Acoustic and mechanical properties of Nankai accretionary prism core samples

Abstract

[1] We studied undeformed sediment and accreted strata recently recovered by Ocean Drilling Program/ Integrated Ocean Drilling Program (ODP/IODP) drilling in Nankai Trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation. We have derived acoustic (V p ) and mechanical (uniaxial poroelastic compliance, compaction amplitude) properties of samples from various drill sites along the Muroto (ODP 1173) and Kii transects (IODP C0001, C0002, C0006, and C0007) from isotropic loading tests where confining and pore pressure were independently applied. We quantified the dependence of V p on both effective (P eff ) and confining (P c ) pressure, which can be used to correct atmospheric pressure measurements of V p . Experimental V p obtained on core samples extrapolated to in situ conditions are slightly higher than logging-derived velocities, which can be attributed either to velocity dispersion or to the effect of large-scale faults and weak zones on waves with longer wavelength. In the high-porosity (30%-60%) tested sediments, velocities are controlled at first order by porosity and not by lithology, which is in agreement with our static measurements of drained framework incompressibility, much smaller than fluid incompressibility. Rather than framework incompressibility, shear modulus is probably the second-order control on V p , accounting for most of the difference between actual V p and the prediction by Wood's (1941) suspension model. We also quantified the mechanical state of Nankai samples in terms of anisotropy, diagenesis, and consolidation. Both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions, attesting to the very low anisotropy of the tested material. When considering the porous samples of the Upper Shikoku Basin sediments (Site 1173) as examples of diagenetically cemented material, several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation. We also detected incipient cementation in samples from IODP Site C0001 (accretionary prism unit). In terms of consolidation, we distinguished two classes of material response (shallow, deformable samples and deep, hardly deformable ones) based on the amount of compaction upon application of a P eff large with respect to the inferred in situ value, with a transition that might be related to a critical porosity.Components: 15,300 words, 15 figures, 1 table.

Lithologic control of frictional strength variations in subduction zone sediment inputs

“…In the ash-rich USB facies, for example, high silica concentrations of up to 800-1000 µM in pore waters facilitated precipitation of amorphous silica. This effect has led to the formation of a Nankai margin-wide zone of anomalously high porosity, which was suggested to be caused by diagenetic cementation (e.g., Spinelli et al, 2007;Raimbourg et al, 2011;White et al, 2011). However, low measured values of cohesion in intact samples from Sites C0011 and C0012 indicate a lack of cementation (Ikari et al, 2013a).…”

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

“…[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).…”

“…Shipboard P wave velocities were corrected empirically by a fit of log data, which was not fully satisfactory for confinement [Kinoshita et al, 2009]. A P wave velocity correction based upon shore-based P wave measurements was subsequently performed on samples from the accretionary wedge lithological unit at Site C0001 for confining pressures ranging from 3 to 30 MPa [Raimbourg et al, 2011]. Assuming hydrostatic pore pressure conditions, the following correction is proposed (see auxiliary material):…”

The role of compaction contrasts in sediments in décollement initiation in an accretionary prism

“…On the other hand, our experimental results can be considered as coarse indicators of the magnitude of the deformation to be expected, whatever the precise loading conditions, and we use them in this sense in the following. This assumption is further supported by our experimental measurements (Raimbourg et al, 2011), showing that, at least in the poroelastic field, the mechanical anisotropy is very limited, so that measurements done in an arbitrary direction are to some extent representative of sample response in every direction.…”