On the Formation of the Dabashan Orocline, Central China: Evidence from Seismic Ambient Noise Tomography

The gigantic Dabashan orocline, situated in the south Qinling Orogen, is a response of the tectonic uplift to the collision between the North China Craton and the Yangtze Craton in Mesozoic. However, how the orocline formed and deformed is still not well understood mainly due to inaccessible deep crust structures. In this study, we aim to address this question by investigating the detailed crust structures. We process continuous ambient noise data collected from ~170 stations of Chinese provincial networks operating during 2007 and 2009 to extract interstation surface wave dispersion curves. Using these dispersion curves, we generate Rayleigh wave phase velocity maps at 6-60 sec periods. By inverting the resulting Rayleigh wave dispersion maps using a Bayesian Monte Carlo method, we construct a 3D isotropic Vsv model with a lateral resolution of 50-60 km. The 3D Vsv model reveals detailed crust velocity structures which help to decipher the formation mechanism of the Dabashan orocline. Prominent low velocities are observed beneath the Dabashan orocline at 10-15 km depth, which may represent the buried passive continental margin sedimentations of the Yangtze Craton. High velocities are observed on the two sides of the orocline beneath the Shengnongjia-Huangling and Hannan Domes extending from the surface with Archean-aged outcrops to the mid/lower crust. The deep-rooted high-velocity domes very likely reflect the presence of the relative rigid Archean basement which acts as obstacles during the intra-continental shortening; while detachments may develop in the weak passive continental margin of sedimentations beneath the Dabashan during the shortening process, assisting the southward thrusting of nappes from the south Qinling Orogen onto the northern margin of the Yangtze Craton, which finally forms the gigantic Dabashan orocline. This model is also consistent with the thermo-chronological constraints and stratigraphic records which together indicate the initiation of the uplifts in the two domes are possibly earlier than in Dabashan.