The generation, dissipation and geological implication of grain-scale stress variation in metamorphic rock

Metamorphic rocks are composed of minerals formed within a wide range of pressure-temperature (P-T) conditions. These minerals possess distinct physical properties with respect to their thermo-elasticity, viscosity and plasticity etc. When far-field nonhydrostatic stress was present during tectonic deformation or the P-T conditions were changed during burial or exhumation processes, grain-scale stress variation can be developed to maintain mechanical equilibrium. The induced stress variations can also be released over geological time. The magnitude, effect and geological implications of the preserved stress variations have been investigated and better understood in recent years, but a lot remains to be explored. It is important because it may as well open an opportunity to decipher the overseen information stored in the rock that is difficult to be detected or achieved with conventional methods.

The geological implication of grain-scale stress variation is directly manifested by a simple mineral inclusion-host system, such as quartz or zircon inclusion in garnet. The different thermal-elastic response between the inclusion and host upon P-T changes will result in a residual stress stored in an entrapped mineral inclusion. The inclusion stress (strain) can be directly measured with e.g. Raman spectroscopy. Combined with an elastic model, it is possible to obtain constraints on the entrapment P-T conditions. This elastic thermobarometry technique has been applied in many recent petrological studies because no global or local chemical equilibrium assumption is needed. However, it relies on the inclusion-host system being elastic and neglects the non-elastic behavior of minerals. As an example, I will present an integrated observational, experimental and numerical modelling work that highlights the importance of considering non-elastic behaviour of the mineral. The heating experiment shows that under conditions at which free fluid is present, a garnet host will be drastically weakened and partially release the inclusion pressure. This is further correlated with a nappe-scale study in the Adula nappe, Alps. A smooth T gradient is found increasing from the north (500-550 ^oC) to the south (700 ^oC) using the Zr-in-rutile thermometer. However, the entrapment P calculated with the quartz inclusion in garnet barometer demonstrate a GPa level steep drop in the middle-south, where the rocks have been hydrated during retrograde metamorphism and abundant micro-hydrous inclusions (e.g. chlorite, amphibole) are found in the garnet. It is interpreted that a combined effect of temperature and water fugacity will drastically speed up the inclusion pressure relaxation on a regional metamorphic scale. In the end, it is highlighted that mechanics with non-elastic stress-strain (rate) relationships are potentially needed when dealing with the generation and dissipation of the stress variations in metamorphic rocks that underwent retrograde hydration or very high T conditions to better extract geological information.