Serpentinite slickenfibres: analogues for small repeating earthquakes at the slab-mantle interface?

Episodic tremor and slip (ETS) is the close association of slow slip events and swarms of tectonic tremor. Hidden inside the tremor signal is evidence for thousands of small low-frequency earthquakes (LFEs) occurring by repeated shear failure along the plate interface. LFEs occur near the tip of the forearc mantle wedge, where the plate interface is likely to be a wide shear zone dominated by serpentinite. In this context, exhumed serpentinite shear zones hold important information on the rheology of the ETS source region. Using a range of analytical techniques, we present a detailed study of the microstructure and geochemistry of a single "crack-seal"-type serpentinite slickenfibre from a plate boundary-scale shear zone in New Zealand.

The slickenfibre occurs in a dilational jog along a brittle fault surface, which cuts through a 5 m-long pod of massive serpentinite. Textural and geochemical relationships in the slickenfibre suggest development as an antitaxial structure by repeated cracking along the jog-wall rock interface. Thousands of individual crack-seal bands developed parallel to the lateral walls of the jog, and are offset by micro-transforms parallel to the upper and lower boundaries of the jog. Statistical analysis shows a characteristic band thickness with a mean of 30 μm and a range between 5 - 60 μm. However, the inner portion of the slickenfibre has a wider range of band thicknesses and a more irregular thickness distribution, possibly reflecting a transition from irregular to near-constant stress drops as the fault became smoother with displacement. The micro-transforms are dominated by chrysotile, whereas each crack-seal band contains a sequence of serpentine that progresses from an amorphous proto-serpentine phase, to poorly-oriented conical (and cylindrical) serpentine, and finally to well-aligned chrysotile and polygonal serpentine. This sequence suggests initial precipitation in to open fractures far from equilibrium, followed by a gradual return to equilibrium as each band was sealed. Assuming that each "crack-seal" band represents a seismic slip event along the brittle fault, the ratio of band thicknesses to potential rupture length (10^-5 - 10^-6) suggests stress drops between 10-170 kPa, within the range estimated for LFE's along the deep plate interface.