Creeping Gas Hydrate Slides and LWD for Hikurangi Subduction Margin: coring and logging while drilling to unravel the mechanisms of creeping landslides and subduction slow slip events at the Hikurangi subduction margin, New Zealand1
Published February 2017
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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.
1Barnes, P.M., Pecher, I., and LeVay, L., 2017. Expedition 372 Scientific Prospectus: Creeping Gas Hydrate Slides and LWD for Hikurangi Subduction Margin. International Ocean Discovery Program. http://dx.doi.org/10.14379/iodp.sp.372.2017