走滑断裂对超高压变质岩石折返的贡献及青藏高原北部白垩纪隆升之新思考

青藏高原已识别出柴北缘、南阿尔金和高喜马拉雅三条超高压变质带。这些超高压变质带提供了一个不可多得的研究超高压变质岩石形成和折返的机会。柴北缘超高压变质带位于阿尔金断裂的东边,是柴达木—东昆仑地体与祁连—阿尔金微地体和阿拉善—敦煌地体碰撞的产物,由榴辉岩、石榴石橄榄岩和含柯石英片麻岩组成,榴辉岩形成时代500~440Ma,峰期超高压变质年龄440Ma。南阿尔金超高压变质带位于阿尔金断裂带的西边,以产出榴辉岩和石榴石橄榄岩为特征,榴辉岩形成时代为500Ma。南阿尔金超高压变质带被认为是柴北缘超高压变质带的西延,两者被阿尔金断裂左旋位移约400km。阿尔金断裂是巨大的深度>200km的岩石圈走滑断裂,断裂的活动时代至少早到240~220Ma,认为走滑过程中伴随的隆升作用有可能为柴北缘和南阿尔金超高压变质岩石的折返和出露地表做出了贡献,其中阿尔金断裂起到了类似剪刀型断裂的作用。高喜马拉雅超高压变质带在巴基斯坦和印度被发现,以榴辉岩中含柯石英或金刚石为特征,榴辉岩的超高压变质年龄为46Ma,表明超高压变质岩石发生在雅鲁藏布江缝合线关闭后并快速折返。喀喇昆仑断裂走滑过程中伴随的抬升作用则可能对高喜马拉雅地区超高压变质岩石的折返和出露地表做出贡献。在中国东部出露的大别—苏鲁超高压变质带被巨大郯庐断裂左旋走滑位移约500km,可以看作是走滑作用伴随的抬升运动对超高压变质岩石的最后折返和出露地表做出重要贡献的又一例证。青藏高原的隆升通常被认为是印度板块和欧亚大陆新生代以来的碰撞结果。根据高原北部断裂的时代、火山活动和沉积盆地的形成,我们提出高原的隆升是两次俯冲碰撞的结果。第一次发生在中特提斯班公湖-怒江洋盆在白垩纪时期的关闭,其时由于北部来自塔里木盆地和北中国板块及东部来自太平洋板块俯冲产生的抵柱效应,高原北部开始隆升;第二次发生在印度板块的新生代俯冲碰撞作用,造成高原的整体抬升,由此可以解释高原北部平均海拔(5000m)要高于高原南部(平均海拔4000m)。

Contributions of strike-slip faulting to exhumation of ultrahigh pressure metamorphic rocks and the Cretaceous uplift of the northern Qinghai-Tibet plateau

Three ultrahigh pressure metamorphic (UHPM) belts, i.e., the north Qaidam, the south Altun, and the high Himalaya UHPM belts, have been discovered in the Qinghai-Tibet plateau, which offer a unique opportunity to investigate the exhumation of deeply subducted UHPM rocks. The north Qaidam UHPM belt is located on the southeastern side of the Altyn Tagh strike slip fault and was formed by the collision between the Qaidam-East Kunlun terrane on the south and the Qilian-Altun micro-terrane and the Alax-Dunhuang terrane on the north. This belt is about 350 km long and consists of eclogite, garnet peridotite and coesite-bearing gneissic rocks. The eclogites formed at between 500 and 440 Ma with a peak UHPM age of 440 Ma. The south Altun UHPM belt is located on the northwestern side of the Altyn Tagh strike slip fault, and contains eclogites and garnet peridotites of ca. 500 Ma. The south Altun belt is believed to be the western extension of the north Qaidam UHPM belt, but now offset for about 400 km by the Altyn Tagh fault. The Altyn Tagh fault is a major sinistral strike slip fault, which extends to depths of greater than 200 km (Wittinger et al., 1998) and has been active at least since 240-220 Ma (Li Haibing et al., 2001). The Altyn Tagh fault has been recognized as a transpressional fault, and responsible for the uplift of the Qilian mountains and the Altun mountains that border it. We propose that the Altyn Tagh fault is probably responsible for the final exhumation of the UHPM rocks in both north Qaidam and south Altun, a geometric expression perhaps being that the Altyn Tagh fault had a scissor-like kinematic history.The high Himalaya UHPM belt is distributed along the Pakistan and Indian Himalayas, where coesite and diamond have been discovered in eclogites (O'Brien et al., 2001; Sachan et al., 2001;Mukherjee et. al., 2005). Coesite-bearing zircons in the eclogite gave a 46 Ma peak UHPM age (Kaneko et al., 2003), suggesting that the UHPM rocks in the high Himalaya formed soon after the closure of the Neo-Tethys ocean and were rapidly exhumed thereafter. The Karakorum dextral strike-slip fault is located NE of the UHPM rocks. Its faulting and associated deformation occurred between 6.88-8.75 Ma, with an offset of about 135 km (Zhou Yong et al., 2001). We propose that this fault may have controlled the exhumation and exposure of the UHPM rocks in the high Himalayas. The Tanlu sinistral strike-slip fault in eastern China, which offsets the Dabie and Sulu UHPM belts for about 500 km, may provide another good example for the exhumation of UHPM rock along the two sides of the fault. Based on these three examples, we propose that strike-slip faulting may have played an important role in the exhumation of UHPM rocks.The uplift of the Qinghai-Tibet plateau has been attributed to the Cenozoic collision between the India plate and the Eurasia continent. Based on field relationships concerning plateau uplift, volcanic and faulting activities, and basin sedimentation records, we propose that the formation of the plateau occurred in response to two episodes of plate subduction and collision. The first occurred in the Cretaceous due to the closure of the Mid-Tethys Bangong-Nujiang ocean by plate subduction. The northern plateau began to be uplifted at that time because of resistance from the north of the Tarim and North China blocks, and also from the east due to the westward subduction of the Pacific plate under the South China block. The second episode occurred in the Cenozoic as a result of the closure of the Neo-Tethys Yarlung Zangbo ocean, and was responsible for the uplift of the entire plateau. These two episodes may explain why the northern plateau has a higher average elevation (5 000 m) than the southern plateau (4 000 m in average).