Structure of the mantle transition zone under the Yunnan region and its geodynamic implications

Abstract

The mantle transition zone (MTZ) bounded by physical discontinuities at 410 and 660 km depth is believed to play a key role in controlling mantle flow. Mineral physics experiments reveal that phase changes from olivine to β phase and γ -spinel to perovskite+magnesiowustite, at pressures equivalent to the existing at those mantle discontinuities, largely explain the variations in seismic velocity observed by the seismologists. The globally observed seismic velocity discontinuity at 660 km depth marks the bottom of the MTZ, which is the natural boundary between the upper and lower mantle. This discontinuity is considered an important mantle boundary, since it is especially associated with the mantle dynamics, the source of mantle plumes and the sinking of the subducted plates. On the other hand, temperature variations are the cause of thickening and thinning of the MTZ in correspondence with the positive and negative slopes of the Clapeyron curve at the 410 and 660 km mantle discontinuities, respectively. Receiver function analysis is a straightforward and commonly accepted method of studying the crust and upper-mantle structure through the use of teleseismic waveforms recorded at three-component seismic stations. This method requires the rotation of the ZNE displacement components to the radial and transversal components in the horizontal plane; and later the transformation of the radial and vertical components, which lie in the same vertical plane, into the L and Q components of the ground motion. By deconvolving the L component from the Q -component, both the source, far field path and instrument effects are removed. The resulting receiver function waveform is the ground’s impulse response. The analysis of each receiver function recorded at an array station provides, after moveout correction, a detailed estimation of the depth of the interface where the P-to-S seismic phase conversion is produced. However, the converted Ps phases at the discontinuities of 410 and 660 km depth are so weak that stacking of a large amount of waveforms is needed to enhance the signal-to-noise ratio in the receiver functions. In this study, we obtained 8600 teleseismic P-wave receiver functions recorded at 48 permanent broadband stations deployed in Yunnan and surroundings, which later were properly stacked in a single trace in 1°×1°-sized grid cells. Before stacking, the receiver functions were migrated from time domain to depth domain with the help of a reference earth model. Finally, we obtained 6 stacking images of receiver functions at latitude of 28°, 27°, 26°, 25°, 24° and 23°N, being the stacking depth in the 0–800 km range. The results indicate: (1) further north of 26°N, the average depths of the 410 and 660 km mantle discontinuities are in a range of 407–408 and 663–670 km, respectively, while the average thickness of the MTZ is within the range of 255–269 km, which are values very close to the global average thickness of 250 km. (2) South of 26°N, the average depths of the 410 and 660 km discontinuities are in a range of 412–426 and 675–703 km, respectively, and the average thickness of the MTZ is within the range of 262–279 km, which is obviously larger than the global average value of 250 km. The deepening of the 410 and 660 km discontinuities in the Yunnan region is obviously associated with the eastward subduction of the Indian plate below the Burma Arc; then, based on the greater thickness of the MTZ beneath the Yunnan region, we suggest that the eastward subduction of the Indian plate occurs mainly south of 26°N in the Yunnan region.