Crustal radial anisotropy beneath the Dabie orogenic belt from ambient noise tomography

Abstract

In this study, we continue our previous work of imaging isotropic shear velocity to study the crustal radial anisotropy beneath the Dabie orogen and surrounding regions by jointly analysing Rayleigh wave and Love wave dispersion curves. In addition to processing the vertical component data to retrieve Rayleigh waves, we collect horizontal-component continuous ambient noise data and retrieve Love waves from cross-correlations of horizontal-component noise data between station pairs. Then, we measure Love phase velocity dispersion curves at periods from 8 to 35 s using a spectral method and perform surface wave tomography to obtain high-resolution phase velocity maps. Finally, we determine crustal radial anisotropy by inverting the local phase velocity dispersion curves of Rayleigh and Love waves. Our model of crustal radial anisotropy reveals complex anisotropic patterns associated with the dynamic processes of continent–continent collisions and post-collision reworking between the North China Craton and the Yangtze Craton. In the Huabei Basin, the radial anisotropy is positive throughout the entire crust, corresponding to areas with extensional tectonics developed since the middle Mesozoic. In the Jianghan Basin, strong positive radial anisotropy in the middle and lower crust is observed, most likely resulting from subhorizontal alignment of seismic anisotropic materials under the crustal extensional deformation. In the eastern Dabie orogenic belt, the radial anisotropy beneath Northern Dabie complex (NDC) appears positive in the upper and lower crust and negative in the middle crust. The positive radial anisotropy in the upper crust is probably because of the presence of igneous rocks with anisotropic minerals aligned subhorizontally under the action of the extensional tectonics during igneous extrusion and emplacement, whereas the positive radial anisotropy in the lower crust is probably because that subhorizontal alignment of anisotropic materials induced by magmatic underplating formed after the delamination of the orogeny. In the middle crust, finite strains associated with the vertical intrusion of deep magma might cause crystalline anisotropic minerals aligned vertically, resulting in observed negative radial anisotropy. In the western Dabie orogenic belt, positive radial anisotropy is present in the whole crust except beneath the Hong'an ultrahigh pressure (UHP) area where radial anisotropy is negative in the middle crust. Similar to NDC, the Hong'an UHP probably experienced similar thermal geodynamic processes as the eastern Dabie and the negative radial anisotropy might be also the result of vertical alignment of anisotropic minerals caused by the vertical intrusion of deep magma. Strong localized negative radial anisotropy in the middle crust beneath the Xinyang area is imaged, most likely caused by the vertical alignment of anisotropic minerals crystallized from the vertically intruding mafic magma originated from the upper mantle during the volcanism.