Natural examples of olivine lattice preferred orientation patterns with a flow-normal a-axis maximum
Natural examples of olivine lattice preferred orientation patterns with a flow-normal a-axis maximum
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DOI：
10.1038/nature02179
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发表时间：
2004-01
期刊：
影响因子：
64.8
通讯作者：
T. Mizukami;S. Wallis;J. Yamamoto
中科院分区：
文献类型：
--
作者：
T. Mizukami;S. Wallis;J. Yamamoto
构造板块运动被认为引起下伏的上地幔内的固态塑性流动，从而导致组成橄榄石晶体的晶格优选方向的发展，，。由这种优选方向产生的力学各向异性通常会产生与板块运动方向平行的最大地震波速度方向。这可以用橄榄石优先取向的存在来解释，其“a轴”最大值平行于诱导地幔流动方向，，，，。然而，在俯冲带，橄榄石轴被推断为大致垂直于板块运动，，，，，，这通常被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：在高水活度的条件下，可能形成“b型”橄榄石首选取向，其茶轴最大垂直于流动方向。然而，这种b型偏好取向的自然例子几乎完全未知。本文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存在的情况下发育的。我们的发现表明，俯冲带上方的地幔流动可能比通常认为的要简单得多。
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 olivineaaxes 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 thea-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.