Structural characteristics of the northern Okinawa Trough and adjacent areas from regional seismic reflection data: Geologic and tectonic implications

Tectonophysics 522–523 (2012) 198–207

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Structural characteristics of the northern Okinawa Trough and adjacent areas from regional seismic reflection data: Geologic and tectonic implications Ayse Gungor a, 1, Gwang H. Lee a,⁎, Han-J. Kim b, Hyun-C. Han c, Moo-H. Kang c, Jinho Kim c, Don Sunwoo c a b c

Department of Energy Resources Engineering, Pukyong National University, Busan 608-737, South Korea Korea Ocean Research and Development Institute, Ansan 426-744, South Korea Korea Institute of Geoscience and Mineral Resources, Daejon 305-350, South Korea

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Article history: Received 4 July 2010 Received in revised form 9 October 2011 Accepted 27 November 2011 Available online 7 December 2011 Keywords: Okinawa Trough Back-arc basin Transfer zone Transfer fault Taiwan–Sinzi belt East China Sea shelf basin

a b s t r a c t Analysis of regional multi-channel seismic data from the northern Okinawa Trough and adjacent shelf provides some important constraints on the structural development of the area. The sedimentary strata in the northern East China Sea shelf basin, separated from the Okinawa Trough by the Taiwan–Sinzi belt, are affected by the Miocene compressional tectonism and truncated by the Late Miocene unconformity. In contrast, those in the western margin of the northern Okinawa Trough are cut by numerous normal faults and the Late Miocene horizon forms a conformable surface. This suggests that the Taiwan–Sinzi belt acted as a buttress for the northern Okinawa Trough against the compressional tectonism. Our data also reveal the Ho Basin in the western margin of the northern Okinawa Trough, previously known only from proprietary industry data. The Longwan Ridge, lying between the Ho Basin and the northern Okinawa Trough, may be the youngest of the relict arcs or buried ridges in the area. The abrupt along-strike change in the fault polarity near the northern margin of the depocenter of the Ho Basin suggests a NW-trending left-lateral transfer zone or fault. The western boundary of the northern Okinawa Trough is characterized by the gently deepening seafloor, whereas that of the southern Okinawa Trough is marked by steep border faults, suggesting more rapid subsidence in the south. This, together with the well-developed symmetric axial faults in the south and the greater width and the lack of well-developed axial zone in the north, may suggest focused rifting/extension in the southern Okinawa Trough and diffuse rifting/extension in the northern Okinawa Trough. We postulate that the diffuse rifting/extension in the northern Okinawa Trough is due to the tectonic perturbation, caused by the convergence or subduction of the topographically high and buoyant Amami Plateau at the northern Ryukyu arc. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The Okinawa Trough is a back-arc basin to the Ryukyu arc, extending from the shelf off SW Kyushu to near the northern Taiwan (Fig. 1). The Okinawa Trough is the only modern example of an incipient continental back-arc basin perhaps except for the Andaman Sea (Letouzey and Kimura, 1986), providing a unique opportunity to study the early continental back-arc opening. The crust underlying the Okinawa Trough is continental in nature (Hirata et al., 1991; Iwasaki et al., 1990; Sibuet et al., 1995) and its thickness decreases from about 27–30 km in the north near Kyushu to 15 km or less in the south near Taiwan (Iwasaki et al., 1990; Sibuet et al., 1995). The very high (>300 nT) magnetic anomalies (Furukawa et al., 1991) of the narrow axial zone in the southern Okinawa Trough may be related to volcanic ridges (Hirata et al., 1991) or dike intrusions or emplacement of early oceanic crust (Davagnier et al., 1987). In the ⁎ Corresponding author. Tel.: + 82 51 629 6558. E-mail address: [email protected] (G.H. Lee). 1 Current address: Arar Oil & Gas Inc., Ankara, Turkey. 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.11.027

southernmost Okinawa Trough, a cluster of active submarine volcanoes, consisting of pre-back-arc rifting volcanic rocks, forms a belt (Chung et al., 2000). Although similar volcanic belts or ridges have not been reported in the axial region of the northern Okinawa Trough, active volcanoes occur along the eastern margin of the northern Okinawa Trough almost exclusively to the north of Okinawa Island (Sibuet et al., 1998). Paleomagnetic and seismic reflection data suggest that the northern Okinawa Trough began to open in the Middle to Late Miocene (Fabbri et al., 2004; Letouzey and Kimura, 1986; Miki, 1995; Sibuet et al., 1995). The opening of the southern Okinawa Trough probably also started at about the same time (Miki, 1995; Sibuet et al., 1995), but much of its extension appears to have occurred during the Quaternary (Hsu et al., 2001; Kong et al., 2000; Lallemand et al., 1997; Letouzey and Sage, 1988; Park et al., 1998). The 3-D P-wave velocity structure under the Okinawa Trough suggests that the opening of the Okinawa Trough is mainly due to the oblique subduction of the Philippine Sea plate and the extensive dehydration reaction of the subducting plate (Wang et al., 2008). Miki (1995) and Kong et al. (2000) attributed the opening of the southern Okinawa Trough to

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the Luzon–Taiwan collision and subsequent clockwise rotation of the southern Ryukyu arc. The focal mechanisms of the intraplate earthquakes in the crust or upper mantle beneath the Okinawa Trough suggest a strike–slip component of the extension (Fabbri and Fournier, 1999; Fournier et al., 2001; Kao and Chen, 1991). The crustal thinning and extension in the Okinawa Trough is accompanied by faulting, block titling, and subsidence that created grabens and half grabens (Fabbri et al., 2004; Letouzey and Kimura, 1985; Sibuet et al., 1995). Fabbri et al. (2004) mapped a series of en echelon left-stepping grabens and half grabens in the northern Okinawa Trough. These grabens and half grabens are segmented by strike–slip faults that trend NW and cut the northern Taiwan–Sinzi belt (Kong et al., 2000; Lin et al., 2005; Liu, 1992). Proprietary seismic reflection data show a NE-trending, deep (~10 km) and narrow (30 km wide) basin between the northern Taiwan–Sinzi belt and the trough area of the northern Okinawa Trough (Lin et al., 2005). This small basin is bounded seaward by a ridge; the basin and the ridge were named the Ho Basin and the Longwan Ridge, respectively (Lin et al., 2005). The structural features in the northern Okinawa Trough were interpreted mostly from shallow-penetration seismic reflection data. The thick sediment cover and the lack of publicly available regional and deep-penetration seismic reflection data make it difficult to study the structural characteristics of the area (Fabbri et al., 2004). In this study, we processed and interpreted regional multi-channel seismic reflection data (Fig. 2) from the northern Okinawa Trough and adjacent shelf, including the northern Taiwan–Sinzi belt and the northern East China Sea shelf basin, to identify the main structural features. The structural features have provided some important constraints on the structural development of the northern Okinawa Trough and adjacent areas. 2. Tectonic and geologic setting Rifting in the East China Sea shelf basin began in the Late Cretaceous and migrated eastward, forming belts of sedimentary basins in the Cenozoic, separated by buried continental ridges or rifted segments of the proto-Ryukyu arc (Sibuet and Hsu, 1997; Sibuet et al., 1987, 2004). The rifting was driven by crustal stretching of the Eurasian lithosphere (Kimura, 1985; Lee et al., 1980; Letouzey and Kimura, 1985, 1986; Sibuet et al., 1987), caused by the subduction of the Philippine Sea plate under the Eurasian plate. The subduction of the Philippine Sea plate was interrupted in the Late Miocene (Uto, 1995) and resumed in the latest Late Miocene (ca. 6 Ma) (Seno and Maruyama, 1984) when the direction of relative convergence changed from NNW to NW (Nakada and Kamata, 1991; Nakamura et al., 1985). The rate of convergence at the subduction zone has been estimated to be about 4–5 cm/yr (DeMets et al., 1990; El-Fiky et al., 1999; Seno et al., 1993) to about 6 – 8 cm/yr (Seno et al., 1993). The Okinawa Trough is the youngest of the sedimentary belts in the area (Sibuet et al., 1995). The Taiwan–Sinzi belt, separating the Okinawa Trough from the East China Sea shelf basin, is made of Cretaceous or older metamorphic rocks, intruded by Miocene volcanic rocks (Sibuet et al., 2004), and may be the youngest of the relict arcs or buried ridges (Sibuet et al., 1995). The partially emergent Ryukyu arc consists of the non-volcanic frontal arc system in the east (Eguchi and Uyeda, 1983) and a broad volcanic arc zone with active volcanoes in the west (Fabbri et al., 2004). The volcanic arc zone is composed of deformed sedimentary rocks covered or intruded by Plio-Quaternary volcanic deposits, lava flows, and sills (Fabbri et al., 2004). The axis of the Okinawa Trough is curved, largely parallel to the Ryukyu arc (Letouzey and Kimura, 1986). Arc-perpendicular extensional stress is observed in the southern and central Okinawa Trough whereas the direction of extension in the northern Okinawa Trough is

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oblique to both the arc normal direction and the direction of the plate motion (Fabbri and Fournier, 1999; Fournier et al., 2001; Kubo and Fukuyama, 2003). The rates of extension in the southern Okinawa Trough today reach about 4 cm/yr; elsewhere the rates are 1 cm/yr or less (Imanishi et al., 1996). Rifting/extension in the northern Okinawa Trough can be divided into three phases (Sibuet et al., 1995). The first phase (Middle to Late Miocene) is not well constrained and the amount of extension is estimated to be about 60 km. The extension during the first phase created a series of en echelon leftstepping grabens or half grabens (Fabbri et al., 2004). The first phase continued until around 9 Ma, followed by a tectonically quiescent period that lasted until 2 Ma. The second phase (Early Pleistocene) is characterized by crustal tilting; the extension is estimated to be about 25 km. The third phase (Late Pleistocene– present) is characterized by normal faults with very small (a few meters) vertical offsets, changing progressively in orientation along the Okinawa Trough; the amount of extension is about 5 km in the southern part of the northern Okinawa Trough. 3. Data and methods The data used in this study consist of: (1) about 7200 km of multichannel seismic data provided by the Korea Institute of Geoscience and Mineral Resources (KIGAM) and (2) acoustic and density logs from nine exploratory wells provided by the Korea National Oil Corporation (KNOC) (Fig. 2). The seismic data were collected in 2000, 2001, and 2004 on the RV Tamhae II of KIGAM using the Trilogy data management system of Western-Geco. A 1035-in 3, 2000-psi air-gun array (shot interval of 25 m) was fired into a 96-channel streamer (hydrophone group interval of 12.5 m). The source and streamer depths were 5 m and 7 m, respectively. The record length is 7 s and the sampling interval is 2 ms. The Trinav GPS navigation system of Western-Geco with position accuracy of 5 m was used for the navigation and seismic data acquisition. Data processing was carried out by the first author using ProMAX2D ® and included F–K filtering, waveequation multiple rejection, deconvolution, velocity analysis, and stacking. The data quality is fair. Kingdom Suite ® (version 8.3) was used for seismic data interpretation and mapping. The top of the acoustic basement, which is the deepest coherent and continuously observed reflector on seismic profiles, and the Late Miocene unconformity, correlated directly with that of Lee et al. (2006) (their Figs. 3 and 8), were interpreted and mapped. Out of the nine wells, five wells are close enough to the seismic profiles. Among the five wells, only two have reliable sonic and density data that can be used to generate synthetic seismograms for seismicto-well tie. The time–depth relationships were obtained at the two wells from seismic-to-well tie and further averaged to create a single time–depth chart which was used for the entire study area to construct the depth structure maps for the top of the acoustic basement and the Late Miocene unconformity. 4. Results The seismic profiles (Figs. 3–6) and the depth structure maps of the top of the acoustic basement (Fig. 7) and the Late Miocene unconformity (Fig. 8) show the key structural elements and features in the study area: from northwest to southeast, the northern East China Sea shelf basin with folded and faulted sedimentary strata, the nearly flat Taiwan–Sinzi belt, the large normal faults along the western margin of the northern Okinawa Trough, and numerous steep and planar normal faults in the intra-trough region. The thick sedimentary strata in the western margin of the northern Okinawa Trough represent the depocenter of the Ho Basin, which was previously known only from proprietary industry data (Lin et al., 2005). The prominent Late Miocene unconformity leveled the folded and faulted sedimentary strata in the northern East China Sea shelf basin and truncated the Taiwan–

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Fig. 2. Seismic reflection data and well locations. The yellow lines and respective figure numbers indicate the seismic profiles shown in other figures. Well names are not shown. Contours are bathymetry in meters from Sandwell and Smith (1997).

Sinzi belt. Small mounds or mound-like features are seen locally on the Taiwan–Sinzi belt. The Late Miocene unconformity forms a conformable surface in the western margin of the northern Okinawa Trough.

The normal faults in the northern part of the Ho Basin dip dominantly NW or toward the Taiwan–Sinzi belt (Figs. 3 and 7). These faults are rotational as evidenced by the decrease in dip with depth, forming half grabens. On the other hand, the normal faults in the

Fig. 1. (A) Tectonic and structural elements of the East China Sea and the northern West Philippine Basin. Adapted from Zhou et al. (1989), Yang (1992), Sandwell and Smith (1997), Lin et al. (2005), and Lee et al. (2006). JB, Jeju Basin; DB, Domi Basin; HAR, Haijiao Rise; HUR, Hupijiao Rise; JD, Jilong Depression; LR, Longwan Ridge; MD, Minjiang Depression; OD, Oujiang Depression; QD, Qiantang Depression; SC, Socotra Basin; XD, Xihu Depression; YSR, Yushan Rise. (B) Bathymetry of the East China Sea and the northern West Philippine Basin from Sandwell and Smith (1997). Contours are in meters.

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A

B Fig. 3. Seismic reflection profile (A) and its interpretation (B) showing compression in the northern East China Sea shelf basin and extension in the western margin of the northern part of the northern Okinawa Trough, including the northern Ho Basin. Extension in the western margin of the northern part of the northern Okinawa Trough is characterized by deeply-rooted, NW-dipping growth faults. The Late Miocene unconformity that eroded the Taiwan–Sinzi belt forms a conformable surface in western margin of the northern Okinawa Trough. Vertical exaggeration is approximately 10×. See Fig. 2 for location.

depocenter of the Ho Basin dip dominantly SE or away from the Taiwan–Sinzi belt (Figs. 4 and 7). These faults are more-or-less planar and displacement in the acoustic basement appears to be small. The intra-trough region of the northern Okinawa Trough is also cut by numerous normal faults (Figs. 4 and 5). These faults are steep and planar and show hints of conjugate systems, typical for down-to-the basin

faults, but the axial rift zone or graben is not evident. The intratrough faults are not mappable. The local bathymetric highs seen in the bathymetric map (Figs. 1B and 2) appear as submarine volcano-like features in the seismic profiles (Figs. 3–5). These isolated bathymetric highs become common near the Ryukyu arc. The seafloor to the northwest of the bathymetric

A

B Fig. 4. Seismic reflection profile (A) and its interpretation (B) showing compression in the northern East China Sea shelf basin and extension the western margin of the southern part of the northern Okinawa Trough, including the depocenter of the Ho Basin. Extension in the western margin of the southern part of the northern Okinawa Trough is characterized by SE-dipping normal faults. The gentle basement high to the southeast of the Ho Basin probably represents the Longwan Ridge. The down-to-the basin faults in the Okinawa Trough are steep and planar and show hints of conjugate systems but the axial rift zone or graben is not evident. The seafloor to the northwest of the bathymetric high, interpreted as a submarine volcano, is smooth whereas that to the southeast is rugged. Vertical exaggeration is approximately 10×. See Fig. 2 for location.

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A

B Fig. 5. Seismic reflection profile from the northern part of the northern Okinawa Trough (A) and its interpretation (B) showing the very gentle seafloor along the western margin of the northern Okinawa Trough and bathymetric highs, interpreted as submarine volcanoes, and the rugged seafloor in the southeast near the Ryukyu arc. This profile obliquely crosses the northern Okinawa Trough and therefore the faults marking the transfer zone are not clearly identified; only one NW-dipping fault, indicated by an arrow in the northern part of the section, is mappable. The Longwan Ridge appeared to have caused faulting in the overburden. Numerous down-to-the basin faults are observed in the Okinawa Trough; these faults are not mappable. The axial rift zone or graben is not evident. Vertical exaggeration is approximately 10×. See Fig. 2 for location.

highs is smooth whereas the seafloor to the southeast toward the Ryukyu arc is rugged or irregular (Figs. 4 and 5). The seafloor of the western margin of the northern part of the northern Okinawa Trough deepens gradually with minor normal faults (Fig. 5). On the other hand, the western margin of the southern part of the northern Okinawa Trough is steep and bordered by large SE-dipping normal faults (Fig. 6). This along-strike transition from gentle slope to steep border faults can be also recognized in the bathymetric map of the area (Figs. 1B and 2). The depths of the top of the acoustic basement in the mapped area range from less than 1000 m in the northern part of the northern East China Sea shelf basin to over 8000 m in the Ho Basin, outlined approximately by 5000-m contour (Fig. 7). The Longwan Ridge is not evident. The Taiwan–Sinzi belt, outlined by the 2000-m contour, forms a NNE-trending basement high. Local highs (b1500 m) are also seen in the Taiwan–Sinzi belt. The two fault sets with opposite dips mark the western margin of the northern Okinawa Trough. The boundary between these two fault sets can be traced along the

northeastern margin of the depocenter of the Ho Basin and coincides with the minor step-like outer edge of the Taiwan–Sinzi belt and the southwestern margin of the shallow basement in the East China Sea shelf basin that may divide the shelf basin into two depocenters. The fault set boundary trends NW. The Late Miocene unconformity forms a broad platform in the northern East China Sea shelf basin and over the Taiwan–Sinzi belt and deepens (>2000 m) rapidly into the Okinawa Trough (Fig. 8). It is not recognizable in much of the intra-trough region of the northern Okinawa Trough due to poor data quality at depth. The depths of the Late Miocene unconformity in the mapped area range from less than 1000 m in the northeastern part of the East China Sea shelf basin to over 3000 m in the southeast (Fig. 8). 5. Discussion The northern East China Sea shelf basin experienced at least three phases of uplift since the Eocene (Cukur et al., 2011; Lee et al., 2006;

Fig. 6. Seismic reflection profile from the southern part of the northern Okinawa Trough showing the steep border faults along the western margin of the trough, suggesting rapid subsidence. Vertical exaggeration is approximately 10×. See Fig. 2 for location.

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Fig. 7. Depth structure map of the top of the acoustic basement. The Taiwan–Sinzi belt and the Ho Basin are outlined by 2000-m and 5000-m contours, respectively. Two fault sets with opposite dips are seen along the western margin of the northern Okinawa Trough. These two fault sets appear to be divided by a NW-trending transfer zone or fault which marks the northern boundary of the depocenter of the Ho Basin and extends to the southwestern margin of the shallow basement in the East China Sea shelf basin. The grey shaded area outlines the Okinawa Trough by 1000-m bathymetric contour to show where the acoustic basement is mapped in the trough region. Contour interval is 1000 m.

Yang et al., 2004): (1) the Late Eocene–Early Oligocene (Yuquan movement), (2) the Early Miocene, and (3) the Late Miocene (Longjing movement). The Late Miocene event created an extensive thrust– fold belt in the eastern part of the northern East China Sea shelf basin (Lee et al., 2006; Zhou et al., 1989). Subsequent erosion resulted in the very prominent Late Miocene unconformity in the northern East China Sea shelf basin and the Taiwan–Sinzi belt. The conformable Late Miocene horizon in the western margin of the northern Okinawa Trough indicates that the compressional tectonic movement that affected the northern East China Sea shelf basin and perhaps the Taiwan–Sinzi belt did not propagate into the Okinawa Trough, including the Ho Basin. Thus, the Taiwan–Sinzi belt is likely to have acted as a buttress against the tectonic forces. The small mounds on the shallow, eroded top of the Taiwan–Sinzi belt probably represent the post-Late Miocene volcanoes and/or their remnants. The presence of the Late Miocene horizon in the Ho Basin indicates that the oldest strata in the basin are older than the Late Miocene, constraining the timing of the onset of the rifting probably to the Middle Miocene. This may further suggest that the opening of the younger northern Okinawa Trough began in the Middle Miocene or later. The Longwan

Fig. 8. Depth structure map of the Late Miocene unconformity. The Late Miocene unconformity forms a broad platform in the northern East China Sea shelf basin and over the Taiwan–Sinzi belt and deepens rapidly into the Okinawa Trough. The grey shaded area outlines the Okinawa Trough by 1000-m bathymetric contour to show where the acoustic basement is mapped in the trough region. Contour interval is 1000 m.

Ridge may be the youngest relict arc or buried ridge in the East China Sea area. The NW-dipping rotational or growth faults to the north of the depocenter of the Ho Basin appear to be deeply rooted into the basement and may sole out into a subrift detachment surface or zone. These faults were also identified from shallow-penetration seismic reflection data (Sibuet et al., 1987). The Tokara Line fault zone to the northeast of the study area off SW Kyushu consists of NWdipping, listric-shaped growth faults that can be followed downward to more than 6000 m (Fabbri et al., 2004). The Tokara Line fault zone is probably part of the growth fault system observed in the study area. Growth faults are also recognized in the western margin of the southern Okinawa Trough (Park et al., 1998). These faults are dipping SE away from the Taiwan–Sinzi belt and some of them may sole out into a detachment fault located between the Taiwan–Sinzi belt and the Ryukyu arc (Park et al., 1998). The normal faults in the depocenter of the Ho Basin are more-orless planar, dipping SE and do not appear to be deeply rooted into the basement. The boundary between these SE-dipping faults and the NW-dipping growth faults in the north is quite abrupt, suggesting a regional transfer zone or fault or along-axis segmentation that has accommodated the transfer slip of two adjacent crustal blocks

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undergoing differential extension. Regional transfer faults or zones have been reported at many extensional settings including the Atlantic margins, the Basin and Range, the Suez Rift, the rifts of Sudan and back-arc basins (Acocella et al., 2005). The transfer zone or fault in the study area approximates the northern boundary of the depocenter of the Ho Basin. It trends NE and appears to be left-lateral as it can be traced along the small left-lateral offset of the Taiwan–Sinzi belt and the shallow basement farther to the northwest in the northern East China Sea shelf basin. The en echelon grabens and half grabens comprising the Tokara Line fault zone are also left-stepping (Fabbri et al.,

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2004). The tectonic maps of the northern Okinawa Trough (e.g., Hsu et al., 2001; Kong et al., 2000; Wu et al., 2007) show regional, NWtrending left-lateral strike–slip faults that apparently offset the Tokara Line fault zone. Sunwoo (2004) reported small NW-trending strike–slip faults in the western margin of the northern Okinawa Trough, which locally offset normal faults. The transfer zone or fault in the study area is oblique to the rift axis of the northern Okinawa Trough, probably parallel to the extension direction. Major transfer faults are generally oblique to the rift axis (Gibbs, 1984) and often separate listric growth faults with opposing detachment asymmetries

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Fig. 9. Schematic model illustrating the evolution of the northern Okinawa Trough and adjacent areas. (A) In the Early Miocene, the subduction of the Philippine Sea plate beneath the Taiwan–Sinzi belt induced a weak zone in the eastern part of the Taiwan–Sinzi belt. (B) In the Middle Miocene, the breakup of the weak zone formed the Ho Basin as the continental fragment separated from the Taiwan–Sinzi belt. The continental fragment and the volcanic arc made up the paleo-Longwan Ridge. (C) Between the Middle and Late Miocene, rifting in the Ho Basin jumped to the paleo-Longwan Ridge and initiated the northern Okinawa Trough. (D) Rifting/extension of the northern Okinawa Trough has continued since the Late Miocene; the subsidence in the northern East China Sea shelf basin was interrupted by compression that created an extensive thrust–fold belt which was eroded by the Late Miocene erosion event. (D) Rifting/extension in the northern Okinawa Trough today is trough-wide and diffuse probably due to the convergence or subduction of the topographically high and buoyant Amami Plateau at the northern Ryukyu arc.

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during continental breakup (Bosworth, 1986; Lister et al., 1986). Because segmentation of rift margin by transfer faults tends to increase with increase in obliquity of the rift axis (McClay et al., 2002), the oblique extension in the northern Okinawa Trough may have facilitated the transfer zone or fault in the area. The gently deepening seafloor along the western margin of the northern Okinawa Trough is probably due to slow subsidence while the steep border faults along western margin of the southern Okinawa Trough may indicate rapid subsidence. The symmetric faults forming well-defined axial grabens in the southern Okinawa Trough (Fournier et al., 2001; Park et al., 1998; Sibuet et al., 1987) and the widely distributed faults in the intra-trough region of the northern Okinawa Trough may be attributed to the different subsidence rates in the southern and northern Okinawa Trough. We interpret that the well-developed axial graben and the steep border faults in the southern Okinawa Trough is due to focused rifting/extension. The thin crust and the high magnetic anomalies in the axial zone of the southern Okinawa Trough may further suggest an embryonic stage of seafloor spreading. On the other hand, the thick crust and the apparent absence of magnetic anomalies in the northern Okinawa Trough probably indicate that the northern Okinawa Trough is still in a rifting stage. However, the southern Okinawa Trough is much narrower (ca. 100 km wide) than the northern Okinawa Trough (ca. 200 km wide) although the total amounts of extension in both regions, estimated from seismic refraction and gravity data, are almost the same (~ 80 km) (Sibuet et al., 1995, 1998). The greater width of the northern Okinawa Trough may be due to trough-wide or diffuse rifting/extension. We postulate that the convergence or subduction of the Amami Plateau (Fig. 1) at the northern Ryukyu arc, as reported by Nishizawa et al. (2009), has caused the perturbation in the kinematics of the overriding plate by the positive buoyancy and high topography, resulting in diffuse rifting/extension in the northern Okinawa Trough. The arrival or subduction of buoyant ridges and plateaus at the subduction zone locally reduces the rate of subduction rollback (Rosenbaum and Mo, 2011), leading to significant deformation and topographic changes along the margin of the overriding plate (Gerya et al., 2009; Gutscher et al., 2000) and even to trench advance (Schellart et al., 2008). The near convergence of the Daito Ridge (Fig. 1) into the Ryukyu arc may also contribute to the kinematic perturbation in the northern Okinawa Trough. The diffuse or troughwide rifting/extension in the northern Okinawa Trough may have further delayed seafloor spreading in the area. The shallow (5000– 6000 m) depth of the northern Ryukyu Trench compared with the southern part (over 7000 m water depth) of the trench (Sandwell and Smith, 1997) may also be due to the convergence or subduction of the Amami Plateau. The maximum trench depth is known to be correlated with regional seafloor depth near the trench (Grellet and Dubois, 1982; Hilde and Uyeda, 1983). Fig. 9 is a schematic model illustrating the evolution of the northern Okinawa Trough and adjacent areas. In the Early Miocene, the northern East China Sea shelf basin entered the stage of postrift subsidence (Lee et al., 2006) and the subduction of the Philippine Sea plate induced a weak zone in the eastern part of the Taiwan-Sinzi belt (Fig. 9A). The breakup of this weak zone, formed the Ho Basin in the Middle Miocene (Fig. 9B). The NW-trending transfer zone or fault accommodated the differential extension in the opening of the Ho Basin. The continental fragment separated from the Taiwan– Sinzi belt with the opening of the Ho Basin and the volcanic arc formed by the subduction of the Philippine Sea plate comprised the paleo-Longwan Ridge. Between the Middle and Late Miocene, rifting in the Ho Basin jumped to the paleo-Longwan Ridge and initiated the northern Okinawa Trough (Fig. 9C). Rifting/extension in the northern Okinawa Trough continued in the Late Miocene while subsidence in the northern East China Sea shelf basin was interrupted by compressional tectonism, creating an extensive thrust–fold belt

which was subsequently eroded, resulting in the Late Miocene unconformity. The arrival and/or subduction of the Amami Plateau at the northern Ryukyu arc has caused trough-wide or diffuse rifting/extension in the northern Okinawa Trough (Fig. 9D). 6. Summary and conclusions ● The Taiwan–Sinzi belt acted as a buttress for the Ho Basin and the Okinawa Trough against the compressional tectonism that caused uplift and folding in the northern East China Sea shelf basin. ● The back-arc rifting/extension has propagated from the Ho Basin to the northern Okinawa Trough in the Late Miocene. ● The Longwan Ridge may be the youngest relict arc or buried ridge in the area. ● The two fault sets of opposite polarities along the western margin of the northern Okinawa Trough appear to be separated by a NWtrending left-lateral transfer zone or fault that extends across the Taiwan–Sinzi belt and into the northern East China Sea shelf basin. ● The northern Okinawa Trough is characterized by trough-wide or diffuse rifting/extension whereas the southern Okinawa Trough by focused rifting/extension. ● The tectonic perturbation due to the convergence or subduction of the Amami Plateau at the northern Ryukyu arc may have caused the diffuse rifting/extension in the overriding northern Okinawa Trough. Acknowledgements This work was funded by Korea Institute of Geoscience and Mineral Resources (KIGAM) (Study on Coastal Geohazard Factor Analysis) and Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Development of Deepwater Hydrocarbon Exploration, Evaluation, and Assessment Technology). Partial financial support to the first author and the corresponding author was provided respectively by Korea Integrated Ocean Drilling Program (K-IODP) and the Ministry of Land, Transport and Maritime Affairs of Korea (Construction of Carbon Storage Map and Selection of Demonstration Sites in Korean Offshore Areas). We thank KIGAM for permission to publish the data. Comments and suggestions by two anonymous reviewers greatly improved the quality of the paper. Kingdom Suite®, provided by Seismic Micro-Technology, Inc. was used for seismic data interpretation and mapping. References Acocella, V., Morvillo, P., Funiciello, R., 2005. What controls relay ramps and transfer faults within rift zones? Insights from analogue models. Journal of Structural Geology 27, 297–408. Bosworth, W., 1986. Comment and reply on “Detachment faulting and the evolution of passive continental margins”. Geology 14, 890–891. Chung, S.-L., Wang, S.-L., Shinjo, R., Lee, C.-S., Chen, C.-H., 2000. Initiation of arc magmatism in an embryonic continental rifting zone of the southernmost part of Okinawa Trough. Terra Nova 12, 225–230. Cukur, D., Horozal, S., Kim, D.C., Han, H.C., 2011. Seismic stratigraphy and structural analysis of the northern East China Sea Shelf Basin interpreted from multichannel seismic reflection data and cross-section restoration. Marine and Petroleum Geology 28, 1003–1022. Davagnier, M., Marsset, B., Sibuet, J.-C., Letouzey, J., Foucher, J.-P., 1987. Mechnismes actuels d'extension dans le bassin d'Okinawa. Bulletin de la Societe Geologique de France 8, 525–531. DeMets, C., Gordon, R.G., Argus, D.F., Stein, S., 1990. Current plate motions. Geophysical Journal International 101, 425–478. Eguchi, T., Uyeda, S., 1983. Seismotectonics of the Okinawa Trough and Ryukyu Arc. Geological Society of China Memoir 5, 189–210. El-Fiky, G.S., Kato, T., Oware, E.N., 1999. Crustal deformation and interpolate coupling in the Shikoku district, Japan, as seen from continuous GPS observation. Tectonophysics 314, 387–399. Fabbri, O., Fournier, M., 1999. Extension in the southern Ryukyu arc (Japan): Link with oblique subduction and back-arc rifting. Tectonics 18, 486–497. Fabbri, O., Monié, P., Fournier, M., 2004. Transtensional deformation at the junction between the Okinawa Trough back-arc basin and the SW Japan island arc. In: Grocott, J., McCaffrey, K.J.W., Taylor, G., Tikoff, B. (Eds.), Vertical Coupling and Decoupling

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