对日本俯冲带与IBM俯冲带俯冲特征的地球物理研究:来自重力与震源分布数据的启示

日本俯冲带与IBM俯冲带位于太平洋板块、菲律宾海板块和欧亚板块三者的交汇地带,是典型的"俯冲工厂"地区,具有重要的研究意义.本文利用震源分布资料与卫星重力数据对日本俯冲带与IBM俯冲带进行了研究.通过空间重力异常反映了俯冲带地区的区域构造形态,在此基础上基于艾利模式计算了均衡异常以反映地壳均衡特征.利用震源分布资料,分别从垂直俯冲带走向与沿俯冲带走向划定了横截剖面(cross-sections)进行了地震提取,讨论了俯冲带地区的Wadati-Benioff带形态特征,并借助于俯冲带地震等深线图直观描述了俯冲带的俯冲形态.在日本俯冲带与伊豆—小笠原俯冲带各选取了一条典型剖面进行了重力2.5D反演,研究了俯冲带地区的壳幔结构特征.研究结果表明,九州—帕劳海脊与IBM岛弧在均衡异常上存在差异,前者已逐渐趋向于地壳均衡.IBM的Wadati-Benioff带存在明显的南北差异,反映出伊豆—小笠原俯冲板片停留在了660km转换带中,而马里亚纳俯冲板片很可能垂直穿过了这一转换带,造成这种南北差异的原因与板块相对运动、岩石圈黏性和年龄差异以及俯冲板片的重力效应等因素有关.在IBM的中部和南部存在板片撕裂现象.日本俯冲带的俯冲洋壳密度随俯冲深度变化较小,洋幔存在一定程度的蛇纹岩化,地幔楔蛇纹岩化作用不典型,海沟处有一范围较小的含水畸变带;伊豆—小笠原俯冲带俯冲洋壳密度随深度增大而明显增大,洋幔蛇纹岩化程度较日本俯冲带低,地幔楔蛇纹岩化作用强烈,板块交汇处存在明显的蛇纹岩底辟.日本俯冲带与IBM俯冲带一线自北向南板片俯冲变陡,两侧板块耦合度降低,与俯冲带两侧的板块运动速率差异有关.

Characteristics of the Japan and IBM subduction zones:Evidence from gravity and distribution of earthquake sources

The Japan and the IBM subduction zone are located in the place where the Pacific plate, the Philippine Sea plate and the Eurasian plate meet. Both the subduction zones act as typical areas where the "subduction factory" takes effect, bringing on themselves great significance for deep research. Using the distribution of earthquake sources and satellite gravity data, this paper studies the characteristics of these two subduction zones.Firstly, a free-air gravity anomaly map was compiled to delineate the regional structure of the subduction zone. From the free-air gravity anomalies, the isostatic anomalies on the premise of the Airy-Heiskanen model were calculated to reflect isostatic characteristics of the crust. Secondly, information of seismic-source distribution of earthquakes with magnitude greater than 3 was acquired from USGS. Four graphs of epicenters relevant to different ranges of depths were drawn, indicating the characteristics of the distribution of earthquakes of different depths. A correlation between the isostatic anomalies and big earthquakes was detected and discussed. Thirdly, by means of the information of earthquakes collected, the Wadati-Benioff zones of Japan and IBM were analyzed by dint of cross-sections across/along the subduction zone. A contour map of depths of earthquakes in the subduction zone was also prepared to show the shape of the subducted slab. Fourthly, with the purpose of demystifying the crustal structure of the Japan and IBM subduction zones, a 2.5D gravity inversion was conducted on two profiles located in the Japan and the Izu-Bonin subduction zones, respectively using the free-air anomalies available.(1) The free-air anomaly map shows a zone with a great gradient in the subduction zone, with positive anomalies on islands and negative anomalies along the trench. The isostatic anomaly map manifests the large deviation from isostasy in the subduction zone, except for the Kyushu-Palau Ridge which has been away from the locality where subduction takes effect.(2) The distribution of earthquakes delineates a characteristic of "zone" accumulation especially in the arc, and few earthquakes are detected inside the basin. Earthquakes tend to occur in the place with a great gradient on the isostatic anomaly map.(3) The Wadati-Benioff zone of Japan has a large width and a small dipping angle, with a subducting direction of NWW. Sparseness of earthquake distribution at the depth of more than 200 km is recognized. As for the Izu-Bonin Wadati-Benioff zone, the subducting direction is from NWW to SWW, with a narrower breadth and a deeper depth and big dipping angle. The distribution of earthquakes of the intermediate depths is sparse as well. This sparse area becomes larger in the south of the Izu-Bonin Wadati-Benioff zone to the central IBM where deep earthquakes are not detectable. The Wadati-Benioff zone of Mariana shows a wider breadth and a deeper depth from north to south, with the subduction zone trending in SWW to NWW gradually, and NNW at the southernmost of IBM where a relatively smaller dip angle, a lower depth and a shorter breadth are discerned.(4) In the Japan subduction zone, the Pacific plate has a thin sedimentary layer without accretion wedge detected in the trench. Layers 2 and 3 are subducted with a homogeneous distribution of density. The upper mantle of the Pacific plate is serpentinized. The crust of the overlying arc is divided into four layers:a sedimentary layer, basement layer, upper crust and lower crust. No obvious serpentinization is discerned on the mantle wedge. In the Izu-Bonin subduction zone, the Pacific plate has a sedimentary layer of heterogeneous density distribution. Layers 2 and 3 are subducted with a large gradient of density variation in relation to the depth. The upper mantle of the Pacific plate is not serpentinized largely. The crust of the overlying plate is divided to four layers:a sedimentary layer, basement, middle crust and lower crust. The lower crust and mantle wedge are serpentinized so strongly that the Moho discontinuity is not detectable. More earthquakes occur in the Japan subduction zone and are distributed uniformly. Most earthquakes in the Izu-Bonin subduction zone are inside the slab.(1) The Kyushu-Palau ridge is inclined to the state of isostasy.(2) The Wadati-Benioff zones of Izu-Bonin and Mariana have disparate characteristics. The results indicate that the subducted slab of Izu-Bonin finally stays on the 660 km transition zone while that of Mariana penetrates nearly vertically into the lower mantle. Such differences are due to several factors such as the gravity effect of the subducted slab, plate movement, viscosity in the slab and the age of lithosphere subducted.(3) Slab windows exist in the central and southern IBM as a result of collision of the trench and the Ogasawara plateau and of accommodation of the reduced volume the slab must occupy, respectively. The lack of earthquakes in the Yap-Belau subduction zone, however, is due to its proximity to the Euler pole, which results in its slow movement that produces few earthquakes.(4) The subducted crust and mantle in the Japan subduction zone are less coupled than those in the Izu-Bonin subduction zone and the subducted crust in Izu-Bonin has a more obvious phase transition.(5) The coupling between the oceanic plate and the overlying plate is better in the north than in the south along the Japan-IBM subduction zone, due to the movement of plates, which is the same reason accounting for the fact that the dipping angle of the subducted slab becomes smaller from the south to the north along the Japan-IBM subduction zone.