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Title: Deep structure beneath the Central-South Tibet crustal density modelling and azimuthal anisotropy variation inferred from Quasi-Love wases
Authors: Zhang, Sufang
Keywords: AnisotropySouth TibetDeep structureSurface waves
Issue Date: 29-Mar-2010
Publisher: Università degli studi di Trieste
Abstract: The area of the present study is the central part of southern Tibet. It consists of two accreted terranes, Lhasa and Himalaya terranes, which today record the deformation history that originated from the processes of collision between the Eurasia and India plates. Our study of the crust/mantle structure in terms of seismic velocity, density, anisotropy and petrologic composition are undoubtedly significant to deepen the understanding of the continent-continent collision and its dynamics. This PhD thesis can be briefly summarized into four parts that are listed in the following. 1) In order to reveal the characteristics of the crust/mantle deformation that has been generated by the Indian/Eurasia collision in the southern Tibet plateau, we study the propagation of Quasi-Love (QL) waves. Our study is based on the results from numerical modeling, which proved that QL is sensitive to lateral variation of seismic anisotropy, rather than heterogeneity and other factors. The results we obtain from processing locally observed seismograms, reveal a West-East variation of crust/mantle deformation in each terrane of the plateau. 2) A 3D density model of central-south Tibet is produced by modeling the Bouguer gravity field using all existing constraints. 3) Integrating seismic velocity and density models of the crust in the Lhasa and Himalaya terranes, we infer crustal composition models in central and southern Tibet. 4) Combining crustal density, velocity and mineralogical composition models, some important issues, such as the Indian slab subduction angle, and the relationship between crustal density and earthquake occurrences are discussed. Some results based on the gravity modeling are summarized as follows: 1) under the constraint of the geometrical structure defined by seismic data, a 3-D density model and Moho interface are proposed for central-south Tibet; 2) the lower crustal density, smaller than 3.2 g/cm3, suggests the absence of eclogite or partial eclogitization due to delamination under the central-south Tibet; 3) seismicity is strong or weak in correspondence of the most negative Bouguer gravity anomaly, so there is not a relationship between them; 4) the composition of the lower crust, determined after the temperature-pressure calibration of seismic P wave velocity, might be one or a mixture of: 1. amphibolite and greenschist facies basalt beneath the Qiangtang terrane; 2. gabbro-norite-troctolite and mafic granulite beneath the Lhasa terrane. When using the data set published by Rudnick & Fountain (1995), the composition of the middle crust turns out to be granulite facies and might be pelitic gneisses. Granulite facies used to be interpreted as residues of partial melting, which coincides with the previous study by Yang et al. (2002) on partial melting in the middle crust. Amphibolite facies are thought to be produced after delamination, when underplating works in the rebound of the lower crust and lithospheric mantle. From the seismology study, I have made the following conclusions: 1) through numerical simulation of surface wave propagation in heterogeneous media, we find that amplitude and polarization of surface wave only change a little when considering heterogeneity and QL waves, generated by surface wave scattering, are caused by lateral variation of anisotropy. 2) QL waves have been identified from the seismograms of selected paths recorded by the Tibetan station CAD, and are utilized to determine the variation of the uppermost mantle anisotropy of the Tibetan plateau. The location of the azimuthal anisotropy gradient is estimated from the group velocities of Rayleigh wave, Love wave and QL wave. We find that a predominant south-north lateral variation of azimuthal anisotropy is located in correspondence of the Tanggula mountain, and a predominant east-west lateral variation of azimuthal anisotropy is found to the north of the Gandese mountain (near 85°E longitude and 30°N latitude) and near the Jinsha river fault (near 85°E longitude and 35°N latitude).
Description: 2008/2009
NBN: urn:nbn:it:units-8973
Appears in Collections:Scienze della terra

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