Cascadia subducting plate fluids channelled to fore-arc mantle corner: ETS and silica deposition
Cascadia subducting plate fluids channelled to fore-arc mantle corner: ETS and silica deposition
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DOI：
10.1002/2015jb011920
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发表时间：
2015-06-01
影响因子：
3.9
通讯作者：
Ague, J.
中科院分区：
文献类型：
--
作者：
Hyndman, R. D.;McCrory, P. A.;Ague, J.
本研究首先总结了对卡斯卡迪亚俯冲逆冲构造的约束条件，即在大逆冲造震带与进一步向陆的幕式震颤滑动（ETS）之间存在70km的降倾间隙；不存在从不稳定滑动到有条件稳定滑动的连续过渡。地震破裂主要发生在这个热俯冲带的近海。ETS位于岸上。然后，我们提出是什么控制了ETS的下降位置。我们得出结论，下行板块脱水的流体集中上升到弧前地幔角以上，是造成ETS的原因。ETS的位置与整个边缘的这个角之间有显著的对应关系。俯冲洋壳和上地幔中的水合矿物组合随着温度的下降而脱水，地震层析成像资料表明，这些流体对上覆弧前地幔具有强烈的蛇纹石化作用。实验数据表明，这种弧前地幔蛇纹岩渗透率低，可能在下伏可渗透洋壳和俯冲剪切带内阻断垂向排液，限制流体上倾流动。在弧前地幔角，这些流体向上释放到更具渗透性的弧前地壳中。这种流体通量的一个迹象来自于在角落上方发现的低泊松比（和V-p/V-s），这可能是由于石英的浓度具有异常低的泊松比。上升的流体应该是硅饱和的，当它们上升到角落以上时，随着温度和压力的降低，沉淀出石英。
In this study we first summarize the constraints that on the Cascadia subduction thrust, there is a 70km gap downdip between the megathrust seismogenic zone and the Episodic Tremor and Slip (ETS) that lies further landward; there is not a continuous transition from unstable to conditionally stable sliding. Seismic rupture occurs mainly offshore for this hot subduction zone. ETS lies onshore. We then suggest what does control the downdip position of ETS. We conclude that fluids from dehydration of the downgoing plate, focused to rise above the fore-arc mantle corner, are responsible for ETS. There is a remarkable correspondence between the position of ETS and this corner along the whole margin. Hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature, and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore-arc mantle. Laboratory data show that such fore-arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone. At the fore-arc mantle corner these fluids are released upward into the more permeable overlying fore-arc crust. An indication of this fluid flux comes from low Poisson's Ratios (and V-p/V-s) found above the corner that may be explained by a concentration of quartz which has exceptionally low Poisson's Ratio. The rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner.