| Summary: | 博士 === 國立中央大學 === 國際研究生博士學位學程 === 102 === Northern Vietnam is located in the southeasternmost extension of the Himalayan syntaxis, which is mechanically linked to the Tibetan plateau through the Red River fault. Knowledge on the detailed crustal structure in this region is scant. Obtaining knowledge on the crustal structure beneath northern Vietnam is a fundamental objective to understand the tectonic development in this region. Therefore, we deployed 25 broadband seismic stations that were uniformly distributed over northern Vietnam, the records of which revealed more than 210 teleseismic events of Mw > 5.5 for the 2006–2012 period. The seismic data recorded in this period provided a unique opportunity to study the deep structure and crustal deformation of northern Vietnam. This thesis investigates the crustal structure beneath northern Vietnam by using receiver function analysis to enhance our understanding of the effects of tectonic processes on the crustal structure.
In the first part of this work, I report the estimates of crustal thickness and Poisson’s ratios for northern Vietnam, based on a teleseismic receiver function analysis of observations from the dense broadband seismic array. Using the radial receiver functions calculated from teleseismic records at individual stations, I determined the optimum crustal thickness and Vp/Vs (where Vp and Vs are the velocities of P- and S-waves, respectively) ratio beneath each station by using the H–κ (where H is the crustal thickness, and κ is defined as the Vp/Vs ratio) stacking algorithm. The determined values of the crustal thickness range from 26.5 to 36.4 km, with an average of 31.0 ± 2.1 km. The simple pattern of variation in crustal thickness in the northeastern region of the studied area, with a mean of approximately 31 km, suggests that the sector belongs to the craton of the South China block. I found a highly variable crustal thickness over the northwestern region of northern Vietnam, ranging from approximately 29.5 to 36.4 km, implying that complex tectonic processes have occurred in this region. The thinnest crust was found in the Red River Delta, where it ranges from 26.5 to 30.4 km, which is suggestive of a recent rifting process. The determinations of crustal thickness show a good linear correlation with Bouguer gravity anomalies. Lower values of Poisson’s ratio in the northeastern and Red River Delta sectors suggest a more felsic crust, and larger values in the northwestern sector suggest a lithospheric extension in the Song Da depression.
The second part of this work presents estimates of the crustal shear wave velocity structure beneath northern Vietnam, based on the linearized inversion of stacked receiver functions. The shear wave velocity structure in the northwestern region is relatively slow and more complex, whereas the velocity in the Red River Delta and northeastern region is slightly faster and simple. The areas of slow velocities in the crust possibly coincided with the observations of partial melting in the active regions, which may be related to the extension processes of the lithosphere or the rapid eastward flow of the deep crust. Those of faster velocities are consistent with the stable continent, suggesting small changes over the tectonic history in the processes that form the crust. The findings enhance our understanding of the geotectonic architecture of the northern Vietnam region.
|
|---|
