Mantle Transition Zone Structure Around Hainan By Receiver Function Analysis

Hainan and its adjacent areas locate in the intersections of theEurasian, the India-Australia and the Philippine Sea Plates. SinceCenozoic, this area experienced many volcanic eruptions, and formedmore than 100 volcanoes, distribution area of volcanic rock is up to 4000km2. Multiscale seismic tomography results show a mantle plume mayexist near Hainan. Geochemical studies of Leiqiong area also show thedeep volcanic features of Hainan. Geophysical and geochemical resultssuggest that Hainan volcano is different from the other volcanoes inmainland China, it is similar to Hawaii hotspot. Seismological studies arecontroversial about the scope and origin of Hainan mantle plume. It hasvery important scientific signification to confirm its existence and deepstructural character for understanding formation of oil, South China Seaand marginal Sea, and dynamic process of surrounding plates subduction.Mantle transition zone is the area between the 410-km and the660-km discontinuity (hereafter called the 410 and the 660), which aretwo seismic velocity discontinuities in the mantle. It is accepted that the410 is formed by the transformation fromα-olivine toβ-spinel whichhave a positive Clapeyron slope, so when the temperature increases (ordecreases), the depth of 410 will be depressed (or uplifted)correspondingly. The 660 is formed by the transformation fromγ-spinelto perovskite plus magnesiowustite, which have a negative Clapeyronslope, so the depth of 660 will be uplifted (or depressed) with increasing(or decreasing) temperature. When mantle plume pass through thetransition zone, the 410 and the 660 will be depressed and uplifted,respectively, and lead to transition zone thinning. Therefore, the depth ofupper mantle discontinuities and the thickness of the transition zoneprovide important information about temperature variation of the mantle,the structure of upper mantle discontinuities and lateral variation of transition zone thickness can be used to confirm the existence of mantleplume and discuss its scope and origin. This is a frontier topic of Earth'sinterior structure study.Teleseismic P wave receiver function is a time series extracted by thedeconvolution of vertical component of three-component teleseismicrecords to the horizontal component, which avoided the effects of thefocal process and propagation path and mainly related to the structureunder the seismic station. Receiver function method can effectively studythe depth of upper mantle discontinuities and transition zone thickness byusing the differential time between converted wave (Ps) at velocitydiscontinuity of teleseismic P wave and direct P wave. In this study, wehave used teleseismic waveforms recorded by 88 broadband seismicstations from Hainan, Guangdong and Guangxi Regional SeismicNetwork and National Seismic Network during August, 2007 to March,2010. 75 teleseismic events with high signal to noise ratio and clear firstmotion was selected out of 529 events are larger than 5.5. We extract4550 receiver functions using frequency domain deconvolution, thenconduct moveout correction and stack to 1842 receiver functions withsignal to noise ratio greater than 25. We use different Gauss coefficientsto extract receiver functions in frequency domain, and employ linearstack and nth(n=2) root nonlinear stack method to receiver functions withsignal to noise ratio greater than 25, respectively, the results show that thenth(n=2) root nonlinear stack method is more suitable for the stack ofreceiver functions with more high frequency signals, which can improvethe signal to noise ratio of stacking results significantly.We obtained the upper mantle discontinuities structure and mantletransition zone thickness beneath the Hainan and adjacent areas. Ourresults reveal a relatively complicated 410, which depress to 447kmlocally, but a structurally simple a 660 beneath the region, with the deptharound 670km. The mantle transition zone is thinned anomalously by 25±5km within an area approximately 200km in diameter centerednortheast of Hainan island, this anomaly is consistent with an excesstemperature of～180℃. After a comprehensive analysis, we believe thatthe variation of upper mantle discontinuities depth and thinning of themantle transition zone imply that Hainan mantle plume exist in themantle transition zone beneath the region and may originate from lowermantle. This result provided new seismological evidences for the scopeand origin of Hainan mantle plume.