Melt-rich channel observed at the lithosphere-asthenosphere boundary
Melt-rich channel observed at the lithosphere-asthenosphere boundary
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DOI：
10.1038/nature11939
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发表时间：
2013-03-21
期刊：
影响因子：
64.8
通讯作者：
Evans, R. L.
中科院分区：
文献类型：
--
作者：
Naif, S.;Key, K.;Evans, R. L.
岩石圈-软流圈边界（LAB）将刚性大洋板块与下伏的温暖韧性软流圈分隔开。虽然在这个边界之下的粘度降低对于板块构造是必不可少的，但关于其起源的共识仍然难以捉摸。地震研究发现，在被认为与LAB一致的深度处存在明显的速度不连续性，但对其原因存在分歧（1-5），通常援引部分熔融（6）或地幔脱水边界（7）作为解释。在这里，我们使用海底大地电磁数据对尼加拉瓜近海中美洲海沟科科斯板块边缘下方LAB的电导率进行成像。在电阻性海洋岩石圈之下，大地电磁数据显示，在45至70公里深处存在一个高导电层。由于部分熔融在温暖潮湿的地幔（8）中的这些深度是稳定的，因此我们将导体解释为部分熔融层，其上覆盖着不可渗透的冻结盖，这是岩石圈的基础。平行于板块运动的电导率各向异性表明，该熔体已被剪切成流动对齐的管状结构（9）。我们推断，年轻板块下的LAB由一个薄的，部分熔融的，低粘度的通道，其作用是从更深的对流地幔上覆的脆性岩石圈解耦。因为这个边界层有可能作为板块运动的润滑剂，它与海沟的接近可能会对俯冲动力学产生影响。
The lithosphere-asthenosphere boundary (LAB) separates rigid oceanic plates from the underlying warm ductile asthenosphere. Although a viscosity decrease beneath this boundary is essential for plate tectonics, a consensus on its origin remains elusive. Seismic studies identify a prominent velocity discontinuity at depths thought to coincide with the LAB but disagree on its cause(1-5), generally invoking either partial melting(6) or a mantle dehydration boundary(7) as explanations. Here we use sea-floor magnetotelluric data to image the electrical conductivity of the LAB beneath the edge of the Cocos plate at the Middle America trench offshore of Nicaragua. Underneath the resistive oceanic lithosphere, the magnetotelluric data reveal a high-conductivity layer confined to depths of 45 to 70 kilometres. Because partial melts are stable at these depths in a warm damp mantle(8), we interpret the conductor to be a partially molten layer capped by an impermeable frozen lid that is the base of the lithosphere. A conductivity anisotropy parallel to plate motion indicates that this melt has been sheared into flow-aligned tube-like structures(9). We infer that the LAB beneath young plates consists of a thin, partially molten, channel of low viscosity that acts to decouple the overlying brittle lithosphere from the deeper convecting mantle. Because this boundary layer has the potential to behave as a lubricant to plate motion, its proximity to the trench may have implications for subduction dynamics.