Imaging crustal structures of orogenic belts using ambient noise tomography: implications for mountain building processes

Orogenic belts are often characterized by strong crustal heterogeneity, which mainly results from complex tectonic evolutions. Conventional geological methods only sample the volumes near the surface, while seismic tomography enables a comprehensive and three-dimensional sampling of the physical properties of the Earth. The recent emergence of ambient noise tomography, in particular, allows us to study crustal heterogeneity with unprecedented resolution. In this thesis, I have adopted ambient noise tomography and Monte-Carlo inversion methods to construct models of crustal velocity in three orogenic belts, including the northeast Tibetan Plateau, the Qinling orogenic belt in central China, and the Delamerain orogen and its associated Newer Volcanics Province (NVP) in southeast Australia. The major deformation in these three orogenic belts occurred in Cenozoic, Mesozoic and early Paleozoic time respectively, and the relict orogenic structure is well preserved. The ambient noise tomography is subject to minor effects of off-great circle propagation and wavefront healing, and is therefore able to capture reliable velocity features in regional scales. Based on the highly resolved crustal-velocity models, some hotly-debated scientific questions are discussed, which include 1) how low-velocity zones in the mid/lower crust of the northeast Tibetan Plateau are distributed, and how they are related to the growth of the Tibetan Plateau; 2) whether there is seismic evidence for the indentation model on the formation of the Dabashan Orocline, and how the extrusion of the Tibetan Plateau affects deformation in the west Qinling Orogen; 3) what is the crustal tectonic setting of the NVP, and how the magmas of the NVP were stored and migrated through the crust. In addition, I also investigate the azimuthal anisotropy in the Qinling and its neighboring area to complement my derived isotropic velocity model in the region. We integrate velocity models with interpretations from other geological, geochemical and geophysical studies in order to solve the above geological questions, and their answers contribute to a better understanding of the mountain building processes in these orogenic belts.