A new interpretation for the nature and significance of mirror-like surfaces in experimental carbonate-hosted seismic faults
A new interpretation for the nature and significance of mirror-like surfaces in experimental carbonate-hosted seismic faults
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DOI：
10.1130/g40197.1
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发表时间：
2018-07
期刊：
影响因子：
5.8
通讯作者：
G. Pozzi;N. Paola;S. Nielsen;R. Holdsworth;L. Bowen
中科院分区：
文献类型：
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作者：
G. Pozzi;N. Paola;S. Nielsen;R. Holdsworth;L. Bowen
在实验性碳酸盐岩断层中观察到了高反射性、连续的光滑表面，称为“镜状表面”(MS)，这些断层在地震和地震速度下都会发生剪切。高速摩擦实验（>0.1 m s–1）期间产生的 MS 通常被解释为摩擦主滑移面，其中弱化机制由剪切加热激活。我们通过在方解石凿岩上的旋转剪切装置中进行摩擦实验来重新检查该模型，地震速度高达 v = 1.4 m s–1 且法向应力 σn = 25 MPa，以分析随着位移增加微观结构的演变。动态弱化开始后，当摩擦系数较低（μ << 0.6）时，剪切凿孔会持续形成由纳米材料组成的边界明确、无孔隙、恒定有限厚度（几十微米）的主滑移区（PSZ），呈现多边形晶粒形状。 MS 出现在 PSZ 的两个边界处，它们在晶粒尺寸上与 PSZ 两侧的烧结、更粗糙的材料形成鲜明对比。我们的观察表明，随着动态减弱的开始，MS 通过将坚固的烧结围岩与中心弱的、活跃变形的粘性 PSZ 分开来分配变形。因此，MS 并不对应于经典意义上的摩擦滑移表面，而是构成尖锐的流变边界，而在 PSZ 中，剪切力通过热和晶粒尺寸相关机制得到增强。
Highly reflective, continuous smooth surfaces, known as "mirror-like surfaces" (MSs), have been observed in experimental carbonate-hosted faults, which were sheared at both seismic and aseismic velocities. MSs produced during high-velocity friction experiments (>0.1 m s–1) are typically interpreted to be frictional principal slip surfaces, where weakening mechanisms are activated by shear heating. We re-examined this model by performing friction experiments in a rotary shear apparatus on calcite gouge, at seismic velocities up to v = 1.4 m s–1 and normal stress σn = 25 MPa, to analyze the evolution of microstructures as displacement increases. After the onset of dynamic weakening, when the friction coefficients are low (μ << 0.6), sheared gouges consistently develop a well-defined, porosity-free principal slip zone (PSZ) of constant finite thickness (a few tens of micrometers) composed of nanometric material, which displays polygonal grain shapes. MSs occur at both boundaries of the PSZ, where they mark a sharp contrast in grain size with the sintered, much coarser material on either side of the PSZ. Our observations suggest that, with the onset of dynamic weakening, MSs partition the deformation by separating strong, sintered wall rocks from a central weak, actively deforming viscous PSZ. Therefore, the MSs do not correspond to frictional slip surfaces in the classical sense, but constitute sharp rheological boundaries, while, in the PSZ, shear is enhanced by thermal and grain-size–dependent mechanisms.