On Earth, the velocity at which subducting plates are consumed at their trenches (termed “subduction rate” herein) is typically 3 times higher than trench migration velocities. The subduction rate is also 5 times higher than estimated lower mantle slab sinking rates. Using simple kinematic analyses, we show that if this present-day “kinematic state” operated into the past, the subducting lithosphere should have accumulated and folded beneath near-stationary trenches. These predictions are consistent with seismic tomography, which images localized and widened lower-mantle slab piles. They are, however, at odds with most dynamic-subduction models, which predict rapid trench retreat and inclined slabs in the mantle transition zone. We test the hypothesis that a weak asthenospheric layer (WAL), between the lithosphere-asthenosphere boundary and 220 km depth, compatible with geophysical constraints, can remedy the discrepancies between numerical models and observations. The WAL lubricates the base of the lithosphere, increases the subduction rate while reducing trench retreat. As a consequence, simulations featuring a WAL predict slab accumulation at the mantle transition zone, and thicker, folded slabs in the lower mantle. A WAL viscosity only 2–5 times lower than that of the adjacent mantle is sufficient to shift subduction regimes toward a mode of vertical slab sinking and folding beneath near-stationary trenches, across a wide range of model parameters, producing surface and slab velocities close to those observed at the present-day. These findings provide support for the existence of a weak asthenosphere beneath Earth's lithosphere, complementing independent evidence from various geophysical data.

中文翻译：

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弱软流圈层对下地幔表面运动学、俯冲动力学和板块形态的影响

在地球上，俯冲板块在其海沟处被消耗的速度（本文称为“俯冲速率”）通常是海沟迁移速度的 3 倍。俯冲速率也比估计的低地幔板下沉速率高 5 倍。使用简单的运动学分析，我们表明，如果这种当今的“运动学状态”延续到过去，俯冲岩石圈应该在近乎静止的海沟下积累和折叠。这些预测与地震层析成像是一致的，地震层析成像对下地幔板桩进行定位和加宽。然而，它们与大多数动态俯冲模型不一致，后者预测地幔过渡带的沟槽快速退缩和倾斜的板片。我们检验弱软流圈层 (WAL) 的假设，岩石圈-软流圈边界与 220 km 深度之间，与地球物理约束兼容，可以弥补数值模型和观测之间的差异。WAL 润滑岩石圈底部，增加俯冲速率，同时减少海沟后退。因此，以 WAL 为特征的模拟预测了地幔过渡带的板片堆积，以及下地幔中更厚的折叠板片。仅比邻近地幔低 2-5 倍的 WAL 粘度足以将俯冲状态转变为垂直板片下沉和折叠模式，在近静止的海沟下，在广泛的模型参数范围内，产生接近的地表和板片速度到今天观察到的人。这些发现为地球下方存在弱软流圈提供了支持。