深反射地震成像揭示的班公湖-怒江缝合带中段Moho断阶及其大地构造意义

Moho terraces and its tectonic significance of central Bangong-Nujiang suture zone uncovered by deep-reflection seismic imaging

揭示班公湖-怒江(班怒)缝合带Moho(莫霍面)结构对于认识中特提斯洋壳俯冲和南羌塘坳陷成因具有重要地球动力学意义。基于横跨班怒缝合带的深反射地震数据(88°30′E),本文采用了中长波长静校正、噪声压制、优化叠加和叠前深度偏移(PSDM)等地震处理技术,获得了深度域地震反射偏移剖面、层速度场和高分辨率Moho结构。由深度域剖面显示,班怒缝合带Moho位于地表以下65~80 km,呈不连续北向抬升趋势,指示在拉萨地块与南羌塘地块之间存在岩石圈上地幔断阶,最大阶步可达15 km。综合分析缝合带两侧的Moho形态认为,这些断阶受南侧拉萨地体的岩石圈上地幔以19.5°北倾俯冲与北侧南羌塘地块的上地壳抬升驱动,可能与深部存在局部熔融相关。班怒缝合带下的Moho结构表明,随着晚侏罗世—早白垩世中特提斯洋闭合,南羌塘地体由边缘海沉积向前陆盆地转换,形成南羌塘坳陷。

Knowledge of the Moho in the Bangong-Nujiang suture(BNS)is geodynamically of importance to understand the subduction of Meso-Tethys oceanic crust and the genesis of southern Qiangtang depression.Based on deep-reflection seismic data(88°30'E)across the BNS,depth-domain seismic profile and velocity field with high-resolution Moho geometry are obtained by deep-reflection seismic imaging using such techniques as mid/long wavelength static correction,noise elimination,optimal stacking,and PSDM.The depth-domain imaging shows that the Moho of the BNS located at a depth of 65~80 km tends to discontinuously uplift northwards,indicating a few lithospheric mantle terraces(up to 15 km)developed between the Lhasa and Qiangtang terranes.The geometry of the Moho in the BNS demonstrates that the terraces have been driven by southern lithospheric mantle subduction at 19.5°northward and northern crustal uplifting of southern Qiangtang basin due to possible deep partial melting.The Moho structure suggests that southern Qiangtang basin was evolved from marginal marine into foreland basin during the Late Jurassic to the Early Cretaceous along with the Meso-Tethys closure.