Seismic fault weakening via CO2 pressurization enhanced by mechanical deformation of dolomite fault gouges

Carbon dioxide emissions from dolomite decarbonation play an essential role in the weakening of carbonate faults by lowering the effective normal stress, which is thermally activated at temperatures above 600-700 degrees C. However, the mechanochemical effect of low crystalline ultrafine fault gouge on the decarbonation and slip behavior of dolomite-bearing faults remains unclear. In this study, we obtained a series of artificial dolomite fault gouges with systematically varying particle sizes and dolomite crystallinities using a high-energy ball mill. The laboratory-scale pulverization of dolomite yielded MgO at temperatures below 50 degrees C, indicating that mechanical decarbonation without significant heating occurred due to the collapse of the crystalline structure, as revealed by X-ray diffraction and solid-state nuclear magnetic resonance results. Furthermore, the onset temperature of thermal decarbonation decreased to similar to 400 degrees C. Numerical modeling reproduced this two-stage decarbonation, where the pore pressure increased due to low-temperature thermal decarbonation, leading to slip weakening on the fault plane even at 400-500 degrees C; i.e., 200-300 degrees C lower than previously reported temperatures. Thus, the presence of small amounts of low-crystalline dolomite in a fault plane may lead to a severely reduced shear strength due to thermal decomposition at similar to 400 degrees C with a small slip weakening distance.

Automated summary

This study reveals the essential role of dolomite decarbonation in weakening carbonate faults, as well as the influence of low crystalline ultrafine fault gouge on decarbonation and slip behavior. Experimental results show a significant contribution of mechanical decarbonation at low temperatures, and a decrease in the onset temperature of thermal decarbonation.