Plate Tectonics And Geodynamics Of Orogenesis Of The Southwest Tianshan Mountains

Orogenic belt of the Southwest Tianshan Moutains is a Paleozoic composite suture, located between Yili-Middle Tianshan plate and Tarim Plate. It's divided into four tectonic units,Yili-Middle Tianshan plate, late Silurian accretionary Wedge in southern margin of middle Tianshan, late paleozoic initial oceanic basin and Tarim plate. Yili-Middle Tianshan plate and Tarim plate are continental plates, with precambrian crystalline basements. Late Silurian accretionary wedge is a″metamorphic subduction complex″belt which consists of early paleozoic ophiolitic melange belt, blueschist belt and greenschist belt. The late paleozoic initial oceanic basin is a small oceanic basin growing out of the inactive margin of Tarim plate. The evolution processes of the lithospheric tectonic plates of Southwest Tianshan Mountains are summarized into:″proterozoic ocean of South Tianshan″stage (pre-Z₁)→″sedimentation of stable caprocks″stage (Z₂-∈₂)→″active continental margin″stage (∈₃-O₃)″→″early paleozoic back-arc oceanic basin″stage (S)→″late paleozoic initial oceanic basin″stage (D)→″tectonic evolution of inner plate after collision″stage (after D).Deformation characteristics and metamorphic PTDt path of blueschists have predieted the uplift mechanism of blueschists and dynamic processes of orogenesis in southern margin of middle Tianshan. In the early time of orogenesis, plate subduction made the subduction complex suffer prograde metamorphism with pressure accreting rapidly , from laumontite facies (424Ma?) to lawsonite-glaucophane schist facies (420Ma?). At the same time, the subduction complex hah suffered ductile compression deformation under lower tectonic level. After cessatin of subduction, blueschists suffered isothermal decompression retrograde metamorphism at the initial time of uplifting, from glaucophane -greenschist facies (415Ma) to greenschist facies (350Ma), superimposed by ductile flowing deformation. In the late time of orogenesis, large scale ductile nappe made blueschists uplift rapidly, the crust of orogenic belt shorten in large scale. Following, ductile strike—slipping happened along the front areas of nappe belt. In the latest time of orogenesis, broad folds, regoinal overturned folds and friable thrusts of upper tectonic level made the crust of orogenic belt shorten in large scale, the blueschists expose in the orogenic belt.