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Lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone: an example of decreasing plate coupling
Earth and Planetary Science Letters 186 (2001) 471^478 www.elsevier.com/locate/epsl
Lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone: an example of decreasing plate coupling Shu-Kun Hsu * Institute of Geophysics, National Central University, Chung-Li 32054, Taiwan Received 6 November 2000; received in revised form 11 January 2001; accepted 1 February 2001
Abstract The Okinawa Trough is a backarc basin located behind the Ryukyu arc^trench system. The southernmost part of the Okinawa Trough (SPOT) displays different tectonic features from the rest of the Okinawa Trough. The SPOT area includes abundant seamounts with active hydrothermal venting and high heat-flow values. To understand better the rifting and magmatism context of the SPOT area, we examine the lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone. The results show that beneath the SPOT area the continental crust and mantle lithosphere thickness of V25^30 and 120 km, respectively, are thick with little continental thinning. The analysis of mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates. However, the two plates are actually decoupled as indicated by presentday interface earthquakes. This situation indicates that the southernmost Ryukyu subduction zone displays a transitory case of a changing plate coupling, from a strong to a weak plate coupling. Such a coupling/decoupling transition is probably associated with the collision of the Luzon arc with the Asian continental margin. Additionally, the curve of the mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone indicates that the submarine magmatism in the SPOT area is located within the volcanic arc area, suggesting early arc magmatism in the SPOT area. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: Ryukyu trench; subduction zone; Okinawa trough; plate coupling/decoupling; buoyancy
1. Introduction Rifted in the southeast Asian continental margin, the Okinawa Trough (OT) is a backarc basin extending from Kyushu in the north to Taiwan in the south [1^3]. The backarc rifting of the OT is
* Tel.: +886-3-4268316; Fax: +886-3-4222044; E-mail: [email protected]
generally linked to the northward subduction of the Philippine Sea plate beneath the Ryukyu arc (Fig. 1). The westernmost Philippine Sea plate (i.e. the Huatung basin) is bordered by Taiwan in the west. The OT exhibits multiphase rifting. The middle-northern OT has formed since late Miocene [4]; whereas, the southern OT is believed to be formed since early Pleistocene [5]. Moreover, the southernmost part of OT (hereafter SPOT), west of about 123³E, shows di¡erent structural [6,7] and geochemical [8] characters
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from the rest of the OT. To the south of the SPOT, the contiguous southern Ryukyu arc is a non-volcanic arc [9]. The Ryukyu arc west of 123³E shows a NW^SE trend instead of the regular NE^SW trend of the rest of the Ryukyu arc (Fig. 1). Along the south Ryukyu subduction zone, the surface projection of the 100 km deep Wadati^Benio¡ zone is also curved near 123³E (Fig. 1). Magmatism has occurred along the central depression of the trough and rifting in the SPOT area has propagated toward Taiwan with a fast rate of V126 mm/yr [10]. The submarine volcanism is observed in terms of seamounts [4]. Additionally, active hydrothermal venting [11] and high heat-£ow values (as high as 1500 mW/m2 ; Chuen-Tien Shyu, 1999, personal communication) are present. Morphologically, the submarine volcanoes in the SPOT area are backarc in style [4]. Abundant shallow normal faulting earthquakes are associated with the backarc rifting [6]. In this paper, we examine lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone, which help us understand better the context of rifting and magmatism in the SPOT area. 2. Residual topography and crust and mantle lithosphere buoyancies Residual topography is the observed topography minus the crustal contribution to the topography. The residual topography is attributed to the mantle lithosphere buoyancy, if we assume that a region is in isostatic equilibrium. Lachenbruch and Morgan [12] showed that:
O a H c H m 3H 0 ; a1 for O v 0 ba a for O 6 0 b a3b w
1
where O is the surface elevation ; ba and bw are the densities of the asthenosphere and water, respectively; H0 is a reference datum of the buoyant height of sea level and is about 2.4 km estimated at mid-ocean ridges [12]; Hc and Hm are the
buoyancies of the crust and mantle lithosphere, respectively: Hc
1 b a 3 b c Lc ba
2
Hm
1 b a 3 b m Lm ba
3
where bc and bm and Lc and Lm are the densities and the thickness of the crust and the mantle lithosphere, respectively. Thus, if we know the surface elevation (O) and the crust buoyancy (Hc ), we can calculate the mantle lithosphere buoyancy (Hm ) from Eq. 1 and then the mantle lithosphere thickness (Lm ) from Eq. 3. Following Lachenbruch and Morgan [12], Jones et al. [13] and Gvirtzman and Nur [14], ba = 3200 kg m33 is used in this paper. In general, the normal Hm value ranges from 31.5 to 32.5 km [14]. The above relationships are based on a lithosphere freely £oating on the asthenosphere. If asthenospheric material below the overriding plate upwells dynamically toward the plate contact between the overriding and subducting plates due to the thermal e¡ect and thinning of the mantle lithosphere, the topography of the overriding plate near the plate boundary can be uplifted higher than in the statically balanced £oating lithosphere. Similarly, the subducting plate can be lower due to dynamic drag forces from the subducted slab. Because of the non-isostatic addition or diminution of the topography, the calculated Hm value near a convergent plate boundary cannot represent the buoyancy of the mantle lithosphere [14]. However, if Hc is known, the change of surface elevation is re£ected on the calculated Hm (see Eq. 1). In other words, near a convergent plate boundary the change of surface elevation depends on the status of plate coupling and is revealed by the Hm anomaly. 3. Crustal and mantle lithosphere structure In order to calculate Hc , we must know the crustal structure across the southernmost Ryukyu subduction zone. Because numerous marine geo-
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Fig. 1. Topography of the study area. Broad gray line indicates the location of the surface projection of the 100 km deep Wadati^Benio¡ zone. Double dashed lines indicate central depressions of the OT. HC: Hualien canyon. YD: Yaeyama depression. GR: Gagua ridge. HB: Huatung basin. NB: Nanao basin. RA: Ryukyu arc. RT: Ryukyu trench. Triangular symbol indicates submarine volcanism (same volcanism symbol in Figs. 2 and 3).
physical surveys have been conducted o¡ eastern Taiwan, we can compile the crustal structure along pro¢le AAP in Fig. 1. In this study the recent compilation of topographic/bathymetric data
of Liu et al. [15] is used as the surface elevation. The top of the continental or oceanic crust is determined on the basis of available seismic (or deep seismic) re£ection studies [4,16,17] (Fig. 2).
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Fig. 2. Construction of the crustal structure and gravity modeling along pro¢le AAP in Fig. 1. The observed gravity anomaly is extracted from [23]. The unit of density in the structure is Mg m33 .
These data indicate that the sediments (accretionary prism) are very thick (ca. 5 km) at the convergent plate boundary (Fig. 2). The depth to the Moho on the oceanward side of pro¢le AAP is constrained by seismic refraction studies [18^20]. As shown in Fig. 2, the thickness of the oceanic crust in the Huatung basin is about twice that of the normal oceanic crust (6 km). The geometry of the subducted slab is based on the distribution of seismicity [21]. The Moho depth beneath the continental crust is 30 km at the northern end of pro¢le AAP, which corresponds to a continental crust with little or no thinning [1^3,22]. Finally, to verify that the compiled structure is also compatible with gravity data, forward gravity modeling has been performed along pro¢le AAP. As shown in Fig. 2, calculated gravity anomalies are in agreement with the observed gravity anomalies [23]. Gravity modeling is non-unique; however, because the geometry of the used crustal structure has been constrained by seismic results, and as the velocity^density relationship of the Nafe^Drake curve [24] has been used to constrain the gravity model, the non-uniqueness problem of gravity
modeling has little in£uence in the calculation of buoyancy. Two major insights can be obtained from the above 2D model (Fig. 2). Firstly, the density of the upper mantle beneath the continental crust is lighter than beneath the oceanic crust. This implies that the upper mantle is hotter beneath the SPOT area. Indeed, the heat-£ow values are abnormally high in the SPOT area as mentioned previously (Chuen-Tien Shyu, 1999, personal communication). Because of di¡erent upper mantle densities, in this study the average density of the mantle lithosphere (bm ) is estimated from 60% of the upper mantle density plus 40% of the asthenosphere density (3200 kg m33 ). That is, we adopt bm = 3254 kg m33 and bm = 3266 kg m33 for the landward and oceanward part, respectively, in the estimation of the mantle lithosphere thickness. The above derivation of mantle lithosphere density is aimed to have a similar result of usual average mantle lithosphere density (i.e. 3250 kg m33 ). However, slight di¡erence of bm will not change our conclusion. Secondly, the crust is relatively thin below the central trough; however,
Fig. 3. Lithospheric structure along pro¢le AAP in Fig. 1. Black dots indicate the calculated depths of the lithosphere^ asthenosphere boundary. Crosses represent the seismic plate coupling zone (V15^35 km deep) [21]. Hc : calculated crust buoyancy. Hm : calculated mantle lithosphere buoyancy.
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the rifted crust below the central trough is still about 8 km thicker than the crust below the Yaeyama depression in the east (cf. [22,25]). On the basis of Fig. 2, the total crust buoyancy Hc was calculated (Fig. 3). Then, the mantle lithosphere buoyancy Hm was calculated from Eq. 1 (Fig. 3). The calculated thickness of the mantle lithosphere is about 120 km (Fig. 3). 4. Discussion 4.1. Decreasing plate coupling across the southernmost Ryukyu subduction zone Across a subduction zone, the location of the plate boundary is supposed to be at the minimum value of the Hm curve (Fig. 4) [14]. The mantle lithosphere buoyancy Hm can re£ect the coupling status between the overriding and subducting plates [14]. For example, the variation of the Hm curve across the Andes subduction zone is relatively £at (around the normal values), representing a strong plate coupling (Fig. 4). As indicated by the down-dip tendency of Hm toward the plate boundary, the overriding plate is pulled down by the subducting slab. The minimum Hm value at the plate boundary is about 2 km higher than the possible lowest value (35 km), suggesting that the
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subducting plate may su¡er a strong suction preventing slab sinking fast. In contrast, the Hm curve across the Calabria peninsula in southern Tyrrhenian subduction zone (Italy) represents a weak plate coupling (Fig. 4). The Hm of the overriding plate is about 1.5 km higher than normal Hm (between 31.5 and 32.5 km); and the Hm at the plate boundary is about 2.5 km lower than normal Hm (Fig. 4). In weak plate coupling, the overriding plate is decoupled (or detached) from the subducted plate and the wedged asthenospheric material propagates upward toward the plate contact between the overriding and subducting plates ; simultaneously, the surface elevation increases. The subducting plate rolls back rapidly due to heavy slab pull and reduction of suction. The Hm curves across Bonin and Kurile arcs correspond to intermediate coupling examples (Fig. 4). In this study, the variation of the Hm curve across the southernmost Ryukyu subduction zone is relatively small (Fig. 4). It suggests the existence of a strong plate coupling and is in agreement with the 5 km thick accretionary prism deposited at the plate boundary (Fig. 2). However, the interface thrust earthquakes along pro¢le AAP indicate that the plate coupling zone occurs only in the 15 to 35 km depth interval (Fig. 3) [21]. Comparing this seismicity to that of the An-
Fig. 4. Comparison of the Hm (mantle lithosphere buoyancy) curve across the southernmost Ryukyu subduction zone with four other subduction zones (see [14] and references therein). Andes represents a strong plate coupling case. Calabria represents a weak plate coupling case. Bonin and Kurile represent intermediate characteristics. Gray area (32 þ 0.5 km) marks normal contribution of the mantle lithosphere to the Earth's topography. Black dots show locations of volcanic arcs. Open circle indicates the location of submarine magmatism in the SPOT area.
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Fig. 5. Schematic diagram showing the changing plate coupling between the overriding and subducting plates across the southernmost Ryukyu subduction zone. The solid line represents the present-day strong coupling Hm curve and the dashed line represent the Hm curve when the overriding and subducting plates are fairly decoupled. Arrows indicate the changing tendency.
des and Kurile cases where the plate coupling zones reach a depth of V100 km and to the weak plate coupling case of the Calabria where the plate coupling zone only reaches a depth of V30 km [14], the southernmost Ryukyu subduction zone should be in a weak plate coupling regime. In the weak coupling case, the mantle lithosphere of the overriding plate thins toward the plate boundary and the crust may directly overlie the asthenosphere [14]. The direct contact between the asthenosphere and the crust de facto is not apparent beneath the south Ryukyu arc; even thinning of the mantle lithosphere is not signi¢cant (Fig. 3). As thick sedimentary deposits are generally accumulated during long periods of subsidence, strong plate coupling in the southernmost Ryukyu subduction zone probably persisted for several Ma prior to actual plate decoupling. The present-day steep dipping angle (V65³) of the subducted slab (Fig. 3) in fact suggests quick rollback of the subducted slab and rifting of the SPOT. Abnormally higher seismicity in the SPOT area and beneath the Nanao basin than east of 123.5³E [6] may be due to the ongoing plate decoupling. As shown in Fig. 4, away from the plate boundary each subducting plate exhibits a Hm value in the range of normal Hm (between 31.5 and 32.5 km). This Hm value, containing less contribution from slab pull, may represent a speci¢c characteristic of each subducting plate. For example,
although the plate coupling is strong for both the Andes and the South Ryukyu cases, away from the plate boundary their Hm values of the subducting plate are di¡erent (Fig. 4). Because the plate coupling decreases across the southernmost Ryukyu subduction zone, the change of the Hm curve in the South Ryukyu is anticipated. When the overriding and subducting plates are fairly decoupled as the case of Calabria, the minimum value of the Hm (at the plate boundary) may be down to 35 km and the maximum value of Hm (of the overriding plate) may reach zero km (Fig. 5). 4.2. Possible mechanism for decreasing plate coupling in the study area The change from a strong plate coupling to an early stage of plate decoupling in the southernmost Ryukyu subduction zone may be linked to the collision of the Luzon arc with the Asian continental margin [6,26]. Since ca. 9^6 Ma ago the Luzon arc has collided with the proto-south Ryukyu margin in a NW direction. After the Luzon arc collision, the cause of a strong plate coupling in the southernmost Ryukyu subduction zone disappeared, which allowed fast post-collisional rifting in the SPOT area. One way to examine the plate coupling history is in the 5 km thick record of sediments deposited in the Nanao basin. 4.3. Backarc or arc magmatism in the SPOT area? As shown in Figs. 2 and 3, the submarine volcanism in the SPOT area belongs to a low density and/or hot magmatic source. The crust and mantle lithosphere are thick, suggesting that the volcanic magmatism in the SPOT area should have crustal contamination. On the basis of the Hm curve in Fig. 3, the minimum Hm (i.e. the corresponding trench location) is situated at the place where the Hualien Canyon cuts the pro¢le (cf. Figs. 1 and 3). In this case, the submarine magmatism in the SPOT area (the open circle location in Fig. 4) is located in the `arc' area, when compared to other subduction zones (Fig. 4). It implies that arc magmatism is developing beneath the SPOT backarc basin. This remark is consis-
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tent with geochemical analysis [8]. Also, the 100 km deep Wadati^Benio¡ zone, suggestive of arc volcanism, is located below the submarine volcanism. It is noted that the present-day volcanic front of the Ryukyu arc is only correlated with the 100 km deep Wadati^Benio¡ zone from Japan to east of 123.5³E [7]. As indicated by relatively low magnetization, the sedimentary Ryukyu arc west of 123.5³E has no present-day arc volcanism or is an area of very low magmatic activity [7]. Therefore, the submarine volcanoes in the SPOT area may represent early arc magmatism occurring in a backarc regime. 5. Conclusion Across the southernmost Ryukyu subduction zone, the oceanic crust (of the Huatung basin) and the continental crust reach V12 and V25^ 30 km thick, respectively. Thick continental crust and mantle lithosphere without signi¢cant thinning indicates that the backarc spreading in the SPOT area is in an early stage. The weak mantle lithosphere buoyancy and thick accretionary prism suggest the residence of a strong plate coupling across the southernmost Ryukyu subduction zone. However, a shallow seismic coupling zone indicates the plate coupling is actually decreasing, which agrees with fast rifting in the SPOT area. Hence, the southernmost Ryukyu subduction zone displays a peculiar case of changing plate coupling from a strong plate coupling to an early stage of plate decoupling. This change may be linked to the collision and post-collision phase of the Luzon arc with respect to the Asian continental margin from 9^6 Ma to present. Additionally, according to the mantle lithosphere buoyancy curve, the submarine volcanoes in the SPOT area are located in an arc area, suggesting that early arc magmatism is occurring in the SPOT area. Acknowledgements Constructive reviews and/or comments from P. Morgan, J.-C. Sibuet and H. Kao are appreciated.
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This study was supported by the National Science Council, Taiwan.[AC] References [1] C.S. Lee, G. Shor Jr., L.D. Bibee, R.S. Lu, T.W.C. Hilde, Okinawa Trough: origin of a backarc basin, Mar. Geol. 35 (1980) 219^241. [2] J. Letouzey, M. Kimura, The Okinawa Trough genesis of a back-arc basin developing along a continental margin, Tectonophysics 125 (1986) 209^320. [3] J.-C. Sibuet, J. Letouzey, F. Barbier, J. Charvet, J.-P. Foucher, T.W.C. Hilde, M. Kimura, L.-Y. Chiao, B. Marsset, C. Muller, J.-F. Stephan, Backarc extension in the Okinawa Trough, J. Geophys. Res. 92 (1987) 14041^ 14063. [4] J.-C. Sibuet, B. De¡ontaines, S.-K. Hsu, N. Thareau, J.-P. Le Formal, C.-S. Liu ACT party,, Okinawa trough backarc basin: early tectonic and magmatic evolution, J. Geophys. Res. 103 (1998) 30245^30267. [5] J.O. Park, H. Tokyuyama, M. Shinohara, K. Suyehiro, A. Taira, Seismic record of tectonic evolution and backarc rifting in the southern Ryukyu island arc system, Tectonophysics 294 (1998) 21^42. [6] S.-K. Hsu, J.-C. Sibuet, S. Monti, C.-T. Shyu, C.-S. Liu, Transition from the Okinawa trough backarc extension and the Taiwan collision: new insights on the southernmost Ryukyu subduction zone, Mar. Geophys. Res. 18 (1996) 163^187. [7] S.-K. Hsu, J.-C. Sibuet, C.-T. Shyu, Magnetic inversion in the East China Sea and Okinawa Trough: tectonic implications, Tectonophysics (2001), in press. [8] S.-L. Chung, R. Shinjo, C.-S. Lee, S.-L. Wang, C.-H. Chen, Initiation of arc magmatism associated with embryo continental rifting in the southernmost part of Okinawa Trough, Tera Nova (2001), in press. [9] K. Kizaki, Geology and tectonics of the Ryukyu islands, Tectonophysics 125 (1986) 193^207. [10] C.-C. Liu, The Ilan plain and the southwestward extending Okinawa Trough, J. Geol. Soc. China 38 (1995) 329^ 342. [11] C.-S. Lee, C.-F. Tsai, S.-L. Chung SPOT members,, Active interaction of submarine volcanoes and ocean current in the southernmost part of Okinawa Trough, EOS Trans. AGU 79 (1998) F858. [12] A.H. Lachenbruch, P. Morgan, Continental extension, magmatism and elevation; formal relations and rule of thumb, Tectonophysics 174 (1990) 39^62. [13] H.J. Jones, J.R. Unruh, L.J. Sonders, The role of gravitational potential energy in active deformation in the southwestern United States, Nature 381 (1996) 37^41. [14] Z. Gvirtzman, A. Nur, Plate detachment, asthenosphere upwelling and topography across subductions, Geology 27 (1999) 563^566. [15] C.-S. Liu, S.-Y. Liu, S.E. Lallemand, N. Lundberg, D.L. Reed, Digital elevation model o¡shore Taiwan and its
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S.-K. Hsu / Earth and Planetary Science Letters 186 (2001) 471^478 tectonic implications, Terrestrial, Atmospheric and Oceanic Sciences 9 (1998) 705^738. P. Schnu«rle, C.-S. Liu, S.E. Lallemand, D.L. Reed, Structural insight into the south Ryukyu margin e¡ects of the subducting Gagua Ridge, Tectonophysics 288 (1998) 237^ 250. A. Deschamps, S. Lallemand, J.-Y. Collot, A detailed study of the Gagua Ridge: a fracture zone uplifted during a plate reorganisation in the Mid-Eocene, Mar. Geophys. Res. 20 (1998) 403^423. Y.-S. Yang, T.K. Wang, Crustal velocity variation of the western Philippine Sea plate from TAICRUST OBS/MCS line 23, Terrestrial, Atmospheric and Oceanic Sciences 9 (1998) 379^393. T.K. Wang, C.-H. Chiang, Imaging of arc^arc collision in the Ryukyu forearc region o¡shore Hualien from TAICRUST OBS line 16, Terrestrial, Atmospheric and Oceanic Sciences 9 (1998) 329^344. K.D. McIntosh, Y. Nakamura, Crustal structure beneath the Nanao forearc basin from TAICRUST MCS/OBS line 14, Terrestrial, Atmospheric and Oceanic Sciences 9 (1998) 345^362. H. Kao, S.J. Shen, K.-F. Ma, Transition from oblique
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subduction to collision: earthquake in the southernmost Ryukyu arc^Taiwan region, J. Geophys. Res. 103 (1998) 7211^7229. J.-C. Sibuet, S.-K. Hsu, C.-T. Shyu, C.-S. Liu, Structural and kinematic evolution of the Okinawa Trough backarc basin, in: B. Taylor (Ed.), Backarc Basin: Tectonics and Magmatism, Plenum, New York, 1995, pp. 343^378. S.-K. Hsu, C.-S. Liu, C.-T. Shyu, S.-Y. Liu, J.-C. Sibuet, S. Lallemand, C. Wang, D. Reed, New gravity and magnetic anomaly maps in the Taiwan^Luzon region and their preliminary interpretation, Terrestrial, Atmospheric and Oceanic Sciences 9 (1998) 509^532. W.J. Ludwig, J.E. Nafe, C.L. Drake, Seismic refraction, in: L.E. Maxwell (Ed.), New Concept of Sea Floor Evolution, Part 1: Regional Observations, Wiley-Interscience, New York, 1970, pp. 53^84. N. Hirata, H. Kinoshita, H. Katao, H. Baba, Y. Kaiho, S. Koresawa, Y. Ono, K. Jayashi, Report on DELP 1988 cruises in the Okinawa Trough, Part 3: Crustal structure of the Southern Okinawa Trough, Earthquake Res. Inst. Univ. Tokyo Bull. 66 (1991) 37^70. J.-C. Sibuet, S.-K. Hsu, Geodynamics of the Taiwan arc^ arc collision, Tectonophysics 274 (1997) 221^251.
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Lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone: an example of decreasing plate coupling Shu-Kun Hsu * Institute of Geophysics, National Central University, Chung-Li 32054, Taiwan Received 6 November 2000; received in revised form 11 January 2001; accepted 1 February 2001
Abstract The Okinawa Trough is a backarc basin located behind the Ryukyu arc^trench system. The southernmost part of the Okinawa Trough (SPOT) displays different tectonic features from the rest of the Okinawa Trough. The SPOT area includes abundant seamounts with active hydrothermal venting and high heat-flow values. To understand better the rifting and magmatism context of the SPOT area, we examine the lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone. The results show that beneath the SPOT area the continental crust and mantle lithosphere thickness of V25^30 and 120 km, respectively, are thick with little continental thinning. The analysis of mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone shows strong plate coupling between the overriding and subducting plates. However, the two plates are actually decoupled as indicated by presentday interface earthquakes. This situation indicates that the southernmost Ryukyu subduction zone displays a transitory case of a changing plate coupling, from a strong to a weak plate coupling. Such a coupling/decoupling transition is probably associated with the collision of the Luzon arc with the Asian continental margin. Additionally, the curve of the mantle lithosphere buoyancy across the southernmost Ryukyu subduction zone indicates that the submarine magmatism in the SPOT area is located within the volcanic arc area, suggesting early arc magmatism in the SPOT area. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: Ryukyu trench; subduction zone; Okinawa trough; plate coupling/decoupling; buoyancy
1. Introduction Rifted in the southeast Asian continental margin, the Okinawa Trough (OT) is a backarc basin extending from Kyushu in the north to Taiwan in the south [1^3]. The backarc rifting of the OT is
* Tel.: +886-3-4268316; Fax: +886-3-4222044; E-mail: [email protected]
generally linked to the northward subduction of the Philippine Sea plate beneath the Ryukyu arc (Fig. 1). The westernmost Philippine Sea plate (i.e. the Huatung basin) is bordered by Taiwan in the west. The OT exhibits multiphase rifting. The middle-northern OT has formed since late Miocene [4]; whereas, the southern OT is believed to be formed since early Pleistocene [5]. Moreover, the southernmost part of OT (hereafter SPOT), west of about 123³E, shows di¡erent structural [6,7] and geochemical [8] characters
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from the rest of the OT. To the south of the SPOT, the contiguous southern Ryukyu arc is a non-volcanic arc [9]. The Ryukyu arc west of 123³E shows a NW^SE trend instead of the regular NE^SW trend of the rest of the Ryukyu arc (Fig. 1). Along the south Ryukyu subduction zone, the surface projection of the 100 km deep Wadati^Benio¡ zone is also curved near 123³E (Fig. 1). Magmatism has occurred along the central depression of the trough and rifting in the SPOT area has propagated toward Taiwan with a fast rate of V126 mm/yr [10]. The submarine volcanism is observed in terms of seamounts [4]. Additionally, active hydrothermal venting [11] and high heat-£ow values (as high as 1500 mW/m2 ; Chuen-Tien Shyu, 1999, personal communication) are present. Morphologically, the submarine volcanoes in the SPOT area are backarc in style [4]. Abundant shallow normal faulting earthquakes are associated with the backarc rifting [6]. In this paper, we examine lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone, which help us understand better the context of rifting and magmatism in the SPOT area. 2. Residual topography and crust and mantle lithosphere buoyancies Residual topography is the observed topography minus the crustal contribution to the topography. The residual topography is attributed to the mantle lithosphere buoyancy, if we assume that a region is in isostatic equilibrium. Lachenbruch and Morgan [12] showed that:
O a H c H m 3H 0 ; a1 for O v 0 ba a for O 6 0 b a3b w
1
where O is the surface elevation ; ba and bw are the densities of the asthenosphere and water, respectively; H0 is a reference datum of the buoyant height of sea level and is about 2.4 km estimated at mid-ocean ridges [12]; Hc and Hm are the
buoyancies of the crust and mantle lithosphere, respectively: Hc
1 b a 3 b c Lc ba
2
Hm
1 b a 3 b m Lm ba
3
where bc and bm and Lc and Lm are the densities and the thickness of the crust and the mantle lithosphere, respectively. Thus, if we know the surface elevation (O) and the crust buoyancy (Hc ), we can calculate the mantle lithosphere buoyancy (Hm ) from Eq. 1 and then the mantle lithosphere thickness (Lm ) from Eq. 3. Following Lachenbruch and Morgan [12], Jones et al. [13] and Gvirtzman and Nur [14], ba = 3200 kg m33 is used in this paper. In general, the normal Hm value ranges from 31.5 to 32.5 km [14]. The above relationships are based on a lithosphere freely £oating on the asthenosphere. If asthenospheric material below the overriding plate upwells dynamically toward the plate contact between the overriding and subducting plates due to the thermal e¡ect and thinning of the mantle lithosphere, the topography of the overriding plate near the plate boundary can be uplifted higher than in the statically balanced £oating lithosphere. Similarly, the subducting plate can be lower due to dynamic drag forces from the subducted slab. Because of the non-isostatic addition or diminution of the topography, the calculated Hm value near a convergent plate boundary cannot represent the buoyancy of the mantle lithosphere [14]. However, if Hc is known, the change of surface elevation is re£ected on the calculated Hm (see Eq. 1). In other words, near a convergent plate boundary the change of surface elevation depends on the status of plate coupling and is revealed by the Hm anomaly. 3. Crustal and mantle lithosphere structure In order to calculate Hc , we must know the crustal structure across the southernmost Ryukyu subduction zone. Because numerous marine geo-
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Fig. 1. Topography of the study area. Broad gray line indicates the location of the surface projection of the 100 km deep Wadati^Benio¡ zone. Double dashed lines indicate central depressions of the OT. HC: Hualien canyon. YD: Yaeyama depression. GR: Gagua ridge. HB: Huatung basin. NB: Nanao basin. RA: Ryukyu arc. RT: Ryukyu trench. Triangular symbol indicates submarine volcanism (same volcanism symbol in Figs. 2 and 3).
physical surveys have been conducted o¡ eastern Taiwan, we can compile the crustal structure along pro¢le AAP in Fig. 1. In this study the recent compilation of topographic/bathymetric data
of Liu et al. [15] is used as the surface elevation. The top of the continental or oceanic crust is determined on the basis of available seismic (or deep seismic) re£ection studies [4,16,17] (Fig. 2).
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Fig. 2. Construction of the crustal structure and gravity modeling along pro¢le AAP in Fig. 1. The observed gravity anomaly is extracted from [23]. The unit of density in the structure is Mg m33 .
These data indicate that the sediments (accretionary prism) are very thick (ca. 5 km) at the convergent plate boundary (Fig. 2). The depth to the Moho on the oceanward side of pro¢le AAP is constrained by seismic refraction studies [18^20]. As shown in Fig. 2, the thickness of the oceanic crust in the Huatung basin is about twice that of the normal oceanic crust (6 km). The geometry of the subducted slab is based on the distribution of seismicity [21]. The Moho depth beneath the continental crust is 30 km at the northern end of pro¢le AAP, which corresponds to a continental crust with little or no thinning [1^3,22]. Finally, to verify that the compiled structure is also compatible with gravity data, forward gravity modeling has been performed along pro¢le AAP. As shown in Fig. 2, calculated gravity anomalies are in agreement with the observed gravity anomalies [23]. Gravity modeling is non-unique; however, because the geometry of the used crustal structure has been constrained by seismic results, and as the velocity^density relationship of the Nafe^Drake curve [24] has been used to constrain the gravity model, the non-uniqueness problem of gravity
modeling has little in£uence in the calculation of buoyancy. Two major insights can be obtained from the above 2D model (Fig. 2). Firstly, the density of the upper mantle beneath the continental crust is lighter than beneath the oceanic crust. This implies that the upper mantle is hotter beneath the SPOT area. Indeed, the heat-£ow values are abnormally high in the SPOT area as mentioned previously (Chuen-Tien Shyu, 1999, personal communication). Because of di¡erent upper mantle densities, in this study the average density of the mantle lithosphere (bm ) is estimated from 60% of the upper mantle density plus 40% of the asthenosphere density (3200 kg m33 ). That is, we adopt bm = 3254 kg m33 and bm = 3266 kg m33 for the landward and oceanward part, respectively, in the estimation of the mantle lithosphere thickness. The above derivation of mantle lithosphere density is aimed to have a similar result of usual average mantle lithosphere density (i.e. 3250 kg m33 ). However, slight di¡erence of bm will not change our conclusion. Secondly, the crust is relatively thin below the central trough; however,
Fig. 3. Lithospheric structure along pro¢le AAP in Fig. 1. Black dots indicate the calculated depths of the lithosphere^ asthenosphere boundary. Crosses represent the seismic plate coupling zone (V15^35 km deep) [21]. Hc : calculated crust buoyancy. Hm : calculated mantle lithosphere buoyancy.
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the rifted crust below the central trough is still about 8 km thicker than the crust below the Yaeyama depression in the east (cf. [22,25]). On the basis of Fig. 2, the total crust buoyancy Hc was calculated (Fig. 3). Then, the mantle lithosphere buoyancy Hm was calculated from Eq. 1 (Fig. 3). The calculated thickness of the mantle lithosphere is about 120 km (Fig. 3). 4. Discussion 4.1. Decreasing plate coupling across the southernmost Ryukyu subduction zone Across a subduction zone, the location of the plate boundary is supposed to be at the minimum value of the Hm curve (Fig. 4) [14]. The mantle lithosphere buoyancy Hm can re£ect the coupling status between the overriding and subducting plates [14]. For example, the variation of the Hm curve across the Andes subduction zone is relatively £at (around the normal values), representing a strong plate coupling (Fig. 4). As indicated by the down-dip tendency of Hm toward the plate boundary, the overriding plate is pulled down by the subducting slab. The minimum Hm value at the plate boundary is about 2 km higher than the possible lowest value (35 km), suggesting that the
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subducting plate may su¡er a strong suction preventing slab sinking fast. In contrast, the Hm curve across the Calabria peninsula in southern Tyrrhenian subduction zone (Italy) represents a weak plate coupling (Fig. 4). The Hm of the overriding plate is about 1.5 km higher than normal Hm (between 31.5 and 32.5 km); and the Hm at the plate boundary is about 2.5 km lower than normal Hm (Fig. 4). In weak plate coupling, the overriding plate is decoupled (or detached) from the subducted plate and the wedged asthenospheric material propagates upward toward the plate contact between the overriding and subducting plates ; simultaneously, the surface elevation increases. The subducting plate rolls back rapidly due to heavy slab pull and reduction of suction. The Hm curves across Bonin and Kurile arcs correspond to intermediate coupling examples (Fig. 4). In this study, the variation of the Hm curve across the southernmost Ryukyu subduction zone is relatively small (Fig. 4). It suggests the existence of a strong plate coupling and is in agreement with the 5 km thick accretionary prism deposited at the plate boundary (Fig. 2). However, the interface thrust earthquakes along pro¢le AAP indicate that the plate coupling zone occurs only in the 15 to 35 km depth interval (Fig. 3) [21]. Comparing this seismicity to that of the An-
Fig. 4. Comparison of the Hm (mantle lithosphere buoyancy) curve across the southernmost Ryukyu subduction zone with four other subduction zones (see [14] and references therein). Andes represents a strong plate coupling case. Calabria represents a weak plate coupling case. Bonin and Kurile represent intermediate characteristics. Gray area (32 þ 0.5 km) marks normal contribution of the mantle lithosphere to the Earth's topography. Black dots show locations of volcanic arcs. Open circle indicates the location of submarine magmatism in the SPOT area.
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Fig. 5. Schematic diagram showing the changing plate coupling between the overriding and subducting plates across the southernmost Ryukyu subduction zone. The solid line represents the present-day strong coupling Hm curve and the dashed line represent the Hm curve when the overriding and subducting plates are fairly decoupled. Arrows indicate the changing tendency.
des and Kurile cases where the plate coupling zones reach a depth of V100 km and to the weak plate coupling case of the Calabria where the plate coupling zone only reaches a depth of V30 km [14], the southernmost Ryukyu subduction zone should be in a weak plate coupling regime. In the weak coupling case, the mantle lithosphere of the overriding plate thins toward the plate boundary and the crust may directly overlie the asthenosphere [14]. The direct contact between the asthenosphere and the crust de facto is not apparent beneath the south Ryukyu arc; even thinning of the mantle lithosphere is not signi¢cant (Fig. 3). As thick sedimentary deposits are generally accumulated during long periods of subsidence, strong plate coupling in the southernmost Ryukyu subduction zone probably persisted for several Ma prior to actual plate decoupling. The present-day steep dipping angle (V65³) of the subducted slab (Fig. 3) in fact suggests quick rollback of the subducted slab and rifting of the SPOT. Abnormally higher seismicity in the SPOT area and beneath the Nanao basin than east of 123.5³E [6] may be due to the ongoing plate decoupling. As shown in Fig. 4, away from the plate boundary each subducting plate exhibits a Hm value in the range of normal Hm (between 31.5 and 32.5 km). This Hm value, containing less contribution from slab pull, may represent a speci¢c characteristic of each subducting plate. For example,
although the plate coupling is strong for both the Andes and the South Ryukyu cases, away from the plate boundary their Hm values of the subducting plate are di¡erent (Fig. 4). Because the plate coupling decreases across the southernmost Ryukyu subduction zone, the change of the Hm curve in the South Ryukyu is anticipated. When the overriding and subducting plates are fairly decoupled as the case of Calabria, the minimum value of the Hm (at the plate boundary) may be down to 35 km and the maximum value of Hm (of the overriding plate) may reach zero km (Fig. 5). 4.2. Possible mechanism for decreasing plate coupling in the study area The change from a strong plate coupling to an early stage of plate decoupling in the southernmost Ryukyu subduction zone may be linked to the collision of the Luzon arc with the Asian continental margin [6,26]. Since ca. 9^6 Ma ago the Luzon arc has collided with the proto-south Ryukyu margin in a NW direction. After the Luzon arc collision, the cause of a strong plate coupling in the southernmost Ryukyu subduction zone disappeared, which allowed fast post-collisional rifting in the SPOT area. One way to examine the plate coupling history is in the 5 km thick record of sediments deposited in the Nanao basin. 4.3. Backarc or arc magmatism in the SPOT area? As shown in Figs. 2 and 3, the submarine volcanism in the SPOT area belongs to a low density and/or hot magmatic source. The crust and mantle lithosphere are thick, suggesting that the volcanic magmatism in the SPOT area should have crustal contamination. On the basis of the Hm curve in Fig. 3, the minimum Hm (i.e. the corresponding trench location) is situated at the place where the Hualien Canyon cuts the pro¢le (cf. Figs. 1 and 3). In this case, the submarine magmatism in the SPOT area (the open circle location in Fig. 4) is located in the `arc' area, when compared to other subduction zones (Fig. 4). It implies that arc magmatism is developing beneath the SPOT backarc basin. This remark is consis-
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tent with geochemical analysis [8]. Also, the 100 km deep Wadati^Benio¡ zone, suggestive of arc volcanism, is located below the submarine volcanism. It is noted that the present-day volcanic front of the Ryukyu arc is only correlated with the 100 km deep Wadati^Benio¡ zone from Japan to east of 123.5³E [7]. As indicated by relatively low magnetization, the sedimentary Ryukyu arc west of 123.5³E has no present-day arc volcanism or is an area of very low magmatic activity [7]. Therefore, the submarine volcanoes in the SPOT area may represent early arc magmatism occurring in a backarc regime. 5. Conclusion Across the southernmost Ryukyu subduction zone, the oceanic crust (of the Huatung basin) and the continental crust reach V12 and V25^ 30 km thick, respectively. Thick continental crust and mantle lithosphere without signi¢cant thinning indicates that the backarc spreading in the SPOT area is in an early stage. The weak mantle lithosphere buoyancy and thick accretionary prism suggest the residence of a strong plate coupling across the southernmost Ryukyu subduction zone. However, a shallow seismic coupling zone indicates the plate coupling is actually decreasing, which agrees with fast rifting in the SPOT area. Hence, the southernmost Ryukyu subduction zone displays a peculiar case of changing plate coupling from a strong plate coupling to an early stage of plate decoupling. This change may be linked to the collision and post-collision phase of the Luzon arc with respect to the Asian continental margin from 9^6 Ma to present. Additionally, according to the mantle lithosphere buoyancy curve, the submarine volcanoes in the SPOT area are located in an arc area, suggesting that early arc magmatism is occurring in the SPOT area. Acknowledgements Constructive reviews and/or comments from P. Morgan, J.-C. Sibuet and H. Kao are appreciated.
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