Rheological Regime of the Upper Mantle beneath the Taiwan Strait and Its Tectonic Significance

This paper deals with deformation textures and fabrics of mantle-derived xenoliths and dislocation microstructures of olivine in the upper mantle in the Penghu Islands, Taiwan. According to the calculation of the chemical composition of xenolith minerals (pyroxene), the equilibrium temperatures and pressures were 986-1116@ and 1.50-2.60 GPa, respectively. Deformation events in the upper mantle may fall into three sequences' (1) uniform steady-state flow deformation with high temperatures and low stresses, (2) shear flow deformation with high temperatures and relatively high stresses on diapiric margins of the upper mantle, and (3) extraction deformation of {110} glide bands with low temperatures and high strain rates. Deformation events and thermal structure of the upper mantle in the study area show that eastern Fujian and the Penghu Islands are characterized by very similar rheological properties of the upper mantle. Volcanism of basalts in the Penghu Islands is related to hot spots of the upper mantle under neath the Taiwan Strait, and diapirism and upwelling of the upper mantle are the dominant factors responsible for rifting of the continental margins in eastern China.

The Thermal Structure of the Upper Mantle in Eastern China——Inferred from the Petrological Model

Cenozoic basalt in eastern China contains abundant ultramafic xenoliths which are specimens of pyrolitesreleased during basaltic magma eruption. A total of 405 P-T data of pyroxene in the ultramafic rocks have beencollected, which present a more precise pyroxene geotherm. The average geothermal gradient in the upper man-tle represented by the pyroxene geotherm is about 3.3℃ / km, which is much less than that derived from theconductive thermal model (≈14℃ / km), implying the great significance of convective heat transfer. The calcu-lation shows that the contributions of convective and conductive heat transfers are 79% and 21%, respectively.The perturbation in the thermal structure of the upper mantle is an important manifestation of thetectonothermal event of Cenozoic continental rifting and intense basaltic volcanism in eastern China. Based onthe pyroxene geotherm and its comparison with the current geothermal field derived from the measurements ofthe surface heat flows, it is suggested that the Moho may be a secondary thermal boundary. The currentgeothermal field and the thermal structure of the lithosphere in eastern China may mainly reflect the result ofthe tectonothermal disturbance in the Neogene-Quaternary, in other words, the lithosphere has just begun toCool.

QβstructureofthecrustanduppermantleintheeasternSino┐Koreanparaplatform

Based on the long period surface wave data recorded by the China Digital Seismograph Network (CDSN), the Q R of fundamental mode Rayleigh wave with periods from 10 s to 146 s is determined for the eastern Sino Korean paraplatform in this paper. The Q β models of the crust and upper mantle are respectively obtained for the 4 paths, with the aid of stochastic inverse method. It shows that in the eastern Sino Korean paraplatform, the average crustal Q β is about 200, and that there exists a weak attenuation layer in the middle crust (about 10～20 km deep) which is possibly related to earthquake prone layer. A strong attenuation layer (low Q ) of 70 km thick extensively exists in the uppermost mantle, with the buried depth about 80 km. The average Q R of fundamental mode Rayleigh wave is between the value of stable tectonic region and that of active tectonic region, and much close to the latter.

Research progress on layered seismic anisotropy-A review

Seismic anisotropy is an effective feature to study the inner structure of the Earth.In complex tectonic area,the assumption of single-layer anisotropy is sometimes not well consistent with the observed data;thus,the assumption of multi-layered(i.e.stratified)anisotropy should be considered.At present,the main methods to study anisotropy include receiver functions,shear wave splitting from local and teleseismic events(SKS,SKKS,and PKS,hereafter collectively called XKS),P-and Pn wave travel time inversion,surface wave inversion from far-field earthquakes and ambient noise.Each of the above method has its own advantages and limitations.Thus,one or more of the above methods are often combined to characterize multi-layered anisotropy,of which the depth range of anisotropic layers are different.This paper reviews the research progress of multi-layered anisotropy for the purpose of providing a basis for future seismic anisotropy investigations.