青藏高原拉萨块体与羌塘块体三叠纪古地磁研究及其大地构造意义

发布时间：2018-06-28 05:22

本文选题：青藏高原 + 拉萨地块　；参考：《西北大学》2016年博士论文

【摘要】：青藏高原属特提斯构造域的东段,其形成演化过程是当代地球科学前缘的研究课题。三叠纪是青藏高原演化过程中关键时期之一,高原主要地体南北向迁移、古特提斯洋逐步收缩及中(?)特提斯洋渐渐开启等一系列重大地质历史事件可能就发生在这一时期。拉萨地块与羌塘地块位于青藏高原腹地,是组成青藏高原的主要地体,它们的运动与古特提斯洋收缩及中(?)特提斯洋开启及闭合等地质事件密切相关。古地磁学可以为研究块体的运动提供定量依据。本文对青藏高原腹地羌塘块体、拉萨块体的三叠纪地层开展了系统的古地磁研究并讨论其构造意义。在拉萨地块中部措勤地区敌布错湖边新发现的三叠纪海相碳酸盐岩地层中布置了32个采点,采集了327块独立的定向样品。岩石磁学研究显示：样品中主要的载磁矿物为磁铁矿和(或)磁黄铁矿。大部分样品逐步退磁曲线具有明显的三分量特征：低温分量为现代地磁场的低温粘滞剩磁；中温分量应为白垩纪的次生剩磁分量；高温分量(分量C)仅可在为数不多且以磁铁矿为主要载磁矿物的样品中获得。早-中三叠世的分量C可通过95%置信度的倒转检验(B级)与99%置信度的褶皱检验；晚三叠世的分量C可通过95%置信度的褶皱检验。对应的古地磁极位置：早-中三叠世(19.3°N,209.3°E,A95=4.0°,N=8)；晚三叠世(19.6°N,211.8°E,495=10.7°,N=6)；相应的古纬度分别为16.9±4.0°S和18.4+10.7°S。对羌塘地块北部三叠纪地层的26个采点(249块样品)进行了系统的古地磁研究。试验结果表明：样品中主要载磁矿物为赤铁矿和(或)磁铁矿。早三叠世与晚三叠世大部分样品的退磁曲线具有明显的双分量特征,中-晚三叠世图中湖组由于后期重磁化的影响,并未获得可靠的高温分量。本研究揭示出了两组高温特征分量,早三叠世与晚三叠世样品的高温分量均可通过高置信度下的褶皱检验,获得羌塘块体两个古地磁极位置：早三叠世(23.8°N,210.3°E,A95=10.9°, N=5),晚三叠世(70.6°N,275.6°E.A95=6.1,N=10)；对应的古纬度分别为-11.4±10.9°N和14.84±6.1°N。本文结果结合前人古地磁成果推测：(1)拉萨地块晚古生代至早中生代一直稳定于南纬中低纬度地区,可能到了白垩纪才快速北移与欧亚大陆主体碰撞拼合。(2)羌塘地块晚古生代徘徊于南纬中低纬度地区,早三叠世与拉萨块体位置相近,早三叠世至晚三叠世期间羌塘块体开始大规模北向移动,至晚三叠世时已移至北半球的中低纬度地区(14.8士6.1°N),三叠纪末以后持续北移,但其速度及规模已不及三叠纪这一时期。早三叠世至晚三叠世羌塘地体至少北向运动了26°,至少北向发生了2880 km的位移量,估算速度约达6.4cm/a,并伴随55°的逆时针旋转。(3)三叠纪时期拉萨块体与羌塘块体可能并不与冈瓦纳大陆北缘相邻,而是处于冈瓦纳大陆与欧亚大陆之间的独立区域。(4)夹于拉萨块体与羌塘块体之间的班公湖-怒江缝合带所代表的特提斯洋盆很可能于早三叠世打开,早三叠世至晚三叠世期间快速扩张。由于班公湖-怒江-特提斯洋的迅速扩张,推动了羌塘地块的快速北移,促使其与主体欧亚大陆的碰撞拼合以及古特提斯洋收缩闭合。
[Abstract]:The Qinghai Xizang Plateau belongs to the east section of the Tethys tectonic domain. Its formation and evolution process is a research topic in the front of the contemporary geoscience. The Triassic period is one of the key periods of the evolution of the Qinghai Tibet Plateau, the migration of the main body of the plateau, the gradual contraction of the ancient Tethys ocean and the gradual opening of the middle (?) tetitis ocean. The Lhasa and Qiangtang blocks, located in the hinterland of the Qinghai Tibet Plateau, are the main bodies of the Qinghai Tibet Plateau. Their movement is closely related to the paleoTethys ocean contraction and the opening and closing of the mid (?) Tethys ocean. The paleomagnetism can provide a quantitative basis for the movement of the research block. The Triassic paleomagnetic study of the Qiangtang block in the original hinterland and the paleomagnetic study of the Triassic in the Lhasa block were carried out and its tectonic significance was discussed. In the newly discovered Triassic marine carbonate rocks at the edge of the enemy cloth Lake in the central Cuoqin area of the Lhasa massif, 32 pieces were arranged and 327 independent directional samples were collected. The main magnetic bearing minerals are magnetite and (or) pyrrhotite. Most of the progressive demagnetization curves have obvious three component characteristics: low temperature component is low temperature viscous remanence of modern geomagnetic field; middle temperature component should be the secondary remanence component of Cretaceous; the high temperature component (component C) is only a few and magnetite is the main load. It is obtained in the samples of magnetic minerals. The component C of the early middle three fold can be tested by the 95% confidence reversal test (B grade) and 99% confidence fold; the late three fold component C can be tested by the 95% confidence fold. The corresponding paleomagnetic pole position: the early mid three world (19.3 N, 209.3 degree E, A95=4.0, N=8); and the late three (19.6 / 19.6, 211.8 E,) 495=10.7, N=6); the corresponding paleo weft is 16.9 + 4 degree S and 18.4+10.7 degree S. respectively. The paleomagnetic study of the Triassic strata of the northern Qiangtang block (249 samples) was systematically studied. The results showed that the main magnetic minerals in the samples were hematite and (or) magnetite. Most of the early three and late three world samples were retreated. The magnetic curve has obvious double component features. The lake group in the middle late three fold world map has not obtained a reliable high temperature component because of the influence of later re magnetization. This study revealed two groups of high temperature characteristic components. The high temperature components of the early three fold and late three fold world samples can be tested by the high confidence fold, and two ancient Qiangtang blocks were obtained. The position of geomagnetic pole: early three fold (23.8 N, 210.3 E, A95=10.9, N=5), late three fold (70.6 degree N, 275.6 E.A95=6.1, N=10), the corresponding palaeoglatities were -11.4 + 10.9 and 14.84 + 6.1 = N., respectively, combined with the previous PALEOMAGNETIC RESULTS: (1) the Lhasa block was stable in the middle and low latitudes of the south latitude from Late Palaeozoic to early Mesozoic. In the Cretaceous, it was possible to move rapidly north to the main collision of Eurasian subjects. (2) the late Paleozoic in the Qiangtang block hovered in the middle and low latitudes of the south latitude, and the early three fold and Lhasa block positions were similar. The Qiangtang block began to move on a large scale during the early period of the three fold to the late three, and was moved to the middle and low latitudes of the northern hemisphere at the end of the three fold. The region (14.8 SL 6.1 N) has been moving steadily North after the Late Triassic, but its speed and scale are less than the period of the Triassic. At least 26 degrees in the north direction of the Qiangtang, at least 26 degrees in the late three fold, at least 2880 km of the north, and the estimated speed of up to 6.4cm/a and with the counter clockwise rotation of 55 degrees. (3) the Lhasa block of the Triassic period and the period of the Triassic period. The Qiangtang block may not be adjacent to the northern margin of the Gondwana continent, but in an independent region between the Gondwana continent and the Eurasia continent. (4) the Tethys basin, represented by the Lhasa block and the Qiangtang block, is likely to be opened in the early three fold period and rapidly expanded during the early period of the three fold and the late three fold. The rapid expansion of the ban Gong Lake - the Nu River - Tethys ocean has promoted the rapid northward movement of the Qiangtang block, prompting its collision with the main Eurasian continent and closing the ancient Tethys ocean.
【学位授予单位】：西北大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P534.51;P318.44

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