Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere
Mid-ocean-ridge seismicity reveals extreme types of ocean lithosphere
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DOI：
10.1038/nature18277
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发表时间：
2016-07-14
期刊：
影响因子：
64.8
通讯作者：
Schmid, Florian
中科院分区：
文献类型：
--
作者：
Schlindwein, Vera;Schmid, Florian
沿着超低扩张海脊，海洋构造板块漂移得非常慢，传导冷却被认为限制了地幔融化(1)，熔体产生被推断为高度不连续(2-4)。沿着这样的扩张中心，没有任何火成岩地壳的长山脊部分与拥有能够发生强烈地震的大规模火山的岩浆部分相间(5)。因此，熔体供应、岩石圈组成和构造结构似乎沿着最慢扩展的山脊的轴线有很大的不同(6)。然而，由于缺乏地震资料，超低脊的岩石圈结构约束较差。本文利用沿390公里超低扩张脊轴的详细地震活动调查，描述了大洋岩石圈的两种端元类型的结构和吸积方式。我们观察到，岩浆区在岩石圈上部15公里处缺乏浅层地震活动，但在35公里以下异常地含有地震。这意味着一个寒冷而厚实的岩石圈，上面的无震区可能反映了大量的蛇纹岩作用。我们发现，岩浆岩石圈区域在火山中心以下显著变薄，并推断由此产生的岩石圈-软流圈边界的地形可能允许沿轴方向的熔体流动，解释了在超低扩张脊处地壳生产的不均匀。地震活动数据表明，大洋岩石圈的变化可能比之前认为的要深得多，对海底变形和流体循环具有重要意义。
Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting(1) and melt production has been inferred to be highly discontinuous(2-4). Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes(5). Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges(6). However, owing to the lack of seismic data, the lithospheric structure of ultraslow ridges is poorly constrained. Here we describe the structure and accretion modes of two end-member types of oceanic lithosphere using a detailed seismicity survey along 390 kilometres of ultraslow-spreading ridge axis. We observe that a magmatic sections lack shallow seismicity in the upper 15 kilometres of the lithosphere, but unusually contain earthquakes down to depths of 35 kilometres. This observation implies a cold, thick lithosphere, with an upper aseismic zone that probably reflects substantial serpentinization. We find that regions of magmatic lithosphere thin dramatically under volcanic centres, and infer that the resulting topography of the lithosphere-asthenosphere boundary could allow along-axis melt flow, explaining the uneven crustal production at ultraslow-spreading ridges. The seismicity data indicate that alteration in ocean lithosphere may reach far deeper than previously thought, with important implications towards seafloor deformation and fluid circulation.