Metamorphism induced strength inversion at high-pressure conditions – Implications for strain localization in eclogite.

The geodynamic evolution of the Earth is highly governed by the mechanical behavior of rocks at plate boundaries. In convergent settings, continental and/or oceanic mafic rocks are subducted to great depths where they experience high pressures and temperatures and transform to eclogite. Accompanied mineral transformations subsequently result in mechanical changes and in density variations. In the last decades, many field, experimental and numerical studies targeted eclogite and aimed at quantifying its mechanical behavior as well as characterizing strain weakening processes. Especially, experimental investigations have shown that eclogite and its main constituents omphacite and garnet are strong phases which are not expected to creep at differential stresses below 1 GPa for tectonically relevant strain rates. Nevertheless, highly localized shear zones and mylonitic fabrics are frequently observed in eclogite, raising the question how and why strain localization occurred.
To characterize processes causing strain localization in eclogite, we investigate an eclogite facies shear zone located at the Hohl locality (Koralpe, Eastern Alps, Austria). The shear zone bears rocks with two distinct eclogite facies mineral assemblages of which one is dominated by clinozoisite, amphibole and garnet. This lithology occurs as foliated sigmoidal lenses hosted by typical eclogite containing omphacite, garnet, clinozoisite, amphibole, quartz, kyanite and rutile. Both lithologies derived from NMORB gabbro which intruded during Permian rifting. Protolith assemblage calculations suggest that lenses have originally been plagioclase-rich cumulates within a clinopyroxene-plagioclase gabbro matrix. Modal-composition based viscosity estimates indicate that previous to the high-pressure metamorphic overprint the cumulate was less competent than the gabbro. However, the sigmoidal shape of lenses surrounded by ultramylonitic eclogite suggests that the lenses were stronger during shear zone development. Microstructural investigations reveal an ultramylonitic fabric dominated by euhedral clinopyroxene (aspect ratio ~1.7) within the host eclogite. Triple- and quadruple-junctions, open grain boundaries and lack of intracrystalline strain suggest that eclogite dominantly deformed by grain boundary sliding. On the other hand, the microstructure of lenses is dominated by elongated clinozoisite (aspect ratio ~4) and elongated sigmoidal amphibole aggregates (aspect ratio ~3). Amphibole aggregates are characterized by coarse-grained highly strained clasts and strain free slightly elongated crystals in strain shadows. These observations indicate that lenses deformed by combined dislocation and dissolution-reprecipitation creep.
Our data show how mineral replacement resulted in strength inversion with lenses, initially weaker than their host, becoming stronger than the surrounding eclogite after metamorphism at eclogite-facies conditions (720 ± 20 °C, 21 ± 3 kbar). The switch in strength caused stress concentration at the lithological contacts and subsequent strain localization in the weaker eclogitic mineral assemblage.