Conclusión

An apparent 20 km thinning of the MTZ is observable beneath the interior of the western Ethiopian Plateau between the latitudes of 10°N and 14°N (Figures 4f and 7).

Because this region is on the edge of the study area and is sampled by a limited number of RFs, we use greater bin radii of 1.5° and 2.0° (Figures S2 and S3) to confirm the existence of MTZ thinning (Figure S4). The thinning can be explained by an upper-mantle LVZ of lower magnitude than that beneath the Afar overlying a positive thermal anomaly which traverses the whole transition zone (Figure 7). The amount of apparent depression of both discontinuities corresponds to a constant -1 per cent Vp and a -2 per cent Vs anomaly and associated temperature increase extending from the surface to below the d660 (model D in Mohamed et al. [2014]). If Clapeyron slope values of +2.9 MPa/K and -2.1 MPa/K are considered for the d410 and d660, respectively [Bina and Helffrich, 1994], then the 20 km thinning corresponds to a temperature anomaly of approximately 170 °C which is well within the heat budget from a plume originating from the core-mantle boundary [Dannberg & Sobolev, 2015]. A plume stem beneath the western Plateau (Figure 7) is consistent with the location of the suggested single-plume hypothesis of Ebinger and Sleep [1998] as well as the location of the oldest shield

volcano within the Western Plateau which Kieffer et al. [2004] attributed to an Oligocene

Figure 7. (a) Stacked RFs for bins of 2° radius along latitude 13°N with bold continuous lines tracing the observed apparent depths of the d410 and d660 for well-defined values and dashed line segments for unusable values. (b) Surface topography profile (top) and an

MTZ discontinuity model (bottom) along latitude 13°N after a hypothetical velocity correction.

plume. Low velocities beneath the Western Plateau detected at depths of >400 km by Bastow et al. [2008] are also consistent with our findings.

Numerical models tested by Dannberg and Sobolev [2015] show that it is possible for sufficiently large, lower mantle-origin plumes characterized by the entrainment of high-viscosity, eclogitic oceanic crust to demonstrate a plume tail with a diameter in excess of 500 km and a longevity of 200 Ma in the MTZ. These features of a low-buoyancy plume stem may account for the broad region of 20-km thinning of the MTZ beneath the Western Plateau (Figure 7). We must note, however, that without appropriate velocity corrections, we cannot resolve whether this apparent thinning is the result of such a plume or possibly a combination of low- and high-velocity material above the d410 and within the MTZ, respectively, as the latter would serve to induce the observed uplift of the d660. Additional data acquired in western Ethiopia away from the rift system would serve to approach this subject more appropriately.

5. CONCLUSIONS

This study has utilized an unprecedented quantity of over 14,500 high-quality radial RFs recorded over the past two decades by stations belonging to a compilation of 12 independent networks within the Afar Depression and adjacent regions to image the apparent depths of the 410 and 660 km discontinuities. Contrary to some of the previous studies utilizing a lower quantity of RFs, we have observed robust P-to-S conversions for both the d410 and d660 throughout the study area. The apparent depths of both

discontinuities regionally exceed the global averages and are generally parallel to each other, and thus indicate a broad upper mantle low-velocity anomaly stretching beneath the entirety of the study area. Parallel MTZ discontinuities and normal transition zone thickness beneath the Afar Depression indicate the absence of thermal anomalies deeper than 410 km, and an ~20 km thinning of the MTZ beneath the western Ethiopian Plateau may suggest focused upwelling associated with an active mantle plume stem originating from the lower mantle. Thickening of the MTZ of about 15 km in magnitude beneath the joint area of the southern Arabian Plate, southern Red Sea, and western Gulf of Aden suggests substantial MTZ hydration, most likely as the result of Precambrian subduction processes.

SUPPLEMENTARY INFORMATION

Table S1. Breakdown of the 12 individual seismic networks used in this study as well as the total number of stations used from each network, the number of receiver functions obtained, and the period of operation.

Network Name

Broadband Experiment XI 2000-2002 27 1247 Nyblade & Langston (2002)

Broadband Experiment YR 1999-2002 5 762 Montagner et al.

(2007)

Table S2. Results of bootstrap resampling averaging of picked MTZ discontinuity depths for all 1° radius circular bins with 10 or more high-quality receiver functions.

Circle

Table S2. Continued.

Table S2. Continued.

Figure S1. Stacked latitudinal traces of time-series receiver functions (RFs) converted into depth-series RFs using the IASP91 Earth model velocities. Dashed lines represent the standard error of the binned trace amplitude from a bootstrap sample of 50 traces.

Black dots with associated standard error mark the depth value obtained from bootstrap resampling of 50 resampled traces for each 1° radius bin.

Figure S1. Continued.

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Figure S2. Same as Figure S1, but for stacked latitudinal traces of time-series RFs for bins of 1.5° radius.

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Figure S3. Same as Figure S1, but for stacked latitudinal traces of time-series RFs for bins of 2.0° radius.

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Figure S4. Resulting apparent depths for bins of 1.5° radius (panels a through c) and 2.0°

radius (panels d through f) for the d410 (a & d) and d660 (b & e) and for the MTZ thickness (c & f) with a minimum hit count of 10 RFs per bin. Dashed circles in panels

(c) and (f) mark the proposed location of the plume stem beneath the Western Plateau from Figure 4. Green dots in (f) mark the bin centers of the profile used in Figure 7a.

Figure S5. Resulting apparent depths in 1° radius bins with minimum RF counts of 100 (panels a through c) and 200 (panels d through f) for the d410 (a & c), d660 (b & e), and

the MTZ thickness (c & f). Dashed circles in panels (c) and (f) mark the proposed location of the plume stem beneath the Western Plateau from Figure 4.

Figure S6. RF Stacks for the Western Plateau (left, 1474 RFs), Afar Depression (center, 7159 RFs), and Gulf of Aden (right, 1260 RFs) binned RFs. The top panel depicts the summed RF stack of all RFs from each respective area, while the three bottom panels demonstrate the binned stacks from each region sorted according to increasing apparent

depth of the d410 for bins of radius 1.0°, 1.5°, and 2.0°.

Figure S7. Resulting apparent depths in 1° radius bins for (a) the d410 using piercing points computed for 410 km and (b) the d660 using piercing points computed for 660 km

with a minimum hit count of 10 RFs per bin. (c) MTZ apparent thickness using the arithmetic difference between the values from panels (a) and (b). Dashed circle in (c)

indicates the proposed location of the plume stem from Figure 4.

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III. PASSIVE RIFTING OF THICK LITHOSPHERE IN THE SOUTHERN EAST AFRICAN RIFT: EVIDENCE FROM MANTLE TRANSITION

ZONE DISCONTINUITY TOPOGRAPHY

ABSTRACT

To investigate the mechanisms responsible for the initiation and early-stage evolution of the non-volcanic southernmost segments of the East African Rift System (EARS), we installed and operated 35 broadband seismic stations across the Malawi and Luangwa rift zones over a two-year period from mid-2012 to mid-2014. Stacking of over 1,900 high-quality receiver functions provides the first regional-scale image of the 410 and 660-km seismic discontinuities bounding the mantle transition zone (MTZ) within the vicinity of the rift zones. When a 1-D standard Earth model is used for time-depth conversion, a normal MTZ thickness of 250 km is found beneath most of the study area.

In addition, the apparent depths of both discontinuities are shallower than normal with a maximum apparent uplift of 20 km, suggesting widespread upper-mantle high-velocity anomalies. These findings suggest that it is unlikely for a low-velocity province to reside within the upper mantle or MTZ beneath the non-volcanic southern EARS. They also support the existence of relatively thick and strong lithosphere corresponding to the widest section of the Malawi rift zone, an observation that is consistent with strain-localization models and fault polarity and geometry observations. We postulate that the Malawi Rift is driven primarily by passive extension within the lithosphere attributed to the divergent rotation of the Rovuma microplate relative to the Nubian plate, and that contributions of thermal upwelling from the lower mantle are insignificant in the initiation and early-stage development of rift zones in southern Africa.

1. INTRODUCTION

The East African Rift System (EARS) represents various stages of continental rifting, from the magmatic rift propagators of the Afar Depression currently transitioning toward incipient seafloor spreading [Bastow et al., 2011; Bridges et al., 2012; Reed et al., 2014] to the incipient amagmatic Okavango Rift Zone of Botswana [Leseane et al., 2015;

Yu et al., 2015a] (Figure 1a), and thus is an ideal tectonic feature for investigating rifting mechanisms. Seismicity patterns as well as kinematic GPS residual velocity analysis [Saria et al., 2014] clearly delineate the rift segments of the Eastern and Western Branches of the EARS, which divide the African lithosphere into the Nubian and Somalian plates and the Victoria and Rovuma microplates (Figure 1a). The Malawi Rift Zone (MRZ) constitutes the southernmost segment of the Western Branch separating the Nubian plate and the Rovuma microplate (Figure 1b), having initiated no earlier than

~8.6 Ma with the onset of volcanics within the Rungwe Volcanic Province in the vicinity of the Nubia-Rovuma-Victoria Triple Junction [Ebinger et al., 1989a; Delvaux et al., 1992]. The MRZ, which is non-volcanic except for its northern tip, demonstrates a succession of alternating polarity half-grabens which display along-strike variations of maturity [Ebinger et al., 1987; Laó-Dávila et al., 2015] and potentially propagates southward into Mozambique via the Urema Graben [e.g., Fonseca et al., 2014]. The Luangwa Rift Zone (LRZ) of Zambia, in contrast, represents the initial continuation of the EARS into the Southwestern Branch and is a Permo-Triassic rift basin which is potentially being reactivated by the current stress regime [Banks et al., 1995]. The area of the current study is sutured together by a number of Proterozoic Pan-African mobile belts, within which the Malawi and Luangwa rift zones reside [Fritz et al., 2013].

Through the application of a variety of seismic tomography [Ritsema et al., 1999;

Mulibo and Nyblade, 2013b], geodynamic and waveform modeling [Ni et al., 2005;

Simmons et al., 2007], and seismic receiver function techniques [Owens et al., 2000;

Huerta et al., 2009; Mulibo and Nyblade, 2013a], it has been suggested that eastern Africa is underlain by a massive low-velocity structure, probably stretching from the core-mantle boundary to beneath the Eastern and Western Branch rift segments. Shear-wave splitting measurements obtained adjacent to the Tanzania Craton, the Main

Ethiopian Rift, and the Arabian Peninsula [Bagley and Nyblade, 2013] are attributed to mantle flow modulated by northward advection of the African Superplume which upwells through the MTZ beneath the northern MRZ. However, shear-wave splitting studies conducted throughout north-central Africa and Saudi Arabia also demonstrate north-south mantle flow patterns which are attributed to basal traction-generated

anisotropy from simple northward plate migration relative to the underlain asthenosphere [Elsheikh et al., 2014; Lemnifi et al., 2015]. Similarly, shear-wave splitting evidence from the Okavango Rift Zone (ORZ, Figure 1a) and southern Africa [Silver et al., 2001; Yu et al., 2015b] indicates that subcrustal mantle anisotropy can be explained by plate motion and lithospheric Archean structures, and are not associated with a contemporary flow system related to the proposed African Superplume.

One method often employed to examine the present-day thermal state of the upper mantle and mantle transition zone (MTZ), and consequently the present-day existence or absence of thermal upwelling through the MTZ, is to study the spatial distribution of the

One method often employed to examine the present-day thermal state of the upper mantle and mantle transition zone (MTZ), and consequently the present-day existence or absence of thermal upwelling through the MTZ, is to study the spatial distribution of the

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