Two years of continuous recordings of ambient seismic noise observed at 354 stations in South China from 2009 to 2010 are used to estimate Rayleigh wave group and phase velocity maps from 6 to 40 s period. These results are merged with Rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame. Eikonal tomography generates the dispersion maps, which, by Monte-Carlo inversion, are used to estimate a 3-D Vsv model of the crust and upper mantle down to a depth of 150km across all of South China with attendant uncertainties. A clear image emerges of the 'West Yangtze Block', a region of the western Yangtze Craton characterized by relatively thick crust (~40 km) overlying a seismic mantle lithosphere that extends to at least 150km that may have been the nucleus for the formation of the Yangtze craton in the Archean and may present a present-day obstacle to the eastward expansion of Tibet. The West Yangtze Block contrasts with the thinner crust (~30 km) and mantle lithosphere (~70-80 km) beneath the eastern Yangtze Craton and South China Foldbelt. These observed differences are consistent with processes associated with flat slab subduction in the Mesozoic that may have eroded the lithosphere of the eastern Yangtze Craton and the South China Foldbelt.