Velocity‐porosity relationships for slope apron and accreted sediments in the Nankai Trough Seismogenic Zone Experiment, Integrated Ocean Drilling Program Expedition 315 Site C0001

Abstract

[1] In this study, we focused on the porosity and compressional wave velocity of marine sediments to examine the physical properties of the slope apron and the accreted sediments. This approach allows us to identify characteristic variations between sediments being deposited onto the active prism and those deposited on the oceanic plate and then carried into the prism during subduction. For this purpose we conducted ultrasonic compressional wave velocity measurements on the obtained core samples with pore pressure control. Site C0001 in the Nankai Trough Seismogenic Zone Experiment transect of the Integrated Ocean Drilling Program is located in the hanging wall of the midslope megasplay thrust fault in the Nankai subduction zone offshore of the Kii peninsula (SW Japan), penetrating an unconformity at ∼200 m depth between slope apron sediments and the underlying accreted sediments. We used samples from Site C0001. Compressional wave velocity from laboratory measurements ranges from ∼1.6 to ∼2.0 km/s at hydrostatic pore pressure conditions estimated from sample depth. The compressional wave velocity-porosity relationship for the slope apron sediments shows a slope almost parallel to the slope for global empirical relationships. In contrast, the velocity-porosity relationship for the accreted sediments shows a slightly steeper slope than that of the slope apron sediments at 0.55 of porosity. This higher slope in the velocity-porosity relationship is found to be characteristic of the accreted sediments. Textural analysis was also conducted to examine the relationship between microstructural texture and acoustic properties. Images from micro-X-ray CT indicated a homogeneous and well-sorted distribution of small pores both in shallow and in deeper sections. Other mechanisms such as lithology, clay fraction, and abnormal fluid pressure were found to be insufficient to explain the higher velocity for accreted sediments. The higher slope in velocity-porosity relationship for accreted sediments can be explained by weak cementation, critical porosity or differences in loading history.

Interpretation of porosity and LWD resistivity from the Nankai accretionary wedge in light of clay physicochemical properties: Evidence for erosion and local overpressuring

“…The correction of interstitial porosity to account for swelling clay content appears to be useful for clarifying the lithological origin of several of the porosity anomalies observed, and more generally, for assessing the compaction state of the sediments. Also, several studies [Doan et al, 2011;Hashimoto et al, 2010], show the importance of using such a porosity when correlating water content with other physical properties (e.g., P wave velocity). However, this model should be tested in sediments with a higher range of cation exchange capacity, since the variations among the lithologies drilled in the Kumano transect are moderate, 0.05 to 0.3 mol kg −1 .…”

“…[7] Five categories of facies are found [Kinoshita et al, 2009] (Figure 3): (1) the slope apron mainly composed of silty clay to clayey silt hemipelagites and volcanic ash, (2) accreted sediments composed of fine-grained terrigenous material and hemipelagites, (3) Kumano fore-arc basin sediments mainly composed of sandy turbidites and hemipelagic mud, (4) trench sediments composed of silt, sand-and gravel-bearing turbidites and muddy marine sediments, and (5) the Upper Shikoku Basin composed of silty clay to clayey silt hemipelagites and volcanic ash. A weak cementation may be present in this facies [Kinoshita et al, 2009], and in the accreted sediments of Site C0001, at least in the cored interval (down to ∼450 m), [Hashimoto et al, 2010;Raimbourg et al, 2011], but it is absent at the other sites [Raimbourg et al, 2011]. [8] Overall, the XRD analysis at the Kumano transect [Kinoshita et al, 2009] indicates that the clay mineral content (relative to silt) increases from the hemipelagic mud of the Kumano fore-arc basin, to slope sediments, to accreted trench sediments, and then to accreted sediments and Upper Shikoku Basin sediments (Figure 4).…”

“…Interstitial Porosity Determination on Core Samples [10] Porosity measurements performed on the cores using standard methods [Blum, 1997] reflect the total water content present within the sediments including the water contained in hydrous minerals, which primarily refers to smectites in the case of the Nankai sediments [Henry and Bourlange, 2004;Underwood and Steurer, 2003]. Previous researches proposed correction for bound water based on smectite content determinations obtained using XRD [Brown and Ransom, 1996;Hashimoto et al, 2010]. We propose a correction based upon cation exchange capacity (CEC) determinations and apply it to shipboard porosity measurements on cores in order to calculate the interstitial porosity.…”

Seismic imaging and velocity structure around the JFAST drill site in the Japan Trench: low Vp, high Vp/Vs in the transparent frontal prism

“…The lithology of these cores was mudstone (Expedition 343/343T Scientists, 2013). The design of the experiment is similar to that employed in the previous studies on saturated marine sediments (e.g., Tobin et al 1994, Hashimoto et al 2010). The pore pressure was kept at 500 kPa for Core343-C0019E-7R (7R), and confining (effective) pressure was stepwise elevated from 1,000 to 18,000 kPa during the measurements.…”

Data report: compressional wave velocity measurements on sediments from the reference site off Kumano, Nankai Trough, NantroSEIZE Expedition 333

“…The design of the experiment is similar to that employed in other studies of saturated marine sediments (e.g., Tobin et al, 1994;Tobin and Moore, 1997;Gettemy and Tobin, 2003;Hashimoto et al, 2010Hashimoto et al, , 2011Raimbourg et al, 2011).…”