Lithostratigraphic controls on dewatering and fluid pressure in the western Nankai subduction zone: Implications for the drainage behavior and consolidation state of the underthrust sequence
Lithostratigraphic controls on dewatering and fluid pressure in the western Nankai subduction zone: Implications for the drainage behavior and consolidation state of the underthrust sequence
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DOI：
10.1130/2018.2534(03
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发表时间：
2018-01-01
期刊：
影响因子：
--
通讯作者：
Kopf, A. J.
中科院分区：
文献类型：
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作者：
Huepers, A.;Saffer, D. M.;Kopf, A. J.
在俯冲带，逆冲下段流体压力的产生是特别令人感兴趣的，因为它控制着沿板块界面的下盘的有效应力条件。只有少数研究系统地探讨了逆冲下沉积的岩性非均质性对流体压力及其分布的影响。我们使用了一个耦合加载和孔隙压力耗散模型，并对砂的性质进行了新的整理，以研究这种非均质性在日本近海南开俯冲带西部板块界面下的排水状态中的作用，在那里，进入的沉积物序列在半深海泥浆基质中包含了许多总厚度接近210 m的砂层。我们的研究结果表明，砂层是压力转换和溶质从更深处向海沟和向海输送的重要通道。模拟孔隙压力主要受集料砂层透过率控制，其次受砂层深度控制，影响流体从周围低渗透泥岩基质进入可渗透砂层的能力。模拟的外俯冲系统砂体渗透率与先前对滑脱带渗透率的估计范围(10（-13）至10（-16)m(2)）一致，而内俯冲系统砂体渗透率则降低了约3个数量级。增强的排水作用使含砂模型的超孔隙压力比不含砂模型低15%。因此，俯冲沉积物的岩石地层差异应该对南开俯冲体系的力学行为产生影响。
In subduction zones, fluid-pressure generation in the underthrusting section is of particular interest because it governs the effective stress conditions of the footwall lining the plate interface. Only a few studies have systematically explored the role of lithological heterogeneity of underthrust sediment on the resulting fluid pressure and its distribution. We used a coupled loading and pore-pressure dissipation model with a new compilation of sand properties to investigate the role of such heterogeneity on the drainage state beneath the plate interface in the western Nankai subduction zone offshore Japan, where the incoming sediment sequence hosts numerous sand layers with a total thickness of up to similar to 210 m within a matrix of hemipelagic mud. Our results show that sand layers act as important conduits for both pressure translation and solute transport from greater depth to the trench and seaward. The simulated pore pressure is mainly controlled by aggregate sand-layer transmissivity, and to second order by sand-layer depth, which affects the ability of fluids to access permeable sands from the surrounding less-permeable mudstone matrix. Modeled sand permeability in the outer subduction system is in the range of previous estimates for decollement zone permeability (10(-13) to 10(-16) m(2)) and evolves to approximately three orders of magnitude lower permeability in the inner subduction system. The enhanced drainage leads to 15% lower excess pore pressures in models with sands than without sands. Thus, differences in the lithostratigraphy of the subducting sediment should have implications for the mechanical behavior along the Nankai subduction system.