Low permeability of deep crust promotes fluid accumulation and fracturing: quantitative evidence of fluid pressure gradients and permeability from metamorphic fluid-rock reaction zones.

Abstract:

High pore fluid pressure has been recognized by geophysical observations and is generally considered as triggers of earthquakes and slow–earthquakes (e.g., Audet and Bürgmann, 2014). Although such high pore fluid pressure is recognized as mineral filled fractures in geologic observations, the actual fluid pressure gradients in the crustal rock have not been evaluated quantitatively and remain largely unknown. Here we propose a new methodology estimating the duration, fluid pressure gradients and permeability recorded in fluid-rock reaction zones, by utilizing thermodynamic analyses in conjunction with halogen (Cl, F) profiles along the reaction zones.

We have analyzed amphibolite and granulite-facies fluid–rock reaction zones at Sør Rodane mountains, East Antarctica. The thermodynamic analyses of granitic dyke–crust reaction zone at 0.5 GPa, 700°C (Uno et al., 2017) and hydration reaction zones around mineral-filled fractures at ~0.3 GPa, 450°C (Mindaleva et al., 2020) reveals extremely high fluid pressure gradients of ~100 MPa/10cm or ~1 MPa/mm across the reaction zones. The reactive transport analysis suggest that fluid activity lasted for 100–250 days and ~10 hours, respectively. These extremely high fluid pressure gradients represent the low permeability of the intact amphibolite and granulite host rocks without fractures. The estimated permeabilities of the host rocks are 10⁻²⁰–10⁻²² m², and are several orders smaller than the widely accepted crustal permeability model (~10⁻¹⁸ m²; e.g., Ingebritsen and Manning, 2010). On the other hand, permeability along the fractures are estimated as high as 10⁻¹¹–10⁻¹⁶ m², which is analogous to the permeability estimated for the hypocenter migration for the crustal earthquake swarms (~10^−14–15 m²; e.g., Nakajima and Uchida, 2018). Our observation support that low permeability of intact crust promotes fluid accumulation and subsequent fracturing in the crust and/or underlying plate boundaries.

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Ingebritsen, S.E., Manning, C.E., 2010. Geofluids 10, 193–205.

Uno, M., Okamoto, A., Tsuchiya, N., 2017. Lithos 284–285, 625–641.

Mindaleva, D., Uno, M., Higashino, F. et al., 2020. Lithos 372–373, 105521.