Geologic Development of the Truong Son Fold Belt In Central Vietnam - Study on the Evolution of Asian Continent (IGCP-411) -

本项目旨在阐明从冈瓦纳大陆分散开始的复杂过程所形成的亚洲大陆的构造演化与日本诸岛发展的关系。认为厘清越南造山带的构造是实现这一目标的关键。项目日本成员在越南地质学家的协助下，在以下地区进行了地质调查；松马缝合带花岗岩的构造分析和取样测定年龄，红河断裂带两侧的古地磁研究取样（2000），张山褶皱带的沉积岩石学研究和越南北部的古生物学和构造研究（2001），越南的印度支那地块与邻近地块关系的构造分析。地质调查结果如下：1)越南红河断裂带北侧被华南地块向南延伸的缝合带划分为两个多块岩；2)张山褶皱带南侧为洋板块物质增生杂岩带，辐合方向为东北。3)印度支那地块的东南边界在越南境内不存在。古地磁、生物地层学和年代学以及沉积岩石学研究正在对从上述地区收集的样品进行研究。到目前为止，明确了以下几点：1)在红河断层两侧的两个地方，白垩纪对应的极点在统计上是无法区分的。2)子马缝合带南侧的增生杂岩发育泥盆系牙形刺，使得子马缝合带北侧的大陆碎片至少在泥盆世从Gondwanaland漂移而来。3)子马缝合带中斜长花岗岩（?）等3个花岗岩样品的年龄分别为238.9+/-1.8 Ma（中三叠世）、280.7+/-3.9 Ma（早二叠世）和256.1+/-1.2 Ma（中二叠世）。4)越南东北部华夏地块与扬子地块之间的缝合带也含有泥盆系牙形刺。4)在砂岩岩相学和地球化学研究的基础上，张山褶皱带中上古生界物源序列揭示了未成熟岩浆弧的形成，物源的明显变化被认为是越南北部及邻近地体古生代构造发育的重要方案。从上述的研究结果，我们可以指出以下几点在这个阶段。1)张山褶皱带是中南陆块之间的缝合带，2)张山缝合带的北部边界为松马缝合带，其南部边界可能对应于岘港断裂，3)张山缝合带包括洋板块和岛弧遗迹。4)越南的华南陆块由扬子地块和华夏地块组成。少

The purpose of this project is to clarify the tectonic evolution of Asian Continent that was formed by the complicated process started from the dispersion of Gondwanaland in the relation with the development of Japanese Islands. It is considered that the clarification of the tectonics of the orogenic belts in Vietnam is a key to this purpose. Japanese member of the project did the geological survey in the following areas by the assistance of Vietnam geologists; structural analysis and sampling of granitic rocks for age determination in the Son Ma Suture Zone, and sampling for paleomagnetic study in both sides of the Red River Fault (2000), sedimentary petrological study in the Truong Son Fold Belt and paleontological and structural study in northern Vietnam (2001), and tectonic analysis of the relationship between Indochina Block and adjacent blocks in Vietnam. The results of the geological survey are as follows; 1) Northern side of the red River Fault in Vietnam is divided into two bl … More ocks by the suture zone which is a southern extension from the South China Block. 2) Zone consisting of accretionary complex of ocean plate materials are recognized southern side of the Truong Son Fold Belt and its vergence is toward northeast. 3) Southeastern boundary of the Indochina Block does not exist inside Vietnam.Paleomagnetic, biostratigraphic and chronological, and sedimentary petrological studies are going on concerning samples collected from the areas mentioned above. Until now, the following points are made clear, 1) The corresponding poles in Cretaceous at two localities in the both sides of the Red River Fault are statistically indistinguishable from each other. 2) Accretionary Complex in the southern side of the Son Ma Suture Zone yields Devonian conodonts so that the continental fragment in the northern side of the Son Ma Suture Zone drifted from the Gondwanaland at least in Devonian. 3) Age of three granitic sample including plagiogranites (?) from the Son Ma Suture Zone is 238.9+/-1.8 Ma (Middle Triassic), 280.7+/-3.9 Ma (Early Permian) and 256.1+/-1.2 Ma (Middle Permian). 4) Suture zone between the Cathaysia and Yantze blocks in northeastern Vietnam is also contains Devonian conodonts. 4) On the basis of the sandstone petrography and geochemistry, the provenance in the Middle-Upper Paleozoic sequence in the Truong Son Fold Belt reveals the formation of the immature magmatic arc and distinct change of the provenance is considered as important scheme of Paleozoic tectonic development in northern Vietnam and adjacent terranes.From above-mentioned results of the study, we could indicate the following points at this stage. 1) The Truong Son Fold Belt is a suture zone between the Indochina and South China Continental Blocks. 2) Northern boundary of the Truong Son Suture Zone is the Son Ma Suture Zone and its southern boundary may be correspond to the Danang Fault. 3) Truong Son Suture Zone includes relicts of ocean-plate and island arc. 4) South China Continental Block in Vietnam is composed of Yantze and Cathaysia Blocks. Less