Creep cavitation bands control porosity and fluid flow in lower crustal shear zones

Abstract

Shear zones channelize fluid flow in the Earth's crust. However, little is known about deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower crustal shear zone. This study investigates the deformation mechanisms, fluid-rock interaction and development of porosity in a monzonite ultramylonite from Lofoten, northern Norway. The rock was deformed and transformed into an ultramylonite under lower crustal conditions (T=700-730° C, P=0.65-0.8 GPa). The ultramylonite consists of feldspathic layers and domains of amphibole + quartz + calcite, which result from hydration reactions of magmatic clinopyroxene. The average grain size in both domains is <25 m. Microstructural observations and EBSD analysis are consistent with diffusion creep as the dominant deformation mechanism in both domains. Festoons of isolated quartz grains define C'-type shear bands in feldspathic layers. These quartz grains do not show a crystallographic preferred orientation. The alignment of quartz grains is parallel to the preferred elongation of pores in the ultramylonites, as evidenced from synchrotron X-ray microtomography. Such C'-type shear bands are interpreted as creep cavitation bands resulting from diffusion creep deformation associated with grain boundary sliding. Mass-balance calculation indicates a 2% volume increase during the protolith-ultramylonite transformation, which is consistent with synkinematic formation of creep cavities producing dilatancy. Thus, this study presents evidence that creep cavitation bands may control deep crustal porosity and fluid flow. Nucleation of new phases in creep cavitation bands inhibits grain growth and enhances the activity of grain-size sensitive creep, thereby stabilising strain localization in the polymineralic ultramylonites.

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

“…9). This is typically interpreted as indicating a transition from GSI creep to GSS creep accommodated by viscous grain-boundary sliding (Mitra, 1978;Etheridge and Wilkie, 1979;Kerrich et al, 1980;Behrmann and Mainprice, 1987;Závada et al, 2007;Kilian et al, 2011;Herwegh et al, 2014;Menegon et al, 2015;Viegas et al, 2016).…”

“…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).…”

“…Many of the geological works cited interpret creep cavities as the product of VGBS that form to accommodate strain incompatibilities (Behrmann and Mainprice, 1987;Ree, 1994;Herwegh and Jenni, 2001;Dimanov et al, 2007;Závada et al, 2007;Rybacki et al, 2008;Fusseis et al, 2009;Kilian et al, 2011;Menegon et al, 2015;Précigout and Stünitz, 2016). In the VGBSbased model of Fusseis et al (2009), cavitation at one grain triple junction is balanced by cavity closure at other grain triple junctions.…”

Millimetre-Scale Localisation of Strain and Dissolution in Oolitic Grainstone

“…Figure 4 -Drawing of cavitation development during GBS (Pilling and Ridley, 1989;Menegon et al, 2015). …”

“…the rate of pressure solution) and therefore, enhanced pressure solution increases the strain rate of GBS. The process of dissolution and precipitation can restrict the development of void spaces that would otherwise be created as grains slide past each other ( Figure 4) (Passchier and Trouw, 1996;Menegon et al, 2015).…”