New paleomagnetic evidences of paleogene tectonic rotation of the Qaidam Basin and adjacent region: mechanism and its tectonic implication

摘要

In order to better understand the tectonic evolution of the Qaidam Basin. The authors carried out a paleomagnetic study on 39 samples of 7 sites from Hongsanhan section, northwest Qaidam Basin. Stepwise thermo-demagnetization isolated a high-temperature component (HTC), which passes the fold and reversal tests at 95% confidence level. It suggests that the HTC should be primary. Unblocking temperature (about 685℃) and the experiment of isothermal remanence indicate that hematite is the mainly carrier of the remanence. A tilt-corrected mean direction is: Ds=7.1°, Is=38.5°, α95=7.4°. Corresponding to a palaeopole at, φ＝250.1°E, λ＝72.0°N, A95=6.8°, Compared with the reference APWP of Eurasia, the sampling area occurred a non-significant rotation (3.4°±5.5°) relative to Eurasia since Eocene due to the affect of left-lateral Altyn fault, Based on the paleomagnetic results of early Cretaceous and Teritary within the Qaidam block, we can obtained the mean clockwise rotations from the samples deposited from 160 Ma to 45 Ma is 24.5°±9.0°, and from 38 Ma to present is -0.5°±7.5° relative to present geomagnetism respectively. During the Mid Eocene (45～38 Ma) rotation phase, there should existing an important early tectonic event in northern part of the present-day Tibetan Plateau, which probably represents one of large-scale strike-slip events of the Altyn strike-slip fault.