Formation And Evolution Of The Kalamaili Fault Zone

The large-scale NWW-striking Kalamaili fault zone in the eastern Junggar area is situated along the collision boundary between the Yemaquan and Harlik arcs, and along the southern margin of the Kalamaili ophiolite zone. On the basis of detailed field investigation and structural analysis, it is demonstrated that the Kalamaili fault zone was subjected to four phases of activities. The first phase activity showed brittle-ductile dextral shearing in the middle Late Carboniferous. The more easily deformed rocks in the brittle-ductile shear zone, such as slate, sandy slate, and low-grade tuff, show densely foliation and tensile veins, and marls was deformed into mylonites. The second phase activity showed top-to-the-south thrusting in the late Middle Permian and the end of Triassic. The thrusts formed along previous foliation of the earlier shear zone, or truncated the earlier shear zone. The third phase activity showed brittle sinistral slipping in the Late Jurassic, and superimposed on the earlier brittle-ductile shear zone and brittle thrusts. The latest phase activity of the Kalamaili fault zone showed brittle thrusting with dextral component at the beginning of Paleogene.A variety of geological phenomena show that deformation temperatures of the Kalamaili brittle-ductile shear zone is 250-300℃, and the shear zone development, regional low-grade metamorphism and tight folds of pre-Permian volcanic-sedimentary rocks in this area occur at the same period. This phase of deformation and metamorphism as well as regional marine regression occured at the same period. These lines of geological evidence show that the Kalamaili brittle-ductile shear zone resulted from the oblique collision between the Yemaquan and Harlik arcs, which suggests that the Kalamaili oceanic plate subducted northwards. The Kalamaili brittle thrusts in the late Middle Permian resulted from intracontinental deformation happening after the post-collisional extension in this area. This phase of deformation should be related to the Middle Permian collision within the Central Asian Orogenic Belt.As the first order boundary fault on the northeastern margin of the Junggar basin, the Kalamaili fault zone controled and affected formation and evolution of the Junggar basin. The Junggar basin formed under the post-collisional extensional setting in the Early Permian. The fault zone, togather with large-scale thrusts in the Kalamaili mountain frontal area on the northeastern margin of the Junggar basin, operated as a foreland thrust belt in the Middle Permian. The basin during the thrusting period belongs to a foreland-type basin. Subsequently, tectonics activity weakened in this area and the Junggar basin entered a stable sedimentary depression stage in Triassic. The Kalamaili fault zone was involved in thrusting again at the end of Triassic, and these two phases of thrusting resulted in a series of NWW-trending, secondary uplifts and sags in the Luliang uplift. The Junggar basin entered a depression stage again in the Early- Middle Jurassic. The Kalamaili fault zone experienced sinistral slipping under the E-W compression in Late Jurassic. The shortening event also produced a series of N-S thrusts and folds in the eastern Junggar basin, leading to formation of several N-S secondary uplifts and sags. The basin turned into a stable sedimentary depression stage in Cretaceous. At the beginning of Paleogene, the Kalamaili fault zone was involved in thrusting under the N-S compression. This compression also resulted in a series of E-W secondary uplifts and sags in the eastern Junggar basin, which superimposed on the earlier N-S uplifts and sags, causing the "grid" basin pattern at last.