Seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches
Seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches
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DOI：
10.1029/2012gc004043
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发表时间：
2012-05-18
影响因子：
3.5
通讯作者：
Grevemeyer, I.
中科院分区：
文献类型：
--
作者：
Lefeldt, M.;Ranero, C. R.;Grevemeyer, I.
在俯冲带，板块向地幔输送的水在构造、岩浆作用、流体和挥发分流动中起着关键作用，很可能在地球海洋和地幔的化学演化中起着关键作用。然而，在俯冲之前将水并入海洋板块是一个鲜为人知的过程。一些研究表明，板块可能在俯冲海沟获得大部分水，因为那里的洋壳和最上地幔是由弯曲和/或板块拉动引起的强烈断层，并显示出异常低的地震速度。低速度被解释为与正常断裂有关的充满流体的裂缝和通过水化作用进行矿物转化的组合。通过将名义上干燥的橄榄岩转化为富水的蛇纹岩而进行的地幔水化作用，可能会在板块中形成最大的流体储集层，因此与地幔深处的水传输最相关。正断层地震的破裂深度通常不会受到很好的限制，但可能会为水创造深层渗流路径，这些路径可能会水合作用长达数十公里进入地幔，但仅受蛇形稳定性的限制。然而，对深部板内矿物蚀变的解释仍然是推测的，因为活动震源地震实验只对地幔最上面的几公里进行了采样，使得异常速度的深度-幅度及其与断层的关系不受限制。在这里，我们使用活动震源地震数据的联合反演，以及局部和区域地震来绘制在海沟海底部署的地震网络下异常速度的三维分布。我们发现，异常速度受制于台网记录的正断层微震活动的深度，比远震、正断层地震的破裂深度或蛇形稳定性极限都要浅得多。伸展微震表明，该地区各断裂每隔2-3个月滑动一次，这可能有利于水的正常渗漏。较深的远震地震相对较少发生，而且可能不会造成重大的裂缝，这种裂缝保持足够长的开放时间，以促进我们的地震研究可以检测到的蚀变。结果表明，蛇纹岩的稳定场并不制约潜在地幔水化的深度。
Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2-3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration.