PRE-EXTRUSION LOCATION OF THE INDOCHINA FRAGMENT AND ITS SHEAR ZONE CICATRIX IN SE TIBET

Extrusion of Indochina (senso lato) has been the subject of considerable controversy, but our recent paleomagnetic and geologic data support early Cenozoic SE-directed extrusion of a complexly fragmented Indochina continental fragment. Paleomagnetic data indicate at least 500 km of southward movement of Indochina relative to South China requiring a larger lateral SE movement of Indochina. Understanding the pre-extrusion position of Indochina is critical to a more complete picture of how Indochina has been extruded. A new compilation of geologic data from SE Tibet and adjacent areas to the SE suggest the Indochina crustal fragments have been extruded from a broad zone of shear that originated between the Qiangtang and Lhasa paleogeographic units. The shear zone is marked by large lenses of rock units defining several different tectonic units juxtaposed by major E-W and WNW-faults; these include the major Bangong suture and the Lancangjiang fault zones. The juxtaposed tectonic lenses are the only remnants left of the different tectonic units that moved to the east and southeast during extrusion around the evolving Eastern Himalayan syntaxis. Shear zone units were progressively shortened north of the syntaxis and sheared right-laterally east of the syntaxis during India's continued penetration into Eurasia. Major faults often follow older sutures, but in many places cut across them, and modification during progressive deformation has made it difficult to recognize sutures NE of the syntaxis. Faults within the shear zone north of the syntaxis diverge eastward and were less modified east of the syntaxis. The presence of the shear zone cicatrix in eastern Tibet suggests it continues westward across southern Tibet. Unfortunately, the placement of the Indochina fragment north and partly west of the Eastern Himalayan syntaxis falls far short of resolving the problem of filling the space with continental crust between NE India and Eurasia at the inception of collision.