四川盆地东西陆块中下地壳结构存在差异

Differences of structure in mid-lower crust between the eastern and western blocks of the Sichuan basin

四川盆地是中上扬子克拉通的主要组成部分.作为我国三大稳定克拉通之一,扬子克拉通经历了自太古代以来的长期演化,直到新元古代晚期与华夏板块发生碰撞拼合前,一直被认为是一个稳定的统一陆块.基底包括了新太古宙-新元古代岩层,其上广泛被新元古代晚期至显生宙地层覆盖,仅有～2.9—2.95Ga基底岩石零星出露于四川盆地的西缘、西南缘和三峡地区,使得对于沉积盖层之下的中下地壳的性质和分布规模的认识十分有限.重力异常则能够宏观揭示区域结构特征.本文通过刨除沉积盖层和莫霍面起伏引起的重力异常而获得了中下地壳的重力异常,反映了四川盆地东西陆块中下地壳存在结构差异,结合深地震反射资料、航磁异常和地球化学资料,证实了该分界线位于重庆—华蓥一线,故而推测中上扬子克拉通在太古宙-古元古代可能存在东西两个陆核.

The Sichuan basin is the main part of the middle-upper Yangtze craton,which has been experienced a long-term tectonic evolution since Archean.It is regarded as a stable block until the collision with the Cathaysia block in late Neoproterozoic.As one of the largest and most complex Archean cratons in China,the Yangtze craton consists of Archean-Paleoproterozoic crystalline basement（mostly covered）outcropped in the western and southwestern margins and the three George area surrounded by late Mesoproterozoic to early Neoproterozoic fold belts,which are locally unconformably overlain by middle Neoproterozoic weakly metamorphosed strata and late Neoproterozoic unmetamorphosed Sinian and Phanerozoic cover. These strata limit the understanding of the nature and distribution characteristics of the middle-lower crust beneath thesedimentary rocks.Gravity anomalies can reveal the overall regional structure.In this paper we obtain the middle-lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho.Gravity anomalies measured on the surface have many possible origins,including deposits,structures,mass excess or defect caused by interfaces in the crust.First,we obtain the Moho depth and sedimentary thickness contours from resampling and interpolating the deep seismic profiles collected.Then we obtain the middle-lower crustal gravity anomalies by removing the gravity anomalies induced by the sedimentary rocks and the mantle beneath the Moho.The middle-lower crust of the Sichuan basin has gravity anomalies ranging form-32 to 206mGal,unevenly distributed.It can be divided into western and eastern parts bounded by the Chongqing-Huaying line.In the western part,there are two high gravity anomaly closures,centered at Neijiang-Suining and Nanchong-Dazhou,respectively.The maximum anomaly value reaches 170 mGal.In the eastern part,there is only one anomaly closure with 170 mGal as the highest value.Therefore,the gravity anomaly structures of the Sichuan basin on either side of the Chongqing-Huaying line are different.Combined with the deep seismic reflection profiling and other geochemical and geophysical data,we propose that the boundary of mid-lower crust between the eastern and western parts is along the Chongqing-Huaying line,which may indicate that there are two continental nucleuses beneath the Yangtze block.