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

Francesco Giuntoli,Giulio Viola

doi:10.1038/s41598-022-08489-2

Abstract

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.

Highlights

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
Behr and Bürgmann[11] and Kirkpatrick et al.[10] suggest a series of potential geological structures related to ETS, reminding us that far there is no universally accepted deformation structure that can be considered positively diagnostic for ETS in the exhumed geological record of subduction zones
The Verrucano bears evidence of brittle dilational hydroshearing and ductile shearing alternating cyclically during subduction at temperatures typical of the brittle-ductile transition zone (350–450 °C in36,37). This zone can occur at greater depths than usual in cold subduction zones, where, as in the case of the Northern Apennines, a low geothermal gradient of 8–10 °C/km was present

Summary

Introduction

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. We present original data from the fossil subduction interface of the Northern Apennines (Italy), from subducted continental metasediments deformed at blueschist facies conditions These metasediments are composed of heterometric, rheologically strong blocks embedded in a weaker matrix containing dilational shear veins and ductile mylonitic shear zones formed broadly coevally. Thermodynamic modelling constrains the formation of both veins and mylonites to high pressure conditions (at least 1 GPa and 350 °C) We conclude that these structures formed due to cyclic brittle and ductile deformation steered by fluctuating pore pressure, transiently reaching near-lithostatic values. These structures might represent a geological record of deep ETS, as defined b y11, in continental metasediments

Methods

Results

Conclusion