西藏喜马拉雅地块显生宙古地磁特征及其构造地质意义

CHARACTERISTICS OF PALEOMAGNETISM AND ITS TECTONIC IMPLICATIONS FOR THE PHANEROZOIC IN THE HIMALAYAN BLOCKS,TIBET

通过对采自西藏南部喜马拉雅地块珠穆朗玛峰北坡地区奥陶系—古近系沉积地层中的2920件古地磁定向样品进行了系统测试研究,结果表明,喜马拉雅地块在显生宙时期向北漂移过程中曾发生过多次顺时针旋转运动。在奥陶纪—晚白垩世喜马拉雅地块在向北漂移过程中发生了大约20.0°的顺时针旋转运动。但在晚泥盆世末到早石炭世初曾发生了0.4°逆时针旋转,在石炭纪和早三叠世分别逆时针旋转了6.0°和8.0°,可能与晚泥盆世末到早石炭世初、石炭纪和早三叠世大陆裂谷的张裂作用有关。在晚三叠世-早侏罗世和晚侏罗世-早白垩世期间,分别发生了古纬度为2.0°和3.8°的向南回返的逆时针旋转,可能与雅鲁藏布新特提斯洋弧后扩张及洋盆开启有关。在始新世到上新世,喜马拉雅地块在相对快速北移的同时产生了约28.0°的顺时针旋转。根据古纬度数据推算,始新世以来的喜马拉雅陆-陆碰撞造山运动导致印度板块-喜马拉雅褶冲构造带-拉萨地块之间的地壳构造缩短量至少为1 000km。通过对珠穆朗玛峰地区显生宙古地磁的系统研究,为进一步深入研究冈瓦纳大陆的演化、古特提斯洋的形成、消亡历史和青藏高原隆升机制提供了科学依据。

2920 oriented paleomagnetic samples are collected from Ordovician-Paleogene sedimentary strata on the north slope of Qomolongma （Everest） in southern Tibet. As a result of thermal demagnetization and statistical analysis,primary components and new paleomagnetic data are obtained. Based on the new paleomagnetic data, the location of the magnetic pole and paleolati- tude data of the Himalayan block from Ordovician-Paleogene are calculated. As result, Ordovician- Paleogene paleomagnetic apparent polar wander path and paleolatitude curve of the Himalayan block are determined. It shows that the Himalayan block rotated several times during its north- ward drifting with a total of --200 clockwise rotations in the Phanerozoic. Also,anticlockwise ro- tations of 0.40,6~ and 8° are recorded during the last phase of upper Devonian to the early,which is likely to be related to the rifting of the Himalayan block. South-directed inversions of 2°and 3.8° in latitude are recorded during the late Triassic-early Jurassic and late Jurassic-early cretaceous,which is likely to be related to the back-arc spreading of the Yarlung Zangbo NeoTethys oceans. The Himalayan block is rotated about -280 clockwise from Eocene to Pliocene,which is likely to be related to the spreading of mid-oceanic ridge of India Ocean that causes the closure and orogenesis of Yarlung Zangbo NeoTethys ocean and to the collision of plates in the western and east tectonic syntax. Based on the estimation of the paleo-latitude data, there is a crustal shortening of at least 1 000 km between the Indian plate, Himalayan fold-thrust belt and Lhasa block,resulted from the Himalayan continent-continent collision orogeny since the Eocene. This study provides scientific evidence for the further study of the formation, evolution and elimination of Gondwanaland and paleoTethys as well as the uplifting mechanism of Qinghai-Tibet plateau.