青藏高原东北缘重力异常多尺度横向构造分析

摘要

We first analyzed the characteristics of the Bouguer gravity anomalies in the northeastern margin of the Tibetan Plateau. We then did multi-scale anomaly separation and transversely structural analyses based on the gravity anomalies respectively by using the preferential filtering and the normalized vertical derivative of the total horizontal derivative. Fromrn the characteristics of the separated multi-scale anomalies, we obtained the following conclusions:rn1) There is a remarkable gravity anomaly gradient belt roughly along 106癊 in the northeastern margin of the Tibetan Plateau, that is the north section of the Helan Shan-Liupan Shan-Sichuan-Yunnan tectonic belt. The characteristics of the Bouguer anomalies are quite different on both side of this gradient belt, indicating the difference of density structures and tectonic features.rn2) To the north of Dingbian in the Ordos block, a high gravity anomaly belt trends from NE to nearly S-N, which implies there is a density or structural interface around the transition zone. Both sides of the interface have different material compositions and tectonic features. Comparisons of the large-scale and small-scale gravity anomalies indicate that these differences are mainly caused by the deep structure of the upper mantle. 3) The tectonic boundaries in the deep northeastern margin of the Tibetan Plateau are quite different from the geological outcrops on the Earth' s surface, reflecting the tectonic complexity and diversity in the deep and shallow structures. 4) Because of the collision between the Indian plate and the Eurasian plate, and the subsequent continuous northward extrusion effects of the Indian plate, the materials of the middle-lower crust in the northeastern margin of the Tibetan Plateau squeeze into the Qinling orogenic belt which is relatively weak, under the interaction of huge northward push force and barrier of the Ordos rigid block. Based on the characteristics of the multi-scale gravity anomalies and their transverse structures, we deduced five deep fault zones in the studied area, i. e. , Qinling North Margin fault, Haiyuan-Liupan Shan fault, Xiang Shan-Tianjing Shan fault, Yantong Shan fault and Longshou Shan fault. And we also analyzed the possible distribution of these five fault zones in the deep crust.%本文研究了青藏高原东北缘地区布格重力异常特征,采用优化滤波法和归一化总水平梯度垂直导数法对研究区重力异常进行多尺度分离和横向构造分析.分离出的多尺度重力异常特征表明:1)青藏高原东北缘地区大致以东经106°线为界,有一条醒目的重力异常梯级带,即贺兰山—六盘山—川滇南北构造带的北段,其东西两侧布格重力异常特征在形态和走向上截然不同,意味着两侧密度结构和构造特征存在明显差异.2)鄂尔多斯地块内部定边以北,重力异常高带走向由北东向转为近南北向,推测定边附近存在一个密度或构造界面,其两侧物质组成和构造特征具有差异,对比大尺度重力异常和中尺度重力异常,表明异常特征的这种差异主要是由上地幔深部结构引起的.3)青藏高原东北部各块体深部边界位置与地表构造分布不同,反映出该区构造复杂,深浅构造差异大.4)由于印度—欧亚板块碰撞及随后印度板块持续向北的挤压作用,造成青藏高原东北缘中、下地壳物质在巨大的北东向推挤力和鄂尔多斯刚性块体阻挡的共同作用下,沿着相对软弱的秦岭造山带方向蠕动.依据多尺度重力异常及其横向构造特征,综合推断出研究区内五条断裂带,即秦岭地轴北缘断裂带、海原—六盘山断裂带、香山—天景山断裂带、烟筒山断裂带和青铜峡—固原断裂带,并分析了它们在地壳深部的可能展布特征.