Fluid overpressure from chemical reactions in serpentinite within the source region of deep episodic tremor

Slow fault slip includes a range of transient phenomena that occur over timescales longer than those of standard earthquakes. Slow slip events are often closely associated with swarms of tectonic tremor. Deep episodic tremor and slip close to the slab-mantle interface in subduction zones has been linked to high fluid pressures produced by dehydration of the subducting slab at greater depths. The slab-mantle interface is a fundamental chemical boundary, where mantle rocks are sheared and mixed with oceanic slab lithologies in a highly reactive environment to form serpentinite. Here we present field and microstructural observations from the plate boundary-scale crustal Livingstone Fault in New Zealand that suggest chemical reactions involving serpentinite can promote rock hardening and generate in situ fluid overpressures. We infer that these processes collectively can result in hydrofracturing and a transition from distributed creep to localized brittle failure and faulting. Serpentinite-related reactions occur over a wide range of pressure and temperature conditions that overlap with those in many forearc mantle wedges. We conclude that the release of fluids derived from such reactions may be an additional and widespread mechanism to generate high fluid pressure patches and brittle failure in the source region of deep tremor along the slab-mantle interface.