PALEOMAGNETISM AND STRUCTURE OF THE LANPING-SIMAO FOLD/THRUST BELT, WESTERN YUNNAN PROVINCE, SOUTHWEST CHINA

Upper Jurassic to lower Tertiary hematite-cemented sandstones and siltstones form most of the shallow crust involved in Cenozoic development of the Lanping/Simao fold thrust belt, in western Yunnan, PRC, and northwest Vietnam, along the eastern margin of the eastern Himalyan syntaxis. Paleomagnetic data have been obtained from these strata, at several,usually artificially-exposed localities (few to no surface exposures due to intense weathering) to test the hypothesis that, beginning in early Cenozoic time, much of the Indochina lithosphere fragment was strongly and probably heterogeneously deformed during some 2500 km of post-collision northward translation of the Indian subcontinent and, specifically, the eastern Himalayan syntaxis. Overall, trends of folds and related faults roughly parallel the north-northwest trending Ailao Shan shear zone, which appears to have acted as a major accommodation zone during left slip between the relatively undeformed South China block and western Yunnan structural elements. Locally, and in particular in the central part of the fold thrust belt some 100-200 km northwest of Jinggu, folds exhibit a more east-west orientation, and geometrically bend around an east-directed promontory where the Baoshan/Linchang assemblage is closest to the Ailao Shan and the fold thrust belt is the narrowest. The high-quality paleomagnetic data obtained from most of the localities are interpreted to show that parts of the fold thrust belt experienced large magnitude (greater than 75^o) clockwise rotations, yet results to date do not show a clear correlation between orientation of contractional structures and magnitude of vertical axis rotation. Our interpretation of the paleomagnetic data from this area, collected from as many parts of the fold thrust belt as possible, is partly compromised by the overall lack of quality three-dimensional exposure to evaluate the effect of plunging structures. We suggest that the area forming the Lanping/Simao fold thrust belt probably originated in an overall more northwest orientation, at latitude some 5-10^o farther north than present.