indentation tectonics in the accretionary wedge of middle manila trench
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Chinese Science Bulletin Vol. 49 No. 12 June 2004 1279
Chinese Science Bulletin 2004 Vol. 49 No. 12 1279 1288
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 Sci-
ences, 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 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.
Keywords: Manila Trench, accretionary wedge, indentation tectonics, oblique subduction, morpho-tectonic analysis.
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. 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. The oblique subduction of Gagua Ridge along the Ryukyu Trench in the Northwestern Philippine Sea not
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.
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, 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, and the Scarborough seamount chain as an extinct spreading ridge has been subducted, indented towards the MT and ex-tended beneath the forearc basin. 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.
1 Data acquisition and study method
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].
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
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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. . 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.
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
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-
ited(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.
2 Subduction zone
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
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.
( ) Trench sedimentation and basement. The 14 18 N segment of MT trends N-S. Based on the reflec-
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,
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.
Chinese Science Bulletin Vol. 49 No. 12 June 2004 1281
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. 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.
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 t