Subducting slabs are an important driver of plate motions, yet the relative importance of different forces in governing subduction motions and styles remains incompletely understood. Basal drag has been proposed to be a minor contributor to subduction forcing because of the lack of correlation between plate size and velocity in observed and reconstructed plate motions. Furthermore, in single subduction system models, low basal drag leads to subduction behaviour most consistent with the observation that trench migration velocities are generally low compared to convergence velocities. By contrast, analytical calculations and global mantle flow models indicate basal drag can be substantial. In this study, we revisit this problem by examining the drag at the base of the lithosphere, for a single subduction system, in 2D models with a free trench and composite non-linear rheology. We compare the behaviour of short and long plates for a range of asthenospheric and lithospheric rheologies. We reproduce results from previous modelling studies, including low ratios of trench over plate motions. However, we also find that any combination of asthenosphere and lithosphere viscosity that produces Earth-like subduction behaviour leads to a correlation of velocities with plate size, due to the role of basal drag. By examining Cenozoic plate motion reconstructions, we find that slab age and plate size are positively correlated: higher slab pull for older plates tends to be offset by higher basal drag below these larger plates. This, in part, explains the lack of plate velocity–size correlation in observations, despite the important role of basal drag in the subduction force balance.

中文翻译：

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基础阻力对二维数值模型俯冲动力学的影响

俯冲板块是板块运动的重要驱动力，但是对支配俯冲运动和样式的不同作用力的相对重要性仍未完全理解。由于在观测的和重建的板块运动中板块大小和速度之间缺乏相关性，因此基础阻力被认为是俯冲强迫的次要贡献者。此外，在单个俯冲系统模型中，低基阻力导致俯冲行为，这与以下观点最一致：沟渠迁移速度通常比收敛速度低。相比之下，分析计算和整体地幔流动模型表明基础阻力可能很大。在这项研究中，我们通过检查岩石圈底部的阻力来研究单个俯冲系统，从而重新探讨了这个问题，在具有自由沟槽和复合非线性流变学的2D模型中。我们比较了各种软流圈和岩石圈流变性的短板和长板的行为。我们重现了先前建模研究的结果，包括沟槽与板块运动的比率较低。但是，我们还发现，由于基底阻力的作用，软流层和岩石圈粘度的任何组合都会产生类似于地球的俯冲行为，从而导致速度与板块尺寸相关。通过研究新生代板块运动重构，我们发现板块年龄和板块大小呈正相关：旧板块的较高板块拉力往往会被这些较大板块下方的较高基础阻力所抵消。这部分地解释了观测中缺乏板速与尺寸的相关性，