Crustal Anisotropy Characteristics Of The Eastern Himalayas Tectonic Junction And Adjacent Areas

The land-continent collision orogeny of the Indian plate and the Eurasian plate created the Tibetan Plateau,which is the highest plateau on earth today.This orogenic movement has made the Tibetan Plateau one of the areas with the most frequent seismic activities,the most violent tectonic activities and topographical changes.The tectonic evolution process and geodynamic characteristics of the Tibetan Plateau are of great significance for solving geophysical problems such as lithospheric deformation,plate collision,plateau uplift,and seismogenic mechanism,and have always attracted the attention of geoscientists.The Himalayan orogenic belt,located at the forefront of the collision between the Indian plate and the Eurasian plate,is the largest and youngest orogenic belt in the world today.The eastern and western ends of the orogenic belt have formed an Eastern Himalayan Syntaxis and a Western Himalayan Syntaxis,respectively.The Eastern Himalayan Syntaxis is located at the core of the plate collision.The geological geomorphology and structural framework in the region have a large turn of nearly 90°.It is one of the areas with the most severe lithospheric deformation,the strongest tectonic stress,and the fastest uplift and denudation in the Himalayan orogenic belt.Due to the drastic topographical changes,complex geological structures,and frequent seismic activities of the Eastern Himalayan Syntaxis,it is a natural experimental field for studying the lithospheric deformation patterns and structural evolution laws during the land-continent collision.The study of the internal structure of the earth and the deformation characteristics of the lithosphere of the Eastern Himalayan Syntaxis provides an important basis for exploring the dynamic evolution process of the land-continent collision orogen in the Tibetan Plateau.Researches on the anisotropy of shear wave splitting in the Tibetan Plateau have been carried out for nearly 30 years,and a large number of shear wave splitting results have been obtained,which has made important contributions to understandings of the deformation characteristics and dynamic interpretation of the crust and mantle anisotropy in the Tibetan Plateau.By summarizing the shear wave splitting results of the anisotropy in the upper crust,crust and upper mantle in the Tibetan Plateau over the past 30 years,and systematically analyzing the shear wave splitting results,it is shown that the XKS wave splitting results are the most stable,the direct S wave splitting results from local earthquakes are better,but the Pms wave splitting results are more discrete.The Pms wave splitting results of different studies in the same study area are often different.The main reason may be the signal-to-noise ratio of the Pms wave is lower than that of the XKS wave and the direct S wave of the local earthquake.The secondary reason may be the differences in methods and processing analysis of various studies.Based on the teleseismic waveform data of 48 broadband seismic stations in the Eastern Himalayan Syntaxis and adjacent areas,we acquired 295 pairs of anisotropy parameters by the Pms wave splitting measurements.The results show that the fast wave polarization directions of the Eastern Himalayas Syntaxis are NE-SW,and rotate clockwise in the adjacent areas.The delay times of the Pms wave splitting in the study area are in the range of 0.11-0.30 s,with an average value of 0.24 s.The comparative analysis of the results of the Pms wave splitting,the direct S wave splitting and the teleseismic SKS wave splitting shows that the crustal anisotropy of the Eastern Himalayan Syntaxis and adjacent areas is mainly from the middle and lower crust,the results of the Pms wave splitting mainly reflect the anisotropy of the middle and lower crust.It also shows that the crust anisotropy has little effect on the SKS wave splitting,the results of SKS wave splitting mainly reflect the anisotropy of the upper mantle.The fast wave polarization directions of the Pms wave splitting are almost consistent with the fast wave polarization directions of the direct S wave splitting and the teleseismic SKS wave splitting,and have a good correlation with the surface structures and deformation characteristics,indicating that the lithospheric deformation of the Eastern Himalayan Syntaxis and adjacent areas may be a Vertical Coherent Deformation mode.