The Rise and Fall of the Himalayan-Tibetan Plateau: An Integrated 3-D Finite Element Modeling

摘要：目前对印支-亚洲碰撞地球动力学的理解主要基于两种端元模型：基于连续介质力学的平面应力粘性薄片模型和基于模拟实验的平面应变塑性泥压痕模型。这两个模型都为理解西藏大地构造提供了重要的框架，但这些模型的二维性和其他简化使许多重要问题处于不确定状态。例如，塑性泥压痕模型预测西藏和东南亚的横向挤压是容纳碰撞后2000公里地壳缩短的主导机制，而粘性薄片模型强调地壳增厚的作用。这些不同的预测是有偏见的模型的局限性：塑性泥压痕模型不允许垂直应变，因此地壳增厚，而粘性薄片模型不包括故障和摩擦（塑性）流变学必要的模拟逃逸构造。自从这些端元模型在大约20年前被引入以来，已经进行了许多改进，但是关于地壳增厚和侧向挤压之间的应变分配以及许多其他问题的持续争论表明需要一种新的方法。本研究拟建立一个全三维有限元模型，以研究喜马拉雅-青藏高原的升降过程。从几十年的深入研究中积累的大量多学科观测数据已准备好在更复杂的地球动力学模型中加以综合和解释。随着计算机技术的进步和有限元方法的成熟，三维有限元模型已经成为可能。PI已经建立了原型三维模型，并成功地应用于青藏高原和安第斯山脉活动构造的研究。本项目的主要模型开发包括在有限应变计算中实现走滑断层，并纳入更现实的流变学。有限应变模型将包括脆性变形内的上地壳，往往忽略了粘性薄片模型，通过使用混合粘弹性流变学。瞬态应力和应变率将在粘弹性和塑性粘弹性流变学的无限小应变模型中进行模拟。有限应变和无穷小应变模型将结合起来，提供一个更完整的模拟碰撞的历史。将进行系统的数值试验，以探索印度-亚洲碰撞的机制。这些结果将导致更好地理解这些问题：喜马拉雅-青藏高原地壳变形的主要驱动力是什么？ 青藏高原是何时、如何隆起的？ 是什么控制了地壳增厚和侧向挤压之间的应变分配？ 是什么导致了西藏的扩张？上地壳变形与下地壳和地幔岩石圈变形的关系如何？通过与美国和中国科学家的合作，包括Yin（加州大学洛杉矶分校），Nabelek（俄勒冈州）和Flower（UIC），数值模拟将与青藏高原正在进行的地质和地球物理研究紧密结合。所提出的三维有限元模型将是大规模大陆变形数值模拟的重要一步，并可用于其他板块边界。PI将记录计算机代码并向公众提供。为这项研究汇编的多学科数据集将编入地理信息系统数据库，供研究界使用。该项目将涉及本科生和研究生;尖端计算与多学科研究的结合为培养新一代地球科学学生提供了一个很好的机会。

Abstract: Much of the current understanding of the geodynamics of the Indo-Asian collision has been based on two end-member models: the plane-stress viscous thin-sheet model based on continuum mechanics, and the plane-strain plasticine indentation model based on analog experiments. Both models have provided important frameworks for understanding the Tibetan tectonics, but the two-dimensionality and other simplifications of these models have left many important issues in uncertainty. For example, the plasticine indentation model predicts lateral extrusion of Tibet and SE Asia to be the dominant mechanism accommodating the 2000 km post-collisional crustal shortening, whereas the viscous thin-sheet model emphasizes the role of crustal thickening. These different predictions are biased by the model limitations: the plasticine indentation model does not allow vertical strain and hence crustal thickening, whereas the viscous thin-sheet model dose not includes the faults and the frictional (plastic) rheology necessary to simulate the escaping tectonics. Since these end-member models were introduced ~20 years ago, numerous improvements have been made, but the continued debate on the strain partitioning between crustal thickening and lateral extrusion and many other questions manifests the need for a new approach. The proposed work is to develop a fully three-dimensional (3-D) finite element model to investigate the rise and fall of the Himalayan-Tibetan plateau. The large volume of multidisciplinary observational data accumulated from decades of intensive studies is ready to be integrated and interpreted in more sophisticated geodynamic models. The 3-D finite element models are now feasible thanks to the advancement of computer technology and maturation of the finite element method. The PI has already built the prototype 3-D model and successful applied it to studies of active tectonics in the Tibetan plateau and the Andes. Major model developments in this project include implementing strike-slip faults in finite-strain calculations and incorporating more realistic rheology. The finite-strain model will include brittledeformation within the upper crust, often omitted in viscous thin-sheet models, by using a mixed viscous-elastic rheology. Transient stress and strain rates will be simulated in infinitesimal-strain models with viscoealstic and plasto-viscoelastic rheology. The finite-strain and infinitesimal-strain models will be combined to provide a more complete simulations of the collision history. Systematic numerical experiments will be conducted to explore the mechanics of the Indo-Asian collision. The results will lead to a better understanding of these questions: What are the major driving forces for crustal deformation in the Himalayan-Tibetan plateau? When and how did the Tibetan plateau uplift? What controlled the strain partitioning between crustal thickening and lateral extrusion? What caused the Tibetan extension? How is the upper crustal deformation related to that in the lower crust and the mantle lithosphere? The numerical modeling will be closely integrated with ongoing geological and geophysical studies in the Tibetan plateau through collaboration with US and Chinese scientists, including Yin (UCLA), Nabelek (Oregon State), and Flower (UIC). The proposed 3-D finite element models will be a major step forward in numerical modeling of large-scale continental deformation and can be used for other plate boundaries. The PI will document the computer codes and make them available to public. The multidisciplinary data sets compiled for this study will be organized into a GIS database and made available to the research community. This project will involve undergraduate and graduate students; the integration of cutting-edge computation with multidisciplinary studies provides a great opportunity for training a new generation of geoscience students.