| 摘要: | 越南北部 (北越) 地區位處由喜馬拉亞山所延伸之連接帶的最東南端。本地區透過紅河斷裂帶銜接隆起的西藏高原，然本地區的詳細地下構造至今仍鮮為人知。為了解本地區的地體構造演化，探查本地區的地殼構造將列為首要目標。為此本研究選取自2006~2012年間平均佈設於北越地區的24部寬頻地震儀所記錄之規模大於5.5的210個遠震資料進行分析。這些資料提供了難得之機會以研究深部構造及地殼變形。本論文將利用接收函數法分析北越地區地殼結構以了解構造運動加諸地殼形貌之影響。
本文之第一部份將報告依據遠震接收函數法分析由高密度地震儀陣列所記錄之資料，推求北越地區地殼厚度及其Poisson比。本研究利用各測站所記錄之資料計算徑向接收函數並採用H-k疊加程式推求位於測站下方之最佳地殼厚度及Vp/Vs比。分析結果顯示本地區地殼厚度介於26.5公里至36.4公里之間，平均厚度為31.0±2.1公里。位於區域東北部份之中國東南地塊其平均厚度約為31公里，只具輕微之地殼厚度改變，位於區域西北部份則具高度地殼厚度改變 (由29.5公里至36.4公里) ，代表本區域的複雜板塊構造作用。最薄的地殼分佈於紅河三角洲地區，代表近期的地殼拉張作用。本研究所推求之地殼厚度與區域之重力布蓋異常值有良好的線性相關。具有低Poisson比值的地區主要分佈於本地區的東北部，紅河三角洲地區顯示富含矽酸鹽質成份，具高Poisson比值的西北地區代表了Song Da 地區的岩石圈拉張。
本研究的第二部份將以疊加後的接收函數透過線性逆推法估算北越地區的地殼剪力波速。分析結果顯示本地區的北部具低剪力波速及複雜速度分佈。相對的紅河三角洲及東北地區則具高速與簡單構造。本地區具低剪力波速地殼代表地殼部份熔融或地殼流的可能存在及活躍的地殼運動。具高速的地殼則與地質上的穩定大陸地塊分佈一致。本研究的成果對於了解北越地區之地體構造具有重要貢獻。
;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. |
