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

Abstract

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.

Depth Extent and Kinematics of Faulting in the Southern Tanganyika Rift, Africa

“…Although spatial gaps remain in our knowledge of the Western rift, the lowest P and S wave velocity regions underlie isolated volcanic provinces (e.g., Accardo et al, ; Mulibo & Nyblade, ; O'Donnell et al, ). Mantle anisotropy patterns determined from SKS‐splitting measurements for the Tanganyika‐Rukwa‐Malawi rift region are consistent with asthenospheric flow around cratonic roots, possibly enhanced by oriented melt pockets beneath the Rungwe Volcanic Province (Tepp et al, ).…”

“…Map of the study area (after Hodgson et al, ; Tepp et al, ) with major basins, border faults, and major faults in the S. Tanganyika‐Rukwa rift zone (after Delvaux et al, ; Ebinger, ; Kervyn et al, ; Morley, ). Purple inverted triangles are the array used in this study.…”

Lowermost Mantle Anisotropy Beneath Africa From Differential SKS‐SKKS Shear‐Wave Splitting

“…Many (~60%) of the stations used in this study are located in the East African Rift. Previous shear‐wave splitting studies in this region (Homuth et al, ; Gao et al, ; Bagley & Nyblade, ; Tepp et al, ) have shown that apparent splitting varies strongly both spatially and with backazimuth. This leads to the conclusion that anisotropy in the upper mantle beneath the East African Rift is highly complex, with strong lateral variation and multiple layers of anisotropy.…”

Kinematics of Active Deformation in the Malawi Rift and Rungwe Volcanic Province, Africa

“…Earthquake distributions, magnitudes, and source mechanisms from the seismically and volcanically active Malawi rift and Miocene‐Recent Rungwe Volcanic Province (RVP), Africa, provide critical constraints on fault kinematics and crustal rheology during rifting of cratonic lithosphere. Extensive crust and mantle imaging in this area has been undertaken as part of the Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) project and reanalyses of earlier data (Accardo et al, ; Borrego et al, ; Grijalva et al, ; McCartney & Scholz, ; Tepp et al, ). The SEGMeNT study area includes the site of a 2‐week‐long sequence of nine Mw 4.9‐6.0 earthquakes in 2009 (Biggs et al, ; Gaherty et al, ; Kolawole et al, , ), and a 2014 Mw 5.2 earthquake recorded by the SEGMeNT array.…”