Abstract Coseismic off-fault damage and pulverization significantly influence the mechanical and transport properties, and in turn the rupture dynamics of faults. Although field-based, laboratory, and numerical studies help elucidate the structure of damage zones adjacent to modern strike-slip faults, the vertical extent of these zones remains an open question. To address this question, we analyzed particle size distribution and microfracture density of fragmented garnets from the Sandhill Corner shear zone, a strand of an ancient, seismogenic, strike-slip fault system exhumed from frictional-to-viscous transition depths (400–500 °C). The shear zone has mutually overprinting pseudotachylyte and mylonite, and juxtaposes quartzofeldspathic and schist units. The inner parts of the quartzofeldspathic and schist units (∼63 m and ∼5 m wide from the lithologic contact, respectively) have two-dimensional D-values ≥1.5, which indicates dynamic fragmentation during rupture propagation. Similar to the particle size distribution analysis, microfracture density data from the garnets show progressive but asymmetric increase toward the core in each unit. Our results suggest that coseismic damage extends down to the base of the seismogenic zone in mature strike-slip faults, and the asymmetric distribution of damage may indicate preferred rupture directivity as proposed for some modern strike-slip faults.

中文翻译：

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同震破坏深入大陆走滑断层

摘要 同震非断层损伤和粉化显着影响断层的力学和输运特性，进而影响断层的破裂动力学。尽管基于现场、实验室和数值的研究有助于阐明现代走滑断层附近损坏带的结构，但这些带的垂直范围仍然是一个悬而未决的问题。为了解决这个问题，我们分析了来自 Sandhill Corner 剪切带的破碎石榴石的粒度分布和微裂缝密度，这是从摩擦到粘性过渡深度（400-500 ° C）。剪切带具有相互叠印的拟速晶石和糜棱岩，并并列石英长石和片岩单元。石英长石和片岩单元的内部（分别距岩性接触面约 63 m 和约 5 m 宽）二维 D 值≥1.5，表明破裂扩展过程中发生了动态破碎。与粒度分布分析类似，石榴石的微裂缝密度数据显示出每个单元中向核心逐渐但不对称的增加。我们的结果表明，在成熟的走滑断层中，同震损伤向下延伸到发震带的底部，并且损伤的不对称分布可能表明某些现代走滑断层所提出的优选破裂方向性。石榴石的微裂缝密度数据显示每个单元的岩心逐渐但不对称增加。我们的结果表明，在成熟的走滑断层中，同震损伤向下延伸到发震带的底部，并且损伤的不对称分布可能表明某些现代走滑断层所提出的优选破裂方向性。石榴石的微裂缝密度数据显示每个单元的岩心逐渐但不对称增加。我们的结果表明，在成熟的走滑断层中，同震损伤向下延伸到发震带的底部，并且损伤的不对称分布可能表明某些现代走滑断层所提出的优选破裂方向性。