THERMAL DISSOCIATION OF CO2, GRANULAR FLOW, AND FAULT WEAKENING ON THE NAUKLUFT THRUST, CENTRAL NAMIBIA

Earthquakes exhibit velocity weakening friction. Fast slip may activate processes which cause velocity weakening friction such as thermal pressurization and nanoparticle lubrication. In carbonate rocks, frictionally heated fault rock will dissociate, emitting CO2 before reaching a melting point. Experimental evidence indicates that pressurization of dissociated CO2 along carbonate-hosted faults may play a role in lubricating faults during earthquakes. The Naukluft Thrust, central Namibia is a ~550 Ma basal foreland thrust fault that hosts granular “gritty dolomite” fault rock which was fluidized during coseismic slip. Here we apply experimental thermal dissociation data of Naukluft Thrust fault rock samples to a dissociation model to investigate the volumes and pressures of CO2 in the fault during coseismic slip. Mineral identification from XRD of Naukluft Thrust fault rocks are compared with post-dissociation experimental samples. The estimated CO2 volumes are used with a fluidization porosity threshold to determine a volume of gritty dolomite generated during coseismic slip. We assume that the CO2 driving the fluidization of gritty dolomite is derived from a condensed 5 - 10 cm, slickenlined, dolo-mylonite layer underlying gritty dolomite. The volumes of gritty dolomite fault rock injections are used to estimate the volume, and then area, of the slip patch required to generate the amount gritty dolomite filling observed fault rock injections. The slip patch area is used to estimate minimum paleoearthquake magnitudes. This technique may be applied to other carbonate faults with injections, and potentially investigate the thermal pressurization of wet gouge layers, to elucidate the seismic history of faults.