Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones

Abstract

The feedback between fluid migration and rock deformation in mid-crustal shear zones is acknowledged as being critical for earthquake nucleation, the initiation of subduction zones and the formation of mineral deposits. The importance of this poorly understood feedback is further highlighted by evidence for shear-zone-controlled advective flow of fluids in the ductile lower crust and the recognition that deformation-induced grain-scale porosity is a key to large-scale geodynamics. Fluid migration in the middle crust cannot be explained in terms of classical concepts. The environment is considered too hot for a dynamic fracture-sustained permeability as in the upper crust, and fluid pathways are generally too deformed to be controlled by equilibrium wetting angles that apply to hotter, deeper environments. Here we present evidence that mechanical and chemical potentials control a syndeformational porosity generation in mid-crustal shear zones. High-resolution synchrotron X-ray tomography and scanning electron microscopy observations allow us to formulate a model for fluid migration in shear zones where a permeable porosity is dynamically created by viscous grain-boundary sliding, creep cavitation, dissolution and precipitation. We propose that syndeformational fluid migration in our 'granular fluid pump' model is a self-sustained process controlled by the explicit role of the rate of entropy production of the underlying irreversible mechanical and chemical microprocesses. The model explains fluid transfer through the middle crust, where strain localization in the creep regime is required for plate tectonics, the formation of giant ore deposits, mantle degassing and earthquake nucleation. Our findings provide a key component for the understanding of creep instabilities in the middle crust.

Superplasticity and ductile fracture of synthetic feldspar deformed to large strain

“…The authors estimated that permeability of the shear zone was increased by an order of magnitude in comparison to the wall rock. Therefore, enhanced fluid flow in high-temperature shear zones [Rutter and Brodie, 1985;Fusseis et al, 2009] and CO 2 degassing of the uppermost mantle [Regenauer-Lieb, 1998] may be attributed to cavitation damage.…”

Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing

“…To date, geological research has identified evidence for creep cavitation in natural ultramylonites from the middle crust (Behrmann and Mainprice, 1987;Mancktelow et al, 1998;Herwegh and Jenni, 2001;Fusseis et al, 2009;Kilian et al, 2011;Rogowitz et al, 2016), the lower crust (Zá-vada et al, 2007;Menegon et al, 2015) and in mantle rocks (Rovetta et al, 1986;Précigout et al, 2017). Experimental work has shown that octachloropropane, quartzite, diabase, feldspar aggregates, anorthite-diopside aggregates, olivineclinopyroxene aggregates and calcite-muscovite aggregates can develop creep cavities (Caristan, 1982;Hirth and Tullis, 1989;Ree, 1994;Dimanov et al, 2007;Rybacki et al, 2008;Delle Piane et al, 2009;Précigout and Stünitz, 2016).…”

“…Models have been proposed for brittle-fracture-based pumping and the advection of fluids (Etheridge et al, 1984), diffusion-dominated granular flow (Paterson, 1995) and more recently a dynamic granular fluid pump that is dominated by fluid advection (Fusseis et al, 2009). Fusseis et al (2009) postulated that in ultramylonites, fine-grained polyphase domains deforming by viscous grainboundary sliding (VGBS) develop the dynamic granular fluid pump. This pump operates during deformation and utilises a synkinematic porosity known as creep cavitation.…”

A comparative study of X-ray tomographic microscopy on shales at different synchrotron facilities: ALS, APS and SLS

“…With its growing importance in the geological field, SRXTM has been applied to various geomaterials such as sandstone (Lindquist et al, 2000), mylonite (Fusseis et al, 2009), peridotite , volcanic rock (Voltolini et al, 2011), meteorites (Friedrich et al, 2008), gypsum (Fusseis et al, 2012) and shale (Lenoir et al, 2007;Kanitpanyacharoen et al, 2011Kanitpanyacharoen et al, , 2012. Shales are of interest owing to the low porosity and permeability, which allow them to serve as cap rocks for hydrocarbon reservoirs (Best & Katsube, 1995), repository sites for nuclear wastes (Mallants et al, 2001;Bossart & Thury, 2007), and storehouses for carbon sequestration (Chadwick et al, 2004;Busch et al, 2008).…”