板块剪式汇聚加地体拼贴——中特提斯消亡的新模式

Shear convergence of plates and suturing of terranes: a new model for the comsumption of the Meso-Tethys

关键词:  板块  剪式汇聚  地体拼贴  中特提斯消亡

Key words:  plate  shear convergence  suturing of terranes  consumption of the Meso-Tethys

摘要:

中特提斯是中生代中晚期存在于南、北大陆之间的海洋。该海洋在晚白垩世消亡后,遗留长千余公里的班公湖-怒江板块结合带。在大量研究成果中,对中特提斯如何消亡这一重大问题至今分歧甚大。不少研究者持洋壳俯冲消亡(东太平洋模式)观点,但在俯冲方向上却有向南或向北之别。笔者则认为中特提斯是一个具有众多互不相通、时代早晚不同的狭窄洋盆的特殊海洋,其消亡过程中根本未发生过大规模的洋壳俯冲,故提出剪式闭合加地体逐次拼贴的盆内聚敛消亡模式,即中特提斯的消亡由早侏罗世晚期羌塘-三江板块与冈底斯-念青唐古拉板块及其中的嘉玉桥-怒山地体,首先在东段无俯冲汇聚开始,中、西段此时基本上仍处于扩张状态,地体裂离与新的狭窄小洋盆产生同时进行,中特提斯呈东闭西张的剪刀状。中晚侏罗世,中西段北侧出现构造地体往羌塘-三江板块拼贴与南部地体裂离出新地体的复杂局面。至早白垩世末,新老地体拼贴、裂离过程结束,呈剪刃状的南、北两个板块逐渐拼合,中特提斯的中、西段主体亦相继消亡,仅余西南部部分海域。白垩纪末,由于新特提斯洋壳向北大规模俯冲的强力推动,冈底斯-念青唐拉板块和羌塘-三江板块完全汇聚并发生陆-陆碰撞,中特提斯在洋壳和过渡型地壳褶皱、冲断的盆内汇聚和碰撞造山中彻底消亡。这一新的消亡模式既符合班公湖-怒江板块结合带洋壳残片的年龄东老西新、海水由东向西退却、有多条时代不同的蛇绿岩带,不整合于蛇绿岩之上的地层东为中上侏罗统,西南为下白垩统和老第三系的实际资料,也符合中特提斯仅有互不相通狭窄小洋盆在“化整为零”的闭合过程中,难以发生大规模洋壳俯冲的动力学机制。还解决了该结合带两侧始终找不到沟、弧、盆体系的俯冲模式这一致命难题。

Abstract:

The Meso-Tethys was a sea lying between Laurasia and Gondwana during the Middle and Late Mesozoic. Following the termination of the sea during the Late Cretaceous, the Bangong-Nujiang suture zone with a length of more than 1000 km was left over. The argument about the extinction of the sea still remains current. Many researchers have insisted on the eastern Pacific model for a long time, i.e.they owed its extinction to the subduction of the oceanic crust. However, the authors have put forward in this paper a new model for the consumption of the Meso-Tethys due to the shear convergence of plates and suturing of terranes, and argued that the Meso-Tethys is an unusual sea in which the narrow basins are isolated and vary in ages. It can be deduced that no large-scale subduction of the oceanic crust occurred at that time. The consumption of the Meso-Tethys commenced during the late Early Jurassic, when the eastern part of the Gangdise-Nyainqentanglha plate began to be sutured together with the Qiangtang-Nujiang-Lancangjiang-Jinshajiang plate without subduction, while the middle and western parts of the Meso-Tethys were still open. Till the Middle and Late Jurassic, the rifting apart from the southern terranes and northward suturing of the tectonic terranes on the northern side of the middle and western parts of the Meso-Tethys onto the Qiangtang-Nujiang-Lancangjiang-Jinshajiang plate took place almost simultaneously. The entire processes came to an end, and the two plates cited above were gradually sutured together during the latest Early Cretaceous. At the same time, the bulk of the middle and western parts of the Meso-Tethys disappeared successively. And during the latest Cretaceous, the large-scale subduction of the Neo-Tethys forced the two plates to be further sutured together, resulting in the continent-continent collision. Finally the Meso-Tethys terminated in response to the intrabasinal convergence including folding and thrusting and collisional orogenesis of the oceanic and intermediate crust.