喜马拉雅造山带造山模式探讨

A new model for the Himalayan orogeny

喜马拉雅是典型的碰撞型造山带,造山带结构构造复杂,可大致划分为以逆冲推覆构造为主的南喜马拉雅造山带和以各种伸展性构造为主的北喜马拉雅造山带;造山带内各类构造均发生过多期变形,且发生过多次缩短与伸展的构造反转;大喜马拉雅结晶杂岩系(GHC)内变形、岩浆及变质作用证明造山过程中存在渠道流作用。据此,本文提出一种由印度一欧亚大陆汇聚速率控制的多阶段造山模式:两大陆汇聚速度快时,青藏高原内形成南北向裂谷系(NSTR),喜马拉雅内经历造山过程,并在造山带中、下地壳形成作为底部拆离层的塑性层;汇聚速率慢时,青藏高原内形成共轭走滑断裂,喜马拉雅造山带内的塑性层发生松弛和重力扩散,形成渠道流,导致藏南拆离系(STDS)的启动、GHC的挤出和北喜马拉雅片麻岩穹窿(NHGD)的形成。上述的增厚与松弛均是在挤压体制下形成的,构造的反转是因挤压速率变化而产生的结构调节作用。

The Himalayas is a typical orogen resulted from continent-continent collision. This orogen has a complicated architecture which can be roughly divided into a southern Himalayan orogen characterized by thrusts and a northern Himalayan orogen featured by different kinds of extensional structures. All the major structures in Himalayas experienced multiple phases of deformation,and tectonic inversions between thickening and extension had happened to this orogenic belt. Deformation,magmatism and metamorphism in the Greater Himalayan Crystalline Complex（GHC） show that a channel flow did exist in the Himalayan orogen. Based on these facts,this paper proposes a multi-phase orogenic model controlled by the India-Asia convergent rate. When the convergence was fast, the north-south trending rifts （NSTRs）were formed in Tibet; meanwhile,the Himalayan region underwent a mountain building process during which a ductile section was produced as the basal detachment of the thrust system. When the rate of India-Asia convergence slowed down, the conjugate strike-slip faults with obtuse angles facing the maximum compression were formed in Tibet. At this time, the slow convergence could not sustain the thickened orogenic wedge built up by the fast convergence. As a result,the ductile section of the Himalayan orogen experienced a relaxation and gravity expansion, forming the channel flow which caused the activities of the South Tibetan Detachment System （ STDS）, the extrusion of the GHC and the formation of the North Himalayan Gneiss Domes（NHGDs）. Both the thickening and relaxation occurred in the same compressive regime, and the tectonic inversions were the structural adjustments according to the changes in convergent rate.