Signature of coseismic decarbonation in dolomitic fault rocks of the Naukluft Thrust, Namibia

Unequivocal geological signatures of seismic slip are rare, exceptionally so in carbonate-hosted faults where carbonate minerals dissociate at temperatures lower than those required for producing a friction melt. This thermal dissociation leads to significant fault weakening by increased fluid pressure and/or nanoparticle lubrication, preventing further heating of the fault surface. Pseudotachylyte is therefore unlikely to form in carbonate-hosted faults, and other evidence for seismic slip must be identified.We studied the lower Cambrian Naukluft Thrust which crops out in central Namibia. It contains a cataclastic dolomite fault rock, referred to as “gritty dolomite”, which we interpret as a signature of coseismic carbonate dissociation and subsequent fluid-rock interactions. The fault was active at ambient temperatures below 200°C.“Gritty dolomite” contains: rounded, low aspect ratio dolomite clasts with a uniform Fe-rich dolomite coating, euhedral to subhedral magnetite, quartz, and K-feldspar in a fine-grained, massive to laminated carbonate matrix of particulate dolomite and crystalline calcite cement. The fault rock texture, combined with evidence of injectites of gritty dolomite into the wallrock, indicates the cataclasite deformed as a fluidized granular flow. At seismic slip velocities, frictional heating caused dissociation of dolomite to CO2 and Ca-, Fe- and Mg-oxides. This release of CO2 decreased the pH of the pore fluid in the fault, causing dissolution and rounding of dolomite clasts within an inertial grain flow, and precipitation of carbonate coatings and euhedral silicates and oxides during subsequent cooling and CO2 escape.Examples of similar rocks having some, if not all of these characteristics have been described from other carbonate-hosted faults. The geological setting of the Naukluft Thrust is unique in spatial extent and quality of exposure, allowing us to eliminate alternative hypotheses for sources of CO2 to drive fluidization.

► We present new seismic indicator rock for carbonate-hosted faults. ► Exposed in Naukluft Thrust, 500 Ma, central Namibia. ► Granular fluidization facilitated by coseismic frictional dissociation of carbonate. ► Dolomite grains with concentric coatings, magnetite and silicate minerals diagnostic.