Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Am Alten Hafen 26, 27568 Bremerhaven, Germany.
Nature. 2016 Jul 14;535(7611):276-9. doi: 10.1038/nature18277. Epub 2016 Jun 29.
Along ultraslow-spreading ridges, where oceanic tectonic plates drift very slowly apart, conductive cooling is thought to limit mantle melting and melt production has been inferred to be highly discontinuous. Along such spreading centres, long ridge sections without any igneous crust alternate with magmatic sections that host massive volcanoes capable of strong earthquakes. Hence melt supply, lithospheric composition and tectonic structure seem to vary considerably along the axis of the slowest-spreading ridges. 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 amagmatic 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.
在超慢速扩张的洋脊上，由于大洋板块缓慢地彼此分离，传导冷却被认为限制了地幔的熔融，并且推断熔融的产生是高度不连续的。在这样的扩张中心，没有任何火成地壳的长脊段与岩浆段交替出现，而岩浆段则存在能够引发强烈地震的大型火山。因此，熔体供应、岩石圈组成和构造结构似乎沿着最慢扩张脊的轴在很大程度上发生变化。然而，由于缺乏地震数据，超慢速脊的岩石圈结构受到很大限制。在这里，我们使用详细的地震活动调查来描述两种极端类型的大洋岩石圈的结构和增生模式，这些调查是沿着 390 公里的超慢速扩张脊轴进行的。我们观察到，无岩浆段在岩石圈的上 15 公里范围内缺乏浅层地震活动，但异常地包含深度达 35 公里的地震。这一观察结果意味着存在一个寒冷、厚实的岩石圈，其上存在一个可能反映大量蛇纹石化的无震区。我们发现，火山中心下方的岩浆岩石圈急剧变薄，并且推断由此产生的岩石圈-软流圈边界的地形可能允许轴向熔体流动，从而解释了超慢速扩张脊上不均匀的地壳产生。地震活动数据表明，海洋岩石圈的变化可能比以前认为的要深得多，这对海底变形和流体循环具有重要意义。
