Stress changes during exhumation of a high-pressure nappe (Phyllite-Quartzite unit, Greece): Insights from quartz microstructures and recrystallized grain size statistics

Abstract

In this study, we carried out microstructural and grain size analysis of recrystallized quartz in quartz-rich samples collected from different structural levels of a 1.5–2.0 km thick high-pressure nappe (Phyllite-Quartzite unit, Greece). Exhumation-related deformation led to recrystallization of quartz by subgrain rotation (SGR) and the formation of different generations of recrystallized quartz with different grain sizes. Applying a new, fully automated histogram-based method, we reconstructed the “hidden” empirical probability density functions included in the histograms of the initial grain size distributions. Statistical analysis showed that most grain-size histograms display bimodal distributions with mean grain sizes in cluster A ranging from 46 to 48 μm, and a cluster B with mean grain sizes between 81 and 93 μm. Few grain size histograms display trimodal distributions and show an additional cluster C with mean grain sizes ranging from 100 to 115 μm. Microstructural observations indicate that the grains of cluster C are the oldest whereas those of cluster A are the youngest. Bimodal and trimodal distributions may reflect an episodic increase in the differential stress from 17 to 19 MPa and then to 31 MPa during exhumation, accompanied by a strain rate increase of at least one order of magnitude (from 10−13 s−1 to 10−12 s−1). This stress increase likely occurred at the early stages of the exhumation and may have lasted less than 0.5 Myr. Stress increase was followed by a stress drop expressed by partial foam structures of quartz in a few samples.