New Geochronological And Paleomagnetic Results Of The Cretaceous Volcanics And Red Beds From The Qiangtang Terrane And Their Paleogeographic Implications

The Tibet Plateau was formed by progressive accretion of several terranes to thestable Asian migrated from north to south, the interaction and the rotation processbetween different terranes provide constraints on the occurrence of various geologicalevents. Tectonic events since the Mesozoic including the collage between the Qiangtangand Lhasa terranes and the collision of India and Eurasia, not only caused the intensecrustal shortening between the Lhasa terrane and the stable Eurasia, but also restrictedthe evolution process of the Mid-Neo Tethys. The Qiangtang and Lhasa terranescollided during the Cretaceous accompanying with the closure of the Bangong-Nujiangocean. At present, some key aspects about the Bangong-Nujiang ocean, such as itsnature of the ocean basin, its evolution and closed time are still open to intense debate.In addition, there has been no consensus on the crustal shortening, rotated model andthe scale of the Neo-Tethys. Therefore, the Cretaceous paleogeography of Qiangtangand Lhasa terranes has become an important window of studying the evolution of theterranes and the Mid-Neo Tethys.Base on the paleomagnetic study of the Zenong volcanics sampled from Cuoqinarea, a combined geochronological and paleomagnetic investigation has been performedon Cretaceous rocks in the west of the Qiangtang terrane near Gaize area. Ageochronologic study of zircon U-Pb dating indicates that the volcanic rocks sampledfrom the Gaize area is103.8±0.46Ma while the red beds belong to103.8-83.5Ma.After thermal demagnetization, the paleopoles obtained from the volcanic rocks and redbeds corrected by the E/I method are79.3°N,339.8°E, A95=5.7°and49.5°N,2.6°E,A95=3.6°, respectively. The results support that the Qiangtang terrane located at29.3°Nduring103.8-83.5Ma. Combined with the geological evidence and previouspaleomagnetic results from the Qiangtang and Lhasa terranes, this study support that thewestern segment of the Bangong-Nujiang ocean closed at~103.8Ma and~580kmcrustal shortening happened during the later Cretaceous. Comparing the paleolatitude ofthe southern margin of the Lhasa terrane (20.1°±4.8°N) with the stable Asia at120Ma,the significant discrepancy (9.8±4.1°) implies that~500km N-S crustal shortening has taken place between them caused by the India-Asia collision which has excluded thecontribution of the Qiangtang-Lhasa terranes collision. Comparing the paleolatitudesbetween stable India and the Lhasa terrane, the paleolatitude difference is59.6±4.2°,indicating the width for the Neotethys ocean was6615.6±466km in the earlyCretaceous. Besides, combined the previous studies (including GPS data andpaleomagnetic data), this study supports that the hinterland of the Tibet plateauexperienced different rotation models from the east to the west since the Cretaceous.The southeast of the Tibet plateau has experienced a large scale clockwise while asignificant count-clockwise rotation happened in the mid-west part. Although theamount of rotation in different regions may be vary influenced by the development ofthe strike-slip faults, they agree with the above rotation models in general mainlytrigged by the India-Asia collision.