Controls on the rheological properties of peridotite at a palaeosubduction interface: A transect across the base of the Oman-UAE ophiolite
Controls on the rheological properties of peridotite at a palaeosubduction interface: A transect across the base of the Oman-UAE ophiolite
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DOI：
10.1016/j.epsl.2018.03.027
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发表时间：
2018-06-01
影响因子：
5.3
通讯作者：
Searle, Michael P.
中科院分区：
文献类型：
--
作者：
Ambrose, Tyler K.;Wallis, David;Searle, Michael P.
对实验变形岩石和小规模自然剪切带的研究表明，体积次要相可以通过限制晶粒生长和促进晶粒尺寸敏感变形机制来控制应变局部化。这些小规模研究通常用于推断次要阶段在板块边界发展中的关键作用。然而，次要相在实际板块边界应变局部化中的作用仍有待通过直接观察来测试。为了检验次要相控制板块边界应变局部化的假设，我们对沿阿曼-阿拉伯联合酋长国 (UAE) 蛇绿岩底部类似 1 公里横断面采集的橄榄岩样品进行了微观结构分析。蛇绿岩的底部以塞梅尔逆冲为标志，它代表了俯冲洋壳和上覆地幔楔之间现已挖出的接触面。因此，蛇绿岩的底部提供了直接检查前板块边界的机会。我们的结果表明，平均橄榄石晶粒尺寸与次要相（主要是斜方辉石，以及单斜辉石、角闪石和尖晶石）的丰度成反比，这与晶界钉扎对晶粒生长的抑制一致。我们的结果还表明，平均橄榄石晶粒尺寸与 CPO 强度成正比（两者通常都向变质底部减小），这表明不同变形机制产生的应变分数在空间上有所不同。实验得出的流动定律表明，在推断的变形条件下，橄榄石的粘度对晶粒尺寸敏感。因此，颗粒尺寸以及次要相的丰度会影响沿地幔楔底部与俯冲相关的变形期间的粘度。我们计算出橄榄石的粘度朝向蛇绿岩底部下降了一个数量级，这表明应变位于俯冲界面附近。我们的数据表明，这种流变减弱主要是蛇绿岩底部附近更丰富的次要相的结果。我们的解释与之前对实验变形岩石和较小规模自然剪切带的研究一致，表明次要相可以对应变局部化发挥主要控制作用。然而，我们的研究首次证明次要相可以控制与主要板块边界相关的尺度上的应变局部化。 (C) 2018 Elsevier B.V. 保留所有权利。
Studies of experimentally deformed rocks and small-scale natural shear zones have demonstrated that volumetrically minor phases can control strain localisation by limiting grain growth and promoting grain-size sensitive deformation mechanisms. These small-scale studies are often used to infer a critical role for minor phases in the development of plate boundaries. However, the role of minor phases in strain localisation at an actual plate boundary remains to be tested by direct observation. In order to test the hypothesis that minor phases control strain localisation at plate boundaries, we conducted microstructural analyses of peridotite samples collected along a similar to 1 km transect across the base of the Oman-United Arab Emirates (UAE) ophiolite. The base of the ophiolite is marked by the Semail thrust, which represents the now exhumed contact between subducted oceanic crust and the overlying mantle wedge. As such, the base of the ophiolite provides the opportunity to directly examine a former plate boundary.Our results demonstrate that the mean olivine grain size is inversely proportional to the abundance of minor phases (primarily orthopyroxene, as well as clinopyroxene, hornblende, and spinel), consistent with suppression of grain growth by grain-boundary pinning. Our results also reveal that mean olivine grain size is proportional to CPO strength (both of which generally decrease towards the metamorphic sole), suggesting that the fraction of strain produced by different deformation mechanisms varied spatially. Experimentally-derived flow laws indicate that under the inferred deformation conditions, the viscosity of olivine was grain-size sensitive. As such, grain size, and thereby the abundance of minor phases, influenced viscosity during subduction-related deformation along the base of the mantle wedge.We calculate an order of magnitude decrease in the viscosity of olivine towards the base of the ophiolite, which suggests strain was localised near the subduction interface. Our data indicate that this rheological weakening was primarily the result of more abundant minor phases near the base of the ophiolite. Our interpretations are consistent with those of previous studies on experimentally deformed rocks and smaller-scale natural shear zones that indicate minor phases can exert the primary control on strain localisation. However, our study demonstrates for the first time that minor phases can control strain localisation at the scales relevant to a major plate boundary. (C) 2018 Elsevier B.V. All rights reserved.