Due to contrasting results between laboratory tests, geophysical data, and field observations, the strength of the plagioclase-rich lower continental crust remains a topic of debate. It has been shown that its strength highly depends on the presence of fluids as they trigger metamorphic reactions that can result in permanent weakening. An important metamorphic reaction in the lower continental crust is the breakdown or hydration of plagioclase and the associated growth of epidote-group minerals, kyanite, quartz, and jadeite/albite. To investigate the impact of this particular reaction on the strength of the lower continental crust, we combined experimental work, i.e., Griggs-deformation tests, with extensive microstructural observations of the recovered experimental samples. Experimental conditions were 1 to 1.5 GPa confining pressure, 550 to 950 °C, and for the deformation tests, we used strain rates ranging from 10^-6 to 10^-5 s^-1. Our results reveal two main findings. First, deformed plagioclase aggregates as well as deformed granulite drill cores show that deformation-induced features in plagioclase grains, e.g., cleavage cracks and twin boundaries, act as nucleation sites for metamorphic reactions and melting. Consequently, reaction can progress faster in deformed samples as the effective reactive surface area is increased relative to undeformed counterparts. Second, when deformed under identical experimental conditions, pure epidote aggregates are consistently stronger or show equal strengths than pure plagioclase aggregates. Hence, a partial plagioclase breakdown, i.e., the exclusive growth of epidote-group minerals at low reaction progress, is not expected to result in permanent weakening. This result further strengthens the idea that a process akin to Zener pinning is a viable mechanism to cause long-term weakening in rocks.

How to cite: Incel, S., Renner, J., Schubnel, A., Labrousse, L., Baisset, M., and Mohrbach, L. K.: Deformation and reaction of plagioclase-rich rocks at conditions of the lower continental crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10806, https://doi.org/10.5194/egusphere-egu24-10806, 2024.

The analysis of deformation and failure in many sedimentary settings hinges upon a fundamental understanding of inelastic behaviour, failure mode of porous carbonate rocks and their implications on fluid flow at various scales. The mechanical compaction behaviour of carbonate rock of a broad range of porosity has been investigated in the laboratory over a wide range of stress conditions in the last decades. The phenomenology of brittle failure and inelastic compaction in this rock type with often bimodal pore size distribution was found similar to that of sandstone. Inelastic compaction in limestone involved primarily cataclastic pore collapse and micromechanical analysis showed the strong influence of the micropore size on the yield stress. Compaction experiments on porous limestones also revealed a broad spectrum of complex failure modes. In situ X-ray Computed Tomography imaging combined with Digital Volume Correlation provided the first observations of discrete compaction bands in a high porosity limestone. Permeability variations in carbonates associated with shear-enhanced compaction and these failure modes were found significantly smaller than variations previously reported in porous sandstones of comparable porosities.

In geophysical applications such 4D reservoir monitoring and the production of geothermal reservoirs, an understanding of the mechanical and chemical effects of pore fluid is fundamental. The mechanical influence of pore fluid on different properties is characterized by effective pressure coefficients. For limestone with dual porosity, both effective stress coefficients for permeability and pore volume change were observed to be consistently greater than unity. This implies that microscopic homogeneity is not a valid approximation for a limestone with dual porosity, and a realistic model must explicitly differentiate between the macropores and micropores, as well as account for their interplay in controlling the hydromechanical behaviour. Recent data showed that a significant weakening effect of water could also be expected in most carbonates. 

How to cite: Baud, P.: Inelastic compaction in porous carbonates, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6574, https://doi.org/10.5194/egusphere-egu24-6574, 2024.