汤加—克马德克俯冲带现今非均匀应力场特征及其动力学意义

汤加—克马德克俯冲带是太平洋板块向澳大利亚板块俯冲碰撞的动力作用区,是全球俯冲带动力学研究的热点区域.本研究基于EHB地震目录,对汤加—克马德克俯冲带(18.5°S—28.5°S)区域进行平面拟合,得到该范围内俯冲带走向约为196°,倾角约为48°;利用该俯冲带研究区域内Global CMT目录,对不同位置、不同深度进行区域应力张量反演,得到汤加—克马德克俯冲带研究区内精细的应力图像.结果显示:(1)俯冲带浅部(60~300km)应力结构非均匀特征明显,主应力轴倾伏角变化多样,并且最大主压应力轴方位在24°S左右发生明显偏转,我们推测这可能与洋底构造路易斯维尔海链俯冲有关;(2)中部(300~500km)最大主压、主张应力轴由北向南逐渐发生偏转,这可能与由北向南流动的地幔流对俯冲板片产生推挤作用有关,并且这种推挤作用向南逐渐减弱;(3)深部(500~700km)最大主压应力轴沿俯冲方向分布;(4)本文的结果还发现了主俯冲带深部西侧"偏移"板片与主俯冲带应力结构不同,表明"偏移"板片与主俯冲带是分离的.

Heterogeneity of present-day stress field in the Tonga-Kermadec subduction zone and its geodynamic significance

The Tonga-Kermadec subduction zone which is a dynamic active region related to subducting from Pacific Plate to Australian Plate, is a key region to the global geodynamic research of subduction zone. The related plate boundary is one of the fastest relative plate motions on Earth and actively deforming region. As the region has complex tectonic feature, a large number of research of subduction zone structure have been studied. We try to solve stress tensor of different sub-region and depth within the slab, and discuss geodynamic questions like the effect of mantle flow for principal stress, the relationship between complex tectonic units and stress field heterogeneity, and the differentiation of stress regime between main subduction zone and the offset slab.#br#There are two steps in this research. Firstly, by using the global seismicity EHB catalog (M_b≥4.7) from 1960 to 2008, we investigate the Tonga-Kermadec subduction zone (18.5°S-28.5°S) geometry by plane fitting. The plane fitting result indicates that the strike of slab is nearly 196°and the dip of slab is nearly 48° in the range of 18.5°S-28.5°S. In the following stress field study, the Tong-kermadec subduction zone is divided into 9 sub-regions to solve stress field based on the strike and dip we obtained. Using focal mechanisms from the Global Centroid Moment Tensor (1993-2012), the present-day stress tensor for different sub-region and depth interval is solved based on grid search method. #br#In shallow part (70~300 km), stress inversion results indicate that the stress regime has strong heterogeneity with the variety of plunges, and the azimuth of maximum principal compression stress is rotated near 24°S, which is associated with the subducting Louisville Seamount Chain. The ridge push within the slab is subjected to resistance due to the Louisville Seamount Chain, which leads to the stress rotation. As the motion of the Chain is southward along trench, the "stress shadow zone" is generated in the southwest of Louisville Seamount Chain.#br#In intermediate part (300~500 km), the azimuths of maximum and minimum principal stress are rotated from north to south gradually. This phenomenon is probably associated with compression due to north-south mantle flow for the slab. The compression for the slab is greater near the northern edge of the slab beneath the Lau Basin. But the compression induced from north-south mantle flow is weakening gradually in southward direction from the northern edge.#br#In deep part (500~700 km), the distribution of principal stress is down dip compression. It may be caused by the resistance of its downward motion due to increases of depth and mantle viscosity.#br#Our results also show different stress plunge between main subduction zone and the western offset slab. The horizontal or vertical maximum compression stress axis in the offset slab shows no down dip compression induced from the upper subduction zone, which confirms the offset slab separation from main subduction zone.