Porosity and permeability evolution in the Tuaheni Landslide Complex at Hikurangi margin from IODP Sites U1517 and U1519

Abstract

Several mechanisms involving excess pore pressure related to gas hydrate have been proposed to explain active creeping at landslides such as the north Hikurangi margin Tuaheni Landslide Complex (TLC). Cores and logging data were retrieved by the International Ocean Discovery Program (IODP) Expeditions 372 and 375 from the South TLC at Site U1517. Here, the evolutions of porosity, pore structure and permeability are determined to assess the compaction state in the landslide and compare it with that of the undeformed sequence at Site U1519. Although no evidence of gas hydrate in the landslide at Site U1517 was identified by the cruise, zones suspected to host gas hydrates below the landslide or at Site U1519 are characterised by higher porosity, pore diameter and permeability. We show that most of the sedimentary section is in hydrostatic conditions, except the base of the TLC at Site U1517 and a zone below the base of the gas hydrate stability zone at Site U1519. These zones might be candidates for excess pore pressure build-up. There is no obvious evidence of the involvement of gas hydrate in active creeping at the TLC, which is more likely induced by hydrogeomechanical processes.

How Clay Mineral Assemblages Affect Instability on the Upper Slope of the Hikurangi Subduction Zone, New Zealand

“…Dutilleul et al. (2022) showed that the base of the Tuaheni landslide coincides with a zone of anomalous porosity, which they attributed to a combination of lithology and elevated pore pressure.…”

“…In other instances, contrasts of hydrogeologic properties across a pronounced lithologic boundary allow pore pressure to build up along a potential failure plane (e.g., Stegmann et al, 2007). Dutilleul et al (2022) showed that the base of the Tuaheni landslide coincides with a zone of anomalous porosity, which they attributed to a combination of lithology and elevated pore pressure. Surprisingly, we found no evidence at Site U1517 for pronounced compositional variations within or across any of the weak layers or inferred slip surfaces.…”

Effective Porosity Profile at IODP Site C0002 in the Heart of the Nankai Accretionary Prism, and Its Use for Predicting In Situ Seismic Velocities

“…It has been shown that in clay‐rich sediments such as those drilled at Site C0002, the water structurally bound to the clay particles affects the porosity measurements, resulting in biased porosity–velocity relationships (Brown & Ransom, 1996; Tudge & Tobin, 2013). Here, we measure cation exchange capacity (CEC) on core samples to differentiate the clay‐bound water content from the effective porosity (representative of the compaction state of the sediments) from the total porosity routinely measured during the cruises (Conin et al., 2012; Dutilleul, Bourlange, Conin, et al., 2020; Dutilleul, Bourlange, & Géraud, 2021; Dutilleul, Bourlange, Géraud, et al., 2020, 2021; Henry, 1997; Henry & Bourlange, 2004). Then, we propose a modified version of the Erickson‐Jarrard model (Erickson & Jarrard, 1998) fitting better the V p –porosity crossplot using effective porosity and shipboard mineralogical data.…”