西藏聂荣变质杂岩及邻区逆冲推覆与构造隆升时代

通过详细的野外观测结合地质填图资料,在聂荣变质杂岩及邻区厘定大规模逆冲推覆构造,不同时代的逆冲岩席自北—北东向南—南西逆冲推覆于上白垩统红层及下伏岩石地层之上,形成大量逆冲断层、滑脱构造、飞来峰、构造窗和褶皱构造。逆冲推覆构造运动导致侏罗纪蛇绿混杂岩、石炭系—二叠系构造层、古生界浅变质岩、变质基底之间发生拆离滑脱,在聂荣变质杂岩内部形成韧性剪切带和高角度斜冲断层。在唐古拉山口南侧形成北西—南东向土门-托纠-杂色右旋走滑断裂,走滑断裂末端转换为近东西向逆冲推覆构造。聂荣变质杂岩顶部逆冲推覆叠加滑覆,导致侏罗系混杂堆积和古生界沉积盖层向南西—西向运移86~110km,在那曲-巴尔达-班戈北形成近东西向长透镜状懂错蛇绿混杂岩逆冲岩席,沿缓倾斜断层发育向西倾斜的构造片理、摩擦镜面和近东西走向的矿物线理。裂变径迹测年表明,聂荣变质杂岩及邻区逆冲推覆及构造隆升时代主要为早白垩世晚期—晚白垩世早期(111±5~91±5Ma)、晚白垩世晚期(89±6~69±5Ma)、古新世晚期—始新世早期(55±4~44±2Ma),估算构造隆升视速率为0.10~0.69mm/a,部分断层逆冲推覆及构造隆升延续至古近纪晚期。综合各类观测资料,建立不同时期构造模式,探讨聂荣变质杂岩及邻区逆冲推覆构造演化过程及形成机理。

Thrust structures and uplift periods of the Nyainrong metamorphic complex and its neighboring areas

Regional thrust systems consisting of thrust faults, tectonic slices and sheets, nappes, structural windows and accompanied folds were discovered in the Nyainrong metamorphic complex and its neighboring areas on the basis of field studies and geological mapping. Tectonic slices and thrust sheets surrounding the Nyainrong metamorphic complex were thrust southwest-westward over the Upper Cretaceous red beds and the Jurassic-Lower Cretaceous marine sedimentary rocks. Regional intensive thrust led to detach-ments among the ophiolite and Jurasssic mélanges, the Carboniferous-Permian clastic-carbonate, the Paleozoic low-grade metamor-phic rocks and the high-grade metamorphic basement, accompanied by ductile shear zones and oblique thrust faults in the Nyainrong metamorphic complex. Dextral-slip fault trending in northwest-southeast direction along the Tumen-Tuojiu-Zase zone trans-formed to regional thrust systems trending in east-west direction in southern Tanggula Range. Regional thrust and gravitational slide resulted in 86~120km southwest-westward movement of the Jurassic mélange and the Paleozoic covering layers, forming the Dong Co tectonic sheets of ophiolite mélanges along the Naqu-Baerda-North Baingoin in the west-southwest of the Nyainrong meta-morphic complex. Tectonic schist, slikensides and linear minerals trending in east-west direction were formed along the detachment faults beneath the Dong Co thrust slices of ophiolite mélanges. Fission-track dating indicates that thrust uplift mainly occurred at the late stage of the Early Cretaceous-the early stage of the Late Cretaceous (111±5~91±5Ma), late stage of the Late Cretaceous (89±6~69±5Ma) and Late Paleocene-Early Eocene (55±4~44±2Ma), and the corresponding rates of thrust uplift range from 0.10mm/a to 0.69mm/a in the Nyainrong metamorphic complex and its neighboring areas. Thrust uplift also occurred in Late Eocene and Oligo-cene in some areas. The process and mechanism of regional thrust in the Nyainrong metamorphic complex and its neighboring areas are discussed on the basis of the data available.