东喜马拉雅构造结陆内变形过程的研究

Intracontinental Deformed Processes of the Eastern Himalayan Syntaxis

在东喜马拉雅构造结识别出“南向挤出”与“北向楔入”两大构造。前者包括倾向北、头朝南的一条正断层和两条分别被称为上部和下部的逆冲断层。南迦巴瓦和派乡杂岩借助这些断层从北部的冈底斯构造单元之下向南折返至地表附近。后者包括一个北东走向的推覆体和两个北北东走向、南倾的低角度推覆体,将东久、南迦巴、派乡等杂岩又北(东)向楔入冈底斯构造单元之中。独居石TIMSU-Th-Pb测年表明“南向挤出”构造中的正断层和下部逆冲断层可能分别在7.9Ma、10.7Ma时活动。锆石SHRIMP与金云母K-Ar测年结果限定上部逆冲断层活动时限介于6.2±0.2Ma和5.5±0.2Ma之间,即“南向挤出"中的上、下两条逆冲断层为逆序的逆冲断层。锆石SHRIMP和角闪石Ar-Ar测年还表明东喜马拉雅构造结核心部位的“北向楔入”很可能发生于3.0Ma和1.5Ma之间。

Structures of ＂ Southward Extrusion＂ and ＂ Northward Indentation＂ have been recently distinguished within the eastern Himalayan syntaxis. The former includes a north-dipping normal fault and two top-to-south thrust zones, named as Upper and Lower Thrust here. The Namche Barwa sequence （high-pressure granulite facies） and the Pei sequence （amphibolite facies） were southwards exhumed from beneath the Gangdise unit by these faults. The latter includes, from west to east, an NE-trending northwest-dipping and two NNE-trending south-dipping nappe sheets. The exposed Dongjiu, Namche Barwa, Pei and Laiguo sequences in the south were displaced to indent into the Gangdise unit in the north by the North Indentation structures. Monazite TIMS U- Th-Pb dating has revealed that the normal fault and the Lower Thrust from the Southward Extrusion structures were probably active at 7.9 Ma and 10.7 Ma, respectively. The Upper Thrust was active between 6.2±0.2 Ma and 5. 5 ±0.2 Ma, suggested by zircon SHRIMP and phlogopite K-Ar dating. This clearly suggests that the thrusts of the Southward Extrusion structures are out-of-sequence structures. The Northward Indentation structures within the core portion of the eastern Himalayan syntaxis were active between 3.0 Ma and 1.5 Ma, also suggested by zircon SHRIMP and amphibole Ar-Ar dating.