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

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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.