海山俯冲对地壳表层构造变形影响:基于离散元模拟的见解

Effect of subduction of seamounts on tectonic deformation of the crust surface:Insights from discrete numerical simulations

本文利用二维离散元数值模拟技术模拟海山俯冲对地表浅层构造变形影响,结果表明沿增生边缘俯冲的海山与粗糙海底地形相互作用,可以赋予上覆地层独特特征:(1)海山之上发育大偏移逆冲断层,断层发育规模大、变形强、形态复杂甚至卷曲,逆冲脊在海山垂直上方,代表变形最强烈区域. (2)海山俯冲导致楔形体形状更加复杂以及不规则化,海山显著阻碍楔形体横向扩展. 通过设定不同俯冲角度进行对比试验,证明了含海山俯冲角度35°楔形体与马里亚纳型俯冲带俯冲角度大,俯冲板块与上覆板块脱离,大洋沉积物大量被削减并向下俯冲,形成窄的增生楔相似;俯冲角度25°楔形体几何形状增生楔宽度更宽,高度低,坡角略低于含海山俯冲角度35°形成的增生楔,这与智利型俯冲带俯冲角度小,俯冲板块和上覆板块耦合紧密,不利于大洋沉积物的俯冲消减,可形成宽阔的增生楔相对应. (3)海山俯冲会导致地表高速带出现以及有助于深部俯冲的沉积物向上搬运到浅部. 由此产生的断裂有助于增强跨板块边界耦合程度,增加在这种背景下发生大地震的可能性. 表明需要重新评估海山与地壳表层的相互作用,这对在增生边缘中评估海山俯冲危害具有启示意义.

2D discrete element numerical experiments are used to investigate the effect of seamounts subduction on shallow surface tectonic deformation in this paper. It is proved that the interaction between seamounts subduction along the accretionary margins and rough seabed topography give the unique stratigraphic evolution variations: (1) The seamounts develop large offset thrust faults, which are characterized by complex deformation, even with curling in shape;Reversed ridges are located directly above the seamounts, representing the area with the most intense deformation. (2) Seamounts subduction leads to more complex and irregular shapes of the wedge. Seamounts significantly hinder the lateral extension of the wedge. By conducting comparative experiments with different dipping angles, it was proved that the wedge-shaped body with a dipping angle of 35° has a larger dipping angle than the Marianas-type subduction zone. In this case, the subducting plate separates from the overlying plate, and a large amount of ocean sediments is eroded and subducted, thus forming a narrow accretion wedge. In contrast, the accretion wedge formed by the wedge-shaped geometry at a dipping angle of 25° is wider and lower in elevation, with a slope slightly lower than that of the accretion wedge formed by the wedge with a dipping angle of 35° that is composed of oceanic islands. This phenomenon corresponds to the Chile-type subduction zone, where the smaller dipping angle causes the subducting plate and overlying plate to be closely coupled, which is not conducive to the erosion and subduction of ocean sediments, thus allowing for the formation of a wide accretion wedge;(3) Seamounts subduction can lead to the emergence of high-velocity zones at the Earth’s surface and contribute to the upward transport of deep subducted sediments to shallow levels. The resultant variations and related fractures are conducive to enhancing the coupling degree across plate boundaries and augmenting the possibility of major earthquakes in this context. This indicates the necessity of reevaluating the interaction between seamounts and the crustal surface, and holds enlightenment significance for assessing the hazards of seamount subduction in accretionary margins.