Cook, Ann E.; Paganoni, Matteo; Clennell, Michael B.; McNamara, David D.; Nole, Michael A.; Wang, Xiujuan; Han, Shuoshuo; Bell, Rebecca E.; Solomon, Evan A.; Saffer, Demian M.; Barnes, Philip M.; Pecher, Ingo A.; Wallace, Laura M.; LeVay, Leah J.; Petronotis, Katerina E.
The Pāpaku Fault Zone, drilled at International Ocean Discovery Program (IODP) Site U1518, is an active splay fault in the frontal accretionary wedge of the Hikurangi Margin. In logging-while-drilling data, the 33-m-thick fault zone exhibits mixed modes of deformation associated with a trend of downward decreasing density, P-wave velocity, and resistivity. Methane hydrate is observed from ~30 to 585 m below seafloor (mbsf), including within and surrounding the fault zone. Hydrate accumulations are vertically discontinuous and occur throughout the entire logged section at low to moderate saturation in silty and sandy centimeter-thick layers. We argue that the hydrate distribution implies that the methane is not sourced from fluid flow along the fault but instead by local diffusion. This, combined with geophysical observations and geochemical measurements from Site U1518, suggests that the fault is not a focused migration pathway for deeply sourced fluids and that the near-seafloor Pāpaku Fault Zone has little to no active fluid flow.
Barnes, Philip M.; Wallace, Laura M.; Saffer, Demian M.; Bell, Rebecca E.; Underwood, Michael B.; Fagereng, Ake F.; Meneghini, Francesca M.; Savage, Heather M.; Rabinowitz, Hannah S.; Morgan, Julia K.; Kutterolf, Steffen K.; Hashimoto, Yoshitaka H.; Engelmann de Oliveira, Christie H.; Noda, Atsushi N.; Crundwell, Martin P.; Shepherd, Claire L.; Woodhouse, Adam D.; Harris, Robert T.; Wang, Maomao W.; Henrys, Stuart H.; Barker, Daniel H.N.; Petronotis, Katerina E.; Bourlange, Sylvain M.; Clennell, Michael B.; Cook, Ann E.; Dugan, Brandon E.; Elger, Judith E.; Fulton, Patrick M.; Gamboa, Davide G.; Greve, Annika G.; Han, Shuoshuo H.; Hüpers, Andre H.; Ikari, Matt J.; Ito, Yoshihiro I.; Kim, Gil Y.; Koge, Hiroaki K.; Lee, Hikweon L.; Li, Xuesen L.; Luo, Min L.; Malie, Pierre R.; Moore, Gregory F.; Mountjoy, Joshu J.; McNamara, David D.; Paganoni, Matteo P.; Screaton, Elizabeth J.; Shankar, Uma S.; Shreedharan, Srisharan S.; Solomon, Evan A.; Wang, Xiujuan W.; Wu, Hung-Yu W.; Pecher, Ingo A.; LeVay, Leah J.; Nole, Michael A.
Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.
Natural gas hydrate is often found in marine sediment in heterogeneous distributions in different sediment types. Diffusion may be a dominant mechanism for methane migration and affect hydrate distribution. We use a 1-D advection-diffusion-reaction model to understand hydrate distribution in and surrounding thin coarse-grained layers to examine the sensitivity of four controlling factors in a diffusion-dominant gas hydrate system. These factors are the particulate organic carbon content at seafloor, the microbial reaction rate constant, the sediment grading pattern, and the cementation factor of the coarse-grained layer. We use available data at Walker Ridge 313 in the northern Gulf of Mexico where two ~3-m-thick hydrate-bearing coarse-grained layers were observed at different depths. The results show that the hydrate volume and the total amount of methane within thin, coarse-grained layers are most sensitive to the particulate organic carbon of fine-grained sediments when deposited at the seafloor. The thickness of fine-grained hydrate free zones surrounding the coarse-grained layers is most sensitive to the microbial reaction rate constant. Moreover, it may be possible to estimate microbial reaction rate constants at other locations by studying the thickness of the hydrate free zones using the Damköhler number. In addition, we note that sediment grading patterns have a strong influence on gas hydrate occurrence within coarse-grained layers.
Fagereng, A F.; Savage, H.M.S.; Morgan, J.K.M.; Wang, M.W.; Meneghini, F.M.; Barnes, P.M.B.; Bell, R.B.; Kitajima, H.K.; McNamara, D.D.M.; Saffer, D.M.S.; Wallace, L.M.W.; Petronotis, K.P.; LeVay, L.L.; Author, No A.; Nole, Michael A.
Geophysical observations show spatial and temporal variations in fault slip style on shallow subduction thrust faults, but geological signatures and underlying deformation processes remain poorly understood. International Ocean Discovery Program (IODP) Expeditions 372 and 375 investigated New Zealand’s Hikurangi margin in a region that has experienced both tsunami earthquakes and repeated slow-slip events. We report direct observations from cores that sampled the active Papaku splay fault at 304 m below the seafloor. This fault roots into the plate interface and comprises an 18-m-thick main fault underlain by ~30 m of less intensely deformed footwall and an ~10-m-thick subsidiary fault above undeformed footwall. Fault zone structures include breccias, folds, and asymmetric clasts within transposed and/or dismembered, relatively homogeneous, silty hemipelagic sediments. The data demonstrate that the fault has experienced both ductile and brittle deformation. As a result, this structural variation indicates that a range of local slip speeds can occur along shallow faults, and they are controlled by temporal, potentially far-field, changes in strain rate or effective stress.