Morgan et al.， 2013的介绍很好地总结了您所询问的特征，以及提出了为什么会发生这种情况的机制。

导读:

地壳构造板块下方的地幔，通常被称为软流圈，是上地幔粘度最低的区域。几种可能的机制被认为是造成这种低粘度带的原因。这些机制……包括:(1)地幔流变学对温度和压力的依赖可能导致70 ~ 200 km深度之间的粘度最小;(2)少量不流动的部分熔体可能削弱地幔;(3)干固相层以下的湿地幔(深度约70 km)可能比在洋中脊部分融化时脱水的浅地幔更弱，但这一机制并不能解释软流圈底部的物理起源;(4)软流圈内地幔颗粒尺寸减小。< / p >

The paper's proposed cause for this is that:

the sub-oceanic asthenosphere forms because it is the ‘graveyard’ for rising (i.e. hotter-than-average mantle) plumes. In this view, below the plate-age-dependent 60–100 km-thick oceanic lithosphere there exists a pool of hot plume material that has risen as far as the overlying thermal and/or compositional lithosphere will allow.

Whether that mechanism is correct or not, only time will tell. In conclusion, due to the temperature and pressure characteristics of the asthenosphere, it is less viscous than the chemically identical mesosphere. This may be because of its place on temperature and pressure curves, because it contains pockets of partially molten rock, because rising mantle plumes make the asthenosphere actually hotter than the mesosphere, or otherwise.