Abstract
AbstractAlthough 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.
Highlights
Earth’s continental plates are heterogeneous in composition, thickness, and rheology, leading to differences in their response to mantle flow as well as forces applied at their boundaries
Based on the combined networks and earlier data sets, we find little evidence that the East African rift imparts a significant rift perpendicular strain fabric
We measured shear wave splitting parameters for the SEGMeNT, TANGA14, and KIVU12 arrays in the weakly magmatic Western rift to characterize the upper mantle anisotropy of the region and to better understand cratonic rifting processes
Summary
Earth’s continental plates are heterogeneous in composition, thickness, and rheology, leading to differences in their response to mantle flow as well as forces applied at their boundaries. The primary upper mantle constituent, is strongly anisotropic, and its fast direction often aligns parallel to the plate transport direction or other sustained strain (e.g., Hansen et al, 2014; Hess, 1964; Kendall et al, 2006; Silver & Chan, 1991). The mantle lithosphere may contribute to observed anisotropy through oriented melt pockets with a preferential orientation due to a sustained stress field (e.g., Gao et al, 1997; Holtzman & Kendall, 2010; Kendall et al, 2006) and to fossil strain fabrics accrued during orogenesis (e.g., Savage, 1999; Silver & Chan, 1991; Tommasi et al, 2009; Walker et al, 2004). The direction of anisotropy can be used to discriminate between these potential sources of anisotropy, providing insight to the lithospheric structure of the study area (e.g., Yu et al, 2015)
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