六盘山地区地壳重力均衡与隆升机制研究

Crustal isostasy and uplifting mechanism of the Liupanshan area

2014年8月跨越六盘山东麓断裂带,在长度约为200公里的剖面上展开了流动重力/GPS联合剖面观测.观测结果表明,测线东端的布格重力异常约为-190 m Gal(10^(-5)ms^(-2)),西端则为-250 m Gal左右.在假设地壳均一的前提下,基于Airy均衡模型,利用布格重力异常和GPS观测数据,分别计算了测线所在剖面的莫霍面深度与均衡面深度,发现六盘山地区处于正均衡异常状态.使用布格重力异常数据反演六盘山地区的地壳密度结构,并据此地壳分层结构,计算了六盘山地区均衡面与莫霍面深度,对比显示该区域亦处于均衡正异常状态.为了确定青藏高原东北缘岩石圈有效弹性厚度和六盘山隆升机制,我们利用EMG2008自由空气异常和SRTM V18.1 DEM数据,使用自由空气重力异常导纳方法,研究了以六盘山地区为中心的青藏高原东北缘岩石圈有效弹性厚度(Te)和加载机制,发现六盘山地区的Te为5 km,岩石圈加载主要来自于地表,占总加载的95%.最后,对比六盘山地区Airy均衡异常与弹性板均衡异常,发现六盘山东麓断层处地壳承载梯度值最大,表明该断裂带吸收了较多的应变能.

In this study we investigate the isostasy state and the uplifting mechanism of the Liupan Shan area which belongs to the convergence region between Tibetan Plateau and Ordos block.Based on the observed and modeled gravity data,we find that the isostasy anomalies of Liupan Shan area are positive as a whole. We deduce that the uplifting of the Liupan Shan area is due to the outward growth of high terrain of Tibetan Plateau.We investigate the isostasy state and the uplifting mechanism using Airy isostasy theory and gravity admittance method.Specifically,we first conduct gravity and GPS hybrid observations at a survey line nearly 200 kilometers which is made up of 51 observation stations across the fault of eastern Liupan shan. Then,under the precondition of assuming homogeneous crust,we calculate the Moho and isostasy depth beneath the survey line,using our observation data and Airy isostasy theory. Next,we inverse the density structure of the crust of Liupan Shan area using bouguer gravity anomalies. And then,we calculated the Moho and isostasy depths based on our inversed density structure of the crust. At last,using gravity admittance method we estimate the effective elastic thickness（ Te） of the lithosphere of northeastern margin of Tibetan Plateau and the uplifting mechanism of Liupan Shan area. The gravity admittance method is a spectrum analysis method which based on the relationship between gravity anomaly and topography in wave number domain. The gravity and topography data that used in this study are come from form EGM2008 gravity model V23. 1 and SRTM V18. 1 DEM.Our observations show that the bouguer gravity anomalies of the study area are negative in total,which change gradually from about- 190 mG al in the east end to- 250 mG al in the west end. The difference between the Moho and isostasy depths represents that the Liupan Shan area is in the state of positive isostasy anomalies.Meanwhile,the results of considering density structure of the crust represent also that the area is in the state of positive isostasy anomaly. Such positive isostasy anomalies indicate that the root of the Liupan Shan is not deep enough to compensate the uplifting height of this area. The results of gravity admittance method show that Te of the Liupan Shan area is 5km,and the load mainly from the surface of lithosphere,accounting for 95 percent of the total load. The differences between Airy isostasy anomalies and elastic plate isostasy anomalies reveal that the largest gradient of the crust load is located at the position of the fault of the eastern Liupan Shan,as indicates that the fault zone absorbed more strain energy than adjacent area.