Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone
© Tsuji et al.; licensee Springer. 2014
Received: 26 February 2014
Accepted: 9 September 2014
Published: 19 September 2014
We evaluated the influence of the trench-parallel component of plate motion on the active fault system within the Nankai accretionary wedge from reflection seismic profiles, high-resolution seafloor bathymetry, and deep-towed sub-bottom profiles. Our study demonstrated that a large portion of the trench-parallel component of oblique plate subduction is released by strike-slip motion along a fault located just landward of and merging down-dip with a mega-splay fault. The shallow portion of the splay fault system, forming a flower structure, seems to accommodate dominant strike-slip motion, while most of the dip-slip motion could propagate to the trenchward décollement. Numerous fractures developed around the strike-slip fault release overpressured pore fluid trapped beneath the mega-splay fault. The well-developed fractures could be related to the change in stress orientation within the accretionary wedge. Therefore, the strike-slip fault located at the boundary between the inner and outer wedges is a key structure controlling the stress state (including pore pressure) within the accretionary prism. In addition, the strike-slip motion contributes to enhancing the continuous mega-splay fault system (outer ridge), which extends for approximately 200 km parallel to the Nankai Trough.
Although the Philippine Sea Plate currently subducts obliquely beneath the accretionary prism in the Nankai Trough (Seno et al. 1993; Miyazaki and Heki 2001), it had subducted perpendicular to the trench until approximately 2 to 4 Ma (Kimura et al. 2005; Takahashi 2006; Ikeda et al. 2009). The change in plate motion reactivated some faults within the accretionary wedge as strike-slip faults that could accommodate trench-parallel shear strain due to the oblique plate subduction. The Median Tectonic Line (MTL), located approximately 250 km landward from the trough axis, is well known as a large strike-slip fault that releases trench-parallel shear strain (Figure 1; Ikeda et al. 2009). A few recent studies (Ashi et al. 2007; Martin et al. 2010) have demonstrated that the lineament at the seaward edge of the Kumano forearc basin (just landward of seafloor trace of the mega-splay fault, see Figures 1 and 2) is the surface expression of an active strike-slip fault, and it can be interpreted to have resulted from strain partitioning. In other words, a portion of the trench-parallel component of oblique convergence is accommodated by the active right-slip fault at the wedge-forearc boundary (i.e., the transition zone). Indeed, borehole breakout analysis has demonstrated that the stress state within the accretionary wedge is a strike-slip regime in the outer ridge region (Chang et al. 2010). Because the strike-slip fault is located at the transition zone between the inner and outer wedges from the region off Shionomisaki to the region off Kumano (Figure 2), we call the strike-slip fault at the wedge boundary the ‘wedge boundary strike-slip fault’ (WBSF) in this study. Moore et al. (2009) named this fault system in the Kumano basin area (eastern side of our survey area; Figure 1) the Kumano basin edge fault zone. At the Ryukyu margin, the southern continuation of the Nankai Trough, Dominguez et al. (1998) showed that a strike-slip fault is developed at the outer ridge and that it accommodates some of the lateral components of oblique plate convergence.
Strike-slip motion within the accretionary wedge (e.g., along the WBSF) would influence the stress (and strain) state and interplate earthquake mechanisms. However, the degree of strain partitioning at the outer ridge has not been well quantified. Furthermore, the role of the strike-slip motion caused by oblique subduction in the evolution and kinematics of seismogenic mega-splay faults and in stress buildup within the accretionary wedge has not been fully explored. In this study, we estimate the extent of the strike-slip motion along the WBSF by integrating seismic-derived images and seafloor topography and evaluate the influence of the strike-slip fault on the stress state in the Nankai accretionary wedge and continuous mega-splay fault system.
Methods and results
We used multi-channel seismic reflection data acquired by 3D seismic surveys and also used several 2D seismic surveys to delineate the mega-splay fault system in a wide area of the Nankai Trough. The 3D seismic data over the Nankai accretionary wedge off Kumano (Figure 2a) were acquired by M/V Nordic Explorer in 2006 (Moore et al. 2009). The 12 km × 56 km survey area included both the inner and outer wedges. We applied 3D prestack depth migration to the 3D seismic data using a tomography-based approach. Because the seafloor multiple overlapped the structures of the transition zone (or outer ridge), the strong seafloor multiples were attenuated before migration processing. Detailed information about the data acquisition and processing procedures is described in Moore et al. (2009). To characterize the stress state and fractures around the transition zone, we further referred to P-wave velocity derived from waveform tomography (Figure 2b; Kamei et al. 2012, 2013, 2014), pore pressure distribution (Figure 2c; Tsuji et al. 2014), fast S-wave velocity anisotropy (Figure 2d), and S-wave velocity difference between the trench-parallel direction and trench-normal direction (Figure 2e; Tsuji et al. 2011a). As abnormal pore pressure estimated from seismic velocity (Figure 2d) cannot be distinguished from open fractures (Tsuji et al. 2014), the estimated overpressured region has the potential to represent well-developed fractures.
The deeper part of the mega-splay fault system (>6 km below the seafloor) is well defined as a prominent single reflector on the analyzed seismic profiles off Kumano, as shown in Figures 2 and 6a,b (Park et al. 2002; Tsuru et al. 2005; Bangs et al. 2009; Moore et al. 2009). The seismic velocity derived from waveform tomography (Figure 2b) demonstrates that the footwall of the deep mega-splay fault has low P-wave velocity due mainly to an overpressured state and a lithology difference (Figure 2c; e.g., Tsuji et al. 2014). On the other hand, the deep mega-splay fault seems to branch trenchward from beneath the WBSF, but it becomes less reflective (Figure 2a) and has lower velocity contrast around the outer ridge (Figure 2b; Kamei et al. 2012). Therefore, it is difficult to identify the geometry of the shallower part of the mega-splay fault system. In this study, we determined the detailed 3D geometries of the shallower parts of the branched faults from a dense 2D seismic profile (ODKM-03) partially using phase information from seismic signals (i.e., instantaneous phase; Taner et al. 1979; Tsuji et al. 2005). The extracted fault geometry is discontinuous and en echelon with faults branching trenchward from beneath the outer ridge (Figure 4a,c). These shallow discontinuous splay faults seem to merge into the deeper, single mega-splay fault at the transition zone (Figure 2). The WBSF could not be imaged clearly on the multi-channel seismic profiles, likely because the heterogeneous geological structures including fractures exclude coherent reflection events and because the dip angle of the WBSF is probably too steep to be imaged. The WBSF seems to form a flower structure (Ashi et al. 2007; Martin et al. 2010), although the relationship between the WBSF and the mega-splay thrust fault cannot be clearly revealed from reflection seismic profiles. Because the WBSF exists parallel to and close to the shallow branched mega-splay thrust fault, it seems likely that the WBSF and the mega-splay fault merge down-dip at the transition zone (black arrows in Figure 2).The characteristics of the mega-splay fault and WBSF change at a point off Cape Shionomisaki (Figure 6c,e), where the WBSF intersects the rupture segment boundary between the 1944 Tonankai and 1946 Nankai earthquakes. However, the lineament associated with the WBSF exists continuously across the segment boundary. Off Cape Shionomisaki, the mega-splay fault does not have strong reflection amplitude, which indicates that the splay fault is not a well-developed boundary of pore pressure or lithology (Figure 6c,e). Instead, steeply dipping faults branch directly from the basal décollement that developed close to the sediment/crust interface, and it is difficult to distinguish the splay fault from the WBSF. Thus, it seems likely that the shallow branched splay fault itself has a strike-slip component and functions as the WBSF off Cape Shionomisaki (Figure 6d,f).
Deep-tow sub-bottom profiler
Strike-slip rate at the WBSF
In this section, we approximately estimate the slip rate of the WBSF from the offset of Shionomisaki Canyon (Figure 3). First, we estimate a minimum rate of slip. The 6-km offset of the western sidewall of the canyon was caused by the cumulative right-slip on the WBSF. It is evident that the channel offset has been increasing progressively by incision of the continental slope since Shionomisaki Canyon was established. Therefore, the maximum age of the canyon offset is the age when the accretionary wedge front was at the present-day offset point. By considering the growth rate of the accretionary prism calculated mainly from the shape of the prism imaged on seismic profiles (Figures 2 and 6) and the thickness of incoming sediment (Westbrook 1994), we can approximately calculate the depositional age of the accretionary prism at the offset point at 20 km distance from the trough axis as 1.7 Ma. Anma et al. (2011) studied seafloor outcrop samples of the accretionary wedge at the offset region and estimated the depositional age at approximately 2 Ma (yellow star in Figure 3a). The present mega-splay fault off Kumano was initially a frontal thrust at 1.95 Ma (Strasser et al. 2009). These ages give the maximum age of the channel offset; therefore, the minimum rate of strike-slip along the WBSF can be calculated as approximately 3 mm/year by dividing the offset length by the age (=6 km/2 Ma). Because this rate is a minimum value, we estimate a more realistic slip rate using other information as follows.
Another estimation can be made on the basis of the local sedimentation rate. The seismic reflection profile across the offset region shows that a deep piggy-back basin has developed beneath the offset channel on the downthrown side of the mega-splay fault (Figure 6e,f). The thickness of the basin fill derived from the upper slope is approximately 1,000 m. The lower part of the basin fill becomes progressively tilted landward with depth (Figure 6f), while the upper basin fill (lying unconformably on the lower basin fill) does not. This indicates that the lower basin fill is coeval with, and the upper basin fill post-dates, activity of the trenchward imbricate thrust fault. On the other hand, the mega-splay fault system has likely been active throughout the deposition of the basin fill because the basin-fill strata thicken toward and terminate against the mega-splay fault system. The deposition rate of the slope sediment in this piggy-back basin has not been estimated, but it could be high because of the considerable amount of sediment that can be supplied from Shionomisaki Canyon, which is one of the major supply routes of terrestrial sediment. Using the depth of the slope sediment beneath the offset deposits (1,000 m) at the landward side of the trough axis, the average deposition rate of the slope sediment can be estimated to be greater than 0.5 mm/year by dividing the sediment thickness (approximately 1,000 m) by the age of the accretionary prism (approximately 2 Ma). Because the height of the channel wall (scarp) of the canyon at the offset point is approximately 150 m and because the present canyon center is almost the same depth (approximately 200 m deeper than the surrounding seafloor; Figure 3), the offset floor did not receive significant sediment during the 6-km strike-slip movement. To preserve the 150-m scarp of the previous canyon wall at 6 km from the canyon center with the high sedimentation environment (>0.5 mm/year), the WBSF would have had to slip 6 km in less than approximately 0.3 Ma (=150 m/0.5 mm/year). Therefore, we can approximately estimate the slip rate as 20 mm/year (=6 km/0.3 Ma). Although our estimation of the slip rate based on several assumptions (e.g., local sedimentation rate) contains large error, the slip rate of the WBSF is of similar order with the plate convergence rate. The slip rate would be faster than that of the landward MTL (approximately 5 mm/year), which is a large strike-slip fault that partitions oblique Philippine Sea Plate motion west of the Kii Peninsula and Shikoku Island (Ikeda et al. 2009).
The convergence vector between the Philippine Sea Plate and the Eurasian Plate deviates by approximately 35° to 50° counterclockwise from the perpendicular to the trench off Cape Shionomisaki (arrow in Figure 1; Seno et al. 1993; Miyazaki and Heki 2001). If the WBSF accommodates all the trench-parallel component of relative plate motion, then the rate of strike-slip on the WBSF would be 38 ± 12 mm/year in the offset region off Cape Shionomisaki. Therefore, our slip rate estimation (approximately 20 mm/year) suggests that the WBSF at the outer ridge releases a large portion (approximately 52%) of the trench-parallel component of plate motion.
The change in the subduction direction of the Philippine Sea Plate could influence our slip rate estimation (described above). The Philippine Sea Plate was subducting beneath the Japanese islands in the NNW direction at 15 Ma (Seno and Maruyama 1984; Takahashi 2006); this direction differs from the current plate motion direction (Miyazaki and Heki 2001; Ide et al. 2010). The time of the change in subduction direction has not been determined precisely, but it was probably between 2 and 4 Ma (e.g., Kimura et al. 2005; Takahashi 2006; Ikeda et al. 2009). Therefore, the slip rate estimated in this study (approximately 20 mm/year) is not influenced by the change in plate subduction direction.
Within the Kumano forearc basin sediment on the landward side of the WBSF, there does not appear to be a continuous strike-slip fault (Gulick et al. 2010). Therefore, the strike-slip motion has not been significant at the inner wedge area since the construction of the Kumano basin (approximately 2 Ma). Shear wave splitting analysis along the Kumano transect (Tsuji et al. 2011a) shows the highest seismic anisotropy around the WBSF (approximately 4%; Figure 2d), and the fast polarization of shear waves is in the trench-parallel direction (i.e., the strike of the WBSF; Figure 2e and yellow bars in Figure 4b). This seismic anisotropy suggests that fractures with trench-parallel orientations are well developed from the outer ridge to approximately 10 km landward of the WBSF (red zone in Figure 4b). The shallow overpressured zone (or lower velocity zone; Kamei et al. 2012; Tsuji et al. 2014) in the accretionary prism beneath the WBSF could also reflect the presence of open fractures or extensional faults around the WBSF (Figure 2c). Therefore, the strike-slip fault with intensive fractures could be observed only around the WBSF within our study area.
Influence of strike-slip faulting on the stress state within the accretionary wedge
The strike-slip motion and well-developed fractures along the WBSF must be related to the stress state (including pore pressure) within the accretionary wedge. The deep mega-splay fault landward of the transition zone off Kumano has been imaged as a single reflector with strong reflection amplitude (Figure 2; Park et al. 2002; Bangs et al. 2009), indicating a large contrast in seismic velocity (Tsuji et al. 2006; Kamei et al. 2012), which in turn originates from large pore pressure contrast or difference in lithology. Overpressured fluid may be trapped by a low-permeability barrier along the deep mega-splay fault (e.g., Brown et al. 1994). On the other hand, the mega-splay fault branches into several fault planes on the seaward side of the transition zone (Figures 2 and 6), and the reflection amplitudes of the shallow splay faults are lower. Therefore, the pore pressure contrast or total displacement along the shallower thrusts should be small compared with the deep mega-splay fault. Fault branching may be related to the decreasing pore pressure contrast. These intensive fractures along the WBSF could release the overpressured pore fluid trapped by the low-permeability barrier along the mega-splay fault. Indeed, we found several cold seepages at the seafloor traces of the mega-splay fault and WBSF (Figures 3a and 4a; Ashi et al. 2002; Ashi et al. 2003; Toki et al. 2004; Anma et al. 2011), suggesting leakage of overpressured fluid along the well-developed fractures.
Borehole breakouts and core sample observations have revealed the change in the stress state (principal horizontal stress orientation) across the WBSF off the Kumano area (e.g., Tobin et al. 2009; Lin et al. 2010; Figure 4b). The maximum principal horizontal stress is perpendicular to the trench seaward of the WBSF and parallel to it landward of the WBSF, and it changes again further landward to become nearly parallel to plate convergence (red bars in Figure 4b). Therefore, the principal stress is locally oriented in the trench-parallel direction around the WBSF (red zone in Figure 4b). These variations in stress orientation could be hypothetically explained by static stress variations during the earthquake cycle (i.e., dynamic Coulomb wedge theory; Wang and Hu 2006). In addition to this interpretation, the stress change could be partially influenced by strain partitioning because strike-slip motion on the WBSF would release trench-parallel shear stress of the outer wedge caused by oblique plate subduction. Therefore, the strike-slip motion realizes the trench-normal principal horizontal stress in the outer wedge (e.g., Site C0001) and inner wedge (Site C0009).
It is difficult to explain the drastic change (approximately 90°) in stress orientation within the short distance around the WBSF (approximately 10 km; red bars in Figure 4b) because the continuous lithology does not allow significant change in principal stress orientation within a short distance (does not allow change in trench-normal stress). Although this anomalous stress state at the WBSF cannot be clearly explained, we have some interpretations as follows. At the seaward edge of the Kumano forearc basin (i.e., transition zone), the accretionary prism is locally bent (Figure 2). The surface of the inner accretionary wedge beneath the forearc basin has a landward dip of approximately 2°, whereas the outer wedge has a seaward dip of approximately 5°. The local bending of the accretionary prism generates extensional stress within the shallow accretionary prism at the transition zone. Gravitational instability of such a bathymetric high at the transition zone may further generate extensional stress. The extension originates open fractures oriented in the trench-parallel direction, as estimated by seismic anisotropy (Figure 2d,e). The pore pressure distribution, which is also interpreted as fracture intensity (Figure 2c; Tsuji et al. 2014), further suggests that open fractures are distributed widely around the WBSF. The apparent fracture zone around the WBSF mechanically divides the accretionary wedge into outer and inner wedges and may function as a stress (and strain) boundary within the accretionary prism.
Furthermore, studies on dislocation fault models have indicated that stress discontinuity across a fault can occur where slip decreases or increases along the fault plane (e.g., Maruyama 1969). Conversely, if slip is uniform over the fault, then the stress field is continuous across it. Moreover, from analogy with the dislocation theory, the changes in slip (i.e., distributed dislocation) produce stress concentration on the fault plane (e.g., Maruyama 1969), which would result in plastic deformation or the development of a fracture zone along the fault. Therefore, the significant stress change could occur at the intensive fracture zone along the WBSF. Furthermore, a stress anomaly could occur at the edge of the coseismic deep mega-splay fault if coseismic rupture from the deep mega-splay fault is abruptly terminated there. Although we have some other interpretations for the significant change in stress orientation (e.g., extension at the hanging-wall side of the seismogenic fault), but cannot clarify a dominant mechanism, future deep drilling could provide new insights into the abrupt change in stress state.On the seismic profile off Cape Shionomisaki, on the other hand, the accretionary wedge is not significantly bent, but is separated by a deep piggy-back basin filled with thick slope sediments on the seaward side of the WBSF (Figure 6d,f). The strike-slip motion off Cape Shionomisaki is larger than that off the Kumano region because of the smaller angle between the strike of the WBSF and the subduction direction. Although the stress state off Cape Shionomisaki has not been measured directly, the stress (and strain) discontinuity across the WBSF could be enhanced by the marked change in rock properties due to soft slope sediment and many dislocation planes (fractures).
Strike-slip motion of the continuous mega-splay fault system
Discontinuous and branching characteristics are observed in the shallower part of the splay fault on the trenchward side of the outer ridge (Figures 4c and 6). Thrust faults generally have discontinuous features along strike and generally change geometry over time (e.g., Yamada et al. 2013) as the result of processes such as segment linkage (Cartwright et al. 1995), propagation across barriers (Ellis and Dunlap 1988), and subsequent deformation after fault formation (e.g., Yamada et al. 2006). However, the mega-splay fault system as a whole (including the WBSF) continues for as long as 200 km in an east–west direction along the trench axis and functions as a transition zone between the inner and outer accretionary wedges (Figures 1 and 3c). Such good continuity of the mega-splay fault system could be explained by strike-slip motion along the WBSF caused by oblique plate subduction. After the mega-splay fault was initiated and the Coulomb wedge was allowed to thicken, the later strike-slip motion emphasized the continuous mega-splay fault in the trench-parallel direction. Analog experiments related to the subduction process demonstrate that strike-slip faults that are continuous for a long distance, like the WBSF, tend to develop in oblique subduction regimes (e.g., Martinez et al. 2002). The continuous mega-splay fault system enhanced by strike-slip motion and its down-dip extension could be the source of a large interplate earthquake, the rupture area of which may extend across the segment boundary between the 1944 Tonankai and 1946 Nankai earthquakes (Park and Kodaira 2012).
At plate convergent margins, where the trench-parallel component of plate motion is dominant (e.g., the Sumatra subduction zone; McCaffrey et al. 2000; Subarya et al. 2006), long, continuous strike-slip faults in the trench-parallel direction develop mainly because (1) a strike-slip fault tends to be linear and continuous along the strike, based on the simple geometrical postulate that a fault plane should be smooth in the slip direction, and (2) rupture energy along a steep strike-slip fault is lower than that along a reverse fault with a low dip angle (and large rupture plane) (Woodcock 1986). Therefore, the WBSF has played an essential role in the evolution of the long and continuous mega-splay fault (outer ridge) system as well as in strain partitioning in the plate convergent zone.
We characterized a strike-slip fault associated with oblique subduction from reflection seismic profiles, seafloor bathymetry, and deep-towed sub-bottom profiles in the Nankai Trough off the Kii Peninsula. The results provide new insights into the influence of strike-slip motion on mega-splay fault systems.
From the seafloor bathymetry of Shionomisaki Canyon, we could approximately estimate the strike-slip rate along the WBSF. The majority of strain due to the oblique subduction is released at the WBSF.
The strike-slip motion caused by oblique plate subduction could have contributed to the generation of the continuous mega-splay fault system in the trench-parallel direction.
Overpressured fluid is released at the well-developed fracture zone along the WBSF, suggesting that several faults could be branched at the seaward side of the WBSF.
The horizontal principal stress changes completely within the intensive fracture zone along the WBSF off the Kumano area.
The shallower portion of the mega-splay fault system including the WBSF could function mainly as a strike-slip fault. The majority of dip slip could propagate from the deep mega-splay fault of the inner wedge to the seaward décollement of the outer wedge.
Multi-channel seismic data were acquired by JAMSTEC. C. Moore (UCSC) and an anonymous reviewer provided constructive comments. This study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Japan Society for the Promotion of Science (JSPS) (21107003). T. Tsuji gratefully acknowledges the support of the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), sponsored by the World Premier International Research Center Initiative (WPI), MEXT, Japan.
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