COMPLICATIONS FOR LARGE MAGNITUDE EARLY CENOZOIC EXTRUSION OF INDOCHINA

Perhaps the most accepted model for the early Cenozoic tectonics around the Eastern Himalayan syntaxis was first proposed by Tapponnier and others (l982), since modified but with the initial emphasis largely unchanged. In this model Indochina moved to the SE ~700 km as an essentially rigid crustal fragment with ~14° of clockwise rotation, bounded on the north by the left-lateral Ailao Shan shear zone and on the west by the right-lateral Gaolingong shear zone, to make way for the northern penetration of India into Eurasia. Our paleomagnetic studies of the Lanping-Simao tectonic unit in the northern part of Indochina crust indicates complex rotations of from 30-90° clockwise during early Cenozoic time that results from rotation of smaller fragments. Structrual patterns south of the Lanping-Simao also suggest breaking of crust into smaller clockwise rotated fragments to be test by further paleomagnetic studies. Our work has shown this southern part of Indochina is cut by the poorly known NW-trending Chong Shan shear zone which shows both left- and right-lateral Cenozoic shear indicating that relative movements of crustal fragments within Indochina was complex. Paleomagnetic data from the LP-S belt do suggest at least 500 km of SE movement relative to South China. Compilation of a detailed tectonic map for the entire SE Tibetan region and adjacent foreland has raised doubt on the northwestward continuation of some of the shear zones responsible for large magnitude extrusion. Across the supposed northern boundary Ailao Shan shear zone Upper Triassic and Paleogene rocks are very similar. Other tectonic units with the Three Rivers area are not obviously offset across projected boundary faults and the location of large magnitude offsets within SE Tibet are difficult to locate at present. This region has been complicated by offsets that may be Mesozoic in age and significant Cenozoic crustal shortening. Detailed studies are on going to resolve these problems.