Velocity structure of the continental upper mantle: evidence from southern Africa

Transcription

1 Ž. Lithos Velocity structure of the continental upper mantle: evidence from southern Africa K. Priestley Department of Earth Sciences, Bullard Laboratories, UniÕersity of Cambridge, Cambridge CB3 0EZ, UK Received 14 April 1998; received in revised form 20 November 1998; accepted 15 December 1998 Abstract The velocity model for southern Africa of Qiu et al. wqiu, X., Priestley, K., McKenzie, D., Average lithospheric structure of southern Africa. Geophys. J. Int. 127, x is revised so as to satisfy both the regional seismic waveform data and the fundamental mode Rayleigh wave phase velocity data for the region. The revised S-wave model is similar to the original model of Qiu et al. except that the high velocity, upper mantle lid extends to 160 km depth in the revised model rather than to 120 km in the original model. Sensitivity tests of the regional seismic data show that the minimum velocity in the S-wave low velocity zone can be as high as 4.45 km s y1 compared to 4.32 km s y1 in the Qiu et al. model. The vertical S-wave travel time for the revised south African model is compared with the vertical S-wave travel times for the global tomographic models S12WM13 and S16B30, and they are found to be similar. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Velocity structure; Continental upper mantle; Southern Africa 1. Introduction The southern African region is particularly important in understanding upper mantle properties because of the large number of kimberlite eruptions, primarily on the Kaapvaal craton, which have brought up nodules from upper mantle depths. Such rocks are the only direct information available on the composition of the lower lithosphere; hence it is important to determine whether these upper mantle samples are derived from typical continental mantle or are unique to the kimberlite source region below the craton. Qiu et al. Ž whereafter referred to as QPM96x published an average compressional and shear wave velocity model for southern Africa. The crust of their model was constrained by published seismic refraction results and receiver function analyses. The upper mantle of the model was constrained by inversion of multi-mode waveforms using the differential locked-mode seismogram technique of Gomberg and Masters Ž and with further forward modeling of the waveforms using reflectivity ŽFuchs and Muller, Seven of the three-component waveforms analyzed were taken from the long period channel of the SLR DWWSSN station Ž Fig. 1.; the remaining waveforms analyzed were taken from the broadband channel of the SUR, BOSA, LBTB and LSZ seismographs and were lowpass filtered at 0.05 Hz with a 3-pole butterworth filter. The average velocity model r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 46 ( ) K. PriestleyrLithos Fig. 1. Source receiver paths for regional earthquake seismograms used in this study superimposed on the major crustal subdivisions of southern Africa. Triangles denote seismograph stations; stars denote earthquake locations; and dotted lines denote paths for events used in the regional waveform modeling. Events 1 8 were used by QPM96; event 9 is the September 21, 1997 earthquake Ž7.258S, E, 30 km deep.. The phase velocity for fundamental mode Rayleigh waves has been measured for the SUR-BOSA and BOSA-LBTB paths. of the crust and upper mantle derived from the analysis of waveforms from 12 paths largely confined to the Kaapvaal and Zimbabwe cratons and the Limpopo belt Ž Fig. 1. has the following features. Below the 42 km thick crust there is an 80 km thick, high S-wave velocity upper mantle lid. The P and S velocities are 8.09 and 4.62 km s y1, respectively, beneath the Moho and the compressional and shear velocity gradients in the lid are s y1 and s y1, respectively. The high velocity upper mantle lid is required to explain both the high velocity observed in southern Africa and the propagation

3 ( ) K. PriestleyrLithos of long period fundamental mode Love and Rayleigh waves from the regional events observed on the paths shown in Fig. 1. Below the lid, QPM96 found a shear wave LVZ in which the S-wave velocity drops to 4.32 km s y1 at 250 km depth. The S-wave LVZ is required to match the arrival time and amplitude of the higher mode waveforms. Forward modeling of Pnl waveforms using reflectivity synthetics shows that an upper mantle P-wave LVZ is not required by the data. Below 125 km depth the P-wave gradient increases to s y1 and increases again to s y1 between 250 km depth and the 410 discontinuity. QPM96 compared the seismic velocity of their upper mantle model with velocities they determined from the analysis of peridotite nodules from the kimberlite volcanics on the Kaapvaal craton. The velocities determined from the nodules coming from the upper mantle lid depth agree well with the seismic structure, but velocities determined from the nodules coming from deeper depths are higher than the seismic velocity at those same depths. The agreement in the lid velocity determined from seismology and petrology suggests that the nodules are representative of the upper mantle lid composition beneath the wider region of southern Africa and are not merely characteristic of the kimberlite source region. Two issues are addressed here. In Section 2, the south African upper mantle velocity model of QPM96 is modified. QPM96 noted that the Rayleigh wave phase velocity curve for their model underestimates the observed dispersion curve of Bloch and Hales Ž for southern Africa. New Rayleigh wave phase velocity measurements are presented which give results similar to those of Bloch and Hales Ž and the QPM96 shear wave model is modified so as to agree with both the regional waveform data and the Rayleigh wave phase velocity data. Section 3 deals with the apparent differences in upper mantle structure between the south African model of QPM96 and those of the global tomographic models. The tomographic models ŽSu et al., 1995; Masters et al., show higher than average S-wave velocities to depths as great as 500 km in a wide region beneath southern Africa. This differs from the QPM96 model and other upper mantle models for southern Africa derived from regionally Ž recorded seismic data Bloch et al., 1969; Cichowicz. and Green, 1992 which show high S-wave velocities extending only to 120 to 160 km depth. The vertical S-wave travel times through the upper mantle of the QPM96 model and the global tomographic models are compared and found to be similar. 2. Seismic constraints on the southern Africa upper mantle Bloch and Hales Ž measured interstation fundamental mode Rayleigh wave phase velocities across the Kaapvaal and Zimbabwe cratons; these measurements, along with fundamental and higher mode Rayleigh and Love wave group velocities, were interpreted by Bloch et al. Ž Their S-wave models derived by a combination of inversion and forward modeling include a high S-wave velocity upper mantle lid overlying a LVZ; the sub-moho shear wave velocity is about 4.70 km s y1, the lid has a positive shear wave velocity gradient, and the base of the upper mantle lid in their model is at 120 km depth. QPM96 used this mantle shear wave model as the starting mantle model for the regional waveform inversion and their resulting velocity model is similar to that of Bloch et al. Ž QPM96 also used one of the Cichowicz and Green Ž South Africa S-wave velocity models as a starting model for the waveform inversion so as to examine the influence of the initial model on the inversion results. Using this starting model, QPM96 obtained an inversion result similar to that obtained when the Bloch et al. Ž model was used as an initial model, suggesting that the QPM96 inversion model was not strongly dependent on the Bloch et al. Ž starting model. However, the QPM96 velocity model underestimates the Bloch and Hales Ž Rayleigh wave phase velocity curve. QPM96 noted this discrepancy, showed that synthetics seismograms computed for the Bloch et al. Ž model did not match the observed seismograms, and concluded that the inversion of the multi-mode waveform data coupled with the observed Sn velocity provided a more stringent constraint on the S-wave velocity structure than did the fitting of the dispersion data on its own. Fundamental mode Rayleigh wave phase velocities have been measured from two events for the Ž. LBTB-BOSA path ^ 400 km and two events for

4 48 ( ) K. PriestleyrLithos the BOSA-SUR path Ž ^ 597 km. Ž Fig. 1. using the transfer function method of Gomberg et al. Ž The upper and lower bounds for these dispersion data, the average dispersion curve of Bloch and Hales Ž 1968., and the theoretical dispersion curve for the QPM96 model are shown in Fig. 2. The Bloch and Hales Ž curve lies within the bounds of the new measurements; the QPM96 dispersion curve underestimates both the Bloch and Hales Ž phase velocity curve and lies below the lower bounds of the dispersion measured here for periods s. This suggests that the S-wave velocity in the km depth range of the southern Africa model of QPM96 is too low. QPM96 conducted a number of tests to determine the sensitivity of their data to the features of the upper mantle velocity structure. From the multi-mode waveform inversions the average depth to the base of the lid was found to be 120 km. However, their Fig. 2. Comparison of observed fundamental and higher mode Rayleigh wave dispersion curves and the theoretical curves for the QPM96 and revised southern African velocity models. The upper and lower bounds of the fundamental mode Rayleigh wave phase velocity measured for the paths SUR-BOSA and BOSA-LBTB are shown as the two thick solid lines. The observed dispersion curves of Bloch and Hales Ž and Bloch et al. Ž are denoted by the thin solid lines. The theoretical phase velocity curve for the QPM96 model Ž dot dash line. lies below the Bloch and Hales Ž phase velocity curve and below the lower bound of the phase velocity measured in this study. Increasing the depth to the top of the S-wave LVZ to 160 km results in the theoretical dispersion curves Ž dashed lines. which match the observed fundamental mode phase velocity curves and the fundamental and higher mode group velocity curves. sensitivity tests indicate that the upper mantle lid could persist to a depth of ;150 km without significantly degrading the fit of the synthetic seismograms to the regional seismic waveforms. A more extensive test of the sensitivity of the data to the depth of the base of the lid is shown in Fig. 3a c. These tests compare synthetic seismograms for four lid models with the observed seismograms at three distance ranges. Increasing the depth to the top of the S-wave LVZ to 160 km results in a small additional increase in the higher mode amplitude at 1038 km distance Ž Fig. 3a.. There is only a small additional increase in amplitude if the depth to the LVZ is increased. None of these lid models produce a significant change in the travel time at this distance. The S-wave at this distance range consists of energy turning in the lid, but the distance range is not great enough to be sensitive to the top of the LVZ. The comparison at 1480 km distance Ž Fig. 3b. shows that increasing the depth to the top of the S-wave LVZ advances the travel time and increases the amplitude of the synthetic waveform with respect to the observed waveform. Increasing the top of the LVZ to 160 km results in a small travel time advance of the synthetic with respect to the observed waveform. The match is still acceptable but not as good as that of the original QPM96 model. Increasing the top of the LVZ depth to greater depths results in both an earlier and larger amplitude synthetic with respect to the observed waveform. At this distance range most of the S-wave consists of energy turning in the lower lid, and increasing the lid thickness increases both the arrival time and the amplitude of the S-wave. The comparison at 2106 km distance Ž Fig. 3c. shows an acceptable match of the synthetic and observed waveform for a 160 km deep LVZ, but thicker lids advance the arrival time of the waveform although the waveform shape is not altered. At this distance range the S-wave consists of energy turning in the vicinity of the 410-km discontinuity. Increasing the lid thickness causes an advance in the arrival time but no change in the amplitude. QPM96 also comment that the S-wave LVZ beneath southern Africa was necessary to match the seismograms but that the velocity structure of the S-wave LVZ was not well constrained. Fig. 3d shows that increasing the minimum LVZ velocity to 4.4 km s y1 has little effect on the synthetic fits to the

5 ( ) K. PriestleyrLithos Fig. 3. Ž. a Sensitivity test for the depth to the base of the upper mantle lid for the SLR seismogram of the 18 July 1986 earthquake ŽFig. 1, No. 2.. The solid line is the observed waveform, the dotted line is the synthetic for the southern Africa velocity model of QPM96, and the dashed line is the synthetic for the same velocity model but with the lid base increased to the depth indicated at the left of each seismogram. Ž. b Same as Ž. a but for the SLR seismogram of the 10 March 1989 earthquake Ž Fig. 1, No. 5.Ž.. c Same as Ž. a but for the SUR seismogram of the 24 July 1991 earthquake Ž Fig. 1, No Ž d. Sensitivity test of the SUR seismogram of the 24 July 1991 earthquake to the minimum S-wave velocity of the LVZ. The solid line is the observed waveform, the dotted line is the synthetic for the southern Africa velocity model, and the dashed line is the synthetic for the same velocity model but with the S-wave velocity of the LVZ increased to the value indicated at the left of each seismogram. observed waveforms at 2106 km distance, but increasing the minimum velocity to 4.5 km s y1 or greater results in an early higher mode arrival for the synthetic seismogram compared to the observed seismograms at this distance. These tests show that even though the average mantle lid thickness from the waveform inversions is 80 km and the minimum LVZ S-wave velocity is ;4.3 km s y1, the thickness of the seismic lithosphere Ž crust plus upper mantle lid. can be increased to ;160 km and the minimum LVZ S-wave velocity can be increased to ;4.45 km s y1 without significantly degrading the match of the synthetic and observed waveforms. Fig. 4 compares synthetic seismograms computed for the revised southern Africa velocity model with the observed seismograms in three distance ranges. The seismograms at increasing epicentral distance range constrain the increasingly deeper structure in the model. The simultaneous match of the Love Ž SH. and Rayleigh Ž P-SV. waveforms with the same isotropic velocity model show that if anisotropy is present, its effects are not sufficiently strong to create a discrepancy between the Love and Rayleigh waves. Increasing the thickness of the seismic lithosphere to 160 km brings the theoretical fundamental mode Rayleigh wave phase velocity curve into agreement with the observed phase velocity curve Ž Fig. 2. and improves the agreement between the seismically determined velocity structure and the velocity estimates from the

6 50 ( ) K. PriestleyrLithos Fig. 4. Three-component waveform fits at three distance ranges for synthetic seismograms Ž dotted line. computed from the revised southern African velocity model and the observed seismograms Ž solid lines.. The Love and Rayleigh wave seismograms are fit with the same velocity model, implying that at least the upper mantle lid is isotropic. Event is Fig. 1, event no. 2, is event no. 10, and is event no. 8. deeper garnet peridotite nodules. The revised southern African shear-wave velocity model is compared in Fig. 5 with the model of QPM96 and the velocities they estimate from the upper mantle nodules. QPM96 examined how uncertainties in earthquake location and source mechanism resulted in uncertainties in their velocity model. The accuracy of the ISC and PDE hypocenters in southern Africa is not known. QPM96 compared the ISC locations of South African rock bursts with locations determined by the South African Geological Survey and estimated the ISC location errors to be about "10 km. They then perturbed the ISC or PDE hypocenter "10 km and reinverted the regional waveforms to evaluate the effects of this magnitude location error on the inversion structure and found that the distance error produced an insignificant change in the inversion model. However, they found that depth errors resulted in large changes in the synthetic waveforms but that for five of the eight events analyzed, the ISC depth resulted in well-fit waveforms; the other three events required less than a 6 km shift in the focal depth from the ISC or PDE focal depth. The study of Zhao et al. Ž helps to better evaluate the earthquake location errors. The August 18, 1994 Lake Rukwa earthquake Ž Event 8, Fig. 1. was well recorded by 20 broadband digital seismographs at distances ranging from 160 to 800 km. Zhao et al. Ž used teleseismic depth phases to constrain the focal depth to 25"2 km, similar to the depth Ž 27" 2 km. Foster and Jackson Ž found from modeling the teleseismic P- and SH-waveforms. Zhao et al. Ž then used the Pn arrival times at the regional stations to determine the origin time and epicentral location. They found the origin time for this event to be 00:45:48:79 UTC and the

7 ( ) K. PriestleyrLithos Fig. 5. Comparison of the revised shear wave velocity model for southern Africa with the shear wave velocity model of QPM96 and the shear wave velocities estimated in QPM96 from the upper mantle nodules. The thin solid lines denote estimates of the uncertainties in the revised shear wave velocity model from the waveform fitting tests described in the text, the earthquake location errors as described in QPM96, and ;2% anisotropy as proposed by Vinnik et al. Ž epicenter to be Ž "4.2 km. S, Ž "5.3 km. E, similar to the ISC Ž00:45:47 Ž "1.4. s, Ž " S, Ž " E, 25 km depth. and PDE Ž 00:45:47.2 UTC, 7.438S, E, 25 km depth. locations. QPM96 found that the regional waveforms were best fit for a focal depth of 30.9 km. The Zhao et al. Ž location for the Lake Rukwa earthquake suggests that the "10 km location error assumed by QPM96 was reasonable for at least the later events they analyzed. QPM96 made similar tests for the effects of errors in the CMT source mechanisms and found that "108 variations in the strike, dip and slip made little difference in the inversion velocity structure. QPM96 note that any significant anisotropy in the lithosphere or upper mantle could bias their results. The effect of anisotropy on seismic waves is manifest in two ways: azimuthal anisotropy in which waves of the same type have a wave speed which is a function of azimuth; and polarization anisotropy in which waves of different polarization Žeg., Love and Rayleigh waves. propagate with different wave speeds along the same path. Low-frequency surfacewave observations ŽNataf et al., 1984; Montagner, show weak anisotropy in the upper mantle beneath southern Africa. Vinnik et al. Ž measured shear-wave splitting at seven sites on the Kaapvaal Craton and found an average delay of 0.9"0.5 s. From this, they conclude ;2% anisotropy localized between 150 and 400 km depth and oriented with the fast direction approximately parallel to the direction of the current plate motion, implying that the material in this depth range has been deformed by plate motion in geologically recent time. A similar result has been observed for Australia Ž Debayle, Anderson and Dziewonski Ž comment that in many cases, isotropic earth models cannot provide a uniform fit to Love and Rayleigh wave data and that in such cases, if the Rayleigh wave data were inverted separately, a pronounced LVZ can occur in the resulting velocity model. They also note that areas such as the Canadian Shield show little or no discrepancy between Love and Rayleigh wave dispersion Ž Brune and Dorman, QPM96 used the Sn velocities they measured from earthquakes and Sn velocities from refraction ŽDurrheim and Green, to constrain the shear wave velocity beneath the Moho. Using this constraint, they simultaneously inverted the Love and Rayleigh regional waveforms for the upper mantle velocity structure. They found that the regional Love and Rayleigh waveforms were fit with a single isotropic velocity structure, suggesting that anisotropy was weak. This is consistent with Bloch et al. Ž who also found no discrepancy between the Love and Rayleigh waves in southern Africa. Thus, while QPM96 could not make any quantitative statements about the level of polarization anisotropy, there was nothing in their data to indicate the presence of polarization anisotropy and that their results were strongly biased by the isotropic assumption. However, the earthquakes examined in both QPM96 and this study all lie north to northeast of the seismographs and the paths studied sample only a small range of azimuths. As a result, their model may be influenced by azimuthal anisotropy. If

8 52 ( ) K. PriestleyrLithos ; 2% anisotropy exists in the km depth range as suggested by Vinnik et al. Ž 1995., this could modify the velocity structure of the LVZ shown in Fig Comparison with global mantle models for southern Africa Global tomographic models ŽSu et al., 1995; Masters et al., show higher than average shear wave velocities extending to depths as great as 500 km beneath the cratons. These features have been cited as evidence for high velocity cratonic roots and as support of the deep tectosphere model ŽJordan, Models for the cratons based on regional seismic data Ži.e., Brune and Dorman, 1963; Cara, 1979; QPM96. show a high S-wave velocity upper mantle overlying a S-wave LVZ. In this section the vertical S-wave travel time for the revised southern Africa model is compared to the vertical travel time through the global tomographic models S12WM13 Ž Su et al., and S16B30 Ž Masters et al., to assess the similarity of the integrated S-wave structure between the base of the crust and 400 km depth in these models. Model S12WM13 Ž Su et al., was derived from the inversion of body and mantle waveforms and differential and absolute travel time residuals. The model is expanded to spherical harmonic degree 12 to describe the horizontal variation and to Chebyshev polynomial order 13 to describe the radial variation. The model is given in terms of a relative deviation from PREM ŽDziewonski and Anderson, S12WM13 shows higher shear wave velocities than in PREM extending to about 500 km depth beneath southern Africa. An average southern African upper mantle velocity model was determined from S12WM13 by averaging six velocity profiles computed from S12WM13 for an area encompassing the whole cratonic region of southern Africa Ž Fig. 1.. Even though S12WM13 shows high shear wave velocities extending over a broad region encompassing southern Africa, only velocity profiles beneath the cratonic region shown in Fig. 1 were averaged. The vertical S-wave travel time between 42 and 400 km depth through the southern African velocity model from S12WM13 is s. Making the same crustal correction as that made for S12WM13 Ž Woodhouse and Dziewonski, 1984., the vertical S-wave travel time through the revised southern Africa S-wave model is s. The mean vertical S-wave travel time for the shields in model S12WM13 is Ž "0.82. s. Fig. 6 shows synthetic seismograms computed for a modified S12WM13 model compared with seismograms of the 14 August 1994 earthquake ŽFig. 1, no. 8. recorded at three distance ranges between 2000 and 3000 km across southern Africa. The velocity model consists of the revised seismic lithosphere described in Section 2 to a depth of 160 km and the southern African upper mantle model from S12WM13 below 160 km depth. The seismic lithosphere of southern African results in the fit of the fundamental mode surface waves. At these distance ranges, the higher mode surface wave corresponds to the S-wave turning in the mantle transition zone; hence, its arrival time is controlled by the S-wave delay time across the upper mantle. The synthetic mantle S-wave arrives early with respect to the observed mantle S-wave. Lowering the shear wave speed of the upper mantle lid to ;4.55 km s y1 as in S12WM13 would improve the match of the synthetic and observed mantle S-wave but would result in a poor fit of the fundamental mode surface Sn waves and also disagree with the high frequency travel time observation and the lid velocities determined from the garnet peridotite nodules. Model S16B30 Ž Masters et al., was derived from the inversion of absolute and differential travel time residuals, surface wave dispersion and polarization measurements, and mode structure coefficients. The model is parameterized laterally by spherical harmonics expanded up to degree 16 and by 30 natural cubic B-splines in radius. Model S16B30 is specified in terms of the percent deviation from an average spherical velocity model which is not retained in the inversion; hence, the velocity structure cannot be determined and a direct comparison of the vertical S-wave travel times for S16B30 and the southern African model cannot be made. Instead, two indirect comparisons are made. The velocity profile for southern Africa from S16B30 was determined as for S12WM13 except using earth model 1066a Ž Gilbert and Dziewonski, as the reference earth model. For this case the vertical S-wave

9 ( ) K. PriestleyrLithos Fig. 6. Three-component waveform fits at three distance ranges for synthetic seismograms Ž dotted lines. computed from composite velocity model consisting of the seismic lithosphere of the revised southern African model above 160 km depth and the southern Africa upper mantle model derived from the global tomographic model S12WM13 below 160 km depth compared to the observed seismograms Ž solid lines. of the 14 August 1994 earthquake Ž Fig. 1, No. 8. recorded at three distance ranges. travel time between 42 and 400 km depth is s. Masters et al. Ž correct their data using the global crustal model of Mooney et al. Ž Assuming this crustal correction is effectively the same as the crustal correction of Woodhouse and Dziewonski Ž but using the Mooney et al. Ž south African crust rather than the crust of PEM-C Ž Dziewonski et al., 1975., the vertical S-wave travel time for the revised southern Africa upper mantle model is s. The mean vertical S-wave travel time for the shields in model S16B30, assuming the average spherical model 1066a is the reference model, is Ž "0.89. s. The degree of similarity between the models can also be indicated by comparing differential S-wave travel times through models for southern Africa and an old ocean basin. Using the old ocean basin Ž ) 110 mya. S-wave model of Nishimura and Forsyth Ž 1989., the difference between the vertical S-wave travel time between 42 and 400 km depth beneath southern Africa and an old ocean basin is y1.00 s for model S16B30; for the revised southern Africa model it is y2.01 s. These comparisons show that the vertical S-wave travel time through the southern Africa shear wave models derived from the regional and global data is similar and lies within the scatter of the shields in each of the global models. However, the models derived from regional seismic data have a high velocity upper mantle lid overlying a shear wave LVZ, whereas the global models have a much smoother velocity structure. 4. Discussion and conclusions The shear wave structure of the QPM96 velocity model for southern Africa has been revised so as to agree with both the fundamental mode Rayleigh

10 54 ( ) K. PriestleyrLithos wave phase velocity data and the regional waveform data. The revised S-wave velocity structure is similar to that of the original QPM96 model except that the high velocity upper mantle lid extends to 160 km depth in the revised model rather than to 120 km depth as in the original model. Increasing the depth to the top of the shear wave LVZ results in fitting of the Rayleigh wave phase velocity data while retaining a satisfactory match to the regional waveform data. The increased lid thickness also improves the agreement between the seismic velocity structure and the velocity estimates at these depths from the garnet peridotite nodules. No revision of the P-wave model of QPM96 is required. The high velocity lid is necessary to fit both the observed high frequency Sn arrivals and the propaga- tion of the low frequency fundamental mode Love and Rayleigh waves. The observed velocity over southern Africa is ;4.65 km s y1, but higher velocities have been observed in more restricted regions of the Kaapvaal craton Ž Durrheim and Green, 1992., suggesting that 4.65 km s y1 is a lower bound on the cratonic velocity. These high velocities observed in the upper mantle lid do not result from neglecting anisotropy since the waveform inversion of QPM96 simultaneously fits both the Love and Rayleigh waveforms with a single model. The high S-wave velocities of the lid are further substantiated by their similarity to the S-wave velocities determined from the peridotite nodules from the same depth range. The high lid S-wave velocities require lower velocities beneath the lid to fit the higher mode surface waves. However, the distribution of the low S-wave velocities is not well constrained. The sensitivity tests in Section 2 indicate that the top of the S-wave LVZ is at ;160 km depth and the minimum S-wave velocity in the LVZ is 4.30 to 4.45 km s y1 Vinnik et al. Ž also find low S-wave velocities in the upper mantle beneath southern Africa using receiver function analysis but place the low S-wave velocities in the region starting about 50 km above the 410-km discontinuity. We place the low shear wave velocities at shallow depth beneath the high velocity lid since tests of thicker lid structures result in earlier arriving and larger amplitude lid phases than are observed in the regional seismograms. The vertical S-wave travel times across the top 400 km of the mantle of the revised southern Africa model are similar to the vertical S-wave travel times through the global tomographic models S12WM13 Ž Su et al., and S16B30 Ž Masters et al., However, the details of the upper mantle S-wave structure are different in the regional and global models. The shear wave models for southern Africa derived from regionally recorded seismic data ŽBloch et al., 1969; Cichowicz and Green, 1992; QPM96. all show high shear wave velocities beneath the Moho with a shear wave LVZ lying below the high velocity lid. The southern African structure from S12WM13 has lower velocities below the Moho, no high velocity lid structure, and no LVZ. All seismological techniques are prone to some averaging when used to determined velocity structure. The velocity model derived from regional seismic data such as that of QPM96 average the velocity structure laterally across the Kaapvaal craton, the Limpopo belt, and the Zimbabwe craton and portions of the surrounding mobile belts. Analysis of the regional seismic data show that the base of the high S-wave velocity upper mantle lid beneath southern Africa lies at ;160 km depth. This is an average for the region and the seismic lithosphere is probably somewhat thicker beneath the craton than beneath the mobile belts as suggested by Boyd and Gurney Ž This depth for the seismic lithosphere is supported by petrological studies of the garnet peridotite nodules and diamond inclusions from the kimberlites ŽBoyd et al., 1985; Boyd and Gurney, 1986; Boyd, 1989; Gurney, Modeling of regional waveform data for the Australian shield suggests that the high velocity continental roots there extend to km depth Ž Kennett et al., 1994., similar to that observed in southern Africa but shallower than observed in the global tomographic models for the Australian shield. The similarity of the vertical S-wave travel times in the regional and global models for southern Africa suggest that much of the deep, broad, high velocity root beneath this region in the global tomographic models may be the result of both lateral and vertical averaging. Averaging of the velocity structure is evident in other parts of the global models. For example, both S12WM13 and S16B30 show low S-wave velocity structures associated with the midocean ridges extending to over 300 km depth. It is generally believed that the mid-ocean ridges are

11 ( ) K. PriestleyrLithos passive features caused by lithospheric stretching. Hot asthenospheric material rises to fill the void resulting from the stretching and the magmatism observed along the ridge crest is caused by decompression melting at shallow Ž ; 60 km. depths ŽMc- Kenzie and Bickle, Hence, the low S-wave velocities associated with the mid-ocean ridges are thought to be confined to shallow depths and should not be distinguished in the velocity models to ;350 km depth. The high velocity continental roots of the cratons extending to ;200 km depth are prone to similar distortion in the global tomographic models. The other difference between the regional and global seismic models is in the way that the crustal effect is handled. In modeling regional seismic data, the effect of the crust is explicitly accounted for in the modeling. In most of the global analysis, a crustal correction is applied to the data before inversion using a standard continentrocean crustal model or a global crustal model. As Masters et al. Ž indicate, the crust has a significant effect on most of the global data sets while being too thin to be resolved by them. However, the crustal correction increases the variance of most of the data, which leads to large amplitude velocity perturbations in the uppermost mantle. Acknowledgements I would like to thank E. Debayle, J. Haines, G. Laske, D. McKenzie, and G. Masters for many helpful discussions and C. Langston, D. McKenzie, H. Patton and R. Saltzer for constructive reviews of the manuscript. I would also like to thank Guy Masters for support while I was on sabbatical leave at the Scripps Institution of Oceanography, where most of this study was undertaken. References Anderson, D.L., Dziewonski, A.M., Upper mantle anisotropy: evidence from free oscillations. Geophys. J. R. Astron. Soc. 69, Bloch, S., Hales, A.L., New technique for the determination of surface wave phase velocities. Bull. Seismol. Soc. Am. 58, Bloch, S., Hales, A.L., Landisman, M., Velocities in the crust and upper mantle of southern Africa from multi-mode surface wave dispersion. Bull. Seismol. Soc. Am. 59, Boyd, F.R., Compositional distinction between oceanic and cratonic lithosphere. Earth Planet. Sci. Lett. 96, Boyd, F.R., Gurney, J.J., Diamonds and the African lithosphere. Science 232, Boyd, F.R., Gurney, J.J., Richardson, S.H., Evidence for a km thick Archaean lithosphere from diamond inclusion thermobarometry. Nature 315, Brune, J.N., Dorman, J., Seismic waves and earth structure in the Canadian shield. Bull. Seismol. Soc. Am. 53, Cara, M., Lateral variation of S velocity in the upper mantle from higher Rayleigh modes. Geophys. J. R. Astron. Soc. 57, Cichowicz, A., Green, R., Tomographic study of uppermantle structure of the South African continent using waveform inversion. Phys. Earth Planet. Inter. 72, Debayle, E., SV-wave azimuthal anisotropy in the Australian upper mantle: preliminary results from automated Rayleigh waveform inversion, Geophys. J. Int., submitted. Durrheim, R.J., Green, R.W.E., A seismic refraction investigation of the Archaean Kaapvaal Craton, South Africa, using mine tremors as the energy source. Geophys. J. Int. 108, Dziewonski, A.M., Anderson, D.L., Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, Dziewonski, A.M., Hales, A.L., Lapwood, E.R., Parametrically simple Earth models consistent with geophysical data. Phys. Earth Planet. Inter. 10, Foster, A.N., Jackson, J.A., Source parameters of large African earthquakes: implications for crustal rheology and regional kinematics. Geophys. J. Int. 134, Fuchs, K., Muller, G., Computation of synthetic seismograms with the reflectivity method and comparison with observations. Geophys. J. R. Astron. Soc. 23, Gilbert, F., Dziewonski, A.M., An application of normal mode theory to the retrieval of structural parameters and source mechanism from seismic spectra. Philos. Trans. R. Soc. London A 278, Gomberg, J.S., Masters, T.G., Waveform modelling using locked-mode synthetics and differential seismograms: application to determination of structure of Mexico. Geophys. J. Int. 94, Gomberg, J.S., Priestley, K., Masters, T.G., Brune, J.N., The structure of the crust and upper mantle of Northern Mexico. Geophys. J. Int. 94, Gurney, J.J., The diamondiferous roots of our wandering continents. South. Afr. J. Geol. 93, , Jordan, T.H., The continental tectosphere. Rev. Geophys. 13, Kennett, B.L.N., Gudmundsson, O., Tong, C., The uppermantle S and P velocity structure beneath northern Australia from broad-band observations. Phys. Earth Planet. Inter. 86, Masters, T.G., Johnson, S., Laske, G., Bolton, H., A

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