Frictional Properties and Healing Behavior of Tectonic Mélanges:
Implications for the Evolution of Subduction Fault Zones
Abstract
The frictional velocity dependence and healing behavior of subduction
fault zones play key roles in the nucleation of stick-slip instabilities
at convergent margins. Diagenetic to low-grade metamorphic processes
such as pressure solution are proposed to be responsible for the change
in frictional properties of fault materials along plate interfaces;
pressure solution also likely contributes to the acceleration of healing
according to previous studies. Here, we report velocity-step experiments
using rocks collected from ancient subduction fault zones, the Lower
Mugi and Makimine mélanges of the Cretaceous Shimanto belt. The two
mélanges preserve paleotemperature records corresponding to the updip
and downdip limits of the seismogenic zone and deformation recording a
lower versus higher degree of pressure solution. Our data show that the
Lower Mugi mélange sample exhibits velocity-weakening to
velocity-neutral behavior under low normal stress, and the Makimine
mélange sample shows velocity-strengthening behavior under high normal
stress. This is consistent with the slip behavior observed at the depths
they have been subducted to along the plate interface. We also perform a
series of slide-hold-slide experiments under different hydrothermal
conditions using the Lower Mugi mélange sample to evaluate the role of
pressure solution in fault healing and its dependency on temperature.
The results show that healing rates increase in tests operated at higher
temperatures. The microstructures related to pressure solution found in
the postexperimental gouges support the idea that the elevated healing
rate can be related to pressure solution.