喜马拉雅造山带北缘大反转逆冲断层(GCT)东段的活动时限及构造演化

Active Timing and Tectonic Evolution of the Eastern Segment of the Great Counter Thrust in the Northern Margin of the Himalayan Orogen Belt

喜马拉雅造山带北缘的大反转逆冲断层（GCT）位于印度-亚洲的碰撞边界，平行雅鲁藏布江缝合带展布，向北逆冲使特提斯喜马拉雅地体向北叠覆在冈底斯地体之上。大反转逆冲断裂制约了中新世Kailas式系列磨拉石盆地（Kailas、秋乌、大竹曲、罗布莎以及朗县）的发育，自西向东磨拉石盆地中砾石变形逐渐增强。本文通过GCT东段“罗布莎-郎县砾岩”的研究，获得罗布莎-朗县变形砾岩中的碎屑锆石的沉积上限年龄为早渐新世（28．7～29．1Ma）。罗布莎-郎县砾岩具有基质糜棱岩化、发育地震化石假熔岩和向北剪切指向的特征，表明其经历了受GCT断裂控制的砾岩盆地在浅部形成、插入10-15km深部并受地震影响形成假熔岩，然后折返到地表的过程。

The Great Counter Thrust （GCT） in the northern margin of the Himalayan orogen belt is located in the boundary of India-Asia collision and is also parallel to the Yarlung Tsangpo suture zone （Yq2SZ）. Northward thrusting of the GCT resulted in the superimposition of the Tethys-Himalaya over the Gandese magmatic belt. The GCT constrains the development of the Miocene Kailas-type molass basin （including Kailas, Qiuwu, Dazhuqu, Luobusha and Langxian）, with molass deformation gradually increasing from west to east. This study carried out the geochrological analysis of the conglomerate from the east section of the GCT. The results show that sedimentary age of detrial zircon from the Luobusha-Langxian conglomerate is early Oligoeene （28. 7-29. 1 Ma）. The pseudotaehylyte, metamorphosed and mylonitic conglomerates are extremely well-developed in the Luobusha-Langxian formation. The Luobusha-Langxian conglomerate is characterized by matrix mylonitization, pseudotachylyte and northward shearing, suggesting that the molass basin controlled by the GCT faulting has experienced three stages of tectonic evolution, （1） formed at shallow depths, （2） underthrusted into depths about 10-15 km and with the pseudotachylyte formation, （3） exhumed into surface.