Cyclic Brittle‐Ductile Oscillations Recorded in Exhumed High‐Pressure Continental Units: A Record of Deep Episodic Tremor and Slow Slip Events in the Northern Apennines

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 brittleductile 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.