GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 23, PAGES 3405-3408, NOVEMBER 15, 1996 Paleomagnetic resultsfrom the Upper Silurian of the Shan-Thai-Malay block, southwest Yunnan, China Haihong Chen, 1 DalaiZhong, 1 Friedrich Heller, 2 and Jon P. Dobson 3 Abstract. 77 paleomagnetic samples from the Upper Silurian reddish limestone and marlstone of the Shan-Thai-Malay block nearBaoshan ( 99.1øE,24.8øN ), western Yunnan,China, reveal a pre-foldingremnantmagnetization, with a mean direction of D=49.6 ø,I= -3.2 ø,k=9.7, a95=5.5 ø, corresponding a paleopole at 207.0øE, 36.8øN. These results, unlikethe upper Paleozoic data with steeper inclinations published earlier, suggest that the block was located at equatorial latitude during late Silurian time, probably adjacent to the northern marginof Gondwanaland. Introduction The Three-Riverregion is a tectonic collageof displaced fragments from Gondwana and Cathaysia continents during Paleotethys evolution stage( Mitchell, 1981; Stauffer, 1983; Zhong, 1995). The Shah-Thai-Malay block,or Baoshan blockin Chinese part, has long been referred to being a piece of continental fragment from Gondwana, indicated by the Carboniferous tillite andGondwana fauna( Ren et al., 1980; Cao, 1986 ). While the Simao blockandLinchang Arc in the eastern vicinity,demarcated by the Changning-Menglian melange Zone ( Main Paleotethys SutureZone ), are interpreted to be rifted blocksfrom the Cathaysia continent ( Hsu, 1994 ) in the Late Paleozoic. Nevertheless, these blocks are still poorly constrained by pa•eomagnetic data, especially the Lower Paleozoic. The data published for the Shan-Thai-Malay block are mainly derived from the Upper Devonian, Carboniferous and Lower Permian ( McElhinny et al., 1974;Fang et al., 1989; Huang andOpdyke, 1991 ) andthese results locate the block at southern hemisphere, probably adjacent to Australia during the Late Paleozoic. Several recent papers postulated that the majority of the Chinese continental blocks were derived from Paleozoic Gondwanaland ( e.g., Burrett et al., 1990). The earlyPaleozoic paleomagnetic results from Australia suggest that Australia was located at northern intermediate to low latitude from the Cambrian to Silurian andafterwards moving southward ( Goleby, 1981;Piper, 1987 ). Till Devonian time, it shifted to southern low latitude. The lower Paleozoic paleomagnetic data from the Yangtzeand North China blocks alsoshowan equatorial inclination ( Lin et al., 1985a;Zhaoet al., 1993 ). Similarities in Cambrian trilobites, early Paleozoicfossil assemblages, phosphorites as well as 1 Institute ofGeology, Chinese Academy ofSciences, P.O.Box 9825, Beijing 100029, China. 2 Institut furGeophysik, ETH-Honggerberg, CH-8093 Zurich, Switzerland. 3 Department of Physics, University of Western Australia, Nedlands, WA6907, Australia. Precambrian stratigraphybetween South China ( including southwest ) and Australia reinforce the explanation by paleomagnefists ( Lin et al., 1985b ). To testthishypothesis, it is critical to determine paleolafitude of the block and paleogeographic relationships with other Chinese blocks by means of paleomagnefism. Geology, sampling and measurements The Paleozoic andTriassic sequence in theBaoshan Block is a continuous marine succession with a total thickness over 8000m ( Geological Bureau of Yunnan,1990 ). The folding took place in late Triassic to Jurassic time whenmarine sedimentation was ceased and continental reddish clastic rocks started accumulation.An unconformity is widely seen between the Jurassicand underlain marine strata. The folding was most probably caused by the collision event of the Baoshan Block with the Lincang arc terrene in the eastand CentralBurmaBlock in the west.The folded Paleozoic sedimentary coverof the block,as a forelandfold and thrustzoneof the neighboring orogenic belts, is well exposed in the region. The UpperSilurian is composed of marine carbonates, and dominated by purple limestone and marlstone in the upperpart with abundant bioclasts and burrows as well as small ripple laminations,which indicate a shallow water and oxidizedenvironment. The age of the stratais well determined by conodont fossils of Spathognothodus sp., S. steinhornensis ? eosteinhornensis, Trichchonodella syminertrica, •300' 24000 , 23ø07 ali CEN I TRAL-BURMA X-• Vongplng .•i 3 •x BLOCK '" ': ,.. t••5•;• :•[• YANGTZE BLOCK glI I•SHAN-THAI- ' ' '• ..... :•:• Mojiang Me•n• ..... ,%•,• B ============================ ::':'• - -•i•::• a L 0 C K • ................ ••••:•:••• Copyright 1996 by theAmerican Geophysical Union. Paper number96GL03154. 0094-8534/96/96GL-03154505.00 Figure 1. Simplified tectonic map of the Three-Riverregio. n showing geographic locations of the major tectonic units and localities of paleomagnetic sampling. Numerals by solid circles are site numbers. 3405