Resolving the problem of shallow magnetizations of Tertiary age in Asia: insights from paleomagnetic data from the Qiangtang, Kunlun, and Qaidam blocks (Tibet, China), and a new hypothesis

We present new paleomagnetic results obtained at 39 sampling sites from fiv e sections of Tertiary red bed formations: two Eocene formations from the Q iangtang block of Tibet (Xialaxiu locality; 32.8 degrees N, 96.6 degrees E) and the Xining basin of Qaidam (Xining locality; 36.5 degrees N, 102.0 deg rees E) and three Neogene formations from the Xining basin (Jungong localit y; 34.7 degrees N, 100.7 degrees E) and the Kunlun block (Tuoluo lake and W est Yushu localities; 35.3 degrees N, 98.6 degrees E and 33.2 degrees N, 96 .7 degrees E, respectively). Thermal demagnetization of the rocks isolated a high-temperature component that we interpret as the primary magnetization in four localities. The paleopoles lie at 52.6 degrees N/352 degrees E (dp /dm=6.0 degrees/10.7 degrees) for Xialaxiu, 61.6 degrees N/211.3 degrees E (dp/dm=9.7 degrees/16.1 degrees) for Xining, 66.0 degrees N/228.6 degrees E (dp/dm=3.6 degrees/6.9 degrees) for Jungong, and 53.9 degrees N/205.4 degr ees E (dp/dm=5.6 degrees/10.0 degrees) for West Yushu. As in previous studi es of Tertiary formations from Asia, the inclinations we obtained are shall ower (by 18 degrees to 26 degrees) than the magnetic field computed from th e Eurasian apparent polar wander path (APWP) at 10 and 20 Ma for Neogene ro cks and at 40 and 60 Ma for Eocene rocks. On the basis of a compilation of Eocene data from the South China Block, Tibet, central Asia and Kyrgyzstan, we conclude that this inclination anomaly reflects erroneous predictions o f positions of the Siberian craton when based on the APWP of Eurasia. The m ain reason for this discrepancy might be nonrigid behavior of the Eurasian plate in the Tertiary. Combination of this with intracontinental shortening of Asia under the penetration of India provides a full explanation for the anomaly. Verification of this new interpretation of the "inclination anoma ly" will require new geologic and paleomagnetic data from the northern part s of these remote regions in Mongolia and Siberia.