PRELIMINARY PALEOMAGNETIC RESULTS FROM CRETACEOUS TO TERTIARY RED BEDS OF NORTHEASTERN TIBET

The tectonic development of the Tibetan Plateau since ~50 Ma has a wide range of interpretations. The inability to decipher between contrasting models is largely due to lack of kinematic constraints. Whereas much is learned about present crustal motion through GPS and active tectonics studies, paleomagnetic data from Cretaceous to Tertiary red beds can constrain Vertical-Axis (V-A) rotation of crustal blocks in the past. This study focuses on eastern Tibet where paleomagnetic sampling was carried out at 16 localities (134 sites) between Xining (N36.4°; E102.1°) and Dangchang (N34.1°; E104.5°). At Xining and Dangchang we also sampled for magnetostratigraphy to constrain the age of deposition and related deformation. Stepwise thermal demagnetization of pilot samples at these two localities yielded the following preliminary results. At Xining, demagnetization of samples from the Hekou group (Cretaceous) yielded a Characteristic Remanent Magnetization (ChRM) carried mainly by magnetite. All ChRM directions determined from 29 samples are of normal polarity. These directions pass fold and conglomerate tests indicating primary magnetization. The mean ChRM direction (I=51.0°; D=18.8°; a95=5.5°) is concordant with the expected direction derived from the Eurasian reference pole indicating that this locality has not undergone V-A rotation since Cretaceous time. At Dangchang, demagnetization of Cretaceous red beds yielded a ChRM carried by magnetite and hematite. Normal and reverse polarity ChRM directions from 32 samples pass reversal and fold tests indicating primary magnetization. The mean ChRM direction (I=50.8°; D=50.4°; a95=4.7°) compared with the expected direction indicates a strong (34.1 ± 6.8°) clockwise V-A rotation. Although study of the remaining samples is needed to constrain the extent in space and time of this pattern of rotation, the above results have several implications. First, they are in agreement with GPS studies indicating clockwise V-A rotation of northeastern Tibet. Second, they confirm our previous analysis in the Qaidam basin suggesting a major change of tectonic regime across the Kunlun fault. Whereas the Dangchang locality south of the Kunlun-Qinling suture is rotating around the eastern Himalayan syntaxis, the Xining locality to the north is being extruded eastward without rotation.