough and the West meyan terrains, and the Togo Structural Unit. The estimates come African passive margin. The only published estimates of Moho depths from Moho Ps arrival times in P-wave receiver functions (PRFs) in Nigeria come from regional gravity studies (e.g., Fairhead and (Zandt et al., 1995), applying the H–k stacking method of Zhu and Okereke, 1987, 1988; Okereke, 1988; Fairhead et al., 1991) or continen- Kanamori (2000) to PRFs, and a joint inversion of PRFs with Rayleigh tal (Tugume et al., 2013) and global (Mooney et al., 1998; Bassin et al., wave group velocities (Julià et al., 2000, 2003). The new estimates are 2000; Laske et al., 2013) models of crustal structure. Ghana has experi- used to examine crustal structure in Nigeria and Ghana by comparing enced historically large earthquakes, yet information on the crustal them with the structure of similar age crust in other parts of Africa, structure within the country is generally lacking. Previous information and with Moho depth estimates from previously published studies in about the Moho depths in Ghana has come from regional gravity studies Nigeria and Ghana. (Ako and Wellman, 1985), seismological studies (Bacon and Quaah, 2. Background

⁎ Corresponding author at: Centre for Geodesy and Geodynamics, Toro, Nigeria. Tel.: +234 8036141300. Nigeria consists of three major tectonic units, the Neoproterozoic E-mail address: [email protected] (O. Akpan). basement complex, the Jurassic Younger Granites complex, and the

Cretaceous to Recent sedimentary successions comprised of the Niger separation of South America and Africa during the opening of the Delta, the Benue Trough, and the Borno, Dahomey, Bida and Sokoto South Atlantic Ocean in the Early Cretaceous (Ofoegbu and Okereke, basins (Obaje, 2009; Fig. 1). The seismological stations used in this 1990; Binks and Fairhead, 1992; Guiraud and Maurin, 1992). After the study are located in the basement complex and Benue Trough. Santonian tectonic and magmatic events, the major depositional axis The basement complex is a component of the West African Pan- in the Lower Benue Trough was shifted to the northwest, leading to African mobile belt (Black, 1980; Wright et al., 1985; Ajibade and the formation of the Anambra Basin (Wright et al., 1985). Therefore, Fitches, 1988; Ekwueme, 1990). In addition to the Pan-African the Anambra Basin is regarded as a part of the Lower Benue Trough con- tectonothermal event (Burke and Dewey, 1972; Ajibade and Fitches, taining post-deformational Campanian to Eocene deposits (Obaje, 1988; Obaje, 2009), several older orogenies are recorded in the base- 2009). Several magmatic events affected the Benue Trough (Agagu ment complex, including the Liberian (2700 ± 200 Ma), Eburnian and Adighije, 1983), most prominently the ones during the late Albian (2200 ± 200 Ma) and Kibaran (1100 ± 100 Ma) orogenies (Ajibade and Turonian (Offodile, 1976). and Fitches, 1988; Obaje, 2009; Ogezi, 1988; Rahaman, 1988; Ajibade Moho depth estimates of 20 to 26 km beneath the Benue Trough et al., 1988; Dada, 1998). The basement complex, which was later in- and its adjoining rifts in Nigeria are reported in regional gravity studies truded by the Younger Granites, is present throughout the country, un- (e.g. Fairhead and Okereke, 1987, 1988; Okereke, 1988; Fairhead et al., derlying the sedimentary basins listed earlier (Avbovbo, 1980; Obaje, 1991). Within the rifted parts of the Benue Trough in Cameroon i.e. 2009). The rocks commonly found in the basement complex range in the Garoua rift, Moho depth estimates of 23 to 28 come from regional metamorphic grade and include migmatites, gneisses, schists, quartz- gravity studies (e.g. Poudjom Djomani et al., 1995; Nnange et al., ites, granulites, amphibolites, phyllites, marbles, and igneous rocks 2000; Kamguia et al., 2005) and seismological studies (e.g. Stuart such as calc-silicates, granites, syenites, granodiorites, adamellite, et al., 1985; Tokam et al., 2010). Continental and global models of crustal quartz monzonites and charnockites (Rahaman, 1988). structure show Moho depths of 25 to 42 km on average beneath the The Benue Trough is oriented in a NE–SW direction and is a compo- Cretaceous Benue Trough and Precambrian basement complex, respec- nent of the West and Central African Rift System with a length and tively (e.g. Mooney et al., 1998; Bassin et al., 2000; Tugume et al., 2013; width of about 800 km and 150 km, respectively (Benkhelil, 1989; Laske et al., 2013). Ofoegbu and Okereke, 1990; Obaje, 2009)(Fig. 1). It developed as a The geologic framework of Ghana consists of ﬁve major tectonic failed arm of the RRR Triple junction (aulacogen) following the units, (1) the Paleoproterozoic Complex, including the Birimian and

Fig. 1. Geological map of Nigeria showing the major tectonic features, seismological stations, Moho depths and Poisson's ratios (the ﬁrst and second numbers close to each station).

Tarkwaian terrains, (2) the Dahomeyan Terrain, (3) the Togo and 3. Data and method of study Buem Structural Units, (4) the Voltaian Basin and (5) other sedimentary basins of Paleozoic to Recent age. The seismological stations 3.1. Data used for this study are located in the Birimian Terrain, the Dahomey- an Terrain and the Togo Structural Unit in the southern part of the Eleven seismological stations in Nigeria and Ghana were used in this country (Fig. 2). The Birimian Terrain was last deformed during the study. The stations belonging to the Nigeria National Seismic Network Eburnean orogeny (Barritt and Kuma, 1998) and consists of phyllites, are IFE, TOR, KAD, NSU and AWK, and those in the Ghana National Seis- volcaniclastics, chemical sedimentary rocks, wackes and granitoids. mic Network are KUKU, MRON, SHAI, AKOS, KLEF and WEIJ (Figs. 1 and The Dahomeyide Belt (consisting of the Dahomeyan Terrain, and 2, Table 1). The IFE, KAD and NSU stations were installed in August, 2008 Togo and Buem Structural Units) forms the eastern border of the while TOR and AWK stations were installed in November, 2010. The sta- West African Craton (Jones, 1990)andrepresentsthesuturebe- tions in Nigeria are equipped with the Eentec DR-4000 24 bit three- tween the Birimia and Dahomeya blocks during the Pan-African channel data acquisition system, three-component seismometers orogeny (Burke and Dewey, 1972; Bacon and Quaah, 1981; Quaah, (Eentec EP-105 or Eentec SP-400) and Global Positioning System 1982). The rocks of the Dahomeyan Terrain consist mainly of (GPS) clocks. Each of them is powered by three solar panels (80 W gneisses, granulites and schists (Wright et al., 1985). The Togo Struc- each) connected to a 200 Ah battery. The stations in the Ghana National tural Unit is made up of supra-crustal sediments that were deformed Seismic Network, which are equipped with Nanometrics Trident 305 by the northwesterly directed thrusting of Dahomeyan basement data loggers, and either Trillium Compact or Trillium 120P seismome- rocks onto the West African Craton (Wright et al., 1985). The major ters, were installed in October 2012. Two AfricaArray stations in rocks are shales, quartz sandstones, schists and strongly folded Ghana (KUKU and SHAI), which are equipped with Reftek RT130 data and deformed quartzites and phyllites (Bacon and Quaah, 1981; loggers, and either a Guralp CMG 3 T or 40 T seismometer, were Amponsah et al., 2009). installed in July 2009. Two of the new stations belonging to the national Regional gravity and seismological studies report Moho depths of 38 network were collocated with the AfricaArray stations. Data at all sta- to 42 km for southern Ghana (Bacon and Quaah, 1981; Ako and tions were recorded at 40 samples per second. Station information is Wellman, 1985), and crustal thickness estimates of 30 to 45 km are re- provided in Table 1. ported in continental and global models of crustal structure (Mooney The data used for this study came from teleseismic earthquakes with et al., 1998; Tugume et al., 2012; Laske et al., 2013). moment magnitudes greater than or equal to 5.5 that occurred between

Fig. 2. Geological map of southern Ghana showing the major tectonic features, seismological stations, Moho depths and Poisson's ratios (the ﬁrst and second numbers close to each station).

Table 1 Locations of seismological stations in Nigeria and Ghana.

Network Name of Station Latitude Longitude Elevation Geology Tectonic region Age Instrumentation code station code (degree) (degree) (m)

NG Ile-Ife IFE 7.5334 4.5336 289 Gray gneiss Migmatite Gneiss Complex Archean Seismograph: DR-4000 Seismometer: EP-105 NG Kaduna KAD 10.4334 7.6334 668 Granite Older Granite Complex Neoproterozoic Seismograph: DR-4000 Seismometer: EP-105 NG Toro TOR 10.1168 9.1167 882 Granite Older Granite Complex Neoproterozoic Seismograph: DR-4000 Seismometer: EP-105 NG Nsukka NSU 6.8667 7.4167 430 Sandstone Benue Trough Cretaceous Seismograph: DR-4000 Seismometer: EP-105 NG Awka AWK 6.2335 7.1001 50 Shale Benue Trough Paleocene Seismograph: DR-4000 Seismometer: SP-400 GH/AF Kukurantumi KUKU 6.1924 −0.3687 240 Granite Birimian Terrain Paleoproterozoic Seismograph: Trident 305 (GH) Seismometer: Trillium 120P (GH) Seismograph: Reftek RT 130 (AF) Seismometer: Guralp CMG 3 T (AF) GH Lake Bosomtwe MRON 6.4647 −1.4371 361 Phyllite Birimian Terrain Paleoproterozoic Seismograph: Trident 305 Seismometer: Trillium Compact GH/AF Shai Hills SHAI 5.9371 0.0627 107 Granite gneiss Dahomeyan Terrain Neoproterozoic Seismograph: Trident 305 (GH) Seismometer: Trillium 120P (GH) Seismograph: Reftek RT 130 (AF) Seismometer: Guralp CMG 40 T (AF) GH Akosombo AKOS 6.2984 0.0681 217 Quartzite Togo Structural Unit Neoproterozoic Seismograph: Trident 305 Seismometer: Trillium Compact GH Ho KLEF 6.6142 0.4407 313 Quartzite Togo Structural Unit Neoproterozoic Seismograph: Trident 305 Seismometer: Trillium 120P GH Weija WEIJ 5.5885 −0.3333 203 Quartzite Togo Structural Unit Neoproterozoic Seismograph: Trident 305 Seismometer: Trillium Compact

June 2009 and April 2014 at epicentral distances between 30° and 95° Ammon, 1999). In the second approach, events in which the tangential from the stations. A list of events used is given in the Supplementary receiver functions exhibited large amplitudes relative to the radial re- material, and the azimuthal coverage provided by them is illustrated ceiver functions were not selected, even if they met the first criteria. in Figs. 3 and 4. Ammon et al. (1990) pointed out that transverse receiver functions should be zero for isotropic, laterally homogenous media. The quality 3.2. Receiver functions of the receiver functions for all the stations computed using a Gaussian width of 1.0 is shown in the Supplementary material. Receiver functions are time series that contain P-to-S conversions generated when an incident P-wave interacts with a seismic interface 3.3. Modeling of receiver functions beneath a seismological station (Fig. 5). Receiver functions are computed by deconvolving the vertical component waveform from the radial In this section, we briefly describe the three methods used to model and tangential components, thereby removing the effects of the source the data. time function and instrument response. The radial receiver function is made up of P-to-S converted waves from the Moho (and other seismic 3.3.1. Forward modeling of Moho Ps arrival times discontinuities beneath a station), and the multiples from those conver- In this method, the equation of Zandt et al. (1995), which uses the sions with the free surface (Fig. 6)(Langston, 1979). travel time difference between the Ps and P arrivals, was used to esti- In this study, the seismograms selected for receiver function analysis mate crustal thickness. The equation is as follows: were first windowed between 10 s before and 110 s after the first P arrival. The data were then detrended, tapered and bandpass ltered be- tPs tp fi H À 1 tween 0.05 and 8 Hz to remove low and high frequency noise, and ¼ 2 2 2 2 ð Þ V sÀ p V pÀ p were thereafter decimated to 10 samples per second. The horizontal À À À rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffirffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi components of the seismograms were then rotated into the great circle path to obtain the corresponding radial and tangential components. where H is the crustal thickness, tPs is the Ps arrival time, tp is the direct The receiver functions were computed using the iterative, time- P-wave arrival time, Vp is the velocity of the P-wave, Vs is the velocity of domain deconvolution technique (Ligorría and Ammon, 1999) using S-wave and p is the ray parameter of the incident P-wave. In using 500 iterations. Radial and tangential receiver functions were computed this technique, the following values were assumed in the computation: for each teleseismic event at two different Gaussian width factors of 1.0 Vp/Vs = 1.75, Vp = 6.5 km/s and Vs = 3.714 km/s (Zandt et al., 1995). (f ≤ 0.5 Hz) and 2.5 (f ≤ 1.25 Hz). Modeling high- and low-frequency Radial receiver functions using a Gaussian width factor of 1.0 were receiver functions simultaneously helps discriminate sharp from grada- used for this computation. The arrival times of the Ps waves (tPs ) rela- tional velocity changes in the resulting velocity–depth profiles (Julià, tive to the direct P-wave (tp) (see Fig. 6) were hand-picked and used 2007). in calculating the Moho depth for each individual waveform. The differ- The quality of the receiver functions was checked using two inde- ent values of H obtained were then averaged to derive an estimate of the pendent approaches. The first approach involved a least squares misfit Moho depth beneath each station. criterion, which consisted of calculating the difference between the An advantage of this method is that since the Ps conversion point original radial waveforms and an estimated radial waveform from the normally lies close to the seismological station (the waves sample convolution of the corresponding vertical waveform with the already only ~10 km from the station), it is less affected by lateral variations computed radial receiver function. Receiver functions that were recov- in velocity and thus provides a good estimate of the crustal thickness ered at 85% and above were selected for further analysis (Ligorría and (Zandt et al., 1995). There is however a problem in the trade-off

20o

40o

60o

80o

100o

Fig. 3. Map showing the distribution of earthquakes recorded at seismological stations in Nigeria used for this study. The black triangle represents the center of the seismic network, the red circles show the earthquake epicenters while the large black circles show the epicentral distance in 20° increment from the center of the network. Map was plotted using the Generic Map- ping Tool (Vessel and Smith, 1998). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

between the Moho depth and crustal velocities (Zhu and Kanamori, the amplitude of the radial receiver function; t1, t2 and t3 are the 2000). Since tPs is the difference in the travel time of the S-wave with re- arrival times of the phases and N is the number of receiver functions spect to the P-wave, the value of H depends to a large extent on the Vp/Vs used. ratio (Zhu and Kanamori, 2000). Radial receiver functions with Gaussian width factors of 1.0 were The uncertainties in H were obtained from the range of the different used in this method. The a priori parameters used in the H–k stacking values of H obtained from each waveform. The uncertainty in Moho were the weights and average crustal P-wave velocity (Vp). Because all depth estimates obtained using this method is ~3–4 km. the phases were clearly identiﬁed in the receiver functions, weights

w1 = 0.4, w2 = 0.3 and w3 = 0.3 were used to give similar importance 3.3.2. H–k stacking to each phase. A P-wave velocity (Vp) of 6.5 km/s was used, as this rep- The H–k stacking method developed by Zhu and Kanamori (2000) resents a reasonable average value of Vp for Precambrian crust was used to estimate crustal thickness (H) and Vp/Vs ratio (k) beneath (Christensen and Mooney, 1995). each station using receiver functions that have clear Ps waves and one The bootstrapping method of Efron and Tibshirani (1991) was used or more multiple phases (PpPs, PpSs + PsPs, PsSs) [Fig. 6]. The tech- to estimate uncertainties in H and k. This involved re-sampling the re- nique transforms the receiver functions from the time-amplitude ceiver function datasets with replacement 500 times for each station, domain into the H–k stacking domain through applying the H–k stacking procedure to the re-sampled dataset, and computing the average and standard deviation from the resulting 500

estimates. To determine the errors in H (Moho depth) and k (Vp/Vs N ratio) arising from the assumed average value for crustal Vp, the H–k s H; k ∑ w1r j t1 w2r j t2 w3r j t3 2 ðÞ¼ j 1 ðÞþ ðÞÀ ðÞ ð Þ stacks were re-calculated using Vp values between 6.3 km/s and ¼ 6.8 km/s. The uncertainties estimated from the bootstrap method were then combined with those obtained from different average crustal where w1, w2 and w3 are the weights assigned to the Ps, PpPs and Vp values, giving an overall uncertainty in H of ~2–3 km and in k of PpSs phases, respectively, the sum of the weights being unity; rj is ±0.05.

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40o

60o

80o

100o

Fig. 4. Map showing the distribution of earthquakes recorded at seismological stations in Ghana used for this study. The black triangle represents the center of the seismic network, the red circles show the earthquake epicenters while the large black circles show the epicentral distance in 20° increment from the center of the network. Map was plotted using the Generic Map- ping Tool (Vessel and Smith, 1998). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.)

Fig. 5. A simpliﬁed ray diagram showing the ray paths of the major P-to-S converted phase and associated multiples that comprise a radial receiver function for a single layer over a half-space (redrawn from Ammon et al., 1990). Ps is the Moho converted phase, and PpPs, PpSs, PsPs and PsSs are the reﬂections from the Moho and the Earth's surface.

Fig. 6. Receiver function waveform (redrawn from Ammon et al., 1990).

3.3.3. Joint inversion smoothing parameter with values of 0.2–0.4 to obtain smooth depth– The joint inversion technique developed by Julià et al. (2000, 2003) velocity profiles that provided a good fit to the data. was used to model shear-wave velocities in the crust and upper mantle The starting model consisted of a 37.5 km thick crust with a linear beneath each station. This method involves jointly inverting the receiv- S-wave velocity increase with depth from 3.4 to 4.0 km/s, overlying a er functions and surface wave dispersion curves using an iterative, least- flattened PREM (Preliminary Reference Earth Model) (Dziewonski and squares algorithm with a roughness norm. The input for the joint Anderson, 1981). The model parameterization consisted of a stack of inversion consists of an initial model, the observed receiver functions, constant velocity layers that increased in thickness with depth. Thick- and the Rayleigh-wave group–velocity curves. nesses of 1 and 2 km were used for the first two layers of the model, Radial receiver functions computed at Gaussian width factors of 1.0 2.5 km for layers between 3 and 60.5 km depth, 5 km for layers between and 2.5 were used for the joint inversion. The selected receiver functions 60.5 and 260.5 km depth, and 10 km below a depth of 260.5 km. Note were grouped according to their respective back-azimuths and ray pa- that, although the starting model was parameterized down to transition rameters. Grouping the receiver functions by ray parameters helps to zone depths, only velocities above 260.5 km depth were inverted for. account for the phase move-out due to varying angles of incidence Velocities at larger depths were kept at PREM values in order to account (Cassidy, 1992; Gurrola and Minster, 1998). To get a shear-wave veloc- for partial sensitivity of long-period dispersion velocities to deep struc- ity model for each of the stations, each receiver function group was ture (Julià et al., 2003). Poisson's ratio was fixed to 0.25 for the crust and jointly inverted with the corresponding dispersion curve for each sta- to PREM values (0.28 to 0.30) for mantle layers during the inversion. tion. Rayleigh wave group velocities for periods between 10 and 100 s Densities (ρ) were derived from P-wave velocities (Vp) using the empir- used in this study were derived from the group velocity measurements ical relationship of Berteussen (1977) expressed as follows: of Raveloson et al. (2015). In the inversion, an influence factor that controls the relative contri- ρ 0:32Vp 0:77: 3 ¼ þ ð Þ butions of the receiver functions and the group–velocity curves must be set a priori, as well as a smoothness parameter that controls the trade- The uncertainties in the shear-wave velocity models were obtained off between fitting the data and model smoothness (Julià et al., 2003). using the method of Julià et al. (2005), which involved carrying out We used an influence factor of 0.5, giving equal weight to the receiver the inversion process repeatedly with different inversion parameters, functions and group velocities and we used a depth-dependent such as smoothing parameters and Poisson's ratios. The overall

Fig. 7. Result from H–k stacking for KUKU station. To the left of each receiver function, the top number represents the event back azimuth and the bottom number gives the epicentral distance of the events in degrees. Contours map out percentage values of the objective function (Equation 2) given in the text.

Please cite this article as: Akpan, O., et al., Crustal structure of Nigeria and Southern Ghana, West Africa from P-wave receiver functions, Tectonophysics (2016), http://dx.doi.org/10.1016/j.tecto.2016.02.005 8 O. Akpan et al. / Tectonophysics xxx (2016) xxx–xxx uncertainties in shear-wave velocities for each layer in the models are layer in the lower crust. A number of studies (e.g., Holbrook et al., 0.1–0.2 km/s, resulting in an uncertainty of ~2–3 km for the crustal 1992; Christensen and Mooney, 1995; Rudnick and Fountain, 1995; thickness estimates. and Rudnick and Gao, 2003) show that common lower crustal mafic lithologies, such as amphibolites, garnet-bearing and garnet-free 4. Results mafic granulite, and mafic gneiss, have higher shear-wave velocities (N3.9 km/s) while intermediate-to-felsic lithologies have lower shear- Figs. 7 and 8 show the examples of the H–k stacking and joint inver- wave velocities (b3.9 km/s). Therefore, we define the maficlowercrust sion modeling for one station (KUKU), and the results from all three as layers in the model with shear-wave velocities between 4.0 km/s methods are summarized in Tables 2 to 4, including uncertainties. H–k and 4.3 km/s (Table 4). stacking and joint inversion results for the other stations are provided For stations where more than one estimate of crustal thickness was in the Supplementary material. Results using the H–k stacking and obtained, the estimates are in good agreement, and an average of the es- joint inversion methods were obtained for 9 stations (Tables 3 and 4). timates is given in Table 5. The stations in the Benue Trough have Moho For the two stations in the Benue Trough (NSU and AWK), crustal thick- depths of 22 to 23 km compared to 33 to 40 km for the three stations in ness estimates were obtained only from the Moho Ps arrival times the Precambrian basement of Nigeria (Fig. 1). Crustal thickness beneath (method 1). the stations in Ghana is more uniform, ranging from 41 to 45 km (Fig. 2).

To estimate crustal thickness from the joint inversion results, the For stations with H–k stacking results, Vp/Vs ratios from 1.65 to 1.76 depth of the Moho was defined as the depth at which the shear-wave were obtained. In addition to the Precambrian crust in Ghana being velocity was equal to or exceeded 4.3 km/s. Studies by Christensen and somewhat thicker than in Nigeria, the crust in Ghana is also character- Mooney (1995) and Christensen (1996) have shown that shear-wave ized by a thick mafic lower crust. The thickness of lower crustal layers velocities for lower crustal lithologies derived from experimentally de- with shear-wave velocities between 4.0 and 4.3 km/s ranges from termined P-wave velocities and Vp/Vs ratios cannot exceed 4.3 km/s. Ad- 12 km at station SHAI to 17 km at stations MRON and WEIJ. In compar- ditionally, from the models, we obtained an average crustal shear-wave ison, the thickness of the mafic lower crust beneath the three stations in velocity beneath each seismic station and the thickness of the mafic the Nigerian basement complex is 3 to 5 km.

1.00 3, 62.0, 0.060 3, 115.1, 0.076 0 0.75

0.50

0.25

0.00 1.00 inverted model 3, 191.9, 0.057 2, 147.3, 0.060 starting model 0.75

0.50 0.25

0.00 12345 1.00 Vs (km/s) 3, 204.6, 0.052 12, 231.9, 0.049 0.75

0.50 5.0 0.25 4.5

0.00 4.0 1.00 3.5 7, 237.9, 0.053 5, 244.4, 0.055 0.75 3.0 0.50 2.5 0.25 2.0 0.00 0 25 50 75 100 125 1.00 4, 252.2, 0.051 0 10 20 30 40 0.75 synthetic 0.50 observed

0.25

0.00 1.00 0 10 20 30 40

Fig. 8. Results from joint inversion for station KUKU. The left panel shows the P-wave receiver functions (PRFs), top right, the shear-wave velocity model, and bottom right, the Rayleigh wave group velocity curve. The numbers at the top of each PRF panel give the number of PRFs contained in the stack, the average back azimuth, and the average ray parameter.

Table 2 Table 4 Estimates of crustal thickness using Moho Ps arrival times. Crustal structure from joint inversion.

Station Number of Minimum Ps Maximum Ps Crustal Station Average Crustal Uppermost Thickness of crustal layers

code receiver arrival time (s) arrival time (s) thickness code crustal Vs thickness mantle Vs having Vs ≥ 4.0 km/s (km) functions (km) (km/s) (km) (km/s)

IFE 24 4.0 4.5 35 ± 3.0 IFE 3.7 35 ± 2.0 4.6 3 ± 2.0 TOR 17 4.5 5.2 40 ± 4.0 TOR 3.7 40 ± 2.0 4.5 5 ± 2.0 KAD 10 3.8 4.0 32 ± 3.0 KAD 3.7 33 ± 2.0 4.5 3 ± 2.0 KUKU 30 4.7 5.8 44 ± 4.0 KUKU 3.9 45 ± 2.0 4.7 15 ± 2.0 MRON 20 4.5 5.4 40 ± 3.0 MRON 3.9 42 ± 2.0 4.7 17 ± 2.0 AKOS 15 5.0 6.0 46 ± 4.0 SHAI 3.8 42 ± 2.0 4.7 12 ± 2.0 SHAI 15 4.8 5.8 45 ± 3.0 AKOS 3.7 45 ± 2.0 4.6 15 ± 2.0 KLEF 9 5.0 5.5 43 ± 4.0 KLEF 3.8 42 ± 2.0 4.4 12 ± 2.0 WEIJ 15 4.5 5.5 42 ± 4.0 WEIJ 3.7 42 ± 2.0 4.7 17 ± 2.0 NSU 4 2.0 3.0 23 ± 4.0 AWK 5 2.0 2.7 22 ± 3.0

about 13 km based on the average basement complex crustal thickness 5. Discussion of 36 km.

In discussing the results, we ﬁrst examine crustal structure in Nigeria 5.2. Ghana and then in Ghana. For both countries, the variability of crustal structure (or lack thereof) is discussed and then compared to crustal structure for The average crustal thickness beneath the six stations in southern similar age crust in other parts of Africa. Ghana is 44 km. The crust under these stations have Poisson's ratio of

0.24 to 0.26 (Vp/Vs ratios of 1.65 to 1.76) indicating a bulk felsic to inter- 5.1. Nigeria mediate composition for the crust. The crust at all six stations is charac- terized by a thick mafic lower crust, with layers that have shear-wave The average crustal thickness of the stations in the basement com- velocities above 4.0 km/s ranging in thickness from 12 to 17 km. The plex is 36 km, and the crust under these stations has a Poisson's crustal structure of the Birimian Terrain (stations KUKU and MRON) is ratio of 0.26 (Vp/Vs ratios of 1.75 to 1.76) indicating a bulk felsic to similar to the average crustal structure of other Paleoproterozoic intermediate composition for the crust. The crustal structure of the terrains in Africa. The range of average crustal thickness for the Okwa basement complex is similar to the average crustal structure of other Terrain, Kheis Province, Ubendian Belt, Usagaran Belt, and Rehoboth Neoproterozoic terrains in Africa. The range of average crustal thickness Province in eastern and southern Africa is 38 to 44 km, the average for the Oubanguides Belt, Zambezi Belt, Damara Belt, Mozambique Belt, crustal Poisson's ratios range from 0.25 to 0.27, and the thickness of and Lufilian Arc is 35 to 43 km, the average crustal Poisson's ratios range the mafic lower crust ranges from 2 to 13 km (Kgaswane et al., 2009; from 0.23 to 0.28, and the thickness of the mafic lower crust ranges from Tugume et al., 2012, 2013; Kachingwe et al., 2015). Thus, there appears 2 to 7 km (Tokam et al., 2010; Tugume et al., 2013; Kachingwe et al., to be no major differences in crustal structure of the Birimian Terrain 2015). Thus, there appears to be no major differences in crustal struc- compared to many other Paleoproterozoic terrains in Africa. ture of the basement complex compared to many other Neoproterozoic In contrast, there appears to be a substantial difference in the crustal terrains in Africa. structure of the Neoproterozoic Dahomeyan Terrain and Togo Structural The average Moho depth for stations in the Benue Trough is 23 km, Unit compared to the average crustal structure of other Neoproterozoic which is in good agreement with previous estimates of crustal thickness terrains in Africa reviewed above. The Neoproterozoic crust in Ghana for the Benue Trough of 20 to 26 km using gravity data (Fairhead and appears to be thicker by several kms and its mafic lower crustal section Okereke, 1987, 1988; Okereke, 1988; Fairhead et al., 1991), as well as is about 10 km thicker than that found in many other African 24 km for the Garoua rift, a northeasterly extension of the Benue Trough Neoproterozoic terrains, including the basement complex in Nigeria. into Cameroon (Kamguia et al., 2005). The Moho depth beneath the As summarized above, the Dahomeyan Terrain and Togo Structural Garoua rift derived from passive seismology is 26 km (Tokam et al., Unit represent the suture between the Birimia and Dahomeya blocks 2010) and 23 km from seismic refraction profiling (Stuart et al., 1985). during the Pan-African orogeny (Burke and Dewey, 1972; Quaah, This is in contrast to the Cenozoic East African rift, where crustal thick- 1982). Both the thickened crust and the thick mafic lower crustal layer ness beneath many parts of the rift system is ~30–35 km (e.g., Fuchs may be relict features from this collisional event during the formation et al., 1997; Julià et al., 2005; Dugda et al., 2005; and references therein). of Gondwana. In Precambrian sutures elsewhere, such as found along As the Benue Trough is underlain by Neoproterozoic basement complex the margins of the Kaapvaal Craton (Kgaswane et al., 2009), the crust (Avbovbo, 1980), it appears that the crust has been thinned by Superior Province (Gibb et al., 1983), the Tanzania Craton (Nyblade

Table 3

Estimates of Moho depth and Vp/Vs ratio from H–k stacking method.

Station code H(1) [km] k1 H(2) [km] k2 H(3) [km] k3

IFE 35.2 ± 0.8 1.76 ± 0.03 33.9 ± 0.8 1.77 ± 0.03 37.0 ± 0.9 1.75 ± 0.03 TOR 39.1 ± 0.9 1.76 ± 0.04 37.7 ± 0.8 1.76 ± 0.04 41.0 ± 1.0 1.75 ± 0.04 KAD 32.1 ± 0.9 1.75 ± 0.05 31.0 ± 1.0 1.75 ± 0.05 33.9 ± 0.9 1.74 ± 0.04 KUKU 45.4 ± 0.4 1.68 ± 0.02 43.8 ± 0.4 1.68 ± 0.02 47.8 ± 0.4 1.67 ± 0.02 MRON 46.6 ± 2.5 1.65 ± 0.06 45.0 ± 2.3 1.65 ± 0.05 49.1 ± 2.1 1.64 ± 0.04 SHAI 43.3 ± 1.6 1.73 ± 0.05 41.9 ± 0.9 1.73 ± 0.03 45.6 ± 4.6 1.72 ± 0.03 AKOS 44.6 ± 1.9 1.75 ± 0.06 43.1 ± 1.9 1.76 ± 0.06 47.1 ± 1.9 1.74 ± 0.06 KLEF 47.7 ± 3.1 1.70 ± 0.08 46.1 ± 2.9 1.70 ± 0.08 50.1 ± 2.7 1.69 ± 0.08 WEIJ 40.6 ± 2.3 1.76 ± 0.04 39.1 ± 3.1 1.77 ± 0.05 42.9 ± 1.1 1.75 ± 0.02

H(1,2,3) = Moho depth. k (1,2,3) = Vp/Vs ratio.

1, 2, 3 = Average Vp used for the H–k stacking method where 1 = 6.5 km/s, 2 = 6.3 km/s and 3 = 6.8 km/s.

Table 5 Summary of crustal structure.

Station code H(1) km H(2) km H(3) km Average crustal thickness (km) k Poisson's ratios

IFE 35 ± 3.0 36.0 ± 2.0 35 ± 2.0 35 ± 2.0 1.76 0.26 TOR 40 ± 4.0 40.0 ± 2.0 40 ± 2.0 40 ± 2.0 1.76 0.26 KAD 32 ± 3.0 33.0 ± 1.0 33 ± 2.0 33 ± 2.0 1.75 0.26 NSU 23 ± 4.0 ––23 ± 4.0 1.75 0.26 AWK 22 ± 3.0 ––22 ± 3.0 1.75 0.26 KUKU 44 ± 4.0 45.0 ± 2.0 45 ± 2.0 45 ± 3.0 1.68 0.24 MRON 40 ± 3.0 46.0 ± 3.0 42 ± 2.0 43 ± 3.0 1.65 0.24 SHAI 45 ± 3.0 43.0 ± 3.0 42 ± 2.0 43 ± 3.0 1.73 0.25 AKOS 46 ± 3.0 45.0 ± 2.0 45 ± 3.0 45 ± 3.0 1.75 0.26 KLEF 43 ± 4.0 47.0 ± 3.0 42 ± 2.0 44 ± 3.0 1.70 0.24 WEIJ 42 ± 4.0 40.0 ± 3.0 42 ± 2.0 41 ± 3.0 1.76 0.26

H = crustal thickness. 1 = forward modeling of Moho Ps arrival time. 2=H–k stacking. 3 = joint inversion. k = Vp/Vs ratio.

and Pollack, 1992; Tesha et al., 1997), the Yilgarn Craton (Mathur, 1974; Acknowledgments Wellmann, 1978), the Indian shield (Subrahmanyam, 1978; Julià et al., 2009) and the Mann shield (Blot et al., 1962; Louis, 1978; Black et al., We gratefully acknowledge the funding of this research by the Cen- 1979), 5–10 km of crustal thickening is observed along with the pres- tre for Geodesy and Geodynamics (CGG), Toro, Nigeria provided to the ence of maﬁc units in a crust commonly affected by granulite facies ﬁrst author as part of his PhD sponsorship, and thank an anonymous re- metamorphism and extraction of a felsic partial melt component. Both viewer for helpful comments. Data from the Nigeria stations were ob- the thicker crust and the large thickness of lower crust with high tained from CGG and those from the stations in Ghana were provided shear-wave velocities in the Dahomeyan Terrain and Togo Structural by AfricaArray, IRIS and Ghana Geological Survey Department, Accra. Unit are consistent with typical ‘suture’ thickened crust found in other Mr. Chimezie Emeka digitized the geological maps of Nigeria and Precambrian terrains, and thus we suggest that this is a viable explana- Ghana used in this study. tion for the nature of crustal structure beneath the Dahomeyan Terrain and Togo Structural Unit. Appendix A. Supplementary data

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