塔西南甫沙地区被动顶板双重构造和乌泊尔地区背驮盆地的数值模拟

The Different Origins Between Passive-roof Duplex in the Fusha Area and Piggyback Basin in the Wupoer Area in Southwestern Tarim Basin Based on Numerical Modelling Study

塔里木盆地西南缘前陆褶皱冲断带发育被动顶板双重构造(甫沙地区)和背驮盆地(乌泊尔地区)两种明显不同的构造类型。为了探讨它们的不同成因,利用有限差分软件FLAC开展了一系列二维数值模拟研究。采用遵循平面应变的黏弹塑性本构模型,设置两个滑脱层和三个能干层。同时考虑基底沉降、同构造剥蚀和沉积,缩短速率为8mm/a,剥蚀速率为每年侵蚀基准面以上高程的3×10-7(相当于每1000m每年剥蚀0.3mm)。在基底水平的情况下,采用均一的沉降速率1.6mm/a并以填平补齐的方式进行沉积时,3.5 Ma后发育成与甫沙地区相似的被动顶板双重构造。而当模型采用中部小两端大的拱形沉降速度以及填平补齐的沉积时最终发育成背驮盆地,和乌泊尔地区地质原型接近。模拟结果表明,同构造沉积地层对褶皱冲断带的影响巨大,当沉积物大量堆积在褶皱冲断带前缘时有利于被动顶板双重构造的形成,而沉积物大量堆积在逆冲楔顶与斜坡时则更有利于背驮盆地的发生。模拟结果认为甫沙地区和乌泊尔地区都接受了填平补齐的沉积方式,但基底沉降差异造成了两者的构造样式明显不同。在小范围内(后陆至前陆小于80km),甫沙地区基底以水平方式发生沉降,褶皱冲断带前缘接受了大量沉积;而乌泊尔地区基底在挤压下发生弯曲,使得后陆发生了更大的沉降从而接受了更多的沉积。

In the foreland fold thrust zone in the southwestern margin of the Tarim basin develops two types of different tectonics： passive-roof duplex in the Fusha area and piggyback basin in the Wupoer area. In order to investigate their origins, a series of two-dimension numerical modeling was conducted using finite difference program FLAC. Through following the visco-elasto-plastic constitutive model of plane strain, two detachment layers and three competent layers were designed. In combination to basement subsidence, syntectonic erosion and sedimentation, shortening rate is 8 mm/a and erosion rate is 3×10-7 a-1 above erosion datum plane （equivalent to an erosion of 0.3 mm for each 1000 m per year）. When the basement is horizontal and deposited in an average sedimentation rate of 1.6 mm/a with the filling leveling up sedimentation mode, it is likely that passive-roof duplex similar to the structural style in the Fusha area will form after 3.5 Ma. Whereas a piggyback basin will be formed when laterally varied subsidence velocity and a filling-and-leveling-up sedimentation mode are used in the model,which is similar to the structural style in the Wupoer area. Simulation results indicate syntectonic sedimentation has a huge impact on the FTB： a lot of sediment accumulation in the front of FTB is beneficial to the formation of passive-roof duplex,whereas accumulation at the top and slope of FTB is more conductive to the formation of piggyback basin. Simulation results also show that both Fusha area and Wopoer area have the same filling-and-leveling-up sedimentation mode, but different subsidence of basement has resulted to different depositing. In a small area from hinterland to foreland with an area of less than 80 km, the basement of the Fusha area was dominated by horizontal subsidence, with the foreland FTB deposited by a large volume of sediments. The basement of the Wupoer area was curved under the compression condition, resulting in that the hinterland subsided greatly and thus accumulating much sediment.