Timescales for the growth of sediment diapirs in subduction zones
Timescales for the growth of sediment diapirs in subduction zones
复制标题
俯冲带沉积物底辟生长的时间尺度
DOI：
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发表时间：
2012
期刊：
影响因子：
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通讯作者：
M. Behn
中科院分区：
文献类型：
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作者：
N. Miller;M. Behn
在这项研究中，我们计算了俯冲带下行板上沉积层中形成的重力不稳定增长的时间尺度。俯冲的变质沉积物相对于上覆地幔漂浮，并可能形成底辟，从板块中分离出来，向上上升到地幔楔形中。我们使用粒子格内有限差分方法来计算在漂浮的、湿的石英变质沉积层中形成的不稳定性的增长率，该层位于由湿橄榄石组成的致密地幔半空间之下。这些增长速度被用来确定在一系列俯冲带热结构上，沉积物底辟在哪里启动和从板块分离。我们发现，在给定足够的层厚(200-800m，取决于板片表面和地幔楔形温度)的情况下，沉积底辟在∼80公里处开始快速增长，并在1-3Myr内与板片分离，温度为≤900MyC，深度大致对应于板片在弧下的位置。底辟生长对绝对板温最为敏感，但也受沉积层与地幔楔体之间的粘性比以及板层上方粘性衰减的长度尺度的影响。这些二次影响在较冷的俯冲系统中最为明显，具有较旧的板块和较快的俯冲速度。对于广泛的俯冲带热条件，我们发现底辟可以有效地将沉积物输送到地幔楔形区，在那里它们将熔融并并入弧状岩浆。因此，我们得出结论，沉积物底辟作用是许多俯冲带的共同特征，为弧状岩浆化学中的“沉积物特征”提供了潜在的解释。
SUMMARY In this study, we calculate timescales for the growth of gravitational instabilities forming in the sediment layer on the downgoing slab at subduction zones. Subducted metasediments are buoyant with respect to the overlying mantle and may form diapirs that detach from the slab and rise upwards into the mantle wedge. We use a particle-in-cell, finite-difference method to calculate growth rates for instabilities forming within a buoyant, wet-quartz metasediment layer underlying a dense mantle half-space composed of wet olivine. These growth rates are used to determine where sediment diapirs initiate and detach from the slab over a range of subduction zone thermal structures. We find that, given a sufficient layer thickness (200– 800 m, depending on slab-surface and mantle-wedge temperatures), sediment diapirs begin to grow rapidly at depths of ∼80 km and detach from the slab within 1–3 Myr at temperatures ≤900 ◦ C and at depths roughly corresponding to the location of the slab beneath the arc. Diapir growth is most sensitive to absolute slab temperature, however it is also affected by the viscosity ratio between the sediment layer and the mantle wedge and the length-scale over which viscosity decays above the slab. These secondary affects are most pronounced in colder subduction systems with old slabs and faster subduction rates. For a broad range of subduction zone thermal conditions, we find that diapirs can efficiently transport sediments into the mantle wedge, where they would melt and be incorporated into arc magmas. Thus, we conclude that sediment diapirism is a common feature of many subduction zones, providing a potential explanation for the ‘sediment signature’ in the chemistry of arc magmas.