Tectonic-Geomorphic Evidence for "Rolling-Hinge" Style Deformation of an Active Low-Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

The near-surface back-rotation and shallowing of high-slip normal faults by a "rolling-hinge" mechanism is still disputed for continental Metamorphic Core Complexes (MCCs). The domal Mai'iu fault in SE Papua New Guinea is one of the best-preserved and fastest-slipping examples of an active Low-Angle Normal Fault (LANF) on Earth, and is ideally suited to evaluate deformation in the exhumed footwalls of MCCs. We analyzed field-based structural data from the footwall, together with geomorphic data for the exhumed slip surface of this fault based on GeoSAR-derived digital terrain models and aerial photography. The active Mai'iu fault emerges near sea level with a northward dip of 20°N (locally it is as low as 16°), and progressively flattens to the south over the 3 km-high crest of the Dayman Dome. The exhumed fault forms a strongly corrugated, continuous landscape surface that can be traced at least 28 km in the NNE slip-direction. Its southernmost mapped portion dips 12°S. Abundant geomorphic and structural evidence indicates that the fault surface, and an underlying foliation in the footwall, were both progressively back-tilted about a horizontal axis through >26º as they were advected southward across the main divide of the dome. Antithetic (north-side-up) dip-slip on a serial array of steep, m-to-km scale faults that strike parallel to the Mai'iu fault accommodated some of the exhumation-related, inelastic footwall bending. Antithetic faults have scarps that offset and deflect stream channels, and are therefore inferred to be active. Near the crest of Dayman Dome, antithetic faults are inferred to be prior normal faults that have since been back-rotated through the vertical, and acquired a reverse sense of dip-slip. Other evidence for a late-stage of slip-parallel contractional strain in the exhumed footwall is: 1) a pervasive, post-metamorphic crenulation foliation with hingelines that trend parallel to the curved Mai'iu fault trace; and 2) folding resulting in shortening of Pliocene bedding in a nearby uplifted rider-block in the footwall of the active detachment. Geodynamic modelling predicts such near-surface contractional strain to be a result of unloading-related flexure in the exhumed footwall.