Microstructural characterisation of ultracataclastic zones in quartzites by electron microscopy

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The temporal and spatial association of (ultra)fine-grained quartz with pseudotachylytes (fossil earthquake structures) frequently appear in nature and seems to be an important feature characteristic for seismic processes. A detailed microstructural characterisation of this relationship was worked out for pseudotachylyte-bearing quartzite of the Schneeberger Zug (Southern Tirol, Italy) during the first period of DFG project WA 1010/11-1. Crystal plastic deformation in host rock has produced ultrafine-grained (<2 μm) quartz microshear zones prior or during the co-seismic event. Since two variables (strain rate and temperature) mainly control the microstructural development the microstructures by its own can not be used to constrain the ambient conditions. Thus it is crucial to determine at least the deformation temperature during the co-seismic event. For this purpose we intend to apply the Ti-in-quartz (TitaniQ) geothermometer. This method was recently tested to constrain deformation temperatures of quartz mylonites in the temperature range of 300-500°C. Further information can be constrained by Ar-Ar dating of the Schneeberg pseudotachylyte with respect to the exhumation history in order to estimate the depth of frictional melting. Preliminary analysis reveals that the deformation microstructures of quartz in the well-analysed pseudotachylyte-bearing Adamello tonalite (Italian Southern Alps) are very similar to those of the Schneeberger pseudotachylyte-bearing quartzite. Thus existing samples of this area are planned to use as a comparison study for microstructural controlled TitaniQ analyses.