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

It is now recognized that there must be a wealth of interaction between processes of vastly different strain rates. One “type” example of a deformation phenomenon encompassing deformation of different rates are Slow Earthquakes (SEs). In SEs, slip occurs more slowly than in regular earthquakes, but significantly faster than can be attributed to normal plate motion. SEs have been shown to be closely associated with a range of deformation processes active at different deformation rates. 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 shape- and crystallographic-preferred orientations, in-grain compositional variations and grain boundary alignment. Fast brittle, hydrofracturing and local granular flow is enabled by episodic high fluid pressures induced by mineral dehydration reactions. These transient deformation processes act at much faster strain rates but occur for only short time periods.