拉分盆地形成机制三维数值模拟——以海原断裂带老龙湾盆地为例

拉分盆地是走滑断层系中受拉伸作用形成的断陷盆地.一般在两条平行断层控制下发育.盆地形似菱形,几何形态主要受两条主控走滑断层错距和叠接长度影响.本文以青藏高原东北缘海原断裂带老龙湾拉分盆地第四纪所处的构造环境为基础,参考盆地周围断层几何分布,建立了三维有限元数值模型,模拟该拉分盆地的演化过程;进一步分析了断层力学性质、地壳分层结构等各因素对盆地形成和演化的影响.模拟结果显示,盆地地表沉降伴随有下地壳物质的上涌,此上涌对盆地地表沉降存在阻碍作用.各因素的影响具体表现为:(1)断层力学性质(弹性模量和黏滞系数)越弱,其对构造应力较低的传递效率导致盆地两端差异性运动越明显,从而形成较大的盆地地表沉降和明显的上地壳减薄.(2)平行主控断层的叠接长度反映盆地形成的拉伸作用范围,叠接长度越大,相同的差异性运动在单位面积形成的拉伸应力越小,盆地地表沉降较小.(3)下地壳流变性影响其物质的上涌量,下地壳黏滞系数越小,其对上部拉伸作用的响应越明显,上涌量越大,此上涌对上地壳沉降形成的阻碍作用也越明显.根据老龙湾拉分盆地所处的构造格局,将平行断层的叠接长度取20km,当断层黏滞系数取值为周围基岩的1/10,参考该盆地第四纪构造演化历史,模拟得到的盆地第四纪下沉量与盆地内第四系沉积层厚度在规模上近似,下地壳黏滞系数取值在(2.5~5.0)×1021 Pa·s范围内时,盆地下沉量模拟结果与老龙湾拉分盆地第四系地层厚度吻合较好.

Three-dimensional numerical simulation of pull-apart basins: an example of the Laolongwan basin in the Haiyuan fault zone

A pull-apart basin is formed by the extension in a strike-slip fault system, generally controlled by two parallel faults. Such a pull-apart basin is rhombic-shaped, whose geometry is determined by the distance and overlap between the two faults. There are many factors controlling the formation and evolution of pull-apart basins. Numerical simulation is an efficient way to explain the mechanism of pull-apart basin generation. This work studies the major factors that control the formation of pull-apart basins based on 3D finite element modeling. As the formation and evolution of a pull-apart basin is a long geological process, we construct a 3D finite element visco-elastic model based on the tectonic setting during Quaternary of the Haiyuan fault zone, northeast of the Tibet plateau. In the numerical model, the two master faults are thin zones of weaker mechanical property. Furthermore, we analyze the effect of each factor, such as mechanical property of weak fault zones, layered structure of the crust and overlap of master parallel faults. According to the simulation results, the extension of upper crust leads to surface subsidence in the basin and upwelling of lower crust. The upwelling to some extent hinders surface subsidence. The effects of all factors are summarized as follows: (1) The weaker faults transfer tectonic stress less efficiently, which can result in more obvious differences in movement, large surface subsidence and thinning of upper crust. (2) The overlap of two master parallel faults determines the area of extension, the longer overlap leading to weaker extensional stress and less surface subsidence. (3) The rheology of lower crust affects its upwelling, low viscosity of lower crust resulting in more sensitive response to extension in upper crust and large upwelling of lower crust which hinders surface subsidence more strongly. Based on the tectonic setting and the evolution during the Quaternary period of the Laolongwan pull-apart basin, we set up a model in which the overlap of faults is 20 km, viscosity of the fault is 1/10 of the surrounding rock. The simulated surface subsidence of the basin is similar to the sedimentary thickness during the Quaternary period. When the viscosity of lower crust in (2.5~5.0)×10²¹ Pa·s, the simulated surface subsidence accords well with the Quaternary sedimentary thickness. The weaker fault zones and overlap of two master parallel faults all play significant roles in the formation of the pull-apart basin, controlling differential tectonic movement through the weak fault zones. The rheology of lower crust affects the surface subsidence of the pull-apart basin by its upwelling. As to the Laolongwan pull-apart basin during the Quaternary, we establish an optimal model according to the tectonic setting. The simulated surface subsidence matches well with the Quaternary sedimentary thickness, with the viscosity of lower crust ranging (2.5~5.0)×10²¹ Pa·s.