T51E-2947
Preliminary Results on the Mechanics of the Active Mai’iu Low Angle Normal Fault (Dayman Dome), Woodlark Rift, SE Papua New Guinea
Friday, 18 December 2015
Poster Hall (Moscone South)
Timothy A Little1, Carolyn J Boulton2, Marcel Mizera1, Samuel McKeever Webber1, Juergen Oesterle1, Susan M Ellis3, Kevin P Norton4, Laura M Wallace5 and James Biemiller6, (1)Victoria University of Wellington, Wellington, New Zealand, (2)University of Liverpool, Earth, Ocean and Ecological Sciences, Liverpool, United Kingdom, (3)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand, (4)Victoria University of Wellington, School of Geography, Environment and Earth Sciences, Wellington, New Zealand, (5)University of Texas at Austin, Institute for Geophysics, Austin, TX, United States, (6)University of Texas, Austin, United States
Abstract:
Rapid slip on the Mai’iu low-angle normal fault (LANF) has exhumed a smooth, corrugated fault surface contiguous for >24 km up-dip, rising from near sea level to ~2900 m. The fault emerges from the ground dipping ~21° N and flattens over the crest of the dome to dip south. Geomorphic analysis reveals a progressive back-tilting of the surface during exhumation accompanied by cross-cutting antithetic-sense high-angle faults—features that we attribute to “rolling-hinge” deformation of a once more steeply-dipping fault. Near the scarp base, the footwall exposes mafic mylonites that deformed at ~400-450°C. The younger Mai’iu fault cross-cuts this ductile mylonite zone, with most brittle slip being localized into a ~20 cm-thick, gouge-filled core. Near the range front, active faults bite across both the hangingwall and footwall of the Mai’iu fault and record overprinting across a dying, shallow (<~1 km deep) part of the fault by more optimally oriented, steeper faults. Such depth-dependent locking up of the fault suggests it weakens primarily by friction reduction rather than cohesion loss. Outcrop-scale fractures in the exhumed footwall reflect formation in an Andersonian stress regime. Previous campaign GPS data suggest the fault slips at up to ~1 cm/yr. To improve resolution and test for aseismic creep, we installed 12 GPS sites across the fault trace in 2015. Quantitative XRD indicates the gouges were derived primarily from mafic footwall, containing up to 65% corrensite and saponite. Hydrothermal friction experiments on two gouges from a relict LANF strand were done at varying normal stresses (30-120 MPa), temperatures (50-200oC), and sliding velocities (0.3-100 μm/s). Results reveal very weak frictional strength (μ=0.13-0.15 and 0.20-0.28) and velocity-strengthening behavior conducive to fault creep. At the highest temperatures (T≥150oC) and lowest sliding velocities (<3 μm/s), a transition to velocity-weakening behavior indicates the potential for unstable slip.
