下地壳流变与造山带同挤压期地壳伸展的动力学关系

应用不可压缩非牛顿粘性流体的本构关系,对造山带同挤压期下地壳流变及其与上地壳构造伸展的动力学关系进行了二维有限元数值模拟. 结果表明,在板块侧向挤压下,当造山带山根下陷和地表隆起达一定程度后,地壳不同层圈岩石将发生复杂的粘性流变. 流变的运动学方式和分布范围不仅与时间有关、同时还受地壳厚度转变带形态的制约. 在构造挤压和山体荷载达到弹性平衡状态后,地壳流变首先发生在造山带下地壳山根,但经一定的Maxwell时间后,流变将不断局限于造山带前缘的厚度转变带. 这一流变方式的变化是导致造山带浅部地壳动力学转变的主要原因. 它造成造山带内上地壳最小主应力从近水平挤压不断转化为近水平拉张,由此使造山带前陆发生挤压冲断的同时,山体的核部发生上地壳的拉张伸展. 最后,应用这一结果讨论了青藏高原南缘南北向地壳伸展的动力学性质.

LOWER-CRUST DUCTILE FLOW AND ITS DYNAMICAL RELATION WITH SYN-COLLISION CRUSTAL EXTENSION IN OROGENIC BELT

In attempt to reveal the dynamical relation between lower crust ductile flow and upper crust extension in collisional orogen, a numerical simulation is carried out using Finite Element Method (FEM), based on the stress strain constitution of incompressible Non Newtonian viscoelastic rheology. Results indicate that, owing to tectonic compression and topographical loading in mountain building process, the crust will undergo complex ductile flow. Patterns of flow are time dependent, and are influenced mainly by the shape of crust transition zone (CTZ) between mountain belt and foreland. Under the mechanic balance of tectonic compression and gravitational loading, the ductile flow is first facilitated in lower crust of mountain root. After several times of Maxwell time, it will be restricted narrowly beneath the CTZ, where the ductile flow is featured by the down flow near the mountain side and the up flow near the foreland side. This flow pattern causes the differential displacement in diverse tectonic layers and finally results in the stress partition in the upper crust of orogen, namely, the minimum principle stress near the center of orogen is transformed from horizontal compression to horizontal tensile. In details, this tensile stress is mainly formed after time of the several Maxwell times, and the magnitude of tensile stress is closely related with the height of mountain range above reference level. It can be inferred from this model result that the crust exension in collisional orogen has the dynamical relation with gravitational collapse, while the collapse is further intensified by the lower crust lateral flow. For this reason, it is concluded that the compression extension coupling in mountain belt is mainly derived from the lower crust ductile flow, which in initiated by the topographical loading the collisional compression. Using this model, the mechanics of crustal extension in southern Qinghai Xizang plateau has been illustrated.