New paleomagnetic results from the Lhasa block: Revised estimation of latitudinal shortening across Tibet and implications for dating the India-Asia collision

Knowing how and when the Tibetan Plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths. Previous paleomagnetic work suggested that intracontinental shortening from deformation related to the India-Asia collision played a major role in building the Tibetan Plateau. However, these studies were derived from red bed facies, which are often afflicted by inclination shallowing, leading to overestimates of continental shortening. We report new paleomagnetic data from the Lhasa block, the southern continental margin of Asia before the India-Asia collision. A high temperature, characteristic remanent magnetization (ChRM) was isolated from 43 sites of Late Cretaceous red beds, 32 sites of Late Cretaceous lava flows, and nine sites of Eocene tuffs. All the ChRM directions are significantly different from the present-day geomagnetic field, and the mean directions of red beds, lava flows and tuffs pass the fold test. The distribution patterns of ChRM directions derived from volcanic rocks are consistent with a secular variation model, suggesting that secular variation was likely averaged out. In contrast, the red bed ChRM directions exhibit an east-west elongated distribution pattern, indicative of inclination flattening. The mean ChRM inclination of the red beds, 23.5 degrees +/- 2.5 degrees, is consistent with previous results, but significantly shallower than that of 41.9 degrees +/- 4.4 degrees carried by the coeval lava flows. An elongation inclination analysis of the red bed ChRM directions yielded an unflattened mean inclination of 39.7 degrees, with 95% confidence limits between 37.7 degrees and 41.9 degrees. The paleolatitudes of the Late Cretaceous volcanic rocks and inclination-corrected sediments suggest about 10 degrees less shortening occurred between Lhasa and Siberia than previous estimates, with total shortening lying between 3.3 degrees +/- 2.6 degrees and 7.3 degrees +/- 5.4 degrees since the Late Cretaceous. The mean ChRM inclination derived from the Eocene tuffs, 51.8 degrees +/- 5.2 degrees, is indistinguishable from the expected reference direction. Our new results imply that India-Asia convergence was accommodated mainly through consumption of the India plate and yield a much younger age (ca. 43 Ma) for the initial India-Asia collision than previously estimated by paleomagnetic data. (C) 2010 Elsevier B.V. All rights reserved.