DAMAGE IN POLYCRYSTALLINE ROCKS BY THERMAL AND MECHANICAL LOADING IN ASSOCIATION WITH MAFIC DIKE INTRUSION INTO GRANITE ROCKS

The role of tensile microfracturing in compressive failure of brittle polycrystalline materials has been widely accepted. However, the effect of grain-scale heterogeneity on the tensile stress still requires quantitative evaluation. The major rock forming minerals including quartz and feldspar contain anisotropic elastic moduli and thermal expansion coefficients. Variations in macroscale thermal or macroscopic mechanical loading cause interactions among anisotropic minerals resulting in grain-scale heterogeneity in stress and strain, and eventually generating microfractures when the loads are large enough.

We use granite rocks mostly consisting of quartz and feldspar intruded by mafic dikes from Schoodic Point, Maine, USA to better understand mechanisms and overall evolution of polycrystalline material failure. Electron backscatter diffraction maps of the granite microstructures are subjected to macroscale thermal and mechanical loading, and the resulting brittle damage is analyzed using a recently-developed numerical toolbox. Both conditions produce high tensile stress, causing microfractures in the rocks. This indicates that (a) mismatches in thermal expansion and elastic stiffness between grains of different composition and (b) anisotropy in thermal expansion and elastic stiffness play critical roles in rock damage. Our findings suggest that the mafic dike intrusion caused brittle deformation by thermal and mechanical loadings.