To investigate the controls on continental rifting in the western branch of the East Africa Rift System, we conduct shear velocity imaging of the crust and uppermost mantle beneath the weakly extended Malawi Rift and the Rungwe Volcanic Province (RVP). We use local‐scale measurements of Rayleigh wave phase velocities between 9‐ and 100‐s periods combined with constraints on basin architecture and crustal thickness to invert for shear velocity from the surface to ~135 km. Our resulting 3‐D model reveals a localized low‐velocity anomaly associated with the RVP extending from the crust and through the upper mantle, which can be explained with modestly elevated temperatures. Away from the RVP, velocities within mantle flanking the rift are fast (>4.6 ± 0.1 km/s), suggesting depleted lithospheric mantle to depths of ~100 and >135 km to the west and east of the rift, respectively. The upper mantle beneath the rift axis is characterized by thinned lithosphere with slower velocities than the surrounding plateau, suggestive of thermal and/or chemical modification by the rifting process. Slowest velocities are mildly asymmetric about the rift axis, with the lowest velocities observed beneath the rift and adjacent footwall escarpments. The underlying asthenosphere is only moderately slow (~4.25 ± 0.1 km/s), including beneath the RVP, precluding the presence of significant volumes of partial melt. The positions of localized lithospheric modification and basin‐bounding border faults correlate with the location of Proterozoic mobile belts, suggesting that these sutures provide lithospheric‐scale weakening mechanisms necessary for localizing strain and allowing extension to occur in the Malawi Rift.

中文翻译：

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受剪切速度成像约束的北马拉维裂谷下上地幔的热化学修饰

为了研究东非裂谷系统西部分支对大陆裂谷的控制，我们对弱延伸的马拉维裂谷和Rungwe火山省（RVP）下的地壳和最高地幔进行了剪切速度成像。我们使用9到100 s周期之间的瑞利波相速度的局部量度，并结合盆地构造和地壳厚度的约束，将地表的剪切速度反演到约135 km。我们生成的3D模型揭示了与RVP关联的局部低速异常，该异常从地壳一直延伸到上地幔，并可以通过适度升高的温度来解释。距RVP较远的地方，裂谷两侧的地幔内部速度很快（> 4.6±0.1 km / s），这表明岩石圈地幔枯竭至约100和> 100的深度 裂谷以西和以东分别为135公里。裂谷轴下方的上地幔的特征是岩石圈变薄，其速度比周围高原的速度要慢，这表明裂谷作用过程会引起热和/或化学作用的改变。最低的速度围绕裂谷轴线略有不对称，而最低的速度则在裂谷和相邻的下盘壁陡崖之下观察到。潜在的软流圈只有适度缓慢（〜4.25±0.1 km / s），包括RVP之下，这排除了存在大量熔体的可能性。岩石圈局部变质的位置和盆地边界的边界断层与元古代活动带的位置有关，