Hikurangi Subduction Margin Coring and Observatories:
unlocking the secrets of slow slip through drilling to sample and monitor the forearc and subducting plate1
Published January 2017
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Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expedition 375 aims to investigate the processes and in situ conditions that underlie subduction zone SSEs at northern Hikurangi through coring of the frontal thrust, upper plate, and incoming sedimentary succession and through installation of borehole observatories in the frontal thrust and upper plate above the slow slip source area. Logging-while-drilling (LWD) data for this project will be acquired as part of Expedition 372 (beginning in November 2017; see the Expedition 372 Scientific Prospectus for further details on the LWD acquisition program).
Northern Hikurangi subduction margin SSEs recur every 2 years and thus provide an excellent setting to monitor deformation and associated chemical and physical properties surrounding the SSE source area throughout the slow slip cycle. Sampling material from the sedimentary section and oceanic basement of the subducting plate and from the primary active thrust in the outer wedge near the trench will reveal the rock properties, composition, and lithologic and structural character of the material transported downdip to the known SSE source region. A recent seafloor geodetic experiment shows the possibility that SSEs at northern Hikurangi may propagate all the way to the trench, indicating that the shallow fault zone target for Expedition 375 may lie within the SSE rupture area.
Four primary sites are planned for coring, and observatories will be installed at two of these sites. Expedition 375 (together with the Hikurangi subduction component of Expedition 372) is designed to address three fundamental scientific objectives: (1) characterize the state and composition of the incoming plate and shallow plate boundary fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth; (2) characterize material properties, thermal regime, and stress conditions in the upper plate above the SSE source region; and (3) install observatories at the frontal thrust and in the upper plate above the SSE source to measure temporal variations in deformation, fluid flow, and seismicity. The observatories will monitor deformation and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of slow slip events and their relationship to great earthquakes along the subduction interface.
1Saffer, D.M., Wallace, L.M., and Petronotis, K., 2017. Expedition 375 Scientific Prospectus: Hikurangi Subduction Margin Coring and Observatories. International Ocean Discovery Program. http://dx.doi.org/10.14379/iodp.sp.375.2017
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