龙门山中段山前带构造楔的发现及其几何学、运动学特征:对青藏高原东南缘隆升动力学机制的约束

运用断层相关褶皱几何学原理,对龙门山中段山前的地震反射剖面进行解释。研究发现,龙门山中段山前带垂向上具有多套滑脱层。其中深层次滑脱层位于前震旦系基底深度约(19±2)km的地方,其上发育双重构造和叠加构造楔,构造楔是龙门山中段山前带重要的构造样式之一。工区A剖面深层构造楔模型以后展破裂式叠加形成,断层产生的滑移量达45.5 km;B剖面深层构造楔模型以前展破裂式叠加形成,断层产生的滑移量约16.6 km;构造楔的形成导致上部断裂和岩层隆升并褶皱变形,A、B剖面山前地层相对川西平原最大抬升量分别为8 km和3 km。构造楔沿龙门山中段山前带走向平面上呈带状分布,具有不同的规模和几何形态;分析其形成时间较晚,可能形成于喜马拉雅期。通过对深层叠加构造楔几何学和运动学的定量计算,其正演运动学模型与实际剖面解释相吻合。深层叠加构造楔的形成在龙门山中段前山带有着其相应的成因机制,在所能限定的空间范围内,其特殊的地理位置、构造特征与成因,可以为研究青藏高原东南缘隆升的动力机制提供一定的约束。

The discovery of structural wedges in the central Longmen Mountains front belt and its geometric and kinematic characteristics: Constraints on the dynamical mechanism of uplifting in the southeast margin of Qinghai-Tibet Plateau

In this paper,the fine interpretations of seismic profiles in the central Longmen Mountains have been accomplished with the application of fault-related folding geometry principles.Our studies found that there are multi-detachment layers with different structural deformation in the mountain front belts.Among them,the deep detachment,which is below the Pre-Cambrian Formation at the depth of(19±2) km,developed the duplex structures and superimposed wedges.The structural wedges are one of the most important structural styles in this area.The deep structural wedge model in section A was formed by superposition of break-backward style and slipped more than 45.5 km.In contrast,structural wedge model in section B was formed by superposition of break-forward style and slipped only 16.6 km.Besides,the upper faults and strata were uplifted and folded because of the motion of the structural wedges.The largest vertical uplift of strata in the A and B section are 8 km and 3 km relative to the Western Sichuan Basin,respectively.The Superimposed wedges strike along the Longmen Mountains with zonal distribution in plan,having different sizes and geometric shapes.Our analysis indicated that they were formed during the Himalayan periods.Meanwhile,through the quantitative geometric and kinematic analysis,it is indicated that the forward modeling of wedges is in coincidence with the seismic sectional characteristics.The formation of the wedges had its own mechanism which is unclear yet.Owing to the special location,the structural characteristics and the genesis,the study of the formation of the wedges could provide the constraints on the dynamical mechanism of uplifting in the southeast margin of the Qinghai-Tibet Plateau.