Structural Deformation And Tectonic Evolution Of Harlik Mountain, In Xinjiang Since The Paleozoic

Being one major content of orogenic evolution, study of deformation history is an effective way to reveal the kinematics and/then discuss the dynamic process of the orogen. In the view of the plate-evolution, orogen is commonly divided into two types: subduction and collision ones. Being the northeastern segment of the Tianshan orogen, the Harlik range is located at the northeastern margin of the Tu-Ha basin, east Xinjiang, NW China. In the view of tectonics, the Harlik Mountain is situated at the junction area of the Junggar orogenic system and Tienshan orogenic system, which had been belong to the overriding plate of a subduction zone during the entire Paleozoic period. Thus, the Harlik range provides an excellent natural laboratory for studying the deformation features and crust growth of a subduction-type orogen.During working in this area for my master degree, I found that the deformation -metamorphism age of the Xiaopu metamorphic rocks is ca. 290 Ma, exactly bracketing within the age scope of 320 to 280 Ma, which was though as the post-collision stage in the north Xinjiang (north of the Tarim basin). However, my structural observations did not provide kinematic pattern to support such a tectonic model. Obviously, further kinematic analyses are required to solve this contradiction.Here I reported new structural data to answer above question, including field section observations and geologic map with large scale in a key area, accompanying with detail microstructural observations, quartz C-axial fabric analyses, and tectono-thermogeochronology (40Ar/39Ar and zircon SHRIMP U-Pb method). Taking available previous data into account, following progression is achieved:1. Completely rebuilding the post late-Paleozoic deformation history of the Harlik MountainFive stages of deformation were identified: (1)pre-Permian compression deformation which may have resulted from pre-Permian subduction; (2) earlier Permian post-collision extension; (3) middle Permian west-east directed dextral slip shearing; (4) later Permian nearly N-S compression deformation; and (5) later Cenozoic intra-plate N-S compressional deformation. Of the five stages deformation, the exact ages of the (2) and (3) were determined by 40Ar/39Ar method as 290 and 259 Ma respectively.2. New results on the metamorphic belts and associated metamorphism of the Xiaopu metamorphic rocksBased on detailed microtextural observations and occurrence of typical metamorphic minerals, five metamorphic belts in the Xiaopu metamorphic rocks are identified. From the north to the south, they are biotite belt, garnet belt, staurolite belt, andalusite belt, and sillimanite belt. I suggested that the metamorphic belts occurred in the Xiaopu area belongs to the high temperature/low pressure (HT/LP) series, may have resulted from the later Permian crust extension at ca. 290 Ma.3. I suggest that there may have been a Carboniferous backarc basin along the southern slope of the Harlik Mountain.Detrital Zircon SHRIMP U-Pb dating indicated that the sedimentary age of the Julideneng formation lithic sandstone must be later than late Devonian, most likely is Carboniferous rather than previously thought Precambrian. Analyses on its geologic setting, location of provenance, and sedimentary environment suggest the Carboniferous strata may form in a lagged backarc basin bounded by backarc uplifts.5. Reconstruction of the post-Paleozoic tectonic evolution history of the Harlik MountainThe tectonic evolution of Harlik mountains since the Paleozoic were reconstructed as followings: (1) arc-affinity magmative activities occurred during the middle-Ordovician to early-Silurian; (2) the middle-Silurian to late-Devonian was a back-arc uplift stage; (3) back-arc extension featured the Carboniferous period; (4) post-collisional extension and/then compression during Permian; (5) at the Mesozoic, differential uplift developed; and (6) the late Cenozoic is the intra-continental re-orogen stage.6. Primary estimation of the crust shortening is madeThe folded pre-Mesozoic strata were used to estimate crust shortening along the Harliknanshankou-Koumenzi, Shichengzi-Baishitou, and Qincheng-Xiaopu sections. Their shortening ratios are 16.2%, 19.8%, and 20.1% respectively corresponding crust shortening are 4.3, 9, and 11.3 km. These results suggest a clear eastward increasing in the late Paleozoic crust shortening of the Harlik Mountain.7. Primary summary of the crust growth feature of the subduction-type orogenBeing a subduction-type orogenic belt, the Harlik mountains experienced several stages of tectonic evolution, including back-arc spreading during contraction of oceanic basin, continent-continent or arc-continent collision after oceanic closure, post-collision extension and compression, and intra-plate compression. No obvious difference exists between the later two stages tectonic evolution and that of a collisional orogenic belt...