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Fragmentation of wall rock garnets during deep crustal earthquakes

机译:地壳深地震中壁石榴石的破碎

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摘要

Fractures and faults riddle the Earth’s crust on all scales, and the deformation associated with them is presumed to have had significant effects on its petrological and structural evolution. However, despite the abundance of directly observable earthquake activity, unequivocal evidence for seismic slip rates along ancient faults is rare and usually related to frictional melting and the formation of pseudotachylites. We report novel microstructures from garnet crystals in the immediate vicinity of seismic slip planes that transected lower crustal granulites during intermediate-depth earthquakes in the Bergen Arcs area, western Norway, some 420 million years ago. Seismic loading caused massive dislocation formations and fragmentation of wall rock garnets. Microfracturing and the injection of sulfide melts occurred during an early stage of loading. Subsequent dilation caused pervasive transport of fluids into the garnets along a network of microfractures, dislocations, and subgrain and grain boundaries, leading to the growth of abundant mineral inclusions inside the fragmented garnets. Recrystallization by grain boundary migration closed most of the pores and fractures generated by the seismic event. This wall rock alteration represents the initial stages of an earthquake-triggered metamorphic transformation process that ultimately led to reworking of the lower crust on a regional scale.
机译:破裂和断层在各个尺度上都困扰着地壳,并且据推测与它们相关的变形对其岩石学和结构演化具有重大影响。但是,尽管有大量直接可观察到的地震活动,但沿古断层的地震滑移率的明确证据很少,通常与摩擦融化和假速滑石的形成有关。我们报道了大约4.2亿年前在挪威西部的卑尔根弧地区发生中等深度地震时横穿下地壳颗粒的地震滑动面附近的石榴石晶体形成的新颖微观结构。地震载荷导致大量的位错形成和围岩石榴石的破碎。在加载的早期阶段进行了微压裂和硫化物熔体的注入。随后的膨胀导致流体沿微裂纹,位错,亚晶粒和晶界的网络无处不在地输送到石榴石中,从而导致破碎的石榴石内部大量矿物包裹体的生长。晶界迁移引起的再结晶封闭了地震事件产生的大部分孔隙和裂缝。这种围岩蚀变代表了地震触发的变质作用转变过程的初始阶段,最终导致了区域范围内下地壳的重新构造。

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