Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust, Shimanto Belt, southwest Japan
Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust, Shimanto Belt, southwest Japan
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DOI：
10.1111/iar.12257
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发表时间：
2018-05
期刊：
影响因子：
1.5
通讯作者：
M. Hamahashi;W. Tanikawa;Y. Hamada;Y. Hashimoto;S. Saito;G. Kimura
中科院分区：
文献类型：
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作者：
M. Hamahashi;W. Tanikawa;Y. Hamada;Y. Hashimoto;S. Saito;G. Kimura
为了研究俯冲带中片理化断层岩的物性各向异性，重点研究了日本四万十带中沿野冈逆冲断层（一个典型的无序逆冲断层）出露的上沿着壁千枚岩和下盘碎裂岩。使用从野冈逆冲钻探项目获得的岩心样品，采用地质坐标（深度平行方向、走向方向和叶理的最大倾角方向）进行离散物理性质（电阻率、P波和S波速度和孔隙度）测量，并将数据与钻孔地球物理测井数据进行比较。与测井相比，较高的样品P波速度（Vp）、较低的S波速度（Vs）、较高的Vp/Vs以及较低的样品孔隙度和电阻率被推断是由测井的较大采样规模和较低的井眼流体饱和度造成的。千枚岩和碎裂岩表现出Vp（分别为0.4- 17.3%和2.7- 13.8%）、Vs（分别为0.5- 56%和7.7- 43%）和电阻率（分别为0.9- 119%和2.0- 65.9%）的显著垂直和水平各向异性。物性各向异性主要受叶理倾角的影响。具有较缓倾角的断层岩在走向和最大倾角方向上表现出较高的Vp，而在深度平行方向上表现出较低的Vp。相反，具有陡倾构造的断层岩在走向和深度平行方向上显示出较高的Vp，在最大倾角方向上具有较低的速度。电阻率各向异性与Vp随倾角的变化趋势相反。我们的研究结果表明，较低的Vp各向异性比在以前的研究中获得的，其中测量的波速垂直或平行于叶理围压下。这项研究突出了地球物理勘探中的垂直属性的意义，倾角横跨片理化断层岩。
To investigate the physical property anisotropies of foliated fault rocks in subduction zones, the hanging wall phyllites and footwall cataclasites exhumed along the Nobeoka Thrust, a fossilized out‐of‐sequence‐thrust in the Shimanto Belt, Japan, was focused. Discrete physical property (electric resistivity, P‐ and S‐wave velocities, and porosity) measurements were conducted employing geologic coordinates (depth‐parallel direction, strike direction, and maximum dip direction of foliation), using the core samples obtained from the Nobeoka Thrust Drilling Project and compared the data to borehole geophysical logs. A higher sample P‐wave velocity (Vp), lower S‐wave velocity (Vs), higher Vp/Vs, and lower sample porosity and resistivity compared to the logs, are inferred to have been caused by the larger sampling scale of the logs and lower fluid saturation of the borehole. The phyllites and cataclasites exhibited substantial vertical and horizontal anisotropy of Vp (0.4–17.3 % and 2.7–13.8 %, respectively), Vs (0.5–56 % and 7.7–43 %, respectively), and resistivity (0.9–119 % and 2.0–65.9 %, respectively). The physical property anisotropies are primarily affected by the dip angles of foliation. The fault rocks that have gentler dip angles exhibit a higher Vp in the strike and maximum dip direction and a lower Vp in the depth‐parallel direction. In contrast, the fault rocks that have steeply dipping structures show a higher Vp in the strike and depth‐parallel directions with a lower velocity in the maximum dip direction. Resistivity anisotropy show a trend opposite to that of the Vp in relation to the dip angles. Our results show lower Vp anisotropy than those obtained in previous studies, which measured wave speeds perpendicular or parallel to foliation under confining pressure. This study highlights the significance of dip angles on vertical properties in geophysical surveys across foliated fault rocks.