摘要
近年来新的研究成果反应出鄂尔多斯地块岩石圈并不是一个具有深根的完整的刚性块体,尤其在鄂尔多斯北部以及河套地堑发现有大范围的下地壳-上地幔低速低阻物质,如果这一情况属实,那么人们对鄂尔多斯地块的认识将发生大的变化.为此,我们在华北克拉通西部布设了一条穿过鄂尔多斯地块、河套地堑和阴山造山带的南北向大地电磁剖面,试图通过深部电性结构的探测提供更多信息.该剖面全长约850 km,共布设54个宽频测点和17个长周期测点.二维和三维反演结果均表明:鄂尔多斯地块内部以38°N为界,南部和北部电性结构存在明显差异.鄂尔多斯地块南部地壳至上地幔150 km深度范围内整体表现为高阻,具有刚性克拉通的特征;鄂尔多斯地块北部到河套地堑之间下地壳出现低阻层,特别是鄂尔多斯北端与河套地堑接壤地段,深部存在一个规模较大的下地壳-上地幔低阻异常体,该异常体从河套地堑开始,横向上向南延伸到鄂尔多斯地块内部约200 km,纵向上从下地壳向下延伸到上地幔(约100 km深度).根据该异常体的空间特征,参考该区地震波低速异常体的分布,我们认为鄂尔多斯北部及河套地堑中下地壳到上地幔存在热物质,其原因与深部的构造活动有关(软流圈热物质上涌、侧向流动等),这一情况可能反映出鄂尔多斯地块北部岩石圈深部正处于被改造(或者破坏)阶段,这对进一步认识青藏高原东北缘与华北克拉通之间的深部关系具有一定的启示作用.
Recent studies revealed that beneath the northern Ordos Block and Hetao Graben,there is a large-scale low velocity/low resistivity body in lower crust and upper mantle. This challenges the conventional view that Ordos Block is a rigid block with deep root. To understand the deep structures of the study area, a north-south magnetotelluric profile, crossing the whole Ordos Block, Hetao Graben and Yinshan Belt in western North China Craton, was deployed to image the deep electric structures. The 2 D and 3 D inversions of the profile shows a striking boundary around 38°N. The south part of Ordos Block has a rigid and thick(150 km) cratonic lithosphere with high resistivity; while extensive conductive anomaly exists beneath the north part of Ordos Block and Hetao Graben. The conductive anomaly extends horizontally southward from Hetao Graben to the interior of Ordos Block about 200 km, and down from lower crust to upper mantle about 100 km. According to the spatial features of the conductive complex and by referring to the low velocity anomaly, we suggest that the conductive anomaly might be hot materials in the lower crust and upper mantle,which reflect an undergoing process of reactivation or destruction in deep lithosphere of north Ordos Block and Hetao Graben. This may help further understanding the deep tectonic relationship between the northeastern Tibetan plateau and western North China Craton.