Methane-hydrogen-rich fluid migration may trigger seismic failure in subduction zones at forearc depths.

Metamorphic fluids, faults, and shear zones are carriers of carbon from the deep Earth to shallower reservoirs. Some of these fluids are reduced and transport energy sources, like H2 and light hydrocarbons. Mechanisms and pathways capable of transporting these deep energy sources towards shallower reservoirs remain unidentified. Here we present geological evidence of failure of mechanically strong rocks due to the accumulation of CH4-H2-rich fluids at deep forearc depths, which ultimately reached supralithostatic pore fluid pressure. These fluids originated from adjacent reduction of carbonates by H2-rich fluids during serpentinization at eclogite-to-blueschist-facies conditions. Thermodynamic modeling predicts that the production and accumulation of CH4-H2-rich aqueous fluids can produce fluid overpressure more easily than carbon-poor and CO2-rich aqueous fluids. This study provides evidence for the migration of deep Earth energy sources along tectonic discontinuities, and suggests causal relationships with brittle failure of hard rock types that may trigger seismic activity at forearc depths.

A likely geological record of deep tremor and slow slip events from a subducted continental broken formation.

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. Only the detailed and further characterization of exhumed fossil geological settings can help fill this knowledge gap. Here we propose that fluctuating pore pressure linked to metamorphic dehydration reactions steered cyclic and ETS-related brittle and ductile deformation of continental crustal rocks in the subduction channel of the Apennines. Dilational shear veins and ductile mylonitic shear zones formed broadly coevally at minimum 1 GPa and 350 °C, corresponding to ~ 30-40 km depth in the subduction zone. We identify carpholite in Ca-poor metasediments as an important carrier of H2O to depths > 40 km in cold subduction zones. Our results suggest that the described (micro)structures and mineralogical changes can be ascribed to deep ETS and provide a useful reference for the interpretation of similar tectonic settings worldwide.

Feedback between high-pressure genesis of abiotic methane and strain localization in subducted carbonate rocks.

Fluid-rock interactions exert key control over rock rheology and strain localization. Redox may significantly affect the reaction pathways and, thereby, the mechanical properties of the rock. This effect may become critical in volatile-rich, redox sensitive rocks such as carbonate-rich lithologies, the breakdown of which can significantly modify the net volume change of fluid-mediated reactions. Subduction focus the largest recycling of crustal carbonates and the most intense seismic activity on Earth. Nevertheless, the feedbacks between deep carbon mobilization and deformation remain poorly investigated. We present quantitative microstructural results from natural samples and thermodynamic modeling indicating that percolation of reducing fluids exerts strong control on the mobilization of carbon and on strain localization in subducted carbonate rocks. Fluid-mediated carbonate reduction progressed from discrete domains unaffected by ductile deformation into localized shear zones deforming via diffusion creep, dissolution-precipitation creep and grain boundary sliding. Grain-size reduction and creep cavitation along localized shear zones enhanced fluid-carbonate interactions and fluid channelization. These results indicate that reduction of carbonate rocks can exert an important positive feedback on strain localization and fluid channelization, with potential implications on seismic activity and transport of deep hydrocarbon-bearing fluids.