NEW CRETACEOUS AND EARLY TERTIARY PALEOMAGNETIC RESULTS FROM XINING-LANZHOU BASIN, KUNLUN AND QIANGTANG BLOCKS, CHINA - IMPLICATIONS ON THEGEODYNAMIC EVOLUTION OF ASIA

We present the results of a paleomagnetic study of 360 cores, drilled at three different areas from the Tibetan Plateau: 13 sites from Lower Cretaceous red beds in the Xining-Lanzhou basin, around Xining and La nzhou cities, southwest of the Qilian mountains (36.2 degrees N, 103.5 degrees E); 13 sites from Cretaceous red beds in the Kunlun block, ne ar Maqin (34.5 degrees N, 100.1 degrees); and 9 sites from Paleocene/l ower Eocene red beds in the Qiangtang block, near Fenghuoshan (34.5 de grees N, 92.8 degrees E). Thermal demagnetization of the samples allow ed us to isolate a high-temperature component which passes both positi ve reversal and fold tests for all formations. The corresponding paleo poles lie at 50.3 degrees N, 195.5 degrees E (A(95)=4.6 degrees) for X ining-Lanzhou, 80.1 degrees N, 281.2 degrees E (dp=7.8 degrees, dm=12. 7 degrees) for Maqin, and 62.6 degrees N, 210.5 degrees E (dp=3.9 degr ees, dm=6.8 degrees) for Fenghuoshan. We discuss these in the frame of a new paleogeographic reconstruction of the Cretaceous paleoposition of the blocks forming the Asian mosaic. We conclude that the Xining-La nzhou area could not be part of the North China Block but rather was a ssociated with the Tarim-Qaidam assemblage. Paleomagnetic data argue i n favor of a Qaidam-Kunlun-Tarim-Junggar assemblage in the Cretaceous, significantly to the south of its current position with respect to th e Asian continent (Siberia, Mongolia and North China). The large N-S c onvergence (800+/-500 km) implied since the Cretaceous appears to be f ar larger than could be absorbed in the Altay range to the north and Q ilian Shan to the east (of the order of 300 km). Part of this motion C ould have occurred along a large left-lateral strike slip fault system , which may connect with, the Mongol-Okhotsk suture to the northeast. Verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions.