Scaly fabrics in sheared clays from the décollement zone of the Barbados accretionary prism
Scaly fabrics in sheared clays from the décollement zone of the Barbados accretionary prism
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来自巴巴多斯增生棱柱脱脱区的剪切粘土中的鳞状织物
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
S. Takizawa
中科院分区：
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作者：
P. Labaume;A. Maltman;A. Bolton;D. Tessier;Y. Ogawa;S. Takizawa
在巴巴多斯北部增生棱柱顶端的滑脱层中，鳞状结构出现在厘米厚的区域，被解释为构造位移集中的层位。使用光学、扫描（二次和背散射模式）和透射电子显微镜对鳞状织物进行了详细的微观结构研究。这些观察结果表明，鳞状织物基本上包括三种类型的微观结构，它们是由剪切和压扁共同作用产生的。在这些微观结构中，应变集中在微米到毫米厚的变形带中，这是由粘土颗粒旋转引起的，伴随着孔隙坍塌（压实塑性应变），导致粘土颗粒形成具有明显优先取向的区域。然而，这种优先取向只影响了一小部分涉及鳞构带的沉积物，也只影响了滑脱总厚度的一小部分。在微观结构关联模式的基础上，提出了鳞片-织物区的运动演化模型。在这些构造中，变形始于与平坦带阵列相对应的间隔片理的形成，然后在S-C （schistosite-cisaillement [schistosite- shear]）带中剪切应变的集中，在几何上类似于变质剪切带中常见的S-C构造岩。变形的分区导致S-C带外围的裂缝网络形成较晚，这些裂缝网络可能是S-C带加宽的前兆。变形带中的压实应变是正常或欠压实沉积物的典型特征，意味着孔隙流体的排出。粘土颗粒的择优取向使变形带成为变形带内流体循环的潜在通道，但压实应变要求变形带在超孔隙压力作用下被扩张，从而具有显著的渗透性。我们推断出在鳞状结构带中应力状态的循环变化与孔隙压力变化有关。在较低的孔隙压力和较大的剪切应力条件下，压缩塑性剪切应变可以形成鳞状织物，而高孔隙压力会抑制进一步的压缩应变，同时增加渗透性。在高孔隙压力期，构造位移可能发生在鳞片构造带的尖锐边界处；因此，鳞状结构的形成只占累积位移的一部分。
Scaly fabrics in the decollement at the toe of the northern Barbados accretionary prism occur in centimeter-thick zones interpreted as the horizons where tectonic displacement is concentrated. Detailed microstructural investigations of the scaly fabrics have been carried out, using optical, scanning (secondary and backscattered modes), and transmission electron microscopy. These observations show that the scaly fabrics essentially comprise three types of microstructures, which arise from a combination of shear and flattening. In these microstructures, strain is concentrated in micrometer- to millimeter-thick deformation bands, caused by clay-particle rotation associated with porosity collapse (compactional plastic strain), resulting in the formation of domains with marked preferred orientation of clay particles. However, this preferred orientation affects only a minor part of the sediment involved in the scaly-fabric zones and only a small proportion of the total decollement thickness. On the basis of the mode of microstructure associations, we propose a model for the kinematic evolution of the scaly-fabric zones. In these, deformation initiates by the formation of a spaced foliation corresponding to flattening band arrays, then continues by concentration of shear strain in S-C (schistosite-cisaillement [schistosity-shear]) bands geometrically analogous to the S-C tectonites common in metamorphic shear zones. Partitioning of deformation results in the late formation of fracture networks at the periphery of the S-C bands, the fractures networks being possible precursors of S-C band widening. Compactional strain in the deformation bands is typical of normally or undercompacted sediments and implies expulsion of pore fluid. Preferred orientation of clay particles makes the deformation bands potential pathways for fluid circulation in deformation zones, but compactional strain requires the bands to be dilated by excess pore pressure to have significant permeability. We infer cyclic variations of stress state in the scaly-fabric zones, related to pore-pressure variations. Formation of scaly fabrics by compactional plastic shear strain would be achieved under relatively low pore pressure and significant shear stress, whereas high pore pressure would inhibit further compactional strain while increasing permeability. Tectonic displacement is likely to be favored at the sharp boundaries of the scaly-fabric zones during high pore-pressure episodes; formation of the scaly fabrics thus would account for only part of the cumulative displacement.