华北地区新生代岩石圈伸展减薄机制的数值模拟

Numerical simulation of the mechanism of Cenozoic lithospheric extension-thinning in North China

新生代时期华北东部裂谷的伸展减薄机制及其周边的构造应力场,西部鄂尔多斯克拉通的抬升和周边断陷盆地的形成机制是目前研究的热点问题,但是较少有人从数值模拟的角度进行探讨。笔者采用有限元程序FEVPLIB对该地区5个剖面进行了模拟,初步取得如下认识:①在太平洋俯冲带的附近岩石圈伸展减薄较强,这与剖面经过的冲绳海槽正在拉开是吻合的,而太平洋的俯冲对较远的华北盆地的伸展减薄的影响较弱;②火山喷发时期,华北盆地有大的软流圈物质上涌造成华北裂谷的伸展减薄,符合纯剪切的机制,现今华北地区已趋于均衡,动力正趋于稳态;③六盘山逆冲在鄂尔多斯块体之上,代表着青藏高原东北缘的挤压,对华北是一个大的推挤力,可诱发鄂尔多斯块体的隆升,而鄂尔多斯向东北方向移动时提供了周边盆地的拉张的背景;④华北地区岩石圈的伸展减薄是六盘山处的挤压和东部太平洋板块俯冲两者联合的影响。模拟的结果与研究区GPS、重力异常以及岩石圈三维结构是吻合的。

The extension-thinning mechanism of the eastern North China rift and the tectonic stress field at its peripheries during the Cenozoic and the mechanism of the uplift of the Ordos craton and formation of its peripheral down-faulted basins in the west are the hot topics of the current research, but they are seldom approached from the angle of numerical simulation. The authors used the finite element program FEVPLIB to perform numerical simulations of five sections in this region and present the following views according to the results. （1） The extension-thinning of the lithosphere near the Pacific subduction zone are strong, which coincides with pulling-apart of the Okinawa trough that the section passes through, while the effect of the Pacific subduction on the extensionthinning of the North China basin that is relatively far away from the subduction zone is weak. （2） Upwelling of asthenospheric material beneath the North China basin during volcanic eruptions resulted in the extension-thinning of the North China rift, which conforms to the mechanism of pure shear. At present the lithosphere beneath the North China region has tended to be in isostatic balance and the dynamic system tends to be in a steady state. （3） The Liupan Mountains were thrust over the Ordos block, indicating the compression on the northeastern margin of the Qinghai-Tibet Plateau, which was a great force of pushing North China and might induce the uplift of the Ordos block, while the movement of the Ordos block to the northeast provided the background of extension of the peripheral basins. （4） The extension-thinning of the lithosphere beneath the North China region is the combined effect of the compression from the Liupan Mountains and the subduction of the Pacific plate in the east. The simulation results are in agreement with the GPS data, gravity anomalies and 3D lithospheric structure in the study region.