Rayleigh Wave Tomography Beneath The Southwestern Qin-Hang Metallogenetic Belt And Its Adjacent Area

Qinzhou Bay-Hangzhou Bay metallogenetic belt?Qin-Hang metallogenetic belt for short?is a tectonic metallogenetic belt between the Yangtze block and the Cathaysian block.It records the process of multiple amalgamation and splitting of the two adjacent blocks,and constitutes the central axis of South China tectonic domain.The metallogenetic belt is of great scientific significance for revealing the tectonic evolution of South China continent,and has always been a research hotspot of global geologists.However,the geological structure of the metallogenetic belt was formed for a long time,overlaid by thick superstratum after the late Paleozoic,transformed and displaced by later structure,and destroyed by magma intrusion.Therefore,there are many unresolved geological problems,especially in Guangxi.Therefore,it is necessary to carry out deep geophysical exploration in the region.Seismic tomography can obtain the velocity structure of the crust and upper mantle,which is a powerful tool to study the internal structure of the crust and upper mantle.It mainly includes body wave tomography and surface wave tomography.Compared with body wave,surface wave has the advantages of slow attenuation,strong energy,easy identification,high vertical resolution and so forth.The velocity structure of South China has been obtained by surface wave tomography.But due to the uneven distribution of stations and large aperture of stations,the resolution of velocity structure in the horizontal direction is low with larger than1.0°×1.0°grid.From 2013 to 2018,mobile seismic array was set up in southwestern Qin-Hang metallogenetic belt.In this paper,based on the seismic data acquired by the array,the surface wave velocity structure and S-wave velocity structure are obtained using surface wave tomography,which provides a scientific basis for the study of the deep dynamics in this area.The main research contents and achievements are as follows:?1?The distance factor is first defined to improve the matching condition of interstation and correct the measured group velocity.It is proved that improved matching condition of interstation by both azimuth difference a and distance factor is superior to the traditional matching condition.At present,the interstation distance is mainly used to calculate group velocity in interstation method.But the interstation distance is not the true propagation distance.Using distance factor to correct the group velocity calculated by interstation distance can reduce the measurement error of group velocity.?2?The time-frequency analysis method of measuring group velocity is studied.Based on the synthetic surface wave data of different epicenter distance,the error of group velocity measured by multiple filter technique?MFT?is analyzed.MFT is firstly compared with continuous wavelet transform?CWT?.It is found that MFT in frequency domain should be equivalent to CWT with Morlet wavelet in time domain when?=?₀²?12.5,selecting the value of Gaussian filter parameter?in MFT is equivalent to choosing of the central frequency?₀ of Morlet wavelet in CWT.Based on theoretical synthetic surface wave data and observed surface wave data,it is proved that ony when the admissibility condition is satisfied,group-velocities would be well estimated by MFT or CWT.?3?Using seismic data acquired by dense mobile seismic array for in study area,5410group velocity curves are picked up and 1570 high quality group velocity curves are finally obtained after averaging group velocities from the same path.Based on group velocity data of5?30 s period,imaging research with horizontal grid scale of 0.5°×0.5°is carried out by SIRT.And then S-wave velocity structure of the study area is established by using genetic inversion algorithm and it fills in the blank of deep high-resolution geophysical data in this area.?4?According to the velocity structures in the study area,faults and structural units are preliminarily discussed.It is infered that the boundary between Yangtze block and the Cathaysian block may be along Longsheng-Yongfu-Laibin-Nanning-Pingxiang line and Kunlunshan-Nandan fault may be the northeastern boundary of Youjiang trough.