tectonophysics - africaarrayafricaarray.psu.edu/publications/pdfs/akpan_et_al... · cretaceous to...

Crustal structure of Nigeria and Southern Ghana, West Africa fromP-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, Nigeriab Department of Geology, University of Calabar, Calabar, Nigeriac Department of Geosciences, Pennsylvania State University, University Park, PA 16802, USAd School of Geosciences, The University of the Witwatersrand, Johannesburg, South Africae Departamento de Geofísica & Programa de Pós-Graduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grando do Norte, Natal, Rio Grande do Norte, Brazil

a b s t r a c ta r t i c l e i n f o

Article history:Received 20 November 2015Received in revised form 1 February 2016Accepted 2 February 2016Available online xxxx

We report new estimates of crustal thickness (Moho depth), Poisson's ratio and shear-wave velocities for elevenbroadband seismological stations in Nigeria and Ghana. Data used for this study came from teleseismic earth-quakes recorded at epicentral distances between 30° and 95° and with moment magnitudes greater than orequal to 5.5. P-wave receiver functions were modeled using the Moho Ps arrival times, H–k stacking, and jointinversion of receiver functions and Rayleigh wave group velocities. The average crustal thickness of the stationsin the Neoproterozoic basement complex of Nigeria is 36 km, and 23 km for the stations in the Cretaceous BenueTrough. The crustal structure of the Paleoproterozoic Birimian Terrain, and Neoproterozoic Dahomeyan Terrainand Togo Structural Unit in southern Ghana is similar, with an average Moho depth of 44 km. Poisson's ratiosfor all the stations range from0.24 to 0.26, indicating a bulk felsic to intermediate crustal composition. The crustalstructure of the basement complex inNigeria is similar to the average crustal structure of Neoproterozoic terrainsin other parts of Africa, but the two Neoproterozoic terrains in southern Ghana have a thicker crust with a thickmafic lower crust, ranging in thickness from 12 to 17 km. Both the thicker crust and thick mafic lower crustalsection are consistent with many Precambrian suture zones, and thus we suggest that both features are relictfrom the collisional event during the formation of Gondwana.

© 2016 Elsevier B.V. All rights reserved.

Keywords:NigeriaGhanaNeoproterozoicPaleoproterozoicCrustal structureSuture zones

1. Introduction

In this paper we report the first seismological estimates of crustalstructure in Nigeria and Ghana using broadband data from the Nigeriaand Ghana national seismic networks. In spite of the prominent roleNigeria and Ghana play in supplying the world with petroleum andother natural resources, very little is known about crustal structurewithin these countries as it relates to the geologic development of keytectonic features, such as the Cretaceous Benue Trough and the WestAfrican passive margin. The only published estimates of Moho depthsin Nigeria come from regional gravity studies (e.g., Fairhead andOkereke, 1987, 1988; Okereke, 1988; Fairhead et al., 1991) or continen-tal (Tugume et al., 2013) and global (Mooney et al., 1998; Bassin et al.,2000; Laske et al., 2013) models of crustal structure. Ghana has experi-enced historically large earthquakes, yet information on the crustalstructure within the country is generally lacking. Previous informationabout theMohodepths in Ghana has come from regional gravity studies(Ako and Wellman, 1985), seismological studies (Bacon and Quaah,

1981), as well as continental and global models of crustal structure(Mooney et al., 1998; Tedla et al., 2011; Tugume et al., 2013; Laskeet al., 2013).

Data from eleven broadband seismic stations have been used to ob-tain new point estimates of crustal thickness, Vp/Vs ratios, and crustalshear-wave velocities in two different tectonic regions of Nigeria, thePrecambrian basement complex and the Cretaceous Benue Trough,and three different tectonic regions in Ghana, the Birimian and Daho-meyan terrains, and the Togo Structural Unit. The estimates comefrom Moho Ps arrival times in P-wave receiver functions (PRFs)(Zandt et al., 1995), applying the H–k stacking method of Zhu andKanamori (2000) to PRFs, and a joint inversion of PRFs with Rayleighwave group velocities (Julià et al., 2000, 2003). The new estimates areused to examine crustal structure in Nigeria and Ghana by comparingthem with the structure of similar age crust in other parts of Africa,and with Moho depth estimates from previously published studies inNigeria and Ghana.

2. Background

Nigeria consists of three major tectonic units, the Neoproterozoicbasement complex, the Jurassic Younger Granites complex, and the

Tectonophysics xxx (2016) xxx–xxx

⁎ Corresponding author at: Centre for Geodesy and Geodynamics, Toro, Nigeria.Tel.: +234 8036141300.

E-mail address: [email protected] (O. Akpan).

TECTO-126947; No of Pages 11

http://dx.doi.org/10.1016/j.tecto.2016.02.0050040-1951/© 2016 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Tectonophysics

j ourna l homepage: www.e lsev ie r .com/ locate / tecto

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

mailto:[email protected]

http://dx.doi.org/10.1016/j.tecto.2016.02.005

www.elsevier.com/locate/tecto


Cretaceous to Recent sedimentary successions comprised of the NigerDelta, the Benue Trough, and the Borno, Dahomey, Bida and Sokotobasins (Obaje, 2009; Fig. 1). The seismological stations used in thisstudy are located in the basement complex and Benue Trough.

The basement complex is a component of the West African Pan-African mobile belt (Black, 1980; Wright et al., 1985; Ajibade andFitches, 1988; Ekwueme, 1990). In addition to the Pan-Africantectonothermal event (Burke and Dewey, 1972; Ajibade and Fitches,1988; Obaje, 2009), several older orogenies are recorded in the base-ment complex, including the Liberian (2700 ± 200 Ma), Eburnian(2200 ± 200 Ma) and Kibaran (1100 ± 100 Ma) orogenies (Ajibadeand Fitches, 1988; Obaje, 2009; Ogezi, 1988; Rahaman, 1988; Ajibadeet al., 1988; Dada, 1998). The basement complex, which was later in-truded by the Younger Granites, is present throughout the country, un-derlying the sedimentary basins listed earlier (Avbovbo, 1980; Obaje,2009). The rocks commonly found in the basement complex range inmetamorphic grade and include migmatites, gneisses, schists, quartz-ites, granulites, amphibolites, phyllites, marbles, and igneous rockssuch as calc-silicates, granites, syenites, granodiorites, adamellite,quartz monzonites and charnockites (Rahaman, 1988).

The Benue Trough is oriented in a NE–SW direction and is a compo-nent of the West and Central African Rift System with a length andwidth of about 800 km and 150 km, respectively (Benkhelil, 1989;Ofoegbu and Okereke, 1990; Obaje, 2009) (Fig. 1). It developed as afailed arm of the RRR Triple junction (aulacogen) following the

separation of South America and Africa during the opening of theSouth Atlantic Ocean in the Early Cretaceous (Ofoegbu and Okereke,1990; Binks and Fairhead, 1992; Guiraud and Maurin, 1992). After theSantonian tectonic and magmatic events, the major depositional axisin the Lower Benue Trough was shifted to the northwest, leading tothe formation of the Anambra Basin (Wright et al., 1985). Therefore,the Anambra Basin is regarded as a part of the Lower Benue Trough con-taining post-deformational Campanian to Eocene deposits (Obaje,2009). Several magmatic events affected the Benue Trough (Agaguand Adighije, 1983), most prominently the ones during the late Albianand Turonian (Offodile, 1976).

Moho depth estimates of 20 to 26 km beneath the Benue Troughand its adjoining rifts in Nigeria are reported in regional gravity studies(e.g. Fairhead and Okereke, 1987, 1988; Okereke, 1988; Fairhead et al.,1991). Within the rifted parts of the Benue Trough in Cameroon i.e.the Garoua rift, Moho depth estimates of 23 to 28 come from regionalgravity studies (e.g. Poudjom Djomani et al., 1995; Nnange et al.,2000; Kamguia et al., 2005) and seismological studies (e.g. Stuartet al., 1985; Tokamet al., 2010). Continental and globalmodels of crustalstructure show Moho depths of 25 to 42 km on average beneath theCretaceous Benue Trough and Precambrian basement complex, respec-tively (e.g. Mooney et al., 1998; Bassin et al., 2000; Tugume et al., 2013;Laske et al., 2013).

The geologic framework of Ghana consists of five major tectonicunits, (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).

2 O. Akpan et al. / Tectonophysics xxx (2016) xxx–xxx



Tarkwaian terrains, (2) the Dahomeyan Terrain, (3) the Togo andBuem Structural Units, (4) the Voltaian Basin and (5) other sedimen-tary basins of Paleozoic to Recent age. The seismological stationsused for this study are located in the Birimian Terrain, the Dahomey-an Terrain and the Togo Structural Unit in the southern part of thecountry (Fig. 2). The Birimian Terrain was last deformed during theEburnean orogeny (Barritt and Kuma, 1998) and consists of phyllites,volcaniclastics, chemical sedimentary rocks, wackes and granitoids.The Dahomeyide Belt (consisting of the Dahomeyan Terrain, andTogo and Buem Structural Units) forms the eastern border of theWest African Craton (Jones, 1990) and represents the suture be-tween the Birimia and Dahomeya blocks during the Pan-Africanorogeny (Burke and Dewey, 1972; Bacon and Quaah, 1981; Quaah,1982). The rocks of the Dahomeyan Terrain consist mainly ofgneisses, granulites and schists (Wright et al., 1985). The Togo Struc-tural Unit is made up of supra-crustal sediments that were deformedby the northwesterly directed thrusting of Dahomeyan basementrocks onto the West African Craton (Wright et al., 1985). The majorrocks are shales, quartz sandstones, schists and strongly foldedand deformed quartzites and phyllites (Bacon and Quaah, 1981;Amponsah et al., 2009).

Regional gravity and seismological studies report Moho depths of 38to 42 km for southern Ghana (Bacon and Quaah, 1981; Ako andWellman, 1985), and crustal thickness estimates of 30 to 45 km are re-ported in continental and global models of crustal structure (Mooneyet al., 1998; Tugume et al., 2012; Laske et al., 2013).

3. Data and method of study

3.1. Data

Eleven seismological stations in Nigeria and Ghanawere used in thisstudy. The stations belonging to the Nigeria National Seismic Networkare IFE, TOR, KAD, NSU and AWK, and those in the Ghana National Seis-mic Network are KUKU, MRON, SHAI, AKOS, KLEF andWEIJ (Figs. 1 and2, Table 1). The IFE, KAD andNSU stationswere installed in August, 2008while TOR and AWK stationswere installed in November, 2010. The sta-tions in Nigeria are equipped with the Eentec DR-4000 24 bit three-channel data acquisition system, three-component seismometers(Eentec EP-105 or Eentec SP-400) and Global Positioning System(GPS) clocks. Each of them is powered by three solar panels (80 Weach) connected to a 200 Ah battery. The stations in the Ghana NationalSeismic Network, which are equipped with Nanometrics Trident 305data loggers, and either Trillium Compact or Trillium 120P seismome-ters, were installed in October 2012. Two AfricaArray stations inGhana (KUKU and SHAI), which are equipped with Reftek RT130 dataloggers, and either a Guralp CMG 3 T or 40 T seismometer, wereinstalled in July 2009. Two of the new stations belonging to the nationalnetwork were collocated with the AfricaArray stations. Data at all sta-tions were recorded at 40 samples per second. Station information isprovided in Table 1.

The data used for this study came from teleseismic earthquakeswithmomentmagnitudes 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 eachstation).

3O. Akpan et al. / Tectonophysics xxx (2016) xxx–xxx



June 2009 and April 2014 at epicentral distances between 30° and 95°from the stations. A list of events used is given in the Supplementarymaterial, and the azimuthal coverage provided by them is illustratedin Figs. 3 and 4.

3.2. Receiver functions

Receiver functions are time series that contain P-to-S conversionsgenerated when an incident P-wave interacts with a seismic interfacebeneath a seismological station (Fig. 5). Receiver functions are comput-ed by deconvolving the vertical component waveform from the radialand tangential components, thereby removing the effects of the sourcetime function and instrument response. The radial receiver function ismade up of P-to-S converted waves from the Moho (and other seismicdiscontinuities beneath a station), and themultiples from those conver-sions with the free surface (Fig. 6) (Langston, 1979).

In this study, the seismograms selected for receiver function analysiswere first windowed between 10 s before and 110 s after the first P ar-rival. The data were then detrended, tapered and bandpass filtered be-tween 0.05 and 8 Hz to remove low and high frequency noise, andwere thereafter decimated to 10 samples per second. The horizontalcomponents of the seismograms were then rotated into the great circlepath to obtain the corresponding radial and tangential components.

The receiver functions were computed using the iterative, time-domain deconvolution technique (Ligorría and Ammon, 1999) using500 iterations. Radial and tangential receiver functions were computedfor each teleseismic event at two different Gaussian width factors of 1.0(f ≤ 0.5 Hz) and 2.5 (f ≤ 1.25 Hz). Modeling high- and low-frequencyreceiver functions simultaneously helps discriminate sharp from grada-tional velocity changes in the resulting velocity–depth profiles (Julià,2007).

The quality of the receiver functions was checked using two inde-pendent approaches. The first approach involved a least squares misfitcriterion, which consisted of calculating the difference between theoriginal radial waveforms and an estimated radial waveform from theconvolution of the corresponding vertical waveform with the alreadycomputed radial receiver function. Receiver functions that were recov-ered at 85% and above were selected for further analysis (Ligorría and

Ammon, 1999). In the second approach, events in which the tangentialreceiver functions exhibited large amplitudes relative to the radial re-ceiver functions were not selected, even if they met the first criteria.Ammon et al. (1990) pointed out that transverse receiver functionsshould be zero for isotropic, laterally homogenous media. The qualityof the receiver functions for all the stations computed using a Gaussianwidth of 1.0 is shown in the Supplementary material.

3.3. Modeling of receiver functions

In this section, we briefly describe the three methods used to modelthe data.

3.3.1. Forward modeling of Moho Ps arrival timesIn this method, the equation of Zandt et al. (1995), which uses the

travel time difference between the Ps and P arrivals, was used to esti-mate crustal thickness. The equation is as follows:

H ¼ tPs " tpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiVs

"2 " p2" #

"r ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

Vp"2 " p2

" #r ð1Þ

where H is the crustal thickness, tPs is the Ps arrival time, tp is the directP-wave arrival time, Vp is the velocity of the P-wave, Vs is the velocity ofS-wave and p is the ray parameter of the incident P-wave. In usingthis technique, the following values were assumed in the computation:Vp/Vs = 1.75, Vp = 6.5 km/s and Vs = 3.714 km/s (Zandt et al., 1995).

Radial receiver functions using a Gaussian width factor of 1.0 wereused for this computation. The arrival times of the Ps waves (tPs ) rela-tive to the direct P-wave (tp) (see Fig. 6) were hand-picked and usedin calculating theMoho depth for each individual waveform. The differ-ent values of H obtainedwere then averaged to derive an estimate of theMoho depth beneath each station.

An advantage of this method is that since the Ps conversion pointnormally lies close to the seismological station (the waves sampleonly ~10 km from the station), it is less affected by lateral variationsin velocity and thus provides a good estimate of the crustal thickness(Zandt et al., 1995). There is however a problem in the trade-off

Table 1Locations of seismological stations in Nigeria and Ghana.

Networkcode

Name ofstation

Stationcode

Latitude(degree)

Longitude(degree)

Elevation(m)

Geology Tectonic region Age Instrumentation

NG Ile-Ife IFE 7.5334 4.5336 289 Gray gneiss Migmatite Gneiss Complex Archean Seismograph: DR-4000Seismometer: EP-105

NG Kaduna KAD 10.4334 7.6334 668 Granite Older Granite Complex Neoproterozoic Seismograph: DR-4000Seismometer: EP-105

NG Toro TOR 10.1168 9.1167 882 Granite Older Granite Complex Neoproterozoic Seismograph: DR-4000Seismometer: EP-105

NG Nsukka NSU 6.8667 7.4167 430 Sandstone Benue Trough Cretaceous Seismograph: DR-4000Seismometer: EP-105

NG Awka AWK 6.2335 7.1001 50 Shale Benue Trough Paleocene Seismograph: DR-4000Seismometer: 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 305Seismometer: 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 305Seismometer: Trillium Compact

GH Ho KLEF 6.6142 0.4407 313 Quartzite Togo Structural Unit Neoproterozoic Seismograph: Trident 305Seismometer: Trillium 120P

GH Weija WEIJ 5.5885 −0.3333 203 Quartzite Togo Structural Unit Neoproterozoic Seismograph: Trident 305Seismometer: Trillium Compact




between the Moho depth and crustal velocities (Zhu and Kanamori,2000). Since tPs is the difference in the travel time of the S-wavewith re-spect to the P-wave, the value of H depends to a large extent on theVp/Vsratio (Zhu and Kanamori, 2000).

The uncertainties in H were obtained from the range of the differentvalues of H obtained from each waveform. The uncertainty in Mohodepth estimates obtained using this method is ~3–4 km.

3.3.2. H–k stackingThe H–k stacking method developed by Zhu and Kanamori (2000)

was used to estimate crustal thickness (H) and Vp/Vs ratio (k) beneatheach station using receiver functions that have clear Ps waves and oneor more multiple phases (PpPs, PpSs + PsPs, PsSs) [Fig. 6]. The tech-nique transforms the receiver functions from the time-amplitudedomain into the H–k stacking domain through

s H; kð Þ ¼ ∑N

j¼1w1r j t1ð Þ þw2r j t2ð Þ "w3r j t3ð Þ ð2Þ

where w1, w2 and w3 are the weights assigned to the Ps, PpPs andPpSs phases, respectively, the sum of the weights being unity; rj is

the amplitude of the radial receiver function; t1, t2 and t3 are thearrival times of the phases and N is the number of receiver functionsused.

Radial receiver functions with Gaussian width factors of 1.0 wereused in this method. The a priori parameters used in the H–k stackingwere the weights and average crustal P-wave velocity (Vp). Because allthe phases were clearly identified in the receiver functions, weightsw1 = 0.4, w2 = 0.3 andw3 = 0.3 were used to give similar importanceto each phase. A P-wave velocity (Vp) of 6.5 km/s was used, as this rep-resents a reasonable average value of Vp for Precambrian crust(Christensen and Mooney, 1995).

The bootstrapping method of Efron and Tibshirani (1991) was usedto estimate uncertainties in H and k. This involved re-sampling the re-ceiver function datasets with replacement 500 times for each station,applying the H–k stacking procedure to the re-sampled dataset, andcomputing the average and standard deviation from the resulting 500estimates. To determine the errors in H (Moho depth) and k (Vp/Vs

ratio) arising from the assumed average value for crustal Vp, the H–kstacks were re-calculated using Vp values between 6.3 km/s and6.8 km/s. The uncertainties estimated from the bootstrap methodwere then combinedwith those obtained fromdifferent average crustalVp values, giving an overall uncertainty in H of ~2–3 km and in k of±0.05.

20o

40o

60o

80o

100o

Fig. 3.Map showing the distribution of earthquakes recorded at seismological stations inNigeria used for this study. The black triangle represents the center of the seismic network, the redcircles show the earthquake epicenterswhile the large black circles show the epicentral distance in 20° increment from the center of the network.Mapwas plotted using theGenericMap-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.)




20o

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 redcircles show the earthquake epicenterswhile the large black circles show the epicentral distance in 20° increment from the center of the network.Mapwas plotted using theGenericMap-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 themajor P-to-S converted phaseand associated multiples that comprise a radial receiver function for a single layer over ahalf-space (redrawn from Ammon et al., 1990). Ps is the Moho converted phase, andPpPs, 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 inversionThe joint inversion technique developed by Julià et al. (2000, 2003)

was used to model shear-wave velocities in the crust and upper mantlebeneath each station. This method involves jointly inverting the receiv-er functions and surfacewave dispersion curves using an iterative, least-squares algorithm with a roughness norm. The input for the jointinversion consists of an initial model, the observed receiver functions,and the Rayleigh-wave group–velocity curves.

Radial receiver functions computed at Gaussian width factors of 1.0and 2.5were used for the joint inversion. The selected receiver functionswere grouped according to their respective back-azimuths and ray pa-rameters. Grouping the receiver functions by ray parameters helps toaccount for the phase move-out due to varying angles of incidence(Cassidy, 1992; Gurrola and Minster, 1998). To get a shear-wave veloc-ity model for each of the stations, each receiver function group wasjointly inverted with the corresponding dispersion curve for each sta-tion. Rayleigh wave group velocities for periods between 10 and 100 sused in this study were derived from the group velocity measurementsof Raveloson et al. (2015).

In the inversion, an influence factor that controls the relative contri-butions of the receiver functions and the group–velocity curvesmust beset a priori, as well as a smoothness parameter that controls the trade-off between fitting the data and model smoothness (Julià et al., 2003).We used an influence factor of 0.5, giving equal weight to the receiverfunctions and group velocities and we used a depth-dependent

smoothing parameter with values of 0.2–0.4 to obtain smooth depth–velocity profiles that provided a good fit to the data.

The starting model consisted of a 37.5 km thick crust with a linearS-wave velocity increase with depth from 3.4 to 4.0 km/s, overlying aflattened PREM (Preliminary Reference Earth Model) (Dziewonski andAnderson, 1981). The model parameterization consisted of a stack ofconstant velocity layers that increased in thickness with depth. Thick-nesses of 1 and 2 km were used for the first two layers of the model,2.5 km for layers between 3 and 60.5 kmdepth, 5 km for layers between60.5 and 260.5 km depth, and 10 km below a depth of 260.5 km. Notethat, although the startingmodel was parameterized down to transitionzone depths, only velocities above 260.5 km depth were inverted for.Velocities at larger depths were kept at PREMvalues in order to accountfor partial sensitivity of long-period dispersion velocities to deep struc-ture (Julià et al., 2003). Poisson's ratio was fixed to 0.25 for the crust andto PREM values (0.28 to 0.30) for mantle layers during the inversion.Densities (ρ)were derived fromP-wave velocities (Vp) using the empir-ical relationship of Berteussen (1977) expressed as follows:

ρ ¼ 0:32Vp þ 0:77: ð3Þ

The uncertainties in the shear-wave velocity models were obtainedusing the method of Julià et al. (2005), which involved carrying outthe inversion process repeatedly with different inversion parameters,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 epicentraldistance of the events in degrees. Contours map out percentage values of the objective function (Equation 2) given in the text.




uncertainties in shear-wave velocities for each layer in the models are0.1–0.2 km/s, resulting in an uncertainty of ~2–3 km for the crustalthickness estimates.

4. Results

Figs. 7 and 8 show the examples of the H–k stacking and joint inver-sion modeling for one station (KUKU), and the results from all threemethods are summarized in Tables 2 to 4, including uncertainties. H–kstacking and joint inversion results for the other stations are providedin the Supplementary material. Results using the H–k stacking andjoint inversion methods were obtained for 9 stations (Tables 3 and 4).For the two stations in the Benue Trough (NSU and AWK), crustal thick-ness estimates were obtained only from the Moho Ps arrival times(method 1).

To estimate crustal thickness from the joint inversion results, thedepth of the Moho was defined as the depth at which the shear-wavevelocity was equal to or exceeded 4.3 km/s. Studies by Christensen andMooney (1995) and Christensen (1996) have shown that shear-wavevelocities for lower crustal lithologies derived from experimentally de-termined P-wave velocities and Vp/Vs ratios cannot exceed 4.3 km/s. Ad-ditionally, from the models, we obtained an average crustal shear-wavevelocity beneath each seismic station and the thickness of the mafic

layer in the lower crust. A number of studies (e.g., Holbrook et al.,1992; Christensen and Mooney, 1995; Rudnick and Fountain, 1995;and Rudnick and Gao, 2003) show that common lower crustal maficlithologies, such as amphibolites, garnet-bearing and garnet-freemafic granulite, and mafic gneiss, have higher shear-wave velocities(N3.9 km/s) while intermediate-to-felsic lithologies have lower shear-wave velocities (b3.9 km/s). Therefore, we define the mafic lower crustas layers in the model with shear-wave velocities between 4.0 km/sand 4.3 km/s (Table 4).

For stations where more than one estimate of crustal thickness wasobtained, the estimates are in good agreement, and an average of the es-timates is given in Table 5. The stations in the Benue Trough haveMohodepths of 22 to 23 km compared to 33 to 40 km for the three stations inthe Precambrian basement of Nigeria (Fig. 1). Crustal thickness beneaththe stations inGhana ismore uniform, ranging from41 to 45 km (Fig. 2).For stations with H–k stacking results, Vp/Vs ratios from 1.65 to 1.76were obtained. In addition to the Precambrian crust in Ghana beingsomewhat thicker than in Nigeria, the crust in Ghana is also character-ized by a thick mafic lower crust. The thickness of lower crustal layerswith shear-wave velocities between 4.0 and 4.3 km/s ranges from12 km at station SHAI to 17 km at stations MRON andWEIJ. In compar-ison, the thickness of themafic lower crust beneath the three stations inthe Nigerian basement complex is 3 to 5 km.

0.00

0.25

0.50

0.75

1.00

0.00

0.25

0.50

0.75

1.00

0.00

0.25

0.50

0.75

1.00

0.00

0.25

0.50

0.75

1.00

0.00

0.25

0.50

0.75

1.00

1.000 10 20 30 40

0 10 20 30 40

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 25 50 75 100 125

0

1 2 3 4 5

Vs (km/s)

syntheticobserved

inverted modelstarting model

3, 62.0, 0.060

2, 147.3, 0.060

3, 204.6, 0.052

7, 237.9, 0.053

4, 252.2, 0.051

3, 115.1, 0.076

3, 191.9, 0.057

12, 231.9, 0.049

5, 244.4, 0.055

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 Rayleighwave 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.




5. Discussion

In discussing the results,wefirst examine crustal structure inNigeriaand then in Ghana. For both countries, the variability of crustal structure(or lack thereof) is discussed and then compared to crustal structure forsimilar age crust in other parts of Africa.

5.1. Nigeria

The average crustal thickness of the stations in the basement com-plex is 36 km, and the crust under these stations has a Poisson'sratio of 0.26 (Vp/Vs ratios of 1.75 to 1.76) indicating a bulk felsic tointermediate composition for the crust. The crustal structure of thebasement complex is similar to the average crustal structure of otherNeoproterozoic terrains in Africa. The range of average crustal thicknessfor the Oubanguides Belt, Zambezi Belt, Damara Belt, Mozambique Belt,and LufilianArc is 35 to 43 km, the average crustal Poisson's ratios rangefrom0.23 to 0.28, and the thickness of themafic lower crust ranges from2 to 7 km (Tokam et al., 2010; Tugume et al., 2013; Kachingwe et al.,2015). Thus, there appears to be no major differences in crustal struc-ture of the basement complex compared to many other Neoproterozoicterrains in Africa.

The average Moho depth for stations in the Benue Trough is 23 km,which is in good agreementwith previous estimates of crustal thicknessfor the Benue Trough of 20 to 26 km using gravity data (Fairhead andOkereke, 1987, 1988; Okereke, 1988; Fairhead et al., 1991), as well as24 km for theGaroua rift, a northeasterly extension of the Benue Troughinto Cameroon (Kamguia et al., 2005). The Moho depth beneath theGaroua rift derived from passive seismology is 26 km (Tokam et al.,2010) and 23 km from seismic refraction profiling (Stuart et al., 1985).This is in contrast to the Cenozoic East African rift, where crustal thick-ness beneath many parts of the rift system is ~30–35 km (e.g., Fuchset al., 1997; Julià et al., 2005; Dugda et al., 2005; and references therein).As the Benue Trough is underlain by Neoproterozoic basement complexcrust (Avbovbo, 1980), it appears that the crust has been thinned by

about 13 km based on the average basement complex crustal thicknessof 36 km.

5.2. Ghana

The average crustal thickness beneath the six stations in southernGhana is 44 km. The crust under these stations have Poisson's ratio of0.24 to 0.26 (Vp/Vs ratios of 1.65 to 1.76) indicating a bulk felsic to inter-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-wavevelocities above 4.0 km/s ranging in thickness from 12 to 17 km. Thecrustal structure of the Birimian Terrain (stations KUKU and MRON) issimilar to the average crustal structure of other Paleoproterozoicterrains in Africa. The range of average crustal thickness for the OkwaTerrain, Kheis Province, Ubendian Belt, Usagaran Belt, and RehobothProvince in eastern and southern Africa is 38 to 44 km, the averagecrustal Poisson's ratios range from 0.25 to 0.27, and the thickness ofthe mafic lower crust ranges from 2 to 13 km (Kgaswane et al., 2009;Tugume et al., 2012, 2013; Kachingwe et al., 2015). Thus, there appearsto be no major differences in crustal structure of the Birimian Terraincompared to many other Paleoproterozoic terrains in Africa.

In contrast, there appears to be a substantial difference in the crustalstructure of theNeoproterozoic DahomeyanTerrain andTogo StructuralUnit compared to the average crustal structure of other Neoproterozoicterrains in Africa reviewed above. The Neoproterozoic crust in Ghanaappears to be thicker by several kms and its mafic lower crustal sectionis about 10 km thicker than that found in many other AfricanNeoproterozoic terrains, including the basement complex in Nigeria.As summarized above, the Dahomeyan Terrain and Togo StructuralUnit represent the suture between the Birimia and Dahomeya blocksduring the Pan-African orogeny (Burke and Dewey, 1972; Quaah,1982). Both the thickened crust and the thick mafic lower crustal layermay be relict features from this collisional event during the formationof Gondwana. In Precambrian sutures elsewhere, such as found alongthe margins of the Kaapvaal Craton (Kgaswane et al., 2009), theSuperior Province (Gibb et al., 1983), the Tanzania Craton (Nyblade

Table 4Crustal structure from joint inversion.

Stationcode

Averagecrustal Vs

(km/s)

Crustalthickness(km)

Uppermostmantle Vs

(km/s)

Thickness of crustal layershaving Vs ≥ 4.0 km/s (km)

IFE 3.7 35 ± 2.0 4.6 3 ± 2.0TOR 3.7 40 ± 2.0 4.5 5 ± 2.0KAD 3.7 33 ± 2.0 4.5 3 ± 2.0KUKU 3.9 45 ± 2.0 4.7 15 ± 2.0MRON 3.9 42 ± 2.0 4.7 17 ± 2.0SHAI 3.8 42 ± 2.0 4.7 12 ± 2.0AKOS 3.7 45 ± 2.0 4.6 15 ± 2.0KLEF 3.8 42 ± 2.0 4.4 12 ± 2.0WEIJ 3.7 42 ± 2.0 4.7 17 ± 2.0

Table 3Estimates 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 ± 0.03TOR 39.1 ± 0.9 1.76 ± 0.04 37.7 ± 0.8 1.76 ± 0.04 41.0 ± 1.0 1.75 ± 0.04KAD 32.1 ± 0.9 1.75 ± 0.05 31.0 ± 1.0 1.75 ± 0.05 33.9 ± 0.9 1.74 ± 0.04KUKU 45.4 ± 0.4 1.68 ± 0.02 43.8 ± 0.4 1.68 ± 0.02 47.8 ± 0.4 1.67 ± 0.02MRON 46.6 ± 2.5 1.65 ± 0.06 45.0 ± 2.3 1.65 ± 0.05 49.1 ± 2.1 1.64 ± 0.04SHAI 43.3 ± 1.6 1.73 ± 0.05 41.9 ± 0.9 1.73 ± 0.03 45.6 ± 4.6 1.72 ± 0.03AKOS 44.6 ± 1.9 1.75 ± 0.06 43.1 ± 1.9 1.76 ± 0.06 47.1 ± 1.9 1.74 ± 0.06KLEF 47.7 ± 3.1 1.70 ± 0.08 46.1 ± 2.9 1.70 ± 0.08 50.1 ± 2.7 1.69 ± 0.08WEIJ 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 2Estimates of crustal thickness using Moho Ps arrival times.

Stationcode

Number ofreceiverfunctions

Minimum Psarrival time (s)

Maximum Psarrival time (s)

Crustalthickness(km)

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




and Pollack, 1992; Tesha et al., 1997), the Yilgarn Craton (Mathur, 1974;Wellmann, 1978), the Indian shield (Subrahmanyam, 1978; Julià et al.,2009) and the Mann shield (Blot et al., 1962; Louis, 1978; Black et al.,1979), 5–10 km of crustal thickening is observed along with the pres-ence of mafic units in a crust commonly affected by granulite faciesmetamorphism and extraction of a felsic partial melt component. Boththe thicker crust and the large thickness of lower crust with highshear-wave velocities in the Dahomeyan Terrain and Togo StructuralUnit are consistent with typical ‘suture’ thickened crust found in otherPrecambrian terrains, and thus we suggest that this is a viable explana-tion for the nature of crustal structure beneath the Dahomeyan Terrainand Togo Structural Unit.

6. Summary and conclusions

In this study, we report new estimates of crustal structure (Mohodepths, Poisson's ratios and shear-wave velocities) for Nigeria andGhana using data from the Nigeria and Ghana national seismic net-works. Moho depth estimates from seismological studies are generallylacking in the study area. The only published estimates of Moho depthsin the study area come from regional gravity studies, and continentaland global models of crustal structure.

The data used for this study came from teleseismic earthquakes re-corded on eleven broadband seismic stations at epicentral distances be-tween 30° and 95° and with moment magnitudes greater than or equalto 5.5. In Nigeria, three stations are located in the basement complexand two in the Benue Trough. In Ghana, two stations are located inthe Birimian Terrain, one in the Dahomeyan Terrain and three in theTogo Structural Unit. P-wave receiver functions were modeled usingMohoPs arrival times, H–k stacking, and joint inversionof receiver func-tions and Rayleigh wave group velocities.

The average crustal thickness of the stations in the basement com-plex in Nigeria is 36 km and 23 km for stations in the Benue Trough.The average crustal thickness beneath the stations in Ghana is 44 km.The crust under all the stations has a Poisson's ratio of 0.24 to 0.26(Vp/Vs ratios of 1.65 to 1.76), indicating a bulk felsic to intermediatecomposition for the crust. The crustal structure of the basementcomplex in Nigeria is similar to the average crustal structure ofNeoproterozoic terrains in other parts of Africa, but the twoNeoproterozoic terrains in southern Ghana have crust that is severalkms thicker as well as a thicker mafic lower crust, ranging in thicknessfrom 12 to 17 km. Both the overall thicker crust and the thicker maficlower crust are consistent with typical suture thickened crust found inmany Precambrian suture zones, and thuswe suggest that both featuresare relict from the collisional event during the formation of Gondwana.

Acknowledgments

We gratefully acknowledge the funding of this research by the Cen-tre for Geodesy and Geodynamics (CGG), Toro, Nigeria provided to thefirst author as part of his PhD sponsorship, and thank an anonymous re-viewer for helpful comments. Data from the Nigeria stations were ob-tained from CGG and those from the stations in Ghana were providedby AfricaArray, IRIS and Ghana Geological Survey Department, Accra.Mr. Chimezie Emeka digitized the geological maps of Nigeria andGhana used in this study.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2016.02.005.

References

Agagu, O.K., Adighije, C.I., 1983. Tectonic and sedimentation framework of the lowerBenue Trough, south eastern Nigeria. J. Afr. Earth Sci. 1 (3–4), 267–274.

Ajibade, A.C., Fitches, W.R., 1988. The Nigerian Precambrian and the Pan African orogeny.In: Oluyide, P.O., Mbonu, W.C., Ogezi, A.E., Egbuniwe, I.G., Ajibade, A.C., Umeji, A.C.(Eds.), Precambrian Geology of Nigeria. Geological Survey of Nigeria, Kaduna,pp. 45–53.

Ajibade, A.C., Rahaman, M.A., Ogezi, A.E.O., 1988. The Precambrian of Nigeria: a geochro-nological summary. In: Oluyide, P.O., Mbonu, W.C., Ogezi, A.E., Egbuniwe, I.G.,Ajibade, A.C., Umeji, A.C. (Eds.), Precambrian Geology of Nigeria. Geological Surveyof Nigeria, Kaduna, pp. 313–324.

Ako, J.A., Wellman, P., 1985. The margin of the West African Craton: the Voltaian Basin.J. Geol. Soc. Lond. 142, 625–632.

Ammon, C.J., Randall, G.E., Zandt, G., 1990. On the nonuniqueness of receiver functioninversions. J. Geophys. Res. 95 (B10), 15303–15318.

Amponsah, P.E., Banoeng-Yakubo, B.K., Panza, G.F., Vaccari, F., 2009. Deterministic seismicground modelling of the Greater Accra metropolitan area, southeastern Ghana. S. Afr.J. Geol. 112 (3–4), 317–328.

Avbovbo, A.A., 1980. Basement geology in the sedimentary basins of Nigeria. Geology 8,323–327.

Bacon, M., Quaah, A.O., 1981. Earthquake activity in southeastern Ghana (1977–1980).Bull. Seismol. Soc. Am. 71 (3), 771–785.

Barritt, S.D., Kuma, J.S., 1998. Constrained gravity models and structural evolution of theAshanti Belt, south west Ghana. J. Afr. Earth Sci. 26 (4), 539–550.

Bassin, C., Laske, G., Masters, G., 2000. The current limits of resolution for surface wave to-mography in North America. EOS Trans. AGU 81, 897.

Benkhelil, J., 1989. The origin and evolution of the Cretaceous Benue Trough, Nigeria.J. Afr. Earth Sci. 8, 251–282.

Berteussen, K.A., 1977. Moho depth determination based on spectral ratio analysis ofNORSAR long-period P wave. Phys. Earth Planet. Int. 15, 13–27.

Binks, R.M., Fairhead, J.D., 1992. A plate tectonic setting for Mesozoic rifts of West andCentral Africa. Tectonophysics 213, 141–151.

Black, R., 1980. Precambrian of West Africa. Episodes 4, 3–8.Black, R., Caby, R., Moussine-Pouchkine, A., Bertad, J.M., Boullier, A.M., Fabre, J., Lesquer, A.,

1979. Evidence for late Precambrian plate tectonics in West Africa. Nature 278,223–227.

Table 5Summary 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 0.26TOR 40 ± 4.0 40.0 ± 2.0 40 ± 2.0 40 ± 2.0 1.76 0.26KAD 32 ± 3.0 33.0 ± 1.0 33 ± 2.0 33 ± 2.0 1.75 0.26NSU 23 ± 4.0 – – 23 ± 4.0 1.75 0.26AWK 22 ± 3.0 – – 22 ± 3.0 1.75 0.26KUKU 44 ± 4.0 45.0 ± 2.0 45 ± 2.0 45 ± 3.0 1.68 0.24MRON 40 ± 3.0 46.0 ± 3.0 42 ± 2.0 43 ± 3.0 1.65 0.24SHAI 45 ± 3.0 43.0 ± 3.0 42 ± 2.0 43 ± 3.0 1.73 0.25AKOS 46 ± 3.0 45.0 ± 2.0 45 ± 3.0 45 ± 3.0 1.75 0.26KLEF 43 ± 4.0 47.0 ± 3.0 42 ± 2.0 44 ± 3.0 1.70 0.24WEIJ 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.





http://refhub.elsevier.com/S0040-1951(16)00101-3/rf0005



































Blot, C., Crenn, Y., Rechenmann, R., 1962. Elements apportes par la gravimetrie a laconnaissance de la tectonique profonde du Senegal. CR Acad. Sci. Paris 254,1131–1133.

Burke, K.C., Dewey, J.F., 1972. Orogeny in Africa. In: Dessauvagie, T.F.J., Whiteman, A.J.(Eds.), African Geology. Department of Geology, University of Ibadan, Ibadan,pp. 583–608.

Cassidy, J.F., 1992. Numerical experiments in broad-band receiver function analysis. Bull.Seismol. Soc. Am. 82, 1453–1474.

Christensen, N.I., 1996. Poisson's ratios and crustal seismology. J. Geophys. Res. 101 (B2),3139–3156.

Christensen, N.I., Mooney, W.D., 1995. Seismic velocity structure and composition of thecontinental crust: a global view. J. Geophys. Res. 100, 9761–9788.

Dada, S.S., 1998. Crust-forming ages and Proterozoic crustal evolution in Nigeria: a reap-praisal of current interpretations. Precambrian Res. 87, 65–74.

Dugda, M.T., Nyblade, A.A., Julia, J., Langston, C., Ammon, C., Simiyu, S., 2005. Crustalstructure in Ethiopia and Kenya from receiver function analysis: implications for riftdevelopment in eastern Africa. J. Geophys. Res. 110. http://dx.doi.org/10.1029/2004JB003065.

Dziewonski, A.M., Anderson, D.L., 1981. Preliminary Reference EarthModel (PREM). Phys.Earth Planet. Int. 25, 297–356.

Efron, B., Tibshirani, R., 1991. Statistical-data analysis in the computer age. Science 253,390–395.

Ekwueme, B.N., 1990. Pb–Sr ages and petrologic features of Precambrian rocks from theOban massif, south eastern Nigeria. Precambrian Res. 47, 271–286.

Fairhead, J.D., Okereke, C.S., 1987. A regional gravity study of theWest African Rift Systemin Nigeria and Cameroon and its tectonic implication. Tectonophysics 143, 141–159.

Fairhead, J.D., Okereke, C.S., 1988. Depths to major density contrasts beneath the WestAfrican rift system in Nigeria and Cameroon based on spectral analysis of gravitydata. J. Afr. Earth Sci. 7, 769–777.

Fairhead, J.D., Okereke, C.S., Nnange, J.M., 1991. Crustal structure of the Mamfe basin,West Africa based on gravity data. Tectonophysics 186, 351–358.

Fuchs, K., Altherr, B., Muller, B., Prodehl, C., 1997. Structure and dynamic processes in thelithosphere of the Afro-Arabian rift system. Tectonophysics 278, 1–352.

Gibb, R.A., Thomas, M.D., Lapointe, P., Mukhopadhay, M., 1983. Geophysics of proposedProterozoic sutures in Canada. Precambrian Res. 19, 349–384. http://dx.doi.org/10.1016/03019268(83)90021-9.

Guiraud, R., Maurin, J.C., 1992. Early Cretaceous rifts of Western and Central Africa: anoverview. Tectonophysics 213, 153–168.

Gurrola, H., Minster, J.B., 1998. Thickness estimates of the upper mantle transition zonefrom bootstrapped velocity spectrum stacks of receiver functions. Geophys. J. Int.133, 31–43.

Holbrook, W.S., Mooney, W.D., Christensen, N.I., 1992. The seismic velocity structure ofthe deep continental crust. In: Fountain, D.M., Arculus, R., Kay, R.W. (Eds.), Continen-tal Lower Crust. Elsevier, Amsterdam, pp. 1–43.

Jones, W.B., 1990. The Buem volcanic and associated sedimentary rocks, Ghana: a fieldand geochemical investigation. J. Afr. Earth Sci. 11 (3–4), 373–383.

Julià, J., 2007. Constraining velocity and density contrasts across the crust–mantle bound-ary with receiver function amplitudes. Geophys. J. Int. 171, 286–301.

Julià, J., Ammon, C.J., Hermann, R.B., Correig, A.M., 2000. Joint inversion of receiverfunctions and surface-wave dispersion observations. Geophys. J. Int. 143, 99–112.

Julià, J., Ammon, C.J., Hermann, R.B., 2003. Lithospheric structure of the Arabian Shieldfrom the joint inversion of receiver functions and surface-wave group velocities.Tectonophysics 371, 1–21.

Julià, J., Ammon, C.J., Nyblade, A.A., 2005. Evidence for mafic lower crust in Tanzania, EastAfrica from joint inversion of receiver functions and Rayleigh wave dispersion veloc-ities. Geophys. J. Int. 162, 555–569.

Julià, J., Jagadeesh, S., Rai, S., Owens, T.J., 2009. Deep crustal structure in the Indian shieldfrom joint inversion of P-wave receiver functions and Rayleigh-wave group veloci-ties: implications for Precambrian crustal evolution. J. Geophys. Res. 114, B10313.http://dx.doi.org/10.1029/2008JB006261.

Kachingwe, M., Nyblade, A., Julià, J., 2015. Crustal structure of Precambrian terranes insouthern African subcontinent with implication for secular variation in crustal gene-sis. Geophys. J. Int. 202, 533–547.

Kamguia, J., Manguelle-Dicoum, E., Tabod, C.T., Tadjou, J.M., 2005. Geological models de-duced from gravity data in the Garoua basin, Cameroon. J. Geophys. Eng. 2, 147–152.

Kgaswane, E.M., Nyblade, A.A., Julià, J., Dirks, P.H.G.M., Durrheim, R.J., Pasyanos, M.E.,2009. Shear wave velocity structure of the lower crust in southern Africa: evidencefor compositional heterogeneity within Archaean and Proterozoic terrains.J. Geophys. Res. 114, B12304. http://dx.doi.org/10.1029/2008JB006217.

Langston, C.A., 1979. Structure under Mount Rainier, Washington, inferred fromteleseismic body waves. J. Geophys. Res. 84, 4749–4762.

Laske, G., Masters, G., Ma, Z., Pasyanos, M., 2013. Update on CRUST1.0— a 1-degree globalmodel of the Earth's crust. Geophys. Res. 2568 Abstracts EGU 15.

Ligorría, J.P., Ammon, C.J., 1999. Iterative deconvolution and receiver function estimation.Bull. Seismol. Soc. Am. 89, 1395–1400.

Louis, P., 1978. Gravimetrie et geologie en Afrique occidentale et centrale. Mem. Bur. Rech.Geol. Min. 91, 53–61.

Mathur, S.P., 1974. Crustal structure in southwestern Australia from seismic and gravitydata. Tectonophysics 24, 151–182.

Mooney, W.D., Laske, G., Masters, T.G., 1998. CRUST 5.1: a global crustal model at 5°×5°.J. Geophys. Res. 103 (B1), 727–747.

Nnange, J.M., Ngako, V., Fairhead, J.D., Ebinger, C.J., 2000. Depths to density discontinuitiesbeneath the Adamawa Plateau region, Central Africa, from spectral analyses of newand existing gravity data. J. Afr. Earth Sci. 30 (4), 887–901.

Nyblade, A.A., Pollack, H.N., 1992. A gravity model for the lithosphere in western Kenyaand northeastern Tanzania. Tectonophysics 212, 257–267.

Obaje, N.G., 2009. Geology and Mineral Resources of Nigeria. Springer Verlag, Berlin,p. 221.

Offodile, M.E., 1976. A review of the geology of the Cretaceous of the Benue Valley. In:Kogbe, C.A. (Ed.), Geology of Nigeria. Elizabethan Publishers, Lagos, pp. 319–329.

Ofoegbu, C.O., Okereke, C.S., 1990. Rifting in West and Central Africa. In: Ofoegbu, C.O.(Ed.), The Benue Trough, Structure and Evolution. Vieweg Friedrich and Sohn, Mich-igan, pp. 3–18.

Ogezi, A.E., 1988. Origin and evolution of the basement complex of northwestern Nigeriain the light of new geochemical and geochronological data. In: Oluyide, P.O., Mbonu,W.C., Ogezi, A.E., Egbuniwe, I.G., Ajibade, A.C., Umeji, A.C. (Eds.), Precambrian Geologyof Nigeria. Geological Survey of Nigeria, Kaduna, pp. 301–312.

Okereke, C.S., 1988. Contrasting modes of rifting: the Benue Trough and Cameroon volca-nic line, West Africa. Tectonics 7 (4), 775–784.

PoudjomDjomani, Y.M., Nnange, J.M., Diament, M., Ebinger, C.J., Fairhead, J.D., 1995. Effec-tive elastic thickness and crustal thickness variations in west central Africa inferredfrom gravity data. J. Geophys. Res. 100 (B11), 22047–22070.

Quaah, A.O., 1982. A study of past major earthquakes in Southern Ghana using intensitydata. Tectonophysics 88, 175–188.

Rahaman, M.A., 1988. Recent advances in the study of the basement complex of Nigeria.In: Oluyide, P.O., Mbonu, W.C., Ogezi, A.E., Egbuniwe, I.G., Ajibade, A.C., Umeji, A.C.(Eds.), Precambrian Geology of Nigeria. Geological Survey of Nigeria, Kaduna,pp. 11–44.

Raveloson, A., Nyblade, A., Fishwick, S., Mangongolo, A., Master, S., 2015. The upper man-tle seismic velocity structure of south-central Africa and the seismic architecture ofPrecambrian lithosphere beneath the Congo Basin. In: de Wit, M.J., Guillocheau, F.,de Wit, M.C.J. (Eds.), Geology and Resource Potential of the Congo Basin, Chapter 1.Springer-Verlag, Berlin, pp. 3–18.

Rudnick, R.L., Fountain, D.M., 1995. Nature and composition of the lower crust: a lowercrustal perspective. Rev. Geophys. 33 (3), 267–309.

Rudnick, R.L., Gao, S., 2003. Composition of the continental crust. In: Holland, H.D.,Turekian, K.K. (Eds.), Treatise on Geochemistry, The Crust. Elsevier, Amsterdam,pp. 1–64.

Stuart, G.W., Fairhead, J.D., Dorbath, L., Dorbath, C., 1985. A seismic refraction study of thecrustal structure associated with the Adamawa Plateau and Garoua rift, Cameroon,West Africa. Geophys. J. R. Astron. Soc. 81, 1–12.

Subrahmanyam, C., 1978. On the relation of gravity anomalies to geotectonics of the Pre-cambrian terrains of the south Indian shield. J. Geol. Soc. India 19, 251–263.

Tedla, G.E., van der Meijde, M., Nyblade, A.A., van der Meer, F.D., 2011. A crustal thicknessmap of Africa derived from a global gravity field model using Euler deconvolution.Geophys. J. Int. 187, 1–9.

Tesha, A.L., Nyblade, A.A., Keller, G.R., Doser, D.I., 1997. Rift localization in suture-thickened crust: evidence from Bouguer gravity anomalies in northeasternTanzania. Tectonophysics 287, 315–328.

Tokam, K.A.P., Tabod, C.T., Nyblade, A.A., Julià, J., Wiens, D.A., Pasyanos, M.E., 2010. Struc-ture of the crust beneath Cameroon, West Africa from the joint inversion of Rayleighwave group velocities and receiver functions. Geophys. J. Int. 183, 1061–1076.

Tugume, F., Nyblade, A., Julià, J., 2012. Moho depth and Poisson's ratios of the Precambriancrust in East Africa: evidence for similarities in Archean and Proterozoic crustal struc-ture. Earth Planet. Sci. Lett. 355, 73–81. http://dx.doi.org/10.1016/j.epsi.2012.08.041.

Tugume, F., Nyblade, A., Julià, J., van der Meijde, M., 2013. Precambrian crustal structure inAfrica and Arabia: evidence lacking for secular variation. Tectonophysics 609,250–266.

Vessel, P., Smith, W.H.F., 1998. New improved version of generic mapping tools released.EOS Trans. AGU 79 (47), 579.

Wellmann, P., 1978. Gravity evidence for abrupt changes in the mean crustal density atthe junction of Australian crustal blocks. Aust. Bur. Miner. Resour. Geol. Geophys.Bull. 3, 153–162.

Wright, J.B., Hastings, D.A., Jones, W.B., Williams, H.R., 1985. Geology and MineralResources of West Africa. George Allen and Unwin, London, p. 187.

Zandt, G., Myers, S.C., Wallace, T.C., 1995. Crust and mantle structure across the Basin andRange-Colorado Plateau at 37° N latitude and implication for Cenozoic extensionalmechanism. J. Geophys. Res. 100, 10529–10548.

Zhu, L., Kanamori, H., 2000. Moho depth variation in Southern California. J. Geophys. Res.105, 2969–2980.

















http://dx.doi.org/10.1029/2004JB003065

http://dx.doi.org/10.1029/2004JB003065
















http://dx.doi.org/10.1016/03019268(83)90021-9

http://dx.doi.org/10.1016/03019268(83)90021-9





















http://dx.doi.org/10.1029/2008JB006261






http://dx.doi.org/10.1029/2008JB006217

































































http://dx.doi.org/10.1016/j.epsi.2012.08.041

















tectonophysics - africaarrayafricaarray.psu.edu/publications/pdfs/akpan_et_al... · cretaceous to...

Documents