Paleomagnetism of early Paleogene marine sediments in southern Tibet, China: Implications to onset of the India-Asia collision and size of Greater India

We report a paleomagnetic study of Paleocene marine sediments in the Gamba area of the Tethyan Himalayan terrane, southern Tibet, which aims to accurately locate the position of the northern margin of Greater India and further constrain timing of initial contact between India and Asia. Following detailed rock magnetic and paleomagnetic experiments on a total of 675 drill-core samples collected from the Zongpu and Upper Jidula formations, characteristic remanent magnetizations (ChRMs) were successfully isolated from most samples following alternating field (AF) or integrated thermal and AF demagnetization. The ChRMs are of dual polarity and pass fold and reversal tests indicating a pre-folding origin. Together with detailed biostratigraphic investigation of the sampling section, a magnetic polarity sequence is constructed from data at 167 sampled horizons and satisfactorily correlates with polarity chrons C24r to C27r dating the Zongpu Formation between similar to 56.2 and 61.8 Ma on the geomagnetic polarity time scale. The ChRM directions from the Zongpu Formation are grouped stratigraphically into 33 sites and yield two paleopoles of 71.6 degrees N, 277.8 degrees E (A(95)=2.5 degrees) and 67.3 degrees N, 266.3 degrees E (A(95)=3.5 degrees) for the time intervals comprising 56 to 59 Ma and 59 to 62 Ma, respectively. These new paleopoles imply that the Tethyan Himalayan terrane was sited at low latitudes in the Northern Hemisphere during the interval similar to 62-56 Ma, suggesting that initial contact between the Tethyan Himalaya and Lhasa terranes was established before 60.5 +/- 1.5 Ma, and very likely occurred near the Cretaceous-Tertiary boundary, at least in the central part of the suture zone. The results also indicate that at least 1500 +/- 480 km of post-collisional crustal shortening occurred within the Himalayas. From the analysis of available paleomagnetic data obtained on both sides of the suture zone, we propose a conceptional collision model for the India-Asia collision. (C)2011 Elsevier B.V. All rights reserved.