【摘要】 龙门山褶皱冲断带位于青藏高原东缘与四川盆地之间,中生代以来主要经历了晚三叠世和新生代两期重要的地壳缩短变形,形成了典型的逆冲推覆构造带。龙门山的形成和演化与青藏高原隆升以及地震活动有着密切的关系,因此龙门山褶皱冲断带南北两段的差异性被众多学者所关注。文章在前人工作的基础之上从3个角度来阐述龙门山南北两段的差异性:1)地质调查和地震反射剖面解释表明,晚三叠世龙门山北段变形强烈而南段变形不明显,南北分段格局就此形成;2)汶川地震同震地表破裂的差异,北段以右旋斜向逆冲为主,南段则主要以逆冲为主,兼有少量的走滑分量;3)同震断裂的三维构造建模揭示出,同震断层的三维几何形态同样存在南北差异,北段同震断层只有一条且不能向下延伸至深部的主滑脱层,而南段同震断层有两条分支,二者在大约10km的深度合并成一条,并向下延伸至约17km深度与主滑脱层相连。因此,认为汶川地震同震破裂的南北分段性是由于晚三叠世龙门山先存的南北构造差异性所引发的。更多还原

【Abstract】 The Longmenshan fold-and-thrust belt between the eastern margin of the Tibetan Plateau and the west margin of the Sichuan Basin experienced at least two major periods of crustal shortening in the Late Triassic and Cenozoic, forming a typical nappe system in China.This paper focused on the segmentation of the Longmenshan belt and its significance to the Wenchuan earthquake.The Longmenshan belt can be divided into two different structural segments: the north segment,which extends from the intersection of the Longmenshan Mountains and the Micangshan Mountains to Anxian County,and the south segment,which extends from Anxian County to Baoxing County.In the north segment,that remnants of the Lower Jurassic rocks unconformably overlie folded and thrust Triassic and Paleozoic rocks supports that the first thrusting and folding there occurred in the Late Triassic.In the south segment,the conformable relationship between Upper Triassic and Lower Jurassic rocks indicates that unlike the north segment,the deformation of the same time was weak or absent there.Field investigations reveal that the Wenchuan earthquake produced two visible surface rupture zones: one along the Beichuan-Yingxiu thrust fault (260km in length)and another along the Pengguan thrust fault(70km in length).These coseismic surface ruptures can also be divided into two segments: the south segment(Beichuan-Yingxiu and Pengguan surface ruptures)and the north segment(Beichuan-Qinchuan surface rupture).The slip on ruptures of the south segment is characterized mainly by thrusting with a little amount of dextral oblique-slip,whereas the slip on ruptures of the north segment is characterized mainly by dextral strike-slip accompanied by a thrusting component.Similar,the 3-D geometry structural models of co-seismic faults,which integrate surface ruptures,relocated aftershocks,logs from petroleum wells,and seismic reflection profiles,also show a south-north segmentation: only a co-seismic fault exists in the north segment and does not reach the main detachment structure,whereas two co-seismic faults exist in the south segment.They have a complex geometry and appear to merge at 17km depth.Thus it can be believed that the segmentation of co-seismic surface ruptures of the Wenchuan earthquake should result from the pre-existing segmentation of the Longmenshan belt,i,e.the Late Triassic segmentation.更多还原