青藏高原东南缘存在连通的下地壳流吗?

Is There an Interconnected Lower Crustal Channel Flow beneath Southeastern Margin of Tibetan Plateau?

利用国家数字地震台网的137个固定台站以及332个ChinArray流动台站的数据,基于背景噪声和远震面波成像方法,共同约束瑞利面波相速度,并通过非线性方法(马尔科夫链蒙特卡洛方法)反演青藏高原东南缘的地壳三维剪切波速度结构。反演结果表明,青藏高原东南缘存在大范围连通的下地壳流,表现为下地壳存在连通的波速小于3.55km/s的近水平的剪切波低速区,且与地表地形有很好的对应关系。推断青藏高原东南缘存在3支连通的地壳流,第一支位于攀枝花一带以西,第二支位于攀枝花一带以东,第三支位于攀枝花一带的下地壳。来自青藏高原的下地壳流受到四川块体坚硬下地壳的阻挡而转向,通过峨眉山一带向南流动,与第三支通过峨眉山大火成岩省(ELIP)内带(攀枝花一带)向南流动的下地壳流一起,改造了峨眉山大火成岩省中带南部的下地壳,并使地壳增厚。推断青藏高原东南缘连通的下地壳流的南端目前大约在北纬24°附近,并将随着时间的推移向南迁移,即通过下地壳流导致该地区地壳增厚的范围也随时间的推移向南扩展。

A 3-D shear wave velocity model of Southeastern Margin of Tibetan Plateau crust was constructed by Markov Chain Monte Carlo inversion,based on Rayleigh wave phase velocity tomography results which are obtained from ambient noise interferometry and tele-seismic two plane wave analysis,using seismic data of 137 permanent stations from China digital seismic network and 332 portable stations from ChinArray.The velocity model indicates the presence of an interconnected lower crustal channel flow in southeastern margin of Tibetan plateau,which is represented by an interconnected low shear wave velocity zone with Vs<3.55 km/s.It consists of three parts which respectively locates beneath the Panzhihua area and to its west and east.The lower crustal channel flow from Tibetan Plateau is blocked by rigid lower crust of the Sichuan block and turns to flow through Emeishan area to the south direction,which alters the lower crust of Emeishan large igneous province(ELIP)together with the lower crustal channel flow beneath Pahzhihua area,thickening the crust in this area.It is infered that the lower crustal channel flow beneath the southeast margin of Tibetan Plateau ends at about 24°N,and has the potential to extend further south,which has the role to extend the scope of crustal thickening in the future.