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

Abstract

We present new paleomagnetic results obtained at 39 sampling sites from five sections of Tertiary red bed formations: two Eocene formations from the Qiangtang block of Tibet (Xialaxiu locality; 32.8°N, 96.6°E) and the Xining basin of Qaidam (Xining locality; 36.5°N, 102.0°E) and three Neogene formations from the Xining basin (Jungong locality; 34.7°N, 100.7°E) and the Kunlun block (Tuoluo lake and West Yushu localities; 35.3°N, 98.6°E and 33.2°N, 96.7°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°N/352°E (dp/dm = 6.0°/10.7°) for Xialaxiu, 61.6°N/211.3°E (dp/dm = 9.7°/16.1°) for Xining, 66.0°N/228.6°E (dp/dm = 3.6°/6.9°) for Jungong, and 53.9°N/205.4°E (dp/dm = 5.6°/10.0°) for West Yushu. As in previous studies of Tertiary formations from Asia, the inclinations we obtained are shallower (by 18° to 26°) than the magnetic field computed from the Eurasian apparent polar wander path (APWP) at 10 and 20 Ma for Neogene rocks 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 of positions of the Siberian craton when based on the APWP of Eurasia. The main 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 anomaly” will require new geologic and paleomagnetic data from the northern parts of these remote regions in Mongolia and Siberia.