龙门山中北段构造变形特征新认识——对多层次滑脱变形的指示意义

龙门山中北段处于中段与北段过渡转换位置，前期的构造变形特征研究相对薄弱。基于重新处理的二维地震长线资料，综合运用野外基础地质观测、钻井结构分析和构造解析等手段方法，系统厘定了龙门山中北段构造变形特征。龙门山中北段厚皮带一侧变形层次深，以韧性变形为主，表现为一系列紧闭倒转向斜和背斜组合样式；过渡Ⅰ带发育多条逆冲断裂呈叠瓦状冲断至地表，主干断裂下盘三叠系软弱层中通常派生次级平缓断裂，控制着内部叠瓦状不对称断层传播褶皱的发育；过渡Ⅱ带整体为山前隐伏带，发育1号隐伏断裂及其上盘强烈变形的叠瓦状逆冲构造和下盘弱挤压褶皱变形的原地体等。高角度逆冲主干断裂下盘的相对软弱层中通常派生发育次级平缓断裂，两者构成倒“γ”型变形样式。综合变形特征和成因机制分析认为，叠瓦状和倒“γ”型复合冲断样式构成研究区基本冲断变形样式，其形成受控于龙门山地区由深到浅、自后缘向前缘的多层次滑脱变形。 

New insights into structural characteristics in central-northern Longmen Mountains:implications for multipledecollement deformation

The central-northern segment of the Longmen Mountains is located in the transfer zone,for which the structural characteristics are little studied.Based on the reprocessed twodimensional seismic data,this work used methods such as field geological investigation,drilling analysis and structural interpretation to comprehensively study the structural characteristics in the central-northern segment of the Longmen Mountains.The thick belt is characterized by deep and ductile deformation and shows the combination styles of a series of tight anticlines and synclines.The transition belt I developed many imbricate faults which cut to the ground surface.The main faults in the transition belt I derived many secondary gently dipping faults in the footwall,which controlled the develop ment of the relative faultpropagation folds.The transition beltⅡis a buried structure that includes the No.1 fault.The intense imbricate thrust structures occurred in its hanging wall and the autochthonous structure with weak fold deformation developed in its footwall.This study provides new insight that the highangle thrust faults commonly derive secondary gently dipping faults in the footwall,and both of them formed the fallingγtype of the structure style.According to the analysis of the structure characteristics and mechanism origin,it is suggested that the composite structure styles combining the imbricate and fallingγtype are the basic structure styles in the study area,which is controlled by the multipledecollement deformation from the deep layer to the shallow layer as well as from the trailing edge to the leading edge.