接收函数揭示的兴蒙造山带西南部的地壳结构特征

针对构造演化历史甚为复杂的兴蒙造山带的地壳结构探测较为有限的问题,本研究利用2016-2018年布设在兴蒙造山带西南部的第3期NECsaids地震台阵和固定地震台及NECESSArray的流动台数据,采用时间域最大熵谱反褶积方法提取到研究区的24027条高质量P波接收函数,进而采用H-κ和共转换点(CCP)叠加方法以及分层剥离转换震相分析地壳各向异性的方法来辨识研究区的地壳结构特征.分析表明:研究区地壳各向异性整体呈近NW向和近EW向,与区域速度场和板块绝对运动方向一致,表明现今地壳结构主要受控于太平洋板块构造域;大兴安岭-太行山重力梯级带是明显的地壳结构差异过渡区,其西侧地壳厚度明显高于东侧且呈降低趋势,而平均波速比则呈现东西两侧盆地较高的分布特征;局部地区的高波速比和不同的各向异性特征则显示受到新生代火山和断裂活动的改造作用,二连盆地的厚地壳和复杂的各向异性特征则展现出古亚洲洋闭合及蒙古-鄂霍茨克和古太平洋等多期构造作用的可能影响.

Crustal structure beneath the southwestern Xing'an-Mongolia Orogenic Belt revealed by receiver function analysis

The Xing'an-Mongolian Orogenic Belt (XMOB) is tectonically located in the area surrounded by the Siberian Craton to the north, the North China Craton to the south and the western Pacific Plate to the east. The tectonic evolution in the area was recognized closely related to the Paleo-Asian Ocean and Paleo-Pacific Ocean regimes during the Paleozoic-Early Mesozoic and related to the western Pacific Ocean and Mongol-Okhotsk Ocean regimes during and after the Mesozoic. Therefore, the XMOB is a key area to understand the NE Asian tectonic evolution and its tectonic transition from contractional thickened crust to extensional thinning. Several tectonic models, often conflicting, have been proposed to explain the evolution of the XMOB. The solution of such problems requires a better understanding of deep structure of the crust and mantle. In this study, we use the maximum entropy deconvolution method to obtain 24027 high-quality P-wave receiving function data recorded by our deployed temporary broadband seismic stations in the phase 3 of the NECsaids array, regional permanent stations and temporary stations from NECESSArray covering the southwestern XMOB. We applied the H-κ and CCP stacking techniques and the joint inversion scheme to estimate crustal thickness (H), velocity ratio V_P/V_S(κ) and crustal anisotropy (fast polarization direction φ and splitting time δt). Our results show that the dominant directions of fast wave polarity in the study area are near NW and EW, which is consistent with the regional velocity field and the absolute plate motion direction, and it reveals that the crustal structure of the southwestern XMOB is dominated by the subducting western Pacific Plate. The Great Xing'an-Taihangshan or the North-South Gravity Lineament (NSGL) is an important transition zone of the southwestern XMOB crustal structure. The crustal thickness is generally high in the west side of the NSGL and low in the east side of the NSGL, while the average crustal velocity ratio (V_P/V_S) is low on both sides of the NSGL. Some stations with relative high V_P/V_S and complex anisotropic observations may represent regional fault influences and/or localized imprints of the Cenozoic volcanic activities such as the Shuangliao and Abaga volcanic cluster. The thickened crust and the complex anisotropic observations in the Erlian Basin may reflect the overprinting of the closure of the Paleo-Asian Ocean and the evolution process of the Mongolian-Okhotsk and the Paleo-Pacific tectonic regime.