Mantle convection with strong subduction zones
Mantle convection with strong subduction zones
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DOI：
10.1046/j.1365-246x.2001.00321.x
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发表时间：
2001-02-01
影响因子：
2.8
通讯作者：
Hager, BH
中科院分区：
文献类型：
--
作者：
Conrad, CP;Hager, BH
由于地幔粘度是温度依赖的，冷俯冲岩石圈应该是强大的，这意味着快速，局部变形与俯冲应强烈抵制板块运动。由于计算限制。俯冲板块的变形在地幔尺度对流模型中无法精确求解，因此难以研究其对对流的影响。我们已经开发出一种新的方法来实现俯冲参数化的变形的海洋岩石圈内的一个小区域的有限元网格。通过强加：在俯冲带附近的速度边界条件下，我们强制俯冲的几何形状，产生具有真实热结构的板。为了使模型动态一致，我们指定了一个俯冲率，平衡对流的能量预算，其中包括一个表达式所需的能量变形的海洋岩石圈，因为它俯冲。该表达式是根据强粘性岩石圈的局部弯曲模型确定的。通过以这种方式实现俯冲，我们已经证明了与地球上观察到的板块和板片的对流，但其中高达40%的地幔总对流阻力与俯冲带内发生的变形有关。与具有弱板的模型相比，这种额外的阻力使板速度减慢了近两倍。对于足够强的岩石圈，弯曲变形减慢了板块表面的速度，使它们不再积极参与全球尺度的对流，而是发生在“缓慢的盖子”下面。通过在大洋板块下引入一个低粘度的软流圈，我们证明了大洋岩石圈底部的小尺度对流可能限制了板块的厚度，从而限制了对弯曲的抵抗力，并导致板块的速度取决于弯曲岩石圈的强度，而不是底层地幔的粘度。对于一个冷却的地球，岩石圈的有效粘度应该是几乎恒定的，但地幔的粘度应随时间而增加。因此，俯冲抵抗对流应产生几乎恒定的板块速度和热流随着时间的推移？这对地球的热演化有影响。我们估计，如果弯曲岩石圈的有效粘度大于约10（23）Pa·s，但只有当一些机制，如小尺度对流，阻止弯曲阻力完全停止板块，这种类型的对流应该适用。这种机制可能是板块构造和地球热历史的基础。
Because mantle viscosity is temperature-dependent, cold subducting lithosphere should be strong, which implies that the rapid, localized deformation associated with subduction should strongly resist plate motions. Due to computational constraints. the deformation of a subducting plate cannot be accurately resolved in mantle-scale convection models, so its effect on convection is difficult to investigate. We have developed a new method for implementing subduction that parametrizes the deformation of the oceanic lithosphere within a small region of a finite element grid. By imposing: velocity boundary conditions in the vicinity of the subduction zone, we enforce a geometry for subduction, producing a slab with a realistic thermal structure. To make the model dynamically consistent, we specify a rate for subduction that balances the energy budget for convection, which includes an expression for the energy needed to deform the oceanic lithosphere as it subducts. This expression is determined here from a local model of bending for a strong viscous lithosphere. By implementing subduction in this way, we have demonstrated convection with plates and slabs that resemble those observed on Earth, but in which up to 40 per cent of the mantle's total convective resistance is associated with deformation occurring within the subduction zone. This additional resistance slows plate velocities by nearly a factor of two compared to models with a weak slab. For sufficiently strong lithosphere, the bending deformation slows surface plates sufficiently that they no longer actively participate in global-scale convection, which occurs instead beneath a 'sluggish lid'. By introducing a low-viscosity asthenosphere beneath the oceanic plate, we demonstrate that small-scale convection at the base of oceanic lithosphere may limit plate thickness, and thus the resistance to bending, and cause plate velocities to depend on the strength of the bending lithosphere rather than on the viscosity of the underlying mantle. For a cooling Earth, the effective lithosphere viscosity should be nearly constant, but the mantle viscosity should increase with time. Thus, subduction-resisted convection should produce nearly constant plate velocities and heat flow over time? which has implications for the Earth's thermal evolution. We estimate that this style of convection should apply if the effective viscosity of the bending lithosphere is greater than about 10(23) Pa s, but only if some mechanism, such as small-scale convection, prevents the bending resistance from stopping plates altogether. Such a mechanism could be fundamental to plate tectonics and Earth's thermal history.