The geological record of deep fossil seismogenesis in subduction zones is limited due to common rock overprinting during exhumation and only a few regions expose well-preserved deeply exhumed structures. We investigated a mesoscopic contractional duplex formed at blueschist facies conditions in continental metasediments in the Northern Apennines (Italy). Field observations reveal strain partitioning within the duplex between metapelite bands, corresponding to high-strain zones, and metaquartzarenite bands, which form low-strain, imbricated metric horses. Dilational shear veins occur in both lithotypes and are composed of quartz and carpholite fibers defining a stretching lineation parallel to the regional SW-NE transport direction. Geometrical, cross cutting and petrographic relationships suggest that veins formed broadly syn-mylonitization. Thermodynamic modeling constrains the formation of the mylonitic foliation to >0.7 GPa and ∼400°C and the dilational shear veins vein to ∼1.1 GPa and 350°C. Therefore, we document a top-to-the-E-NE thrust that formed at the deepest conditions recorded by this unit of the Northern Apennines. In the thrusted continental metasediments, aqueous fluid locally released by metamorphic dehydration reactions transiently increased pore pressure, in turn triggering brittle-ductile cyclicity. We propose that blueschist facies dilational shear veins and mylonitic foliation represent a geological record of deep episodic tremor and slow slip events. To the best of our knowledge, this is the first time that these events are genetically associated with brittle and ductile structures of a mesoscopic duplex. We suggest that these structures could be common features of the high-pressure metamorphic units of the Apennine orogenic belt.

中文翻译：

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在挖掘出的高压大陆单元中记录的循环脆性-延展性振荡：北亚平宁山脉的深层间歇性震颤和慢滑事件的记录

由于挖掘过程中常见的岩石叠印，俯冲带深部化石地震发生的地质记录有限，只有少数地区暴露出保存完好的深部挖掘结构。我们研究了在北亚平宁山脉（意大利）大陆变沉积岩中蓝片岩相条件下形成的细观收缩双链体。现场观察揭示了变形泥岩带之间的应变分配，对应于高应变区，和变形石英带，形成低应变的叠瓦公制马。扩张剪切脉出现在两种岩型中，由石英和卡佛石纤维组成，形成平行于区域 SW-NE 输运方向的拉伸线。几何、横切和岩相关系表明，矿脉形成了广泛的同糜棱化。热力学模型将糜棱片理的形成限制在 >0.7 GPa 和 ~400°C，将扩张剪切脉脉限制在 ~1.1 GPa 和 350°C。因此，我们记录了在北亚平宁山脉该单元记录的最深条件下形成的从顶部到 E-NE 的推力。在逆冲大陆变质沉积物中，变质脱水反应局部释放的含水流体瞬时增加孔隙压力，进而引发脆韧循环。我们认为蓝片岩相扩张剪切脉和糜棱叶理代表了深部间歇性震颤和缓慢滑动事件的地质记录。据我们所知，这是这些事件第一次与细观双链体的脆性和韧性结构在遗传上相关。