Deformation Sequence And Regional Tectonic Evolution In The North Margin Of South Qinling Daba Mountain

The Daba Mountain structural belt is a part of south Qinling Mountains which has been regarded as a foreland thrusting belt of the collisional belt of the north and south China plates.The Ankang structural trust is in the north boundary of the Daba Mountain structural belt which is an important NW-NWN regional fault in the south Qinling trust nappes.It has the smilar trend with the whole Qinling orogenic belt.Our research choosed the closed part of Ankang fault envelope surface where nearby Guihua village to do detailed field tectonic deformation characteristics investigation.Based on the macro-and micro-structural observations,our research obtained the main structual deformation characteristic of the north Daba Mountain structural belt that regional tendency north nappes,penetrative superimposed foliation and ductile shear zone.Combined with the regional study of the earler formed fornix tectnoic that as the core of volcannic rocks uplifted,which is nearby the Pingli and Phoenix Mountains,we focus on the extensional tectonics and thrusting nappe formation process of the north boundary of the Daba Mountain.The results of the researches show that the north of Daba Mountain curvilinear structural belt nearby the Ankang thrust fault has undergone strong superimposed deformation during the Early Paleozoic.Inner rift basin of north Daba Mountain structural belt,developed nearly East-West extensional tectnoics mechanim formation such as the bedding shear folds(F1),penetrative foliation S0//S1 and ductile shear structure,especially development in the Cambrian-Ordovician rocks.The Pingli and Phonixshan vault structure which has the core of Upper Proterozoic volcanic rocks that causing by the stretching mechanism.In the inner rift basin of south Ankang trust the nappes formation mainly observed in the Silurian Meiziya Formation which was combained with block and stepped folds in the foreland thrust-fold belts and strong squeeze contraction under the system of longitudinal structural replacement fomationed phylite.The longitudinal structurals which were sandwiched between a number of sub-thrust faults is the deformation in the main deformation period of North Daba Mountain structural belt.Our reserch did experiment to research the formation phylite quartz C-axis fabric about qualitative analysis by EBSD technique and calculated the kinematic vorticity.Revealed the existence of at least two stages of ductile deformation superimposed,the quantitative research methods constructed in phases to provide evidence.The 40Ar/39Ar dating of syntec-tonic microthermal metamorphic muscovite showed that Ankang fault had an intense deformation during Indosinian period(about 230 Ma).Combined with previous chronology and the tectonic evolution model simulation of the Daba Mountain thrust belt,we inferred that 230Ma maybe representatives of the start period of the south extension of nappes structure.Based on the aboved-mentioned study and the previous research on the Daba Mountain thrust belt,our research explored the evolutionary history of the South Qinling and provided some new basis for the regional tectonic history of the Qinling orogenic belt.