Characterization of porosity from Cation Exchange Capacity and resistivity data at IODP Expedition 358 Site C0024: insights on compaction state, stress and deformation history at the frontal thrust of the Nankai margin

In subduction zones, megathrust seismicity depends on the hydrogeological, thermal, physical and mechanical properties of the sediments before they enter the subduction zone and how these properties evolve through the subduction process. In particular, fluids are progressively released by compaction and/or mineral dehydration reactions as burial increases, resulting in the build-up of pore fluid pressure in low-permeability sediments that strongly affects fault behavior through its control on effective normal stress.

We use porosity, logging and chemical data to characterize compaction state and bound water content of sediments at Site C0024. This site was drilled in March 2019 during International Ocean Discovery Program Expedition 358 in the anticline overlying the frontal thrust of the Nankai margin, a few kilometers landward of Sites C0006 and C0007 that were previously drilled during the NanTroSEIZE project. Sites C0024, C0006 and C0007 transect the décollement and overlying accreted Upper Shikoku Basin and wedge slope deposits. At Site C0024, the main frontal thrust at ~820 mbsf, the top of its footwall (up to ~870 mbsf) and its hanging-wall were logged. Two intervals were cored in the hanging-wall (~0-320 mbsf and ~510-652 mbsf). Four sedimentary facies were identified : (1) the slope apron (~0-4 mbsf) composed of silty clay to clayey silt hemipelagites, (2) accreted trench wedge sediments (~4–519 mbsf) composed of hemipelagites with volcanic ash layers, silt and sand turbidites, (3) outer trench-wedge sediments (~519-555 mbsf) mainly composed of hemipelagites with rare silt turbidites and volcanic ash layers and (4) accreted Upper Shikoku Basin (>555mbsf) composed of hemipelagites and volcanic ash layers. The Pliocene to Miocene accreted Upper Shikoku Basin deposits at the frontal thrust are correlated with undeformed Upper Shikoku Basin deposits at reference Sites C0011 and C0012 seaward in the incoming Shikoku Basin.

Following previous studies, we use Cation Exchange Capacity (CEC) to determine bound water content and interstitial porosity in the cored interval. Unlike total porosity commonly measured onboard, interstitial porosity is representative of the compaction state of sediments. We use interstitial porosity data to calibrate resistivity-derived porosity through the hanging-wall, the décollement and below. Resistivity-derived porosity is obtained with a resistivity model accounting for the high surface conductivity of clays based on CEC, exchangeable cation composition, LWD resistivity and gamma ray logs. We also document the evolution of the structure of micro- to macropores with depth using low pressure nitrogen adsorption/desorption, mercury injection capillary pressure and nuclear magnetic resonance.  Finally, we compare the porosity dataset at Site C0024 with that of Sites C0006 and C0007 in the frontal thrust and reference Sites C0011 and C0012 in the entering Shikoku Basin to characterize the evolution of the compaction state of sediments during accretion.