Strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates

High‐velocity friction tests were conducted on solid and hollow cylinders of Carrara (calcite) marble, dolomite marble, silicate‐bearing calcite marble, and calcite gouge to investigate the strength of carbonate faults during seismic slip. The experiments, performed at normal stresses of 0.6–14.7 MPa, slip rates of 0.03–1.60 m/s, and room temperature in a rotary‐shear friction testing machine, yielded an extraordinarily low steady state friction coefficient (<0.1) at slip rates of ∼1.1–1.2 m/s. The slip‐weakening distance of 4–28 m became shorter at higher normal stress or frictional work rate. Strong velocity weakening was observed not only in steady state but also in nonsteady state friction, while the slip rate was changing; thus slip deceleration was accompanied by fault strength recovery. Large, rapid temperature rises in narrow shear localization zones (less than a few micrometers) induced carbonate decomposition, such as the breakdown of calcite into aggregates of CaO nanograins and CO 2 in Carrara marble. Scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains (mostly <1 μ m in diameter). These microstructures and the measured high permeability (∼10 −14 m 2 ) of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication. Powder rheology at high slip rates is not yet well understood, but the frictional behavior of nanograins appears to be strongly velocity dependent. If decarbonation occurs during seismic slip in natural carbonate faults, powder lubrication may make the faults slippery even under fluid‐drained conditions.