Surface wave imaging of the weakly extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers

Accardo, N. J.; Gaherty, J. B.; Shillington, D. J.; Ebinger, C. J.; Nyblade, A. A.; Mbogoni, G. J.; Chindandali, P. R. N.; Ferdinand, R.; Mulibo, G. D.; Kamihanda, G.; Keir, Derek; Scholz, Christopher A.; Selway, Kate; O’Donnell, J. P.; Tepp, Gabrielle; Gallacher, R. J.; Mtelela, K.; Salima, J.; Mruma, A.

doi:10.1093/gji/ggx133

Cited by 48 publications

(67 citation statements)

References 75 publications

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“…Although there is no surface expression of magmatism in the Tanganyika rift, Hodgson et al () determine high crustal v P / v S ratios from receiver function analyses and interpret new results and existing data as evidence for active magmatism in the lower crust beneath the southern Tanganyika rift. From ambient noise tomography, the lowest shear wave velocities in the upper mantle occur beneath the RVP and southern Rukwa rift, but they do not continue beneath the Malawi rift (Accardo et al, ). O'Donnell et al () also find low Pn and Sn beneath the RVP and Rukwa rift region, but they do not extend beneath the Malawi and Tanganyika rifts.…”

Section: Discussionmentioning

confidence: 99%

“…Holocene Plinian eruptions at Rungwe volcano and an historic eruption at Kiejo volcano within the RVP indicate active magma reservoirs beneath at least two of the volcanoes (e.g., Fontijn et al, 2012 (Accardo et al, 2017). O'Donnell et al (2013) also find low Pn and Sn beneath the RVP and Rukwa rift region, but they do not extend beneath the Malawi and Tanganyika rifts.…”

Section: Oriented Melt Pocketsmentioning

confidence: 99%

“…Receiver functions from the northern and central basins of the Malawi rift indicate thinning from ~44 to 33 km or ~25% extension (Borrego, ). Surface wave tomography indicates that lithospheric thinning is greatest beneath the Rungwe volcanic province, and results suggest that the zone of mantle lithospheric thinning and heating is 50–100 km broader than the width of the fault‐bounded basins (Accardo et al, ). Extensional strains associated with Miocene‐Recent rifting are therefore predicted to be weak and oriented E‐W.…”

Section: Tectonic Settingmentioning

confidence: 99%

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Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

et al. 2018

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Although the East African rift system formed in cratonic lithosphere above a large‐scale mantle upwelling, some sectors have voluminous magmatism, while others have isolated, small‐volume eruptive centers. We conduct teleseismic shear wave splitting analyses on data from 5 lake‐bottom seismometers and 67 land stations in the Tanganyika‐Rukwa‐Malawi rift zone, including the Rungwe Volcanic Province (RVP), and from 5 seismometers in the Kivu rift and Virunga Volcanic Province, to evaluate rift‐perpendicular strain, rift‐parallel melt intrusion, and regional flow models for seismic anisotropy patterns beneath the largely amagmatic Western rift. Observations from 684 SKS and 305 SKKS phases reveal consistent patterns. Within the Malawi rift south of the RVP, fast splitting directions are oriented northeast with average delays of ~1 s. Directions rotate to N‐S and NNW north of the volcanic province within the reactivated Mesozoic Rukwa and southern Tanganyika rifts. Delay times are largest (~1.25 s) within the Virunga Volcanic Province. Our work combined with earlier studies shows that SKS‐splitting is rift parallel within Western rift magmatic provinces, with a larger percentage of null measurements than in amagmatic areas. The spatial variations in direction and amount of splitting from our results and those of earlier Western rift studies suggest that mantle flow is deflected by the deeply rooted cratons. The resulting flow complexity, and likely stagnation beneath the Rungwe province, may explain the ca. 17 Myr of localized magmatism in the weakly stretched RVP, and it argues against interpretations of a uniform anisotropic layer caused by large‐scale asthenospheric flow or passive rifting.

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Section: Discussionmentioning

confidence: 99%

Section: Oriented Melt Pocketsmentioning

confidence: 99%

Section: Tectonic Settingmentioning

confidence: 99%

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Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

et al. 2018

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“…We incorporate measurements of fundamental‐mode Rayleigh‐wave velocity for frequencies 0.02–0.05 Hz from earthquake sources from a separate study (Janiszewski et al, ). These phase‐velocity maps are calculated via Helmholtz‐equation inversion from Rayleigh waves, sampled by arrays of stations to account for focusing effects (Accardo et al, ; Jin & Gaherty, ; Lin & Ritzwoller, ). Uncertainties for these measurements are calculated from the standard deviations of all individual earthquake phase‐velocity maps at each node.…”

Section: Methodsmentioning

confidence: 99%

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

et al. 2019

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Mount St. Helens (MSH) lies in the forearc of the Cascades where conditions should be too cold for volcanism. To better understand thermal conditions and magma pathways beneath MSH, data from a dense broadband array are used to produce high‐resolution tomographic images of the crust and upper mantle. Rayleigh‐wave phase‐velocity maps and three‐dimensional images of shear velocity (Vs), generated from ambient noise and earthquake surface waves, show that west of MSH the middle‐lower crust is anomalously fast (3.95 ± 0.1 km/s), overlying an anomalously slow uppermost mantle (4.0–4.2 km/s). This combination renders the forearc Moho weak to invisible, with crustal velocity variations being a primary cause; fast crust is necessary to explain the absent Moho. Comparison with predicted rock velocities indicates that the fast crust likely consists of gabbros and basalts of the Siletzia terrane, an accreted oceanic plateau. East of MSH where magmatism is abundant, middle‐lower crust Vs is low (3.45–3.6 km/s), consistent with hot and potentially partly molten crust of more intermediate to felsic composition. This crust overlies mantle with more typical wave speeds, producing a strong Moho. The sharp boundary in crust and mantle Vs within a few kilometers of the MSH edifice correlates with a sharp boundary from low heat flow in the forearc to high arc heat flow and demonstrates that the crustal terrane boundary here couples with thermal structure to focus lateral melt transport from the lower crust westward to arc volcanoes.

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“…Numerous recent studies have used joint inversion of phase velocities from ambient noise and teleseismic surface waves in regional tomography (the central Andean plateau, Ward et al, 2016;Madagascar, Pratt et al, 2017; and the Malawi rift, Accardo et al, 2017). Receiver functions have also been incorporated into 10.1029/2018GC007962 Geochemistry, Geophysics, Geosystems such inversions (e.g., Porritt et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

Seismic Imaging of the Alaska Subduction Zone: Implications for Slab Geometry and Volcanism

Martin‐Short

Allen

Bastow

et al. 2018

Geochem Geophys Geosyst

118

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Alaska has been a site of subduction and terrane accretion since the mid‐Jurassic. The area features abundant seismicity, active volcanism, rapid uplift, and broad intraplate deformation, all associated with subduction of the Pacific plate beneath North America. The juxtaposition of a slab edge with subducted, overthickened crust of the Yakutat terrane beneath central Alaska is associated with many enigmatic volcanic features. The causes of the Denali Volcanic Gap, a 400‐km‐long zone of volcanic quiescence west of the slab edge, are debated. Furthermore, the Wrangell Volcanic Field, southeast of the volcanic gap, also has an unexplained relationship with subduction. To address these issues, we present a joint ambient noise, earthquake‐based surface wave, and P‐S receiver function tomography model of Alaska, along with a teleseismic S wave velocity model. We compare the crust and mantle structure between the volcanic and nonvolcanic regions, across the eastern edge of the slab and between models. Low crustal velocities correspond to sedimentary basins, and several terrane boundaries are marked by changes in Moho depth. The continental lithosphere directly beneath the Denali Volcanic Gap is thicker than in the adjacent volcanic region. We suggest that shallow subduction here has cooled the mantle wedge, allowing the formation of thick lithosphere by the prevention of hot asthenosphere from reaching depths where it can interact with fluids released from the slab and promote volcanism. There is no evidence for subducted material east of the edge of the Yakutat terrane, implying the Wrangell Volcanic Field formed directly above a slab edge.

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Surface wave imaging of the weakly extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers

Cited by 48 publications

References 75 publications

Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

Seismic Anisotropy of the Upper Mantle Below the Western Rift, East Africa

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

Seismic Imaging of the Alaska Subduction Zone: Implications for Slab Geometry and Volcanism

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