Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench, off Tohoku
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Multichannel seismic reflection lines image the subducting Pacific Plate to approximately 75 km seaward of the Japan Trench and document the incoming plate sediment, faults, and deformation front near the 2011 Tohoku earthquake epicenter. Sediment thickness of the incoming plate varies from <50 to >600 m with evidence of slumping near normal faults. We find recent sediment deposits in normal fault footwalls and topographic lows. We studied the development of two different classes of normal faults: faults that offset the igneous basement and faults restricted to the sediment section. Faults that cut the basement seaward of the Japan Trench also offset the seafloor and are therefore able to be well characterized from multiple bathymetric surveys. Images of 199 basement-cutting faults reveal an average throw of approximately 120 m and average fault spacing of approximately 2 km. Faults within the sediment column are poorly documented and exhibit offsets of approximately 20 m, with densely spaced populations near the trench axis. Regional seismic lines show lateral variations in location of the Japan Trench deformation front throughout the region, documenting the incoming plate’s influence on the deformation front’s location. Where horst blocks are carried into the trench, seaward propagation of the deformation front is diminished compared to areas where a graben has entered the trench. We propose that the décollement’s propagation into the trench graben may be influenced by local stress changes or displacements due to subduction of active normal faults. The location and geometry of the up-dip décollement at the Japan Trench is potentially controlled by the incoming outer-rise faults.
KeywordsJapan Trench Subduction Normal faults Outer rise Trench Oceanic plate
When large bathymetric features, such as seamounts, fracture zones, ridges, and oceanic plateaus, are subducted at convergent margins, they strongly deform the landward trench slopes (e.g., McCann and Sykes 1984; Lallemand and Le Pichon 1987; Dominguez et al. 1998; Taylor et al. 2005). Smaller features, such as subducting horst and graben structures, were once considered to play a role in sediment subduction and upper plate abrasion by horst blocks (Hilde 1983), but better imaging of the subducting plate suggested the subduction plane was well above the top of such features (von Huene and Culotta 1989). Sediment thickness and lithostratigraphy of the incoming plate may further influence the landward trench slope by controlling the physical properties of the margin and inducing lateral heterogeneities in prism formation (Underwood 2007; Ike et al. 2008). Although large-scale features can dominate the upper plate morphology, small-scale features are likely more common at most trenches and may also play a role in modifying the upper plate over time.
Normal faults, found on the outer rise of many Pacific trenches (e.g., Masson 1991), are one type of small-scale feature with potential impacts on the upper plate morphology. As the plate enters a subduction zone, flexural bending of the plate produces the outer rise (e.g., Bodine et al. 1981). Brittle failure manifested by normal faults permanently deforms the surface of the downgoing plate. Faults can form parallel to the trench axis in subduction zones. However, in regions where the abyssal hill fabric, formed at oceanic spreading centers, strikes at a low angle (<25° to 30°) to the trench, the abyssal hill faults are reactivated instead of forming new faults (Masson 1991). Reactivated abyssal hill faults are identified by their strike, which is parallel to the abyssal hill fabric or magnetic lineations seaward of the trench (Billen et al. 2007). Flexural bending structures and seafloor spreading fabrics along the outer rise are the major contributors to strikes of incoming topographic features on the Northwestern Pacific Plate (Nakanishi 2011). Furthermore, normal faults produced by plate flexure seaward of the trench penetrate into the mantle are thought to allow hydration of the upper mantle and crust (Ranero et al. 2003; Grevemeyer et al. 2007) leading to a potential role in overpressuring and hydrofracturing of the upper plate at depth (von Huene et al. 2004).
The dominant features observed on the incoming Pacific Plate are bending-related normal faults, abyssal hill fabric, fracture zones, seamounts, and elongated ridges (Nakanishi 2011). The incoming outer-rise faults are largely active structures before and during subduction. Following a large megathrust earthquake at a subduction zone, shallow intraplate earthquakes are often detected along the outer rise (Christensen and Ruff 1988); these earthquakes might be caused by the Coulomb stress change from the main thrust event (e.g., Sato et al. 2013). After the 2011 Mw 9.0 Tohoku earthquake, the outer rise of the Japan Trench was dominated by normal fault earthquakes of varying size, depth, and spatial location. These earthquakes may still be potential tsunamigenic hazards, as several Mw >7.0 events have occurred since the main shock, including a large outer-rise event of Mw 7.7 (Lay et al. 2011) and two Mw 7.2 earthquakes in 2012 (Obana et al. 2014). Historically, large outer-rise events within this region include the 2005 Mw 7.0 (Hino et al. 2009) and the 1933 Mw 8.4 (Kanamori 1971) events. Precise measurements from ocean bottom seismographs (OBSs) detected numerous intraplate earthquakes after the 2011 event ranging in location from approximately 120 km seaward of the trench to approximately 90 km landward of the trench with mainly normal fault focal mechanisms in the uppermost subducting Pacific Plate (Obana et al. 2012; Obana et al. 2013).
Geophysical analysis of the Tohoku earthquake revealed large coseismic slip that extended to the trench (Ide et al. 2011; Ito et al. 2011; Fujiwara et al. 2011; Fujii et al. 2011). Differential bathymetry from before and after the earthquake also revealed the largest vertical displacements near the trench axis (Fujiwara et al. 2011), and seismic reflection data imaged frontal thrust development after the event (Kodaira et al. 2012). Seismic data collected in 2011 suggested that the décollement steps down into a local graben at the trench axis (Kodaira et al. 2012; Nakamura et al. 2013) rather than remaining at a constant depth above the overriding plate and ‘bulldozing’ the sediment from the top of the incoming horst blocks (e.g., Hilde 1983). Subduction of active bending-related normal faults may have larger implications for deformation of the near-trench upper plate and may be a small contributor to tsunamigenesis, with large seafloor displacements but small areal extent. To better determine the role of the incoming plate on near-trench coseismic processes, we first examine the nature of the crust that is subducting at the Japan Trench.
This study focuses on the basement and sediment structure of the incoming Pacific Plate off Tohoku. We investigate the morphology of outer-rise normal fault systems and their lateral variations along the Japan Trench. These fault systems are particularly important in the Japan Trench because the lack of significant trench sediment fill means that, along many parts of the margin, the morphologic trench is defined where a graben has entered the trench. Therefore, it is important to define the characteristics of the subducting normal fault system. We use recently collected seismic reflection data seaward of the Tohoku earthquake epicenter to image the sediment and basement structure of the Pacific Plate. In the present study, we analyze the bending-related normal faults off Tohoku with a goal of establishing relationships among the subducting horst-graben structures, their sediment cover, and deformation in the Japan Trench and the adjacent landward trench slope.
We use four two-dimensional (2D) multichannel seismic (MCS) surveys carried out by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) for a total of 28 dip lines and 19 strike lines along the Japan Trench. JAMSTEC’s vessel R/V Kairei conducted three MCS surveys in March, April to May, and August to September 2011, collecting a total of 15 regional lines. The seismic system used a 444-channel hydrophone streamer, 12.5-m group interval, 6,000-m-long receiver cable, with a 7,800 in.3 air gun array fired at 50-m intervals, producing a common mid-point (CMP) interval of 6.25 m. R/V Kaiyo collected 32 high-resolution seismic lines in October to November 2011. The acquisition system used a 192-channel, 6.25-m group interval, 1,200-m-long receiver cable, with a 320 in.3 cluster gun array fired at 37.5-m intervals. Line spacing for the Kaiyo seismic grid was approximately 500 m with a CMP interval of 3.125 m. Both regional and high-resolution lines were processed through conventional workflows (e.g., Yilmaz 2001) that included trace edit, CMP binning, band-pass filter, velocity analysis, normal moveout correction, and CMP stacking. We performed Kirchhoff post-stack depth migration (PoSDM) on the regional Kairei lines using previous local refraction and reflection studies (e.g., Tsuru et al. 2000) to constrain our velocity model. For the high-resolution Kaiyo lines, we performed Kirchhoff pre-stack depth migration (PSDM). Due to the short streamer-cable length, the velocity model for the PSDM is poorly constrained, so our starting velocity models were based on velocity values of PoSDM migrations for each line. We performed 2 to 4 iterations of velocity analyses for PSDM creating a final model that produced the clearest image. Because of the great water depths (>6,000 m), the water velocity (which does not vary greatly) has a large effect on the migrations. We estimate that within the sediment section, which is less than 1,000-m-thick, our velocity errors are no more than approximately 200 to 400 m s−1 based on PSDM sediment velocity sensitivity in the high-resolution lines (e.g., Tsuru et al. 2000; Costa Pisani et al. 2005). Such velocity errors would result in a maximum of approximately 5% to 10% error in depth that would not affect our structural interpretations.
Distribution of outer-rise normal faults off Tohoku
Incoming Pacific Plate sediments
The Japan Trench margin has long been known for its relatively thin incoming sediment column of approximately 400 m from both seismic and DSDP drill cruises (e.g., Ludwig et al. 1966; Shipboard Scientific Party 1980). Our data provide new insight into regional variation in sediment thickness and internal deformation within the sediments.
Near-trench, sediment-restricted faults
The deformation front along the Japan Trench
Earthquakes and bathymetric trends demonstrate that bending-related faults form at a maximum of approximately 120 km east of the Japan Trench axis (Obana et al. 2012; Nakanishi 2011). A simple subduction calculation with the Pacific Plate subducting at a rate of approximately 8 to 10 cm/year (DeMets et al. 2010; Apel et al. 2006; Argus et al. 2011; Niitsuma 2004) nearly orthogonally under Honshu, predicts the oldest normal faults about to be subducted to be approximately 1.2 to 1.5 Ma. Assuming steady slip on the normal faults, maximum throws of 500 m divided by the age of the oldest faults gives a maximum throw rate of 333 to 417 m/Ma. Faults with smaller offsets must be much younger or less active. The increase in the density of the faults westward, towards the trench, indicates that new faults continue to form as the plate nears the trench, which indicates that the assumption of steady-state deformation is incorrect. This overall pattern is similar to normal fault development in other trenches, such as the Middle America Trench (Ranero et al. 2003). Furthermore, average fault spacing of 2 km is on the same order as predicted in some modeling studies (Faccenda et al. 2009), although other models generate larger fault spacing of up to 10 km (Naliboff et al. 2013), which is the maximum fault spacing we observe. The increase in the number of faults towards the trench and the range of fault spacing suggests continuous formation of new faults occurs as the Pacific Plate approaches the trench rather than apportioning strain to only major, more widely spaced faults.
The fault traces in our survey area is similar to patterns developed in other locations and in analog models. Onshore three-dimensional (3D) normal fault propagation through basalt shows linkage of surface fractures that produce a fault with an irregular trace (Kaven and Martel 2007). Similar zig-zag fault traces have been generated in analog fault experiments where different stress directions were applied to the sediments (e.g., Henza et al. 2011). The 3D stress field of the Japan Trench outer rise is poorly known. A bend in the Japan Trench trend at 38° N will create 3D effects not accounted for in 2D models. Furthermore, coseismic rupture of thrust segments south and north of 38° N will create a heterogeneous stress field on the outer rise. This is the only region along the Japan Trench where topographic structures on the subducting plate lie collinear with the fracture zone strike (Nakanishi 2011). At approximately 30° from the trend of the northern part of the trench (Figure 1), the fracture zone trend is slightly more than the approximately 25° reactivation cutoff determined by Billen et al. (2007). However, that study considered only reactivation of abyssal hill trends, but this may still imply that fracture zones are still weak with a lower coefficient of friction than the surrounding rock. Following initial surface breaks parallel to the trench, large fault-tip stress concentrations may help reactivate the pre-existing fabrics to develop the fault trace geometry found in the survey area (Figure 1).
Branches, relays, and conjugate faults are found in many normal fault populations (Peacock 2002; Morley et al. 2007; Nicol et al. 1995; Walsh et al. 1999). The presence of such features in our study area indicates poor lateral fault connectivity. Regional north to south differences in cumulative throw (Figure 4) may be related to such lateral effects. Fault interactions, especially in areas with closely spaced faults, may also contribute to variations in slip distributions along a single fault (Martel and Shacat 2006). Therefore, our throw analysis should be viewed cautiously due to effects of nearby faults and location of our MCS lines within individual fault segments.
Incoming plate sediment thickness
Sediment thickness and lithology of the incoming plate plays a dominant role in development and growth of accretionary prisms. Accurate understanding of the geometry of the TIC and overlying sediments is vital to measuring sediment influx and fault throw. Although basement-cutting faults offset the TIC, the TIC is not always a continuous, strong reflector in between basement-cutting faults within the study area, as seen on the eastern section of line D02 (Figure 5A). This discontinuous nature of the TIC in our seismic lines is due to several factors: 1) seismic imaging through complex geology scatters ray paths, resulting in an inadequately imaged subsurface. In our study area, this is largely caused by to the rough seafloor and the chert layer overlying the TIC. The high-resolution survey data likely show greater scattering (due to the higher frequency content), as the regional lines image the TIC more consistently. 2) The velocity of the overlying chert creates little impedance contrast with the TIC. Although no formation velocity for DSDP Site 436 was measured for the chert unit, velocities of porcellanite and chert samples generally range from 2.6 to 4.8 km/s (Shipboard Scientific Party 1980), which may be near the velocity of the TIC. 3) Drilling at Site 436 did not reach the igneous basement, leaving the composition and alteration of the basement unknown near our survey location. Mineralogic analysis of oceanic basalts elsewhere reveals significant alteration with a large potential for fluid incorporation (Kameda et al. 2011), which may lower the acoustic impedance across the TIC. 4) The igneous basement may differ compositionally throughout the survey area. The seismic character of a sill or sheet flow may produce a smoother and stronger reflector than pillows and flows (Abrams et al. 1993), creating variations in reflector strength for the TIC. 5) Continued faulting along the outer rise will further reduce continuous reflections from the TIC. This can be from development of a new fault breach through the TIC, secondary faulting, or off-fault, nonelastic deformation, including fissure and cavity formation, as the fault tip propagates towards the surface (Holland et al. 2006; Martel and Langley 2006). Even with sections of discontinuous TIC, we are confident in our interpretation because we observe relative amplitude and frequency differences between the sediment and underlying basalt along with regions of a prominent TIC horizon.
Our results reveal variations of as much as 600 m in thickness of the sediments overlying the TIC (Figure 1B). Correlation of sediment thickness between regional lines was not possible because the seismic line spacing is too wide and there are complications imposed by variation in fault strikes. Basement topography is a dominant factor in variation of sediment thickness in two ways. 1) Older structures contribute to sediment variations. For example, the abyssal hill imaged in line D19 (Figure 2) has thinner sediments above it than on its flanks (Figures 1 and 2). Interpreted petite spot volcanic fields found in the southeast of the survey (Figure 1) have little to no sediment cover owing to their likely young age, based on ages of nearby fields (Hirano et al. 2008). Similarly, sediment overlying the tops of seamounts is generally much thinner than on the seamounts’ flanks. 2) Basement-cutting normal faults influence sediment thickness. This may be caused by erosion of topographic highs by bottom currents or by mass wasting (Figure 6) and filling of topographic lows, such as along graben systems (Figure 5). Sediment thickness generally increases towards the trench but with fluctuations as large as approximately 400 m on individual lines. No regional trends in sediment thickness are found in other directions. However, the large grid spacing between regional lines will mask small-scale trends. Because the Japan Trench accretes a large portion of sediment at the toe (Figure 7; Kodaira et al. 2012; Nakamura et al. 2013), sediment variations of 600 m may still affect prism morphology because they are a large percentage of the total sediment thickness. However, due to the fluctuations in sediment thickness and lack of regional trends, prism morphology and growth in our survey area owing to the incoming sediments is difficult to constrain.
As a graben passes beneath the near-trench edge of the overriding plate, the tensile stress from the buried normal fault may increase. The stress field of a fault tip breaching the seafloor versus being buried differs drastically (e.g., Martel and Langley 2006). This increase in tensile stress may promote the décollement to step-down into the graben.
Subduction of horst-graben structures may be similar to seamount subduction. Coulomb wedge mechanics argues that the change in basement slopes due to subducting a seamount promote both overcritical and undercritical wedge deformation (Lallemand and Le Pichon 1987). For an incoming horst block, the increase in basement slope could promote undercritical wedge deformation and affected by shortening. For an incoming graben, the decrease in the basement slope could promote overcritical deformation within the prism, creating listric faulting or slumps into the graben. There are three morphologic differences between subducting seamounts and normal faults: their vertical scales, slopes, and associated sediment. Seamounts studied near active margins tend to be greater than 16-km-wide and more than 1.5 km above the seafloor with slopes less than 20° (Dominguez et al. 1998). They also have significantly less sediment cover than their surroundings. However, horst-graben structures entering the Japan Trench are much smaller, lower-relief features (average throw of approximately 122 m and fault spacing of approximately 2 km), have steeper flanks (approximately 50° to 80° fault dips) and have smaller variations in sediment thickness. The large broad change in both sediment type and thickness, may play an important role for seamount subduction as a mechanically distinctive zone, but the horst-graben lithology varies little. These differences may create a different upper plate deformation for a horst-graben than a seamount.3) Normal faults restricted to sedimentary units 1 and 2 (Figure 6) may become more active, with increasing offsets, as the plate moves closer to the trench. By breaking through the layered sediments, these sediment-restricted faults could act as a pathway for the décollement step-down from the top of the horst blocks into the adjacent graben.
As the décollement propagates seaward, either on top of the graben or within the graben sediments, the buried active basement-cutting normal fault will continue propagating upward, and the décollement will eventually be offset by the basement-cutting fault. These two intersecting faults may continue to offset the fault plane of the other if they remain active. The effects of active normal faults further landward, beneath the prism, are less easily resolved. However, such basement-cutting faults may continue to influence the décollement geometry, as OBS data reveal, they remain active under the prism (Obana et al. 2013).
Horst block subduction also affects the up-dip geometry of the décollement (Kimura et al. 2012) and potentially lateral décollement connectivity. Our regional lines image subducted horsts near the trench and show that the décollement ascends over the subducting horst along a fault ramp (Figure 5B). A seafloor bathymetric high above the ramp is an anticline caused by sediments being folded over the fault bend (Figure 9). Fault slip over this 20° dipping décollement fault ramp will create greater seafloor uplift than in sections of shallow dips. Furthermore, the sediment column overlying the décollement ramp shows no continuous reflections, thus indicates internal deformation (Figure 5). In the high-resolution lines, the décollement stops within a graben near the incoming horst block (Figure 7). Seafloor topography within this graben deepens to the south. This morphology change can be attributed to varying internal deformation and uplift along strike, in that the décollement propagates seaward to a horst where the prism topography steepens before creating a ramp over the horst. If décollement propagation does vary in horst-graben systems, subduction of intersecting or linked normal fault sections would create lateral décollement segments that are linked at depth.
Faults within the sediment column may play an additional role in décollement propagation. DSDP Site 436 (Figure 1C) is the only nearby site where sediments on the Japan Trench’s outer rise have been sampled; drilling there only penetrated to the chert layer (Shipboard Scientific Party 1980). At Site 436, a small layer of pelagic clay overlies the chert. In this deep water setting, the overall lithology is not expected to vary much laterally, allowing for a general correlation among our seismic lines. Ogawa et al. (1997) discovered cracks in the seafloor along the outer rise and proposed that a combination of gravitational slope instability and earthquake shaking for the cracks formation. These surface cracks may be from the early stages of sediment-restricted normal faulting at depth, imaged in the high-resolution survey (Figure 6). These sediment-restricted faults may have a basal sliding plane between the basement-chert, chert-clay boundaries, and/or within the upper unit, deforming the sediment along strata. The creation of these sliding surfaces before subduction may play a role during seismic rupture to the trench. The Japan Trench Fast Drilling Project (Integrated Ocean Drilling Program Expedition 343 and 343 T) drilled to the chert layer further landward (Figure 1) and found that the décollement is located within a thin band of pelagic clay (Chester et al. 2012), with localized deformation occurring within 5 m of this pelagic clay. Thus, this lithological unit may be a regional control on rupture to the trench (Chester et al. 2013), as the low coseismic shear stress and low coseismic friction of the clay may have allowed the 2011 Tohoku earthquake to rupture to the trench (Fulton et al. 2013; Ujiie et al. 2013). However, Expedition 343 drilled only above a horst block. Seismic results agree with the décollement’s location above the horst. However, within the graben, the décollement located higher in the sediment column (Figure 7). The pelagic clay within a graben may have a more dominant role in sediment sliding and sediment-restricted faulting (Figure 6) than for décollement propagation. Near-trench faults in sediment may play an important role in weakening the sediment before subduction.
The accretionary prism at the toe of the Japan Trench initially accumulates a major proportion of incoming sediment. Previous models indicated that all of the sediments within a graben were subducted, as the décollement propagated over the top of the entire graben sediment column (e.g., Hilde 1983). This would allow for significant sediment subduction. However, the décollement appears to initially scoop out the majority of sediments within a graben (Figure 7). Furthermore, both seismic and drilling (e.g., Chester et al. 2013) indicate that the décollement is near the base of the sediment column when located above a horst. These results reduce the amount of sediment available for subduction under the toe of the frontal prism. Variations in both the incoming sediment column thickness (Figure 1) and the amount of long-term sediment accretion cause potential difficulties in calculating the amount of sediment subducted (e.g., Clift and Vannucchi 2004). Additionally, instead of underthrust sediment-filled graben blocks being only vertically loaded, initial sediment accretion may affect overall sediment porosity due to the addition of both vertical and horizontal strains on the subducting sediment (e.g., Moore et al. 2011). This may affect the sediment dehydration further landward and, as a result, whether or not subduction erosion might occur at depth. Other water-rich sources may contribute more to the fluid budget including both the igneous basement and chert, where the dehydration range for siliceous sediments starts at approximately 40 km landward of the Japan Trench (Kimura et al. 2012). Sediment cracks and faults along with basement faults may provide necessary pathways for fluid transport and may play a vital role in hydration below the sediment column. These near-trench processes may influence different subduction processes occurring at depth at the Japan Trench subduction zone.
Although generally included in discussion of subducting topography (e.g., Bilek 2007), bending-related normal faults are typically not considered to directly deform the overriding plate at the toe of the landward trench slope. Our depth-migrated regional and high-resolution seismic lines that image the incoming Pacific Plate along the Japan Trench off Tohoku show trench-parallel faults that offset the igneous crust on the outer-rise seaward of the trench. As the plate moves landward, reactivation of pre-existing seafloor structures, due to either increasing bending stresses or interaction with trench-parallel faults, further deforms the plate. Throw analysis indicates more offset in the north than south of our survey area with continuing formation of faults towards the trench. Sediment thickness of the Pacific Plate is not uniform but fluctuates between 0 and 600 m with a general increase in thickness towards the trench. Recent sediment deposits are present in some normal fault footwalls and near-trench topographic lows. Furthermore, the sediment column contains densely spaced fault populations with only a few meters of offset. We expand on previous studies (e.g., Kodaira et al. 2012; Nakamura et al. 2013) to document up-dip décollement propagation over both horst and graben systems along the Japan Trench and found the deformation front located further landward during initial subduction of a horst block compared to graben subduction. We propose a relationship between the décollement seaward propagation and normal faults that reflect stresses and displacements at the tips of actively subducting active normal faults. Other margins where horst blocks are subducting, such as the Middle America Trench off Costa Rica (Moore et al. 1986; Moore and Shipley 1988) should also be considered for anomalous seafloor displacement during coseismic rupture to the trench axis. Furthermore, our results have additional implications for the hydration of the plate and upper mantle (Ranero et al. 2003; Garth and Rietbrock 2014) from both large- and small-scale faulting, prism development (Underwood 2007; Ike et al. 2008), and coseismic near-trench anomalous vertical displacement (e.g., Kodaira et al. 2012). Thus, bending-related normal faults may play an integral role in near-trench subduction zone processes.
Funding for this project was through the National Science Foundation (grant numbers OCE-1260718 and OCE-1138051). The authors are grateful to S. Martel, P. Fryer, and C. Conrad for their piercing comments that greatly improved the manuscript. We thank Paradigm Geophysical and Landmark Graphics (Halliburton) for their academic software licenses. The authors would like to thank the scientists and crews of KR11-05 Leg 2, KR11-E03, KR11-E05, and KY11-E05.
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