New paleomagnetic results of Paleocene volcanic rocks from the Lhasa block: Tectonic implications for the collision of India and Asia

A combined geochronological and paleomagnetic investigation has been performed on Paleocene volcanic sequences in the Lhasa block near the localities of Mendui (30.1 degrees N/90.9 degrees E). A total of 15 sites have been sampled from rhyolitic tuffs. Stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions. The tilt-corrected mean direction is D/I=359.0 degrees/26.1 degrees with alpha(95)=9.2 degrees and N=14 sites, corresponding to a paleopole at 73.6 degrees N, 274.3 degrees E with A(95)=7.3 degrees. Positive fold tests suggest a primary origin for the characteristic remanence. In order to provide a more accurate pole, we propose to combine site-mean directions from this study and Achache et al.'s (1984). The combined average palaeomagnetic direction from early Paleocene volcanic rocks is D=356.6 degrees, I=25.9 degrees, kappa=21.7, alpha(95)=6.8 degrees after tilt correction, N=22 sites, corresponding to a pole at 73.2 degrees N, 282.4 degrees E with A(95)=5.4 degrees. The paleomagnetic results yield a paleolatitude of 13.6 +/- 5.4 degrees N for the southern margin of Eurasia at similar to 55 Ma. Compared with expected paleomagnetic directions from the stable India and Eurasia blocks, significant crustal shortening of 1400 +/- 600 km and 2000 +/- 550 km respectively may have occurred between the southern margin of Eurasia and the stable India, and within Eurasia since the collision of India and Eurasia. (C) 2010 Elsevier B.V. All rights reserved.