Mizukami Tomoyuki, Wallis Simon R, Yamamoto Junji
Department of Geology and Mineralogy, University of Kyoto, Sakyo-ku, Kyoto, 606-8501, Japan.
Nature. 2004 Jan 29;427(6973):432-6. doi: 10.1038/nature02179.
Tectonic plate motion is thought to cause solid-state plastic flow within the underlying upper mantle and accordingly lead to the development of a lattice preferred orientation of the constituent olivine crystals. The mechanical anisotropy that results from such preferred orientation typically produces a direction of maximum seismic wave velocity parallel to the plate motion direction. This has been explained by the existence of an olivine preferred orientation with an 'a-axis' maximum parallel to the induced mantle flow direction. In subduction zones, however, the olivine a axes have been inferred to be arranged roughly perpendicular to plate motion, which has usually been ascribed to localized complex mantle flow patterns. Recent experimental work suggests an alternative explanation: under conditions of high water activity, a 'B-type' olivine preferred orientation may form, with the a-axis maximum perpendicular to the flow direction. Natural examples of such B-type preferred orientation are, however, almost entirely unknown. Here we document widespread B-type olivine preferred orientation patterns from a subduction-type metamorphic belt in southwest Japan and show that these patterns developed in the presence of water. Our discovery implies that mantle flow above subduction zones may be much simpler than has generally been thought.
板块运动被认为会导致下伏上地幔内部的固态塑性流动，从而导致组成橄榄石晶体的晶格优选取向的发展。由这种优选取向产生的力学各向异性通常会产生一个与板块运动方向平行的最大地震波速度方向。这可以通过存在一个“a轴”最大方向与诱发的地幔流动方向平行的橄榄石优选取向来解释。然而，在俯冲带中，橄榄石的a轴被推断大致垂直于板块运动方向排列，这通常归因于局部复杂的地幔流动模式。最近的实验工作提出了另一种解释：在高水活度条件下，可能会形成一种“B型”橄榄石优选取向，其a轴最大方向垂直于流动方向。然而，这种B型优选取向的自然实例几乎完全不为人知。在这里，我们记录了日本西南部俯冲型变质带中广泛存在的B型橄榄石优选取向模式，并表明这些模式是在有水的情况下形成的。我们的发现意味着俯冲带上地幔流动可能比一般认为的要简单得多。
