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.