Rapid fluid infiltration and permeability enhancement during middle-lower crustal fracturing: Evidence from fluid-rock reaction zones, Sør Rondane Mountains, East Antarctica

Aqueous fluid flow cause hydration reactions, which induce mass transport, and changes rheology of rocks. It is generally accepted that fluids play a key role in the generation of earthquakes, tremors, and slow slip events. It was suggested that tremors are related to fluids released during dehydration processes in the subduction slab (e.g., Abers et al., 2009; Katsumata and Kamaya, 2003; Obara et al., 2004). H₂O released due to dehydration reactions increases fluid pressure. However, quantitative constraints on fluid pressure gradients and crustal permeability are limited, particularly with regards to its temporal evolution. Therefore, it is important to constrain timescales of fluid infiltration to understand fluid pressure gradient, permeability, and tectonic evolution. Here we constrain the permeability evolution in the middle-lower crust, based on metamorphic processes associated with fluid infiltration and crustal fracturing.

We investigated fluid-rock reaction zones in hydrated metamorphic rocks samples from the Sør Rondane Mountains (SRM), East Antarctica. Millimetre-scale hydrous reaction zones occur along fractures. Previous studies (e.g., Higashino et al., 2013; Higashino et al., 2018; Uno et al 2017) suggested Cl-bearing fluid infiltration in the SRM. Chlorine concentrations in apatite and amphibole grains show a gradual decrease from the fractures toward the wall rocks. Chlorine concentration profiles were analyzed by a reactive-transport model to define transport mechanism. Fitting results suggest that advection with minor diffusion is dominant. The timescales of fluid infiltration are constrained to tens of hours. The fluid pressure gradient across the reaction zones was estimated from the H₂O activity to be 0.4-1.4 MPa/mm. The permeability of the wall rock and fractures were estimated to be 10⁻²⁰-10⁻²² and 10⁻⁸-10⁻⁹ m², respectively. Our results show that rapid infiltration of Cl-bearing fluids occurred due to a limited fluid flux. Low permeable wall rock (10⁻²⁰-10⁻²² m²) was fractured due to fluid accumulation and rising of the fluid pressure. The spatio-averaged permeability then significantly increased (10⁻¹⁰-10⁻¹⁶ m²). The contrasting permeability reveals enhancements associated with crustal fracturing on timescales comparable to geophysical observations.