High-stress creep preceding coseismic rupturing in amphibolite-facies ultramylonites
High-stress creep preceding coseismic rupturing in amphibolite-facies ultramylonites
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角闪岩相超糜棱岩同震破裂前的高应力蠕变
DOI：
10.1016/j.epsl.2020.116260
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发表时间：
2020
影响因子：
5.3
通讯作者：
Papa S
中科院分区：
文献类型：
--
作者：
Papa S
同期的假速溶岩（地震滑移过程中产生的凝固熔体）和糜棱岩通常被认为是主要延性流过程中瞬态地震事件的地质记录。热失控已被提出作为解释假速石-糜棱岩组合的模型。在蒙特玛丽单元（西阿尔卑斯山），假速岩断层脉沿着副片麻岩的角闪岩相（约 550 °C；0.35 GPa）超糜棱岩叶理出现。这些矿脉是在与超糜棱岩相同的变质条件下形成的，因此可能记录了热失控。我们分析了超糜棱岩中石英和假速晶岩中超糜棱岩碎屑的微观结构，以研究热失控发生的可能性。石英聚集体在恒温下表现出超细晶再结晶晶粒尺寸 (2.5 μm) 的演变，反映了高差应力 (> 200 MPa) 和高应变率 (10−9s−1) 下沿着非常窄的叶状平行层的蠕变。在超细聚集体中，粘性晶界滑动占主导地位，并促进空化，导致石英聚集体崩解，并在孔隙空间中沉淀黑云母，方向平行于主要的超糜棱岩叶理。应变限速过程为抗震流体辅助黑云母沉淀。通过数值模拟研究了在蒙特玛丽超糜棱岩变形条件下纯石英层中热失控的潜在发生情况。该模型预测，对于与岩石脆性强度相当的临界差应力，背景应变率会快于 10−9s−1，从稳定流转变为热失控。超糜棱岩的变形发生在接近热失控发生的条件下，但根据微观结构记录，我们得出的结论是，蒙玛丽假速晶石-糜棱岩组合最好是通过脆性破坏来解释，脆性破坏是由高差应力和应变率的瞬变触发的，导致脆性转变向下偏转。
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 (10−9s−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 10−9s−1for 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.
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