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Abstract. P-wave and S-wave delay times from the broadband data of the southern Africa seismic experiment have been inverted to obtain three-dimensional images of velocity perturbations in the mantle beneath southern Africa. High velocity mantle roots appear to extend to depths of at least 250 km, and locally to depths of 300 km beneath the Kaap-

Abstract. The assertion of cratonic stability put forward in the model for deep continental structure can be tested by examining upper mantle structure beneath the Tanzania Craton, which lies within a tectonically active region in east Africa. Tomographic inversions of about 1200 teleseismic P and $ travel times indicate that high-velocity lithosphere beneath the Tanzania Craton extends to a depth of at least 200 km and possibly to 300 or 350 km. Based on the thickness of mantle lithosphere beneath Archean cratons elsewhere, it appears that the mantle lithosphere of the Tanzania Craton has not been extensively disrupted by the Cenozoic tectonism in east Africa, thus corroborating the assertion of cratonic stability in the model for deep continental structure. The presence of thick, high-velocity structure beneath the Tanzania Craton implies relatively low temperatures within the cratonic mantle lithosphere, consistent with relatively low surface heat flow. The thick cratonic keel is surrounded by low seismic velocity regions beneath the east African rifts that extend to depths below 400 km. Our models show a shear velocity contrast between the cratonic lithosphere and the uppermost mantle beneath the eastern branch of the rift system of about 5% to 6%, but from resolution experiments we infer that this contrast could be underestimated by as much as a factor of 1.5. We attribute about half of this velocity contrast to the depleted composition of the cratonic keel and the other half to thermal alteration of upper mantle beneath the rifts. Low-density structures that may be required to provide buoyant support for the elevation of the Tanzania Craton must reside at depths greater than about 300-350 km.

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[1] We present the result from teleseismic travel time inversions for P and S wave data mainly recorded at portable broadband stations in SE Brazil. The stations were deployed at 45 sites within an area 1000 Â 1700 km during the years 1992-1999. More than 10,000 relative P and S wave arrival times, including core phases, were obtained from the waveforms using a new coherence functional. These P and S relative phase times are independently inverted for slowness perturbations, earthquake relocations, and station corrections. The final models represent the least amount of structure required to explain the residuals within a defined standard error. The robust and consistent features in the velocity anomaly models are interpreted and their resolution is tested with synthetic case inversions. We confirm the existence of a cylindrical low-velocity anomaly beneath the Paraná basin, which has been interpreted by VanDecar et al. [1995] as the fossil conduit through which the initial Tristan da Cunha plume head traveled to generate the Paraná -Etendeka flood basalts about 130 Ma. We now show that this low-velocity cylindrical structure seems to be confined to the upper mantle. Beneath the upper mantle, the velocity anomalies show a N-S oriented pattern, which we interpret as due to the Nazca plate subducted slab. At lithospheric depths, the Archean, southern part of the São Francisco craton, shows high velocities down to 200-300 km. All areas with Late Cretaceous postrift alkaline intrusions are characterized by low velocities at lithospheric depths.INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 8180 Tectonophysics: Tomography; 9360 Information Related to Geographic Region: South America; KEYWORDS: seismic regional tomography, SE Brazil, Paranã-Etendeka flood basalts, Tristan da Cunha plume, subducted Nazca plate, alkaline intrusions Citation: Schimmel, M., M. Assumpção, and J. C. VanDecar, Seismic velocity anomalies beneath SE Brazil from P and S wave travel time inversions,

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[1] P and S wave travel times from teleseismic earthquakes recorded by the Transantarctic Mountains Seismic Experiment (TAMSEIS) have been used to tomographically image upper mantle structure beneath portions of the Transantarctic Mountains (TAM), the East Antarctic (EA) craton, and the West Antarctic rift system (WARS) in the vicinity of Ross Island, Antarctica. The TAM form a major tectonic boundary that divides the stable EA craton and the tectonically active WARS. Relative arrival times were determined using a multichannel cross-correlation technique on teleseismic P and S phases from earthquakes with m b ! 5.5. 3934 P waves were used from 322 events, and 2244 S waves were used from 168 events. Relative travel time residuals were inverted for upper mantle structure using VanDecar's method. The P wave tomography model reveals a low-velocity anomaly in the upper mantle of approximately dVp = À1 to À1.5% in the vicinity of Ross Island extending laterally 50 to 100 km beneath the TAM from the coast, placing the contact between regions of fast and slow velocities well inland from the coast beneath the TAM. The magnitude of the low-velocity anomaly in the P wave model appears to diminish beneath the TAM to the north and south of Ross Island. The depth extent of the low-velocity anomaly is not well constrained, but it probably is confined to depths above $200 km. The S wave model, within resolution limits, is consistent with the P wave model. The low-velocity anomaly within the upper mantle can be attributed to a 200-300 K thermal anomaly, consistent with estimates obtained from seismic attenuation G 3