【摘要】 在区域地质构造研究中,龙门山断裂带也称为龙门山褶皱-冲断带或推覆构造带。许多研究者认为,2008年汶川8级地震的发震构造是这条断裂带或其中央映秀—北川断裂。笔者在深入分析龙门山断裂带的构造演化和岩石圈结构构造特征的基础上,着重探讨8级地震的发震构造,提出不同的认识。龙门山断裂带经历了松潘—甘孜造山带的前陆褶皱-冲断带(T3-J)、造山带(K-E)和青藏高原边缘隆起带(N-Q)3个动力学条件不同的演化阶段,在前两个阶段断裂带递进发展,第三阶段断裂带则被改造。从三维空间看,龙门山断裂带位于松潘—甘孜地块东南缘的上地壳内,并被推覆到扬子陆块上;而松潘—甘孜地块的中—下地壳和岩石圈地幔发生韧性增厚,而且向扬子陆块壳下俯冲,从而使浅、深部构造在垂向上形成"吞噬"扬子地块的"鳄鱼嘴"式结构。虽然在平面上汶川8级地震的主余震分布与映秀—北川断裂一致,但从剖面上看其震源所构成的震源破裂体位于龙门山断裂带之下的扬子陆块内。这种不一致性表明,8级地震的发震构造不是龙门山断裂带,而是扬子陆块内新生的高角度断裂,其走向基本与龙门山断裂带一致。推测这一震源断裂的形成过程是:当松潘—甘孜地块向东南推挤时,其前缘"鳄鱼嘴"构造咬合并错断被吞噬的扬子陆块部分,形成具有右旋逆平移性质的新断裂,导致汶川8级地震的发生。更多还原

【Abstract】 In tectonic studies,the Longmen Mountains fault zone is also called the Longmen Mountains fold-thrust zone or nappe structure.Many researchers suggested that the 2008 Wenchuan Ms 8 earthquake was generated by the Yingxiu-Beichuan fault which is the central branch of the Longmen Mountains fault zone.Based on analysis of tectonic evolution and structural features in shallow subsurface of this fault zone,we did a new study of the relationship between the fault zone and the Ms 8 quake,and proposed a different view on this issue.Geological data indicate that the Longmen Mountains fault zone had undergone three evolution stages with varied dynamic conditions,they were the foreland fold-thrust of Songpan-Ganzi orogenic belt(T3-J),the orogene(K-E) and the uplift at the eastern margin of the Tibetan Plateau(N-Q).During the first and second stages,the fault zone developed in a progressive manner.And in the third stage,the fault zone was strongly reformed to the current shape.Now the Longmen Mountains fault zone lies in the upper crust of the Songpan-Ganzi Block and overthrusts onto the Yangtze Block in the east,where the ductile middle and lower crust as well as lithospheric mantle were thickened and plunged beneath the Yangtze Block,forming a structural style of a crocodile mouth biting the Yangtze Block in view on the vertical profile.Although the epicenters of the main shock and aftershocks of the Wenchuan Ms 8 event were distributed along the Yingxiu-Beichuan fault,its entire source body comprising these ruptures was located in the Yangtze Block below the Longmen Mountains.Such an inconsistency suggested that the causative structure of the Ms 8 quake was not the Longmen Mountains fault zone,and instead,it should be a high-angle fault in the crust of the Yangtze Block.As the Songpan-Ganzi Block moved towards southeast and its frontal crocodile-mouth-shaped structure bit the Yangtze Block,the latter was cut off to generate a new high-angle fault along which the right-lateral and thrust movement occurred,causing the Wenchuan Ms 8 event in 2008.更多还原