欧亚大陆东部白垩纪两期伸展穹隆构造及其动力学机制探讨

Cretaceous two stage extensional tectonic in eastern Eurasia continent and its geodynamics

以变质核杂岩或伸展穹隆为代表的晚中生代伸展构造在欧亚大陆东部广泛发育。与北美西部的科迪勒拉型变质核杂岩既相似,又存在很大的不同。区域上可以进一步划分为次一级的伸展构造带,由北向南依次为泛贝加尔-蒙古-鄂霍茨克带、华北西部带、华北东部带、华北南缘及秦岭-大别带及华南中部带。与北美地区显著不同的是它们并非平行于俯冲带展布,而是呈面状分布于太平洋西部广大的地区,不仅发育在岩石圈薄弱带或造山带相关的构造单元之上,而且还发育在稳定的“克拉通”之上。这些穹隆构造记录了NW-SE向的区域伸展方向,构成了全球最大的伸展构造发育区。通过对各带伸展穹隆的结构样式、时空分布和发育过程的系统分析、归纳和总结,我们将这些伸展穹隆分为早晚两期。两期伸展构造所具有的不同特点决定了他们的动力学机制的不同。早期伸展构造发生在早白垩世早期,其具有“对称性”、“等时性”和“等深性”的特点,决定了其动力学机制以“沉坠”作用(foundering)为主导,是对华北克拉通破坏的峰期响应。晚期伸展构造形成时间为早白垩世晚期-晚白垩世早期,时空分布上具有向S或SW迁移的规律,或指示了古太平洋板块的俯冲回撤(roll-back)过程对欧亚大陆板块的渐次影响。

The Cretaceous tectonics of the North China Craton(NCC)and its adjacent regions were characterized by a general lithospheric extension and remarked by extensional dome structures.This makes the largest crustal scale extensional tectonic realm in the world.Based on our field and laboratory work,five extensional domains have been delineated,namely Transbaikalia-Mongol-Okhotsk,western part of NCC,eastern part of NCC and Korea,Qinling-Dabie and its neighbouring,and the interior of South China Block(SCB),respectively.These domains are featured by a NW-SE extensional direction with intensive extensional exhumation of middle to lower crust rocks to the surface along detachment faults.Two stage extension tectonics have been suggested,which took place at ca.130~120Ma and ca.118~85Ma.The early stage constrains a narrow activity period,and the foundering of the lower part of the lithosphere could be a possible mechanism for this continent-scale extensional tectonics.This geodynamic model could help us to enhance the knowledge of the time,scale,and mechanism of the NCC destruction from the view of structural analysis.The timing of late-stage extensional tectonics is suggested by a younging trend from the northeast(Liaodong Peninsula)to the southwest(Yuechengling massif in SW Hunan Province).The slab roll-back of westward subducted‘Palaeo-Pacific’or Izanagi plate may lead to this regional extension event.