用陆块旋转解释藏东南渐新世—中新世伸展作用——来自点苍山及邻区变质核杂岩的证据

Rotation of crustal blocks as an explanation of Oligo-Miocene extension in Southeastern Tibet—evidenced by the Diancangshan and nearby metamorphic core complexes

沿红河断裂带(RRFZ)分布的点苍山变质核杂岩是一个不完整的变质核杂岩,它由两个特征迥异的单元组成,包括被同构造二长花岗岩侵入角闪岩相构造岩组成的下盘和绿片岩相的拆离断层带。下盘岩石包括具有高温构造组合,具有指示左行走滑剪切运动方向的L型糜棱岩或LS型糜棱岩。拆离断层带是一个上盘向E到SE伸展剪切的低温剪切带,由具有剪应变和压应变的典型S-L糜棱岩构成。低温构造岩也包括发育于下盘的几个糜棱岩化似斑状二长花岗岩侵入体。变质核杂岩与西侧覆盖未变质的中生代沉积岩并置,东部受第四纪断层作用影响为沿洱海分布的更新世—全新世沉积盆地。通过对点苍山变质核杂岩的构造研究,结合邻区变质核杂岩的地质年代学及古地磁学分析,我们认为:位于东南亚红河断裂和实皆断裂带之间的扇形区域内出现的变质核杂岩与渐新世—中新世时期区域性伸展作用有关,而伸展作用是由印支地块的差异性旋转产生的,其原因是由于约33Ma开始斜向俯冲的印度板块的顺时针旋转和回退所致。

The Diancangshan （DCS） metamorphic core complex （mcc） along the Red River Fault Zone （RRFZ） is an incomplete race composed of two units with distinct characteristics, i.e. a lower plate of amphibolite grade teetonites intruded by synkinematie monzogranite, and a greensehist grade detachment fault zone. Lower plate rocks possess high temperature （high-T） structures and L or L〉〉S mylonites with left-lateral strike-slip shear sense. The detachment fault zone is a low temperature （low-T） shear zone with top to the E to SE extensional shearing and has typical S-L mylonites with both shearing and flattening strains. Low-T teetonites also include several mylonitized porphyritic monzogranite intrusions in the lower plate. The race is juxtaposed by non-metamorphosed Mesozoic sedimentary rocks to the west, and cut by Quaternary faults in the east, presenting a Pleistocene to Holoeene sedimentary basin along Er Hal Lake. Through structural analysis of the DCS race, together with geochronological and paleornagnetie analyses of race＇s in contiguous areas, we would argue that the occurrence of mee＇s in a fan-shaped area between the Red River fault zone and Sagaing fault zone in SE Asia is related to regional extension during Oligo-Mioeene period. The extension is attributed to differential rotation in the Indoehina block, caused by clockwise rotation and rollback of the obliquely subdueting Indian plate since about 33 Ma.