Thermochronology of three major faults in the Chifeng area, Inner Mongolia of China

On the basis of investigation of the macroscopic and microscopic characteristics of deformation and kinematics of the Heilihe-Songsanjia and the Xilamulun near E-W-trending faults and the Xiaochengzi-Balihan NNE-trending fault in the Chifeng area, Inner Mongolia, this paper performed biotite, K-feldspar, plagioclase, and whole rock internal Rb-Sr isochron dating of mylonitic rocks, and single-grain laser fusion 40Ar/39Ar dating of biotite from these faults. Deformational features of the two E-W-trending faults have shown dextral shearing, whereas the Xiaochengzi-Balihan fault is characterized by sinistral shear on the horizontal plan and normal slip in the cross-section. According to the difference of feldspar porphyroclast and ductile matrix in grain size, the mylonitic rock samples collected from the three faults were broken to successively finer grains from which K-feldspar, plagioclase, and biotite were separated after removal of feldspar porphyroclasts. Mineral internal Rb-Sr isochron dating yielded reliable results. It shows that mylonitization has caused complete isotope resetting of the ductile matrix from the fault mylonitic rocks. Age of deformation along the Heilihe-Songsanjia fault is dated at 232 Ma, whereas the shearing and sliding of the Xilamulun fault is dated at 165 Ma. The collisional suturing of the Mongolian Paleozoic magmatic are with the North China block in the early Mesozoic, and far field effect of the collisional suturing of the combined North China-Mongol block with the Siberia plate from Late Jurassic to Early Cretaceous are responsible for the above two stages of deformation, respectively. Ages of the sinistral shear and normal slip of the Xiaochengzi-Balihan fault are dated at about 125 Ma and 117 Ma. Both ages are concurrent with the rapid uplift of the Harkin metamorphic core complex. We concluded that the mantle-crust upwelling prompt ductility in the middle-low crust and detachment of cover rocks from the basement. These conditions eventually resulted in the sinistral shear and normal slip along the NNE-trending faults.