地球物理综合观测揭示秦岭-桐柏-大别复合造山带地壳及上地幔结构

The crustal and uppermost mantle structure of the Qinling-Tongbai-Dabie orogenic belt from integrated geophysical observations

秦岭—桐柏—大别复合造山带(以下称为秦岭大别造山带)属于中国中央造山带的一部分,由华北克拉通与扬子克拉通汇聚形成.对于秦岭大别造山带及其周缘地区的研究,可以为这一大陆碰撞造山带的形成与演化过程提供重要信息.本文整合研究区域的接收函数与背景噪声数据,采用H-κ叠加分析、接收函数与背景噪声联合反演、克希霍夫偏移成像等方法,得到了沿秦岭东西方向具有高分辨率的地壳及上地幔结构.研究结果显示:(1)莫霍面深度由西向东逐步抬升,由剖面西侧最深约55 km上升至剖面东侧最浅约30 km;莫霍面于东西秦岭之间起伏明显;桐柏以及东大别下方莫霍面局部加深.(2)西秦岭中下地壳观测到的高速异常阻隔了青藏高原东北缘地壳低速异常的向东扩张,反映了青藏高原东北缘的中下地壳流没有通过西秦岭继续向东流动.(3)西秦岭岩石圈地幔顶部高速异常延伸至100 km深度(剖面底部),桐柏—西大别岩石圈地幔顶部高速延伸至70 km深度,东大别、东秦岭岩石圈地幔顶部未见较大深度范围的高速异常.

The Qinling-Tongbai-Dabie orogenic belt(QD)is the important element of the China Central Orogenic Systems,which was formed by the convergence of the North China Craton and the Yangtze Craton.Due to its key position in the Central Orogenic Systems,the QD is an ideal place to study the orogenic processes between collided continents.To reveal the fine crustal and uppermost mantle structure with high resolution beneath the QD and surrounding areas,this work integrates seismic data available around the QD,which consist of raw seismic data,ambient noise data,and receiver function data.We adopted the Kirchhoff migration method to obtain a preliminary image of the Moho,used the H-κanalysis to calculate the crustal thickness and V P/V S,and utilized a joint inversion of ambient noise and receiver functions to construct a high resolution V S velocity model beneath the QD.Finally,we obtained a profile crossing the QD from west to east with detailed structures in the crust and uppermost mantle.The results reveal that:(1)Moho depth gradually becomes shallower from west to east,from the deepest about 55 km in the west to the shallowest about 30 km in the east of the profile.(2)The high velocity anomaly in the middle and lower crust of the West Qinling blocks the eastward expansion of the low velocity anomaly in the crust of the Northeast Tibetan Plateau,which provides unambiguous evidence that the crustal ductile flow within the Northeast Tibetan Plateau does not continue to flow eastward through the West Qinling.(3)In the uppermost mantle,the West Qinling is underlain by high velocity down to at least 100 km depth,the Tongbai and East Dabie are underlain by high velocity down to 70 km depth,however,the high velocity is not shown beneath East Qinling or East Dabie.