Evolution of a rapidly slipping, active low-angle normal fault, Suckling-Dayman metamorphic core complex, SE Papua New Guinea

The active Mai'iu low-angle normal fault in the Woodlark Rift dips as low as 16 degrees -20 degrees at the surface and has formed by extensional inversion of the Paleogene Owen-Stanley thrust fault. The Mai'iu Fault has slipped at centimeter-per-year rates for at least 3.3 m.y., in the process exhuming a >29 km width of a largely uneroded fault surface, and uplifting the Suckling-Dayman metamorphic core complex to elevations up to 3.7 km. The exhumed fault surface is overlain by one rider block. This formed where the main fault locally became so shallowly dipping in a synformal megacorrugation that it was no longer frictionally viable for slip. Tectonic geomorphology, structural geology, and microseismicity provide evidence for a convex-upward shape and rolling hinge style of evolution for this strongly back-warped normal fault, and for an approximately Andersonian state of stress in the footwall at depth. Flexure of the exhuming footwall as a result of tectonic unloading apparently caused a late increment of extension-parallel horizontal contraction, together with a constrictional stress state. During exhumation, fault megacorrugations amplified as folds--even in the near surface. Glassy pseudotachylytes in a meter-thick foliated cataclasite unit yield (super 40) Ar/ (super 39) Ar maximum ages of 2.2-3.0 Ma. The pseudotachylytes and interleaved ultracataclasites developed in a zone of mixed-mode seismic/aseismic slip behavior that formed near the brittle-ductile transition along a frictionally strong, high-stress ramp at 10-12 km depth. The exhumed mafic mylonite zone narrows upward and probably deformed at strain rates of up to 2.1 X 10 (super -10) s (super -1) in its narrowest, upper part. Observed microseismicity at 10-25 km depth attests to current (interseismic) fault activity downdip of the fault trace. Rolling-hinge-style deformation causes the Mai'iu Fault to dip at low angles (<22 degrees ) in the near surface. The shallowest part of the Mai'iu Fault contains abundant saponite and is thus probably weak enough (mu <0.2) to slip at such a poorly oriented fault dip. At depth, this microseismically active structure slips at moderate dip angles (dip 30 degrees -40 degrees ) akin to conventional normal faults.