Bercovici David, Mulyukova Elvira
Department of Earth & Planetary Sciences, Yale University, New Haven, CT 06511
Department of Earth & Planetary Sciences, Yale University, New Haven, CT 06511.
Proc Natl Acad Sci U S A. 2021 Jan 26;118(4). doi: 10.1073/pnas.2011247118.
How subduction-the sinking of cold lithospheric plates into the mantle-is initiated is one of the key mysteries in understanding why Earth has plate tectonics. One of the favored locations for subduction triggering is at passive margins, where sea floor abuts continental margins. Such passive margin collapse is problematic because the strength of the old, cold ocean lithosphere should prohibit it from bending under its own weight and sinking into the mantle. Some means of mechanical weakening of the passive margin are therefore necessary. Spontaneous and accumulated grain damage can allow for considerable lithospheric weakening and facilitate passive margin collapse. Grain damage is enhanced where mixing between mineral phases in lithospheric rocks occurs. Such mixing is driven both by compositional gradients associated with petrological heterogeneity and by the state of stress in the lithosphere. With lateral compressive stress imposed by ridge push in an opening ocean basin, bands of mixing and weakening can develop, become vertically oriented, and occupy a large portion of lithosphere after about 100 million y. These bands lead to anisotropic viscosity in the lithosphere that is strong to lateral forcing but weak to bending and sinking, thereby greatly facilitating passive margin collapse.
俯冲——冷的岩石圈板块沉入地幔的过程——是如何开始的，是理解地球为何有板块构造的关键谜团之一。俯冲触发的首选位置之一是被动大陆边缘，即海底与大陆边缘相邻的地方。这种被动大陆边缘的坍塌存在问题，因为古老、寒冷的海洋岩石圈的强度应使其无法在自身重量作用下弯曲并沉入地幔。因此，需要某种机械弱化被动大陆边缘的方式。自发的和累积的颗粒损伤能够使岩石圈显著弱化，并促进被动大陆边缘的坍塌。在岩石圈岩石中矿物相之间发生混合的地方，颗粒损伤会加剧。这种混合是由与岩石学非均质性相关的成分梯度以及岩石圈中的应力状态驱动的。在一个正在扩张的海洋盆地中，由于洋中脊推力施加的侧向压缩应力，混合和弱化带能够形成，变得垂直定向，并在大约1亿年后占据很大一部分岩石圈。这些带导致岩石圈中出现各向异性粘度——对侧向作用力强，但对弯曲和下沉作用弱——从而极大地促进了被动大陆边缘的坍塌。
