Visual inspection of CT-scans collected at S1, S2 and S3 reveal the variation of fracture aperture. Similarly, aperture analyses confirm that Bm varied from 190+/-110, to 72+/-43 and 84+/-45 µm during the S1, S2, and S3, respectively (Fig. 3ABC). During the same stages, the number of voxel counted within the fracture varied respectively from ~934,000 to ~581,000 and ~797,000. Microphotography of sample MT07 at stage S3, shows that in several loci, the fracture collapsed, and a fine-grained amorphous mass infilled the fracture (Fig. 3DE).
Figure 3. CT-scan and transmitted light microphotography of sample MT07 sample after fracturing. Panels A, B, and C report CT-scan data at stages S1, S2, and S3, respectively. Each top-left inset of panels A-C show x-z sections of the CT-scan model after CT-number normalization, whose distribution is shown in the inset below. The red vertical line indicates the binarization threshold, and the percentages on the left and the right of such a line indicate the relative quantity of voxels representing air and solid rock, respectively. The right top inset in each A-C panel shows the binarized 3D model, where voxels collected within the fracture are shown in blue. The bottom right insets show the aperture distribution (B), the calculated average and standard deviation (Bm - red bars) and the total count of voxel within the fracture between parenthesis. Panel D is a microphotography of the thin section at stage S3. Panel E is a zoom of panel D highlighted in red. Insets 1 to 4 show fracture infill, which are highlighted by red arrows along with open fractures.
4 Discussion
We provide porosity-permeability relationships for rock samples from the subaerial northern HM under a range of confining pressures. Ultrasonic velocities of dry samples are similar to the seismic velocities estimated offshore New Zealand by the SHIRE project (Gase et al., 2021). The seismic reflectivity imaged along the transect MC10 shows the decollement along the prism base and several splay faults that may partly accommodate the convergence (Fig. 4A). Inside the prism, Vp increases gradually from 2.0 km/s near the surface to 4.5 km/s at the prism base ~7 km below sea level. A comparison between the seismic velocities in figure 4B and our ultrasonic velocities shows that samples GB13 and FB12 are representative of the modern slope basins on the outer prism, which is consistent with their depositional environment. The physical properties of sample MT07 of the Tinui Group correspond well to the velocities of the deep part of the prism, where Vp reaches 4.5 km/s. Compaction and diagenesis must contribute to the increase in Vp with depth (Dvorkin & Nur, 1996; Saxena & Mavko, 2014). We measured an ultrasonic Vp of 4.8 km/s at 150 MPa in the Torlesse basement sample MO02, which is higher than what we imaged in the deep prism on Line MC10 (Fig. 4), suggesting that there may not be a deep offshore portion of the Torlesse basement offshore northern HM (Bassett et al., 2022; Gase et al., 2021).
When comparing seismic and ultrasonic velocities, we should note that they have been estimated at frequencies around 20 Hz and 800 kHz, respectively. Considering such a frequency range, a typical P-wave quality factor range of 30 to 150, and a nearly-constant Q model (Liu et al., 1976; Tisato et al., 2021), we should expect velocities to increase between 2.3 and 12%. Another difference between SHIRE and laboratory data is that the latter are measured on dry samples, whereas the accretionary prism must be saturated. We show that P-wave velocities increase by ~10% when sample GB13 is saturated, suggesting that the effects of fluid saturation and anelasticity on velocities should counteract each other. Given the similarity in P-wave velocities and depositional environment, we suggest that the Tinui and Tolaga group rocks (samples MT07 and GB13) are good lithological proxies for the deep and shallow offshore Hikurangi prism, respectively.