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Earth and Planetary Science Letters 355-356 (2012) 73–81 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Moho depths and Poisson’s ratios of Precambrian crust in East Africa: Evidence for similarities in Archean and Proterozoic crustal structure Fred Tugume a, Andrew Nyblade a,n, Jordi Julia b a b Department of Geosciences, Penn State University, University Park, PA 16802, USA ~ em Geodinâmica e Geofı́sica, Universidade Federal do Rio Grande do Norte, Natal, Brazil Departamento de Geofı́sica & Programa de Pós-Graduac- ao a r t i c l e i n f o abstract Article history: Received 11 January 2012 Received in revised form 20 August 2012 Accepted 22 August 2012 Editor: P. Shearer We investigate the structure of Precambrian crust in East Africa by using estimates of Moho depth and Poisson’s ratio for 37 new seismic stations in Uganda and Tanzania and for 32 previously deployed seismic stations in Kenya and Tanzania. The dataset includes estimates of crustal structure distributed between all of the major Archean and Proterozoic terrains obtained from modeling P-wave receiver functions. The average crustal thickness for the Ubendian belt is 42 km and between 37 and 39 km for all of the terrains. The average Poisson’s ratio for all of the terrains is either 0.25 or 0.26, indicating a felsic to intermediate bulk crustal composition. A similar composition and thickness of the crust in East Africa for terrains spanning some 4 byr of Earth history is different than in many other Precambrian regions. Our results suggest that there may have been few changes over Earth’s history in the processes that formed the East African crust, or else that processes have been at play to homogenize crustal structure, such as the flow of lower crustal material in orogenic systems or through the foundering of eclogites into the mantle. The finding that crustal structure is similar between the Archean and Proterozoic terrains indicates that crustal structure, through its influence on lithospheric rheology and strength, has not had a first-order effect on the location of rifting. We also find that there is little correlation between Moho depth and elevation across the East African Plateau, indicating that variations in crustal thickness exert few, if any, influences on topography in East Africa. This conclusion lends support to many studies arguing that mantle structure and processes provide the primary buoyant support for the plateau. & 2012 Elsevier B.V. All rights reserved. Keywords: crust Africa tectonics Precambrian 1. Introduction In this paper, we investigate the Precambrian crustal structure of East Africa by using estimates of Moho depth and Poisson’s ratio for 37 new seismic stations in Uganda and Tanzania and for 32 previously deployed seismic stations in Kenya and Tanzania. Advancing our understanding of crustal Poisson’s ratio and Moho depths in East Africa is important because this information provides fundamental constraints on the formation and evolution of the continental crust, most of which formed in the Precambrian (e.g., Goodwin, 1996). 94% of African crust is Precambrian, representing about 29% of Precambrian crust globally (Goodwin, 1996). In comparison to many other continents, little is known about the Precambrian crustal structure of Africa, including East Africa, and consequently African crustal structure is poorly represented in global studies of crustal structure. The results presented in this study not only advance our understanding of East African crustal structure, but also help to address the bias inherent within global studies because of a lack of data on crustal structure from Africa. In addition, the results from this study are important for understanding the role crustal structure may or may not have played in the development of the East African rift valleys. The estimates of Moho depth and Poisson’s ratio for the 37 new stations come from modeling P-wave receiver functions using the H–k stacking method (Zhu and Kanamori, 2000), while the results from the previous studies were obtained by modeling P-wave receiver functions using both the H–k method (Dugda et al., 2005) and a slant-stack method (Last et al., 1997). The dataset, which includes estimates of crustal structure distributed between all of the major Precambrian terrains in East Africa, provides new insights into crustal composition and thickness for many areas of East Africa that have not been affected by Cenozoic rifting and volcanism. 2. Background 2.1. Geological setting n Corresponding author. E-mail address: [email protected] (A. Nyblade). 0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2012.08.041 The Archean Tanzania Craton forms the nucleus of the Precambrian framework of eastern Africa and is surrounded by 74 F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81 that were partially derived from reworking and recycling of the Tanzania Craton (Cahen et al., 1984; Schulter, 1997; De Waele et al., 2008). The Mozambique Belt is the longest terrain, extending along the East African coast from Mozambique in the south to Egypt in the north (Schulter, 1997). This belt is believed to represent a Himalayan-type continental collision zone formed by multiple collisional events dated between 1200 Myr and 450 Myr (Cahen et al., 1984; Shackleton, 1986). The northern part of the Mozambique Belt consists of juvenile terrains and blocks of reworked Archean to Mesoproterozoic continental crust, while the southern part (i.e., basement of southern Tanzania and northern Mozambique) consists of Paleoproterozoic to Neoproterozoic gneisses metamorphosed during the Pan-African Orogeny (Begg et al., 2009). Superimposed on the Precambrian tectonic framework is the Cenozoic East African rift system (EARS). The EARS is a classic example of a continental rift, exhibiting characteristic patterns of rifting, volcanism, and plateau uplift. The development of the rift system has likely been influenced by the Precambrian tectonic framework, with both the eastern and western branches forming in the Proterozoic mobile belts surrounding the Archean Tanzania Craton (Ebinger, 1989; Hetzel and Strecker, 1994; Tesha et al., 1997; Nyblade and Brazier, 2002) (Fig. 1). The eastern branch of the EARS starts at the Afar triple junction, continues southwards through Ethiopia and Kenya, and terminates in northern Tanzania in a wide zone of block faulting. The western branch of the EARS extends from northern Uganda to Mozambique, running along the borders of Uganda, the Democratic Republic of Congo, Rwanda, Burundi, Tanzania, Malawi and Zambia. 2.2. Previous studies of crustal structure Fig. 1. Map showing elevation, the tectonic framework of East Africa, and the locations of temporary and permanent broadband seismic stations. The black squares and diamonds give locations of the AfricaArray East Africa broadband seismic experiment (AAEASE) phase I and II stations, respectively. Black triangles and hexagons give locations of stations from the 1994–1995 Tanzania broadband seismic experiment (TBSE) and the 2001–2002 Kenya broadband seismic experiment (KBSE), respectively. The black circles and inverted triangles represent the permanent AfricaArray (AF) and GSN stations, respectively. several Proterozoic mobile belts (Cahen et al., 1984) (Fig. 1). To the east of the craton is the Neoproterozoic Mozambique Belt, to the southeast and southwest are the Paleoproterozoic Usagaran and Ubendian Belts, respectively, and to the northwest and north of the craton are the Mesoproterozoic Kibaran and Rwenzori Belts, respectively. The northern part of the Tanzania Craton, referred to as the Nyanzian terrain, is comprised of greenstone belts and granites. The Dodoman terrain, located in the southern part of the craton, consists of granodiorites, granitic gneisses, migmatites, and other associated high-grade metamorphic rocks (Cahen et al., 1984; Schulter, 1997; Manya and Maboko, 2003). The Rwenzori Belt is an approximate E–W trending belt, also referred to as the Buganda-Toro–Kibaran Belt, consisting of metasedimentary rocks and tholeites, folded and cut by 1850 Myr granitoids and thrust onto the northern margin of the Tanzania Craton ( Begg et al., 2009). The Kibaran Belt forms a linear NE–SW oriented terrain of amphibolite grade rocks formed during the Kibaran orogeny which affected large areas of central, eastern and southern Africa (Klerkx, 1987). The Ubendian Belt consists of granulite and amphibolite facies gneisses and metasedimentary rocks which formed during two orogenic episodes, the first between 2100 and 2000 Myr and the second around 1860 Myr, which also involved the emplacement of numerous granitoids and exhumation of granulites and eclogites (Cahen et al., 1984; Lenoir et al., 1994; Schulter, 1997). The Usagaran Belt consists mainly of granitoids and orthogneisses Many previous investigations of crustal structure in East Africa have focused on the eastern and western branches of the rift system and by contrast much less work has been done on crustal structure away from the rifts. Early studies used seismic refraction data and observations from teleseismic and regional earthquakes to examine crustal structure (Bonjer et al., 1970; Griffiths et al., 1971; Long et al., 1972; Mueller and Bonjer, 1973; Bram and Schmeling, 1975; Nolet and Mueller, 1982; Hebert and Langston, 1985), yielding estimates of Moho depths of 40–48 km beneath unrifted Precambrian crust, and of 20–32 km under the rift valleys. More detailed work on crustal structure in and around the Kenya rift was undertaken by the Kenya Rift International Seismic Project (KRISP) (Prodehl et al., 1994; Fuchs et al., 1997, and references therein). Away from the rift, they obtained crustal thicknesses of 34–40 km beneath the Tanzania Craton and 35–42 km beneath the Mozambique Belt, and an average Vp for both terrains between 6.4 and 6.6 km/s. More recent investigations of crustal structure in Tanzania (Last et al., 1997; Julia et al., 2005), and Kenya (Dugda et al., 2005, 2009) using P-wave receiver functions and Rayleigh wave dispersion measurements, have provided additional information about crustal structure in the mobile belts and craton. Data used in those studies came from the Tanzania broadband seismic experiment (Nyblade et al., 1996) and the Kenya broadband seismic experiment (Nyblade and Langston, 2002) (Fig. 1). For the Tanzania Craton, Last et al. (1997) obtained Moho depths of 37–42 km and Poisson’s ratios of 0.24–0.26. For the Mozambique Belt, they obtained Moho depths between 36 and 39 km and Poisson’s ratios between 0.24 and 0.27, and for the Ubendian Belt they obtained Moho depths between 40 and 45 km. Results similar to those of Last et al. (1997) for Tanzania were obtained by Julia et al. (2005), and Dugda et al. (2005, 2009) obtained Moho depths of 37–42 km for the Mozambique Belt in Kenya, similar to the KRISP results. For Uganda, Wolbern et al. (2010) reported F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81 75 Moho depths of 20–28 km for the Rwenzori block, a horst within the Albertine rift, and Moho depths of 30 km along the eastern flank of the Albertine rift. 3. Data and methodology 3.1. Data The data used for this study were recorded between August 2007 and June 2010 by the AfricaArray East African Broadband Seismic Experiment (AAEASE), which consisted of portable broadband seismometers installed in Uganda and southern and western Tanzania (Fig. 1). Each station was equipped with a broadband seismometer (Guralp CMG-3T, Guralp CMG3-ESP, or Streckeisen STS-2), a 24-bit Reftek data logger and a GPS (Global Positioning System) clock. Data were recorded continuously at 40 samples per second. During the first phase of the deployment, twenty stations were installed in August 2007 in Uganda and northwestern Tanzania and operated until December 2008 (Fig. 1). In the second phase of the deployment from January 2009 to June 2010, eighteen of the stations were removed from Uganda and northwestern Tanzania and redeployed in southern Tanzania. Station spacing was between 100 and 200 km. Data from the permanent AfricaArray and IRIS/GSN stations in the region were also used (Fig. 1). 3.2. Receiver functions Receiver functions are time series containing secondary P-to-S converted waves generated by the interaction of a teleseismic P-wavefront with seismic discontinuities under the recording station. The main amplitudes after the direct P wave are generally a P-to-S conversion upon refraction across the Moho (Ps) and multiple reverberated phases between the Moho and the freesurface (PpPs and PpSs). Analysis of the S–P travel-times, therefore, can be utilized to infer average crustal properties such as crustal thickness and bulk Vp/Vs ratio (e.g., Zandt et al., 1995; Zhu and Kanamori, 2000). Receiver functions are created by deconvolving the vertical component waveforms from the corresponding radial component waveforms to isolate the receiver site effects from other information contained in the teleseismic P wave coda (Langston, 1979; Ligorria and Ammon, 1999). The deconvolution effectively removes the signature of the source and instrument response from the originally recorded waveforms keeping only information on the structure local to the station. For this study, P-wave receiver functions were computed for each station using teleseismic events with magnitudes equal to or greater than 5.5, and at epicentral distances between 301 and 901 (Fig. 2). A table of events used for computing receiver functions is provided in the Supplementary material. For computing receiver functions, the waveforms were windowed between 10 s before and 100 s after the leading P arrival, de-trended, tapered, high pass filtered above 0.05 Hz to remove the low frequency and instrument noise and decimated to 10 samples per second, after low-pass filtering below 8 Hz to avoid aliasing. The horizontal seismograms were rotated into the greatcircle path to obtain radial and transverse components, and then the vertical component was deconvolved from both radial and transverse components using an iterative time domain deconvolution with 500 iterations (Ligorria and Ammon, 1999). For each teleseismic event, receiver functions were computed using two overlapping frequency bands ( o0.5 Hz and o1.25 Hz) with Gaussian bandwidths of 1.0 and 2.5, respectively. Lower frequency bands imply longer wavelengths and may amplify the signal from the Moho in the presence of gradational crust–mantle Fig. 2. Map showing the distribution of earthquakes used in this study. Black circles and open circles represent earthquake locations used for the computation of receiver functions for phase I and phase II AAEASE stations, respectively, plotted using an equal distance projection. The large circles are plotted at distances of 30 and 90 degrees from the center of the seismic network. boundaries, especially for the multiply reverberated phases (Owens and Zandt 1997; Julia, 2007). The quality of the receiver functions for one station is illustrated in Fig. 3 and additional examples can be found in the Supplementary materials. The receiver functions were evaluated for quality using a least squares misfit criterion to assess the percentage of recovery of the original radial component. The misfit criterion provides a measure of closeness of the receiver function to the ideal case, and it is calculated by using the difference between the radial component seismogram and the convolution of the vertical component seismogram with the already determined radial receiver function. Receiver functions with a fit of 85% and above were considered for further processing. In addition, events with large amplitude tangential receiver functions were not used, even if they passed the 85% threshold. 3.3. H–k stacking method The H–k stacking method was applied to the receiver functions to obtain an estimate of crustal thickness (H) and Vp/Vs (k) ratio under each seismic station (Zhu and Kanamori, 2000). In this method, receiver function amplitudes are stacked along theoretical P-to-S phase moveout curves obtained after assuming an a priori value for P-wave velocity (Vp). An objective function, s(H,k), is constructed from the weighted sum given by sðH,kÞ ¼ N X w1 r j ðt 1 Þ þ w2 r j ðt 2 Þw3 r j ðt 3 Þ ð1Þ j¼1 where ti are the S–P travel-times for the three main P-to-S converP sions at the Moho (Ps, PpPs, and PpSs), wi are weights ð wi ¼ 1Þ assigned to each conversion, rj(ti) is the receiver function amplitude at ti for the j-th receiver function waveform, and N is the number of receiver functions. For a simple layer-over-half space model, s(H,k) reaches a maximum when H coincides with the layer parameters 76 F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81 necessary to observe at least two P-to-S conversions (Zhu and Kanamori, 2000). For these stations, we estimated Moho depth using a Poisson’s ratio of 0.25, which is an average value for most of the Precambrian terrains in East Africa, the Moho Ps arrival time, and Eq. (2) from Zandt et al. (1995). In addition, for stations BUTI, KATE and KYLA, which were installed on rift sediments, the receiver functions were too noisy to identify any Ps phases, and therefore results for these stations are not reported. 4. Results Fig. 3. A plot of radial and tangential receiver functions against ray parameter for station BEND computed using a Gaussian filter of 1.0. The lines shows the theoretical move-out curves calculated for each phase (Ps, PpPs, PsPsþ PpSs) for a 40 km thick crust. (Zhu and Kanamori, 2000), which are then taken as estimates of crustal thickness and Vp/Vs ratio under the station. In order to apply the H–k method, weights must be assigned to each phase in Eq. (1), and an average crustal Vp must be selected. A Vp of 6.5 km s 1, which is consistent with average crustal P wave velocities from previous studies of Precambrian crust in East Africa (Julia et al., 2005; Fuchs et al., 1997 and references therein), and a weighting system of w1 ¼0.6, w2 ¼ 0.3, and w3 ¼0.1 were used. However, at a few stations the PsPsþPpSs phase was not observed, and the weights were re-adjusted so that w3 ¼ 0 (Tugume, 2011). Results are illustrated for one station in Fig. 4, and for the other stations the results are provided in the Supplementary material. Receiver functions computed with a Gaussian filter of 1.0 were used for all stations except SAKA, MLBA, BIHA, NJOM, MGOR, DODT and KGMA, where a Gaussian filter of 2.5 was used. For these stations, the Moho Ps phase and its multiples were more clearly seen on the higher frequency receiver functions. Uncertainties in H and k were obtained using a bootstrap method, which involved repeating the stacking procedure 200 times with a resampled data set selected at random from the original dataset, with replacement (Efron and Tibshirani, 1991). Examples of the uncertainties are shown in Fig. 4, and the uncertainty for each station using a mean crustal Vp of 6.5 km/s for the H–k stacking is given in Table 1. In addition, for assessing uncertainty in H and k resulting from the choice of the mean crustal P wave velocity, the H–k stacks were recomputed using mean crustal P wave velocities of 6.3 km s 1 and 6.7 km s 1 (Table 1). To obtain an overall uncertainty in H and k, the uncertainty obtained from the bootstrap method was combined with the range of H and k values obtained when using different mean crustal P wave velocities. Combining the formal uncertainties with the range of H and k values obtained by varying the mean crustal Vp from 6.3 to 6.7 km/s yields an overall uncertainty in Moho depth for each station that is 3–4 km. Results from this study, together with the results from studies by Last et al. (1997) and Dugda et al. (2005) are summarized in Table 1 and Fig. 5. As mentioned above, the overall uncertainty in Moho depth for each station is 3–4 km. For the Rwenzori Belt (stations BEND, ROTI, MALE, PIGI, and SAKA), crustal thickness varies between 34 and 40 km with an average of 37 km, and Poisson’s ratio varies between 0.24 and 0.27, with an average of 0.25. For the Kibaran Belt (stations BKBA, MLBA, BIHA, KIBO, MKRE, SULU, MBAR and KBLE), the average crustal thickness and Poisson’s ratio is 39 km and 0.25, respectively, but with larger variability than in most of the other terrains. For example, a significantly thinner crust and a higher Poisson’s ratio (0.28) are found beneath station MBAR (33 km), while the thickest crust within this terrain (station MLBA) is 44 km. In the Ubendian Belt (stations UVZA, MBEY, PNDA, NAMA, INZA, GOMA, KGMA, SUMB, PAND, LAEL, LOSS and TUND), the thickest crust anywhere within East Africa is found at stations LOSS (49 km), NAMA (48 km) and LAEL (47 km). On average, the crustal thickness for this terrain is 42 km and Poisson’s ratio is 0.26. Our results for station MBEY, within the reported uncertainties, are consistent with those reported for nearby station PAND by Last et al. (1997). For TUND station, Last et al. (1997) did not report any results and therefore we only show values obtained in this study for that station (Table 1). Stations IRIN, MAFI, MAKA, NJOM, WINO, SONG, MGOR, CHIM and MIKU are located in the Usagaran belt. The crustal thickness in this terrain varies from 32 to 39 km and Poisson’s ratios range from 0.24 to 0.28. The average crustal thickness is 37 km and Poisson’s ratio is 0.26. For the Nyanzian terrain within the Tanzania Craton, we report new results from stations HAMA, GEIT and JNJA and combine them with previous results for stations BASO and PUGE (Last et al., 1997). Crustal thickness ranges from 37 km to 44 km and Poisson’s ratio ranges from 0.25 to 0.28, with average values of crustal thickness and Poisson’s ratio of 39 km and 0.26, respectively. For the Dodoman terrain, we combine results for station DODT (this study) with results for stations MBWE, MITU, MTAN, MTOR, RUNG, and SING (Last et al., 1997). Crustal thickness ranges from 37 to 42 km and Poisson’s ratio ranges from 0.25 to 0.26, with average values of crustal thickness and Poisson’s ratios of 39 km and 0.26, respectively. In the Mozambique Belt, we combine results from station MAUS with results from Last et al. (1997) and Dugda et al. (2005) for stations HALE, KIBA, KIBE, KOMO, KOND, LONG, TARA, ANGA, KAKA, KITU, KMBO, KR42, NAI and TALE. Crustal thickness varies between 36 and 42 km, with a mean Moho depth of 38 km. Poisson’s ratio ranges from 0.24 to 0.27, with a mean value of 0.25. 3.4. Special cases 5. Discussion For stations SUMB, MIKU, FOPO, CHIM and JNJA reverberations from the Moho could not be easily identified on either lower or higher frequency receiver functions. In order to constrain crustal thickness and Vp/Vs ratio with the H–k stacking method it is The main finding of this study is that crustal structure is similar across all of the Precambrian terrains in East Africa. Within the uncertainties of our estimates, there is no discernable difference in F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81 77 Fig. 4. H–k stack of receiver functions for station BEND. To the left of each receiver function the top number gives the event backazimuth and the bottom number gives the event distance in degrees. Contours map out percentage values of the objective function given in the text. the crustal thicknesses or Poisson’s ratios between the Archean and Proterozoic terrains, or within the Proterozoic terrains themselves. The average crustal thickness for all but one of the terrains is between 37 and 39 km. The exception is in the Ubendian terrain, which has an average crustal thickness of 42 km. In all of the terrains, the average Poisson’s ratio is similar, either 0.25 or 0.26. This is in contrast to other areas in Africa, as well as within other continents, where the Precambrian crustal structure on a regional basis exhibits larger variability (i.e., Nair et al. (2006) and Kgaswane et al. (2009) for southern Africa; Tokam et al. (2010) for west Africa; Chevrot and van der Hilst (2000) and Kennett et al. (2012) for Australia; Chen et al. (2010) for China; and Thompson et al. (2010) for Canada). Using Poisson’s ratio as a proxy for bulk crustal composition, the average Poisson’s ratio of 0.25 to 0.26 indicates a felsic to intermediate composition for the crust. For example, granitic rocks have a Poisson’s ratio of 0.24, while intermediate composition rocks (e.g., diorite) have values around 0.27, and mafic rocks (e.g., Gabbro) have values around 0.30 (Christensen, 1996; Tokav and Vavakin, 1982). Poisson’s ratios of 0.28 or 0.29 are found at stations SULU, MBAR, UVZA, PNDA, KGMA, MAKA, WINO and HAMA (Table 1). However, when the uncertainties in these estimates are considered, they are not necessarily anomalous with respect to the average Poisson’s ratio of 0.25–0.26 (Table 1) for the region. Although there is much similarity in the average crustal structure between terrains, there are a few stations where the crustal thickness departs from the 37–39 km average. Thinner crust is observed beneath stations BEND (35 km) and MBAR (33 km). Studies by Dugda et al. (2005) and Wolbern et al. (2010) obtained Moho depths of 33 and 30 km, respectively, for station MBAR, consistent with our results. As noted by Dugda et al. (2005), there are no obvious tectonic explanations as to why the crust is thinner than average at station MBAR, nor is there an obvious explanation for the thinner than average crust at station BEND. Thicker than average crust is observed beneath stations NAMA, LAEL, LOSS and SUMB, which are all located within the Ubendian Belt. We suggest that the thickened crust beneath these stations resulted from the collision of the Tanzania Craton with the Bangweulu Block during the Ubendian Orogeny ( Lenoir et al., 1994; Begg et al., 2009). A summary of Moho depths and Poisson’s ratio for Precambrian terrains in East Africa as well as globally is given in Table 2. In comparison to a global investigation of crustal structure by Durrheim and Mooney (1991, 1994), who compiled Moho depth estimates from 18 studies using a variety of seismic methods, we find somewhat different results. Durrheim and Mooney (1991, 1994) concluded that average Proterozoic crustal thickness ranges from 40 to 55 km, while Archean crustal thickness ranges from 27 to 40 km. We do not find such differences in the crustal thickness between the Archean and Proterozoic terrains in East Africa. 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. Our results concur in this respect with the findings of Rudnick and Fountain (1995), however the mean crustal thickness for all but one of the Precambrian terrains in East Africa (37–39 km) is somewhat less than the mean crustal thickness of 43 and 44 km reported by Rudnick and Fountain (1995) for Archean and Proterozoic terrains, respectively. In the study by Zandt and Ammon (1995) using receiver functions from 114 stations distributed across the globe, they found the crustal thickness of shields and platforms to be between 37 and 42 km and Poisson’s ratios for the bulk crust to be 0.27. Our results are consistent with the findings of Zandt and Ammon (1995), and also with the global compilation of Precambrian crust by Christensen and Mooney (1995). The finding that on average the Precambrian crust in all of the terrains within East Africa is similar has important implications for understanding processes of crustal formation and evolution, as 78 F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81 Table 1 Results from H–k stacking of receiver functions. Tectonic region Station name N rf Lat. (deg.) Long. (deg.) Elev. (m) H (1) (km) k (1) Poisson’s ratio H, k (2) (km) H, k (3) (km) M S Rwenzori Belt BEND MALE ROTI SAKA PIGI Average 1 1 1 1 1 40 35 40 19 28 0.588 1.070 1.626 0.315 0.231 31.392 34.167 33.600 31.730 32.319 1351 1127 1108 1263 1252 34.5 þ / 0.6 38.0 þ / 0.5 40.2 þ / 1.3 37.0 þ / 1.6 36.7 þ / 0.7 37 1.73 þ / 0.02 1.70 þ/ 0.02 1.76 þ / 0.04 1.73 þ / 0.06 1.77 þ / 0.03 1.74 0.25 þ/ 0.01 0.24 þ/ 0.01 0.26 þ/ 0.02 0.25 þ/ 0.02 0.27 þ/ 0.01 0.25 36.4, 39.3, 41.8, 38.8, 38.2, 1.71 1.69 1.75 1.72 1.76 33.8, 36.5, 38.9, 36.5, 35.4, 1.73 1.71 1.76 1.71 1.78 A A A A A 1 1 1 1 1 Kibaran Belt BKBA MLBA BIHA KIBO MKRE SULU MBAR KBLE Average 1 1 1 1 1 1 4 1 19 17 28 31 32 29 – 21 1.364 1.838 2.638 3.583 4.282 4.573 0.602 1.254 31.812 31.670 31.316 30.713 30.424 30.087 30.738 29.992 1274 1237 1459 1485 1175 1359 1390 1879 42.4 þ / 0.9 44.4 þ / 1.3 39.2 þ / 0.6 39.1 þ / 0.6 39.4 þ / 0.3 36.7 þ / 0.7 33.4 þ / 0.8 37.0 þ / 1.2 39 1.70 þ/ 0.02 1.70 þ/ 0.03 1.70 þ/ 0.02 1.72 þ / 0.03 1.75 þ / 0.01 1.84 þ / 0.02 1.82 þ / 0.02 1.75 þ / 0.03 1.75 0.24 þ/ 0.01 0.24 þ/ 0.01 0.24 þ/ 0.01 0.24 þ/ 0.01 0.26 0.29 þ/ 0.01 0.28 þ/ 0.01 0.26 þ/ 0.01 0.25 43.9, 46.1, 40.6, 40.5, 40.5, 38.2, 34.4, 38.6, 1.69 1.69 1.70 1.72 1.75 1.83 1.81 1.73 40.8, 42.9, 37.9, 37.7, 37.8, 36.9, 31.8, 35.9, 1.71 1.71 1.70 1.73 1.76 1.83 1.83 1.75 A A A A A A A A 1 1 1 1 1 1 1 1 Ubendian Belt UVZA INZA PNDA NAMA LAEL LOSS TUND MBEY PAND GOMA KGMA SUMB Average 1 2 1 1 1 1 1 5 2 2 1 1 16 – 26 20 11 12 7 – – – 19 – 5.104 5.117 6.352 7.510 8.566 8.417 9.295 9.000 8.983 4.839 4.878 7.953 30.393 30.399 31.061 31.041 32.059 33.158 32.771 33.250 33.242 29.693 29.633 31.620 992 975 1071 1559 1596 1195 1697 1331 1248 880 821 1837 39.0 þ / 0.9 42 40.9 þ / 1.3 47.5 þ / 2.1 46.7 þ / 1.1 49.2 þ / 2.0 43.8 þ / 1.9 40.1 þ / 3.6 35 44 39.6 þ / 1.7 44.6 42 1.80 þ/ 0.04 – 1.82 þ / 0.05 1.70 þ/ 0.05 1.72 þ / 0.03 1.79 þ / 0.03 1.77 þ / 0.05 1.69 þ / 0.07 1.72 1.74 1.81 þ / 0.17 – 1.76 0.28 þ/ 0.02 – 0.28 þ/ 0.02 0.24 þ/ 0.02 0.24 þ/ 0.01 0.27 þ/ 0.01 0.27 þ/ .02 0.23 þ/ 0.03 0.24 0.25 0.28 þ/ 0.06 – 0.26 40.4, 1.79 – 42.3, 1.82 48.9, 1.69 48.9, 1.71 51.1, 1.78 45.4, 1.76 – – – 41.0 1.8 – 37.6, 1.81 – 39.4, 1.83 45.6, 1.70 45.6, 1.72 47.5, 1.80 42.2, 1.78 – – – 38.1 1.82 – A C A A A A A A B B A C 1 2 1 1 1 1 1 1 2 2 1 1 Usagaran Belt IRIN MAFI MAKA NJOM WINO SONG MGOR MIKU CHIM Average 1 1 1 1 1 1 1 1 1 17 20 7 9 12 17 10 35.686 35.313 34.830 34.791 35.300 35.651 37.670 36.990 34.028 1561 1866 1685 1949 1511 1119 501 518 1100 39.4 þ / 1.4 38.8 þ / 1.4 39.1 þ / 2.3 32.3 þ / 1.3 39.6 þ / 2.5 37.0 þ / .8 36.1 þ / 2.4 37.2 38.1 37 1.74 þ / 0.04 1.74 þ / 0.04 1.80 þ/ 0.07 1.73 þ / 0.05 1.82 þ / 0.09 1.71 þ / 0.04 1.76 þ / 0.09 – – 1.76 0.25 þ/ 0.02 0.25 þ/ 0.02 0.28 þ/ 0.03 0.25 þ/ 0.02 0.28 þ/ 0.03 0.24 þ/ 0.02 0.26 þ/ 0.03 – – 0.26 41.0, 40.1, 40.8, 36.6, 41.0, 38.5, 37.7, – – 38.1, 37.3, 37.9, 31.2, 38.1, 35.5, 35.1, – – 1.74 1.74 1.79 1.73 1.83 1.72 1.76 – 7.762 8.306 8.847 9.367 9.757 10.670 6.828 7.400 8.826 A A A A A A A C C 1 1 1 1 1 1 1 1 1 Tanzania Craton (Nyanzian) HAMA GEIT BASO PUGE JNJA Average 1 1 2 2 1 6 13 – – – 3.832 2.881 4.324 4.175 0.446 32.642 32.217 35.138 33.184 33.182 1227 1281 1694 1350 1133 37.0 þ / 1.3 36.6 þ / 2.1 41 þ/ 4 37 þ/ 4 44.4 39 1.82 þ / 0.06 1.73 þ / 0.07 – 1.76 – 1.77 0.28 þ/ 0.02 0.25 þ/ 0.03 – 0.26 – 0.26 38.5, 1.80 38.7, 1.71 – – – 35.9, 1.81 36.0, 1.73 – – – A A B B C 1 1 2 2 1 Tanzania Craton (Dodoman) DODT MBWE MITU MTAN MTOR RUNG SING Average 5 2 2 2 2 2 2 31 – – – – – – 7.762 4.961 6.019 7.907 5.251 6.937 4.640 35.748 34.346 33.406 33.320 35.401 33.518 34.732 1114 1100 1566 1393 1100 1230 1462 40.9 þ / 0.5 37 þ/ 4 38 þ/ 4 37 þ/ 4 38 þ/ 4 42 þ/ 4 37 þ/ 4 39 1.71 þ / 0.02 1.74 1.76 1.76 1.71 1.76 1.76 1.74 0.24 þ/ 0.01 0.25 0.26 0.26 0.24 0.26 0.26 0.26 42.7, 1.70 – – – – – – 39.6, 1.71 – – – – – – A B B B B B B 1 2 2 2 2 2 2 Mozambique Belt MAUS LONG HALE KIBA KIBE KOMO KOND TARA ANGA KAKA KITU KMBO KR42 NAI TALE Average 1 2 2 2 2 2 2 2 3 3 3 4 3 4 3 31 – – – – – – – – – – – – – – 2.741 2.725 5.302 5.322 5.378 3.842 4.904 3.889 2.500 0.559 1.373 1.127 0.038 1.127 0.979 36.704 36.698 38.617 36.570 37.476 36.719 35.797 36.006 36.800 34.796 38.002 37.352 35.726 36.804 34.976 1334 1380 230 1500 997 1114 1419 1268 1000 1477 1129 1940 2157 1692 1821 39.1 þ / 1.0 37 þ/ 4 39 þ/ 4 36 þ/ 4 37 þ/ 4 36 þ/ 4 37 þ/ 4 37 þ/ 4 39 þ/ 3.0 37 þ/ 2.0 40 þ/ 3.9 41 þ/ 2.6 38 þ/ 4.8 42 þ/ 2.3 38 þ/ 3.2 38 1.67 þ / 0.02 1.78 – – 1.74 1.72 1.78 1.74 1.70 þ/ 0.07 1.67 þ / 0.07 1.73 þ / 0.07 1.74 þ / 0.05 1.81 þ / 0.16 1.75 þ / 0.04 1.76 þ / 0.04 1.74 0.24 þ/ 0.01 0.27 – – 0.25 0.24 0.27 0.25 0.24 þ/ 0.03 0.24 þ/ 0.03 0.25 þ/ 0.03 0.26 þ/ 0.02 0.26 þ/ 0.06 0.26 þ/ 0.02 0.27 þ/ 0.02 0.25 – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – A B C C B B B B A A A A A A A 1 2 2 2 2 2 2 2 3 3 3 3 3 3 3 Western Branch FOPO 1 – 0.663 30.282 1535 35 – – – – C 1 S¼ Source. 1¼ This study; 2¼ Last et al. (1997); 3¼ Dugda et al. (2005). N ¼Network. 1 ¼AAEASE; 2¼ TBSE; 3 ¼KBSE; 4¼ GSN; 5¼ AF. M ¼Methods. A ¼ H–k stack; B ¼ slant stack; C ¼Ps travel time. rf ¼no. of receiver functions used. H (1,2,3), k (1,2,3); ave. Vp used for H–k stack. (1) Vp ¼ 6.5 km/s; (2) Vp¼ 6.7 km/s; (3) Vp¼ 6.3 km/s. 1.73 1.73 1.80 1.72 1.81 1.70 1.74 F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81 well as the influence crustal structure may have exerted on the location and development of the East African rift system. A similar composition and thickness of the crust in East Africa for terrains spanning some 4 byr of Earth history is different than in many other regions of Precambrian crust, as noted previously. Our results for East Africa suggest that there may have been few changes over Earth’s history in the processes that formed the East African crust, or else that processes have been at play to homogenize crustal structure, such as the flow of lower crustal material in orogenic systems (e.g., Costa and Rey, 1995; Rey et al., 2001) or 79 through the foundering of eclogites into the mantle (e.g., Arndt, 1989; Jarchow and Thompson, 1989; Austrheim, 1991; Zandt et al., 2004). With regard to the rifting in East Africa, the fact that crustal structure is similar between the Archean and Proterozoic terrains indicates that variations in crustal structure, through its influence on lithospheric rheology and strength, have not exerted terrainscale control on the location of rifting. It has long been noted that the Cenozoic rift valleys have developed within the Proterozoic mobile belts surrounding the Archean craton (e.g., McConnell, 1972; Ebinger, 1989; Nyblade and Brazier, 2002), and the results of this study indicate that any major influence that the Precambrian lithosphere has had on the development of the rift system must primarily come from variations in Archean versus Proterozoic lithospheric mantle and not crustal structure. However, within individual terrains, some control on rift development could have come from small ( o5 km) variations in crustal thickness (e.g., Tesha et al., 1997). Lastly, we use Airy isostasy to examine if variations in crustal thicknesses have an influence on topography across the East African plateau. Assuming a crustal underplating layer, h1, beneath an uplifted region with elevation h2, and taking rc, ru and rm as densities of the crust, underplating layer, and uppermost mantle layer, respectively, then h1 and h2 can be related using the Airy relation h1 ¼h2 (rm ru)/rc. If we take h2, rc, ru and rm as 1 km, 2670 kg/m3, 3000 kg/m3 and 3300 kg/m3, respectively, then for an uplift of 1 km about 10 km of crustal thickening is required. Fig. 6 shows a plot of elevation against Moho depth for all the stations in Table 1. While there is Z1 km variation in elevation between the stations, there is no obvious trend between elevation and Moho depth. The lack of correlation between Moho depth and elevation in Fig. 6 indicates that variations in crustal thickness do not have a first-order influence on topography in East Africa, lending support to models showing mantle structure and processes provide the primary buoyant support for the plateau elevations (e.g., Moucha and Forte, 2011). 6. Summary Fig. 5. Map of the study region showing major geological features, Moho depths, and Poisson’s ratios (the first and the second numbers next to each station name) obtained from this study, Last et al. (1997) and Dugda et al. (2005). Solid circles show Moho depths of 35–39 km, open circles Moho depths from 40 to 44 km and solid triangles Moho depths of 45 km or deeper. Solid squares show stations for which no results are reported. Geological features are the same in Fig. 1. P-wave receiver functions have been modeled using the H–k stacking method to investigate crustal Poisson’s ratio and Moho depth for Precambrian terrains in East Africa in areas away from the Cenozoic East African rift system. The average crustal thickness for all but one of the terrains is between 37 and 39 km. An exception is the Ubendian terrain, which has an average crustal thickness of 42 km. In all of the terrains, the average Poisson’s Table 2 Comparison of crustal structure obtained in this study with global averages. Source Country Terrain Moho depth range (km) Poisson’s ratio range This paper Uganda Uganda–Tanzania Tanzania Rwenzori Belt Kibaran Belt Ubendian Belt Usagaran Belt Mozambique Belt Tanzania Craton (Nyanzian) Tanzania Craton (Dodoman) 35–40 33–44 34–49 36–39 36–42 37–41 37–42 0.25–0.27 0.24–0.29 0.25–0.28 0.25–0.28 0.24–0.28 0.25–0.28 0.24–0.26 Dugda et al. (2005) Christensen and Mooney (1995) Rudnick and Fountain (1995) Kenya Global Global Mozambique Belt Precambrian Archean Proterozoic 37–42 41.5 7 5.8 (s.d.) 43.0 7 6.3 (s.d.) 43.6 7 4.6 (s.d.) 0.24–0.28 Zandt and Ammon (1995) Global Shields Platforms Durrheim and Mooney (1994) Global Archean Proterozoic 0.29 7 0.02 (s.d.) 0.27 7 0.03 (s.d.) 27–40 40–55 80 F. Tugume et al. / Earth and Planetary Science Letters 355-356 (2012) 73–81 anonymous reviewers for helpful reviews. This study was funded by the National Science Foundation (Grants OISE-0530062, EAR0440032, EAR-0824781). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.epsl.2012.08.041. References Fig. 6. A plot of Moho depth versus elevation for stations listed in Table 1. ratio is either 0.25 or 0.26. The main finding of this study is that crustal structure is similar across all of the terrains. There is no discernable difference in the crustal thicknesses or Poisson’s ratios between the Archean and Proterozoic terrains, or within the Proterozoic terrains. This finding is in contrast to Precambrian crustal structure elsewhere in Africa and in other continents, where, on a terrain-by-terrain basis, discernable variations in crustal structure can be found. The finding that on average the Precambrian crust in all of the terrains within East Africa is similar has important implications for understanding processes of crustal formation and evolution, as well as the influence crustal structure may have exerted on the location and development of the East African rift system. A similar composition and thickness of the crust in East Africa for terrains spanning some 4 byr of Earth’s history is different than in many other Precambrian regions. Our results suggest that there may have been few changes over Earth’s history in the processes that formed the East African crust, or else that processes have been at play to homogenize crustal structure, such as the flow of lower crustal material in orogenic systems or through the foundering of eclogites into the mantle. With regard to the rifting in East Africa, the finding that crustal structure is similar between the Archean and Proterozoic terrains indicates that crustal structure, via its influence on lithospheric rheology and strength, has not had a first-order influence on the location of rifting, at least not compared to the influence that variations in Precambrian lithospheric mantle structure have had on the development of the rift system. However, this conclusion does not preclude the possibility that within individual terrains some control on rift development may have come from small ( o5 km) variations in crustal thickness. Finally, the lack of correlation between Moho depth and elevation across the East African Plateau indicates that variations in crustal thickness do not have a first-order influence on elevation in East Africa. 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