Paleogeographic and tectonic evolution of southern Tibet since Middle Cretaceous time: NEw paleomagnetic data and synthesis

During the second French-Chinese paleomagnetic campaign in Tibet, in the summer of 1981, we sampled the middle Cretaceous red beds of the Takena formation and the Cenozoic volcanics of the Lingzizong formation. A detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization N in the north of the Lhasa block (north of the Nyainqentanglha range) and two stable components in the south (LT, HT) where the red beds are more strongly folded and are unconformably overlain by the Lingzizong volcanics. All three components are carried by hematite. Blocking temperatures range from 400° to 600° C for LT and up to the Néel temperature of hematite for both N and HT. LT is shown to postdate part of the folding of the Takena, whereas N and HT both indicate a positive fold test at the 99% probability level. In addition, a small (less than 10% of the natural remanent magnetization) antiparallel component is superimposed in most samples and may be consistent with a model of long-term acquisition of a chemical remanent magnetization for the red beds. Analysis of the volcanics reveals a single stable component with mixed polarities, carried either by hematite or magnetite. The paleomagnetic directions indicate that (1) the Lhasa block stood at 12.5°±3° N by Upper Cretaceous and at 13.5°±6.5° N in the Paleocene-Eocene, thus confirming the limited displacement of the margin of Eurasia between Albian-Aptian (deposition of the red beds) and Eocene (onset of the India/Asia collision). These values imply crustal shortening by as much as 1900±850 km north of the block (since the Upper Cretaceous) and by 1400±1200 km south of the block (since the onset of the collision), (2) Southern Tibet formed a single tectonic unit by middle Cretaceous time, and (3) differential rotations are observed within the block and are related to the collision. These rotations are discussed in the frame of a recent propagating extrusion tectonics model. This study confirms the close paleogeographic relationship between Indochina (Khorat plateau) and Southern Tibet prior to the India/Asia collision.