青藏高原南部南北向裂谷成因机制的数值模拟研究

The N-S rifts are widely distributed in southern Tibet, which is one of the most typical features during the Cenozoic tectonic evolution of Tibetan Plateau. The presence of N-S rifts in southern Tibet suggests that this region is under pronounced E-W extension along with N-S compression, behind the collision between the Eurasia and India plates. The related researchers have proposed numerous mechanisms of N-S rifting, based on the deep structure, Cenozoic tectonic evolution, and magmatic activities of N-S rifts in southern Tibet. However, the formation and mechanism of N-S rifting are still controversial and should be further investigated. To comprehensively analyze the mechanism of rifts in southern Tibet, we will adopt the 3-D viscous-plastic thermomechanical numerical method, and simulate the formation of N-S rifts driven by the bi-directional N-S compression and E-W extension, constrained by the tectonic settings of southern Tibet. By systematically comparative numerical tests, we will quantitatively analyze the effects of the layered structure and rheology of the thickened lithosphere, partially molten zone in the middle to lower crust and the mode of E-W extension on the formation and spatial pattern of rifts. Furthermore, we will comprehensively analyze the mechanism and the key influential factors of N-S rifting, which will promote the understanding of Cenozoic geodynamic settings of southern Tibet, and have significant implications for the Cenozoic tectonic evolution of Tibetan Plateau.

青藏高原南部发育了一系列南北向裂谷，是青藏高原新生代尤为特殊的构造特征之一。在印度-欧亚大陆碰撞挤压背景下，藏南南北向裂谷的存在，说明该区域在南北向挤压作用下同时经历明显的东西向拉张变形。目前，相关学者基于藏南南北向裂谷深部结构、构造演化和岩浆活动等特征提出了众多裂谷成因模式，但尚未形成统一认识，需要进一步开展研究。为综合分析藏南裂谷动力学成因，本项目拟采用三维粘塑性热力学耦合数值模拟方法，以藏南区域动力学背景资料为约束，动态模拟在南北向挤压和东西向拉张双向驱动下裂谷的形成。通过系统地数值实验，定量化分析藏南增厚岩石圈分层结构及其流变特性、中-下地壳部分“熔融体”和岩石圈伸展变形模式等因素对裂谷形成及其空间展布的控制作用，进而，综合分析裂谷的动力学成因及关键控制因素。预期成果将有助于深化对藏南新生代动力学环境的认识，进而对理解青藏高原新生代构造演化过程具有重要意义。

青藏高原南部近南北向裂谷是陆陆碰撞造山带内尤为特殊的伸展构造之一，增厚地壳内裂谷的形成及其与深部地幔岩石圈的关联尚存在争议。本研究利用高性能数值模拟手段，建立了增厚岩石圈缓慢伸展变形模型，开展了一系列对比实验，分别分析了地壳软弱层和软流圈物质上涌对增厚岩石圈伸展构造变形演化的影响，重点探讨了深部软流圈物质上涌和软弱中地壳共同作用下缓慢伸展的增厚岩石圈变形演化及裂谷形成过程。数值模拟结果显示在缓慢伸展背景下，低粘滞系数和低密度的地壳软弱层促进了上地壳范围内裂谷的快速形成。软流圈物质上涌（可能由俯冲的印度地幔岩石圈撕裂过程诱发）对上覆增厚地壳施加了较大的水平伸展应力，进一步促进了增厚地壳范围内裂谷的形成。地壳软弱层和软流圈上涌共同作用的模型结果显示，较厚的地壳低速层对上地壳与岩石圈地幔上涌具有较强的解耦作用，降低了深部过程的水平拉张应力，阻碍了地幔上涌区上方裂谷的形成；而应变集中会穿过解耦作用较弱的薄地壳软弱层，形成贯穿整个地壳的裂谷。地壳软弱层和软流圈物质上涌共同驱动下，增厚地壳内产生延伸上地壳和增厚地壳内的多条裂谷，对应模型演化结果与藏南增厚岩石圈结构异常体及裂谷分布近似。因此，参考一系列数值模拟结果，本研究认为受深部地幔动力学过程的影响，藏南一系列南北向裂谷不仅存在集中在上地壳范围内，又存在贯穿整个地壳范围的裂谷。尽管本研究的模型较为简化，但模拟结果对于揭示藏南裂谷成因机制及壳、幔伸展变形的动力学过程关联提供了重要参考依据。