Tectonic Dynamics Research And Subducting Characteristics Comparison Between Middle And Northern Part Of Manila Subducting Belt

Associative interpretation by all-covered high-resolution multi-beam topographical & geomorphic maps and high-definition seismic profiles with other geophysical data can promote resolution and stereo analytical capacities of submarine tectonics extremely. This method has special predominance in regional submarine tectonic research especially to young submarine tectonics and active submarine tectonics, and has been an important method in studying tectonics and formation mechanism of mid-ocean ridges, subducting belts and marginal basins. As an active young subducting belt, eastern Manila Subducting Belt (MSB) is one of the most important margins of South China Sea (SCS). Tectonic and evolutionary study to it is the key point in study to SCS. While as one of the biggest marginal basins of west Pacific, deepening and refining to the study of SCS is the key to understand the tectonic of East Asia continental margin and the formation mechanism of whole west Pacific basins.Based on high-resolution multi-beam topographical & geomorphic maps and multiple seismic profiles at middle and northern part of Manila Trench acquired by multi-time investigations, combining with data-known boreholes and other collected geophysical data of southwest Taiwan area, we analyzed and contrast the diving tectonic features deeply to the middle and northern part of Manila Trench. We placed focus on planar tectonic characteristics and deep 3D-tectonic features of the subducting belt, integrating with evolutionary histories of surrounding terrains such as rifting history of eastern sub-basin of SCS, kinetic history of West Philippine Sea crust and orogenic history of Taiwan arc-continent collision, and discussed extensively to the geomorphic appearance, tectonic origin, the formation and evolutionary history of middle and northern part of MSB at last. Multiple dominant research products had been obtained as followed:1. We analyzed and contrasted the delicate tectonic features of accretionary wedges at middle & northern part of MSB, disclosed the planar zonal features and deep layered characteristics of two regions, and found that the trapped basins were developed extensively in Lower Tectonic Zone of accretionary wedge. After morphostructure analysis to the tectonic appearance, emerging place and formation process of the trapped basins, we confirmed that the formation, development and extinction of trapped basins are corresponding to the tectonic dynamic process of front fold and deformation, mid thrust and break, top nappe and uplift of accretionary wedge respectively. It also reflected the feature of strengthened horizontal compression stress of accretionary wedge from deformation front to Ridge Tectonic Zone, and represented a new developing pattern of accretionary wedge. The trapped basins could be found both at northern part and middle part of MSB which demonstrate that it represents a widespread developing method of accretionary wedge at subducting belt;2. The NE to NNE compressive fracture zone, accretionary wedge tectonic belt widening to north, super-thick and large extent uplift at top wedge area, which were found on morphostructure map of northern part of MSB proved that the sea area at South Taiwan near 21°N is the interaction area between Manila subducting activity and Taiwan orogenic movement where it has characteristics both of ocean crust subduction and arc-continent collision. Because of the influence of Taiwan Orogenic activity to the northern part of MSB, the surficial bound of subducting activity had changed greatly. The deformation front extend NW along Penghu Canyon after the disappearance of Manila Trench, turned NE on Chinese continental shelf area, and connected to the western piedmont belt of Taiwan after getting on land at south of Tainan city. The fore-arc basin extended along North Luzon Trough and connected to Taitung Longitudinal Valley after crossing the Taitung Trough. As a part of accretionary wedge of MSB, Hengchun Ridge is also the extended area of Taiwan Orogen. It is characterized by the features of both thrust and nappe of subducting belt and collision and uplift of Orogen; 3. The surficial appearance and extended direction of northern and middle part of Manila Trench have great differences. The trench shows"V"-shape down-cut appearance in southwestern Taiwan sea area and extends meanderingly until its disappearance. But at the middle part of northwest Luzon Island, it shows flat and regular straight appearance. By analysis, we confirmed that the middle part of trench is controlled by deep subducting activity while northern part is influenced by Taiwan arc-continent collision and the sedimentation of Chinese continental margin. Therefore, northern part of the trench is influenced mostly by bottom turbidity current with abundant marginal sediments and less influenced by deep subducting activity. Thus it shows entirely different appearance against middle part.4. Deduced by seismic, borehole and other information, we are certain that Manila Subducting Belt formed at about 16Ma, that is, early stage of mid-Miocene. The multi-channel seismic profile crossing northern part of Manila trench passed by the"A-1B"borehole drilled by Taiwan Petroleum Corporation in slope area of northern SCS margin which the detailed strata information is known. Other adjacent boreholes can provide reference information for the strata determination to the seismic profile. Extending the stratigraphic sequence to the southwest area of Taiwan from continental margin, we found the detaching surface of the subducting belt is the interface between mid-Miocene and low-Miocene. This indicates that the subducting activity started not early than mid-Miocene. As the cretionary wedge of subducting belt, we know by geological tectonic information, the Hengchun ridge was formed at about mid-Miocene, so we deduced that the Manila subducting belt is formed at about early stage of mid-Miocene (16Ma) after constraint to its formation time by above information.5. The dynamic force of MSB formation is originated from NW movement of Philippine Sea Plate but has no straight causal relation with the spreading of eastern sub-basin of SCS.