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

Abstract

[1] 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 timeintegrated fluid flux is calculated as 3 Â 10 5 m 3 m À2 , 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.

Slow‐to‐Fast Deformation in Mafic Fault Rocks on an Active Low‐Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

“…Although this is a fluid‐assisted reaction, we did not find evidence for sustained high pore‐fluid pressures in the foliated cataclasites. Emplacement of calcite veins indicate that fluid pressures were at times high enough to induce hydrofracturing (i.e., P f >σ 3 ); however, fluid influx was not high enough to completely retrogress the metabasaltic mineral assemblage as observed in other MCCs such as the Moresby Seamount Detachment in the eastern Woodlark Rift (Speckbacher et al., 2012, 2013). We interpret temporal variations in fluid pressure, the formation and folding of the foliation in the foliated cataclasites, and multiple generation of pseudotachylite veins hosted in these rocks in the context of a seismic cycle‐model as explained below (e.g., Scholz, 2002; Sibson, 1992, Figures 11b–11d).…”

Orientation, composition, and entrapment conditions of fluid inclusions in the footwall of the northern Snake Range detachment, Nevada

“…Coseismic events that cause cyclical fluctuations in pore pressure between lithostatic and hydrostatic conditions may allow fluids to infiltrate shear zone rocks of MCCs beneath the seismogenic zone (Parry and Bruhn, 1986;McCaig, 1988;Parry et al, 1991;Montolini et al, 2001;. Subsequent fault sealing along the detachment system enhances hydration reactions and mass transfer, resulting in strain softening, fluid overpressure and/or embrittlement (Axen 2004;Famin et al, 2004;Keller et al, 2004;Smith et al, 2008;Micklethwaite et al, 2010;Speckbacher et al, 2012;. Fault sealing may also promote buffering of the system between the infiltrated exotic fluids and the mid to lower crustal rocks (e.g., , or fluids from underlying rocks may be released following the onset of detachment faulting (Famin and Nakashima, 2005;Siebenaller et al, 2013).…”

Using Syntectonic Calcite Veins to Reconstruct the Strength Evolution of an Active Low‐Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

“…We note that calcite veins in the fault rocks may indicate fluid pressures sufficiently high at times to induce local hydrofracturing (i.e., P f > σ 3 ); however, fluid influx was not high enough to completely retrogress the metabasaltic mineral assemblage as observed in other MCCs such as the Moresby Seamount Detachment in the eastern Woodlark rift (cf. Speckbacher et al, 2012Speckbacher et al, , 2013. Overall, there is little evidence for sustained fluid flow or high pore fluid pressure in the foliated cataclasites, which contains ductilely sheared calcite veins (e.g., Figures 5b and 5c; Mizera et al, 2020).…”