综合数据分析青藏高原东北缘六盘山地区构造形变及其构造成因独特性探讨

青藏高原边缘是研究青藏高原构造生长的重要场所.然而,青臧高原各边界却呈现出不同的地貌形态响应.尤其是青藏高原东北缘的六盘山地区,与青藏高原东缘相比,它与邻近稳定鄂尔多斯地台之间表现出了截然不同的地形变化.青藏高原东边界所对应的龙门山构造带呈现出高陡的地貌形态:在100 km范围内,海拔高程从四川盆地的500 m陡升至临近的龙门山构造带的3500 m.而青藏高原东北边界所对应的六盘山构造带则与邻近的鄂尔多斯盆地表现为宽缓的地形变化.之前由于缺少高精度的数据资料,对造成这一地表形态差异所对应的地壳结构缺少必要的了解.在本次研究中,将着重利用前期在青藏高原东北缘六盘山地区所获得的165 km长高分辨率深反射地震数据,并结合在此区域所获得的航磁数据资料进行该地区地壳结构的综合解释,得出青藏高原东北缘一鄂尔多斯地块构造转换带的地壳结构变形模型.研究表明六盘山地区主要物质组成为构造增生楔,其两侧分别存在陇西火山岛弧和鄂尔多斯结晶基底.高原生长所产生的构造应力并不能使相对松散的构造增生楔无限制的抬高而是容易发生重力坍塌,从而造成六盘山地区比较宽缓的地形结构.同时本文还将此地壳结构研究结果与前人在青藏高原东缘所获得的地壳结构及变形机制进行对比分析,探讨这两个地区的构造变形模式,并找出两个地区的构造变形共性和差异.研究结果也将为了解青藏高原侧向构造生长过程提供理论和数据支持.

Integrated analysis on the deformation of the Liupan Shan fold-thrust belt,NE Tibet,and its tectonic attribution

The margin of the Tibetan Plateau is an ideal place to study the growth of the plateau. However, topographic differences exist in the margins of the plateau, especially for the eastern margin and the northeastern margin. The eastern margin rises from a 500 m-elevation of the Sichuan basin to above 3500 m in the Longmen Shan block within less than 100 km distance. The northeastern margin, on the contrary, shows a very gentle topography. Owing to lack of high resolution data set, the mechanism behind the difference in topographic relief remains enigmatic. In this study, we will focus on the analysis of the newly obtained 165 km-long deep seismic reflection data across the Tibet-Ordos transition zone, i.e. the Liupan Shan fold-thrust belt. Together with aeromagnetic anomalies of the research area and previous studies in geology and geophysics, a crustal geometry of the tectonic transition zone between Tibet and the Ordos block will be constructed. We suggest that a tectonic accretionary prism exists beneath the Liupan Shan fold-thrust belt, which is the key responsible for the gentle topography of this area due to the less dense character of the accretionary prism and thus easily to incur gravitational collapse. In addition, the crustal architecture also indicates that there is the Longxi island arc and the Ordos crystalline basement on the two sides of the Liupan Shan fold-thrust belt. Along with previous studies on the eastern margin, a comparison will be made regarding the deformation mode differences and features in common. The results will provide substantial support to understanding the lateral growth of the plateau.