基于激光扫描和构造分析的紫云苗厅“双穹顶结构”成因机理初步研究

隶属于贵州格必（凸）河伏流洞穴系统的紫云苗厅,是世界上已知探明体积和表面积最大的洞厅,平面投影面积仅次于马来西亚的沙捞越大厅。2014年英国研究人员采用Riegl VZ-400三维激光扫描仪首次对整个苗厅进行了地面激光扫描,本研究通过激光扫描数据并结合地质构造背景分析,对苗厅基本形态特征及其控制因素获得了如下新认识:（1）苗厅整体由两个洞厅与一段大型廊道（极有可能是世界上跨度最大的洞穴廊道）共同构成,在形态上则是两个穹顶与一个拱顶相连接,总体呈“凹”字形。点云切片显示苗厅为鞍部（上部）水平截面呈两个独立又相互关联的多边形且底座并联的“双穹顶”特殊结构;（2）苗厅鞍部的多边形系多期地质构造错动所致,产生了不同方向、不同等级的断裂和节理体系,在区域内形成了具有格子状构造的格局,密集的裂隙交汇于苗厅使得溶蚀作用最为发育;（3）苗厅发育在交麻向斜核部下二叠厚层—块状石灰岩地层中,向斜底部的开张裂隙为地下水进入提供了良好的构造条件,是苗厅能发育成世界级巨大洞厅一个不可忽略的关键因素;（4）苗厅还存在上层洞道,与区域内的层状洞穴共同反映了地质历史时期构造抬升与河流下切的双重作用。推测早期有其它多股水流从高处共同汇聚于苗厅,共同掏蚀出巨大的地下空间,并经历了从潜流带到包气带的转换,才最终形成现今所见的宏大洞厅;（5）根据地貌形态与地质构造的关系,“双穹顶”的发育暗示了有利于形成超大地下洞厅的特殊地质构造也是相邻成对的。

Mechanism of Miao Chamber in Ziyun county based on terrestrial laser scanning and structural analysis

Located in Ziyun county of Guizhou Province, Miao Chamber belongs to the Gebihe（another name Getuhe）underground river cave system. It is the largest chamber in the world as known by volume and surface area, and its planar projected area is second only to Sarawak Chamber in Malaysia. In 2014, for the first time, British researchers conducted the terrestrial laser scanning and collected the entire point cloud of Miao Chamber by using Riegl VZ-400 3D laser scanner. Combining the point cloud with the analysis of the geological structure, researchers have obtained the following new understanding of the basic morphological characteristics and control factors of Miao Chamber, （1） The integrity structure of Miao Chamber consists of two chambers and a section of large gallery, and this arched gallery probably becomes the world's largest span cave gallery currently registered. Morphologically, two domes connect with one vault, which shapes like the Chinese character "concave". The upper layer plan view of Miao Chamber presents a double-dome structure with two nearly regular polygons and their pedestals in parallel connection. These two polygons are independent but connect with each other. （2） The polygons of upper Miao Chamber are caused by the dislocation of multiple geological structures. This dislocation produced a set of fracture and joint systems in different directions and grades, and formed a pattern with lattice structure in this region, and then the dense fissures converged in Miao Chamber, which makes the dissolution the most developed. （3） Miao Chamber developed in the core of Jiaoma syncline with thick-massive limestone belonging to lower Permian stratum. The opening fissure at the bottom of the syncline provides good structural conditions for the entry of groundwater, which is a key factor that cannot be ignored for the development of Miao Chamber, one of the world’s hugest. （4） The upper layer gallery in Miao Chamber, together with the layered caves in the region, reflects the dual effects of tectonic uplift and river incision in geological history. It is speculated that in the early period many other streams of water from the high place converged in Miao Chamber toerode a huge underground space. Miao Chamber experienced the transition from hyporheic zone to vadose zone, and finally formed the huge cave chamber that accommodates a large number of collapsing boulders  due to stress adjustments. （5） According to the relationship between geomorphic form and geological structure, the development of "double dome" implies that the special geological structure is conducive to the formation of the huge underground caves in pairs, but the exact structure remains to be ascertained.