Abstract Coeval pseudotachylytes (solidified melts produced during seismic slip) and mylonites are generally regarded as the geological record of transient seismic events during dominant ductile flow. Thermal runaway has been proposed as a model to explain the pseudotachylyte-mylonite association. In the Mont Mary unit (Western Alps), pseudotachylyte fault veins occur along the amphibolite-facies (ca. 550 °C; 0.35 GPa) ultramylonitic foliation of paragneisses. These veins formed at the same metamorphic conditions of the ultramylonites, thus potentially recording thermal runaway. We analysed the microstructure of quartz in ultramylonite and of ultramylonite clasts in pseudotachylyte to investigate the possible occurrence of thermal runaway. Quartz aggregates show an evolution under constant temperature to ultrafine-grained recrystallised grain size (2.5 μm), reflecting creep under high differential stresses (> 200 MPa) and high strain rates (10−9 s−1), along very narrow foliation-parallel layers. In the ultrafine aggregates, viscous grain boundary sliding became dominant and promoted cavitation leading to disintegration of quartz aggregates and precipitation, in the pore space, of biotite, oriented parallel to the main ultramylonitic foliation. The strain rate-limiting process was aseismic fluid-assisted precipitation of biotite. The potential occurrence, at the deformation conditions of the Mont Mary ultramylonites, of thermal runaway in pure quartz layers was investigated by numerical modelling. The models predict a switch from stable flow to thermal runaway at background strain rates faster than 10−9 s−1 for critical differential stresses that are comparable to the brittle strength of rocks. Deformation of ultramylonites occurred close to the conditions for thermal runaway to occur, but based on the microstructural record we conclude that the Mont Mary pseudotachylyte-mylonite association is best explained by brittle failure, triggered by transients of high differential stress and strain rate causing a downward deflection of the brittle-ductile transition.

中文翻译：

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角闪岩相超糜棱岩同震破裂前的高应力蠕变

摘要 同期假速凝岩（地震滑动过程中产生的凝固熔体）和糜棱岩通常被认为是主导韧性流动期间瞬态地震事件的地质记录。热失控已被提议作为解释伪速凝石-糜棱岩关联的模型。在 Mont Mary 单元（西阿尔卑斯山），假长石断层脉沿斜长闪岩相（约 550 °C；0.35 GPa）超糜棱岩叶理出现。这些矿脉形成于与超糜棱岩相同的变质条件下，因此可能记录热失控。我们分析了超糜棱岩中石英的微观结构和伪速凝液中超糜棱岩碎屑的微观结构，以研究可能发生的热失控。石英聚集体在恒温下演化为超细晶粒再结晶晶粒 (2.5 μm)，反映了在高差应力 (> 200 MPa) 和高应变率 (10−9 s−1) 下沿非常窄的平行叶理的蠕变层。在超细骨料中，粘性晶界滑动占主导地位，促进了空化，导致石英骨料崩解和孔隙空间中黑云母的沉淀，平行于主要的超糜棱岩叶理。应变限速过程是黑云母的抗震流体辅助沉淀。通过数值模拟研究了在 Mont Mary 超糜棱岩变形条件下纯石英层中热失控的潜在发生。对于与岩石的脆性强度相当的临界差应力，模型预测在背景应变速率快于 10-9 s-1 的情况下，从稳定流动到热失控的转变。超糜棱岩的变形发生在接近发生热失控的条件下，但根据微观结构记录，我们得出结论，Mont Marypseudotachylyte-mylonite 组合最好用脆性破坏来解释，由高差分应力和应变率的瞬变引发，导致向​​下脆-韧转变的偏转。