大陆深俯冲的动力学机制:观测和模拟结果

Dynamic mechanism of continental deep subduction:results from observation and simulation.

兴都库什-帕米尔-中国西部1975～2003年期间的地震活动记录、地表地质构造和地壳速度结构数据证明,沿特提斯陆-陆碰撞带正在进行大陆深俯冲作用。帕米尔地区大陆地壳的下部物质与上地幔一起俯冲到200km 以下,而中、上地壳在不同深度上被反冲断层所剥离。帕米尔地区向南的大陆深俯冲作用限于西部恰曼左行走滑断裂和东部喀喇昆仑右行走滑断裂之间。沿深俯冲带存在上、中、下3个地震群。上地震群出现在30～50km 附近,对应于中、上地壳的反冲剥离构造作用,地震成因与长英质地壳的脆-韧转换和“二相变形”机制有关。中地震群大体出现在90～120km 深度上,与帕米尔深俯冲岩板向下由缓倾变陡的深度大体相当。下地震群的主体出现在180～220km,代表深俯冲岩板的最前端。帕米尔大陆深俯冲岩板为上宽下窄、上缓(20～30°)下陡(60～70°),转变深度在80～120km 的楔形体,深度超过200km 的走向宽度只有500～600km。在探讨大陆深俯冲的动力学过程中采用了2种模拟方法。利用考虑温度场和负浮力的二维数值模拟表明:(1)地幔对流拖曳力对俯冲深度和俯冲速度有重要控制作用,从100MPa 到20MPa 的变化将导致俯冲深度由231km 减到151km,速率由10.79mm/a 减到5.46mm/a。(2)俯冲角30°与45°相比,前者的俯冲深度要深约25～50km。(3)俯冲板块厚度越大,则俯冲深度越浅。(4)在俯冲板块的负浮力、洋脊推力为10～30MPa 及地幔对流拖曳力为100MPa 的综合作用下,陆壳俯冲实际垂向位移可达120km,最终俯冲深度达到150km,而洋壳实际垂向位移约170km,最终俯冲深度达到230km。在考虑岩石圈和软流圈相互耦合的俯冲模拟中,块体间的接触判断采用了 LDDA 方法的接触判断准则和区域分解方法求解。其特点是边界条件比较简单,并能自动实现俯冲过程中岩石圈和软流圈之间的相互作用,但需要确定研究区域不同深度和不同板块的力学参数,且计算量很大。

Seismic record from 1975 to 2003, surface tectonics and crustal seismic velocity in Hindu kushi-Pamier-west China area show that a continental deep subduction is acting along Tethys continent-continent collision zone. Both lower crust and upper mantle are brought down to a depth more than 200 km, while mid-upper crust is overburdened by multiple back-thrusting in Pamier area. The continental deep subduction is restricted by Qiaman left-lateral strick slip faults in the west and Karakoron right-lateral strick slip faults in the east. Three seismic swarms occur along the deep subduction zone in vertical direction. The upper seismic swarm presents in depth between 30 and 50 km, responding to the back-thrusting overburden of mid-upper crust. The middle swarm is concentrated around 90 - 120 km in depth comparable to a change position where the dipping angle of subducted slab becomes steeper downwards. The lower swarm occurs in a depth from 180 to 200 km, representing the edge of the slab. Geometry of deep subducted slab in Pamier area shows a wedge narrowing downwards, with a change in subduction angle from 20～30 to 60～ 70 in 80～120 km. Two numerical simulations have been used in the discussion about dynamics of the continental deep subduction. Two-dimensional numerical simulation with a given temperature field and negative buoyancy indicates that ： （ 1 ） the mantle convection drag force plays an important control role to subducted depth and subducted rate. Its change from 100 MPa to 20 MPa results in decreases of the subducted depth from 231km to 151km and subducted rate from 10.79 mm/a to 5.46 mm/a; （2） The subducted depth of a subducted slab with a 30 degree angle is 25 ～ 50 km more than that with a 45 degree angle. （3） Under the combined action of three kinds of dynamic load including a 10 ～ 30MPa push force of the mid-ocean ridge, negative buoyancy and the mantle convection drag force of 100MPa, the actual vertical displacement of the subducted continental crust can reach 117km and its final depth is 147km; the corresponding data for the subducted oceanic crust are 162km and 231km, respectively. In another model of coupled lithosphere and asthenosphere the LDDA and regional division methods are used. The model is characterized by the simple border conditions and the automatically acted effects between lithosphere and asthenosphere during subduction, but the mechanic parameters of different depths in different slabs must be given and calculating process is more complex.