Numerous processes such as metamorphic reactions, fluid and melt transfer and earthquakes occur at a subducting zone, but are still incompletely understood. These processes are affected, or even controlled, by the magnitude and distribution of stress and deformation mechanism. To eventually understand subduction zone processes, we quantify here stresses and deformation mechanisms in and around a subducting lithosphere, surrounded by asthenosphere and overlain by an overriding plate. We use 2-D thermomechanical numerical simulations based on the finite difference and marker-in-cell method and consider a 3200 km wide and 660 km deep numerical domain with a resolution of 1 km by 1 km. We apply a combined visco-elasto-plastic deformation behaviour using a linear combination of diffusion creep, dislocation creep and Peierls creep for the viscous deformation. We consider two end-member subduction scenarios: forced and free subduction. In the forced scenario, horizontal velocities are applied to the lateral boundaries of the plates during the entire simulation. In the free scenario, we set the horizontal boundary velocities to zero once the subducted slab is long enough to generate a slab pull force large enough to maintain subduction without horizontal boundary velocities. A slab pull of at least 1.8 TN m(-1) is required to continue subduction in the free scenario. We also quantify along-profile variations of gravitational potential energy (GPE). We evaluate the contributions of topography and density variations to GPE variations across a subduction system. The GPE variations indicate large-scale horizontal compressive forces around the trench region and extension forces on both sides of the trench region. Corresponding vertically averaged differential stresses are between 120 and 170 MPa. Furthermore, we calculate the distribution of the dominant deformation mechanisms. Elastoplastic deformation is the dominant mechanism in the upper region of the lithosphere and subducting slab (from ca. 5 to 60 km depth from the top of the slab). Viscous deformation dominates in the lower region of the lithosphere and in the asthenosphere. Considering elasticity in the calculations has an important impact on the magnitude and distribution of deviatoric stress; hence, simulations with increased shear modulus, in order to reduce elasticity, exhibit considerably different stress fields. Limiting absolute stress magnitudes by decreasing the internal friction angle causes slab detachment so that slab pull cannot be transmitted anymore to the horizontal lithosphere. Applying different boundary conditions shows that forced subduction simulations are stronger affected by the applied boundary conditions than free subduction simulations. We also compare our modelled topography and gravity anomaly with natural data of seafloor bathymetry and free-air gravity anomalies across the Mariana trench. Elasticity and deviatoric stress magnitudes of several hundreds of MPa are required to best fit the natural data. This agreement suggests that the modelled flexural behaviour and density field are compatible with natural data. Moreover, we discuss potential applications of our results to the depth of faulting in a subducting plate and to the generation of petit-spot volcanoes.

中文翻译：

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俯冲带的应力和变形机制：二维热机械数值模拟的见解

许多过程，如变质反应、流体和熔体转移以及地震发生在俯冲带，但仍不完全清楚。这些过程受到应力和变形机制的大小和分布的影响甚至控制。为了最终理解俯冲带过程，我们在这里量化了俯冲岩石圈内部和周围的应力和变形机制，被软流圈包围并被上覆板块覆盖。我们使用基于有限差分和标记单元法的二维热机械数值模拟，并考虑一个 3200 公里宽和 660 公里深的数值域，分辨率为 1 公里乘 1 公里。我们使用扩散蠕变的线性组合来应用组合粘弹塑性变形行为，位错蠕变和 Peierls 蠕变用于粘性变形。我们考虑两种最终成员俯冲情景：强制俯冲和自由俯冲。在强制场景中，在整个模拟过程中，水平速度被应用于板的横向边界。在自由情​​景中，一旦俯冲板片足够长以产生足够大的板片拉力以在没有水平边界速度的情况下维持俯冲，我们将水平边界速度设置为零。在自由情​​景中继续俯冲需要至少 1.8 TN m(-1) 的平板拉力。我们还量化了重力势能 (GPE) 的沿剖面变化。我们评估了地形和密度变化对俯冲系统中 GPE 变化的贡献。GPE 变化表明沟槽区域周围的大规模水平压缩力和沟槽区域两侧的延伸力。相应的垂直平均差分应力介于 120 和 170 MPa 之间。此外，我们计算了主要变形机制的分布。弹塑性变形是岩石圈上部区域和俯冲板块（距板块顶部约 5 至 60 公里深度）的主要机制。粘性变形在岩石圈下部区域和软流圈中占主导地位。在计算中考虑弹性对偏应力的大小和分布有重要影响；因此，增加剪切模量的模拟，为了降低弹性，表现出相当不同的应力场。通过减小内摩擦角来限制绝对应力大小会导致板块脱离，从而使得板块拉力无法再传递到水平岩石圈。应用不同的边界条件表明强制俯冲模拟受所应用边界条件的影响比自由俯冲模拟更强。我们还将模拟的地形和重力异常与横跨马里亚纳海沟的海底测深和自由空气重力异常的自然数据进行了比较。需要数百兆帕的弹性和偏应力大小才能最好地拟合自然数据。这种一致性表明建模的弯曲行为和密度场与自然数据兼容。而且，