Deformation of granulite at high pressure and temperature: an investigation into the hypothesized eclogitization-induced deep crustal earthquakes

Abstract:

High resolution seismic waveform studies show that intra-continental earthquakes can occur at depths of 70-80km. This has puzzled researchers for decades because the high pressure and high temperature conditions at these depths should prohibit brittle failure. Indeed, in most continental regions, earthquakes are restricted to the upper half of the crust. Beneath southernmost and northwest Tibet, the crust is up to 50-90 km thick, where rocks under thermodynamic equilibrium conditions should be eclogitic. However, receiver functions show a well-defined Ps conversion continuously from ~40 km depth in northern India to ~90 km in northwest and southern Tibet. This suggests that the deep crust cannot be eclogite, which has similar seismic velocities to the upper mantle peridotite and cannot result in a sharp seismic discontinuity. For these reasons, as well as related gravity, topography and density data, it is suggested that the deep lower crust beneath southern and northwest Tibet is likely metastable granulite. Very few studies on the mechanism of deep crustal earthquakes have been reported. Eclogitic psuedotachylites observed in the Western Gneiss Region of Norway were attributed to faulting in the deep crustal rocks in dry granulite (Austrheim, 1994). Deep crustal earthquakes under Tibet may be related to a similar transformation of metastable granulite to eclogite. We have conducted preliminary deformation experiments on a metastable felsic-granulite in the stability field of eclogite using a modified Griggs-type apparatus. In contrast to mafic granulite that showed ductile behaviors at P = 1GPa, T = 1273K and strain rates between 1×10^-4 and 8.77×10^–6 s^–1 (Wang et al., 2012). The felsic-granulite specimen deformed within the eclogite stability field, at P=3GPa, T=1273K and strain rate of 2×10^-4/s, shows well developed faults. Ultra-fine grain materials (grain size < 1 mm), as the products of eclogitic phase transformation, are observed in fault gauges. These results reveal that the eclogitization of granulite may be a viable mechanism for certain deep crustal earthquakes under Tibet. In situ deformation experiments using the D-DIA are underway.