PRELIMINARY PALEOMAGNETIC RESULTS FROM THE CENOZOIC ROCKS OF INDOCHINA: EVIDENCE FOR OLIGOCENE–MIDDLE MIOCENE ROTATION OF THE SOUTHEAST TIBETAN PLATEAU

Paleomagnetic studies were conducted on the Paleocene to Miocene strata in the central part of the Indochina Block in order to constrain the crustal deformation induced by continuous penetration of the Indian plate into Eurasia during the Cenozoic. Characteristic remanent magnetizations (ChRMs) from Paleocene to Oligocene sediments were isolated by thermal demagnetization with unblocking temperature of ~ 680 ºC. The syn-folding or positive fold test suggests that the magnetizations of Paleocene to Oligocene sediments are primary. ChRMs of middle Miocene sediments isolated by thermal/alternating field demagnetization exhibited dual polarity and passed both the reversal and fold test at the 95% confidence level, indicating the primary nature. Comparing these results with previous data and coeval Asia paleomagnetic poles suggest that the clockwise rotation of Indochina can be divided into two periods: a large, rapid rotation between Oligocene and Miocene and a small, slow rotation after mid-Miocene. The earlier rapid rotation between Oligocene and Miocene may related to block extrusion in early Cenozoic, while the latter slow rotation suggest that other mechanism, for example, crustal flow, may controlled the crustal deformation in SE Tibetan Plateau after mid-Miocene.