Upper Mantle Velocity Structure Beneath The South China And Tibetan Plateau From Triplicated Seismic Waveforms

Velocity structures of upper mantle,especially the Mantle Transition Zone(MTZ),as well as lithosphere,provide important constraint on thermal,compositional properties of the earth,and help to understand the mechanisms of mantle convection and lithospheric failure.The South China Block,formed by the collision between Yangtze and Cathaysia blocks during late Neoproterozoic,experienced multi-episode intensive tectonic activity,is one best place to study lithospheric thinning mechanisms related to oceanic plate subduction.In the meantime,the Tibetan Plateau is a natural laboratory for studying the evolution of continental collision.The Himalayan-Tibetan orogen has absorbed at least 1700km of north-south shortening since the Indo-Asian collision at Cenozoic.The mechanisms of the continental uplift and the lithospheric deformation are of key importance to reconstruct the plateau evolution models.In the first part of the thesis,I first briefly introduced the upper mantle discontinuities of standard earth models and explain their significance for seismology studies.Then I summarized the concepts and steps of some commonly used techniques to study upper mantle structure,of which I put much focus on is triplicated waveform modeling.Some key steps will be summarized as following.Correctness of source parameters and initial model is essential for the success of the triplicated waveform modeling.To determine the source parameters,I calculated the source time function and source depth using teleseimic event records with iterative time-domain deconvolution technique.To determine the initial model,I developed a noval relative travel time inversion method.With the well determined source parameters and initial model,waveforms of synthetic and real data are fit in a trial and error manner.The methods developed in thesis provide an effective way for triplicated waveforms data processing.In the following part of the thesis,triplicated waveforms ranging from 10° to 30° recorded by 350 temporary seismic stations(ChinArray)and Chinese Digital Seismic Network(CDSN)were used to obtain high-resolution velocity models of MTZ and lithosphere beneath South China Block(with the newly developed method)and Tibetan Plateau(with the classic method).Combing with the geological and geophysical results of previous studies,the possible dynamic mechanisms are proposed.Results and discussions will be summarized as following.In the South China study,I found that a high velocity anomaly is observed in the MTZ beneath the Middle Yangtze Craton,which is interpreted to be the stagnant Pacific slab.The anomaly stopped 27°N to the south and North-South Gravity Lineament(NSGL)to the west.The subducted slab didn't penetrate into the lower mantle but depressed the 660-km discontinuity by 11 km as a result of negative Clapeyron slope of post-spinel phase transition.Without obvious high velocities,the upper mantle structure beneath Cathasyia is similar to the IASP91 model.A low-velocity layer atop the 410-km discontinuity with the P velocity reduced by 1.38%-2.29%compared with IASP91 model in the region is mainly caused by partial melting of upper mantle materials.Within the study area,both the Middle Yangtze Craton and Cathaysia Block have a much thinner seismic lithosphere of 80-km in thickness on average.The lithospheric thinning is likely due to delamination related to the subduction and fast retreat of Pacific plate.The thinner lithosphere and somewhat lower asthenospheric velocity in the Cathaysia Block reflect a slightly weaker and perhaps warmer uppermost mantle than in the Middle Yangtze Craton.From late Cretaceous,the Pacific Plate retreated eastward,resulting in an extensional setting,the effects of which were enhanced by regional thinning of the lithosphere,commonly attributed to delamination.The delamination of lithosphere results in upwelling of asthenosphere and triggered extensive and intense magmatic activity in Southern China area during late Mesozoic.In the Tibetan Plateau study,based on the results from waveform modeling,an upper mantle structure along the N-S profile across the Tibetan Plateau was sketched.The high velocity anomaly observed at the bottom of the MTZ beneath Lhasa and Qiangtang Terranes were interpreted as the remnants of the subducted India slab.I suggested that the Indian mantle lithosphere thrusted beneath the Tibetan Plateau as far as the Bangong-Nujiang suture.The high velocities depressed the 660-km discontinuity by about 3-8 km.In addition,the upper mantle velocities over 200 km decrease gradually from south to north.The velocities of lids beneath Qiangtang and Songpan-Ganzi Terranes are lower than that beneath Lhasa Terrane,and even missing beneath the Songpan-Ganzi Terrane.The low velocities between the depths of 100 and 200 km under the Qiangtang and Songpan-Ganzi Terranes are likely caused by the delamination of the thickened Asian lithosphere due to a thermal instability or small-scale mantle convections.Some of the delaminated lithosphere is now resting on top of the 410-km discontinuity,and others may have penetrated into the MTZ.The high velocity in the lower MTZ beneath Songpan-Ganzi Terrane probably corresponds to the delamination of thickened Eurasian lithosphere.The delaminated and cold lithosphere causes the uplift of 410-km by about 10km and depression of 660-km by about 8km,resulting in an increase in the thickness of MTZ.