Precambrian Crustal Structure in Africa and Arabia: Evidence Lacking for Secular Variation

TECTO-125880; No of Pages 17 Tectonophysics xxx (2013) xxx–xxx

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Review Article Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation

Fred Tugume a,b, Andrew Nyblade a,c,⁎, Jordi Julià d, Mark van der Meijde e a Department of Geosciences, Penn State University, University Park, PA 16802, USA b Geological Survey Department, Ministry of Mines, Entebbe, Uganda c School of Geosciences, The University of the Witwatersrand, Johannesburg, South Africa d Departamento de Geofísica & Programa de Pós-Graduação em Geodinâmica e Geofísica, Universidade Federal do Rio Grande do Norte, Natal, Brazil e University of Twente, Faculty of Geo-Information Sciences and Earth Observations (ITC), Enschede, The Netherlands article info abstract

Article history: We review the thickness and shear wave velocity structure of Precambrian crust in Africa and Arabia, where Received 6 May 2012 over 90% of the landmass is comprised of Archean and Proterozoic terranes, and examine the data for Received in revised form 11 April 2013 evidence of secular variation. The data come from many published 1D shear wave velocity profiles obtained Accepted 22 April 2013 by jointly inverting receiver functions and surface wave dispersion measurements, 35 new 1D shear wave Available online xxxx velocity profiles for locations in eastern Africa, and a new map of crustal thickness for Africa and Arabia derived from modeling satellite gravity data. We find for both Archean and Proterozoic terranes a similar Keywords: – – Africa range of crustal thicknesses (~33 45 km), similar mean crustal shear wave velocities (~3.6 3.7 km/s), and Arabia similar amounts of heterogeneity in lower crustal structure, as reflected in the thickness of lowermost Crust crust with shear wave velocities ≥4.0 km/s. There is little evidence for secular variation in crustal structure, Moho indicating that there may have been few changes over much of Earth's history in the processes that form the Archean continental crust. Post-formation tectonic events also may have modified many of the terranes to such an Proterozoic extent that secular trends arising from crustal genesis may be difficult to recognize. © 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction ...... 0 2. Geologic background ...... 0 2.1. Precambrian tectonic framework and crustal structure of the southern region ...... 0 2.2. Precambrian tectonic framework and crustal structure of the central region ...... 0 2.3. Precambrian tectonic framework and crustal structure of the northeastern region ...... 0 3. New shear wave velocity models for eastern Africa ...... 0 3.1. Data ...... 0 3.2. Receiver functions ...... 0 3.3. Joint inversion of receiver functions and Rayleigh wave group and phase velocities ...... 0 3.4. Results ...... 0 4. Comparison of crustal shear wave velocities ...... 0 4.1. Archean terranes ...... 0 4.2. Archean/Paleoproterozoic terranes ...... 0 4.3. Mesoproterozoic terranes ...... 0 4.4. Neoproterozoic terranes ...... 0 5. Gravity modeling of crustal thickness ...... 0 5.1. Satellite gravity data ...... 0 5.2. 3D gravity inversion method ...... 0 5.3. Gravity model benchmarking ...... 0 5.4. Crustal thickness estimates ...... 0 5.5. Comparison with other maps of crustal thickness ...... 0

⁎ Corresponding author at: Department of Geosciences, Penn State University, University Park, PA 16802, USA. Fax: +1 814 863 7823. E-mail address: [email protected] (A. Nyblade).

6. Discussion ...... 0 7. Summary and conclusions ...... 0 Acknowledgments ...... 0 References ...... 0

1. Introduction northwestern (West African), and northeastern (Arabian-Nubian Shield) (Goodwin, 1996). We briefly summarize the Precambrian geolo- Secular variation in Precambrian crustal structure has long gy (terranes and sub-terranes) of the regions for which crustal shear been debated and is important for understanding the genesis and wave velocity profiles are available (Table 1), as well as review results evolution of continental crust because most continental crust from previous work on seismic imaging of crustal structure. We refer worldwide formed during the Precambrian (Goodwin, 1996). the reader to Begg et al. (2009) for a review of the geology of the other Is Archean crust thinner and less mafic than Proterozoic crust, for regions. example, as suggested by Durrheim and Mooney (1991, 1994), reflecting a change in mantle temperature and/or the style of plate tectonics through time, or does Archean and Proterozoic 2.1. Precambrian tectonic framework and crustal structure of the crust have similar thickness and composition, suggesting that tec- southern region tonic processes affecting crustal genesis and evolution have not changed significantly during the Precambrian (e.g., Rudnick and At the core of the Precambrian framework of southern Africa is the Fountain, 1995)? Archean Kaapvaal and Zimbabwe Cratons sutured together by the In this study, we address that question by examining the shear Archean and Paleoproterozoic Limpopo Belt (Fig. 1). To the west of wave velocity structure and thickness of Precambrian crust in Africa the Zimbabwe Craton lies the Paleoproterozoic Okwa-Magondi Belt, and Arabia obtained from seismic and gravity models. Point estimates and to the south and southwest of the Kaapvaal Craton lies the – of crustal thickness and velocity structure are taken from 1D shear Mesoproterozoic Namaqua Natal Belt and the Kheis Province (de Wit wave velocity models constructed by jointly inverting receiver func- et al., 1992)(Fig. 1). tions and surface wave dispersion measurements. Using velocity The Kaapvaal Craton is an Archean granite-greenstone terrane that models constructed with the same inversion method applied to sim- formed between 3.7 to 2.7 Ga (deWitetal.,1992). It can be divided ilar kinds of data (i.e., receiver functions and surface wave dispersion) into several sub-terranes based on the age distribution of outcropping makes it simpler to determine the variability, or the lack thereof, in rocks and major structural boundaries. The major sub-terranes include – – crustal structure between terranes. To increase the number of the Kimberly (3.0 2.8 Ga), Pietersburg (3.0 2.8 Ga), Witwatersrand – – terranes for which 1D shear wave velocity models are available, we (3.6 3.1 Ga), and Swaziland (3.6 3.1 Ga) blocks. The Tokwe terrane also present new velocity models for 35 locations in eastern Africa. forms the core of the Zimbabwe Craton and consists of granite- The number of Precambrian terranes for which 1D shear wave veloc- greenstone belts that formed between 3.6 and 2.5 Ga (Dirks and ity models are available far exceeds the number of terranes for which Jelsma, 2002). The Limpopo Belt consists of highly metamorphosed other kinds of velocity models are available, in particular P wave granite-greenstone and granulite terranes that underwent a series of models derived from seismic refraction profiles. orogenic events between 2.0 and 3.0 Ga during the collision of the For examining Precambrian crustal structure in regions where Kaapvaal and Zimbabwe Cratons (Krammers et al., 2006; McCourt and there are no seismic constraints, we use a model of crustal thickness Armstrong, 1998). derived from satellite gravity data benchmarked against many (377) The Magondi Belt formed between 2.0 and 1.8 Ga and is dominat- receiver-function estimates of crustal thickness. Combining results ed by the passive margin shelf sediments of the Magondi supergroup from both methods (seismic and gravity) enables us to examine in thrust eastwards onto the Zimbabwe Craton during the Magondi greater detail than previously possible similarities and differences Orogeny (McCourt et al., 2001). In the Okwa Belt, which formed between Archean and Proterozoic crustal structure over large parts about 2.05 Ga, rocks correlate to the Magondi Belt suggesting a of Africa and Arabia. continuous northeast trending orogenic belt. – The results of this study, which show no obvious secular trends in The Namaqua Natal Belt is comprised of igneous and supracrustal Precambrian crustal structure, lend support to previous studies argu- rocks accreted against the Kaapvaal Craton during the Namaqua – ing that few differences exist, if any, between Archean and Proterozo- Orogeny (1.2 1.0 Ga) (Cornell et al., 2006). The Kheis Province – ic crust. This study also serves as a review of Precambrian crustal separates the Kaapvaal Craton from the Namaqua Natal Belt and structure in Africa and Arabia, which represents 29% of Precambrian is comprised of siliciclastic rocks of the Olifantshoek supergroup – crust globally (Goodwin, 1996). For clarity, we refer to “crustal thick- (1.2 1.0 Ga). ness” in this paper as the total thickness of the crust from the surface Early studies of the crust in southern Africa mainly used seismic to the Moho, and we use the term “Moho depth” to indicate the recordings of mine tremors associated with gold mining activity in distance from sea level to the Moho. the Witwatersrand basin (e.g., Gane et al., 1949, 1956; Hales and Sacks, 1959; Willmore et al., 1952). Hales and Sacks, 1959 describe a two-layered crust in the eastern Kaapvaal Craton with a crustal 2. Geologic background thickness of 37 km and a ~24 km thick upper crustal layer with P and S wave velocities of 6.0 and 3.6 km/s, respectively. They also The Precambrian terranes in Africa and Arabia are diverse, includ- found a lower crustal layer ~13 km thick with P and S wave velocities ing a number of Archean cratons of various size and numerous of 7.0 and 4.0 km/s, respectively. In an early surface wave study, Paleoproterozoic, Mesoproterozoic, and Neoproterozoic mobile belts. Bloch et al. (1969) inverted Rayleigh and Love wave group and Several interior basins hosting Precambrian through Cenozoic sediments phase velocities from regional earthquakes and obtained a Poisson's cover parts of many of these terranes. The Neoproterozoic Pan-African ratio of 0.28 for the lower crust in the northern Kaapvaal Craton orogenic system is extensively developed across most of Africa and and a crustal thickness in the range of 40–45 km. The first seismic re- Arabia and separates the Precambrian framework into five regions, fraction studies in and around the Witwatersrand basin yielded a southern (Kalahari), central (Congo), north-central (East Saharan), crustal thickness of 35 km and lower crustal P wave velocities in the

Table 1 Summary of crustal structure for Precambrian terranes for which 1D shear wave velocity models are available.

Age Region Terrane Sub-terrane Average crustal Number of Average Vs Average Vs Average thickness thickness ± standard stations of the crust below 20 km of layers with deviation (km) (km/s) depth (km/s) Vs ≥ 4.0 km/s

Archean Southern Africa Kaapvaal Craton Witwatersrand 37 ± 2 10 3.7 3.9 7 Swaziland 39 ± 2 2 3.7 4.0 14 Pietersburg 39 ± 5 2 3.6 4.0 12 Kimberley 36 ± 2 19 3.7 3.8 2 Zimbabwe Craton Western part of 36 ± 1 3 3.6 3.9 4 the Tokwe terrane eastern part of 36 ± 1 3 3.6 4.0 12 the Tokwe terrane Eeastern Africa Tanzania Craton Dodoman 38 ± 2 7 3.7 3.9 3 Nyanzian 38 ± 1 4 3.7 3.9 4 Western Africa Congo Craton Ntem 45 ± 2 5 3.9 4.1 23 Archean Paleoproterozoic Southern Africa Limpopo Belt Southern Marginal 40 1 3.7 4.0 15 Zone Central Zone 44 ± 3 9 3.7 4.0 11 Northern Marginal 38 1 3.7 4.1 15 Zone Paleoproterozoic Southern Africa Kheis Province 39 ± 3 6 3.7 3.9 7 Okwa Province 43 1 3.7 3.9 13 Eastern Africa Ubendiain Belt 43 ± 5 10 3.7 3.9 4 Usagaran Belt 37 ± 3 7 3.6 3.9 6 Mezoproterozoic Southern Africa Namaqua–Natal Belt 33 ± 6 6 3.8 4.1 12 Eastern Africa Kibaran Belt 40 ± 3 8 3.7 3.9 4 Rwenzori Belt 38 ± 2 5 3.7 3.9 2 Neoproterozoic Eastern Africa Mozambique Belt 38 ± 2 13 3.6 3.8 1 Western Africa Oubanguides Belt 39 ± 3 4 3.8 3.9 6 Arabia Asir Belt 39 ± 1 2 3.6 3.9 4 Nabitah Belt 39 ± 1 2 3.6 3.9 4 Aﬁf Belt 39 ± 1 3 3.6 3.9 2 Af-Rayn Belt 43 1 3.6 3.9 4

range of 6.4 to 6.7 km/s (Durrheim and Green, 1992). A similar study Ntem Complex. Similar Moho depths were reported by Gallacher by Green and Durrheim (1990) of the Namaqua–Natal Belt obtained a and Bastow (2012) and Stuart et al. (1985). crustal thickness of 42 km and lower crustal P wave velocities in the range of 6.6 to 6.9 km/s. 2.3. Precambrian tectonic framework and crustal structure of the More recently, crustal structure in southern Africa has been investi- northeastern region gated using data from the Southern African Seismic Experiment (SASE) (Carlson et al., 1996). A compilation of results from Harvey et al. (2001), The Archean Tanzania Craton lies at the center of Precambrian Nguuri et al. (2001), Stankiewicz et al. (2002), Niu and James (2002), framework of eastern Africa and is surrounded by several Proterozoic James et al. (2003), Kwadiba et al. (2003), Wright et al. (2003), Nair et mobile belts (Cahen et al., 1984)(Fig. 1). To the northwest and north al. (2006) and Kgaswane et al. (2009) show crustal thicknesses of of the craton are the Mesoproterozoic Kibaran and Rwenzori Belts, re- 35–45 km and 34–37 km, respectively, for the Kaapvaal and Zimbabwe spectively. To the southeast and southwest are the Paleoproterozoic cratons. These studies also reported Moho depths for the Kheis Usagaran and Ubendian Belts, respectively, and to the east of the cra- Province, Limpopo Belt, Okwa Belt, and Namaqua–Natal Belt of 40 km, ton is the Neoproterozoic Mozambique Belt. Precambrian crustal 37–55 km, 40–45 km, and 40–50 km, respectively. structure in the horn of Africa is not described because in most of the areas for which there are seismic constraints the Precambrian 2.2. Precambrian tectonic framework and crustal structure of the terranes are covered with flood basalts. The age of the basement central region rocks is uncertain and crustal structure may have been modified by the Cenozoic volcanic activity. The Precambrian tectonic framework of central Africa has been The Tanzania Craton can be divided into the Dodoman terrane in studied by several investigators (e.g., Deruelle et al., 1991; Fairhead the southern part of the craton, consisting of granodiorites, granitic and Okereke, 1987; Hedberg, 1968; Tchameni et al., 2001). The only gneisses, migmatites, and other associated high-grade metamorphic available estimates of crustal shear wave velocities come from Camer- rocks, and the Nyanzian terrane in the northern part, consisting of oon. In Cameroon, the Precambrian terranes include the Oubanguides greenstone belts and granites (Cahen et al., 1984; Manya and Maboko, Belt and the Congo Craton (Fig. 1)(Nzenti et al., 1988; Toteu et al., 2003; Schulter, 1997). To the west of the Tanzania Craton is the Kibaran 1987). The Congo Craton occupies a large part of central Africa, and Belt, a linear NE–SW oriented terrane of amphibolite grade rocks its northern edge in southern Cameroon is referred to as the Ntem formed during the Kibaran orogeny which affected large areas of central, Complex. The Ntem Complex is composed of Archean rocks preserved eastern and southern Africa (Klerkx et al., 1987). The Rwenzori Belt is an in greenstone belts and characterized by the intrusion of mafic approximate E–W trending belt, also referred to as the Buganda– doleritic dykes associated with a tectonothermal event ca. 1.8 Ga Toro–Kibaran Belt, consisting of metasedimentary rocks of passive- (Tchameni et al., 2001). To the north of the Congo Craton is the Pan margin affinity and tholeites, folded and cut by 1850 Ma granitoids, African Oubanguides Belt, which includes several mylonitic shear all thrust onto the northern margin of the Tanzania Craton (Begg et zones. Tokam et al. (2010) reported seismic estimates of crustal struc- al., 2009). ture for the central region from jointly inverting receiver functions The Usagaran Belt consists mainly of granitoids and orthogneisses and surface wave group velocities, including crustal thickness esti- that were partially derived from reworking of the Tanzania Craton mates of 35–39 for the Oubanguides Belt and 43 to 48 km for the (Cahen et al., 1984; de Waele et al., 2008; Schulter, 1997). The Ubendian

Arabia (all Proterozoic) i. Af Ryan terrane ii. Afif terrane 30 iii. Nabitah terrane iv. Asir terrane

ii i iv 20 iii

Y X Proterozoic 0 Archean C East Africa G A. Nyanzian terrane C. Rwenzori Belt D A B. Dodoman terrane D. Kibaran Belt E. Ubendian Belt E B F. Usagaran Belt F Latitude -10 G. Mozambique Belt Cameroon X. Congo Craton Y. Oubanguides belt Southern Africa 8 -20 1. Kimberley terrane 8. Okwa terrane 9 7 6a 2. Witwatersrand terrane 9. W. Tokwe 3. Swaziland terrane 5 6b 6c terrane 4 4. Pietersburg terrane 10. Kheis 5. Magondi Belt Province 10 1 6a. Limpopo Belt - 11. Namaqua- 3 N. Marginal Zone Natal Belt 2 -30 6b. Limpopo Belt - 11 Central Zone Elevation (m) 6c. Limpopo Belt - S. Marginal Zone 7. E. Tokwe terrane 0 1000 2000 -40 340 350 0 10 20 30 40 50 Longitude

Fig. 1. Map showing topography and Precambrian terranes in eastern, southern and western Africa and Arabia referred to in this study. Solid black lines show terrane boundaries, and the dotted line shows the outline of the Arabian Shield.

Belt consists of granulite and amphibolite facies gneisses and 1970; Bram and Schmeling, 1975; Griffiths et al., 1971; Hebert and metasedimentary rocks which formed during two orogenic epi- Langston, 1985; Long et al., 1972; Mueller and Bonjer, 1973; Nolet and sodes, the first between 2100 and 2000 Ma and the second around Mueller, 1982), yielding estimates of crustal thickness of 40–48 km 1860 Ma, which also involved the emplacement of numerous gran- beneath unrifted Precambrian crust. More detailed work on crustal itoids and exhumation of granulites and eclogites (Cahen et al., structure in and around the Kenya rift was undertaken by the Kenya 1984; Lenoir et al., 1994; Schulter, 1997). Rift International Seismic Project (KRISP) (Prodehl et al., 1994; The Mozambique Belt extends from Mozambique in the south to Fuchs et al., 1997, and references therein). Away from the rift, they Egypt and Arabia in the north (Schulter, 1997). This belt is believed obtained crustal thicknesses of 34–40 km beneath the Tanzania to represent a Himalayan-type continental collision zone formed by Craton and 35–42 km beneath the Mozambique Belt, and an average multiple collisional events dated between 1200 Ma and 450 Ma Vp for both terranes between 6.4 and 6.6 km/s. (Cahen et al., 1984; Shackleton, 1986). The Mozambique Belt consists More recent investigations of crustal structure in Tanzania (Julià of juvenile terranes and blocks of Archean to Mesoproterozoic conti- et al., 2005; Last et al., 1997), and Kenya (Dugda et al., 2005, 2009) nental crust in the northern part, and in the southern part (i.e., base- using P wave receiver functions and Rayleigh wave dispersion ment of southern Tanzania and northern Mozambique) it consists of measurements have provided additional information about crustal Paleoproterozoic to Neoproterozoic gneisses metamorphosed during structure in the mobile belts and craton. Data used in those studies the Pan-African Orogeny (Begg et al., 2009). The Precambrian crust came from the Tanzania broadband seismic experiment (Nyblade et in Arabia formed between 800 and 620 Ma and is comprised of an al., 1996) and the Kenya broadband seismic experiment (Nyblade amalgamation of several island-arc and backarc terranes (Asir, Afif, and Langston, 2002). For the Tanzania Craton, Last et al. (1997) Nabitah and Af-Ryan). obtained crustal thicknesses of 37–42 km. For the Mozambique Belt, Many previous investigations of crustal structure in East Africa they obtained thicknesses between 36 and 39 km, and for the have focused on the eastern and western branches of the Cenozoic Ubendian Belt they obtained thicknesses between 40 and 45 km. Re- East African rift system and by contrast much less work has been sults similar to those of Last et al. (1997) for Tanzania were obtained done on Precambrian crustal structure away from the rifts. Early stud- by Julià et al. (2005). Dugda et al. (2005, 2009) obtained crustal thick- ies used seismic refraction data and observations from teleseismic nesses of 37–42 km for the Mozambique Belt in Kenya, similar to the and regional earthquakes to examine crustal structure (Bonjer et al., KRISP results. For Uganda, Wolbern et al. (2010) reported crustal

3. New shear wave velocity models for eastern Africa To develop S-velocity models for the selected stations, the receiver functions were jointly inverted with Rayleigh wave group and phase ve- In this section, we present new shear wave velocity models for many locities. Fundamental-mode Rayleigh-wave group velocities between locations in eastern Africa to supplement existing models and allow for a 10 and 50 s period were obtained from an updated model for the one more comprehensive comparison of crustal structure between Precam- presented by Pasyanos and Nyblade (2007), while fundamental-mode brian terranes in eastern Africa and other parts of Africa and Arabia. Rayleigh wave phase velocities for periods between 45 and 143 s were obtained from Adams et al. (2012). 3.1. Data The joint inversion was performed using the method developed by Julià et al. (2000, 2003). We used three groups of receiver functions, The data used for obtaining the new shear wave velocity models in each corresponding to a range of ray parameters from 0.05 to 0.059, eastern Africa were recorded between August 2007 and June 2010 by 0.06 to 0.069 and 0.07 to 0.079. For each group of receiver functions, the AfricaArray East African Broadband Seismic Experiment (AAEASE). we computed and stacked two sets that have overlapping frequency During the first phase of the deployment, twenty stations were installed bands; i.e., a low frequency of f b 0.5 Hz and high frequency of in August 2007 in Uganda and northwestern Tanzania and operated f b 1.25 Hz, corresponding to Gaussian bandwidths of 1.0 and 2.5, re- until December 2008 (Fig. 2). In the second phase of the deployment spectively. Inverting receiver functions at several frequency bands from January 2009 to July 2010, eighteen of the stations were removed helps discriminate sharp discontinuities from gradational transitions from Uganda and northwestern Tanzania and redeployed in southern in the velocity models during the inversion process (Julià, 2007). Tanzania. Station spacing was between 100 and 200 km. Data recorded The starting model used in the joint inversion consisted of an by the permanent AfricaArray and IRIS/GSN stations in the region also isotropic medium with a 37.5 km thick crust and with a linear shear have been used (Fig. 2). wave velocity increase in the crust from 3.4 to 4.0 km/s. The crust overlays a flattened PREM (Preliminary Reference Earth Model) 3.2. Receiver functions model for the mantle (Dziewonski and Anderson, 1981). The model parameterization consisted of constant velocity layers that increase The teleseismic P wave coda contains S waves generated by P-to-S in thickness with depth. Layer thicknesses were 1 and 2 km at conversions at seismic discontinuities in the crust and upper mantle. the top of the model, 2.5 km between 3 and 60.5 km depth, 5 km Receiver functions are time-series containing the P-to-S converted between 60.5 and 260.5 km depth, and 10 km below a depth of and multiple reverberated phases, and are created by deconvolving 260.5 km. Poisson's ratio was fixed at 0.25 for crustal layers and at the vertical component waveforms from the horizontal component PREM values for mantle layers. waveforms to isolate the near-receiver structure from other informa- Even though the velocity model was parameterized down to tion contained in the teleseismic P wave coda (Ammon et al., 1990; 400 km depth, the S-velocity structure was modeled only to a depth Langston, 1979). The deconvolution effectively removes the signature of 290 km. Including deeper structure is necessary to account for of the source and instrument response from the waveforms preserv- the partial sensitivity of long-period dispersion velocities to this ing information on the structure local to the station. part of the velocity model (Julià et al., 2003). To determine the best P wave receiver functions (PRF) were computed for each station velocity to use below 290 km depth, we performed suites of inver- using teleseismic events with magnitudes equal to or greater than sions for several stations, where the structure below 290 km was 5.5, and at epicentral distances between 30° and 90° (Fig. 3). For com- constrained to be equal to PREM and 5% lower than PREM. An exam- puting receiver functions, the velocity waveforms for the selected ple for station DODT is shown in Fig. 4. It is noted that the best fitto events were windowed between 10 s before and 100 s after the lead- the longest periods is achieved for a five percent lower than PREM ing P arrival, de-trended, tapered and high-pass filtered to remove model. For all inversions, velocities below a depth of 290 km were low-frequency noise. The waveforms were then decimated to 10 sam- fixed to 5% lower than PREM. ples per second after low-pass filtering below 8 Hz to avoid aliasing. To illustrate the quality of the fit to the data obtained from the joint The horizontal seismograms were rotated into the great circle path inversions, Fig. 5 shows results from stations BEND and PIGI. Inversion to obtain the corresponding radial and transverse seismograms. The results for the other stations are provided in the supplemental mate- vertical component was deconvolved from both radial and transverse rials. To estimate the uncertainties in our model results, we followed components using an iterative time-domain deconvolution with 500 the approach by Julià et al. (2005) and repeatedly performed inversions iterations (Ligorría and Ammon, 1999). The advantage of performing using a range of weighting parameters, constraints, and Poisson’s ratio. the deconvolution in the time domain is that it has higher stability We found uncertainties in layer velocities of about 0.1 km/s for the with noisy data compared to frequency domain methods. crust and about 0.2 km/s for the upper mantle, which leads to uncer- The quality of the receiver functions was assessed using a least tainties in crustal thickness estimates of +/−2.5 km. squares misfit criterion on the original radial component. This misfit criterion consists of reconstructing the radial waveform for each 3.4. Results event by convolving the receiver function back with the corresponding vertical waveform and comparing it with the originally recorded The results obtained are discussed together with published results radial waveform. PRF's with a fit under 85% were not used in further obtained using the same joint inversion method for other stations in

Please cite this article as: Tugume, F., et al., Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.04.027 6 laect hsatcea:Tgm,F,e l,Peabincutlsrcuei fiaadAai:Eiec akn o eua variation, secular for lacking Evidence Arabia: and Africa in structure crustal Precambrian al., et F., Tugume, as: (2013), article Tectonophysics this cite Please http://dx.doi.org/10.1016/j.tecto.2013.04.027 .Tgm ta./Tcoohsc x 21)xxx (2013) xxx Tectonophysics / al. et Tugume F.

Table 2 Crustal structure of new stations in eastern Africa.

Terrane Station Crustal Thicknessa Terrane averaged crustal Average crustal Terrane averaged Average crustal Vs Terrane averaged Average thickness of Terrane averaged Source Crustal Thicknessb name (km) thickness +/− standard Vs (km/s) crustal Vs (km/s) (km/s) below 20 km Crustal Vs (km/s) crustal layers with Vs thickness of crustal (km) deviation (km) below 20 km ≥4.0 km/s layers with Vs ≥4.0 km/s

Rwenzori Belt BEND 36 38 +/−2 3.7 3.7 3.8 3.9 3 2 1 34.5 MALE 38 3.7 3.9 3 1 38.0 ROTI 41 3.7 3.9 3 1 40.2 SAKA 41 3.8 3.9 3 1 37.0 PIGI 38 3.7 3.8 0 1 36.7 Kibaran Belt BKBA 41 3.8 3.9 5 1 42.4 MLBA 38 3.8 4.0 8 1 44.4 – BIHA 38 3.7 3.9 5 1 39.2 xxx KIBO 41 40 +/−3 3.7 3.7 3.9 3.9 5 4 1 39.1 MKRE 38 3.7 3.8 0 1 39.4 SULU 41 3.7 3.8 0 1 36.7 MBAR 36 3.7 3.9 3 1 33.4 KBLE 43 3.8 3.9 5 1 37.0 Ubendian Belt UVZA 41 3.7 3.8 3 1 39.0 PAND 43 3.7 3.9 5 1 40.9 NAMA 46 3.8 4.0 8 1 47.5 LAEL 43 3.7 3.9 3 1 46.7 LOSS 51 43 +/−4 3.8 4.0 15 1 49.2 TUND 45 3.7 3.9 5 2 – TUND 43 3.7 3.7 3.9 3.9 3 4 1 43.8 MBEY 37 3.6 3.7 0 1 40.1 PNDA 38 3.6 3.8 0 2 35.0 SUMB 46 GM –– – 1 44.6 KGMA 41 3.7 3.8 0 1 39.6 GOMA 45 3.6 3.9 3 2 44.0 Usagaran Belt IRIN 36 3.7 3.9 5 1 39.4 MAFI 36 3.6 3.9 5 1 38.8 NJOM 33 GM –– – 1 32.3 WINO 41 35 +/−2 3.6 3.6 4.0 3.9 15 5 1 39.6 SONG 36 3.6 3.9 3 1 37.0 laect hsatcea:Tgm,F,e l,Peabincutlsrcuei fiaadAai:Eiec akn o eua variation, secular for lacking Evidence Arabia: and Africa in structure crustal Precambrian al., et F., Tugume, as: (2013), article Tectonophysics this cite Please MGOR 33 3.5 3.8 3 1 36.1 MAKA 41 3.6 3.9 0 1 39.1 CHIM ––––1 38.1 MIKU 33 GM –– – 1 37.2 Tanzania Craton HAMA 36 3.7 3.9 0 1 37.0 Nyanzian GEIT 36 3.7 3.9 3 1 36.6 BASO 40 38 +/−1 3.7 3.7 4.0 3.9 10 4 2 41.0 PUGE 38 3.7 3.8 3 2 37.0 JNJA 45 GM –– – 1 44.4 Tanzania Craton DODT 38 3.6 3.9 3 1 40.9

http://dx.doi.org/10.1016/j.tecto.2013.04.027 Dodoman MBWE 38 3.6 3.8 0 2 37.0 MITU 38 3.6 3.8 0 2 38.0 MTAN 35 38 +/−2 3.7 3.7 3.9 3.9 3 3 2 37.0 MTOR 40 3.7 3.9 3 2 38.0 RUNG 40 3.7 3.9 10 2 42.0 SING 40 3.7 3.9 3 2 37.0 URAM 38 GM –– – 2 – Mozambique Belt MAUS 38 3.6 3.8 3 1 39.1 LONG 40 3.6 3.7 3 2 37.0 HALE 38 3.7 3.9 3 2 39.0 KIBA 40 3.7 3.9 5 2 36.0

KIBE 38 3.6 3.9 0 2 37.0 xxx (2013) xxx Tectonophysics / al. et Tugume F. KOMO 38 3.6 3.8 0 2 36.0 KOND 35 3.6 3.8 0 2 37.0 TARA 40 38 +/−2 3.6 3.6 3.8 3.8 3 1 2 37.0 ANGA 38 3.6 3.8 0 3 39.0 KAKA 38 3.6 3.8 0 3 37.0 KITU 35 3.6 3.8 0 3 40.0 KMBO 40 3.6 3.8 3 3 41.0 TALE 38 3.7 3.8 0 3 38.0

GM = gradational Moho. Source: 1 = this paper; 2 = Julià et al. (2005);3=Dugda et al. (2009). a Crustal thickness from joint inversion of receiver functions and surface wave dispersion measurements. b Crustal thickness from H-k stacking of receiver functions (Tugume et al., 2012). – xxx 7 8 F. Tugume et al. / Tectonophysics xxx (2013) xxx–xxx

Fig. 2. Map showing elevation, the tectonic framework of eastern Africa, and the locations of temporary and permanent broadband seismic stations. The squares and diamonds give locations of AfricaArray East Africa broadband seismic experiment (AAEASE) phase I and II stations, respectively. Triangles and hexagons give locations of stations from the 1994 Tanzania broadband seismic experiment (TBSE) and the 2002 Kenya broadband seismic experiment (KBSE), respectively. The circles and inverted triangles represent the permanent AfricaArray (AF) and GSN stations, respectively. Station BUTI, FOPO, KATE and KYLA are in Cenozoic rifts and are not used in this study. eastern Africa (Dugda et al., 2009; Julià et al., 2005)(Fig. 2). Fig. 6 find average shear wave velocities between 3.6 and 3.7 km/s for the shows the new results grouped by tectonic terrane and Table 2 pro- whole crust, between 3.8 and 3.9 km/s for the crust below 20 km vides a summary of the models for all of the stations in eastern Africa. depth, and between 4.4 and 4.6 km/s for the uppermost mantle. Following the approach of Kgaswane et al. (2009) for southern Africa These results are consistent with the estimates of crustal thickness and Tokam et al. (2010) for western Africa, crustal thickness beneath and Poisson's ratio for the same stations reported by Tugume et al. each station was determined by placing the Moho at the depth where (2012). Crustal thickness estimates from Tugume et al. (2012) are the shear wave velocity exceeds 4.3 km/s. Shear wave velocities for given in Table 2, and the crustal Poisson's ratios that they obtained typical lower crustal lithologies obtained from experimentally deter- are 0.25 or 0.26 for all stations, indicating a felsic to intermediate mined Vp/Vs ratios show that in the lower crust they cannot be bulk crustal composition. The similar Poisson's ratio for all of the sta- higher than 4.3 km/s and that shear wave velocities exceeding tions is consistent with the similar mean crustal shear wave velocities 4.3 km/s reflect lithologies typical of the mantle (Christensen, 1996; for all of the stations obtained in this study. Christensen and Mooney, 1995). For many of the stations, there is a significant increase in the velocity at the depth at which the shear ve- 4. Comparison of crustal shear wave velocities locity exceeds 4.3 km/s, except for stations SUMB, MIKU, JNJA, NAMA and NJOM, where the change in shear velocity is gradational from the Including the new results from eastern Africa, there are 18 ter- lowermost crust to the uppermost mantle. ranes and 11 sub-terranes in Africa and Arabia for which there are Across all Precambrian terranes in eastern Africa, the shear wave crustal shear wave velocity profiles obtained using a joint inversion velocity structures are similar (Table 2, Fig. 6). For all terranes, we of receiver functions and surface wave dispersion measurements.

There is considerable variability in the thickness of the lower crustal layers with shear wave velocities ≥4.0 km/s, which ranges from only 4 km for the Ubendian Belt to 13 km for the Okwa Province and 15 km in the Limpopo Belt. The range in average crustal thickness is 37 to 44 km.

4.3. Mesoproterozoic terranes

For the Namaqua–Natal Belt in southern Africa and Kibaran and Rwenzori Belts in eastern Africa, the shear wave velocity for the whole crust is similar (3.7–3.8 km/s), but for the crust below 20 km, the shear velocity in the Namaqua–Natal Belt is 0.2 km/s faster than what is observed in the Rwenzori and Kibaran Belts. A 12 km thick high shear velocity layer (Vs ≥ 4.0 km/s) characterizes the lower crust in the Namaqua–Natal Belt compared to layers of 4 and 2 km thickness observed in the Kibaran and Rwenzori Belts, respectively. The range in average crustal thickness is 33 to 40 km.

4.4. Neoproterozoic terranes

Comparing results for the Neoproterozoic terranes, we find average crustal shear velocities that are 0.2 km/s faster in the Oubanguides Belt than in the other terranes, and that for the crust below 20 km, shear wave velocities in all of the terranes is comparable. On average, a 6 km thick layer with high shear wave velocity (≥4.0 km/s) is observed beneath the Oubanguides Belt compared to a 1 km thick layer for the Mozambique Belt, a 4 km thick layer for the Asir, Nabitah and Ar-Rayn Fig. 3. The distribution of earthquakes used for computing receiver functions plotted terranes, and a 2 km thick layer for the Afif terrane. The range in aver- using an equal distance projection. The large circles give distances of 30, 90, and – 150° from the center of eastern Africa. age crustal thickness is 38 43 km. To summarize, we find that while there is little difference in the mean shear wave velocities for the entire crust across all of the Pre- Geological descriptions of the terranes were provided in Section 2, cambrian terranes (~3.6–3.7 km/s), there exists substantial variabili- and a summary of the crustal structure of each terrane and/or ty in the lower crustal structure. The variability is reflected primarily sub-terrane from published Vs models is given in Table 1. Results in the thickness of the lower crustal layers with shear wave velocities from southern Africa are from Kgaswane et al. (2009), for western ≥4.0 km/s. The variability is found in both Archean and Proterozoic Africa they are from Tokam et al. (2010), and for Arabia they are terranes. We also find that there is little variability in the range from Julià et al. (2003). of crustal thicknesses for each of the age grouping; 36–45 km for Archean, 37 to 44 km for Archean/Paleoproterozoic, 33 to 40 km for 4.1. Archean terranes Mesoproterozoic, and 38–43 km for Neoproterozoic.

The average crustal shear wave velocities (3.7 km/s) for the 5. Gravity modeling of crustal thickness Dodoman and Nyanzian sub-terranes of the Tanzania Craton are similar to the average crustal shear wave velocities (3.6–3.7 km/s) for To examine the thickness of Precambrian crust in regions where Archean terranes and sub-terranes in southern Africa. However, the there are no seismic constraints on Moho depth, we have developed average shear wave velocity of the Ntem Complex in western Africa a map of crustal thickness for Africa and Arabia from modeling satel- is somewhat faster (3.9 km/s). lite gravity data. The map is benchmarked against 377 crustal thick- In contrast to the similarity in average crustal shear wave veloci- ness estimates from receiver functions studies across Africa and ties, considerable variability is observed in the lower crustal veloci- Arabia and provides new information on crustal thickness, in particu- ties. The average shear wave velocity below 20 km depth for the lar for many parts of central, western and northern Africa not previ- Nyanzian, Dodoman, Kimberly, and Witwatersrand terranes, as well ously well studied. as the western part of the Tokwe terrane, is 3.8–3.9 km/s, while for the other Archean terranes (Swaziland, Pietersburg, eastern part of 5.1. Satellite gravity data Tokwe, Ntem, Limpopo Belt) it is 4.0–4.1 km/s. Similarly, there is a substantial difference in the thickness of the lower crust layers with The global gravity model EIGEN-6c (Förste et al., 2011) has been shear wave velocities ≥4.0 km/s between the two groupings of used for constructing our map of crustal thickness. It is a combined terranes. For the former group, the average thickness of layers with model based on data from the GOCE (Gravity ﬁeld and steady-state shear wave velocities ≥4 km/s is ≥7 km, while for the latter group Ocean Circulation Explorer) satellite together with data from previ- it is ≥11 km, reaching a maximum of 23 km for the Ntem Complex. ous satellite gravity missions, radar altimetry data and terrestrial The range in average crustal thickness is 36–45 km. data (see Förste et al., 2011 for details). The Bouguer gravity anomaly derived from the EIGEN-6c model was downloaded, at a grid spacing 4.2. Archean/Paleoproterozoic terranes of 0.1° and at its full resolution, from the International Centre for Global Earth Models (Fig. 7). The Bouguer gravity anomaly is deﬁned For the Archean/Paleoproterozoic terranes, (Limpopo, Ubendian (Barthelmes, 2009) by the classical gravity anomaly, the magnitude of and Usagaran Belts, Kheis and Okwa Provinces) the average shear the gradient of the downward continued potential on the geoid wave velocity structure of the crust is 3.6–3.7 km/s, and the average minus the magnitude of the gradient of the normal potential on the shear wave velocities in the crust below 20 km is 3.9–4.1 km/s. ellipsoid, minus the attraction of the Bouguer slab. The topographic

Please cite this article as: Tugume, F., et al., Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.04.027 10 ie n rdce rdln)gopadpaevlct uvs otmpnl ha aevlct oesotie rmtejitivrin(e ie n h RMserwave shear PREM the and best line) the (red give inversion depth joint km the 290 from (black below observed obtained velocities panel; models Middle PREM velocity functions. than wave receiver less line) shear (red 5% panel; predicted The Bottom and reference. line) curves. for (black velocity observed line) phase panel; (blue and Top model PREM. group than velocity line) less (red 5% and predicted PREM and from line) velocities using depth km 290 below 4. Fig. laect hsatcea:Tgm,F,e l,Peabincutlsrcuei fiaadAai:Eiec akn o eua variation, secular for lacking Evidence Arabia: and Africa in structure crustal Precambrian al., et F., Tugume, as: (2013), article Tectonophysics this cite Please iga o tto OTilsrtn h rcdr sdfrdtriigsrcueblw20k ntejitivrin hw r ifrn oestse o h structure the for tested models different are Shown inversion. joint the in km 290 below structure determining for used procedure the illustrating DODT station for Diagram

Dispersion Velocity (km/s) rayparameter (s/km) 0.055 0.065 0.075 http://dx.doi.org/10.1016/j.tecto.2013.04.027 2.5 3.0 3.5 4.0 4.5 { { { 5051 52 53 35 30 25 20 15 10 5 0 −5 55 51015150 125 100 75 50 25 0

Depth (km) DODT (-5%PREM) 400 300 200 100 0 2468 period(s) time(s) Vs(Km/s) .Tgm ta./Tcoohsc x 21)xxx (2013) xxx Tectonophysics / al. et Tugume F. Synthetic Synthetic Data Data 40 2.5 3.0 3.5 4.0 4.5 400 300 200 100 55 51015150 125 100 75 50 25 0 0 2468 01 02 03 40 35 30 25 20 15 10 5 0 ﬁ – DODT (PREM) otelnetpro hs velocities. phase period longest the to t xxx Vs(Km/s) period(s) time(s) Synthetic Data Synthetic Data F. Tugume et al. / Tectonophysics xxx (2013) xxx–xxx 11

Fig. 5. Results from the joint inversion for stations BEND and ROTI. For each station, the three panels shown are (top) receiver functions, (middle) dispersion curves and (bottom) shear wave velocity models. heights H(λ,φ) are calculated from the spherical harmonic model 5.2. 3D gravity inversion method DTM2006 used up to the same maximum degree as the gravity ﬁeld model. The density contrasts used in the Bouguer correction were The modeling approach applied to obtain crustal thickness esti- 2.67 g/cm3 for rocks and 1.025 g/cm3 for oceans. The data used for mates is based on a Parker–Oldenburg iterative inversion method the inversion for crustal thickness covers both continental and ocean- and is independent of point constraint data (Oldenburg, 1974; ic regions, but only the continental part is shown (Fig. 7). Parker, 1973). The implementation of the method by Gómez-Ortiz

Fig. 6. New shear wave velocity proﬁles for eastern Africa grouped by tectonic terrane. Crustal thicknesses are indicated with horizontal lines and numbers in km. Reference lines at 4.0 km/s (solid) and 4.3 km/s (dashed) are shown in each proﬁle. a) Kibaran Belt. b) Usagaran Belt. c) Ubendian Belt. d) Mozambqiue Belt. e) Rwenzori Belt. f) Nyanzian terrane (Tanzania Craton). g) Dodoman terrane (Tanzania Craton). and Agarwal (2005), as described by van der Meijde et al. (2013), has in the model and should be compensated for or corrected for, a priori. been used here. The inversion results in a simple two-layer model The effect of surface topography has been removed by the Bouguer with Moho topography as the interface, and is based on the assump- correction. A correction also has been applied for sedimentary tion that the entire gravity signal is due to Moho topography. Any basins. For this correction, sediment thickness information has been surface topography or subsurface inhomogeneities are not considered retrieved from a global sediment thickness map on a 1 × 1 degree

thickness, most of them in eastern Africa, southern Africa, Arabia, and a few in western and northern Africa.

5.4. Crustal thickness estimates

Model parameters selected for our final (best) model are 22 km for the initial starting Moho depth and a density contrast across the Moho of 0.45 g/cm3. Topography from ETOPO1 (Amante and Eakins, 2009) has been added to the gravity derived Moho depths to convert from Moho depth to crustal thickness. Our model (Fig. 8) shows variations in crustal thickness from around 30 km in some coastal regions to 45 km in the Ethiopian highlands. In general, the thickest crust, around 40 km, is found in the southern and eastern part of Africa and in southwestern Arabia. In central, western and northern Africa the crust is slightly thinner, around 35 km on average. In Fig. 8 we also show the results of the gravity model in comparison to seismic estimates of crustal thickness. The estimates of crustal thickness from the gravity inversion agree to within 6 km of the seismic estimates at 75% of the locations, of which 62.5% have a misfitofless than ±3 km (Fig. 8 inset), illustrating that there is reasonably good agreement overall between the gravity derived and the seismically constrained estimates of crustal thickness. The average misfit between model and receiver functions is −1.1±5.6km.Thismisfitisusedas a threshold for our comparison. This is further justified by uncertainty Fig. 7. Bouguer corrected gravity anomaly from the EIGEN-6C model. analysis of the two datasets used. Changing input parameters within reasonable bounds, ±0.05 g/cm−3 for density and ±3 km for the starting Moho depth, can lead to variations in modeled Moho depths scale (Laske and Masters, 1997). A density contrast 0f 0.2 g/cm3 (or of 3 km (van der Meijde et al., 2013). Crustal thickness estimates from 200 kg/m3) has been used and assumed constant. Values between receiver functions often come with uncertainties of ±3 km. Combining 0.1 and 0.3 g/cm3, with intervals of 0.05 were tested and didn't result that uncertainty with the sensitivity (~3 km) of the gravity derived in much variation in the final crustal thicknesses. A value of 0.2 g/cm3 estimates to variations in the modeling parameters justifies using a can be considered a good average between different ages of sedi- threshold of 6 km for our comparison. For areas where the crustal thick- ments and different thicknesses of sediments. A depth dependent ness estimates differ by >6 km, the areas typically have a gradational density contrast was considered but with the variety (in thickness Moho, as imaged seismically, or else are in narrow tectonic regions, and age) of basins present in Africa this would require much more such as the rift valleys in eastern Africa and the Gibraltar Straight. detailed information about basin structure than is available. Within the 6 km uncertainty in crustal thickness estimates, there As the inversion is unstable at high frequencies (Gómez-Ortiz and appears to be little variation in crustal thickness across much of Africa Agarwal, 2005), a high-cut filter is included in the inversion proce- and Arabia. As already noted, Fig. 8 shows that the regions with larger dure to ensure convergence of series. The upper boundary was set negative Bouguer anomalies and higher elevation (southern and east- to a wavelength of approximately 200 km. Two parameters have ern Africa, western Arabia) have somewhat thicker crust, about been varied in the inversion; the starting (initial) Moho depth and 3–5 km thicker on average, than other parts of Africa. As this variability the density contrast between the lowermost crust and uppermost falls within our Moho depth uncertainties, it is not necessarily a robust mantle. Starting values have been chosen to reflect realistic values. feature of the model. However, the visual correlation between the areas Initial Moho depth was varied between 20 and 28 km, in line with of thickest crust, highest elevation and largest Bouguer (negative) the average crustal thickness for the region, which contains both oce- anomaly suggests that in our map of crustal thickness (Fig. 8)there anic and continental crust, with steps of 0.5 km. The density contrast could be a long wavelength influence on the crustal thickness estimates at the Moho was varied between 0.25 and 0.50 g/cm3 in steps of from mantle structure. However, due to the long wavelength character 0.05 g/cm3. Final model selection was based on the comparison of of the crustal thickness features this is not possible to remove without estimated crustal thickness with point observations from receiver also removing information on actual crustal thickness. function analysis (see below), as well as the misfit between the For examining Precambrian crustal thickness in areas without seis- input gravity anomaly and the forward modeled gravity anomaly mic constraints on Moho depth, we have overlain the terrane map based on the computed Moho depths. Further details on the data from Begg et al. (2009) on our map of crustal thickness (Fig. 8), and processing can be found in van der Meijde et al. (2013). noted the Archean and Proterozoic terranes. Visual inspection of the map shows crustal thickness of ~33–45 km for these terranes, similar 5.3. Gravity model benchmarking to the range of crustal thicknesses found from comparing the 1D shear wave velocity profiles (Section 4). Because of the similarity in Precam- For benchmarking of the gravity derived Moho depths, a database brian crustal thickness between areas with and without seismic con- of point constraints on crustal thickness has been compiled (see sup- straints, we argue that the conclusion reached previously in Section 4 plemental materials). This database consists of published crustal holds for most of Africa and Arabia. That is, to first order, there are thickness observations from receiver function analysis, including few, if any, significant variations in terrane-averaged estimates of crust- those reported in previous sections of this paper. For a few locations al thickness between the Precambrian terranes in Africa and Arabia. multiple observations of crustal thickness were available. In these cases the observations were assessed qualitatively based on the reli- 5.5. Comparison with other maps of crustal thickness ability or accuracy of the crustal thickness observation. In cases where both above criteria were equal, the most recent result was There are some notable similarities and differences between our map selected. The total database contains 377 point estimates of crustal and the Crust 2.0 model (Laske and Masters, 1997)andagravity-derived

30˚ 26 10 13 1 27 24 28 15˚ 25

11 14 2 21

4 5 diff<-12 km 33 0˚ -12

-15˚ 0˚ 15˚ 30˚ 45˚ 60˚ Longitude (deg)

Fig. 8. Map with gravity derived crustal thickness (km) based on 3D inversion of the EIGEN-6C model. Circles give seismic station locations (see supplemental materials for station information). Green circles indicate stations for which the difference between the receiver function derived and the gravity derived crustal thickness estimates are within ±6 km. Orange and blue circles show stations for which the differences are between 6 and 12 km. Larger deviations are shown by the red and black circles. Solid black lines shows terrane boundaries from Begg et al. (2009), and numbers correspond to speciﬁc terranes. Archean: 1, Reguibat Shield; 2, Man-Leo Shield; 3, Gabon-Cameroon Shield; 4, Bomu-Kibalian Shield; 5 Ugandan Craton; 6, Kasai Shield; 7, Tanzania Craton; 8, Kaapvaal Craton; 9, Zimbabwe Craton. Archean crust with Proterozoic reworking: 10, North of Taoudeni; 11, Man south of Taoudeni; 12, Angolan Shield; 13, Tuareg Block; 14, Benin Nigerian Block; 15, Bangweleu Block. Proterozoic: 16, Kibaran; 17, Irumide and Southern Irumide; 18, Magondi; 19, Kheis; 20, Namaqua–Natal; 21, Obanguides; 22, Mozambique; 23, Congo; 24, Pharusian; 25, Mauritanides; 26, Tindouf; 27, Taudeni; 28, Arabian-Nubian Shield; 29, Damara; 30, Rehoboth; 31, Usagaran; 32, Ubendian; 33, Rwenzori; 34, Saldania. Inset shows a cross-plot of crustal thickness estimates from receiver functions and the gravity derived values. The red lines show +/−6 km and the black lines show +/−3 km.

Fig. 9. Difference maps for the gravity derived crustal thickness and two other crustal thickness maps. Left panel: Difference map with Crust 2.0 model (Laske and Masters, 1997). Right panel: Difference map with Tedla et al. (2011) model.

Please cite this article as: Tugume, F., et al., Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.04.027 F. Tugume et al. / Tectonophysics xxx (2013) xxx–xxx 15 map of crustal thickness from Tedla et al. (2011). The differences for most of the Precambrian terranes in Africa and Arabia is less between our crustal thickness estimates and the Crust 2.0 estimates than 40 km (Table 1), somewhat lower than the mean crustal thick- are substantial (Fig. 9). Significant differences (i.e., >6 km) are found ness of 43 and 44 km reported by Rudnick and Fountain (1995) for throughout parts of Africa and Arabia, most notably in Ethiopia and Archean and Proterozoic terranes, respectively. In the study by across western Africa (Fig. 9). We attribute these differences to the fact Zandt and Ammon (1995) using receiver functions from 114 stations that there are few point constraints on crustal thickness used for Africa distributed across the globe, they found the crustal thickness of in the Crust 2.0 model. shields and platforms to be between 37 and 42 km. Our results are In comparison to the results from Tedla et al. (2011), our map consistent with the findings of Zandt and Ammon (1995), and also shows overall slightly thinner crust by a few kms (Fig. 9). For much with the global compilation of Precambrian crust by Christensen of eastern, southern, and central Africa, in addition to the southwest- and Mooney (1995). ern part of Arabia, the differences between the two models is less Mafic lithologies commonly found in continental crust, such as than 6 km. However, for significant portions of western and northern amphibolites, garnet-bearing and garnet-free mafic granulites, and Africa, the models are discrepant by more than 6 km (Fig. 9). We mafic gneisses, have high shear wave velocities (>3.9 km/s) while attribute these differences in western and northern Africa to an intermediate-to-felsic lithologies have lower shear wave velocities overestimation of the crustal thickness in the model of Tedla et al. (b3.9 km/s). Rudnick and Fountain (1995) and Rudnick and Gao (2011). Tedla et al. (2011) used Euler deconvolution to estimate (2003) argued that high velocity (Vp ~ 7 km/s and Vs ~4.0 km/s) crustal thickness. This is different from the inversion technique layers, irrespective of the age of the crust (i.e., Archean versus Prote- applied in this paper since Euler deconvolution searches for discon- rozoic), indicate the presence of mafic rocks in the lower crust, and tinuous structure in the gravity data by means of spatial and vertical that such layers are common in most Precambrian terranes globally. derivatives. The specific application of this technique by Tedla et al. In contrast, studies by Durrheim and Mooney (1991, 1994) reported (2011) was commented on by Reid et al. (2012). In particular, the large differences in lower crustal composition between Archean and choice of parameters such as grid interval, Euler window size, and Proterozoic terranes. structural index was questioned by Reid et al. (2012). Although we Our results indicate larger variability in the lower crustal structure don't believe that the choice of parameters can explain the differ- of Precambrian terranes of all ages than suggested by either the ences observed, we do believe that the filtering of solutions by Rudnick and Gao (2003) or Durrheim and Mooney (1991, 1994). Tedla et al. (2011) might have caused many of the differences This is illustrated by a plot of crustal thickness versus thicknesses of shown in Fig. 9. The shallowest solutions shown in Tedla et al. the high shear wave velocity layers in the lower crust (Fig. 10). The (2011) are around 33.25 km (see Fig. 4 and Table S1 in Tedla et al., plot illustrates that for Precambrian terranes of all ages there is signif- 2011), indicating that a cut-off of 33.25 km in the Euler solutions icant variability in the thickness of the high velocity lower crust and was used. Our model (Fig. 8) shows crustal thicknesses that are less that there is no apparent correlation with crustal thickness. than the minimum cut-off used in Tedla et al. (2011), therefore The similar bulk crustal compositions, ranges in crustal thickness, resulting in significant differences between the two models. and amounts of lower crustal heterogeneity that characterize Arche- an and Proterozoic terranes in Africa and Arabia suggest few changes 6. Discussion over much of Earth’s history in the processes that form continental crust. An alternative possibility is that similar kinds of tectonic In summary, we find little evidence for variations in crustal thick- processes that can signi ficantly modify the crust have been active ness between Archean and Proterozoic crust, similar to the conclusion throughout the Precambrian. The flow of lower crustal material in reached by Al-Damegh et al. (2005) using results from Arabia, as well orogenic systems (e.g., Costa and Rey, 1995; Rey et al., 2001)or as little evidence for variations in bulk crustal composition, as the foundering of eclogites into the mantle (e.g., Arndt, 1989; reflected in mean crustal shear wave velocity. We also find significant Austrheim, 1991; Jarchow and Thompson, 1989; Zandt et al., 2004), variability in the velocity structure of the lower crust, but it is not sec- are examples of such processes. If many of the terranes included in ular in nature. Similar variability is observed in lower crustal struc- this study have been significantly modified by such processes, then ture within both Archean and Proterozoic terranes. We discuss the that could explain why there is little evidence for secular trends in implications of these findings for understanding crustal genesis and crustal structure reflecting crustal genesis. In other words, if enough evolution by comparing our results to several previous studies that terranes have had sufficient modifications made to their original have examined the structure of Precambrian continental crust globally, as well as regionally. With regard to global studies, our results are somewhat different to those reported by Durrheim and Mooney (1991, 1994), who compiled Moho depth estimates from 18 studies using a variety of seismic methods. They found that average Proterozoic crustal thickness ranges from 40 to 55 km, while Archean crustal thickness ranges from 27 to 40 km. Similar results showing age-dependent variations in crustal thickness in northern Canada were reported by Thompson et al. (2010), who found thinner crust beneath the Archean Rae Domain compared to thicker crust beneath the Paleoproterozoic Trans-Hudson Orogen. We do not find such clear differences in crustal thickness between the Archean and Proterozoic terranes in Africa and Arabia. In comparison, Rudnick and Fountain (1995), using results from 29 seismic refraction investigations, found no significant difference between the thickness of Archean and Proterozoic crust, and in a comprehensive study of crustal structure across Australia, Kennett et al. (2011) concluded that patterns of Moho depth do not show any clear correlation with basement age. Our results are in better Fig. 10. Plot showing average crustal thickness for each terrane versus the thickness of agreement with these studies, however the mean crustal thickness crustal layers with Vs ≥ 4.0 km/s.

Please cite this article as: Tugume, F., et al., Precambrian crustal structure in Africa and Arabia: Evidence lacking for secular variation, Tectonophysics (2013), http://dx.doi.org/10.1016/j.tecto.2013.04.027 16 F. Tugume et al. / Tectonophysics xxx (2013) xxx–xxx crustal structure, then secular variation arising from crustal genesis Barthelmes, F., 2009. Definition of functionals of the geopotential and their calculation from spherical harmonic models. Helmholtz Centre Potsdam, GFZ (http://icgem. may not be apparent. gfz-potsdam.de/ICGEM/theory/str-0902.pdf). It is also possible that secular variation arising from crustal genesis Begg, G.C., Griffin, W.L., Natapov, L.M., O’Reilly, Suzanne Y., Grand, S.P., O’Neill, C.J., may be evident only between unmodified crust that is older and Hronsky, J.M.A., Djomani, Y.P., Swain, C.J., Deen, T., Bowden, P., 2009. 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Springer Verlag, Belin, pp. 274 327. between unmodi ed crust that is older and younger than ~3 Ga, Dirks, P.H.G.M., Jelsma, H.A., 2002. Crust-mantle decoupling and growth of the Archean when plate tectonics characterized by Wilson cycles may have Zimbabwe Craton. Journal of African Earth Science 34, 157–166. began (Shirey and Richardson, 2011). Dugda, M.T., Nyblade, A.A., Julia, J., Langston, C.A., Ammon, C.J., Simiyu, S., 2005. Crustal structure in Ethiopia and Kenya from receiver function analysis: Implications for Supplementary data to this article can be found online at http:// Rift development in eastern Africa. Journal of Geophysical Research 110, B01303. dx.doi.org/10.1016/j.tecto.2013.04.027. http://dx.doi.org/10.1029/2004JB003065. Dugda, M.T., Nyblade, A.A., Julià, J., 2009. S-wave velocity structure of the crust and upper mantle beneath Kenya in comparison to Tanzania and Ethiopia: Implication Acknowledgments or the formation of the East African and Ethiopian plateaus. 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