The Effect of a Weak Asthenospheric Layer on Surface Kinematics, Subduction Dynamics and Slab Morphology in the Lower Mantle
The Effect of a Weak Asthenospheric Layer on Surface Kinematics, Subduction Dynamics and Slab Morphology in the Lower Mantle
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DOI：
10.1029/2022jb024494
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发表时间：
2022-08-01
影响因子：
3.9
通讯作者：
Mihalynuk, Mitchell G.
中科院分区：
文献类型：
--
作者：
Cerpa, Nestor G.;Sigloch, Karin;Mihalynuk, Mitchell G.
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下地幔板块下沉速率高5倍。使用简单的运动学分析，我们表明，如果这个今天的“运动状态”的操作到过去，俯冲岩石圈应该积累和折叠下近静止的沟槽。这些预测是一致的地震层析成像，图像本地化和扩大下地幔板桩。然而，它们与大多数动力俯冲模型不一致，动力俯冲模型预测了地幔过渡区的快速海沟后退和倾斜板片。我们测试的假设，一个弱的软流圈层（WAL）之间的岩石圈，软流圈边界和220公里的深度，兼容的地球物理约束，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，具有WAL的模拟预测地幔过渡区的板片堆积，以及下地幔中较厚的褶皱板片。一个WAL粘度只有2-5倍低于相邻的地幔是足够的俯冲制度转向垂直板块下沉和折叠模式下的近静止的沟槽，在广泛的模型参数，生产表面和板块速度接近那些在今天观察到的。这些发现为地球岩石圈下存在弱软流圈提供了支持，补充了来自各种地球物理数据的独立证据。
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.