Earthquakes at lower crustal depths are common during continental collision. However, the coseismic weakening mechanisms required to propagate an earthquake at high pressures are poorly understood. Transient high-pressure fluids or melts have been proposed as a viable mechanism, but verifying this requires direct in situ measurement of fluid or melt overpressure along fault planes that have hosted dynamic ruptures. Here, we report direct measurement of highly overpressurized frictional melts along a seismic fault surface. Using Raman spectroscopy, we identified high-pressure quartz inclusions sealed in dendritic garnets that grew from frictional melts formed by lower crustal earthquakes in the Bergen Arcs, Western Norway. Melt pressure was estimated to be 1.8–2.3 GPa on the basis of an elastic model for the quartz-in-garnet system. This is ~0.5 GPa higher than the pressure recorded by the surrounding pseudotachylyte matrix and wall rocks. The recorded melt pressure could not arise solely from the volume expansion of melting, and we propose that it was generated when melt pressure approached the maximum principal stress in a system subject to high differential stress. The associated palaeostress field demonstrates that a strong lower crust accommodated up to 1 GPa differential stress during the compressive stage of the Caledonian orogeny.

在大陆碰撞期间，地壳深处的地震很常见。然而，人们对在高压下传播地震所需的同震减弱机制知之甚少。瞬态高压流体或熔体已被提议作为一种可行的机制，但验证这一点需要沿具有动态破裂的断层面直接原位测量流体或熔体超压。在这里，我们报告了沿地震断层表面的高度超压摩擦熔体的直接测量。使用拉曼光谱，我们确定了密封在树枝状石榴石中的高压石英包裹体，树枝状石榴石是由挪威西部卑尔根弧下地壳地震形成的摩擦熔体生长而成的。根据石榴石中石英系统的弹性模型，熔体压力估计为 1.8-2.3 GPa。这是〜0。比周围假速溶岩基质和围岩记录的压力高 5 GPa。记录的熔体压力不能仅由熔体的体积膨胀产生，我们建议当熔体压力接近承受高差应力的系统中的最大主应力时产生。相关的古应力场表明，在加里东造山运动的压缩阶段，强大的下地壳容纳了高达 1 GPa 的差异应力。