Trench motion, slab geometry and viscous stresses in subduction systems
Trench motion, slab geometry and viscous stresses in subduction systems
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俯冲系统中的海沟运动、板片几何形状和粘性应力
DOI：
10.1111/j.1365-246x.2006.03079.x
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发表时间：
2006
影响因子：
2.8
通讯作者：
L. Husson
中科院分区：
文献类型：
--
作者：
L. Royden;L. Husson
一个牛顿粘性上地幔俯冲的半解析三维模型提供了一种计算俯冲系统中粘性应力、海沟运动和板块几何形状的动态一致方法。虽然负板片浮力提供了俯冲的基本驱动力，但从上地幔表面延伸到底部的板片在浅地幔（<100 - 150 km）中受到粘性应力的过度支撑，而在上地幔更深处受到粘性应力的支撑不足。俯冲系统的这些较深的部分作为俯冲的“发动机”，而较浅的部分作为沟运动的“制动器”;沟迁移率和板的几何形状反映了这两种效果之间的竞争。在稳定俯冲过程中，海沟运动速率与板块浮力近似成线性关系，模型的海沟运动速率与观测到的两层上地幔粘度（300 km以上为102 × 1020 Pa·s，300 km以下为105 × 1020 Pa·s）的速率范围吻合得很好。稳态板倾角随着板密度的降低而增加，特别是对于非常低密度的板，其比高密度板倾斜得更陡。在下地幔顶部的水平速度，测量相对于前陆，有一个非常大的影响沟迁移率，媲美甚至超过板浮力。平行于沟槽的平板宽度也对沟槽迁移率有显著影响，这是由于平板周围的环形流的粘性压力。俯冲岩石圈的刚度不会对海沟迁移率或板块几何形状产生显着影响，刚性与大洋岩石圈兼容。非常坚硬的板，弹性板厚度超过40 km或粘度在10 25 - 10 26 Pa s范围内，俯冲速度明显慢于弱板，沟槽迁移率在弱板的一半到三分之一的范围内。大，意想不到的影响，沟槽迁移率和板的几何形状施加的结构和密度的正面棱镜和覆盖板，表明当地的地质可以施加重要的制约板动态。在非稳定状态俯冲过程中，海沟迁移的速度迅速响应，因为超浮力岩石圈进入软流圈。在没有其他汇聚驱动力的情况下，海沟迁移速率可以在200万年至300万年的时间内改变两倍或更多，例如，当大洋板块跟随大陆板块进入俯冲系统时，从35毫米到70毫米/年。变浮力岩石圈的俯冲伴随着板块倾角随深度和时间的变化。
SUMMARY A semi-analytic, 3-D model for subduction within a Newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress, trench motion and slab geometry in subduction systems. Although negative slab buoyancy provides the basic driving force for subduction, slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow (<100‐150 km) mantle and undersupported by viscous stresses at greater depth in the upper mantle. These deeper parts of the subduction system act as an ‘engine’ for subduction while shallower parts act as a ‘brake’ on trench motion; trench migration rates and slab geometry reflect a competition between these two effects. During steady-state subduction, trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of ∼2 × 10 20 Pa s above 300 km and ∼5 × 10 20 Pa s below. Steady-state slab dip increases as slab density decreases, especially for very low-density slabs, which dip significantly more steeply than high-density slabs. The horizontal velocity at the top of the lower mantle, measured relative to the foreland, has a very large effect on trench migration rates, rivalling or even exceeding that of slab buoyancy. Slab width, parallel to the trench, also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab. The stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere. Very stiff slabs, with elastic plate thicknesses more that ∼40 km or viscosities in the range of 10 25 ‐10 26 Pa s, subduct significantly more slowly than weak slabs, with trench migration rates in the range of half to a third that of weak slabs. Large, unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate, indicating that local geology can exert important constraints on slab dynamics. During non-steady-state subduction, rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere. In the absence of other driving forces for convergence, trench migration rates can change by a factor of two or more in as little as 2‐3 Myr, for example, from 35 to 70 mm yr −1 when an oceanic piece of slab follows a continental one into the subduction system. Subduction of variable-buoyancy lithosphere is accompanied by changes in slab dip with depth and through time.