12:00 PM - 12:15 PM
[SSS02-11] Frictional properties of the Nankai Trough accretionary mud samples collected from 1000–3000 mbsf at IODP Site C0002
Keywords:friction, mudstone, accretionary prism, Nankai Trough
XRD analyses of tested mud samples revealed that the content of total clay minerals tends to increase with depth from ~30 to ~60 wt%, while that of smectite tends to decrease with depth from ~30 to ~20 wt%. Thus, the smectite fraction in total clay minerals decreases with depth from ~0.75 to ~0.3. Because the temperature at 3000 mbsf reaches ~100°C, this decrease in smectite fraction with depth is likely due to the progress of smectite dehydration with increasing temperature.
Friction experiments of tested mud samples revealed that the steady-state friction coefficient (µss) has a negative correlation with the content of total clay minerals. µss at Vaxial = 1 µm/s tends to decrease with depth from ~0.5 to ~0.3, according to the increasing content of total clay minerals with depth. Although shallower samples exhibited a clear increase in µss when Vaxial was increased and vice versa, i.e., velocity strengthening, a few deeper samples exhibited a decrease in µss when Vaxial was increased and vice versa, i.e., velocity weakening. Velocity dependence of steady-state friction (dµss/dlnVsliding, where Vsliding is sliding velocity) has a positive correlation with the smectite fraction in total clay minerals. Because the latter decreases with depth, dµss/dlnVsliding also tends to decrease with depth. dµss/dlnVsliding values are relatively large (>0.002) and positive at depths shallower than 2000 mbsf, implying stable faulting at these depths. In contrast, dµss/dlnVsliding values are relatively small (≤0.002) and locally negative at depths deeper than 2000 mbsf, implying conditionally stable faulting including slow slip events at these depths.