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

摘要

High-velocity friction tests were conducted on solid and hollow cylinders of Carrara(calcite) marble, dolomite marble, silicate-bearing calcite marble, and calcite gouge toinvestigate 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 roomtemperature in a rotary-shear friction testing machine, yielded an extraordinarily lowsteady state friction coefficient (<0.1) at slip rates of ~1.1-1.2 m/s. The slip-weakeningdistance of 4-28 m became shorter at higher normal stress or frictional work rate. Strongvelocity weakening was observed not only in steady state but also in nonsteady statefriction, while the slip rate was changing; thus slip deceleration was accompanied by faultstrength recovery. Large, rapid temperature rises in narrow shear localization zones (lessthan a few micrometers) induced carbonate decomposition, such as the breakdown ofcalcite into aggregates of CaO nanograins and CO_2in Carrara marble. Scanning electronmicroscope observation revealed that the shear localization zone in the highly porousdecomposition product was a layer of scattered small grains (mostly <1μm in diameter).These microstructures and the measured high permeability (~10~(-14)m~2)of thedecomposed marble indicate that the dominant weakening mechanism in our experimentswas possibly powder lubrication. Powder rheology at high slip rates is not yet wellunderstood, but the frictional behavior of nanograins appears to be strongly velocitydependent. If decarbonation occurs during seismic slip in natural carbonate faults, powderlubrication may make the faults slippery even under fluid-drained conditions.