Crust And Upper Mantle Structure Of Yunnan Province,China From Receiver Function

Yunnan locates at the southeastern margin of the Tibetan Plateau and is well represented by the massive seismic activities and wide-spread faults here.It's reported that near 42.4%earthquakes with magnitude greater than 6 in mainland China occurred here.Moreover it is also the host of the series of Quaternary volcanoes and hot springs in Tengchong volcanic area in western Yunnan Province,China.The unique geology and geophysics features have been attracting the attention of worldwide geoscientists.Recent studies argue there exist two or more branches of the hypothetic lower crust flow redirected from central Tibetan,while there are big discrepancies about the detailed passageways and the numerical values of the inverted shear wave velocities at almost same depth in their studies.We note that they just use S-wave distribution,expected to capture the specific clue of such a scientific hypothesis,to delineate the extension of the ductile flow in this rather complex zones.Unfortunately,S-wave velocity alone is not a better indicator of the hot,soft material in this strongly deformed and fractured zones.We suggest to use Poisson's ratio obtained from the H-? stacking result to identify the distribution of the lower crust flow and common conversion point sections to show detailed images of the crust and upper mantle within the study area.We also made some numerical synthetic tests about the effects of varying factors for H-? stacking method to give some reasonable explanations for the big disparity of results in previous similar studies.The distribution of crust Poisson's ratio show distinct partition characteristics with most high Poisson's ratio(>0.28)area above 26°N and extreme high values in Tengchong and Chuxiong volcanic areas,which indicate possible partial melt/mafic contents in the crust.We find the Moho depth and lower crust composition along this altitude vary significantly which predicts a possible transition belt.The CCP profiles show there are two upper mantle rises in northern Yunnan and weak and laterally varying Moho interfaces along the transition belt.Some interesting phenomena are that step-like and double Moho were obtained under some stations,which might be related with the lower crust delamination.Delamination leads to low Poisson's ratio and systematic felsic lower crust was usually considered the result of the removal of mafic layer.And we see a band of low Poisson's ratio intersecting the normal to high Poisson's anomalies.Based on the obvious crustal composition transition in northwestern Yunnan and similar features observed by tomography result,S-wave splitting and SKS results along 26°N,we conclude that extensive lower crust flow material was largely constrained above this transition belt.The mechanism of the bended crust and upper mantle interface in volcano area is the influence of the mantle flow from Myanmar and southeastern Tibetan and all the high Poisson's ratio areas presented in our study coincides with the continental thermal heat flow field in Yunnan.