长江三峡巴东复杂斜坡系统成因研究

Study on Genetic Mechanism of the Badong Slope System in the Three Gorges of the Yangtze River

三峡库区巴东县新城区所在的扇形大斜坡是一个复杂斜坡系统。斜坡后缘边界的挤压变形带是斜坡整体沿其底界面(T1j3—T2b1)之间岩层界面向长江发生重力蠕滑作用的结果,不存在“巴东断裂”问题。斜坡系统内部未发现成规模的褶皱,也不存在数量众多的内动力成因的断裂带。巴东斜坡区浅表生变形破坏类型可概括为侧向拉裂、“雪崩式”垮塌、表层风化、侧向滑移张剪破裂、岩层折断变位、软岩膝折剪破裂和压扭破裂又重胶结7种重力卸荷破坏力学模式。斜坡系统内部发育分期分区(块)滑动(滑坡)现象,但不存在构造地质意义上的“断层”形迹。在地貌形态上,巴东大斜坡被4条冲沟分割成5个斜坡单元。在斜坡地质结构、成因类型与空间分布上,巴东大斜坡可划分为3个层次,即表层崩塌滑坡成因为主的堆积层(第一层次),冲沟分割且浅表生地质形迹发育的层状顺倾的中间基岩层(第二层次)和整体连续顺倾的深层基岩层(第三层次)。分形几何计算证明,巴东大斜坡的地貌形态尚处于侵蚀发育的青(幼)年期。FLAC3D数值模拟发现,在长江侵蚀下切的不同阶段,巴东斜坡体前缘和后缘接近底界面位置塑性变形区分布集中,但不具备沿深层界面发生整体滑动剪出的可能性。基本认识是,“巴东复杂斜坡系统”是在官渡口—东氵襄口向斜南翼(单斜山)的地质背景下,持续经受长江快速侵蚀下切外动力作用,河谷岸坡快速临空导致其自身重力产生强烈的侧向卸荷与滑移等浅表生地质改造作用过程而形成的,可简单地概括为“重力成因论”。

The fan-shaped slope for new town of Badong county is a complex system in the Three Gorges reservoir of Yangtze River. Compression deformation zone in the posterior border of slope is resulted from gravity creep action to Changjiang River along its bottom interface （separation plane of bed between T1j^3 and T2b1）, and there is no ＂Badong fracture＂. Neither larger scale folds nor numerous fracture bands is found. Hypabyssal and hypergene deformation failure types may be divided simply into seven unloading mechanics models, i.e. lateral pull-apart failure, breakdown or collapse like snow avalanche, surficial weathering, tenso-shear fissures from sliding, rock layers breaking and shift, soft rock king-band and shear fracture, and blastic deformation after compresso-shear frature. There are no ＂fault zones＂ in a meaning of structural geology, although sliding phenomena is widespread in different geo-periods. Badong large slope has been cut apart five secondary slope unites by four gullies in geomorphologic landscape. Based on the geo-structures, genesis patterns and space distribution, Badong large slope has been divided into three parts, i.e. the upper part to be accumulation horizon （first layer） mainly caused from surface falling or landslide, the middle part to be a stratified and consequent bedrock layer （second layer） with Hypabyssal and hypergene geologic trace, apart by gullies, and the lower part to be a fully continuous and consequent bedrock layer （third layer）. According to fractal geometry dimension calculation, the geomorphologic landscape of Badong slope is in the neanic/infancy stage. In the light of FLAC^3D numerical simulation, there are plastic deformation and stress concentration areas in the frontier and posterior border near bottom in the different geo-historical periods for Yangtze River erosion and undercutting process, but no possibility for whole slope to shear and slide along deep-seated bottom interface. Our basic conclusion is that. ＂Badong complex slope system＂ has been made in itself sustaining gravity action, so called ＂review of gravity genesis＂. The action has been resulted from the River bank slope breakdown to produce unloading and sliding with eroding, such as sustainable hypabyssal and hypergene rebuild, in which happen gradually undercutting by Yangtze River to monocline slope in the southern limb of Guandukou-Dongrangkou syncline.