EST at the site C0023
Figure 3 shows the conversion of drilling parameters recorded at site C0023 into EST, log EST, and background torque (Tb) calculated based on Eq. (3) for each time interval of 300 s, together with the employed coring assembly. The EST values were averaged within 2-m depth intervals and plotted with standard deviations to the EST values in the 2-m intervals. The EST profile was calculated continuously with depth except for at 392.5–419.3, 747.5–761.3, and 792.9–809.0 mbsf where drilling data is lacking. Ranging from 1 to 10 MPa throughout hole C0023A, EST does not show significant discontinuities at depths where coring tools or drill bits were exchanged (e.g., ~ 400 mbsf), confirming that EST is not affected by coring type or bit diameter. The background torque (Tb) increases steadily with depth. The down-hole profile of Tb shows less fluctuations than EST and closely resembles Tm (Fig. 2).
EST does not increase monotonically with depth but shows characteristic ups and downs (Fig. 3). To describe this variability, the EST zones were defined independently of the lithostratigraphic description. The evaluated EST indicates a gradual increasing trend in the shallower half (294–820 mbsf; zone A). In this zone, the baseline of EST shown as a blue broken line increases with depth at a gradient of ~ 3 MPa/km and reaches 2.5 MPa at the bottom of the zone. Zone A includes a high-EST subzone at 602–648 m (high EST zone, HEZ). Some EST values exceed 20 MPa in this subzone. The EST baseline is also higher than that of the adjacent intervals and reaches ~ 7.5 MPa approximately 630–648 mbsf. At a depth of 820 mbsf, a major discontinuity can be seen in the EST trend. In the interval between 820 and 1014 mbsf, defined as zone B, EST is slightly lower compared to the baseline of zone A (blue broken line). The EST profile in zone B appears to be constant at 5 MPa on average. However, the difference from baseline of zone A is unclear because of EST variations of ~ 2 MPa in zone B. At the top of zone B, the EST value significantly decrease from 10 to 2 MPa in the interval between 820 to 836 mbsf. The minimum value reaches 0.9 MPa, which is equivalent to values at approximately 300–400 mbsf. We defined this low-EST zone (LEZ) (820–870 mbsf) as a particularly weak portion inside zone B. Below 1014 mbsf, EST again increases with depth to 1128 mbsf (zone C). EST gradually increases from approximately 5 MPa. The variation in the EST becomes more pronounced as one moves deeper. EST increases abruptly at 1128 mbsf and reaches 80 MPa at 1170 m (zone D; 1150-TD). The lowest value still exceeds 10 MPa in this zone. For most of the borehole intervals, the relations between the defined zones and the background torque are not clear. The Tr trend indicates a monotonic increase with depth from 4 kN-m at 300 mbsf to 7 kN-m at 1150 mbsf. Even in HEZ, where the EST rapidly changes, no significant variation is found in Tb. On the other hand, both EST and Tb increase significantly in zone D.
The low-strength zone below the décollement
Generally, the strength of rocks increase with depth as effective pressure increases. At site C0023, however, this gradual trend is reversed at the boundary between zones A and B (820 mbsf), and in both zones considerable variations occur in the baseline (HEZ and LEZ) (Fig. 3). Figure 4 was compiled in order to compare the EST trend with a cropped seismic image and with lithological, mineralogical, and physical observations on cores taken from site C0023. The mechanical zones defined by the EST profile do not completely correspond to the lithostratigraphic units except for unit V–basement and zone D. Unit V is characterized by varicolored clay and a high abundance of volcaniclastics. The underlying lithologic basement is composed of hyaloclastites and pillow basalts. Zone D is recognized as a sudden increase in EST, and it is plausible that the high EST values reflect the high strength of the volcaniclastics and basalts.
At drilling site C0002 in the Nankai Trough (IODP NanTro SEIZE), calcium carbonate content and EST show a positive relationship in the shallower portion of the boreholes (Hamada et al. 2018). Such a positive correlation is not recognized from site C0023 (Fig. 4), indicating the possibility that cementation by carbonate is not developed to the same extend in this frontal region off Muroto. On the other hand, the depth trend of P-wave velocity (Vp), measured on core samples, corresponds closely to the EST zones (Fig. 4). In particular, both significantly increase in HEZ (600–640 mbsf), corresponding to the bottom of unit III, where repeated thick (~ 25 cm) layers of volcanic ash have been recognized in core samples (Heuer et al. 2017a; Taira et al. 1991). Thus, the increase in both of Vp and EST could reflect the physical property of stiffness in this tuffaceous layer.
Interestingly, EST neither varies clearly at the depth of the décollement zone itself nor can the décollement be characterized as an individual EST zone. EST rather significantly changes below the décollement zone (zones B and LEZ). A similar trend is also seen in the sample Vp. These are somewhat strange, because when there is a weak zone with lower strength than the décollement, strain can concentrate on the weak zone. One possibility to interpret the existence of this weak region is that the décollement zone identified at site C0023 is branching deformation zone, and the LEZ corresponds to the real décollement zone. The depth of the décollement zone found at previous ODP site 1174 is slightly deeper than at C0023, rather closer to LEZ (Fig. 4). On board scientists in Exp. 370, however, reported that the first major fault was found at 758.15 mbsf, interpreted to mark the top of the décollement zone, then no thrust fault zone was identified deeper than 796.395 mbsf (Heuer et al. 2017b). In addition, depth profiles of physical properties around the décollement zone at site C0023 show similarities with the profiles at site 1174; sample porosity increases, and P-wave velocity decreases below defined décollement zone (Taira et al. 1991; Heuer et al. 2017b). These observations and measurements suggest that the décollement zone found at C0023 corresponds to the décollement zone identified at site 1174. Another possibility to explain that LEZ is weaker than the décollement zone is that the actual strength of the décollement zone was not represented in EST due to low resolution of EST and/or lack of EST data. EST was calculated at 300-s intervals. Because the ROP during penetrating the décollement zone was approximately 0.2 m/min (Fig. 2), the calculation interval of EST is approximately every 1 m. Furthermore, the calculated EST was rejected when the data correlation is not good (R < 0.6). EST cannot be calculated accurately if there is strength contrast in the data section of 300 s (Hamada et al. 2018). Thus, the actual strength of the décollement zone may not appear in EST if the thickness of the fault core of the décollement is thin relative to the calculation section. In fact, the décollement zone at site C0023 appears to be composed of alternating intact intervals (approximately several meters in thickness) and thinner fault zones (Heuer et al. 2017b), suggesting that the thickness of the décollement is thinner than the EST resolution. These facts indicate that the latter possibility is plausible, and LEZ is considered to be a low-strength zone existing below the décollement zone.
The décollement zone in the Nankai Trough off the Cape Muroto exists within a hemipelagic mudstone sequence of the lower Shikoku Basin in lithostratigraphic unit IV (Taira et al. 1991) (Fig. 4). No significant difference in host lithology is found above and below the décollement (Heuer et al. 2017b). Thus, the characteristic drop of EST in zone B cannot be attributed to a change in lithology. An alternative explanation is the existence of high pore fluid pressure below the décollement, which has also been noted in previous studies (Gamage and Screaton 2006; Tsuji et al. 2008). Evidence for the existence of high fluid pressure at site C0023 comes from the observation of low seismic velocity zone in seismic reflection data, which was found to be a characteristic feature in subducting sediments below the frontal décollement in this Muroto transect (Figs. 1 and 4) (Tsuji et al. 2008). The normalized pore pressure ratio (λ*) below the décollement is estimated to be 0.4–0.7 (Tsuji et al. 2008). If the pore pressure is higher than the hydrostatic pressure (excess pore pressure), the effective pressure applied to the sediment decreases and its in-situ strength would also decrease.
The EST decreases significantly in LEZ, and this may indicate that this interval is currently under a particularly high pore pressure condition. In the Vp profile, significant decreasing in LEZ is not observed. This is probably because the Vp measurements have not been performed under in-situ pressure conditions. Vp is measured on the core samples recovered on the surface whose pore pressure has been released, thus the measured Vp values do not reflect the effect of high pore pressure at the original position. In addition, upwelling fluid from the borehole was also observed during the T-Limit project (Hirose et al. 2017). The upwelling flow was discovered by underwater TV during a re-entry operation of the drill bit, after cutting Core 110R at 1129.0 mbsf. At this time, a casing was already inserted down to 858.0 mbsf in order to stabilize the borehole. An opening between casing and formation was not filled by concrete during the period. The fluid might possibly flow from casing interval and it reached surface through the narrow gap. It is, however, more plausible that the fluid has injected from the bare hole section between 858.0 and 1129.0 mbsf and passed through casing pipes, because the flow velocity at borehole top is high (~ 0.1 m/s) (Hirose et al. 2017). Thus, the upwelling flow can indicate that over-pressurized pore fluids occur at least near that depth (Hirose et al. 2017). From this observation, it is suggested that the low EST zone corresponds to a low-effective-pressure zone where fluid pressure is high because of fluid influx from clay dehydration and/or influx from a deeper portion.
As the effective pressure beneath the décollement decreases, there is a significant difference in strength between the décollement zone and the underlying sediments as demonstrated by the EST. This strength contrast possibly influences the development process of the décollement itself. Fluid permeability perpendicular to the fault plane tends to decrease with the slip displacement and the fault may act as a barrier (Faulkner et al. 2010; Tanikawa et al. 2014; Yamashita and Tsutsumi 2018). Generated or advected fluids thus flow beneath the décollement plane. As a result, the effective strength of the sediments below the décollement decreases and the strength contrast increases. This feedback effect can work once the décollement forms, promoting décollement development. Moreover, as the décollement development is repeated, the deformation may occur in the high pore pressure zone; the plate boundary fault may shift toward the lower portion. The self-development strength structure may lead to the coupling of the plate boundary to constantly weaken, resulting in a low taper angle of the accretionary prism in the Nankai Trough off the Cape Muroto. As the relationship between the absolute value of EST and the value of in situ shear strength has not been examined so far, it is impossible to evaluate pore fluid pressure, friction property, or other physical properties from EST. Careful examinations based on drilling experiment in laboratory and on field allow us to make a correlation between EST and strength, and to draw pore pressure structure around plate boundary décollement. In addition, comparison of the EST distribution and geochemical profiles of pore fluid or core samples will help to further elucidate the above inferences.