229_240 Hanson et.al

Estimates of crustal and lithospheric thickness beneath ten permanent seismic stations in southern, central, and eastern Africa have been obtained from modeling S-wave receiver functions (SRFs). For eight of the examined stations, the Moho depth estimates agree well with estimates from previous studies using P-wave receiver functions (PRFs). For two stations, TSUM and BGCA, previous PRF estimates are not available, and our results provide new constraints on the Moho depth, indicating crustal thicknesses of 35 and 40 km, respectively. SRFs from four stations, BOSA, SUR, FURI, and ATD, display clear S-to-P (Sp) conversions from the lithosphereasthenosphere boundary (LAB), corresponding to lithospheric thicknesses of 155, 140, 80, and 34 km, respectively. As expected, thicker lithosphere is observed beneath the Precambrian Kaapvaal Craton (station BOSA) and the Namaqua-Natal mobile belt (station SUR) and thinner lithosphere is observed beneath the edge of the Ethiopian rift (station FURI) and the Afar Depression (station ATD). The thinner lithosphere beneath the two latter stations is consistent with the transition from continental to oceanic rifting at the Afar triple junction. For the remaining stations, bootstrap error estimates indicate that the Sp conversion from the LAB cannot be well resolved, calling into question interpretations of lithospheric structure in previous SRF studies using data from these same stations. SOUTH AFRICAN JOURNAL OF GEOLOGY, 2009, VOLUME 112 PAGES 229-240 doi:10.2113/gssajg.112.3-4.229 tomography (James et al., 2001; Fouch et al., 2004; Priestley et al., 2006; 2008; Li and Burke, 2006; Pasyanos and Nyblade, 2007), inversion of surface wave phase velocities (Freybourger et al., 2001; Weeraratne et al., 2003; Larson et al., 2006), regional waveform modeling (Priestley and McKenzie, 2002; Wang et al., 2008), joint inversion of P-wave receiver functions (PRFs) with surface wave dispersion (Julià et al., 2005; Dugda et al., 2007; this volume), thermal estimates based on heat flow data (Jones, 1988; Rudnick and Nyblade, 1999; Artemieva and Mooney, 2001; Deen et al., 2006), and pressure-temperature estimates based on xenolith data (Boyd et al., 1985; Boyd and Gurney, 1986; Deen et al., 2006). Nevertheless, there remain conflicting views on the thickness of the African lithosphere, in part due to previous interpretations of S-wave receiver functions (SRFs; Kumar et al., 2007; Wittlinger and Farra, 2007). SOUTH AFRICAN JOURNAL OF GEOLOGY ESTIMATES OF CRUSTAL AND LITHOSPHERIC THICKNESS IN SUB-SAHARAN AFRICA 230 Figure 1. Stations (triangles) examined in this study in (a) southern Africa and (b) central and eastern Africa. Note that the two maps are not plotted at the same scale. The Sp conversion points at the best-interpreted LAB depth for each station are shown by circles (BOSA: 155 km, LBTB: 155 km, SUR: 140 km, LSZ: 142 km, TSUM: 180 km, ATD: 34 km, FURI: 80 km, KMBO: 140 km, MBAR: 100 km, BGCA: 285 km). The color shading indicates which Sp conversion points correspond to which station. Bold dashed lines outline the boundaries of labeled tectonic terrains while bold solid lines mark faults and rift segments. a S.E. HANSEN, A.A. NYBLADE AND J. JULIÀ SOUTH AFRICAN JOURNAL OF GEOLOGY 231 In this study, we also use the SRF technique (e.g. Farra and Vinnik, 2000; Li et al., 2004; Kumar et al., 2007; Hansen et al., 2007; 2009) to investigate the depth of the crust-mantle boundary (Moho) as well as the lithosphere-asthenosphere boundary (LAB) by identifying S-to-P (Sp) conversions from discontinuities beneath several permanent seismic stations in southern, central, and eastern Africa. Unlike PRFs, where crustal multiples can mask conversions from the LAB, boundary conversions on SRFs can be more clearly identified because they arrive earlier than the direct S phase while all crustal multiples arrive later (Figure 2; e.g. Farra and Vinnik, 2000; Li et al., 2004; Kumar et al., 2007; Hansen et al., 2007; 2009). Our SRF analysis differs from previously published studies (Vinnik et al., 2004; Kumar et al., 2007) and provides new estimates of crustal and lithospheric thickness for several terrains in southern, central, and eastern Africa that take into consideration the often large uncertainties associated with SRFs. Our results place new constraints on the nature of the African lithosphere and help to elucidate age-dependent variations in lithospheric structure. Data and Methodology Teleseismic waveform data recorded at several permanent stations throughout southern, central, and eastern Africa were used in this study (Figure 1). These stations belong to several networks, including the Global Seismographic Network, the Global Telemetered Seismograph Network, and GEOSCOPE. Most of these stations have been operating since the midto late1990’s, providing over ten years of data. To minimize any potentially interfering teleseismic phases (Wilson et al., 2006), we selected S-waves with high signal-tonoise ratios recorded at these stations from earthquakes with magnitudes larger than 5.7, depths less than 250 km, and distances between 60° to 82°. Waveforms were first rotated from the N-E-Z to the R-T-Z coordinate system using the event’s back-azimuth and were visually inspected to pick the S-wave onset. The threecomponent records were then cut to focus on the section of the waveform that is 100 s prior to and 12 s after the S arrival. To detect Sp conversions, the data must be rotated around the incidence angle into the SH-SV-P coordinate system (Li et al., 2004). This second rotation is critical because if an incorrect incidence angle is used, noise can be significantly enhanced and converted phases may become undetectable. To make this second rotation, the approach of Sodoudi (2005) was used to determine the correct incidence angle. The cut R-Z seismograms were rotated through a series of incidence angles to create a set of quasi-SV and quasi-P data. Each quasi-SV component was then deconvolved from the corresponding quasi-P component using Ligorría and Ammon’s (1999) iterative time domain method to create a SRF. To make the SRFs b Figure 1b. Central and eastern Africa.