The dependence of planetary tectonics on mantle thermal state: applications to early Earth evolution
The dependence of planetary tectonics on mantle thermal state: applications to early Earth evolution
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DOI：
10.1098/rsta.2017.0409
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发表时间：
2018-10
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影响因子：
--
通讯作者：
B. Foley
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作者：
B. Foley
为了使板块构造在行星上起作用，地幔对流力必须能够在岩石圈中形成弱的局部剪切带，作为板块边界。否则，一个行星的地幔将在一个停滞的盖层中对流，在那里没有俯冲和板块运动。因此，板块构造何时以及如何在地球上开始与地幔对流形成板块边界的能力内在地联系在一起；然而，这一过程背后的物理原理仍然不确定。大多数地幔对流模型都采用了简单的岩石圈假塑性模型，当岩石圈的应力达到规定的屈服应力时，岩石圈“失效”并形成一个可移动的盖子。具有假可塑性的高地幔温度和高内部加热速率，与早期地球相关的条件，通过减少岩石圈应力来阻碍板块边界的形成，因此有利于早期地球的停滞盖子。然而，当使用基于晶粒尺寸减小的剪切带形成模型时，早期地球热条件不利于停滞盖。岩石圈应力随地幔温度或产热速率的升高而减小，而驱动晶粒尺寸减小的变形功增大。因此，对流形成弱板块边界的能力不受早期地球热条件的阻碍。然而，地幔热状态确实改变了俯冲和岩石圈运动的方式；高地幔温度导致了更为缓慢的滴状俯冲。这种“缓慢的盖子”对流可能能够解释地质记录中保存的早期地壳形成过程的许多关键观察结果。此外，这项工作强调了了解板块边界形成的微物理对于评估早期地球构造的重要性，因为不同的板块边界形成机制以根本不同的方式受到地幔热状态的影响。本文是“地球动力学与板块构造的发展”讨论会的一部分。
For plate tectonics to operate on a planet, mantle convective forces must be capable of forming weak, localized shear zones in the lithosphere that act as plate boundaries. Otherwise, a planet's mantle will convect in a stagnant lid regime, where subduction and plate motions are absent. Thus, when and how plate tectonics initiated on the Earth is intrinsically tied to the ability of mantle convection to form plate boundaries; however, the physics behind this process are still uncertain. Most mantle convection models have employed a simple pseudoplastic model of the lithosphere, where the lithosphere ‘fails’ and develops a mobile lid when stresses in the lithosphere reach the prescribed yield stress. With pseudoplasticity high mantle temperatures and high rates of internal heating, conditions relevant for the early Earth, impede plate boundary formation by decreasing lithospheric stresses, and hence favour a stagnant lid for the early Earth. However, when a model for shear zone formation based on grain size reduction is used, early Earth thermal conditions do not favour a stagnant lid. While lithosphere stress drops with increasing mantle temperature or heat production rate, the deformational work, which drives grain size reduction, increases. Thus, the ability of convection to form weak plate boundaries is not impeded by early Earth thermal conditions. However, mantle thermal state does change the style of subduction and lithosphere mobility; high mantle temperatures lead to a more sluggish, drip-like style of subduction. This ‘sluggish lid’ convection may be able to explain many of the key observations of early Earth crust formation processes preserved in the geologic record. Moreover, this work highlights the importance of understanding the microphysics of plate boundary formation for assessing early Earth tectonics, as different plate boundary formation mechanisms are influenced by mantle thermal state in fundamentally different ways. This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.