Can dolomitic marble be weaker than calcitic marble at lower greenschist facies conditions? Microstructural analyses of co-existing greenschist facies calcitic and dolomitic mylonites

The bedrock of Mt. Hymittos, Greece, exposes two parallel Miocene low-angle extensional faults. Strain along the upper detachment is localized in calcitic marble and schist, deformation associated with the lower detachment is accommodated in a >;100 m thick package of dolomitic mylonite exhibiting consistent top-to-southwest kinematic indicators and pronounced L-S fabrics. Field and geochronometric evidence indicate both detachments deformed under middle to lower greenschist facies conditions, where dolomite is predicted to be stronger than calcite. Evidence of preferential strain localization in dolomitic marble under these conditions conflicts with generally expected predictions of mineral and rock strength; microstructural analysis and electron backscatter diffraction were applied to investigate this anomaly. Microstructures in both dolomitic and calcitic marbles record dislocation creep of dolomite and calcite leading to subgrain rotation recrystallization and grain size reduction of coarse domains (>50 mu m). Grain size reduction initiated a switch in the dominant deformation mechanism from dislocation creep to diffusion creep in fine grained dolomite domains. Zener pinning and limited recovery controlled dolomite grain sizes. The crystallographic preferred orientation in calcitic marble indicates deformation by dislocation creep accommodated grain boundary sliding. Our work suggests that polydeformed dolomitic marble can be weaker than calcitic marble at relatively low temperatures.