应用二维有限元方法恢复阿留申岛弧边缘晚白垩世板块增生附加体古构造应力场

Application of 2D finite element method to reconstruct paleostress field of accretionary prism in Late Cretaceous plate in Aleutian arc

一系列有限单元模型模拟了阿留申岛弧板块边缘附加体在晚白垩世板块增生过程中古构造应力场的演变特征。该地区板块增生过程中的楔形附加体具有不同的长度和厚度,在数学模拟过程中,附加体被假定为弹性连续体,根据有限单元程序计算出数学模型应力场,断裂发育状况是根据库仑—莫尔准则确定的。有限单元模型的边界条件为太平洋板块相对北美大陆板块的俯冲及与板块俯冲作用有关的大洋地壳的逆冲作用。数学模拟实验的结果表明,附加体古构造应力场有一定的演化规律。在板块增生的早期阶段,古应力1σ表现为近水平方向的压应力,逆断层发育于附加体中。大洋地壳的逆冲作用使得附加体应力轨迹重新定向,正断层发育于楔形附加体的底部,平移断层发育于大洋地壳的深部位置。逆冲作用可能导致了楔形附加体的侧向增生和垂直方向上的厚度加大。在板块增生的晚期阶段,楔形附加体中晚白垩统沉积岩尚未固结时,古应力1σ表现为近于垂直方向的压应力;当晚白垩统沉积岩固结之后,1σ为近于水平方向的压应力,逆断层发育于楔形附加体顶部。晚白垩统沉积岩的成岩过程对楔形附加体顶端古应力场分布特征和断层发育状况影响较大。

A series of finite element models are used to simulate the evolving paleostress field within an accretionary prism during Late Cretaceous accretion in the eastern Aleutian arc-trench system, Alaska. The Late Cretaceous accretion is modeled with two types of accretionary wedges of different length and width. The rheology of the accretionary prism is assumed to be elasticity. The stress field is calculated by the finite element program. Fault development is predicted according to Mohr-Coulomb criterion. The subduction of the Pacific plate beneath the North American plate and associated underplating of oceanic crust marks the boundary condition of finite element model. The finite element modeling suggests that the paelostress field within the accretionary prism evolved during Late Cretaceous accretion. During the initial stage of accretion, paleostress σ1 is compressive and nearly horizontal and thrust faults develop within the accretionary prism. Underplating of the oceanic crust causes a reorientation of the paleostress axes within the accretionary prism. Normal faults develop at the base of the accretionary wedge, and strike-slip faults develop in the deeper portion of the oceanic crust. The underplating process possibly results in lateral accretion and thickening of the accretionary wedge. During the final stage of accretion, paleostress σ1 is compressive and nearly vertical at the toe of the wedge, as the Late Cretaceous sediments are poorly consolidated. However, σ1 is compressive and subhorizontal and thrust faults develop at the toe of the wedge for the well consolidated sediments. The degree of lithification of Late Cretaceous sediments controlled the paleostress distribution and fault development at the toe of the accretionary wedge.