[1] The Archean Tanzanian craton, nestled between the eastern and western branches of the East African Rift, presents a unique opportunity to study the interaction of active rifting with stable cratonic lithosphere. The high density of Rayleigh wave paths recorded in a regional seismic array yields unusually precise determinations of phase velocity within the Tanzanian craton. Shear velocities in the cratonic lithosphere are higher than a global average to a depth of 150 ± 20 km. Beginning at 140 km, shear velocity decreases sharply, reaching a minimum of 4.20 ± 0.05 km/s at depths of 200-250 km. The base of the lithosphere, identified by the depth to the center of the maximum negative velocity gradient, is similar to that found beneath other Archean lithospheres. Where Cenozoic rifting crosscuts the southern corner of the craton, velocities up to 130 km depth are reduced, indicating recent disruption of the lithosphere. The anomalously low velocities beneath the Tanzanian craton indicate high temperatures and the presence of melt, consistent with the spreading of a mantle plume head beneath the craton. Tests for the possibility of a radial pattern of azimuthal anisotropy that may indicate outward flow from a plume show that a model with average anisotropy of 0.71 ± 0.17% centered SE of Lake Victoria fits the data significantly better than a uniform, single direction of anisotropy. Thus our results agree with the suggestion that an upper mantle plume, centered beneath the Tanzanian cratonic lithosphere, provides the buoyancy required for uplift of the East African Plateau.

SUMMARY The shear wave velocity structure of the upper mantle beneath the East African plateau has been investigated using teleseismic surface waves recorded on new broadband seismic stations deployed in Uganda and Tanzania, as well as on previously deployed stations in Tanzania and Kenya. Rayleigh wave phase velocities at periods between 20 and 182 s, measured with a two‐plane wave method, have been used to create phase velocity maps, and dispersion curves extracted from the maps have been inverted to obtain a quasi‐3‐D shear wave velocity model of the upper mantle. We find that phase velocities beneath the Tanzania Craton and areas directly north and west of the craton are faster, at all periods, than those beneath the Western and Eastern branches of the East African Rift System. At periods <50 s, the western branch is slower than the Eastern Branch, but at periods greater than 50 s, this relationship is reversed. Anisotropy is found at all periods, with a generally north–south fast polarization direction. The shear wave velocity model shows a seismically fast lithosphere (lid) beneath the Tanzania Craton to depths between 150 and 200 km. The fast velocities in this depth range extend to the north beneath the Uganda Basement Complex and to the east beneath the northern Tanzania divergence zone, indicating that these regions together form a rigid block around which rifting has occurred within weaker mobile belt lithosphere. The Eastern and Western branches are slower than the craton at lithospheric mantle depths, and both branches show variable structure in the upper 200 km of the mantle, with the lowest velocities found beneath areas of Cenozoic volcanism. At depths greater than ∼225 km, a low velocity anomaly is present beneath the entire East African plateau that may extend into the mantle transition zone. Velocities in the low velocity region are reduced by ≥10 per cent relative to lid velocities, and if attributed only to temperature variations, would represent an unrealistic thermal perturbation of >400 K. Consequently, it is likely that the velocity reduction reflects a combination of thermal and compositional changes, and also possibly the presence of partial melt. The width and thickness of the low velocity anomaly is greater than typically expected for a plume head and is more easily attributed to an upward continuation of the lower mantle African superplume structure into the upper mantle.

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a b s t r a c t a r t i c l e i n f oDirect seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges. On the East Pacific Rise, at 17°S, there were two long-term broadband ocean bottom seismometer deployments, the MELT and GLIMPSE experiments. These arrays spanned young Pacific plate seafloor ranging in age from 0 to 8 Ma. Combining these two data sets, we describe the increase in Rayleigh wave phase velocities for 16-100 s period as function of distance from the ridge with two parameterizations: an arbitrary function of seafloor age and a simple polynomial function of age on the Pacific and Nazca plates. Although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the Pacific plate can be resolved at periods of ≤50 s, the three parameter polynomial model fits the data almost as well, indicating a simple pattern of the evolution of structure with age. To compare our observations to the predictions for the conductive cooling of the lithosphere, we convert a thermal half-space cooling model to shear velocity using anharmonic and anelastic contributions. The patterns in the velocities are not consistent with simple conductive cooling. The conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range. We observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend. The asthenospheric low velocity zone is asymmetric, dipping to the west with the lowest velocities beneath the Pacific plate west of the spreading center. The conductive cooling model also over predicts the shear velocities in the lithosphere by~0.2 km/s. These anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required.

[1] We test models for the origin of intraplate volcanic ridges and gravity lineations on young seafloor west of the East Pacific Rise using Rayleigh wave dispersion measured in the Gravity Lineations and Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) seismic experiment. The excellent azimuthal distribution of teleseismic sources recorded over a 12-month period provides resolution of phase velocities at periods up to 100 s. The average phase velocities for the study area reveal a pronounced low-velocity zone reaching a minimum shear velocity of $3.95 km/s. The negative velocity gradient defining the base of the lithosphere, observed at 40 ± 15 km, abruptly reverses at 70 km depth. The underlying positive gradient changes slope at $125 km. We attribute these changes in gradient to the onset of incipient partial melting of upwelling mantle in the presence of water at 125 km, followed by increased melt production at 70 km that leads to dehydration of the residual matrix and migration of melt to the surface spreading center. Rayleigh wave tomography shows that there are anomalously low shear velocities extending to at least 50 km depth beneath the Sojourn Ridge and the Hotu Matua volcanic complex, with relatively high velocities between these volcanic chains. These observations are not consistent with passive models for the origin of the volcanic ridges involving lithospheric extension or thermoelastic cracking. Dynamic models invoking flow in the asthenosphere in the form of small-scale convection or viscous fingering instabilities may explain the observed pattern of seismic velocity anomalies.

Rayleigh wave phase and amplitude data are analyzed to provide new insight into the velocity structure of the upper mantle beneath the Slave craton, in the northwestern Canadian Shield. We invert for phase velocities at periods between 20 s–142 s (with greatest sensitivity at depths of 28–200 km) using crossing ray paths from events recorded by the POLARIS broadband seismic network and the Yellowknife array. Phase velocities obtained for the Slave province are comparable to those from other cratons at shorter periods, but exceed the global average by ∼2% at periods above 60 s, suggesting that the Slave craton may be an end member in terms of its high degree of mantle depletion. The one‐dimensional inversion of phase velocities yields high upper‐mantle S‐wave velocities of 4.7 ± 0.2 km/s that persist to 220 ± 65 km depth and thus define the cratonic lithosphere. Azimuthal anisotropy is well resolved at all periods with a dominant fast direction of N59°E ± 20°, suggesting that upper mantle anisotropy beneath the Slave craton is influenced by both lithospheric fabric and sub‐lithospheric flow.

S U M M A R YWe analysed background surface waves in seismic ambient noise by cross-correlating continuous records of eight ocean bottom seismometers and nine differential pressure gauges deployed in the northwestern Pacific Ocean by the PLATE project. After estimating the clock delay and instrumental phase responses of differential pressure gauges by using cross-correlation functions, we measured average phase velocities in the area of the array for the fundamental-, first higher-and second higher-mode Rayleigh waves, and the fundamental-mode Love waves at a period range of 3-40 s by waveform fitting. We then measured phase-velocity anomalies of fundamental-mode and first higher-mode Rayleigh waves for each pair of stations at a period range of 5-25 s, and corrected the effect of variation in water-depths. The seismic anomalies imply the presence of strong azimuthal anisotropy beneath the eastern part of array. The direction of maximum velocity is approximately N35• E in the fossil seafloor spreading direction perpendicular to magnetic lineations from the ancient triple junction at this location. The peak-to-peak intensity of shear-wave velocity anisotropy in the mantle is ∼7 per cent.

Pn is the high-frequency, scattered P phase guided for great distances within the old oceanic lithosphere. Two arrays of ocean bottom seismometers were deployed on old (150-160 Ma) seafloor in the northwestern Pacific south of Shatsky Rise for the Pacific Lithosphere Anisotropy and Thickness Experiment. We use Pn phases from 403 earthquakes during the 1 year of deployment to measure apparent velocities across the arrays. Each array was deployed on a separate limb of a magnetic bight, formed near a fast-spreading, ridge-ridge-ridge triple junction. Using high-frequency waves (5-10 Hz), we look at variations of Pn velocities as a function of azimuth. In the western array, we find Pn anisotropy with velocities ranging from~8.7 km/s in the back azimuth (θ) direction of 310°to~7.7 km/s at~350°. In the eastern array, the velocity ranges from~8.5 km/s in back azimuth direction of~210°to~7.7 km/s at 260°and~310°. We observe rapid velocity changes with azimuth in the both arrays requiring sinusoidal variations of roughly equal amplitude as a function of both 2θ and 4θ, which is not expected for the orthorhombic symmetry of olivine or orthopyroxene. The fastest directions on the two limbs are roughly orthogonal to each other suggesting the dominance of fossil anisotropy, but the fast directions of the 2θ components are skewed counterclockwise from the spreading directions. We speculate that the rapid azimuthal variations may be caused by vertical stratification with changing anisotropy with depth in the oceanic lithosphere.