Abstract
The deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too.
Highlights
Subduction zones are very large thrust faults, in which seismogenic and aseismic sections can be distinguished, usually based on the degree of coupling along the plate interface (e.g., Pacheco et al, 1993; Hyndman et al, 1997; Agard et al, 2018)
Based on the analysis of crystallographic preferred orientation (CPO), previous studies have focused on discussions of whether dislocation creep or diffusion creep give rise to fabric formation and development
The CPO’s are discussed above, we address the question of the shape preferred orientation (SPO)’s
Summary
Subduction zones are very large thrust faults, in which seismogenic and aseismic sections can be distinguished (including slow slip phenomena; Gomberg et al, 2007), usually based on the degree of coupling along the plate interface (e.g., Pacheco et al, 1993; Hyndman et al, 1997; Agard et al, 2018). One type of rock involved in many subduction zones is the mafic oceanic crust, which undergoes mineral reactions to form eclogites. The formation and development of mineral fabrics in eclogites is an important problem in tectonics, because microstructural studies are used as indicators for deformation and exhumation processes under (ultra)-high-pressure conditions Based on the analysis of crystallographic preferred orientation (CPO), previous studies have focused on discussions of whether dislocation creep or diffusion creep give rise to fabric formation and development. Previous studies of fabric development focused strongly on omphacite while garnet studies are comparatively rare
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