Abstract

Quartz/feldspar fragment (‘clast’) sizes were measured in thin sections of three types of fault zone rocks, e.g., melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and cataclasite (Ct), from two well-studied Precambrian shear/fault zones in the Indian craton (e.g., the Gavilgarh–Tan Shear zone in central India and the Sarwar–Junia Fault zone in western India). Logarithmic plots of clast area vs. cumulative frequency in the pseudotachylytes demonstrate a fractal clast-size distribution (c.s.d.) for the intermediate size range, whereas the finer and coarser clast size fractions clearly deviate from the fractal trend. Under-representation of the finer size clasts in the pseudotachylyte samples may be attributed to their preferential melting and removal from the clast population. The relative paucity of coarse clasts, on the other hand, is possibly due to a sampling bias against coarse clasts. The c.s.d of the cataclastic rock shows a multi-fractal character with two different slopes (i.e., lower D-value for finer clast sizes) and absence of the left-hand (finer size) fall off. This suggests less efficient crushing in the finer clast size fraction. The proportion of clasts, compared to the matrix, is very small in M-Pt, increases in C-Pt and is highest in Ct, suggesting that melting of rock/mineral fragments is a dominant process in forming M-Pt, whereas it is less significant in C-Pt, and is absent in Ct, which corroborates the microscopic observations.

Research Highlights

• Clast size and cumulative frequency were measured in pseudotachylyte and cataclasite samples from two shear zones.

• Size-frequency relationship follows a power law in melting- and crushing-dominated pseudotachylytes.

• The power law does not hold good for the finest and coarsest size ranges in both types of pseudotachylytes.

• For cataclasite, a multi-fractal power law relationship exists between the size and cumulative frequency of clasts.

• The non-uniform power law relationship is explained by selective melting of finer clasts in pseudotachylyte.

• Clast size distribution in a fault rock is likely controlled by the degree of melting involved in its genesis.

中文翻译：

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假速溶质和白云母中幸存碎片的尺寸分布：对地震断裂带破碎和融化过程的影响

摘要

在三种类型的断层带岩石的薄片中测量了石英/长石碎片（“碎屑”）的大小，例如，熔解为主的假速溶质（M-Pt），碎裂为主的假速溶质（C-Pt）和碎石质（Ct），来自印度克拉通的两个经过充分研究的前寒武纪剪切/断层带（例如，印度中部的Gavilgarh-Tan Shear带和印度西部的Sarwar-Junia断层带）。劈裂面积与对数的对数图假速溶质中的累积频率显示出中等大小范围的分形碎屑大小分布（csd），而较细和较粗的碎屑大小分数显然偏离了分形趋势。假速溶样品中较细尺寸的碎屑的代表性不足可能归因于它们优先熔化并从碎屑群体中移出。另一方面，粗碎屑的相对稀少可能是由于对粗碎屑的采样偏差。碎裂岩的csd表现出具有两个不同斜率的多重分形特征（即较低的D-值（用于更细的clast尺寸）和缺少左手（更细的尺寸）会下降。这表明在较细的碎屑粒度部分中粉碎效率较低。与基质相比，碎屑的比例在M-Pt中很小，在C-Pt中增加，而在Ct中最高，这表明岩石/矿物碎片的熔化是形成M-Pt的主要过程。 C-Pt的显着性较低，而Ct中的C-Pt不显着，这证实了微观观察。

研究重点

• 测量了来自两个剪切带的假速溶质和白云母样品中的碎屑大小和累积频率。

• 大小-频率关系遵循以熔解和压碎为主的假速溶电解质中的幂定律。

• 幂律不适用于两种假速溶电解质的最细和最粗尺寸范围。

• 对于白蜡石，碎屑的大小和累积频率之间存在多重分形幂律关系。

• 非均匀幂律关系可以通过伪速溶质中较细碎屑的选择性熔化来解释。

• 断层岩石中的碎屑大小分布可能受其成因的融化程度控制。