The Yingba shear zone on the Sino-Mongolian border: Southwestern extension of the Zuunbayan Fault from Mongolia to China and implications for Late Mesozoic intracontinental extension in Eastern Asia

摘要

Major strike-slip and normal faults are prominent features of Eastern Asia during the Meso-Cenozoic and they are critical to understanding the intracontinental deformation. Among others, the northeast-striking Zuunbayan Fault in southeastern Mongolia is usually documented as a strike-slip fault, but it may be a normal fault during the Late Mesozoic. However, its southwestern extension from Mongolia to China is still uncertain. The northeast-striking Yingba shear zone in western Inner Mongolia, China, just lies in the place where the Zuunbayan Fault probably strikes through southwestward. The gentle northwest-dipping foliation and lineation and top-to-the-northwest shear sense in the granitic mylonite indicate that the Yingba shear zone is a northwest-dipping, low-angle normal fault. The mylonitization occurred mainly at 400-500. °C, as a result of the Early Cretaceous intracontinental extension, as bounded by the zircon U-Pb ages of 145. Ma from two deformed (pre-tectonic) pegmatite veins and of 134. Ma from one undeformed (post-tectonic) pegmatite vein, all of which definitely crosscut the shear zone. The Yingba normal faulting was coeval with the Late Mesozoic extensional phase of the Zuunbayan Fault in southeastern Mongolia, which is characterized by northwest-dip and NW-SE extension from at least 155 to 126. Ma. Therefore, the Yingba shear zone is considered as the southwestern segment of the Zuunbayan Fault in China. The Late Mesozoic intracontinental extension, which was possibly triggered by the Jurassic thrusting in the South Gobi area of Mongolia, generated the Yingba shear zone and the Yagan metamorphic core complex on the Sino-Mongolian border.