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A model of late Cenozoic transcurrent motion and deformation in the fore-arc of northeast Japan: Constraints from geophysical studies
Physics of the Earth and Planetary Interiors 156 (2006) 117–129
A model of late Cenozoic transcurrent motion and deformation in the fore-arc of northeast Japan: Constraints from geophysical studies Yasuto Itoh a,∗ , Tetsuro Tsuru b,1 a
Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Gakuencho 1-1, Sakai, Osaka 599-8531, Japan Japan Agency for Marine-Earth Science and Technology, Showa-machi 3173-25, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan
b
Received 19 October 2005; received in revised form 13 February 2006; accepted 17 February 2006
Abstract We use recently acquired seismic reflection data and reprocessed sections to interpret the structure of the northern part of the NE Japan forearc. We identify a NNW-SSE trending deformation zone bounded by large transcurrent faults (trends T1–T4 from west to east) on the upper continental slope. The lower slope is characterized by a frontal prism and reverse faults. Bathymetric data reveal several en echelon topographic bulges with NW-SE elongation aligned parallel to the fault trends. The bulges are interpreted as a dextral wrench zone that has developed since the Neogene, based on structural contours of two late Cenozoic seismic horizons. Clockwise rotation associated the wrenching is indicated by the paleomagnetism of Paleogene marine sediments obtained from a drill hole within the faulted zone close to trend T1. A previous seismic refraction study shows remarkable contrast in P-wave velocity across trend T2, indicating a considerable rearrangement of crust associated with transcurrent motion along this fault. We propose a paleogeographic reconstruction assuming >200 km southward (trenchward) lateral transportation along trend T2 and its northern extension. In our model, subduction erosion at the northern Japan Trench was enhanced by trenchward migration of the fore-arc sliver, but steadied following rearrangement of the sliver along the transect. The calculated variation in subduction erosion is consistent with subduction histories based on existing well data. © 2006 Elsevier B.V. All rights reserved. Keywords: Convergent margin; Transcurrent fault; Subduction erosion; Northeast Japan; Seismic interpretation; Paleomagnetism
1. Introduction The geological structure of the northern part of the NE Japan forearc (Fig. 1) is understood in terms of a late Cenozoic compressive stress regime associated with subduction of the Pacific Plate. This area has been the site of remarkable tectonic events such as the ∗
Corresponding author. Tel.: +81 72 254 9752; fax: +81 72 254 9752. E-mail addresses: [email protected] (Y. Itoh), [email protected] (T. Tsuru). 1 Tel.: +81 45 778 5964; fax: +81 45 778 5439. 0031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2006.02.009
Oligocene–Miocene opening of the Japan Sea back-arc basin (Otofuji et al., 1994; Jolivet et al., 1994) and subsequent subduction erosion along the Japan Trench (von Huene and Lallemand, 1990; Tsuru et al., 2002). This area provides an opportunity to investigate various tectonic process and features within a subduction zone via an integrated study of the fore-arc architecture. In the present study, we present a set of seismic reflection profiles of the NE Japan forearc. Together with biostratigraphic age determinations from borehole samples, some of which penetrated regional seismic horizons, we identify fault trends and discuss the deformation history of fault-bounded sedimentary basins of the fore-arc. The
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Fig. 1. Index map of the study area. The dashed lines and thin solid lines on the fore-arc are multi-channel seismic survey lines acquired by the Japan National Oil Corporation (JNOC) and the Japan Marine Science and Technology Center (JAMSTEC), respectively. Bold segments of these lines are shown in subsequent figures. Anticlines within recent fore-arc sediments, determined from an acoustic survey by the GSJ (Honza et al., 1978), and transcurrent structural trends around northeast Japan (Oide et al., 1989; Sasaki, 2003; Okamura et al., 1983; Itoh et al., 2000b) are also depicted on the map. Two topographic bulges (B1 and B2) are recognizable from bathymetric data and are visible on multi-channel seismic profiles.
validity of tectonic interpretations that arise from the structural interpretation is tested on the basis of expected rotational motions derived from paleomagnetic data and crust heterogeneity data sourced from a previous seismic refraction study. Considerable transportation of a forearc sliver is proposed to have occurred along one of the major transcurrent faults. 2. Background Northeast Japan is marked by large NNW-SSE transcurrent faults (Fig. 1; Oide et al., 1989; Sasaki, 2003). Based on geological evidence, these faults were activated with left-lateral motion during the Cretaceous (Sasaki, 2003; Otsuki, 1992). Near-shore extensions of such structures (e.g., the Lineament in Fig. 1 (Okamura et al., 1983)) have been identified from marine geological and geophysical surveys conducted by the Geological Survey of Japan (GSJ). Of these near-shore extensions, Itoh et al. (2000b) recognized significant displacement across a NNW-SSE structural trend (Off-
shore Trend A in Fig. 1) that coincides with a marked density contrast. The authors argued that the trend had been activated with left-lateral motion, based on seismic interpretation. In contrast, anticlines developed within recent sediments in the northern fore-arc (Fig. 1; Honza et al., 1978) are aligned subparallel to the trench axis, and some display an en echelon arrangement indicative of right-lateral wrench deformation. Thus, controversial transcurrent motion within the fore-arc region should be verified on the basis of offshore seismic data. 3. Seismic interpretation In this study, we utilized multi-channel seismic reflection data recently acquired by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), as well as reprocessed data originally acquired by the Japan National Oil Corporation (JNOC). The area of structural interpretation covers the region 39◦ 40 N–41◦ 20 N, 141◦ 40 E–144◦ 00 E.
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Fig. 2. E-W seismic profile of the NE Japan forearc. Time-migrated section (upper figure) is a composite of JNOC S52-2 and JAMSTEC HK203. See Fig. 1 for line locations. Transcurrent structural trends (T1–T4) and seismic horizons (A, B) are shown in the interpretation (lower figure).
3.1. Seismic horizons and structural trends We identified two seismic horizons: horizons A and B. The A horizon is an angular unconformity within the upper Paleogene, while the B-horizon represents a Neogene onlapping surface that can be traced throughout the study area (Fig. 2). The A and B are the same horizons as those defined by Itoh and Tsuru (2003, 2005). Here, the ages of the horizons were determined by correlation with well-log data from DSDP sites 438 and 439
that penetrated the Cenozoic sedimentary units (Nasu et al., 1980; DSDP Scientific Party, 1980) on line JNOC 1 (Figs. 1 and 3). Time-structural contours delineate NNW-SSE faults upon the shelf that developed during the late Cenozoic. These faults are referred to as trends T1–T4 (Itoh and Tsuru, 2003, 2005), from west to east (Fig. 2). The westernmost fault strand, T1, is the oldest, and is developed within a highly deformed and truncated Paleogene basin, as shown in a combined seismic profile (Fig. 2) and the
Fig. 3. Correlation of geological boundaries at DSDP sites 438 and 439 and seismic horizons on a crossing seismic profile of line JNOC 1 (DSDP Scientific Party, 1980). See Fig. 1 for line and drilling locations.
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Fig. 4. Time-structural contour maps (contour interval: 250 ms) of the A (a) and B (b) horizons, and an isopach map (contour interval: 200 ms) of a unit between the two horizons (c). Fault trends (T1–T4) are delineated on the maps, and structural bulges (B1, B2) are also indicated.
A horizon structural map (Fig. 4a). The Paleogene basin is in fault contact, across fault strand T2, with a Neogene basin that corresponds to the eastern half of the fore-arc shelf, as identified from the B-horizon contours (Fig. 4b) that indicate Neogene activity. 3.2. Wrench deformation of the fore-arc region Two fault-bounded bulges are aligned within the Neogene basin, marked as B1 and B2 in the A horizon contour map (Fig. 4a). The bulges consist of deformed Cenozoic sedimentary units, and the relief of the bulges is less prominent in the B-horizon contours (see Fig. 4b). The bulges began developing during the Miocene, although relevant changes in sedimentary thickness are not evident in the lower Miocene (Fig. 4c) that may reflect episodic tectonic disturbance associated
with coeval back-arc opening in northeast Japan (Otofuji et al., 1994). An along-arc seismic profile demonstrates that the B-horizon follows the geologic features of older units (Fig. 5); this indicates that deformation continued until relatively recently. The bulges correspond to en echelon anticlines developed within shallow sediments (Fig. 1), and are interpreted as a right-lateral wrench deformation zone in the fore-arc region. 4. Paleomagnetism To evaluate rotational motions related to the forearc wrenching proposed in the previous section, we undertook paleomagnetic measurements of core samples obtained from the MITI Sanriku-oki borehole (40◦ 40 8 N, 142◦ 17 19 E; Fig. 1). Although the core recovery rate for the exploration well was quite low, sam-
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Fig. 5. Time-migrated N-S seismic profile of the NE Japan forearc (JNOC S52-B; upper figure). Seismic horizons (A, B), structural bulges (B1, B2), and DSDP sites 438 and 439 are shown in the interpretation (lower figure). See Fig. 1 for line and drilling locations.
ples from great depths in the near-vertical well preserve unique information on the long-standing rotational history of the fore-arc region. Samples were collected from two horizons. The shallower core (Sample 3; 3101 m) consists primarily of Eocene mudstone, while the deeper sample, from Core 5 (4066 m), is a Late Cretaceous (late Campanian) siltstone. The shallower core was successfully oriented based on correlation between bedding planes on the core surface and side-wall imaging, whereas the deeper core was not initially oriented because bedding was horizontal. Cylindrical rock specimens of 25 mm in diameter and 22 mm in length were cut from the core samples. For all specimens, bulk magnetic susceptibility was measured using a Bartington susceptibility meter (MS-2). Natural remanent magnetization (NRM) was then measured using a 2-G Enterprise three-axis cryogenic magnetometer settled in a magnetically shielded laboratory at Kyoto University, Japan.
tization (IRM) in a forward field increasing from 10 to 3000 mT in 31 steps (Kruiver et al., 2001). Cores 3 and 5 have a sole contributing component with similar ranges of B1/2 (47–56 mT). This suggests that the major ferromagnetic mineral in the samples is magnetite with a moderate coercivity spectrum. Next, we performed thermal demagnetization of IRM for the same specimens. Based on the procedure proposed by Lowrie (1990), composite IRMs were imparted by applying direct magnetic fields of 3.0, 0.4, and then 0.12 T onto the specimens in three orthogonal directions. As shown in Fig. 6, sampled intervals are characterized by the dominant soft coercivity fraction with a minor contribution from the medium fraction, both of which have TUB spectra up to 580 ◦ C. We therefore conclude that the dominant magnetic mineral from Cores 3 (SRC3-05 in Fig. 6a) and 5 (SR-C5-10 in Fig. 6b) is Ti-poor titanomagnetite. 4.2. Demagnetization test
4.1. Magnetic mineralogy To identify the dominant coercivity components in the samples, representative specimens were subjected to the detailed acquisition of isothermal remanent magne-
We conducted a progressive thermal demagnetization (PThD) test on the core samples to isolate the stable component of remanent magnetization. The PThD test was performed in air using a non-inductively wound electric
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TUB component was determined as shown in Fig. 7(b3). As the TUB range (140–300 ◦ C) and intermediate mean inclination (50.8◦ ) are in good accordance with a TVRM acquisition of magnetite (Pullaiah et al., 1975; Middleton and Schmidt, 1982) in recent geologic periods, the low TUB component is assumed to have an in situ northerly declination. Therefore, the high TUB component, as the intersection of remagnetization circles, is restored as shown in Fig. 7(b4). The oriented remanence direction does not show a significant deflection in declination. 4.4. Rotational history of the fore-arc of northeast Japan
Fig. 6. Thermal demagnetization curves of orthogonal IRMs for Cores 3 (a: Eocene) and 5 (b: Late Cretaceous) from the MITI Sanriku-oki borehole.
furnace with an internal residual magnetic field of less than 10 nT up to 600 ◦ C, at which temperature most of the NRM was lost. Results of this test are presented in Fig. 7 (a1) and Fig. 7 (b1) for Core 3 and 5 samples, respectively. The results are characterized by overlapping TUB spectra of primary and secondary components. A remagnetization circle-fitting method (McFadden and McElhinny, 1988) was then adopted to determine the directions of characteristic remanent magnetization (ChRM). The untilted ChRM direction of Core 3 is shown in Fig. 7 (a2), along with statistical parameters (precision parameter, α95 ). 4.3. Correction of NRM components of the Cretaceous sample Late Cretaceous siltstone from Core 5 showed a linear demagnetization trend before PThD treatmentinduced acquisition of unstable and erratic remanence (Fig. 7(b2)). Applying principal component analysis (Kirschvink, 1980) for ChRM calculations, a normal low
The high TUB components of the Eocene and late Cretaceous samples are regarded as primary remanent magnetization carried by magnetite with a unimodal coercivity spectrum. Fig. 8 (right-hand side) provides a summary of the rotational history around the MITI Sanriku-oki borehole. Significant deflection in the declination (D = 42.0◦ , D = 10.3◦ ) of Core 3 requires a clockwise rotation since the Eocene. This rotation may be linked to dextral faulting in northeast Japan related to the Neogene opening of the Japan Sea (Yamaji et al., 1999). Restoring post-Eocene rotation, the mean declination of Core 5 has a significant westerly deflection (D = −47.1◦ , D = 6.2◦ ). Late Cretaceous paleomagnetic data for Benxi, northeastern China (Lin et al., 2003), is adopted here as a reference because the primary nature of NRM has been confirmed for the data set. The amount of counterclockwise rotation and associated uncertainty, as defined by Beck (1980), is R = −87.2◦ and R = 15.2◦ , respectively, pre-dating Eocene time. Paleomagnetic data for the fore-arc shelf of northeast Japan are minimal; consequently, we don’t propose a regional tectonic model of the rotational events. We do recognize, however, that a similar rotation pattern of paleomagnetic data was found for the onshore area of the island arc. Itoh et al. (2000a) analyzed multicomponent remanent magnetization residing within a Cretaceous granitic intrusion at the Kamaishi Mine (Fig. 1), and argued that the intrusive rock underwent syn-cooling counterclockwise rotation (−116◦ ) and subsequent clockwise rotation (50◦ ) (Fig. 8, left-hand side). Although a lack of information on the structural geometry of the intrusion and poor age constraints on the later rotation phase make it difficult to fully characterize the rotational events, it appears that the fore-arc region of northeast Japan has been a site of active transcurrent deformation, as suggested by seismic interpretation in the present study.
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Fig. 7. Results of progressive thermal demagnetization (PThD) for Cores 3 (a: Eocene) and 5 (b: Late Cretaceous) from the MITI Sanriku-oki borehole. The unit used in the vector-demagnetization diagram is bulk remanent intensity. Solid and open symbols are projections of vector endpoints on the horizontal and N-S vertical planes, respectively. For the equal-area projections, solid and open circles are plotted on the lower and upper hemispheres, respectively. Numbers attached on the symbols are levels of PThD in degrees Celsius. Dotted ovals represent the 95% confidence limit of mean paleomagnetic directions. See text for the procedure used to determine the orientation of Core 5.
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Fig. 8. Rotational history of the fore-arc region of northeast Japan inferred from paleomagnetic studies, and related tectonic regimes.
5. Discussion 5.1. Tectonic implications of the geophysical data The nature of the fore-arc upper crust can be estimated from geophysical properties. The data in Fig. 9 show that the structural bulges described in Section 3 are not accompanied by a positive gravity anomaly. Therefore, they are not related to the subduction of high-density asperities such as sea mounts. It should also be noted that a clear discontinuity in the Bouguer anomaly is located at trend T2. A contrast in seismic velocity within the fore-arc crust is confirmed across the trend T2 where it divides the Paleogene and Neogene basins (see Fig. 10; Hayakawa et al., 2002). This implies a considerable rearrangement of crustal blocks associated with movement across the fault. A N-S positive geomagnetic anomaly known as the Kitakami Magnetic Belt (KMB; Finn, 1994) developed solely to the west of trend T1. The anomaly is caused by westerly-magnetized Cretaceous granitic intrusions (Itoh et al., 2000b), whose rock-magnetic properties were investigated in the Kamaishi Mine (Fig. 1; Itoh et al., 2000a). A further circular-shaped geomagnetic anomaly, referred to as the Eastern Magnetic Belt (EMB), exists between trends T2 and T4 (Finn, 1994; Fig. 10), and is distinct from the KMB. Models for the
Fig. 9. Bouguer anomaly (colour scale) for the NE Japan forearc. Fault trends (T1–T4) and structural bulges (B1, B2) are also shown on the map. Aligned orange dots are OBS stations (Hayakawa et al., 2002) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).
origin of the EMB have been proposed (Finn, 1994), including a buried granitic pluton and serpentinite that extends from central Hokkaido. The existence of serpentinized peridotite would result in low levels of seismicity due to weak interplate coupling (Kamiya and Kobayashi, 2000), but the area around the EMB is characterized by intensive interplate seismic activity (Hino et al., 2000; Hayakawa et al., 2002). In this case, therefore, the geomagnetic anomaly is considered to represent Cretaceous granite that is widely distributed on the fore-arc side of northeast Japan (Itoh et al., 2000a,b). 5.2. Reconstruction of fore-arc components Seismic interpretations reveal the existence of several NNW-SSE transcurrent faults in the NE Japan forearc. In making a paleogeographic reconstruction of the study area, we assumed that trend T2 accommodated the largest displacement of the four trends because it is accompanied by a clear discontinuity in seismic velocity. The tectonic model presented in Fig. 11 suggests
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Fig. 10. Geomagnetic anomalies and transcurrent faults (left-hand figure) and velocity structure (right-hand figure) of the NE Japan forearc. There are two evident geomagnetic anomalies, the KMB (Kitakami Magnetic Belt) and EMB (Eastern Magnetic Belt; after Finn, 1994) that have a N-S or NNW-SSE trend. The time-structural contour of the B-horizon (Fig. 4b; contour interval = 250 ms) is overlain on the geomagnetic anomaly map, showing that the faults have the same strike as the geomagnetic anomalies. The velocity structure (right-hand figure) was obtained from an existing seismic refraction experiment (Hayakawa et al., 2002; aligned orange dots in the left-hand figure are OBS stations from their study), strongly indicating spatial variations in tectonic structure. The southern part, located landward of the fault trend T2, represents the general fore-arc crustal structure of the northern Japan Trench, while the northern part, seaward of T2, indicates considerably thickened island arc crust (Hayakawa et al., 2002) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).
that central Hokkaido was a right-lateral shear zone during the late Cenozoic, based on paleomagnetic rotations (Takeuchi et al., 1999), and that the western margin of the rotated blocks (indicated by a dashed line in the figure), where right-lateral shear occurred until the Middle Miocene (Jolivet and Huchon, 1989), was continuous with trend T2. Trend T2 may have been part of a larger tectonic boundary that extended to the western coast of Sakhalin (Jolivet and Tamaki, 1992). The eastern termination of the lateral motion was the Hidaka Main Thrust, around which Late Oligocene dextral faulting has been confirmed (Kusunoki and Kimura, 1998). A zone between the boundary faults as wide as 100 km, which corresponds to the “Cretaceous basin” in Fig. 11 (right-hand side), underwent distributed shear. Formation of the N-S shear zone may be related to the Oligocene–Miocene opening of the Kuril Basin (Kimura, 1994). In the Oligocene reconstruction, a crustal sliver between the boundary faults (1 and 2 in Fig. 11, right-
hand side) at the southern extension of present-day central Hokkaido was restored so that the EMB and KMB form a line constituting a geomagnetic anomaly belt related to Cretaceous volcano-plutonic activity. This reconstruction requires more than 200 km of transportation of the fore-arc, which is concordant with a previous tectonic model of eastern Hokkaido (Jolivet et al., 1994). We note that DSDP sites 438 and 439 were located near western Hokkaido prior to the transcurrent motion. We can therefore use the DSDP data to verify the similarity of those paleoenvironments around the sites and in our reconstruction. DSDP site 439 contains boulders of dacite with Late Oligocene radiometric ages (Moore and Fujioka, 1980). These boulders are considered to have originated from in situ arc volcanism, as the site is located on the northern extension of a volcanic front, as inferred from volcanism in northeast Japan between 23 and 15 Ma (Tatsumi et al., 1989) and volcanism around the northernmost part of the KMB from the late Oligocene to the early
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Fig. 11. Paleogeographic reconstruction of the fore-arc region of northeast Japan. The Oligocene volcanic front is inferred from volcanism in northeast Japan during the period 23–15 Ma (Tatsumi et al., 1989) and volcanism around the northernmost part of the KMB from the Late Oligocene to the Early Miocene (Kurita and Yokoi, 2000). The KMB and EMB are remarkable geomagnetic anomaly trends (Finn, 1994).
Miocene (Kurita and Yokoi, 2000). An upper Oligocene marine sedimentary unit that overlies the volcanic conglomerate at site 439 is characterized by an abundant influx of Cretaceous pollen (Sato, 1980). Such contamination ceases at the beginning of the Miocene, which may be related to the submergence of voluminous Cretaceous sedimentary terranes in central Hokkaido (Fig. 11; Ishiwada et al., 1992) as a result of active faulting in the distributed shear zone and the formation of pullapart basins (Kurita and Yokoi, 2000; Itoh and Tsuru, 2005). 5.3. Temporal variations in tectonic erosion In previous studies, DSDP sites 438 and 439 were fixed to the east of northeast Japan, and paleoenvironmental changes recorded within sediments from the sites were attributed to subduction erosion of the fore-arc region. von Huene et al. (1994) estimated the rate of subduction erosion assuming that the Pacific Plate has been steadily subducting underneath northeast Japan for the past 20 m.y.
In contrast, our model involves more than 200 km of south-southeastward migration of the fore-arc segment. This migration would have increased the rate of convergence between the oceanic and continental plates and accelerated basal erosion at the plate boundary. Furthermore, material at the tip of the continental plate that previously filled the trench would have been transported into the deep Earth interior together with the subducting oceanic crust via frontal erosion, which would have accelerated with the increased migration rate. The rate of subduction erosion would have changed not only with the subduction rate of the oceanic plate but also the migration rate of the fore-arc segment. Here we investigate subsidence rates of the forearc region, as estimated from existing borehole data. Temporal changes in the basal erosion rate would have affected the subsidence rate. Fig. 12 presents subsidence histories from DSDP and industry wells (von Huene and Lallemand, 1990; Itoh et al., 2000b; Ishiwada et al., 1992; Osawa et al., 2002; Japan National Oil Corporation, 1999), indicating clear changes in the subsidence rate between the periods Oligocene – 10 and 10 Ma
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Huchon, 1989). Therefore, subsidence since 10 Ma has been related to the steady subduction of the Pacific Plate. Finally, the subsidence rates recorded at drilling sites increase eastward (trenchward), which may indicate that the studied transect has been actively eroded beneath the upper slope region since the time that the fore-arc components were rearranged. 6. Summary
Fig. 12. Subsidence curves for the base of the Cenozoic basin at drilling sites on the NE Japan forearc (von Huene and Lallemand, 1990; Itoh et al., 2000b; Ishiwada et al., 1992; Osawa et al., 2002; Japan National Oil Corporation, 1999). Shaded areas represent regions of steady subduction erosion following massive rearrangement of the fore-arc components. See Fig. 11 for drilling locations.
– present. Although a quantitative estimate of the erosion rate is beyond the scope of the present study, the subsidence curves demonstrate that the erosion rate during the period Oligocene – 10 Ma was considerably higher than that since 10 Ma. The observed difference in erosion rate is consistent with our segment-transportation model. In addition, geomagnetic data for the fore-arc region do not favor the fixed model, as there is no indication of intrusive bodies to the west of the Oligocene volcanics at DSDP 439. Such intrusives should exist if the trench axis has been retreating westward as a result of steady erosion. From the Oligocene to the Early Miocene, the fore-arc sliver was inevitably transported toward the trench by trenchward motion and coeval spreading of the Japan Sea, as inferred previously (von Huene et al., 1994). During this period, intensive tectonic erosion would have generated rapid subsidence of the fore-arc region, as shown in Fig. 12. In Fig. 12, the four sites other than the DSDP sites belong to the same zone (west of the trend T1; Fig. 11), and basin formation is considered to have continued during the Paleogene, although the pattern of vertical motions differs considerably between sites. Especially rapid subsidence is evident in DSDP sites 438 and 439 since the initiation of transcurrent motion in the forearc region. Our tectonic model indicates that these sites were originally located around present-day Hokkaido; thus, their initial subsidence may be linked to deformation of the migrating crustal sliver. The geological structure of Hokkaido suggests that the transcurrent regime had diminished by the Late Miocene (Jolivet and
Seismic interpretation of the NE Japan forearc indicates a NNW-SSE trending deformation zone bounded by prominent transcurrent faults (trends T1–T4). Rightlateral motions and related deformation modes have been dominant since the Miocene, despite the E-W compressive stress regime caused by the motion of the Pacific Plate. The occurrence of fore-arc wrenching is supported by clockwise rotation in the late Cenozoic inferred from the paleomagnetism of core samples recovered from a borehole within the study area. Of the four transcurrent faults, trend T2 is the largest, and is accompanied by a discontinuity in seismic velocity. A paleogeographic reconstruction of the region requires more than 200 km of southward migration of the fore-arc sliver, which is consistent with paleoenvironmental changes reported from drilling surveys. The large-scale transcurrent motion of the fore-arc sliver would have affected the rate of subduction erosion in the northern part of the Japan Trench fore-arc region. Acknowledgements The authors thank METI (Ministry of Economy, Trade and Industry, Japan) and JOGMEC (Japan Oil, Gas and Metals National Corporation) for permission to publish this work. We also acknowledge: M.K. Thu, T. Hashimoto, T. Murayama (JAMSTEC) and Y. Nagasaki (JOGMEC) for their assistance during the seismic interpretation; N. Ishikawa for the use of paleomagnetic laboratory at Kyoto University. Comments by reviewers greatly helped to improve early version of the manuscript. References Beck Jr., M.E., 1980. Paleomagnetic record of plate-margin tectonic processes along the western edge of North America. J. Geophys. Res. 85, 7115–7131. DSDP Scientific Party, 1980. Initial Reports of the Deep Sea Drilling Project, Legs 56 and 57. U.S. Government Printing Office, Washington, DC (Attachment). Finn, C., 1994. Aeromagnetic evidence for a buried Early Cretaceous magmatic arc, northeast Japan. J. Geophys. Res. 99, 22165–22185.
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Hayakawa, T., Kasahara, J., Hino, R., Sato, T., Shinohara, M., Kamimura, A., Nishino, M., Sato, T., Kanazawa, T., 2002. Heterogeneous structure across the source regions of the 1968 TokachiOki and the 1994 Sanriku-Haruka-Oki earthquakes at the Japan Trench revealed by an ocean bottom seismic survey. Phys. Earth Planet. Int. 132, 89–104. Hino, R., Ito, S., Shiobara, H., Shimamura, H., Sato, T., Kanazawa, T., Kasahara, J., Hasegawa, A., 2000. Afrershock distribution of the 1994 Sanriku-oki earthquake (Mw 7.7) revealed by ocean bottom seismographic observation. J. Geophys. Res. 105, 21697–21710. Honza, E., Tamaki, K., Murakami, F., 1978. Geological Map of the Japan and Kuril Trenches and the Adjacent Areas 1:1,000,000. Geological Survey of Japan, Tsukuba. Ishiwada, Y., Aida, H., Atake, M., Araki, N., Iijima, A., Ikeda, A., Okuda, Y., Kikuchi, Y., Kojima, K., Saito, T., Sato, Y., Tanaka, S., Tono, S., Hirayama, J., Honza, E., Miyazaki, H., Morishima, H., Harada, Y., 1992. Oil and Natural Gas Resources in Japan. Natural Gas Exploration Association, Tokyo, 520 pp. Itoh, Y., Amano, K., Danhara, T., 2000a. Paleomagnetism, rock magnetism, and fission track dating of the Kurihashi Granodiorite in northeast Japan: evidence for a Cretaceous hydrothermal event and differential rotation along the eastern Eurasian margin. J. Geophys. Res. 105, 13519–13532. Itoh, Y., Nagasaki, Y., Matsuda, T., 2000b. Fore-arc transcurrent faults in northeast Japan: hybrid geophysical models of an island-arc upper crust. Tectonophysics 327, 311–333. Itoh, Y., Tsuru, T., 2003. Transcurrent motions and deformation on the fore-arc of northeast Japan. In: Abstracts of, XXIII, General Assembly of the International Union of Geodesy Geophysics, Secretariat General of IUGG 2003, Yokosuka, p. B472. Itoh, Y., Tsuru, T., 2005. Evolution history of the Hidaka-oki (offshore Hidaka) basin in the southern central Hokkaido, as revealed by seismic interpretation, and related tectonic events in an adjacent collision zone. Phys. Earth Planet. Int. 153, 220–226. Japan National Oil Corporation (JNOC), 1999. Report on the Geological Study of MITI Sanriku-oki Borehole. Japan National Oil Corporation, Tokyo. Jolivet, L., Huchon, P., 1989. Crustal-scale strike-slip deformation in Hokkaido, northern Japan. J. Struct. Geol. 11, 509–522. Jolivet, L., Tamaki, K., 1992. Neogene kinematics in the Japan Sea region and volcanic activity of the northeast Japan arc. In: Tamaki, K., Suyehiro, K., Allan, J., McWilliams, M., et al. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 127/128, part 2. Ocean Drilling Program, College Station, TX, pp. 1311–1331. Jolivet, L., Tamaki, K., Fournier, M., 1994. Japan Sea, opening history and mechanism: a synthesis. J. Geophys. Res. 99, 22237–22259. Kamiya, S., Kobayashi, Y., 2000. Seismological evidence for the existence of serpentinized wedge mantle. Geophys. Res. Lett. 27, 819–822. Kimura, G., 1994. The latest Cretaceous–early Paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern Pacific margin. J. Geophys. Res. 99, 22147–22164. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. R. Astr. Soc. 62, 699–718. Kruiver, P.P., Dekkers, M.J., Heslop, D., 2001. Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation. Earth Planet. Sci. Lett. 189, 269–276.
Kurita, H., Yokoi, S., 2000. Cenozoic tectonic settings and a current exploration concept in southern central Hokkaido, northern Japan. J. Jpn. Assoc. Pet. Technol. 65, 58–70. Kusunoki, K., Kimura, G., 1998. Collision and extrusion at the KurilJapan arc junction. Tectonics 17, 843–858. Lin, W., Chen, Y., Faure, M., Wang, Q., 2003. Tectonic implications of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China. J. Geophys. Res. 108 (B6), 2313, doi:10.1029/2002JB002169. Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophys. Res. Lett. 17, 159–162. McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetization circles and direct observations in palaeomagnetism. Earth Planet. Sci. Lett. 87, 161–172. Middleton, M.F., Schmidt, P.W., 1982. Paleothermometry of the Sydney Basin. J. Geophys. Res. 87, 5351–5359. Moore, G.W., Fujioka, K., 1980. Age and origin of dacite boulder conglomerate anomalously near the Japan Trench. In: Scientific Party (Ed.), Initial Reports of the Deep Sea Drilling Project, 56, 57 (part 2). U.S. Government Printing Office, Washington, DC, pp. 1083–1088. Nasu, N., von Huene, R., Ishiwada, Y., Langseth, M., Bruns, T., Honza, E., 1980. Interpretation of multichannel seismic reflection data, Legs 56 and 57, Japan Trench transect, Deep Sea Drilling Project. In: Scientific Party (Ed.), Initial Reports of the Deep Sea Drilling Project, 56, 57 (Part 1). U.S. Government Printing Office, Washington, DC, pp. 489–503. Oide, K., Nakagawa, H., Kanisawa, S., 1989. Regional Geology of Japan, part 2 Tohoku. Kyoritsu Shuppan, Tokyo, 338 pp. Okamura, Y., Nishimura, K., Tanahashi, M., 1983. Explanatory Notes of Geological Map off Kamaishi 1:200,000. Geological Survey of Japan, Tsukuba, 25 pp. Osawa, M., Nakanishi, S., Tanahashi, M., Oda, H., 2002. Structure, tectonic evolution and gas exploration potential of offshore Sanriku and Hidaka provinces, Pacific Ocean, off northern Honshu and Hokkaido, Japan. J. Jpn. Assoc. Pet. Technol. 67, 38–51. Otofuji, Y., Kambara, A., Matsuda, T., Nohda, S., 1994. Counterclockwise rotation of Northeast Japan: Paleomagnetic evidence for regional extent and timing of rotation. Earth Planet. Sci. Lett. 121, 503–518. Otsuki, K., 1992. Oblique subduction, collision of microcontinents and subduction of oceanic ridge: their implications on the Cretaceous tectonics of Japan. The Island Arc 1, 51–63. Pullaiah, G., Irving, E., Buchan, K.L., Dunlop, D.J., 1975. Magnetization changes caused by burial and uplift. Earth Planet. Sci. Lett. 28, 133–143. Sasaki, M., 2003. Early Cretaceous sinistral shearing and associated folding in the South Kitakami Belt, northeast Japan. The Island Arc 12, 92–109. Sato, S., 1980. Palynological study of sediments in Holes 439 and 438A. In: Scientific Party (Ed.), Initial Reports of the Deep Sea Drilling Project 56, 57 (part 2). U.S. Government Printing Office, Washington, DC, pp. 895–901. Takeuchi, T., Kodama, K., Ozawa, T., 1999. Paleomagnetic evidence for block rotations in central Hokkaido-south Sakhalin, Northeast Asia. Earth Planet. Sci. Lett. 169, 7–21. Tatsumi, Y., Otofuji, Y., Matsuda, T., Nohda, S., 1989. Opening of the sea of Japan back-arc basin by asthenospheric injection. Tectonophysics 166, 317–329. Tsuru, T., Park, J.-O., Miura, S., Kodaira, S., Kido, Y., Hayashi, T., 2002. Along-arc structural variation of the plate boundary at the
Y. Itoh, T. Tsuru / Physics of the Earth and Planetary Interiors 156 (2006) 117–129 Japan Trench margin: implication of interplate coupling. J. Geophys. Res. 107, doi:10.1029/2001JB001664. von Huene, R., Klaeschen, D., Cropp, B., Miller, J., 1994. Tectonic structure across the accretionary and erosional parts of the Japan Trench margin. J. Geophys. Res. 99, 22349–22361.
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von Huene, R., Lallemand, S., 1990. Tectonic erosion along the Japan and Peru convergent margins. Geol. Soc. Am. Bull. 102, 704–720. Yamaji, A., Momose, H., Torii, M., 1999. Paleomagnetic evidence for Miocene transtensional deformations at the eastern margin of the Japan Sea. Earth Planets Space 51, 81–92.
A model of late Cenozoic transcurrent motion and deformation in the fore-arc of northeast Japan: Constraints from geophysical studies Yasuto Itoh a,∗ , Tetsuro Tsuru b,1 a
Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Gakuencho 1-1, Sakai, Osaka 599-8531, Japan Japan Agency for Marine-Earth Science and Technology, Showa-machi 3173-25, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan
b
Received 19 October 2005; received in revised form 13 February 2006; accepted 17 February 2006
Abstract We use recently acquired seismic reflection data and reprocessed sections to interpret the structure of the northern part of the NE Japan forearc. We identify a NNW-SSE trending deformation zone bounded by large transcurrent faults (trends T1–T4 from west to east) on the upper continental slope. The lower slope is characterized by a frontal prism and reverse faults. Bathymetric data reveal several en echelon topographic bulges with NW-SE elongation aligned parallel to the fault trends. The bulges are interpreted as a dextral wrench zone that has developed since the Neogene, based on structural contours of two late Cenozoic seismic horizons. Clockwise rotation associated the wrenching is indicated by the paleomagnetism of Paleogene marine sediments obtained from a drill hole within the faulted zone close to trend T1. A previous seismic refraction study shows remarkable contrast in P-wave velocity across trend T2, indicating a considerable rearrangement of crust associated with transcurrent motion along this fault. We propose a paleogeographic reconstruction assuming >200 km southward (trenchward) lateral transportation along trend T2 and its northern extension. In our model, subduction erosion at the northern Japan Trench was enhanced by trenchward migration of the fore-arc sliver, but steadied following rearrangement of the sliver along the transect. The calculated variation in subduction erosion is consistent with subduction histories based on existing well data. © 2006 Elsevier B.V. All rights reserved. Keywords: Convergent margin; Transcurrent fault; Subduction erosion; Northeast Japan; Seismic interpretation; Paleomagnetism
1. Introduction The geological structure of the northern part of the NE Japan forearc (Fig. 1) is understood in terms of a late Cenozoic compressive stress regime associated with subduction of the Pacific Plate. This area has been the site of remarkable tectonic events such as the ∗
Corresponding author. Tel.: +81 72 254 9752; fax: +81 72 254 9752. E-mail addresses: [email protected] (Y. Itoh), [email protected] (T. Tsuru). 1 Tel.: +81 45 778 5964; fax: +81 45 778 5439. 0031-9201/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2006.02.009
Oligocene–Miocene opening of the Japan Sea back-arc basin (Otofuji et al., 1994; Jolivet et al., 1994) and subsequent subduction erosion along the Japan Trench (von Huene and Lallemand, 1990; Tsuru et al., 2002). This area provides an opportunity to investigate various tectonic process and features within a subduction zone via an integrated study of the fore-arc architecture. In the present study, we present a set of seismic reflection profiles of the NE Japan forearc. Together with biostratigraphic age determinations from borehole samples, some of which penetrated regional seismic horizons, we identify fault trends and discuss the deformation history of fault-bounded sedimentary basins of the fore-arc. The
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Fig. 1. Index map of the study area. The dashed lines and thin solid lines on the fore-arc are multi-channel seismic survey lines acquired by the Japan National Oil Corporation (JNOC) and the Japan Marine Science and Technology Center (JAMSTEC), respectively. Bold segments of these lines are shown in subsequent figures. Anticlines within recent fore-arc sediments, determined from an acoustic survey by the GSJ (Honza et al., 1978), and transcurrent structural trends around northeast Japan (Oide et al., 1989; Sasaki, 2003; Okamura et al., 1983; Itoh et al., 2000b) are also depicted on the map. Two topographic bulges (B1 and B2) are recognizable from bathymetric data and are visible on multi-channel seismic profiles.
validity of tectonic interpretations that arise from the structural interpretation is tested on the basis of expected rotational motions derived from paleomagnetic data and crust heterogeneity data sourced from a previous seismic refraction study. Considerable transportation of a forearc sliver is proposed to have occurred along one of the major transcurrent faults. 2. Background Northeast Japan is marked by large NNW-SSE transcurrent faults (Fig. 1; Oide et al., 1989; Sasaki, 2003). Based on geological evidence, these faults were activated with left-lateral motion during the Cretaceous (Sasaki, 2003; Otsuki, 1992). Near-shore extensions of such structures (e.g., the Lineament in Fig. 1 (Okamura et al., 1983)) have been identified from marine geological and geophysical surveys conducted by the Geological Survey of Japan (GSJ). Of these near-shore extensions, Itoh et al. (2000b) recognized significant displacement across a NNW-SSE structural trend (Off-
shore Trend A in Fig. 1) that coincides with a marked density contrast. The authors argued that the trend had been activated with left-lateral motion, based on seismic interpretation. In contrast, anticlines developed within recent sediments in the northern fore-arc (Fig. 1; Honza et al., 1978) are aligned subparallel to the trench axis, and some display an en echelon arrangement indicative of right-lateral wrench deformation. Thus, controversial transcurrent motion within the fore-arc region should be verified on the basis of offshore seismic data. 3. Seismic interpretation In this study, we utilized multi-channel seismic reflection data recently acquired by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), as well as reprocessed data originally acquired by the Japan National Oil Corporation (JNOC). The area of structural interpretation covers the region 39◦ 40 N–41◦ 20 N, 141◦ 40 E–144◦ 00 E.
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Fig. 2. E-W seismic profile of the NE Japan forearc. Time-migrated section (upper figure) is a composite of JNOC S52-2 and JAMSTEC HK203. See Fig. 1 for line locations. Transcurrent structural trends (T1–T4) and seismic horizons (A, B) are shown in the interpretation (lower figure).
3.1. Seismic horizons and structural trends We identified two seismic horizons: horizons A and B. The A horizon is an angular unconformity within the upper Paleogene, while the B-horizon represents a Neogene onlapping surface that can be traced throughout the study area (Fig. 2). The A and B are the same horizons as those defined by Itoh and Tsuru (2003, 2005). Here, the ages of the horizons were determined by correlation with well-log data from DSDP sites 438 and 439
that penetrated the Cenozoic sedimentary units (Nasu et al., 1980; DSDP Scientific Party, 1980) on line JNOC 1 (Figs. 1 and 3). Time-structural contours delineate NNW-SSE faults upon the shelf that developed during the late Cenozoic. These faults are referred to as trends T1–T4 (Itoh and Tsuru, 2003, 2005), from west to east (Fig. 2). The westernmost fault strand, T1, is the oldest, and is developed within a highly deformed and truncated Paleogene basin, as shown in a combined seismic profile (Fig. 2) and the
Fig. 3. Correlation of geological boundaries at DSDP sites 438 and 439 and seismic horizons on a crossing seismic profile of line JNOC 1 (DSDP Scientific Party, 1980). See Fig. 1 for line and drilling locations.
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Fig. 4. Time-structural contour maps (contour interval: 250 ms) of the A (a) and B (b) horizons, and an isopach map (contour interval: 200 ms) of a unit between the two horizons (c). Fault trends (T1–T4) are delineated on the maps, and structural bulges (B1, B2) are also indicated.
A horizon structural map (Fig. 4a). The Paleogene basin is in fault contact, across fault strand T2, with a Neogene basin that corresponds to the eastern half of the fore-arc shelf, as identified from the B-horizon contours (Fig. 4b) that indicate Neogene activity. 3.2. Wrench deformation of the fore-arc region Two fault-bounded bulges are aligned within the Neogene basin, marked as B1 and B2 in the A horizon contour map (Fig. 4a). The bulges consist of deformed Cenozoic sedimentary units, and the relief of the bulges is less prominent in the B-horizon contours (see Fig. 4b). The bulges began developing during the Miocene, although relevant changes in sedimentary thickness are not evident in the lower Miocene (Fig. 4c) that may reflect episodic tectonic disturbance associated
with coeval back-arc opening in northeast Japan (Otofuji et al., 1994). An along-arc seismic profile demonstrates that the B-horizon follows the geologic features of older units (Fig. 5); this indicates that deformation continued until relatively recently. The bulges correspond to en echelon anticlines developed within shallow sediments (Fig. 1), and are interpreted as a right-lateral wrench deformation zone in the fore-arc region. 4. Paleomagnetism To evaluate rotational motions related to the forearc wrenching proposed in the previous section, we undertook paleomagnetic measurements of core samples obtained from the MITI Sanriku-oki borehole (40◦ 40 8 N, 142◦ 17 19 E; Fig. 1). Although the core recovery rate for the exploration well was quite low, sam-
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Fig. 5. Time-migrated N-S seismic profile of the NE Japan forearc (JNOC S52-B; upper figure). Seismic horizons (A, B), structural bulges (B1, B2), and DSDP sites 438 and 439 are shown in the interpretation (lower figure). See Fig. 1 for line and drilling locations.
ples from great depths in the near-vertical well preserve unique information on the long-standing rotational history of the fore-arc region. Samples were collected from two horizons. The shallower core (Sample 3; 3101 m) consists primarily of Eocene mudstone, while the deeper sample, from Core 5 (4066 m), is a Late Cretaceous (late Campanian) siltstone. The shallower core was successfully oriented based on correlation between bedding planes on the core surface and side-wall imaging, whereas the deeper core was not initially oriented because bedding was horizontal. Cylindrical rock specimens of 25 mm in diameter and 22 mm in length were cut from the core samples. For all specimens, bulk magnetic susceptibility was measured using a Bartington susceptibility meter (MS-2). Natural remanent magnetization (NRM) was then measured using a 2-G Enterprise three-axis cryogenic magnetometer settled in a magnetically shielded laboratory at Kyoto University, Japan.
tization (IRM) in a forward field increasing from 10 to 3000 mT in 31 steps (Kruiver et al., 2001). Cores 3 and 5 have a sole contributing component with similar ranges of B1/2 (47–56 mT). This suggests that the major ferromagnetic mineral in the samples is magnetite with a moderate coercivity spectrum. Next, we performed thermal demagnetization of IRM for the same specimens. Based on the procedure proposed by Lowrie (1990), composite IRMs were imparted by applying direct magnetic fields of 3.0, 0.4, and then 0.12 T onto the specimens in three orthogonal directions. As shown in Fig. 6, sampled intervals are characterized by the dominant soft coercivity fraction with a minor contribution from the medium fraction, both of which have TUB spectra up to 580 ◦ C. We therefore conclude that the dominant magnetic mineral from Cores 3 (SRC3-05 in Fig. 6a) and 5 (SR-C5-10 in Fig. 6b) is Ti-poor titanomagnetite. 4.2. Demagnetization test
4.1. Magnetic mineralogy To identify the dominant coercivity components in the samples, representative specimens were subjected to the detailed acquisition of isothermal remanent magne-
We conducted a progressive thermal demagnetization (PThD) test on the core samples to isolate the stable component of remanent magnetization. The PThD test was performed in air using a non-inductively wound electric
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TUB component was determined as shown in Fig. 7(b3). As the TUB range (140–300 ◦ C) and intermediate mean inclination (50.8◦ ) are in good accordance with a TVRM acquisition of magnetite (Pullaiah et al., 1975; Middleton and Schmidt, 1982) in recent geologic periods, the low TUB component is assumed to have an in situ northerly declination. Therefore, the high TUB component, as the intersection of remagnetization circles, is restored as shown in Fig. 7(b4). The oriented remanence direction does not show a significant deflection in declination. 4.4. Rotational history of the fore-arc of northeast Japan
Fig. 6. Thermal demagnetization curves of orthogonal IRMs for Cores 3 (a: Eocene) and 5 (b: Late Cretaceous) from the MITI Sanriku-oki borehole.
furnace with an internal residual magnetic field of less than 10 nT up to 600 ◦ C, at which temperature most of the NRM was lost. Results of this test are presented in Fig. 7 (a1) and Fig. 7 (b1) for Core 3 and 5 samples, respectively. The results are characterized by overlapping TUB spectra of primary and secondary components. A remagnetization circle-fitting method (McFadden and McElhinny, 1988) was then adopted to determine the directions of characteristic remanent magnetization (ChRM). The untilted ChRM direction of Core 3 is shown in Fig. 7 (a2), along with statistical parameters (precision parameter, α95 ). 4.3. Correction of NRM components of the Cretaceous sample Late Cretaceous siltstone from Core 5 showed a linear demagnetization trend before PThD treatmentinduced acquisition of unstable and erratic remanence (Fig. 7(b2)). Applying principal component analysis (Kirschvink, 1980) for ChRM calculations, a normal low
The high TUB components of the Eocene and late Cretaceous samples are regarded as primary remanent magnetization carried by magnetite with a unimodal coercivity spectrum. Fig. 8 (right-hand side) provides a summary of the rotational history around the MITI Sanriku-oki borehole. Significant deflection in the declination (D = 42.0◦ , D = 10.3◦ ) of Core 3 requires a clockwise rotation since the Eocene. This rotation may be linked to dextral faulting in northeast Japan related to the Neogene opening of the Japan Sea (Yamaji et al., 1999). Restoring post-Eocene rotation, the mean declination of Core 5 has a significant westerly deflection (D = −47.1◦ , D = 6.2◦ ). Late Cretaceous paleomagnetic data for Benxi, northeastern China (Lin et al., 2003), is adopted here as a reference because the primary nature of NRM has been confirmed for the data set. The amount of counterclockwise rotation and associated uncertainty, as defined by Beck (1980), is R = −87.2◦ and R = 15.2◦ , respectively, pre-dating Eocene time. Paleomagnetic data for the fore-arc shelf of northeast Japan are minimal; consequently, we don’t propose a regional tectonic model of the rotational events. We do recognize, however, that a similar rotation pattern of paleomagnetic data was found for the onshore area of the island arc. Itoh et al. (2000a) analyzed multicomponent remanent magnetization residing within a Cretaceous granitic intrusion at the Kamaishi Mine (Fig. 1), and argued that the intrusive rock underwent syn-cooling counterclockwise rotation (−116◦ ) and subsequent clockwise rotation (50◦ ) (Fig. 8, left-hand side). Although a lack of information on the structural geometry of the intrusion and poor age constraints on the later rotation phase make it difficult to fully characterize the rotational events, it appears that the fore-arc region of northeast Japan has been a site of active transcurrent deformation, as suggested by seismic interpretation in the present study.
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Fig. 7. Results of progressive thermal demagnetization (PThD) for Cores 3 (a: Eocene) and 5 (b: Late Cretaceous) from the MITI Sanriku-oki borehole. The unit used in the vector-demagnetization diagram is bulk remanent intensity. Solid and open symbols are projections of vector endpoints on the horizontal and N-S vertical planes, respectively. For the equal-area projections, solid and open circles are plotted on the lower and upper hemispheres, respectively. Numbers attached on the symbols are levels of PThD in degrees Celsius. Dotted ovals represent the 95% confidence limit of mean paleomagnetic directions. See text for the procedure used to determine the orientation of Core 5.
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Fig. 8. Rotational history of the fore-arc region of northeast Japan inferred from paleomagnetic studies, and related tectonic regimes.
5. Discussion 5.1. Tectonic implications of the geophysical data The nature of the fore-arc upper crust can be estimated from geophysical properties. The data in Fig. 9 show that the structural bulges described in Section 3 are not accompanied by a positive gravity anomaly. Therefore, they are not related to the subduction of high-density asperities such as sea mounts. It should also be noted that a clear discontinuity in the Bouguer anomaly is located at trend T2. A contrast in seismic velocity within the fore-arc crust is confirmed across the trend T2 where it divides the Paleogene and Neogene basins (see Fig. 10; Hayakawa et al., 2002). This implies a considerable rearrangement of crustal blocks associated with movement across the fault. A N-S positive geomagnetic anomaly known as the Kitakami Magnetic Belt (KMB; Finn, 1994) developed solely to the west of trend T1. The anomaly is caused by westerly-magnetized Cretaceous granitic intrusions (Itoh et al., 2000b), whose rock-magnetic properties were investigated in the Kamaishi Mine (Fig. 1; Itoh et al., 2000a). A further circular-shaped geomagnetic anomaly, referred to as the Eastern Magnetic Belt (EMB), exists between trends T2 and T4 (Finn, 1994; Fig. 10), and is distinct from the KMB. Models for the
Fig. 9. Bouguer anomaly (colour scale) for the NE Japan forearc. Fault trends (T1–T4) and structural bulges (B1, B2) are also shown on the map. Aligned orange dots are OBS stations (Hayakawa et al., 2002) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).
origin of the EMB have been proposed (Finn, 1994), including a buried granitic pluton and serpentinite that extends from central Hokkaido. The existence of serpentinized peridotite would result in low levels of seismicity due to weak interplate coupling (Kamiya and Kobayashi, 2000), but the area around the EMB is characterized by intensive interplate seismic activity (Hino et al., 2000; Hayakawa et al., 2002). In this case, therefore, the geomagnetic anomaly is considered to represent Cretaceous granite that is widely distributed on the fore-arc side of northeast Japan (Itoh et al., 2000a,b). 5.2. Reconstruction of fore-arc components Seismic interpretations reveal the existence of several NNW-SSE transcurrent faults in the NE Japan forearc. In making a paleogeographic reconstruction of the study area, we assumed that trend T2 accommodated the largest displacement of the four trends because it is accompanied by a clear discontinuity in seismic velocity. The tectonic model presented in Fig. 11 suggests
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Fig. 10. Geomagnetic anomalies and transcurrent faults (left-hand figure) and velocity structure (right-hand figure) of the NE Japan forearc. There are two evident geomagnetic anomalies, the KMB (Kitakami Magnetic Belt) and EMB (Eastern Magnetic Belt; after Finn, 1994) that have a N-S or NNW-SSE trend. The time-structural contour of the B-horizon (Fig. 4b; contour interval = 250 ms) is overlain on the geomagnetic anomaly map, showing that the faults have the same strike as the geomagnetic anomalies. The velocity structure (right-hand figure) was obtained from an existing seismic refraction experiment (Hayakawa et al., 2002; aligned orange dots in the left-hand figure are OBS stations from their study), strongly indicating spatial variations in tectonic structure. The southern part, located landward of the fault trend T2, represents the general fore-arc crustal structure of the northern Japan Trench, while the northern part, seaward of T2, indicates considerably thickened island arc crust (Hayakawa et al., 2002) (for interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).
that central Hokkaido was a right-lateral shear zone during the late Cenozoic, based on paleomagnetic rotations (Takeuchi et al., 1999), and that the western margin of the rotated blocks (indicated by a dashed line in the figure), where right-lateral shear occurred until the Middle Miocene (Jolivet and Huchon, 1989), was continuous with trend T2. Trend T2 may have been part of a larger tectonic boundary that extended to the western coast of Sakhalin (Jolivet and Tamaki, 1992). The eastern termination of the lateral motion was the Hidaka Main Thrust, around which Late Oligocene dextral faulting has been confirmed (Kusunoki and Kimura, 1998). A zone between the boundary faults as wide as 100 km, which corresponds to the “Cretaceous basin” in Fig. 11 (right-hand side), underwent distributed shear. Formation of the N-S shear zone may be related to the Oligocene–Miocene opening of the Kuril Basin (Kimura, 1994). In the Oligocene reconstruction, a crustal sliver between the boundary faults (1 and 2 in Fig. 11, right-
hand side) at the southern extension of present-day central Hokkaido was restored so that the EMB and KMB form a line constituting a geomagnetic anomaly belt related to Cretaceous volcano-plutonic activity. This reconstruction requires more than 200 km of transportation of the fore-arc, which is concordant with a previous tectonic model of eastern Hokkaido (Jolivet et al., 1994). We note that DSDP sites 438 and 439 were located near western Hokkaido prior to the transcurrent motion. We can therefore use the DSDP data to verify the similarity of those paleoenvironments around the sites and in our reconstruction. DSDP site 439 contains boulders of dacite with Late Oligocene radiometric ages (Moore and Fujioka, 1980). These boulders are considered to have originated from in situ arc volcanism, as the site is located on the northern extension of a volcanic front, as inferred from volcanism in northeast Japan between 23 and 15 Ma (Tatsumi et al., 1989) and volcanism around the northernmost part of the KMB from the late Oligocene to the early
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Fig. 11. Paleogeographic reconstruction of the fore-arc region of northeast Japan. The Oligocene volcanic front is inferred from volcanism in northeast Japan during the period 23–15 Ma (Tatsumi et al., 1989) and volcanism around the northernmost part of the KMB from the Late Oligocene to the Early Miocene (Kurita and Yokoi, 2000). The KMB and EMB are remarkable geomagnetic anomaly trends (Finn, 1994).
Miocene (Kurita and Yokoi, 2000). An upper Oligocene marine sedimentary unit that overlies the volcanic conglomerate at site 439 is characterized by an abundant influx of Cretaceous pollen (Sato, 1980). Such contamination ceases at the beginning of the Miocene, which may be related to the submergence of voluminous Cretaceous sedimentary terranes in central Hokkaido (Fig. 11; Ishiwada et al., 1992) as a result of active faulting in the distributed shear zone and the formation of pullapart basins (Kurita and Yokoi, 2000; Itoh and Tsuru, 2005). 5.3. Temporal variations in tectonic erosion In previous studies, DSDP sites 438 and 439 were fixed to the east of northeast Japan, and paleoenvironmental changes recorded within sediments from the sites were attributed to subduction erosion of the fore-arc region. von Huene et al. (1994) estimated the rate of subduction erosion assuming that the Pacific Plate has been steadily subducting underneath northeast Japan for the past 20 m.y.
In contrast, our model involves more than 200 km of south-southeastward migration of the fore-arc segment. This migration would have increased the rate of convergence between the oceanic and continental plates and accelerated basal erosion at the plate boundary. Furthermore, material at the tip of the continental plate that previously filled the trench would have been transported into the deep Earth interior together with the subducting oceanic crust via frontal erosion, which would have accelerated with the increased migration rate. The rate of subduction erosion would have changed not only with the subduction rate of the oceanic plate but also the migration rate of the fore-arc segment. Here we investigate subsidence rates of the forearc region, as estimated from existing borehole data. Temporal changes in the basal erosion rate would have affected the subsidence rate. Fig. 12 presents subsidence histories from DSDP and industry wells (von Huene and Lallemand, 1990; Itoh et al., 2000b; Ishiwada et al., 1992; Osawa et al., 2002; Japan National Oil Corporation, 1999), indicating clear changes in the subsidence rate between the periods Oligocene – 10 and 10 Ma
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Huchon, 1989). Therefore, subsidence since 10 Ma has been related to the steady subduction of the Pacific Plate. Finally, the subsidence rates recorded at drilling sites increase eastward (trenchward), which may indicate that the studied transect has been actively eroded beneath the upper slope region since the time that the fore-arc components were rearranged. 6. Summary
Fig. 12. Subsidence curves for the base of the Cenozoic basin at drilling sites on the NE Japan forearc (von Huene and Lallemand, 1990; Itoh et al., 2000b; Ishiwada et al., 1992; Osawa et al., 2002; Japan National Oil Corporation, 1999). Shaded areas represent regions of steady subduction erosion following massive rearrangement of the fore-arc components. See Fig. 11 for drilling locations.
– present. Although a quantitative estimate of the erosion rate is beyond the scope of the present study, the subsidence curves demonstrate that the erosion rate during the period Oligocene – 10 Ma was considerably higher than that since 10 Ma. The observed difference in erosion rate is consistent with our segment-transportation model. In addition, geomagnetic data for the fore-arc region do not favor the fixed model, as there is no indication of intrusive bodies to the west of the Oligocene volcanics at DSDP 439. Such intrusives should exist if the trench axis has been retreating westward as a result of steady erosion. From the Oligocene to the Early Miocene, the fore-arc sliver was inevitably transported toward the trench by trenchward motion and coeval spreading of the Japan Sea, as inferred previously (von Huene et al., 1994). During this period, intensive tectonic erosion would have generated rapid subsidence of the fore-arc region, as shown in Fig. 12. In Fig. 12, the four sites other than the DSDP sites belong to the same zone (west of the trend T1; Fig. 11), and basin formation is considered to have continued during the Paleogene, although the pattern of vertical motions differs considerably between sites. Especially rapid subsidence is evident in DSDP sites 438 and 439 since the initiation of transcurrent motion in the forearc region. Our tectonic model indicates that these sites were originally located around present-day Hokkaido; thus, their initial subsidence may be linked to deformation of the migrating crustal sliver. The geological structure of Hokkaido suggests that the transcurrent regime had diminished by the Late Miocene (Jolivet and
Seismic interpretation of the NE Japan forearc indicates a NNW-SSE trending deformation zone bounded by prominent transcurrent faults (trends T1–T4). Rightlateral motions and related deformation modes have been dominant since the Miocene, despite the E-W compressive stress regime caused by the motion of the Pacific Plate. The occurrence of fore-arc wrenching is supported by clockwise rotation in the late Cenozoic inferred from the paleomagnetism of core samples recovered from a borehole within the study area. Of the four transcurrent faults, trend T2 is the largest, and is accompanied by a discontinuity in seismic velocity. A paleogeographic reconstruction of the region requires more than 200 km of southward migration of the fore-arc sliver, which is consistent with paleoenvironmental changes reported from drilling surveys. The large-scale transcurrent motion of the fore-arc sliver would have affected the rate of subduction erosion in the northern part of the Japan Trench fore-arc region. Acknowledgements The authors thank METI (Ministry of Economy, Trade and Industry, Japan) and JOGMEC (Japan Oil, Gas and Metals National Corporation) for permission to publish this work. We also acknowledge: M.K. Thu, T. Hashimoto, T. Murayama (JAMSTEC) and Y. Nagasaki (JOGMEC) for their assistance during the seismic interpretation; N. Ishikawa for the use of paleomagnetic laboratory at Kyoto University. Comments by reviewers greatly helped to improve early version of the manuscript. References Beck Jr., M.E., 1980. Paleomagnetic record of plate-margin tectonic processes along the western edge of North America. J. Geophys. Res. 85, 7115–7131. DSDP Scientific Party, 1980. Initial Reports of the Deep Sea Drilling Project, Legs 56 and 57. U.S. Government Printing Office, Washington, DC (Attachment). Finn, C., 1994. Aeromagnetic evidence for a buried Early Cretaceous magmatic arc, northeast Japan. J. Geophys. Res. 99, 22165–22185.
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Hayakawa, T., Kasahara, J., Hino, R., Sato, T., Shinohara, M., Kamimura, A., Nishino, M., Sato, T., Kanazawa, T., 2002. Heterogeneous structure across the source regions of the 1968 TokachiOki and the 1994 Sanriku-Haruka-Oki earthquakes at the Japan Trench revealed by an ocean bottom seismic survey. Phys. Earth Planet. Int. 132, 89–104. Hino, R., Ito, S., Shiobara, H., Shimamura, H., Sato, T., Kanazawa, T., Kasahara, J., Hasegawa, A., 2000. Afrershock distribution of the 1994 Sanriku-oki earthquake (Mw 7.7) revealed by ocean bottom seismographic observation. J. Geophys. Res. 105, 21697–21710. Honza, E., Tamaki, K., Murakami, F., 1978. Geological Map of the Japan and Kuril Trenches and the Adjacent Areas 1:1,000,000. Geological Survey of Japan, Tsukuba. Ishiwada, Y., Aida, H., Atake, M., Araki, N., Iijima, A., Ikeda, A., Okuda, Y., Kikuchi, Y., Kojima, K., Saito, T., Sato, Y., Tanaka, S., Tono, S., Hirayama, J., Honza, E., Miyazaki, H., Morishima, H., Harada, Y., 1992. Oil and Natural Gas Resources in Japan. Natural Gas Exploration Association, Tokyo, 520 pp. Itoh, Y., Amano, K., Danhara, T., 2000a. Paleomagnetism, rock magnetism, and fission track dating of the Kurihashi Granodiorite in northeast Japan: evidence for a Cretaceous hydrothermal event and differential rotation along the eastern Eurasian margin. J. Geophys. Res. 105, 13519–13532. Itoh, Y., Nagasaki, Y., Matsuda, T., 2000b. Fore-arc transcurrent faults in northeast Japan: hybrid geophysical models of an island-arc upper crust. Tectonophysics 327, 311–333. Itoh, Y., Tsuru, T., 2003. Transcurrent motions and deformation on the fore-arc of northeast Japan. In: Abstracts of, XXIII, General Assembly of the International Union of Geodesy Geophysics, Secretariat General of IUGG 2003, Yokosuka, p. B472. Itoh, Y., Tsuru, T., 2005. Evolution history of the Hidaka-oki (offshore Hidaka) basin in the southern central Hokkaido, as revealed by seismic interpretation, and related tectonic events in an adjacent collision zone. Phys. Earth Planet. Int. 153, 220–226. Japan National Oil Corporation (JNOC), 1999. Report on the Geological Study of MITI Sanriku-oki Borehole. Japan National Oil Corporation, Tokyo. Jolivet, L., Huchon, P., 1989. Crustal-scale strike-slip deformation in Hokkaido, northern Japan. J. Struct. Geol. 11, 509–522. Jolivet, L., Tamaki, K., 1992. Neogene kinematics in the Japan Sea region and volcanic activity of the northeast Japan arc. In: Tamaki, K., Suyehiro, K., Allan, J., McWilliams, M., et al. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, vol. 127/128, part 2. Ocean Drilling Program, College Station, TX, pp. 1311–1331. Jolivet, L., Tamaki, K., Fournier, M., 1994. Japan Sea, opening history and mechanism: a synthesis. J. Geophys. Res. 99, 22237–22259. Kamiya, S., Kobayashi, Y., 2000. Seismological evidence for the existence of serpentinized wedge mantle. Geophys. Res. Lett. 27, 819–822. Kimura, G., 1994. The latest Cretaceous–early Paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern Pacific margin. J. Geophys. Res. 99, 22147–22164. Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. Geophys. J. R. Astr. Soc. 62, 699–718. Kruiver, P.P., Dekkers, M.J., Heslop, D., 2001. Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation. Earth Planet. Sci. Lett. 189, 269–276.
Kurita, H., Yokoi, S., 2000. Cenozoic tectonic settings and a current exploration concept in southern central Hokkaido, northern Japan. J. Jpn. Assoc. Pet. Technol. 65, 58–70. Kusunoki, K., Kimura, G., 1998. Collision and extrusion at the KurilJapan arc junction. Tectonics 17, 843–858. Lin, W., Chen, Y., Faure, M., Wang, Q., 2003. Tectonic implications of new Late Cretaceous paleomagnetic constraints from Eastern Liaoning Peninsula, NE China. J. Geophys. Res. 108 (B6), 2313, doi:10.1029/2002JB002169. Lowrie, W., 1990. Identification of ferromagnetic minerals in a rock by coercivity and unblocking temperature properties. Geophys. Res. Lett. 17, 159–162. McFadden, P.L., McElhinny, M.W., 1988. The combined analysis of remagnetization circles and direct observations in palaeomagnetism. Earth Planet. Sci. Lett. 87, 161–172. Middleton, M.F., Schmidt, P.W., 1982. Paleothermometry of the Sydney Basin. J. Geophys. Res. 87, 5351–5359. Moore, G.W., Fujioka, K., 1980. Age and origin of dacite boulder conglomerate anomalously near the Japan Trench. In: Scientific Party (Ed.), Initial Reports of the Deep Sea Drilling Project, 56, 57 (part 2). U.S. Government Printing Office, Washington, DC, pp. 1083–1088. Nasu, N., von Huene, R., Ishiwada, Y., Langseth, M., Bruns, T., Honza, E., 1980. Interpretation of multichannel seismic reflection data, Legs 56 and 57, Japan Trench transect, Deep Sea Drilling Project. In: Scientific Party (Ed.), Initial Reports of the Deep Sea Drilling Project, 56, 57 (Part 1). U.S. Government Printing Office, Washington, DC, pp. 489–503. Oide, K., Nakagawa, H., Kanisawa, S., 1989. Regional Geology of Japan, part 2 Tohoku. Kyoritsu Shuppan, Tokyo, 338 pp. Okamura, Y., Nishimura, K., Tanahashi, M., 1983. Explanatory Notes of Geological Map off Kamaishi 1:200,000. Geological Survey of Japan, Tsukuba, 25 pp. Osawa, M., Nakanishi, S., Tanahashi, M., Oda, H., 2002. Structure, tectonic evolution and gas exploration potential of offshore Sanriku and Hidaka provinces, Pacific Ocean, off northern Honshu and Hokkaido, Japan. J. Jpn. Assoc. Pet. Technol. 67, 38–51. Otofuji, Y., Kambara, A., Matsuda, T., Nohda, S., 1994. Counterclockwise rotation of Northeast Japan: Paleomagnetic evidence for regional extent and timing of rotation. Earth Planet. Sci. Lett. 121, 503–518. Otsuki, K., 1992. Oblique subduction, collision of microcontinents and subduction of oceanic ridge: their implications on the Cretaceous tectonics of Japan. The Island Arc 1, 51–63. Pullaiah, G., Irving, E., Buchan, K.L., Dunlop, D.J., 1975. Magnetization changes caused by burial and uplift. Earth Planet. Sci. Lett. 28, 133–143. Sasaki, M., 2003. Early Cretaceous sinistral shearing and associated folding in the South Kitakami Belt, northeast Japan. The Island Arc 12, 92–109. Sato, S., 1980. Palynological study of sediments in Holes 439 and 438A. In: Scientific Party (Ed.), Initial Reports of the Deep Sea Drilling Project 56, 57 (part 2). U.S. Government Printing Office, Washington, DC, pp. 895–901. Takeuchi, T., Kodama, K., Ozawa, T., 1999. Paleomagnetic evidence for block rotations in central Hokkaido-south Sakhalin, Northeast Asia. Earth Planet. Sci. Lett. 169, 7–21. Tatsumi, Y., Otofuji, Y., Matsuda, T., Nohda, S., 1989. Opening of the sea of Japan back-arc basin by asthenospheric injection. Tectonophysics 166, 317–329. Tsuru, T., Park, J.-O., Miura, S., Kodaira, S., Kido, Y., Hayashi, T., 2002. Along-arc structural variation of the plate boundary at the
Y. Itoh, T. Tsuru / Physics of the Earth and Planetary Interiors 156 (2006) 117–129 Japan Trench margin: implication of interplate coupling. J. Geophys. Res. 107, doi:10.1029/2001JB001664. von Huene, R., Klaeschen, D., Cropp, B., Miller, J., 1994. Tectonic structure across the accretionary and erosional parts of the Japan Trench margin. J. Geophys. Res. 99, 22349–22361.
129
von Huene, R., Lallemand, S., 1990. Tectonic erosion along the Japan and Peru convergent margins. Geol. Soc. Am. Bull. 102, 704–720. Yamaji, A., Momose, H., Torii, M., 1999. Paleomagnetic evidence for Miocene transtensional deformations at the eastern margin of the Japan Sea. Earth Planets Space 51, 81–92.