STRESS TENSORS AT THE TOE OF THE NANKAI ACCRETIONARY PRISM : AN APPLICATION OF INVERSE METHODS TO SLICKENLINED FAULTS
STRESS TENSORS AT THE TOE OF THE NANKAI ACCRETIONARY PRISM : AN APPLICATION OF INVERSE METHODS TO SLICKENLINED FAULTS
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南海增生棱柱顶端的应力张量:反演方法在光滑断层中的应用
DOI：
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发表时间：
2006
期刊：
影响因子：
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通讯作者：
P. Henry
中科院分区：
文献类型：
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作者：
S. Lallemant;T. Byrne;A. Maltman;D. Karig;P. Henry
现场808（ODP 131航次）的钻探提供了南海增生杂岩趾部离散脆性结构（小断层和剪切带）的大量记录。脆性破坏发生在整个洞，虽然大部分的变形已经观察到之间的正面推力和décollement（从365至963 mbsf）。脆性破坏发生在浊流沟填充物（0至600 mbsf）中，但也发生在四国盆地上部（600至800 mbsf）的含灰半长岩和四国盆地下部层序的无灰半长岩中，直至滑脱层（963 mbsf）。构造特征的几何形状已记录在与岩心内衬相关的局部框架中，然后通过使用古地磁测量校正到绝对框架。我们使用由此产生的几何形状的均匀人口slickenlined故障估计约化应力张量。主要结果是显示三个一致的应力模式。从前缘逆冲断层到滑脱断层的大部分断层群与具有西北向σj（方位角N305°至N315 °）的挤压体制非常一致。因此，压缩方向大致平行于相对会聚的局部方向。它也垂直于背斜脊的走向。离散故障，局部表现为群集（例如，30 m以上）与一个完全不同的挤压方向（西-西南-东-东北）一致，并仅限于滑脱层之上的半远洋层序。这可能与该层内部变形的不均匀性有关。虽然存在孤立的正断层，但它们的成簇出现仅限于四国盆地半远洋层序，并指示一个东西向至西北-东南向的伸展轴（σ3）。最后，与最佳拟合张量有关的莫尔图的分析显示了浊积岩和半长岩序列的断层群之间的一些相似性和差异。对应于95%数据的摩擦角为30° ± 5°，而包括浊积岩和半长岩中的剩余5%将导致更低的摩擦角（18° ± 8°）。主要的区别在于共轭组之间的夹角，浊积岩中的夹角约为60°，而滑脱层上方200 m厚的区域中的夹角较低（35° ~ 40°），这可以用半深海岩中较高的流体压力或这种半深海物质的奇怪机械性质来解释。
Drilling at Site 808 (ODP Leg 131) provided an extensive record of the discrete brittle structures (small faults and shear bands) at the toe of the Nankai accretionary complex. Brittle failure is occurring throughout the hole, although most of the deformation has been observed between the frontal thrust and the décollement (from 365 to 963 mbsf). Brittle failure occurs in the turbiditic trench fill (0 to 600 mbsf) but also in the ash-bearing hemipelagites from the upper Shikoku Basin (600 to 800 mbsf) and the ash-free hemipelagites of the lower Shikoku Basin sequences, down to the décollement (963 mbsf). The geometry of the tectonic features has been recorded in a local frame related to the core liner and then corrected to an absolute frame by the use of paleomagnetic measurements. We used the resulting geometry of homogeneous populations of slickenlined faults to estimate the reduced stress tensor. The main result is to show three consistent stress patterns. Most of the fault clusters from the frontal thrust down to the décollement are in good agreement with a compressional regime with a northwest-trending σj (azimuth N305° to N315 °). The compression direction is thus roughly parallel to the local direction of the relative convergence. It is also perpendicular to the trend of the anticlinal ridges. Discrete faults, locally appearing as clusters (e.g., 30 m above the décollement) agree with a quite different compression direction (west-southwest-east-northeast) and are restricted to the hemipelagic sequences above the décollement. They could be related to the heterogeneous internal deformation of this layer. Although isolated normal faults exist, their occurrence as clusters is restricted to the Shikoku Basin hemipelagic sequences and indicate a east-west to northwest-southeast extension axis (σ3). Finally, the analysis of Mohr diagrams related to the best-fitting tensors shows some similarities and differences between the fault populations of the turbidite and those of the hemipelagite sequences. The angle of friction corresponding to 95% of the data is 30° ± 5°, whereas including the remaining 5% in turbidites and hemipelagites would result in a lower friction angle (18° ± 8°). The major difference concerns the angle between conjugate sets which are about 60° in the turbidites and lower (35°40°) in a 200 m thick zone above the décollement and can be explained by higher fluid pressure in the hemipelagites or by odd mechanical properties of this hemipelagic material.