Grain-Scale Deformation Mechanisms Reveal Slow-to-Fast Slip on an Actively Exhuming Low-Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

The Mai'iu fault in SE Papua New Guinea is one of the best-preserved and fastest slipping active low-angle normal faults on Earth. The corrugated, convex-upward shaped fault scarp forms a continuous landscape surface traceable for at least 28 km in the NNE slip-direction. Structurally, the Mai'iu fault forms a metamorphic core complex that has exhumed a metabasaltic footwall during 30-45 km of dip slip on a rolling-hinge style detachment fault. Rapid exhumation on the active fault (Late Quaternary dip-slip rates of 12 mm/yr) enabled preservation of early formed microstructures in a stunning >20 km-deep crustal profile. This exhumed section records the spatiotemporal evolution of fault rock deformation mechanisms, including the combined contributions of coseismic slip and aseismic creep to total fault slip.

Field mapping reveals that the <2 m thick fault core, consisting of gouges and cataclasites, overprints an older and structurally underlying carapace of mylonitic rocks reaching at least 60 m in thickness. Detailed investigations with a FEG SEM equipped with EDS and EBSD together with chlorite geothermometry reveal that mylonitic deformation was controlled by rotation, sliding and comminution of pre-existing fine-grained (8-25 µm in diameter) epidote, actinolite and albite grains, together with chlorite precipitation in strain shadows. Slip was accommodated in the mylonites by diffusion-creep and grain-boundary sliding at temperatures 280-400 ºC. At shallower levels (T≥150-280 ºC) on the fault, solution-precipitation creep and fluid-assisted mass transfer processes led to mineral transformation reactions, creating an anisotropic, foliated cataclasite. In this unit, pseudotachylites and ultracataclasites mutually cross cut one another and reflect coseismic slip events. We infer that frictional melting and extreme grain-size reduction took place near the brittle-ductile transition within a frictionally strong, high-stress ramp at 10-12 km depth. At the shallowest crustal levels (T<150ºC), clay-rich gouges contain abundant saponite, promoting creep on the most poorly oriented part of the Mai'iu fault (dipping <22°). Our field and microstructural data support geodetic, seismological, and geomorphic evidence for mixed-mode, seismic-to-aseismic, slip behavior on the Mai'iu fault.