Oceanic transform faults display fewer and smaller-magnitude earthquakes than expected for their length. Several mechanisms have been inferred to explain this seismic slip deficit, including increased fault zone damage resulting in elevated fluid flow, and the alteration of olivine to serpentine. However, to date, these possible mechanisms are not supported by direct observation. We use micro- to kilometre scale observations from an exhumed oceanic transform fault in the Troodos Ophiolite, Cyprus, to determine mineral-scale deformation mechanisms and infer likely controls on seismic behaviour of serpentinised lithospheric mantle in active oceanic transform faults. We document a range of deformation fabrics including massive, scaly and phyllonitic serpentinite, attesting to mixed brittle-ductile deformation within serpentinite shear zones. The progressive development of a foliation, with cumulative strain, is an efficient weakening mechanism in scaly and phyllonitic serpentinite. Further weakening is promoted by a transition in the serpentine polytype from lizardite-dominated massive and scaly serpentinites to chrysotile-dominated phyllonitic serpentinite. The development of a foliation and polytype transition requires dissolution-precipitation processes. Discrete faults and fractures locally crosscut, but are also deformed by, foliated serpentinites. These brittle structures can be explained by local and transient elevated strain rates, and play a crucial role in strain localisation by providing positive feedback for dissolution-precipitation by increasing permeability. We propose that the evolution in structure and deformation style documented within the serpentinised lithospheric mantle of the Southern Troodos Transform Fault Zone is a viable explanation for the dominantly creeping behaviour and long-term weakness of oceanic transform faults.

中文翻译：

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蛇形地幔的剪切带发育：对海洋转换断裂强度的影响

海洋转换断层所显示的地震数量少于预期的长度。已经推断出几种机制来解释这种地震滑动缺陷，包括增加的断层带破坏导致流体流动增加，以及橄榄石变为蛇纹石。但是，到目前为止，直接观察尚不支持这些可能的机制。我们使用塞浦路斯Troodos Ophiolite的一个发掘出的海洋转换断层的微米到千米规模的观测资料，来确定矿物尺度的变形机制，并推断出蛇形化岩石圈地幔在活跃的海洋转换断层中的地震行为的可能控制。我们记录了一系列的变形织物，包括块状，鳞状和千层状蛇纹岩，证明了蛇纹岩剪切区内的混合脆性-延性变形。渐进发展的叶面，具有累积的应变，是鳞片状和千枚性蛇纹岩中一种有效的弱化机制。从蛇纹石为主的块状和鳞状蛇纹岩过渡到温石棉为主的新纪蛇纹岩，蛇纹石多型体的转变进一步促进了这种作用。叶状和多型转变的发展需要溶解-沉淀过程。离散断层和裂缝局部横切，但也因叶状蛇纹岩而变形。这些脆性结构可以用局部和瞬时应变率升高来解释，并通过增加渗透性为溶解沉淀提供正反馈，从而在应变局部化中发挥关键作用。