Paleozoic paleomagnetism of the South China block and the Shan Thai block: The composite nature of Southeast Asia.

Abstract

Southeast Asia has long been recognized as a mosaic of continental fragments which coalesced at various times. However, the derivations of the composing elements are not clear. In order to provide constraints on the tectonic evolutions of the area, Paleozoic paleomagnetic samples were collected from eastern and western Yunnan, southwest China, which represent the South China Block (SCB) and the Shan Thai-Malay Block (STM) respectively. Though a majority of the samples appear to have been strongly overprinted by the recent Earth's field and some yield aberrant results, characteristic remanent components have been obtained from well-behaved samples and these are interpreted as primary or near primary magnetizations. The paleopoles from eastern Yunnan fall at 9$\sp\circ$N, 10$\sp\circ$E (for the Devonian), 23$\sp\circ$N, 205$\sp\circ$E (for the Middle and Late Silurian), 35$\sp\circ$N, 227$\sp\circ$E (for the Early and Middle Ordovician), and 69$\sp\circ$N, 271$\sp\circ$E (for the Cambrian). The paleopoles from western Yunnan fall at 66$\sp\circ$N, 134$\sp\circ$E (for the Devonian), 7$\sp\circ$N, 194$\sp\circ$E (for the Middle and Late Silurian), and 21$\sp\circ$N, 191$\sp\circ$E (for the Middle Ordovician). The results of this study indicate that during the early Paleozoic the SCB and the STM were located in an equatorial area; possibly they were adjacent to each other and on the periphery of Gondwana. The SCB remained near the equator for the entire Paleozoic but may have left the STM and Gondwana in the Silurian. The STM moved together with Gondwana to a higher paleolatitude of 42$\sp\circ$ in the Devonian.