Indentation tectonics in the accretionary wedge of middle Manila Trench

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<ul><li><p>ARTICLES </p><p>Chinese Science Bulletin Vol. 49 No. 12 June 2004 1279 </p><p>Chinese Science Bulletin 2004 Vol. 49 No. 12 1279 1288 </p><p>Indentation tectonics in the accretionary wedge of middle Manila Trench LI Jiabiao1,2, JIN Xianglong1,2, RUAN Aiguo1, WU Shimin3, WU Ziyin1 &amp; LIU Jianhua1</p><p>1. Key Lab of Submarine Geosciences, State Oceanic Administration, Hangzhou 310012, China; </p><p>2. College of Sciences, Zhejiang University, Hangzhou 310027, China; 3. South China Sea Institute of Oceanology, Chinese Academy of Sci-</p><p>ences, Guangzhou 510301, China Correspondence should be addressed to Li Jiabiao (e-mail: jbli@zgb. com.cn) </p><p>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 distri-bution and stress mechanism for the indentation tectonics are analyzed, oblique subduction along Manila Trench with convergent stress of NW55 is presented, and the relation-ship of the ceasing of Eastern Subbasin spreading of South China Sea Basin to the formation of subduction zone of Ma-nila Trench is discussed. By the model analysis and regional research, it is found that the seamount subduction along Ma-nila Trench does not lead to the erosion of the accretionary wedge and the oblique subduction actually is a NWW- trending obduction of Luzon micro-plate that results from the NWW-trending displacement of the Philippine Sea plate. </p><p>Keywords: Manila Trench, accretionary wedge, indentation tectonics, oblique subduction, morpho-tectonic analysis. </p><p>DOI: 10.1360/03wd0412 </p><p>Some important progress has been reached in recent years on the indentation of accretionary wedges by sub-duction of seamounts or ridges on the oceanic plate along trenches[1 8]. More geoscientists pay attention to these researches because of their great significance for deepen-ing the insight into the structural styles, stress mechanism, accretive or erosive effects and plate kinematics of sub-duction 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 indenta-tion tectonics in the subduction zone[2]. By the mor-pho-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 </p><p>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]. </p><p>The subduction zone of the Manila Trench (MT), lo-cated 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 Tai-wan in the north[9], is thought an important active conver-gent boundary with special significance (Fig. 1). Re-searches 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 ex-tended beneath the forearc basin[13]. Because of being lim-ited 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 rela-tionship to the spreading ceasing of SCSB? Where does the original force of subduction come from? What about the response mechanism between local and regional tec-tonics? 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 distribu-tion to reveal the characters of indentation tectonics of seamount subduction in the accretionary wedge, and fur-ther try to discuss the tectonic features, stress field, sub-duction direction and dynamic mechanism of MT subduc-tion zone. </p><p>1 Data acquisition and study method </p><p>The high-resolution multibeam swath sounding tech- nology, combined with other geological and geophysical data, has a unique dominance for analyzing regional tec-tonics 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]. </p><p>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 1999 2000 with the vessel DaYangYiHao and obtained the full-coverage high- resolution 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 </p></li><li><p>ARTICLES </p><p>1280 Chinese Science Bulletin Vol. 49 No. 12 June 2004 </p><p>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 14 18 N. </p><p> bathymetry and a wide-range DGPS system, i.e. SeaStar 3000L with 12 channels for positioning were used. Fol-lowing 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 </p><p>set, we focus on the area of 17 18 N in the subduction zone of middle MT which could best describe the accre-tionary wedge and indentation tectonics. In order to show morpho-tectonic features more clearly, the data are proc-essed to generate the shaded relief images after being ed-</p></li><li><p>ARTICLES </p><p>ited[15](Fig. 2). SIO also took a series of sediment sam-pling 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 re-flection seismic profiles to do morpho-tectonic analysis for the subduction zone of middle MT. </p><p>2 Subduction zone </p><p>The MT is morphologically demonstrated as a long nar-row 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 </p><p>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. </p><p>( ) Trench sedimentation and basement. The 14 18 N segment of MT trends N-S. Based on the reflec-</p><p>tion seismic survey, the sediment thickness becomes smaller from north to south, decreasing from 2.6 km at 18.5 N, 1.7 km at 17.5 N to 0.5 0.3 km in the extinct spreading ridge area in 16.5 15.5 N. South of extinct spreading ridge, the sediments normally are 1.2 km thick, </p><p> 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. </p><p>Chinese Science Bulletin Vol. 49 No. 12 June 2004 1281 </p></li><li><p>ARTICLES </p><p>mainly gathering in some long and narrow trench grooves. This variation was usually interpreted as the trench sedi-ments 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[10 12] reveal that the sediments maybe do not just or mainly come from the north. Because East-ern Subbasin underwent N-S and NNW-SSE-trending spreading during 32 17 Ma, and also being influenced by sedimentation duration, the sediment thickness should become thicker from the extinct spreading ridge to both north and south. </p><p>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 charac-terized by high energy, strong amplitude and dense reflec-tors. 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, con-trolled 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 </p><p>volcanic ashes and turbidites. By analyzing the coring No.149 which is 4.2 m long and collected near the trench in 118 54 E, 16 44 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 vol-canic 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 riv-ers 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. </p><p>There are three sequences of hemipelagic sedimenta-tion with week seismic energy below the T1 boundary (Figs.3 and 4). Each sequence is acoustically more trans-parent and consists of parellel to sub-parellel reflectors. In 70-km-wide area covered by the reflection seismic pro-files 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 </p><p>Fig. 3. Reflection seismic profiles of the Manila Trench and its accretionary wedge. For location of profiles A and B see Fig. 1. </p><p>1282 Chinese Science Bulletin Vol. 49 No. 12 June 2004 </p></li><li><p>ARTICLES </p><p>Chinese Science Bulletin Vol. 49 No. 12 June 2004 1283 </p><p>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). </p><p> 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. </p><p>( ) Subduction dcollement. T2 boundary is a dcollement 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 down-wards but just the hemipelagic sedimentary sequence be-tween 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 to-gether into the trench along the dcollement. Due to the </p><p>seismic profiles of middle MT (Figs. 3, 4 and Fig. 9 of ref. [13]) and sonobuoy 211V28[16], the dcollement 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 6 . And based on earthquake distribution (inset of Fig. 1), such tendency could go down to the depth of 50 km, then the dcollement bends again and extends along the subduction zone at an average dip of 27.7 up 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 mid-dle MT and discussed in detail in the latter text. </p><p>( ) Accretionary wedge. The accretionary wedge is distributed above the dcollement 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, seis-mic reflectors in the thrust slices are characterized by weakening from the trench inner wall of low slope to up-per 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, </p></li><li><p>ARTICLES </p><p>1284 Chinese Science Bulletin Vol. 49 No. 12 June 2004 </p><p>indicating that the wedge has an elder age and stronger deformation from the low slope to the upper slope and has obvious tectonic zona...</p></li></ul>

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