Research On The Segmentation And Its Causes Of Mariana Subduction Zone

The study seeks to understand the characteristics, as well as the factors governing the segmentation of the subduction zones of the Mariana Trench and the surrounding areas. In other to achieve this water depth, terrain, seismic characteristics, gravity anomalies, oceanic crustal age and materials from drilling projects of the Mariana Trench and the surrounding areas were analyzed. The study combined water depth, seismic and gravity data were used to:evaluate depth profile of different regions, morphology of the subduction and Benioff zones and the structure of the exhumed faults in the study area.The results of the study show that the subduction characteristics of different regions are related and significant to segmentation characteristic of the Mariana Trench. The segmentation characteristic of the subduction zones shows a very good reflection of the depth of the trench. The water depth and subduction angle gradually increased from north to south. When compared with the oceanic lithosphere age and depth of the northern Izu-Ogasawara Trench, the main factors governing the development of trench depth are magnitude of seamounts and oceanic rises moving with Pacific plate, rather than the subducting lithospheric age, as traditionally understood.The seamounts and oceanic rises that influenced the depth of the trench can be divided into three types:the first is smaller seamounts. They occupy the shallow depth of trench when they move to the trench axis. They gradually sink into the trench as they undergo subduction, and the trench return to its original form. The second is the relatively isolated large seamounts. As they move towards the ocean side of the trench with the plate, they weaken the surrounding area of the subduction plate and shallow part of trench, and transit from the trench to ocean-sides at the same time. The third is a large range of seamounts and oceanic rises, when they approach the trench axis, they will increase the trench depth, and the phenomenon of depth increases is more significant in the direction of the subducting plate.The water depth at the center of Mariana Trench becomes significantly shallow. While the axis becomes discontinuous and the topography becomes varied at fore-arc in Mariana Trench. Large scale seamount intrusion, which is relatively isolated leads to all these phenomena. This is as a result of the filling of the seamount. The same process that produced by the collision between seamount and overlying plate can form fore-arc basin when they cross the trench and trap the residual trench. With continual subduction, large-scaled seamount is moved in the direction of the volcanic arc, which becomes decomposed upon reaching a relatively stable state. Finally, volcanic arc comes to hyperplasia. In this progressive evolution, different developmental periods of volcanic arc have different scales and depths of fore-arc basin. The five large-scaled depressions in the region may be experiencing an evolution, from deep to shallow and from large to small. The ultimate product of such evolution is that fore-arc disappear regularly and the residual seamount is linked with volcanic arc.The bulging of the Mariana Trench eastward may be explained by the connection between large number of great seamounts, oceanic rises and the trench. Several seamounts keep moving towards the axial part of the trench. This resulted tore treat of the trench and the formation of new subduction zone at the rear side of the trench. It is followed by gradual positional migration to the ocean side where the subducting plate is under-thrust into the mantle. Differences exist in terms of the amount and scales of the invading seamounts in the trench. There are large number of seamounts which ranged widely in the middle area of the trench. The migration rate of the back-arc spreading increased as the amplitude of the trench moving backward amplifies. These behaviors led to the phenomenon that caused the distance of the Mariana Island Arc and Mariana Trench in the middle region to be further from the ocean side than the northern area. At the Southernmost tip of the trench, because the place where the trench should migrate is occupied by new seamounts, shifting of the ocean rise, crossing the trenches, and retreating of the trench is not very obvious. Forecasting without considering other factors (such as the change of the ratio and direction of Pacific plate movement), the trench developments will continue to retreat toward the sea. In the middle part of the trench, where there are many Pacific volcanoes,the speed of trench retreat will improve while the bulge function of the trench and island arc will become more apparent.Importance of the existence of the Challenger deep in the southern part of Mariana trench and the deepest area in the middle of the trench is connected with the cutting block of the subducting plate, which is near the ocean. The seamount chain is composed of huge seamounts. The tabled seamounts, sea otters and sea plateaus can incise the subducting plate and produce some small blocks. Under the same subducting conditions, the small blocks can relatively contribute to the subduction processes and increase the depth of the trench.