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Although there is increasing evidence for wet, monsoonal conditions in Southeast Asia during the late Eocene, it has not been clear when this environment became established. Cenozoic sedimentary sequences constrained by radiometrically dated igneous rocks from the Jianchuan Basin in the southeast flank of Tibetan Plateau now provide a section whose facies and climatic proxies determine this evolution. Semi-arid conditions had dominated the region since Paleocene controlled by the northern sub-tropical high pressure system, culminating in mid Eocene when desert dunes developed. From 36 Ma, the basin began to accumulate swamp sediments with coals, together with synchronous braided river deposits and diversified pollen assemblages, indicating significant increase in precipitation. This remarkable transition from dry to wet conditions precedes the Eocene/Oligocene boundary at 34 Ma, thus excluding general global cooling as the prime driver. We propose that uplift of Tibetan Plateau might have reached a threshold elevation by that time, operating through thermal and dynamic forcing, causing the inception or significant intensification of monsoonal rains to penetrate into this downwind locality. [ABSTRACT FROM AUTHOR]