Hydrostratigraphy as a control on subduction zone mechanics through its effects on drainage: an example from the Nankai Margin, SW Japan
Hydrostratigraphy as a control on subduction zone mechanics through its effects on drainage: an example from the Nankai Margin, SW Japan
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水文地层学通过其对排水的影响来控制俯冲带力学：以日本西南部南海边缘为例
DOI：
10.1111/j.1468-8123.2009.00276.x
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发表时间：
2010
期刊：
影响因子：
1.7
通讯作者：
D. Saffer
中科院分区：
文献类型：
--
作者：
D. Saffer
地质流体 (2010) 10, 114–131
抽象的
在许多俯冲带，当沉积物从俯冲板块上刮落时，就会形成增生复合体，并且当低渗透性海洋沉积物经历快速的构造驱动载荷时，通常会形成超孔隙压力。力学模型表明，孔隙压力通过改变增生楔内及其底部的剪切强度来控制这些系统的整体几何形状。在日本西南部近海的南海边缘，增生楔的锥角沿走向变化显着，从沿东部（室户）横断面的约 4°，到沿西部（足折）横断面的 8-10°。俯冲板块上的沉积地层也有所不同：沿足折断面，该剖面的最下部包括丰富的砂质浊积岩，而沿室户断面则由单调的半远洋泥岩组成。在这里，我使用流体流动的数值模型，结合限制大块泥岩渗透率的实验室测量，来定量检验这样的假设：足折断面沿线的富含浊积岩的部分允许在增生杂岩的底部排水，从而导致机械强度的差异足以引起锥角的差异。我的结果表明，如果富含浊积岩的单元的渗透性是泥岩单元的 2-100 倍，那么地层学的变化确实可以解释观测到的锥角。相比之下，富含浊积岩单元内的渗透率各向异性仅产生很小的影响；需要约 1000:1 的各向异性比才能引起锥角的差异。沿着 Ashizuri 横断面，模拟孔隙压力导致基底剪切强度范围从沟槽处的几 MPa 到 30 公里弧线处的约 20 MPa；室户横断面沿线的抗剪强度明显较低，30 公里处仅为 ∼5 MPa。这项工作表明，岩石地层学可以通过控制超孔隙压力的分布和大小，强烈影响俯冲带断层的力学行为。
Geofluids (2010) 10, 114–131
Abstract
At many subduction zones, accretionary complexes form as sediments are offscraped from the subducting plate, and excess pore pressures commonly develop as low-permeability marine sediments undergo rapid tectonically driven loading. Mechanical models demonstrate that pore pressure controls the overall geometry of these systems by modifying shear strength both within the accretionary wedge and along its base. At the Nankai margin offshore SW Japan, the taper angle of the accretionary wedge varies markedly along-strike, from ∼4° along an eastern (Muroto) transect, to 8–10°along a western (Ashizuri) transect. Sediment stratigraphy on the subducting plate also varies: along the Ashizuri transect, the lowermost part of the section includes abundant sandy turbidites, whereas along the Muroto transect it is composed of monotonous hemipelagic mudstone. Here, I use a numerical model of fluid flow, together with laboratory measurements that constrain the bulk mudstone permeability, to quantitatively test the hypothesis that the turbidite-rich section along the Ashizuri transect allows drainage at the base of the accretionary complex, resulting in differences in mechanical strength sufficient to cause the differences in taper angle. My results demonstrate that if the turbidite-rich units are 2–100 times more permeable than the mudstone units, the variation in stratigraphy can indeed explain the observed taper angles. In contrast, permeability anisotropy within the turbidite-rich units has only a minor effect; anisotropy ratios of ∼1000:1 would be required to cause the differences in taper angle. Along the Ashizuri transect, simulated pore pressures result in a basal shear strength ranging from a few MPa at the trench to ∼20 MPa by 30 km arcward; along the Muroto transect shear strength is substantially lower, reaching only ∼5 MPa by 30 km. This work shows that lithostratigraphy can strongly influence the mechanical behavior of subduction zone faults, through its control on the distribution and magnitude of excess pore pressure.