Cyclic brittle-ductile oscillations recorded in exhumed high-pressure continental units: a record of deep episodic tremor and slow slip events?
Authors: Giuntoli Francesco , Viola Giulio , Eske Sørensen Bjørn 
Published: February 2023

Fluids in subduction zones play a key role in controlling seismic activity, drastically affecting the rheology of rocks, triggering mineral reactions, and lowering the effective stress. Fluctuating pore pressure is one important parameter for the switch between brittle and ductile deformation, thus impacting seismogenesis. Episodic tremor and slow slip events (ETS) have been proposed as a common feature of the geophysical signature of subduction zones. Their geological record, however, remains scanty. We propose that fluctuating pore pressure linked to metamorphic dehydration reactions steered cyclic and ETS-related brittle and ductile deformation of continental metasediments in the subduction zone of the Apennines (Italy).Field observations reveal a metamorphosed broken formation composed of boudinaged metaconglomerate enveloped by metapelite displaying a pervasive mylonitic foliation. Dilational shear veins occur in both lithotypes but are more common and laterally continuous in the metapelite. Veins are generally parallel to the metamorphic foliation and are composed of iso-oriented stretched quartz and carpholite fibres, which form single-grains up to several centimetres long. These fibres define a stretching direction mainly consistent with that of the hosting metaconglomerate and metapelite, which is marked by K-white mica and quartz. Thermodynamic modeling constrains the formation of the high-pressure veins and the mylonitic foliation to ~ 1 GPa and 350&#176;C, corresponding to c. 30-40 km depth in the subduction channel1.Microstructural analysis suggests that dilational hydroshear veins formed by incremental crack-sealing at supralithostatic pore pressure values. Successively, the veins experienced only limited recrystallization of quartz fibres by subgrain rotation recrystallization, with adjacent metapelite bands acting as decollement horizons, likely by slip on the basal plane of phyllosilicates. Blueschist facies mylonites formed mainly by a combination of dissolution-precipitation creep and slip along phyllosilicate bands.Dilational shear veins in subducted metasedimentary successions have been suggested to be potential records of episodic tremors and slip events2. We propose these microstructures and deformation mechanisms to represent a geological evidence of deep episodic tremor and slow slip events in subducted continental metasediments. Pore pressure cyclically reached supralithostatic values triggering tremors causing fracturing of all involved lithotypes. Likely, slow slip was accommodated preferentially by slip on phyllosilicate bands. Aseismic creep occurred mainly by dislocation creep with subgrain rotation recrystallization in vein quartz, slip on the basal plane of phyllosilicates, and dissolution and precipitation creep in the host rock3.Our results suggest reconsidering the role of quartz-carpholite veins forming coevally with metamorphic foliation as a possible record of deep ETS in similar geological settings of other convergent orogens 1.&#160;1 Giuntoli et al. A likely geological record of deep tremor and slow slip events from a subducted continental broken formation. Sci Rep 12, (2022).2 Fagerenget al. Incrementally developed slickenfibers &#8212; Geological record of repeating low stress-drop seismic events? Tectonophysics 510, 381&#8211;386 (2011).3 Giuntoli et al. Deformation Mechanisms of Blueschist Facies Continental Metasediments May Offer Insights Into Deep Episodic Tremor and Slow Slip Events. J Geophys Res Solid Earth 127, (2022).&#160;&#160;This project has received funding from the European Union&#8217;s Horizon 2020 research and innovation programme under the Marie Sk&#322;odowska-Curie grant agreement No 839779.&#160;&#160;&#160;