Figure 4. A) Velocity model along the SHIRE Line MC10 (Gase et al., 2021). MC10 position is reported in figure S1. B) Summary of laboratory result: permeabilities vs porosity and color-coded markers as a function of measured ultrasonic Vp for samples FB12, MO02, and MT07. Color coding is based on the colorbar of panel A. The arrow indicates in which direction the permeabilities vary when tests are performed using water rather than helium gas. Dashed lines indicate empirical permeability vs porosity according to eq. 3. The dotted line represents an average permeability for unconsolidated clays and possibly a lower bound for the permeability of HM sediments (Neuzil, 1994). S data (dark-gray area) are for siltstones (Reece et al., 2012). The continuous line fits our data and agrees with measured mudstone permeabilities indicated by the MN gray-shaded area (Magara, 1978; Neglia, 1979). Such a line also represents an upper bound for the permeability of HM rocks. M, T, and B data are permeabilities measured in boreholes: M by Reisdorf et al. (2016), Yu et al. (2017); T by Boisson et al. (2001); B by Intera Eng. Ltd. (2011), Roberts et al. (2011), Walsh (2011).

The permeability of our samples ranges from 1 nD to 1 mD, with the samples representing the deep part of the prism being the tightest. Neuzil (1994, 2019) compiled data from several studies on unconsolidated clays with a maximum porosity of 80%, and a few consolidated mudstone-siltstones with porosities (Φ) <35%. Saffer & Bekins (1998) followed Neuzil’s work and described the permeability (κ) of the Nankai accretionary complex as:

\(\mathbf{\kappa}\left(\mathbf{\text{nD}}\right)\mathbf{\approx}\mathbf{10}^{\mathbf{1+5.5}\mathbf{\phi}}\)eq. 1

Equation 1 fits the porosity-permeability relationship of unconsolidated sediments and is a lower bound for the permeability of mudstones that are similar to our samples (Magara, 1978; Neglia, 1979; Reece et al., 2012). On the other hand, we found that:

\(\mathbf{\kappa}\left(\mathbf{\text{nD}}\right)\mathbf{\approx}\mathbf{10}^{\mathbf{-1.3+45}\mathbf{\phi}}\)eq. 2

fits our results and is an upper bound for the permeability of mudstones. We suggest that the permeabilities comprised between equations 1 and 2 (Fig. 4B MN,S) are proxies for rock permeabilities in the Northern Hikurangi accretionary prism at depths >1 km, because helium gas limits clay swelling, which would have lowered the measured permeabilities (Villar et al., 2005); At burial depths >1-2 km, the porosity of clay-bearing sediments, mudstones, siltstone, and shales drops below 35% (Griffiths & Joshi, 1989; Skempton, 1969; Magara, 1978); Permeabilities measured in boreholes are typically orders of magnitude higher than those measured in the laboratory due to the presence of fractures (Fig 4B lines M,T,B) (Neuzil, 2019), and numerical models of permeability in microfractured claystones agree with the mudstone porosity-permeability in fig 5B (Vora & Dugan, 2019). We also propose that the permeability of rocks in the Northern Hikurangi accretionary prism can be described by a Kozeny-Carman relation (dashed lines in Fig 4B):

\(\mathbf{\kappa=}\frac{\mathbf{\phi}}{\mathbf{8}\mathbf{\tau}^{\mathbf{2}}}\mathbf{R}^{\mathbf{2}}\)eq. 3

Where \(\mathbf{\tau}\) is tortuosity and R is the median pore diameter (Carman, 1997). We obtained\(\mathbf{R}\left(\mathbf{\text{nm}}\right)\mathbf{=61.02}\mathbf{\phi}^{\mathbf{2}}\mathbf{+56.51}\mathbf{\phi}\)from data reported by Hunt (1996).

Every 1-2 years, the northern HM experiences an SSE that lasts several weeks (Wallace, 2020). Recent analyses of the APG data offshore Gisborne have shown that the 2014 SSE may have experienced up to 30 cm of slip in the center of a ~100 km wide patch, though less displacement is expected along the edges (Yohler et al., 2019). Some authors have suggested that SSEs that originate along the decollement at the base of the wedge are accompanied by slip diverted to thrust faults in the Hikurangi accretionary wedge (Shaddox & Schwartz, 2019). We expect mudstones along these thrust faults to experience fracturing during an SSE. Our laboratory measurements before and after rock failure for sample MT07 show that the deeper prism, where Tinui Group equivalent rocks may be present, may experience large increases in permeability during an SSE.

In a few weeks, the fractured sample MT07 regained its pre-fracturing permeability. Between stages S1 and S2, the permeability recovery was achieved in dry conditions. Although sample MT07 and sample FB12 have different compaction levels and grain sizes, they share a similar mineralogy. Thus, altough limited, we expect plastic deformation also in sample MT07, likely concentrated near clays (Mondol et al., 2008). Between stages S2 and S3, the permeability decreased by a factor of 5, while B_m increased, suggesting clay expansion. Once confined, we expect that the hydrated clays would deform plastically, clogging the fracture more efficiently than dry clays and justifying the observed permeability loss. We propose that such efficient permeability healing is also present along HM faults, given the presence of clays at depth, especially above the 5-7 km deep temperature-controlled smectite-illite transition (Freed & Peacor, 1989; Pecher et al., 2017; Tisato & Marelli, 2013).

In the Hikurangi subduction zone, fluids expelled from pore space and fluids released by dehydration reactions travel along the plate interface or through the accretionary wedge (Ellis et al., 2015). As the fluid pressure increases near the decollement and inside the accretionary wedge, conditions may become favorable for an SSE (Kobayashi & Sato, 2021). Though this mechanism has been proposed for several subduction zones where SSE occur at larger depths (Audet et al., 2009; Kodaira et al., 2004), the analysis of Warren-Smith et al. (2019) shows that the northern HM also can seal fluid pathways after an SSE. The expansion and plastic deformation of clays, which we have observed in our laboratory tests, may provide an efficient mechanism to reduce permeability over weeks or months after an SSE.

5 Conclusions

We provided relationships between porosity, permeability, and confining pressure for rocks that make up the accretionary prism of the northern HM. We suggest an empirical porosity-permeability relationship to model fluid transport and estimate effective stress in shallow subduction zones. Mechanical failure of these rocks enhances permeability, but over the course of several weeks, healing reduces the permeability again, suggesting that after an SSE, mudstones deep in the northern HM accretionary prism can recover permeability efficiently within the time frame of an SSE as a mechanism explaining the regular recurrence of these events.