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

9 Shear zones channelize fluid flow in the Earth’s crust. However, little is known about 10 deep crustal fluid migration and how fluids are channelized and distributed in a deforming lower 11 crustal shear zone. This study investigates the deformation mechanisms, fluid-rock interaction 12 and development of porosity in a monzonite ultramylonite from Lofoten, northern Norway. The 13 rock was deformed and transformed into an ultramylonite under lower crustal conditions 14 (T=700-730° C, P=0.65-0.8 GPa). The ultramylonite consists of feldspathic layers and domains 15 of amphibole + quartz + calcite, which result from hydration reactions of magmatic 16 clinopyroxene. The average grain size in both domains is <25 m. Microstructural observations 17 and EBSD analysis are consistent with diffusion creep as the dominant deformation mechanism 18 in both domains. Festoons of isolated quartz grains define C’-type shear bands in feldspathic 19 layers. These quartz grains do not show a crystallographic preferred orientation. The alignment 20 of quartz grains is parallel to the preferred elongation of pores in the ultramylonites, as evidenced 21 from synchrotron X-ray microtomography. Such C’-type shear bands are interpreted as creep 22 Manuscript Click here to download Manuscript: Menegon_et_al_creep_cavitation_revised_ms.doc