Seismic activity below the standard seismogenic zone is difficult to investigate because the geological records of such earthquakes, pseudotachylytes, are typically reacted and/or deformed. Here, we describe unusually pristine pseudotachylytes in lower-crustal granulites from the Lofoten Archipelago, northern Norway. The pseudotachylytes have essentially the same mineralogical composition as their host (mainly plagioclase, alkali feldspar, orthopyroxene) and contain microstructures indicative of rapid cooling, i.e., feldspar microlites and spherulites and “cauliflower” garnets. Mylonites are absent, both in the wall rocks and among the pseudotachylyte clasts. The absence of features recording precursory ductile deformation rules out several commonly invoked mechanisms for triggering earthquakes in the lower crust, including thermal runaway, plastic instabilities, and downward propagation of seismic slip from the brittle to the ductile part of a fault. The anhydrous mineralogy of host and pseudotachylytes excludes dehydration-induced embrittlement. In the absence of such weakening mechanisms, stress levels in the lower crust must have been transiently high.

中文翻译：

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下地壳中的高瞬态应力：挪威北部罗弗敦群岛花岗石中干假速溶质的证据

在标准地震发生带以下的地震活动很难进行调查，因为这种地震的地质记录（假速溶物）通常会发生反应和/或变形。在这里，我们描述了来自挪威北部罗弗敦群岛下地壳的粒状岩中的原始假速溶物质。假速溶菌具有与它们的主体基本相同的矿物学组成（主要是斜长石，碱金属长石，邻苯二酚），并含有指示快速冷却的微观结构，即长石微晶和球晶以及“花椰菜”石榴石。围岩和假速溶岩碎屑中均不含石。缺少记录先兆延性变形的特征，排除了几种通常用来触发下地壳地震的机制，包括热失控，塑性不稳定性以及地震滑动从断层的脆性到延性部分的向下传播。宿主和假速溶菌的无水矿物学排除了脱水引起的脆化。在没有这种削弱机制的情况下，下地壳的应力水平必须一直很高。