ARTICLES Chinese Science Bulletin 2004 Vol. 49 No. 12 1279ü1288
Indentation tectonics in the accretionary wedge of middle Manila Trench LI Jiabiao1,2, JIN Xianglong1,2, RUAN Aiguo1, WU Shimin3, WU Ziyin1 & LIU Jianhua1 1. Key Lab of Submarine Geosciences, State Oceanic Administration, Hangzhou 310012, China; 2. College of Sciences, Zhejiang University, Hangzhou 310027, China; 3. South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China Correspondence should be addressed to Li Jiabiao (e-mail: jbli@zgb. com.cn)
Abstract Based on the multibeam morpho-tectonic analysis of the Manila Trench accretionary wedge and its indentation tectonics and the contrasting researches with other geological and geophysical data, three tectonic zones of the wedge are established, faulting features, tectonic distribution and stress mechanism for the indentation tectonics are analyzed, oblique subduction along Manila Trench with convergent stress of NW55e is presented, and the relationship of the ceasing of Eastern Subbasin spreading of South China Sea Basin to the formation of subduction zone of Manila Trench is discussed. By the model analysis and regional research, it is found that the seamount subduction along Manila Trench does not lead to the erosion of the accretionary wedge and the oblique subduction actually is a NWWtrending obduction of Luzon micro-plate that results from the NWW-trending displacement of the Philippine Sea plate. Keywords: Manila Trench, accretionary wedge, indentation tectonics, oblique subduction, morpho-tectonic analysis. DOI: 10.1360/03wd0412
Some important progress has been reached in recent years on the indentation of accretionary wedges by subduction of seamounts or ridges on the oceanic plate along trenches[1ü8]. More geoscientists pay attention to these researches because of their great significance for deepening the insight into the structural styles, stress mechanism, accretive or erosive effects and plate kinematics of subduction zones. It is believable that the accretionary wedge in Costa Rica-Nicaragua of Central America is being eroded rather than accreted based on the study of indentation tectonics in the subduction zone[2] . By the morpho-tectonic analysis of Japan and Kuril Trench and its Erimo seamount indentation, the accretionary wedge of this continental margin is thought to be strongly rifted and subsided, the Erimo seamount penetration is a key factor resulting in left-lateral displacement between two trenches and reflects an oblique subduction along the Japan Trench[6]. The oblique subduction of Gagua Ridge along the Ryukyu Trench in the Northwestern Philippine Sea not Chinese Science Bulletin Vol. 49 No. 12
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only shapes the tectonic features of accretionary wedge and controls the distribution of forearc basins in this area, but leads to basement uplifting of the Ryukyu Arc[7]. The subduction zone of the Manila Trench (MT), located on the east of the South China Sea Basin (SCSB) and connected with deep-earthquake tectonic zone of Mindoro in the south and collision tectonic zone of Taiwan in the north[9], is thought an important active convergent boundary with special significance (Fig. 1). Researches show that the SCSB is subducting eastwards along the MT, and formed a tectonic system including nonvolcanic arc (accretionary wedge)-forearc basin (North and West Luzon Trough)-volcanic arc (Luzon volcanic arc). The Eastern Subbasin of SCSB was thought to be formed by the N-S spreading from 32 Ma to 17 Ma[10,11]. Recent research indicates that the spreading of late-stage after 24Ma trends NNW-SSE rather than N-S[12], and the Scarborough seamount chain as an extinct spreading ridge has been subducted, indented towards the MT and extended beneath the forearc basin[13]. Because of being limited by less data and tools, studies on tectonic dynamics, subduction stress, formation mechanism and evolution feature for the subduction zone of MT now are still not enough. Is the subduction mechanism of MT an oceanic subduction or continental obduction? How about its relationship to the spreading ceasing of SCSB? Where does the original force of subduction come from? What about the response mechanism between local and regional tectonics? Clearly it is significant and valuable to answer above questions. Thus we use newly-obtained multibeam swath sounding data of the accretionary wedge along middle MT to do morpho-tectonic analysis, contrast them with reflection seismic profiles and earthquake distribution to reveal the characters of indentation tectonics of seamount subduction in the accretionary wedge, and further try to discuss the tectonic features, stress field, subduction direction and dynamic mechanism of MT subduction zone. 1
Data acquisition and study method
The high-resolution multibeam swath sounding technology, combined with other geological and geophysical data, has a unique dominance for analyzing regional tectonics especially for young and active tectonics on the seafloor and has become an important tool for studying the tectonic features and formation mechanism of mid-ocean ridges, subduction zones[2,14]. In order to study the tectonic features of the MT, Second Institute of Oceanography (SIO) of State Oceanic Administration of China carried out a multibeam sounding survey over eastern SCSB in 1999ü2000 with the vessel “DaYangYiHao” and obtained the full-coverage highresolution bathymetric data of this area at the first time. In the survey a deep-water multibeam sounding system, i.e. SeaBeam 2112 with the working frequency of 12 kHz for 1279
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Fig. 1. Tectonic setting of the eastern South China Sea. The topography is made of SeaBeam sounding data (in the deep sea area) and ETOPO2 global bathymetric data (in Luzon Island and its continental slope). Box is the location of study area for Fig. 2. Thin solid lines indicate the location of single or multi-channel reflection seismic profiles. Heavy solid lines A and B are the location of reflection seismic profiles of Figs. 3, 4 and heavy solid lines C is the location of reflection seismic profile of Fig. 9 of ref. [13]. Solid dots are earthquake epicenters more than Ms 4 from Jan. 1, 1977 to Jul. 30, 2002. The data of earthquake epicenters and focal mechanism resolutions are collected from the Data Center of Chinese Earthquake Network and the HCMT Data Center, USA respectively. Inset shows the sectional distribution of earthquakes and Benioff Zone in 14eü18eN.
bathymetry and a wide-range DGPS system, i.e. SeaStar 3000L with 12 channels for positioning were used. Following a series of corrections, a data precision evaluation indicates that water depth error of repeated test and cross lines is less than 0.3% water depth. From the above data 1280
set, we focus on the area of 17eü18eN in the subduction zone of middle MT which could best describe the accretionary wedge and indentation tectonics. In order to show morpho-tectonic features more clearly, the data are processed to generate the shaded relief images after being edChinese Science Bulletin
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ARTICLES ited[15](Fig. 2). SIO also took a series of sediment sampling such as grabbing and piston coring with the vessel “XiangYangHong14” in the same area in 1998. This paper utilizes above data and other single or multi-channel reflection seismic profiles to do morpho-tectonic analysis for the subduction zone of middle MT. 2
Subduction zone
The MT is morphologically demonstrated as a long narrow trough as deep as 5000 m. It is spatially extended as a N-S trending arc, from the big canyon in southwestern continental shelf of Mindoro in the south, to the collision tectonic zone of Taiwan in the north, with the depth going
shallow. East of it there is an active accretionary wedge of subduction zone, and west of it there is the SCSB. As presently strong earthquake and volcanic activities, the subduction zone of MT is considered as an active plate boundary. (ν) Trench sedimentation and basement. The 14e ü18eN segment of MT trends N-S. Based on the reflection seismic survey, the sediment thickness becomes smaller from north to south, decreasing from 2.6 km at 18.5eN, 1.7 km at 17.5eN to 0.5ü0.3 km in the extinct spreading ridge area in 16.5eü15.5eN. South of extinct spreading ridge, the sediments normally are 1.2 km thick,
Fig. 2. Multibeam shaded relief image of the accretionary wedge of middle Manila Trench. The shaded relief image illuminating from the southwest at low angles, SeaBeam survey lines are mostly N-S trending, partly NE-trending in the east. For location see Fig. 1.
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ARTICLES mainly gathering in some long and narrow trench grooves. This variation was usually interpreted as the trench sediments mainly coming from north and less sediments in the south resulting from blocking of the extinct spreading ridge[16]. Nevertheless the studies on spreading of Eastern Subbasin of SCSB[10ü12] reveal that the sediments maybe do not just or mainly come from the north. Because Eastern Subbasin underwent N-S and NNW-SSE-trending spreading during 32ü17 Ma, and also being influenced by sedimentation duration, the sediment thickness should become thicker from the extinct spreading ridge to both north and south. Three important boundaries could be recognized on the seismic profiles (Figs. 3 and 4). Above T1 boundary is trench-fill sediments, in which seismic faces are characterized by high energy, strong amplitude and dense reflectors. Such a sedimentary sequence grows thicker quickly eastward, from 0.25s at the oceanic basin to more than 1s at trench axis. The wedged trench-fill sediments, controlled by local tectonic subsidence and nearby rich mass supply, are mainly distributed in 40-km-wide deeper trench grooves and folded during subduction. Based on the samples of piston coring, the trench-fill sediments mainly are silt clay interbedded with several layers of
volcanic ashes and turbidites. By analyzing the coring No.149 which is 4.2 m long and collected near the trench in 118e54Ą E, 16e44Ą N at water depth of 4183 m, we find that the calcium carbonate of the turbidites, now situated below CCD, reaches up to 16.06%, thus revealing that the trench turbidites are characterized by terrigenous and volcanic sources and are one kind of quick-mixed product generated by nearby seafloor collapses, volcanic activities and turbidity currents. The sources are mainly from the terrigenous supply by seabed canyons and submarine rivers in the accretionary wedge and nearby volcanic supply. The age of T1 boundary now has not been known yet. But according to the late Pleistocene of this coring bottom age and contrasting of the seismic sequences, the trench-fill sediments could be formed since Pliocene. There are three sequences of hemipelagic sedimentation with week seismic energy below the T1 boundary (Figs.3 and 4). Each sequence is acoustically more transparent and consists of parellel to sub-parellel reflectors. In 70-km-wide area covered by the reflection seismic profiles near the trench, the thickness of sequence between T1 and T2 is more even, but the sequence between T2 and T3 trends towards that the thickness progressively becomes small eastwards, opposite to the tendency of sequence
Fig. 3. Reflection seismic profiles of the Manila Trench and its accretionary wedge. For location of profiles A and B see Fig. 1.
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Fig. 4. Structural interpretation for reflection seismic profiles of the Manila Trench and its accretionary wedge. It is the structural interpretation of Fig. 3. T1, T2, T3 are main reflection boundaries(see the text for details).
above T1. The oceanic basement under the sedimentary sequences, as products of spreading, is normally faulted into a series of tilted faulting blocks, and grows deeper and smoother to both north and south from the extinct spreading ridge. On seismic profiles, the oceanic basement is illustrated by obvious strong-amplitude scatters on the surface and no reflectors exist in the interior. Basement morphology is wavelike with average fluctuation of 500 m, and often complicated by submarine seamounts with maximal fluctuation of 1200 m. When getting into the wedge, the basement becomes smoother and deeper into the trench. (ξ) Subduction décollement. T2 boundary is a décollement between upper and subducting plates. Above this boundary, the sedimentary sequence has an even thickness, and below this boundary, it has a decreasing thickness due to the subduction and compression. Figs. 3, 4 and Fig. 9 of ref. [13] (for location see Fig. 1) indicate that all thrusts in the wedge terminate to this boundary and folding in the trench-fill sequence could influence downwards but just the hemipelagic sedimentary sequence between T1 and T2. So T2 constitutes the boundary between upper and subducting plates. Above it the sediments are folded and thrust to be accreted into the wedge, and below it the sediments and oceanic basement are subducted together into the trench along the décollement. Due to the Chinese Science Bulletin Vol. 49 No. 12
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seismic profiles of middle MT (Figs. 3, 4 and Fig. 9 of ref. [13]) and sonobuoy 211V28[16], the décollement beneath the Deformation Front (DT) in this area is 5.4 km deep. After extending about 5 km subhorizontally into the wedge, it begins to go downwards at a dip of 6e. And based on earthquake distribution (inset of Fig. 1), such tendency could go down to the depth of 50 km, then the décollement bends again and extends along the subduction zone at an average dip of 27.7eup to the depth of 150 km. Such variation is similar to that of the Okinawa and Costa Rica-Nicaragua subduction zone[2, 17], and will be used to establish the boundary of subduction zone model of middle MT and discussed in detail in the latter text. (ο) Accretionary wedge. The accretionary wedge is distributed above the décollement in the east of trench. It consists of a series of eastward-tilted thrust slices and morphologically demonstrates a series of ridges and troughs on seismic profiles (Figs. 3, 4, and Fig. 9 of ref. [13]), in which the thrusts are normally located at the toe of seaward flank of ridges. As different strengths and ages of folding and faulting in different tectonic settings, seismic reflectors in the thrust slices are characterized by weakening from the trench inner wall of low slope to upper slope. In the low slope, some reflectors associated with the trench-fill folds could be recognized, but in the upper slope, not much useful information could be gotten, 1283
ARTICLES indicating that the wedge has an elder age and stronger deformation from the low slope to the upper slope and has obvious tectonic zonation. The processed multibeam shaded relief image (Fig. 2) shows the complicated 2D regional tectonic characteristics of the wedge of subduction zone. Dense linear ridges and troughs parallel to the trench are a typical representation of accretionary tectonics of subduction zone. Fig. 5 is the structural interpretation of Fig. 2 based on reflection seismic profiles and shows the whole tectonic frame of the accretionary wedge. According to structural styles and deformation characteristics, three tectonic zones can be identified from the trench axis to the upper slope: Lower Tectonic Zone (LTZ), Middle Tectonic Zone (MTZ) and Upper Tectonic Zone (UTZ), and between each other, there are three large-scale thrusts: Lower Main Thrust (LMT), Middle Main Thrust (MMT) and Upper Main Thrust (UMT) separating these three zones. The thrusts and anticlines dominate in each tectonic zone and the imbricate thrust slices form the morphology of alternation of ridges and troughs. Western boundary of the LTZ is the DF (DF of Fig.5) of the wedge where presently active accretionary structures are still being formed. In the DF, the accretion is mainly developed by the way which the trench-fill sediments are continually bended to form the folds and the low-angle thrusts are developed from the ramp of these folds to lead to sheet stack, morphologically forming a series of dense linear ridges (Fig. 3). Same as Nankai Trough of Japan[18], the accretion mainly concerns the trench-fill sediments. The first fold in the DF corresponds to a nascent thrust plane that goes down into the décollement. The LTZ is located in the low slope, averagely 7.5 km wide and just 3.5 km wide in the seamount indentation (Fig. 5). It normally consists of 2ü3 large overthrust nappes with the morphology of stepped and linear ridges. The topographic gradient has big change with the average of 5e, maximum of 9e and minimum of 3e. It is a strongly active compressive tectonic zone in the wedge. The MTZ is constituted by a series of large tectonic slices thrusting toward the trench and departed from the LTZ and UTZ by the LMT and MMT (Fig. 5). The tectonic zone width and its topographic gradient are 7 km and 3.5erespectively with a small change. The thrust dips between tectonic slices quickly increase landwards. It is a strongly compressive uplifting tectonic zone in the wedge. The UTZ is mainly constituted by a series of imbricate tectonics and linear folds, where the density of thrusts separating the tectonic slices is apparently less than the LTZ and MTZ. These features are clearly illustrated by a series of linear ridges and troughs related to faults and folds on the morphology. It displays large lateral variation in the structural styles. The faults and folds, trending left-lateral, are dense near the area of seamount indentation and become sparse progressively to both sides of the 1284
north and south. This zone is averagely 17 km wide and has a small average topographic gradient (generally 0.8e). Affected by differential expression, some large poned back-tilted basins distribute in the north and south of the indentation tectonics, but the northern basin is still on the rifting trough due to short supply (Fig. 5). Consequently this zone, in which the overthrust and compression are presently inactive, mainly displays the differential uplifting without significant surface shortening, and the long narrow rise in the east of UMT constitutes the front highland of the Northern Luzon Trough forearc basin. 3
Indentation tectonics
In 14eü18eN study area of SCSB exist the extinct spreading ridge, large amounts of seamounts and NE-trending linear highs. The subduction of these seamounts and highs into the wedge along the trench induced some indentation tectonics and complicated the DF of the trench in geometry. The high-resolution multibeam bathymetric data reveals this special morpho-tectonics formed by rigid block indentation which arouses landward uplifting and seaward slumping. On the other hand, dense paralleling linear structures curving along the trench also are a response to seamount subduction in the wedge. The subducting seamount (seamount A) in the center of Figs. 2 and 5 corresponds to a typical indentation tectonics and its feature is in good agreement with sandbox experiments and digital simulation of conical seamounts[19]. As a big embayment 3ü4 times larger than the present emerged seamount in front of the trench, this seamount has been mostly subducted and the original size could be equal to the embayment. The embayment with a gradient up to 12.4e is formed by back slump due to the seamount subduction. Such seamount subduction also induced local landward uplifting which formed a high peak up to 2136 m above the sea floor. The indentation tectonics chiefly concerns the LTZ and MTZ. In the embayment area where seamount enters, accompanied with slumping, a set of fan-shaped subvertical strike-slip fractures is formed and convergent to the indented rigid seamount. At the same time, from the southern part of Figs. 2 and 5, we can see another curving morphology of indentation tectonics in the DF of the wedge on the east of seamount B, although the seamount is topographically still not contacted with the DF, thus showing that the nascent indentation tectonics is first formed from the roots of seamounts. The seamount indentation in central study area makes the DF move back about 8ü10 km and changes the strike of original NE-trending structures in the north of the indentation tectonics to near SN-trending or NW-trending. In the MTZ, resulting from slumping, two curving faulting cliffs can be identified and distributed in the northern side of two indentation tectonics in central and southern areas. With stepped faulting, they cut off the Chinese Science Bulletin
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Fig. 5. Structural interpretation of the accretionary wedge of middle Manila Trench. 1, Deformation front (DF), 2, main thrusts; 3, compressive faults; 4, extensional faults; 5, faults with unknown characters; 6, faulting cliffs; 7, folds; 8, submarine canyon; 9, submarine seamounts; 10, sedimentary basins on the slope; 11, local uplift due to seamount subduction; 12, profiles and their number for volume statistics of the wedge. A is central subducted seamount, B is southern seamount LMT is Lower Main Thrust, MMT is Middle Main Thrust, UMT is Upper Main Thrust, location is the same as in Fig. 2.
folds and thrusts formed by accretion and are a result of local expression formed by SEE-oriented oblique indentation of seamounts (Fig. 5). In the UTZ, on the southeast of the central indentation tectonics, there exists the most severe uplift which gradually descends to both sides of the Chinese Science Bulletin Vol. 49 No. 12
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south and north and transforms into the sedimentary area of slope basins. The indentation tectonics provide a good indicator for analyzing regional stress field and relative plate motion of convergent margin. Above analysis shows that the 1285
ARTICLES MT now is in the process of NWW-SEE oblique subduction. The three tectonic zones with general NE strike are curved as “S-type” due to the seamount subduction. Their tectonic strikes change from SN-trending in northern part to NE-trending in the central indentation tectonics and southwards again to SN-trending. To the east of seamount B, these tectonic strikes again change to NE-trending, then curve to NW-trending subsequently and enter another large indentation zone. The DF is normally curved and sheared off along the convergent vector of subduction. In the curving DF in front of the central and southern indentated seamounts, this kind of faults, transecting the accretion tectonic zone, can be found with strike of NW55e. Plenty of left-lateral folds and faults existing in these three tectonic zones indicate that the convergent vector is oblique to the trench. As the seamount indentation, lots of folds and thrusts are formed with strike of NE35e in the wedge in front of the subducted seamounts, thus showing that the main compressive stress of subduction zone is along NW55e. This main compressive stress also induces rifting fractures perpendicular to the main tectonic strikes in the wedge. A large “V-type” canyon with cutting down of 300 m and general strike of NW60e in central part is the product by erosion along the transverse normal faults formed by this oblique compressive stress (Fig. 5).
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Discussion and conclusion
What does the indentation and subduction of rigid volcanic bodies into trenches make for the wedges? Is it relative erosion, accretion or primary balance? Therefore we evaluate the budget of input and lose for the wedge matters. Based on the seismic profiles and multibeam bathymetric data, the comprehensive profile of the wedge is constructed as Fig. 6(b). As the study area just covers a small region of latitudinous 1e, the changes of sedimentary supply and plate subducting angle can be ignored, so we can use the sectional areas perpendicular to wedge tectonic strikes to illustrate the variation of wedge volume. The statistical range is the wedge from the DF to the UMT, total 7 sections perpendicular to the tectonic strikes are selected. The calculation result (Fig. 6(a)) indicates that besides the wedge sectional area tending to decrease from the north to south, there is an about 3% local, relative lose corresponding to central indentation tectonics (profile 4 of Fig. 6(a)), showing there is no accretion of the wedge for the seamount indentation. Otherwise the maximum lose is reached at profile 5 located in the southern side of the central indentation tectonics, thus also reflecting the oblique subduction of the seamount. Above slight loss is almost insignificant in comparison with about 38% rela-
Fig. 6. Profile model of subduction zone and sectional area of upper plate (accretionary wedge) (a) Sectional area (km2) from DF to UMT of upper plate (accretionary wedge) of the profiles perpendicular to the tectonic strikes in 17eü18eN. 1ü7 are the profile number, for location see Fig. 5, abscissa is the latitudes (N) at which the profiles intersect DF. (b) Tectonic profile model of subduction zone. 1, 4, 7 are the profile number, profiles start from the points where the profiles intersect UMT.
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ARTICLES tive lose in the subduction zone of Costa Rica-Nicaragua of Central America. So this matter budget of seamount subduction is basically of balance, the slight loss maybe arises by differential erosion of the uplift in front of the indentation tectonics. In order to reveal the deep variation of the stress field obtained by morpho-tectonic analysis, we collect earthquakes more than Ms 4.0 from Jan. 1, 1977 to July 30, 2002 from the Data Centre of Chinese Earthquake Network (Fig. 1). In the depth, the earthquakes of upper plate are mainly distributed at the depth shallower than 60 km and concentrated at the depth of 15ü35 km, but the earthquakes of subducting plate are mainly distributed at the depth of 30ü150 km along the Benioff zone with an angle of 27.7e. In the region, earthquakes are mainly concentrated in MT and its east, and the earthquakes near the trench are associated with the indentation of seamounts (chain). The focal mechanism solutions are selected from the Harvard centroid moment tensor solutions (HCMT) of USA in the region of 14.5eN to 19.5eN in order to avoid the complicated influence of the collision zone and deep-earthquake tectonic zone in the north and south. A total of 117 solutions are chosen in the same period, in which 12 earthquakes are deeper than 60 km and only 8 earthquakes are located on the Benioff zone. From these 8 solutions (Fig. 1), the compressive axis of the stress is considered to be eastward-dip and NWW-SEE trending, which indicates that the mechanism of oblique subduction not only has regional significance but also exists in the depth. Similarly, the quantitative analysis for the crustal motion of Fujian province of China and its adjacent marginal seas by Zhou Shuoyu et al.[20] demonstrated that the southeastern continent of China and northern margin of South China Sea are undergoing a NWW-SEE compressive stress. According to the focal mechanism solution of the earthquake occurring in Nov. 7, 1965 and of which the hypocenter is 5km deep under the seafloor in center of the Southwestern Subbasin of SCSB, S.C.Wang et al.[21] suggested that the compressive stress of this subbasin is horizontal and trends NW52 e, which is consistent with the orientation of rigid seamounts subduction along the trench mentioned in the text. The spreading of Eastern Subbasin of SCSB is thought to cease at 15Ma of the mid Miocene[22]. But why did the spreading of Eastern Subbasin suddenly cease? How about the relation to the formation of MT? Now these questions are still not clear. According to the Miocene sediments existing at the bottom of the Western and Northern Luzon Trough of MT forearc basins[13], the MT subduction zone should be formed in Miocene, which is near the ceasing age of Eastern Subbasin spreading. Furthermore the extinct spreading ridge of Eastern Subbasin has been partially subducted under the accretionary wedge of MT[13], but this subducted extinct spreading ridge does Chinese Science Bulletin Vol. 49 No. 12
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not induce the relative magmatic activities in the wedge above, thus indicating that the subbasin spreading had ceased before starting of the subduction. So maybe it is the formation of MT subduction zone that induced the ceasing of SCSB spreading. According to the stress due to the Philippine Sea plate convergent to Eurasian plate at a rate of 70 km/my with the strike of NW55e[23] and the above regional tectonic analyses. The motion of the Philippine Sea plate is just consistent with the SEE subducting of Eastern Subbasin of SCSB to the MT. Paleo-magnetic research[24] indicates that the location of Eastern Asian plate has not changed since Cenozoic and SCSB become a part of Eastern Asian plate after ceasing of spreading, so SCSB is short of the actual kinematic mechanism of subduction. Thus the convergence between SCSB and Luzon micro-plate should be taken by the NWW displacement of Luzon micro-plate. In fact the subduction of the MT should be an obduction mechanism of Luzon micro-plate, its dynamical source should come from the Philippine Sea plate. Acknowledgements Thank Xu Saiying for drawing some figures of this paper. This work was supported by the National Major Fundamental Research and Development Project of China (Grant No. G2000046704).
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Chinese Science Bulletin
Vol. 49 No. 12 June 2004
