模拟弹塑性上地壳中逆断层盆地的演化

利用黏弹塑性有限元分析方法 ,模拟了厚为 1 0km的上地壳 (假定上地壳具有Byerlee型强度包络 )中以逆断层为边界的盆地的演化 ;假定基底无黏性 ,而盆地中充填的沉积物密度比地壳密度小 40 0kg/m3 。随着每次以 0 .2 5km位移增量逐渐挤压地层 ,塑性破裂区扩大 ,直至挤压位移至约 5 .5km时 ,出现了破裂切穿整个地层的现象。当达到这种状态时 ,盆地的沉降减慢 ,最终停止。地层被挤压至位移 5 .5km后形成一个 30km宽、5km深的盆地。整个演化过程中 ,盆地转折线逐渐向断层移动。切穿破裂出现以后 ,地层被继续挤压到位移 8km ,以便观察研究断层两侧的挤压褶皱构造。这些褶皱是从早期轻微的挠曲起伏演化而来的 ,到挤压位移 3km时 ,它们开始明显增强 ,到挤压位移 8km时 ,已经形成非常大的背斜和向斜 ,其幅度从断层向两侧逐渐减小。

Modeling the evolution of basin bounded by reverse fault in the elastoplastic upper crust

The evolution of basin bounded by reverse faulting in a 10km thick upper crust with a byerlee-type strength envelope has been modeled by elasto-viscoplastic finite element analysis. The substratum is assumed to be inviscid and the basin is filled by sediments 400kg/m3 lower in density than that of the crust. As the layer is shortened by progressive increments of 0.25km, the zones of plastic failure extend until throughgoing failure of the layer eventually occurs. When this stage is reached, subsidence of the basin slows down and finally stops. Throughgoing failure occurs at 5.5km shortening for reverse faulting. A narrower trough 30km wide and 5km deep forms for reverse faulting after 5.5km shortening. The hinge line of the basin progressively migrates towards the fault. The reverse faulted model has been continued to 8km shortening,well beyond throughgoing failure,to demonstrate the formation of compressional folds on both sides of the fault. These develop from the gentle flexural undulations of earlier stages, which start to be visibly amplified by 3km shortening, and form quite strong peripheral anticlines and synclines by 8km shortening, decaying away from the fault.