FRICTIONAL DECARBONATION DURING EARTHQUAKES: TEXTURES AND MINERALS IN CARBONATE FAULT ROCK PRESERVE EVIDENCE FOR SUPERCRITICAL CO2 – H2O – ROCK INTERACTION

Supercritical CO₂ is very efficient at dissolving carbonate minerals, and also increases the solubility of many minerals in H₂O. In the shallow brittle crust at temperatures 30-400°C, liquid water and supercritical CO₂ coexist as immiscible fluids.

Carbonate dissociation by fast slip has been shown in laboratory experiments to release CO₂ (gas) and solid nanoparticles of the cations/oxides remaining. Experiments by Han et al (2007) showed that these nanoparticles can lead to lubrication during frictional sliding. Other workers have predicted that the CO₂ release would cause thermal pressurization, another co-seismic weakening mechanism.

The Naukluft Thrust, central Namibia offset low grade to unmetamorphosed carbonate and siliciclastic strata a total of >50 km during the ~500 Ma Damara Orogeny. Along this fault, we infer that dolomitic fault rock dissociated catastrophically, producing a transiently fluid-overpressured, chemically reactive slurry of mineral grains, H₂O and CO₂. Fast slip on the fault surface during an earthquake is the most likely cause of dissociation.

Evidence for co-seismic frictional dissociation of the carbonate fault rock in the Naukluft Thrust includes:

• Thick (~3m) layers of granular rock with textures consistent with inertial-phase fluidization and granular flow at elevated fluid pressure including off-fault injectites.
• Spherical rounded carbonate mineral grains (but not silicate grains) are consistent with chemical attack by a very corrosive fluid phase during granular flow
• “Leftover” oxides and silicates abandoned by CO₂ escaping the fault zone, forming minerals such as abundant euhedral magnetite and, in places, talc
• Overgrowth cements of microcrystalline albite/quartz similar to Na-Al zeolites produced in CO2-injection experiments by Kasuba et al. (2006) by CO₂-brine reactions.