SUMMARY Several interpretations of shear wave splitting measurements in East Africa have been proposed, but a comprehensive understanding of seismic anisotropy is lacking vis-à-vis geodynamic processes affecting the African Plate, in part because of geographically limited data coverage. Here, we report new shear wave splitting observations using PKS, SKS and SKKS phases recorded on 17 temporary AfricaArray seismic stations in northeastern Uganda and southeastern Tanzania that help fill key gaps in data coverage in non-volcanic regions of East Africa. The stations in northeastern Uganda straddle the Aswa shear zone, which separates the Neoproterozoic Saharan Metacraton (to the east) and the Palaeoproterozoic Northern Uganda Terrane (to the west). The stations in southeastern Tanzania are located mostly in Karoo basins formed within the Neoproterozoic Mozambique Belt. Our results show NNE fast polarization directions in northeastern Uganda, with splitting times between 1.0 and 1.5 s, and NE directions in southeastern Tanzania, with splitting times between 0.6 and 1.3 s. The NNE fast polarization directions in Uganda cannot be explained with oriented melt pocket (OMP) anisotropy in the lithosphere, as the stations are far from any volcanic fields, or fossil anisotropy resulting from the Neoproterozoic East African orogeny. They are, however, parallel to the flow direction of the African superplume and corroborate previous measurements from northern Uganda that were used to argue that the overall northerly orientation of fast polarization directions across East Africa can be attributed to sublithospheric mantle flow associated with the African superplume. The NE fast polarization directions in Tanzania cannot be explained with SE-oriented fossil anisotropy expected from the Mesozoic rifting of Madagascar from Africa or OMP anisotropy in the lithosphere. However, they are consistent with the overall northerly orientation of fast polarization directions in East Africa and can be attributed to either fossil anisotropy in the Mozambique Belt lithosphere resulting from the East African Orogeny or sublithospheric mantle flow associated with the African superplume.

中文翻译：

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乌干达东北部和坦桑尼亚东南部的横波分裂测量：东非地下岩石圈地幔流的确凿证据

总结 已经提出了对东非剪切波分裂测量的几种解释，但是相对于影响非洲板块的地球动力学过程，缺乏对地震各向异性的全面理解，部分原因是地理上的数据覆盖范围有限。在这里，我们使用乌干达东北部和坦桑尼亚东南部的 17 个临时 AfricaArray 地震台站记录的 PKS、SKS 和 SKKS 相位报告了新的剪切波分裂观测，这有助于填补东非非火山地区数据覆盖的关键空白。乌干达东北部的台站横跨阿斯瓦剪切带，该剪切带将新元古代撒哈拉超克拉通（向东）和古元古代乌干达北部地块（向西）分开。坦桑尼亚东南部的台站主要位于新元古代莫桑比克带内形成的卡鲁盆地。我们的结果显示乌干达东北部的 NNE 快速极化方向，分裂时间在 1.0 到 1.5 s 之间，而坦桑尼亚东南部的NE 方向，分裂时间在 0.6 和 1.3 s 之间。乌干达的 NNE 快速极化方向不能用岩石圈中的定向熔袋 (OMP) 各向异性来解释，因为这些台站远离任何火山场，或者新元古代东非造山运动产生的化石各向异性。然而，它们是 平行于非洲超级羽流的流动方向，并证实了乌干达北部先前的测量结果，这些测量结果被用来证明整个东非快速极化方向的整体北向可归因于与非洲超级羽流相关的岩石圈下地幔流。坦桑尼亚的 NE 快速极化方向不能用来自非洲马达加斯加的中生代裂谷预期的 SE 取向的化石各向异性或岩石圈中的 OMP 各向异性来解释。然而，它们与东非快速极化方向的整体偏北方向一致，并且可以归因于东非造山运动引起的莫桑比克带岩石圈中的化石各向异性或与非洲超级地幔相关的岩石圈下地幔流。