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1. Seismic Velocity Heterogeneity of the Hikurangi Subduction Margin, New Zealand: Elevated Pore Pressures in a Region With Repeating Slow Slip Events

   https://doi.org/10.1029/2020JB021605  

   Yarce, Jefferson ; Sheehan, Anne ; Roecker, Steven ; Mochizuki, Kimihiro ( May 2021 , Journal of Geophysical Research: Solid Earth)

   We investigated the seismic velocity structure of the Hikurangi margin in New Zealand to uncover the physical features of the subduction zone and explore the relationships between microearthquake seismicity, seismic velocity structure, and slow slip events. Using local earthquake tomography with data collected from both temporary ocean bottom seismometers and on‐land permanent seismic stations, we used the tomography code TomoFD to iteratively perform a damped least squared inversion of absolute P and S arrival times to obtain relocated hypocenters and generate 3D velocity models for Vp and Vp/Vs. The seismic tomography images show two high Vp/Vs anomalies, one offshore and adjacent to a subducted seamount and the other beneath the North Island of New Zealand. The ∼50‐km wide offshore anomaly extends ∼10 km beneath the plate interface and lies directly beneath the area that slipped at least 50 mm during the 2 week‐long 2014 slow slip event. High Vp/Vs values may be related to high pore fluid pressures from subducted sediments, and such increases in pore fluid pressures have been suggested to trigger the occurrence of slow slip events in active subduction zones. The second onshore high Vp/Vs anomaly is located in the overlying plate and subducting slab and correlates with areas suggested by other geophysical techniques to be rich in fluids. Our seismic imaging supports interpretations that subduction processes in the Hikurangi margin are highly dependent on physical features such as subducted seamounts and fluid‐rich sediments.

    

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2. Expedition 372 Scientific Prospectus: Creeping Gas Hydrate Slides and LWD for Hikurangi Subduction Margin

   https://doi.org/10.14379/iodp.sp.372.2017  

   Barnes, P.M. ; Pecher, I. ; LeVay, L. ( February 2017 , Scientific prospectus)

   null (Ed.)

   International Ocean Discovery Program (IODP) Expedition 372 combines two research topics, slow slip events (SSEs) on subduction faults (IODP Proposal 781A-Full) and actively deforming gas hydrate–bearing landslides (Proposal 841-APL). Our study area on the Hikurangi margin east of New Zealand provides unique locations for addressing both research topics. Gas hydrates have long been suspected of being involved in seafloor failure; not much evidence, however, has been found to date for gas hydrate–related submarine landslides. Solid, icelike gas hydrate in sediment pores is generally thought to increase seafloor strength, as confirmed by a number of laboratory measurements. Dissociation of gas hydrate to water and overpressured gas, on the other hand, may destabilize the seafloor, potentially causing submarine landslides. The Tuaheni Landslide Complex on the Hikurangi margin shows evidence for active, creeping deformation. Intriguingly, the landward edge of creeping coincides with the pinchout of the base of gas hydrate stability (BGHS) on the seafloor. We therefore hypothesize that gas hydrate may be linked to creeping by (1) repeated small-scale sliding at the BGHS, in a variation of the conventional model linking gas hydrates and seafloor failure; (2) overpressure at the BGHS due to a permeability reduction linked to gas hydrates, which may lead to hydrofracturing, weakening the seafloor and allowing transmission of pressure into the gas hydrate stability zone; or (3) icelike viscous deformation of gas hydrates in sediment pores, similar to onshore rock glaciers. The latter two processes imply that gas hydrate itself is involved in creeping, constituting a paradigm shift in relating gas hydrates to submarine slope failure. Alternatively, creeping may not be related to gas hydrates but instead be caused by repeated pressure pulses or linked to earthquake-related liquefaction. We have devised a coring and logging program to test our hypotheses. SSEs at subduction zones are an enigmatic form of creeping fault behavior. At the northern Hikurangi subduction margin (HSM), they are among the best-documented and shallowest on Earth. They recur about every 2 y and may extend close to the trench, where clastic and pelagic sediments about 1.0–1.5 km thick overlie the subducting, seamount-studded Hikurangi Plateau. The northern HSM thus provides an excellent setting to use IODP capabilities to discern the mechanisms behind slow slip fault behavior, as proposed in IODP Proposal 781A-Full. The objectives of Proposal 781A-Full will be implemented across two related IODP expeditions, 372 and 375. Expedition 372 will undertake logging while drilling (LWD) at three sites targeting the upper plate (midslope basin, proposed Site HSM-01A), the frontal thrust (proposed Site HSM-18A), and the subducting section in the trench (proposed Site HSM-05A). Expedition 375 will undertake coring at the same sites, as well as an additional seamount site on the subducting plate, and implement the borehole observatory objectives. The data from each expedition will be shared between both scientific parties. Collectively, the LWD and coring data will be used to (1) characterize the compositional, structural, thermal, and diagenetic state of the incoming plate and the shallow plate boundary fault near the trench, which comprise the protolith and initial conditions for fault zone rock associated with SSEs at greater depth, and (2) characterize the material properties, thermal regime, and stress conditions in the upper plate above the SSE source region. These data will be used during Expedition 375 to guide the installation of CORK observatories at the frontal thrust and in the upper plate above the SSE source to monitor temporal variations in deformation, fluid flow, seismicity, and physical and chemical properties throughout the SSE cycle (Saffer et al., 2017). Together, these data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface. 

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3. Near Trench 3D Seismic Attenuation Offshore Northern Hikurangi Subduction Margin, North Island, New Zealand

   https://doi.org/10.1029/2020JB020810  

   Nakai, Jenny S. ; Sheehan, Anne F. ; Abercrombie, Rachel E. ; Eberhart‐Phillips, Donna ( March 2021 , Journal of Geophysical Research: Solid Earth)

   We image seismic attenuation near the Hikurangi trench offshore New Zealand, using ocean bottom and land‐based seismometers, revealing high attenuation above a recurring shallow slow‐slip event and within the subducting Hikurangi Plateau. The Hikurangi subduction margin east of the North Island, New Zealand is the site of frequent shallow slow slip events. Overpressured fluids are hypothesized to lead to slow slip at shallow depths close to the oceanic trench. Seismic attenuation, energy loss of seismic waves, can be used to detect high temperatures, melt, the presence of fluids, and fractures. We use local earthquake P‐ and S‐waves from 180 earthquakes to invert fort*, and subsequently invert for Qp and Qs, offshore the North Island directly above the area of slow slip. We image Qp and Qs to ∼25 km depth, increasing resolution of previously identified coastal lowQ(100–300), and finding a new region of even higher attenuation (Qp and Qs < 50–100) directly above the shallow slow slip event of 2014–2015, beneath the offshore seismic array. This highest attenuation is downdip of a subducting seamount, and is spatially correlated with a high seismic reflectivity zone and Vp/Vs > 1.85, all of which provide evidence for the presence of fluids. The Qp and Qs is low at the trench (<50–100) and in the subducting plate (100–200), suggesting that seismic wave scattering due to faults, fractures, and the inherent heterogeneous composition of the Hikurangi Plateau, a large igneous province, plays a role in seismic attenuation.

    

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4. 372 Preliminary Report: Creeping Gas Hydrate Slides and Hikurangi LWD

   https://doi.org/10.14379/iodp.pr.372.2018  

   Pecher, I.A. ; Barnes, P.M. ; LeVay, L.J. ( March 2018 , Preliminary report)

   null (Ed.)

   International Ocean Discovery Program (IODP) Expedition 372 combined two research topics, slow slip events (SSEs) on subduction faults (IODP Proposal 781A-Full) and actively deforming gas hydrate–bearing landslides (IODP Proposal 841-APL). Our study area on the Hikurangi margin, east of the coast of New Zealand, provided unique locations for addressing both research topics. SSEs at subduction zones are an enigmatic form of creeping fault behavior. They typically occur on subduction zones at depths beyond the capabilities of ocean floor drilling. However, at the northern Hikurangi subduction margin they are among the best-documented and shallowest on Earth. Here, SSEs may extend close to the trench, where clastic and pelagic sediments about 1.0–1.5 km thick overlie the subducting, seamount-studded Hikurangi Plateau. Geodetic data show that these SSEs recur about every 2 years and are associated with measurable seafloor displacement. The northern Hikurangi subduction margin thus provides an excellent setting to use IODP capabilities to discern the mechanisms behind slow slip fault behavior. Expedition 372 acquired logging-while-drilling (LWD) data at three subduction-focused sites to depths of 600, 650, and 750 meters below seafloor (mbsf), respectively. These include two sites (U1518 and U1519) above the plate interface fault that experiences SSEs and one site (U1520) in the subducting “inputs” sequence in the Hikurangi Trough, 15 km east of the plate boundary. Overall, we acquired excellent logging data and reached our target depths at two of these sites. Drilling and logging at Site U1520 did not reach the planned depth due to operational time constraints. These logging data will be augmented by coring and borehole observatories planned for IODP Expedition 375. Gas hydrates have long been suspected of being involved in seafloor failure; not much evidence, however, has been found to date for gas hydrate–related submarine landslides. Solid, ice-like gas hydrate in sediment pores is generally thought to increase seafloor strength, as confirmed by a number of laboratory measurements. Dissociation of gas hydrate to water and overpressured gas, on the other hand, may weaken and destabilize sediments, potentially causing submarine landslides. The Tuaheni Landslide Complex (TLC) on the Hikurangi margin shows evidence for active, creeping deformation. Intriguingly, the landward edge of creeping coincides with the pinch-out of the base of gas hydrate stability on the seafloor. We therefore hypothesized that gas hydrate may be linked to creep-like deformation and presented several hypotheses that may link gas hydrates to slow deformation. Alternatively, creeping may not be related to gas hydrates but instead be caused by repeated pressure pulses or linked to earthquake-related liquefaction. Expedition 372 comprised a coring and LWD program to test our landslide hypotheses. Due to weather-related downtime, the gas hydrate-related program was reduced, and we focused on a set of experiments at Site U1517 in the creeping part of the TLC. We conducted a successful LWD and coring program to 205 mbsf, the latter with almost complete recovery, through the TLC and gas hydrate stability zone, followed by temperature and pressure tool deployments. 

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5. Mariana Convergent Margin and South Chamorro Seamount

   https://doi.org/10.14379/iodp.proc.366.2018  

   Fryer, P. ; Wheat, C.G. ; Williams, T. ( February 2018 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity in subduction zones and suprasubduction zone lithosphere. International Ocean Discovery Program Expedition 366 was designed to address the nature of these processes in the shallow to intermediate depth of the Mariana subduction channel. Although no technology is available to permit direct sampling of the subduction channel of an intraoceanic convergent margin at depths up to 19 km, the Mariana forearc region (between the trench and the active volcanic arc) provides a means to access materials from this zone. Active conduits, resulting from fractures in the forearc, are prompted by along- and across-strike extension that allows slab-derived fluids and materials to ascend to the seafloor along associated faults, resulting in the formation of serpentinite mud volcanoes. Serpentinite mud volcanoes of the Mariana forearc are the largest mud volcanoes on Earth. Their positions adjacent to or atop fault scarps on the forearc are likely related to the regional extension and vertical tectonic deformation in the forearc. Serpentinite mudflows at these volcanoes include serpentinized forearc mantle clasts, crustal and subducted Pacific plate materials, a matrix of serpentinite muds, and deep-sourced formation fluid. Mud volcanism on the Mariana forearc occurs within 100 km of the trench, representing a range of depths and temperatures to the downgoing plate and the subduction channel. These processes have likely been active for tens of millions of years at the Mariana forearc and for billions of years on Earth. At least 19 active serpentinite mud volcanoes have been located in the Mariana forearc. Two of these mud volcanoes are Conical and South Chamorro Seamounts, which are the farthest from the Mariana Trench at 86 and 78 km, respectively. Both seamounts were cored during Ocean Drilling Program Legs 125 and 195, respectively. Data from these two seamounts represent deeper, warmer examples of the continuum of slab-derived materials as the Pacific plate subducts, providing a snapshot of how slab subduction affects fluid release, the composition of ascending fluids, mantle hydration, and the metamorphic paragenesis of subducted oceanic lithosphere. Data from the study of these two mud volcanoes constrain the pressure, temperature, and composition of fluids and materials within the subduction channel at depths of up to 19 km. Understanding such processes is necessary for elucidating factors that control seismicity in convergent margins, tectonic and magma genesis processes in the volcanic arc and backarc areas, fluid and material fluxes, and the nature and variability of environmental conditions that impact subseafloor microbial communities. Expedition 366 focused on data collection from cores recovered from three serpentinite mud volcanoes that define a continuum of subduction-channel processes to compare with results from drilling at the two previously cored serpentinite mud volcanoes and with previously collected gravity, piston, and remotely operated vehicle push cores across the trench-proximal forearc. Three serpentinite mud volcanoes (Yinazao, Fantangisña, and Asùt Tesoro) were chosen at distances 55 to 72 km from the Mariana Trench. Cores were recovered from active sites of eruption on their summit regions and on the flanks where ancient flows are overlain by more recent ones. Recovered materials show the effects of dynamic processes that are active at these sites, bringing a range of materials to the seafloor, including materials from the crust of the Pacific plate, most notably subducted seamounts (even corals). Most of the recovered material consists of serpentinite mud containing lithic clasts, which are derived from the underlying forearc crust and mantle and the subducting Pacific plate. A thin cover of pelagic sediment was recovered at many Expedition 366 sites, and at Site U1498 we cored through distal serpentinite mudflows and into the underlying pelagic sediment and volcanic ash deposits. Recovered serpentinized ultramafic rocks and mudflow matrix materials are largely uniform in major element composition, spanning a limited range in SiO2, MgO, and Fe2O3 compositions. However, variation in trace element composition reflects interstitial water composition, which differs as a function of the temperature and pressure of the underlying subduction channel. Dissolved gases H2, CH4, and C2H6 are highest at the site farthest from the trench, which also has the most active fluid discharge of the Expedition 366 serpentinite mud volcanoes. These dissolved gases and their active discharge from depth likely support active microbial communities, which were the focus of in-depth subsampling and preservation for shore-based analytical and culturing procedures. The effects of fluid discharge were also registered in the porosity and gamma ray attenuation density data indicated by higher than expected values at some of the summit sites. These higher values are consistent with overpressured fluids that slow compaction of serpentinite mud deposits. In contrast, flank sites have significantly greater decreases in porosity with depth, suggesting that processes in addition to compaction are required to achieve the observed data. Thermal measurements reveal higher heat flow values on the flanks (~31 mW/m2) than on the summits (~17 mW/m2) of the seamounts. The new 2G Enterprises superconducting rock magnetometer (liquid helium free) revealed relatively high values of both magnetization and bulk magnetic susceptibility of discrete samples related to ultramafic rocks, particularly dunite. Magnetite, a product of serpentinization, and authigenic carbonates were observed in the mudflow matrix materials. In addition to coring operations, Expedition 366 focused on the deployment and remediation of borehole casings for future observatories and set the framework for in situ experimentation. Borehole work commenced at South Chamorro Seamount, where the original-style CORK was partially removed. Work then continued at each of the three summit sites following coring operations. Cased boreholes with at least three joints of screened casing were deployed, and a plug of cement was placed at the bottom of each hole. Water samples were collected from two of the three boreholes, revealing significant inputs of formation fluids. This suggests that each of the boreholes tapped a hydrologic zone, making these boreholes suitable for experimentation with the future deployment of a CORK-Lite. 

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