Fluid flow and metasomatic fault weakening in the Moresby Seamount detachment, Woodlark Basin, offshore Papua New Guinea

Abstract

Low‐angle normal faults play a prominent role in discussions about fault strength, as they require significant weakening to remain active at low angles. The submerged Moresby Seamount detachment (MSD) is arguably the best exposed active low‐angle detachment worldwide. We analyzed dredged MSD protoliths, cataclasites and mylonites to investigate deformation mechanisms and fault‐weakening processes. Deformation is accompanied by important syntectonic, fluid‐induced mass transfer, controlling the rheological behavior of the MSD. While the mafic protolith behaves brittlely at the onset of deformation, the metasomatic mineralogical and chemical changes cause a transition to plastic flow as the rock is progressively exhumed. Immobile elements provide a reference frame for total material gains and losses. Si, Ca and K are syntectonically enriched, while Fe, Ti, Mg, and Al are depleted. Mass increase is about 10% in the cataclasites and about 48% in the mylonites. Main mechanism is syntectonic veining, causing enrichment in calcite and quartz, thus making the mylonites capable to flow plastically. Minimum time‐integrated fluid flux is calculated as 3 × 105 m3 m−2, indicating that the MSD is an important fluid conduit. The fluids have a deep crustal source, a bottom water temperature and turbidity anomaly suggests that the hydrothermal system is still active. Syntectonic veining in fault rocks and recent seismic activity both suggest that the MSD is intermittently brittle, implying a brittle‐plastic transition at unusually high temperature and low differential stress. We conclude that fault zone metasomatism is crucial in forming weak detachments at passive margins, and may be a prerequisite for successful crustal breakup.