Signatures of changing deformation rate dynamics in deforming rocks: Examples from the exhumed Slow Earthquake Zone of New Caledonia

Deformation on planetary bodies is characterized by processes that act at strain rates of more than 15 orders of magnitude difference. With the advent of advanced geophysical techniques with ever increasing resolution in time and space, we are now able to detect some of these intriguing dynamics. However, to improve earthquake related hazard assessments, advancing from observations of apparent dynamics of geophysically detected deformation events to in-depth understanding of the underlying physical processes is urgently needed. One “type” example of a deformation phenomenon encompassing deformation at different rates are Slow Earthquakes (SEs). In SEs, slip occurs more slowly than in regular earthquakes, but significantly faster than can be attributed to long-term plate motion. Although SEs are abundant, their geophysically observed characteristics cannot be reconciled with current understanding of how rocks deform: new evidence of slip processes need to be discovered in the geological record.
         Rock outcrops from an example of exhumed subducted crust in New Caledonia are interpreted to contain zones of former SEs. Microstructural characterization combining EBSD and EDS analyses deciphers controlling deformation processes, while phase petrology is used to evaluate stages of fluid ingress, production or egress. Based on our observations, we interpret that several deformation processes directly associated with the presence and movement of fluids governed rock behaviour. Relatively “slow” dissolution-precipitation creep is the main “background” deformation process responsible for the observed shape- and crystallographic-preferred orientations, in-grain compositional variations and grain boundary alignment. Geometric features akin to soft sediment deformation structures and water escape structures that developed at high grade conditions suggest that intermittently, local liquefaction is triggered by episodic high fluid pressures induced by mineral dehydration reactions. Based on these observations, we propose that wet granular flow at high fluid pressure may occur in subduction zone environments. This process is transient and relatively fast contrasting the slow, continuous viscous background flow. Catastrophic failure and flow by wet granular flow represents a viable candidate process for geophysically observed transient high slip rates in fluid rich subduction environments.