The asthenosphere plays a fundamental role in present-day plate tectonics as its low viscosity controls how convection in the mantle below it is expressed at the Earth’s surface above. The origin of the asthenosphere, including the role of partial melting in reducing its viscosity and facilitating deformation, remains unclear. Here we analysed receiver-function data from globally distributed seismic stations to image the lower reaches of the asthenospheric low-seismic-velocity zone. We present globally widespread evidence for a positive seismic-velocity gradient at depths of ~150 km, which represents the base of a particularly low-velocity zone within the asthenosphere. This boundary is most commonly detected in regions with elevated upper-mantle temperatures and is best modelled as the base of a partially molten layer. The presence of the boundary showed no correlation with radial seismic anisotropy, which represents accumulated mantle strain, indicating that the inferred partial melt has no substantial effect on the large-scale viscosity of the asthenosphere. These results imply the presence of a globally extensive, partially molten zone embedded within the asthenosphere, but that low asthenospheric viscosity is controlled primarily by gradual pressure and temperature variations with depth.

软流圈在当今板块构造中起着重要作用，因为它的低粘度控制着其下方地幔中的对流如何在上方的地球表面表现出来。软流圈的起源，包括部分熔融在降低其粘度和促进变形方面的作用，仍不清楚。在这里，我们分析了来自全球分布的地震台站的接收函数数据，以对软流圈低地震速度区的下游进行成像。我们提供了全球范围内广泛存在的证据，证明在约 150 公里的深度处存在正地震速度梯度，这代表了软流圈内速度特别低的区域的底部。该边界最常在上地幔温度升高的区域检测到，最好将其建模为部分熔融层的底部。边界的存在与代表累积地幔应变的径向地震各向异性没有相关性，表明推断的部分熔化对软流圈的大尺度粘度没有实质性影响。这些结果表明在软流圈内存在一个全球广泛的、部分熔融的区域，但软流圈的低粘度主要受压力和温度随深度的逐渐变化控制。