Tectonic Event Of Altyn Tagh At 9.8-9.1 Ma: Evidence From Sedimentary Records Of Honggouzi Section In The Western Qaidam Basin

Sedimentary basins and orogenic belts are the two basic continental structural units, which constituted the interrelated structural system in spatial development and formation mechanism. Sedimentary basins, being the most direct and specific records of the orogenic belts evolution, provided the information and clues to study the development and evolution of the orogenic belts. The sediments were the weathering and erosion orogenic rocks, which were transported by the stream, deposited in the basins and well recorded the history of tectonic movement of the orogenic belts. Qaidam basin is the inland basin under the S-N compressive stress of India-Eurasia plate collision, and bordered by three large faults, the Kunlun Fault to the south, the Altyn Tagh Fault to the west and the Qilian Fault to the north. We chose the Honggouzi section in Western Qaidam basin, which established high-precision paleomagnetic chronology from 16.5 Ma to 4.8 Ma. Based on the sedimentary characteristics, sedimentary facies, paleocurrent, sediment source, magnetic susceptibility, rock magnetism and paleomagnetism tectonic rotation, combination of basin-mountain coupling theory, we discussed the late Cenozoic major tectonic events of the Altyn Tagh in the northern Tibet Plateau, and gained the following views:1. Sedimentary facies of Honggouzi region experienced fan delta in Xia Youshashan Fm (>16.5 Ma), fan delta-shallow lake in Shang Youshashan Fm (16.5-9.8 Ma), sedimentation interruption in 9.8-9.1 Ma, fan delta-shallow lake in Shizigou Fm (9.1-4.8 Ma) and piedmont alluvial fan in Qigequan Fm (<4.8 Ma).2. The direction of flow which was indicated by the direction of the largest axis of the Magnetic susceptibility ellipsoid (angle), A-B surface of gravel, gravel composition, sedimentary construction, and so on, suggested that paleocurrent of Honggouzi area in Western Qaidam Basin mainly came from the NE during 16.5-4.8 Ma, and the source mainly was Altyn Tagh.3. The Magnetic properties of sediments in Honggouzi section showed a phased change, the average magnetic susceptibility values were lower (1.04×10-8m3kg-1) and did not fluctuate severely before 9.8 Ma. After 9.1 Ma, the average magnetic susceptibility values increased 2 times nearly (up to 1.96×10-8m3kg-1) and fluctuated increasingly. The main magnetic minerals in sediments was hematite in 9.8Ma ago, then after 9.1 Ma the hematite content of the sediments drastically reduced and the dominant magnetic mineral was magnetite.4. The Fisher average on the ChRM of the samples in Shang Youshashan Fm and Shizigou Fm revealed that Shizigou Fm clockwise rotated 7.3°relative to Shang Youshashan Fm, and its paleolatitude northward migrated 0.5°.5. Researching on the relationship between deposition and tectonic and the tectonic rotation, indicated that the Altyn Tagh had a major tectonic uplift event in 9.8-9.1 Ma in northern Tibetan Plateau.