Physico-chemical Changes and Dramatic Fault Weakening Induced by Thermal Decomposition in Carbonate Fault Zones: Results from Friction Tests at Seismic Slip Rates

To understand the physico-chemical processes and dynamic frictional strength of carbonate-rich fault zones during seismic faulting, we conducted rotary shear friction tests at seismic slip-rates on carbonate rocks (calcite marble and dolomite marble) and artificial gouges of siderite and siderite-calcite-quartz mixture. Here, we report that thermal decomposition due to frictional heating is an important coseismic process in carbonate- rich fault zones and the fault zones can become very weak during seismic slip. For the calcite marble (Carrara marble), we performed rock-to-rock friction experiments at slip rates (V) of 0.1-1.2 m/s and normal stresses (σn) of 2.5-13.4 MPa. The main results are: (1) The initial friction coefficient (μp) of 0.6 decreases down to the steady-state friction coefficient (μss) of 0.06 at V = 1.1-1.2 m/s; (2) The μss decreases with increasing V at σn = 7.3 MPa, once V exceeds a certain value (about 0.1 m/s); (3) Fault strength recovers very rapidly after the cessation of sliding; (4) CO2 gas emission due to the thermal decomposition of calcite seems simultaneous with the onset of the dramatic fault weakening, which is confirmed by monitoring with two CO2 sensors. Deformed samples were analyzed by XRD and observed by SEM and TEM, and we identified nano-scale lime (CaO) grains as a decomposition product of calcite. Almost the same weakening was also observed in the dolomite marble experiments through thermal decomposition of dolomite. For the siderite gouge and the mixture of siderite, calcite and quartz gouges, we conducted experiments at V = 1.3-2.0 m/s and σn = 0.6-1.3 MPa with a gouge layer confined between a pair of solid or hollow rock cylinders. The overall mechanical behavior was the same as that observed in the friction tests on carbonate rocks mentioned above, except dramatic color change of the gouges from pale brown or brown to black during the experiments. The newly formed, ultrafine magnetite crystals (about 20 nm in diameter) by the decomposition of siderite are responsible not only for the color change but also for anomalously high magnetic susceptibility (49,484 x 10^{-8} m3/kg). In the mixture sample, nanocrystalline lime (CaO) and periclase (MgO) in addition to magnetite formed as decomposition products. The weakening of the carbonate-rich fault zones cannot be explained by the previously reported weakening mechanisms (e.g. thermal pressurization, melting and silica gel formation). The simultaneous onset of CO2 emission and weakening may suggest a possible effect of gas pressurization. However, the effect may not be significant in our cases because almost the same weakening was observed in experiments on totally decomposed rock or gouge samples by pre-heating. We believe that low frictional strength of the ultrafine decomposition products at seismic slip rates is responsible for the dramatic weakening, although it is still not certain why the nanoparticles have low frictional strength at seismic slip-rates. In the light of our experimental results, thermal decomposition due to frictional heating may be an important coseismic process by which physico-chemical properties of carbonate-rich fault zones change dramatically.